JP2025533531A - 3D structured electrodes for electrochemical cells - Google Patents

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application No. 63/409,688, filed September 23, 2022, which is incorporated herein by reference in its entirety.

This disclosure generally relates to 3D structured cathodes for batteries.

Certain batteries use cathodes that undergo large volume changes during electrochemical cycling. For example, batteries that use electrochemically active conversion materials in the cathode experience cathode expansion and contraction as the conversion material transforms from one form to another. A specific example of such a battery is a lithium-sulfur battery, in which the conversion between sulfur and lithium sulfide causes large volume changes in the cathode. The conversion to _Li2S can result in significant volume expansion. Such expansion can adversely affect battery performance. For example, the expansion can displace electrolyte from the bulk of the cathode, which in turn can affect the charging (and subsequent discharging) of the battery. Therefore, a cathode material that mitigates the adverse effects of volume changes during electrochemical cycling is needed for batteries.

Another problem that can exist with certain battery chemistries that utilize electrochemically active conversion materials in the cathode is the formation of insoluble products during electrochemical cycling. Such products often deposit on the surface(s) of the cathode, for example, at the cathode-separator interface. The deposition of such products on such surface(s) reduces mass transport into the bulk of the cathode, which can result in undesirable capacity reductions during subsequent electrochemical cycling. Therefore, there is also a need for cathode materials that mitigate the adverse effects of uncontrolled deposition of insoluble products on subsequent electrochemical cycling.

The present disclosure addresses problems caused by volume changes and/or insoluble product formation during electrochemical cycling through the use of three-dimensional structured cathodes (or "structured cathodes" for short). Structured cathodes may include cathode films having a patterned surface with recesses extending into the film. In certain embodiments, the structured cathode includes an electrochemically active conversion material (referred to as a "structured conversion cathode"). Different structured cathodes may be used in various electrochemical cells with different chemistries. In some embodiments, structured conversion cathodes are used in lithium-sulfur batteries. While sulfur is a common electrochemically active conversion material, other materials, such as other chalcogenides (e.g., Se and Te), can also be used.

A structured cathode can be formed by providing (e.g., forming) a cathode film and then patterning the surface of the cathode film to obtain recesses, such as holes and/or grooves. For example, the cathode film can be deposited on a substrate (e.g., a current collector) and then patterned. The recesses can extend only partially through the film or can extend throughout the entire film. The recesses can be interconnected or separate. The recesses can be regularly or irregularly distributed across the patterned surface. The recesses can be at least partially filled with the electrolyte in the battery. The cathode film can be calendered before patterning. This ordering of steps can improve the final cathode film structure compared to the reverse ordering, where calendering can undesirably alter or destroy the intended recesses (e.g., their morphology). In certain embodiments, patterning is achieved by removing material from the cathode film. Laser ablation is a particularly convenient process for removing material because it allows for a high degree of control using precise laser placement, spot size, exposure time, etc. Furthermore, laser ablation can be easily integrated into cathode manufacturing processes, such as roll-to-roll processes. Cost is a critical consideration, especially for the competitiveness of certain battery technologies, such as lithium-sulfur batteries. The low material cost of lithium-sulfur batteries justifies the additional expense of laser ablation. In some embodiments, laser ablation itself is low cost because it can be easily integrated into existing manufacturing processes. Thus, cost-viable structured conversion cathodes for batteries can be realized. A pulsed laser may be used to perform the laser ablation. In some embodiments, a patterned compression or other debossing process is performed to form recesses in the cathode film.

Unstructured conversion cathodes can expand significantly during electrochemical cycling of the cell (e.g., conversion to _Li2S in a lithium-sulfur battery). In particular, the expansion reduces the pore volume, and electrolyte can be displaced from the pore volume accordingly. In some cells, even if the electrolyte is displaced from the cell stack or into the void volume and the cathode volume is reduced (e.g., if one or more lithium sulfides, such as _Li2S, revert to S), it can be difficult to return the electrolyte to the porosity upon charging the cell. The recesses in the patterned cathode surface can provide localized reservoirs of electrolyte. The electrolyte can easily migrate to such reservoirs during cathode expansion and can more easily return to the bulk of the cathode as the cathode volume decreases.

Additionally or alternatively, recesses in the patterned cathode surface can shorten the mass transport distance of electroactive species (e.g., Li ⁺ ) into and out of the bulk of the cathode. Without intending to be bound by any particular theory, shortening the mass transport distance can enable more effective utilization of the available ion storage capacity within the cathode. In unstructured cathodes, mass transport is primarily determined by the film thickness. In structured cathodes, the recesses extend into the cathode film, thereby shortening the transport distance into the bulk of the film. The recesses in the patterned surface of the cathode film can lead to an increased rate and/or extent of ion transport (e.g., lithium ion transport) into and/or out of the cathode film during electrochemical cycling.

Additionally or alternatively, the electrochemical cell may be configured to form insoluble products (e.g., non-equilibrium products) during electrochemical cycling of the cell. Such products may form at and/or be transported to the cathode-separator interface within the electrochemical cell. In an unstructured cathode, a layer may form across the entire interior surface of the cathode (e.g., the surface not in contact with the current collector) that inhibits transport to and from the bulk of the cathode. The recesses in a structured cathode may provide regions free of insoluble products or regions with reduced concentrations of insoluble products. Thus, while transport through the top of a non-recessed, patterned surface may be hindered by the formation of an insoluble product layer, transport through the recesses may occur at a faster rate or unabated.

Additionally or alternatively, the use of patterned cathode films enables improved mixed electrolyte systems that can leverage the benefits of both solid-state and polymer, gel, or liquid electrolytes. For example, a solid electrolyte can be placed directly on the structured cathode. Due to the recesses in the structured cathode, even when the top surface of the structured cathode is in contact with the solid electrolyte, there is still volume for the polymer, gel, or liquid electrolyte. In this way, electrochemical cells can realize the benefits of both solid-state electrolytes (e.g., reduced polysulfide shuttling in lithium-sulfur batteries) and polymer, gel, or liquid electrolytes (e.g., faster reaction rates).

Advantages of the structured cathode for lithium-sulfur batteries disclosed herein alternatively or additionally include: (i) improved reversible polysulfide transport during cycling by reducing cathode tortuosity while mitigating detrimental power decay due to non-equilibrium redox reactions; (ii) improved electrolyte management by extruding electrolyte from the cathode porosity as sulfur converts to lithium sulfide, expanding the porosity in the process; and (iii) high volumetric capacity achieved by using a low E/S ratio enabled by a highly calendered cathode with improved electrolyte transport from strategically shaped and placed patterned structures (e.g., laser ablated structures) (e.g., microstructures). The present disclosure enables tailored cathodes for lithium-sulfur batteries that balance the design requirements of both high sulfur utilization and high energy (high internal surface area, low tortuosity, porosity compatible with ultra-low E/S ratios, and electrolyte migration due to sulfur expansion during conversion). Those skilled in the art will appreciate that other battery chemistries, such as, but not limited to, sodium-sulfur batteries, may provide similar or identical benefits.

In some embodiments, the structured cathodes disclosed herein can be used to construct batteries that achieve one or more of the following performance metrics: (i) gravimetric energy density (Wh/kg) of at least 550, (ii) volumetric energy density (Wh/L) of at least 900, (iii) charge power/tolerance (kW/kg) of at least 1.3, (iv) performance loss per °C (%, ≦30°C to -20°C) of 0.4 or less, (v) cycle life at at least 90% of initial capacity (80% state of charge (SOC) swing) of at least 750, and (vi) cell cost target ($/kWh) of 60 or less. In some embodiments, additional techniques beyond simply using a structured cathode are utilized to achieve one or more of these performance metrics. For example, thin film solid separator technologies (SwRI) such as sputtered/evaporated LLZO2, atomic layer deposition (ALD) coatings to enhance Li plating rates, prevent dendrites, and ensure chemical stability, and/or integration of a structured cathode and solid protection for the Li anode may be used.

In some aspects, the present disclosure provides an electrode for a secondary lithium battery (e.g., a lithium-sulfur battery), the electrode comprising a film including an electrochemically active material. The film may have a first surface in contact with a substrate (e.g., a current collector) and a patterned second surface opposite the first surface (e.g., the side not in contact with the substrate). The patterned second surface includes recesses extending into the film toward the substrate (e.g., in a direction substantially perpendicular to the first surface). In some embodiments, the electrode is a cathode. In some embodiments, the electrochemically active material is an electrochemically active conversion material.

In some embodiments, the patterned second surface of the film has a repeating geometric pattern of recesses. For example, the repeating geometric pattern may correspond to a hexagonal grid. As another example, the repeating geometric pattern may correspond to an equiangular grid or a square grid.

In some embodiments, at least a portion of the recesses may be interconnected across the second surface (e.g., forming a network of recesses across the extent of the second surface). The recesses in the second surface of the film may comprise holes. In some embodiments, the holes may be substantially circular in cross section.

In some embodiments, the diameter of the holes may be at least 20 nm and no greater than 500 μm (e.g., 20 nm to 50 μm, 20 nm to 100 μm, 20 nm to 200 μm, 20 nm to 300 μm, 100 nm to 100 μm, 100 nm to 200 μm, 1 μm to 100 μm, 1 μm to 200 μm, 10 μm to 100 μm, 10 μm to 200 μm, 50 μm to 100 μm, 50 μm to 200 μm, or 100 μm to 200 μm). The diameter of the holes may correspond to the resolution limit of the laser used to form the holes.

In some embodiments, each hole may have a depth of at least 25% of the thickness of the electrode film (e.g., a depth of at least 50%, at least 75%, at least 80%, or at least 90% of the thickness of the electrode film), and the holes may extend completely through the film from the second surface to the first surface.

Additionally or alternatively, in some embodiments, the recesses of the electrodes of the present disclosure may comprise grooves, and the grooves may have a width and a length across the second surface. For example, the width may be less than the length. In some embodiments, the width is at least 20 nm and no greater than 500 μm (e.g., 20 nm to 50 μm, 20 nm to 100 μm, 20 nm to 200 μm, 20 nm to 300 μm, 100 nm to 100 μm, 100 nm to 200 μm, 1 μm to 100 μm, 1 μm to 200 μm, 10 μm to 100 μm, 10 μm to 200 μm, 50 μm to 100 μm, 50 μm to 200 μm, or 100 μm to 200 μm). The width of the grooves may correspond to the resolution limit of the laser used to form the grooves.

The grooves may have a depth of at least 25% of the thickness of the cathode film (e.g., a depth of at least 50%, at least 75%, at least 80%, or at least 90% of the thickness of the cathode film), or the grooves may extend completely through the film from the second surface to the first surface.

In some embodiments, the recess may be at least partially filled with an electrolyte. For example, the electrolyte may include a polymer. In some cases, the electrolyte may be a liquid or a solid.

In some embodiments, the recesses in an electrode of the present disclosure may be distributed in a regular pattern across the second surface of the film. For example, the recesses may be distributed across the second surface such that no point in the film is 500 μm or less (e.g., 200 μm or less, 100 μm or less, 50 μm or less, 25 μm or less, or 20 μm or less) from at least one edge of at least one of the recesses.

Additionally or alternatively, the recesses may be dispersed across the second surface such that any point in the film is within a distance of no more than 3 times (e.g., no more than 2 times or no more than 1.5 times) the maximum thickness of the film from the nearest point of a recess. In some embodiments, the distance is no more than the maximum thickness of the film.

In some embodiments, the recess of the disclosed electrode can represent at least 5% (e.g., at least 10%, at least 15%, at least 20%, at least 25%, at least 33%, or at least 50%) of the total volume contained between the plane coincident with the top of the second surface and the first surface.

In some cases, the surface of the recess (e.g., the portion of the second surface defined by the recess) may be coated with a solid material having a composition different from the composition of the bulk electrode in the film. For example, the surface of the recess may be coated with a solid epitaxial material (e.g., formed by atomic layer deposition after formation of the recess).

In some embodiments, at least some (e.g., all) of the recesses extend completely through the film. In some embodiments, at least some (e.g., all) of the recesses do not extend completely through the film.

In some embodiments, the film of the disclosed electrode is a first film, and the electrode further includes a second film disposed on the side of the substrate opposite the first film.

In some embodiments, the second film has a patterned second surface including recesses extending into the second film and a first surface opposite the second surface, with the first surface of the second film in contact with the substrate. In some embodiments, the substrate may be porous. The recesses in the first film may extend entirely through the first film and intersect with pores in the substrate. Alternatively or additionally, the pores in the substrate may extend entirely through the substrate (e.g., thereby defining pores that extend entirely through the electrode).

In some embodiments of the electrodes of the present disclosure, the second surface of the film may be patterned after the film is applied to a substrate. For example, the film may be produced by applying a wet slurry to a substrate and then allowing the slurry to dry before patterning.

In some embodiments, the film may be calendered before patterning the second surface.

In some embodiments, the second surface of an electrode of the present disclosure may be patterned by laser ablation.

For example, laser ablation can be patterned on the second surface using a pulsed laser. In some embodiments, the pulsed laser can emit pulses with durations of less than 1000 femtoseconds (e.g., less than 500, 400, 300, 200, 150, 100, 50, 25, 15, 10, 5, 4, 3, 2, or 1 femtosecond).

In some embodiments, the disclosed electrode films may be porous (e.g., a porous collection of individual structures (e.g., particles) (e.g., nanoparticles) (e.g., core-shell particles or yolk-shell particles)).

In some electrodes of the present disclosure, the electrochemically active material (e.g., electrochemically active conversion material) comprises (i) elemental sulfur (e.g., in the form of an octacyclic _S8 molecule), (ii) sulfur in the form of lithium sulfide (e.g., _Li2S2 and/or _Li2S ), (iii) sulfur in the form of _an electrochemically active organosulfur compound, (iv) sulfur in the form of an electrochemically active sulfur-containing polymer, or (v) a combination of two or more of (i)-(iv).

Additionally or alternatively, the disclosed electrode films may further include one or more metal sulfides. For example, at least one of the one or more metal sulfides may be an intercalation electrochemically active material.

Alternatively or additionally, in some embodiments, the disclosed electrodes may further include a conductive additive (e.g., conductive carbon).

Alternatively or additionally, the disclosed electrodes may further include a polymer binder.

In some embodiments, the disclosed electrodes may be substantially free of carbon (e.g., carbon content of 10% by weight or less, carbon content of 5% by weight or less, carbon content of 2% by weight or less, or carbon content of 1% by weight or less).

In some embodiments, electrodes of the present disclosure can provide (i) an average mass transport path to an electrochemically active material (e.g., an electrochemically active conversion material) that is shorter than the average mass transport path to the electrochemically active material in an otherwise equivalent cathode without recesses, (ii) a degree of electrode tortuosity that is reduced compared to the degree of tortuosity of an otherwise equivalent electrode without recesses, or (iii) both (i) and (ii).

Additionally or alternatively, the electrodes of the present disclosure can provide (i) an electrode capacity greater than the capacity of an otherwise equivalent cathode without recesses at the same current density, (ii) an electrode with a high volumetric capacity, or (iii) both (i) and (ii).

In some embodiments, the present disclosure provides secondary batteries (e.g., lithium-sulfur) including the exemplary electrodes disclosed herein.

In some embodiments, the disclosed battery can further include an electrolyte disposed within the film of the disclosed electrode, with the recesses being a local reservoir for a portion of the electrolyte displaced from the bulk of the film during electrochemical cycling of the battery.

Additionally or alternatively, the disclosed batteries may further include a liquid electrolyte that at least partially fills the recesses of the disclosed electrodes (e.g., the liquid electrolyte also directly contacts the second surface that is not the recesses). For example, in some embodiments, the secondary batteries may further include a solid, polymer, or gel electrolyte (e.g., a polymer gel electrolyte) that at least partially fills the recesses.

In some embodiments, the secondary battery may further include a liquid electrolyte in contact with the solid, polymer, or gel electrolyte (e.g., the liquid electrolyte is disposed within a recess in the disclosed electrode and the solid, polymer, or gel electrolyte is in direct contact with the second surface).

Additionally or alternatively, the secondary battery may further include a non-conductive separator, wherein the second surface of the film of the disclosed electrode is in contact with the non-conductive separator (e.g., at a portion of the second surface that is not in the recess) [e.g., thereby defining an interface between the separator and the electrode, and wherein the non-equilibrium insoluble product is disposed (e.g., precipitated) in a higher concentration on the surface of the film at the interface between the separator and the electrode than in the recess].

The secondary battery of the present disclosure may further include a solid electrolyte, and the second surface of the film of the disclosed electrode is in contact with the solid electrolyte (e.g., at a portion of the second surface that is not in the recess).

Additionally or alternatively, the disclosed secondary battery may further include a protected lithium anode, with the second surface (e.g., conversion cathode) of the disclosed electrode film being in contact with the protected lithium anode.

In some embodiments, the secondary batteries of the present disclosure may have an anode-free configuration (e.g., the battery includes a current collector and lithium is deposited on the current collector during the first electrochemical cycle).

In some embodiments, the electrolyte of the secondary battery of the present disclosure may not include a sulfonamide salt (e.g., lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)).

In some embodiments, the disclosed secondary batteries may have a low electrolyte-to-sulfur (E/S) ratio.

In some embodiments, the present disclosure provides a method of operating a secondary battery (e.g., a lithium-sulfur battery) disclosed herein, comprising an electrode (e.g., a conversion cathode) and an electrolyte disclosed herein, the method comprising expanding a film of the disclosed electrode (e.g., by expanding individual structures (e.g., particles) assembled within the film) (e.g., by reducing the porosity of the film) during an electrochemical cycle of the battery (e.g., during discharge of the battery) such that a portion of the electrolyte migrates from the bulk of the film to the recesses.

In some embodiments, the disclosed operating methods may further include returning a portion of the electrolyte to the bulk of the film (e.g., by shrinkage of the film) during further electrochemical cycling of the battery (e.g., during charging of the battery).

In some embodiments, the battery of the method of operation can further include a separator in contact with the second surface of the film of the disclosed electrode (e.g., cathode), thereby defining a separator-electrode interface, and the method includes forming a non-equilibrium insoluble product during electrochemical cycling and disposing (e.g., precipitating) the non-equilibrium insoluble product on the surface of the film at the separator-electrode interface.

In some embodiments, the disclosed method of operation may include disposing the non-equilibrium insoluble product on the surface of the film of the disclosed electrode at the separator-electrode interface at a concentration greater than the concentration at which the non-equilibrium insoluble product is disposed in the recess.

In some embodiments, the disclosed method of operation may include not disposing non-equilibrium insoluble products in the recesses.

In some embodiments, the disclosed method of operation may include reversibly transporting lithium (e.g., in the form of polysulfides) through the recesses to an electrode (e.g., a conversion cathode) during electrochemical cycling.

Additionally or alternatively, the methods of operation disclosed herein may include transporting lithium to an electrode (e.g., a conversion cathode) through the recesses at a rate and/or to a greater extent (e.g., based on the amount of lithium transported) than the rate and/or extent that concurrently transporting lithium through the surface on which the non-equilibrium insoluble product is disposed.

Furthermore, the method of operation may include reducing electrolyte migration into one or more voids of the cell stack and/or battery during electrochemical cycling by migration into the recesses.

In some aspects, the present disclosure provides a method of manufacturing an electrode (e.g., a cathode) for a battery (e.g., a lithium-sulfur battery), the method including providing (e.g., forming) a film including an electrochemically active material (e.g., an electrochemically active conversion material). In some embodiments, the method further includes forming a recess in a surface of the film that extends into the film. In some embodiments, forming the recess can include removing a portion of the film.

In some embodiments, removing can include laser ablating the film. In some embodiments, laser ablation includes irradiating the film with a pulsed laser. For example, the pulsed laser can emit pulses with a duration of less than 1000 femtoseconds (e.g., less than 500, 400, 300, 200, 150, 100, 50, 25, 15, 10, 5, 4, 3, 2, or 1 femtosecond).

In some embodiments, laser ablation may be performed in-line (e.g., during battery manufacturing).

In some embodiments, the manufacturing method can include calendering the film [e.g., on a substrate (e.g., a current collector)]. In some embodiments, forming can occur after calendering. In some embodiments, (i) calendering leaves 40% or less (e.g., 30% or less, 20% or less, or 10% or less) of the initial porosity of the film before calendering, (ii) the maximum thickness of the film is 40% or less (e.g., 30% or less, 20% or less, or 10% or less) of the initial thickness before calendering, or (iii) both (i) and (ii).

In some embodiments, forming can include scraping, cutting, and/or scratching (e.g., with one or more blades). In some embodiments, forming can include debossing (e.g., compressing, imprinting, and/or stamping) the film.

In some embodiments, providing includes forming a film, and forming the film can include assembling individual structures (e.g., particles) that include the electrochemically active material (e.g., the electrochemically active conversion material).

In some embodiments, the assembling may include one or more elements selected from the group consisting of slurry coating, slot die coating, spin coating, spray drying, drawdown coating, doctor blade coating, inkjet printing, comma coating, and inverted comma coating.

In some embodiments, the method may be performed as part of a roll-to-roll manufacturing process (e.g., a roll-to-roll cathode manufacturing process or a roll-to-roll battery manufacturing process).

Any two or more of the features described herein, including the features described in this Summary section, may be combined to form an implementation not specifically described herein.
definition

To make this disclosure more readily understandable, certain terms used herein are defined below. Additional definitions for these and other terms may be found throughout the specification.

In this application, unless otherwise clear from the context or otherwise expressly stated, (i) the term "a" may be understood to mean "at least one," (ii) the term "or" may be understood to mean "and/or," (iii) the terms "comprising" and "including" may be understood to encompass the listed components or steps, whether presented by themselves or with one or more additional components or steps, (iv) the terms "about" and "approximately" may be understood to allow for standard variations as understood by one of ordinary skill in the art, and (v) when ranges are provided, the endpoints are included.

About, Approximately: As used herein, the terms "about" and "approximately" are used equivalently. Unless otherwise specified, the terms "about" and "approximately" can be understood to allow for standard variations as understood by one of ordinary skill in the art. When numerical ranges are provided, the endpoints are included. Any numbers used herein, with or without approximately/about, are intended to encompass any normal variations understood by one of ordinary skill in the art. In some embodiments, the term "about" or "approximately" refers to a range of values that is within 25%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, or less in either direction of (greater than or less than) the stated reference value, unless otherwise specified or otherwise clear from the context (except where such number would exceed 100% of the possible values).

Polymer: As used herein, the term "polymer" generally refers to a substance having a molecular structure consisting primarily or entirely of repeating subunits bonded together, such as the synthetic organic materials used as plastics and resins.

Substantially: As used herein, the term "substantially" refers to the quantitative state of exhibiting the total or near-total extent or degree of a desired characteristic or attribute.

The drawings herein are presented for purposes of illustration and not limitation. The foregoing and other objects, aspects, features, and advantages of the present disclosure will become more apparent and may be better understood by referring to the following description in conjunction with the accompanying drawings.

1 is a scanning electron microscope (SEM) photograph of a structured conversion cathode according to an exemplary embodiment of the present disclosure.

1 is a cross-sectional schematic diagram of a structured conversion cathode according to an exemplary embodiment of the present disclosure. 1 is a cross-sectional schematic diagram of a structured conversion cathode according to an exemplary embodiment of the present disclosure. 1 is a cross-sectional schematic diagram of a structured conversion cathode according to an exemplary embodiment of the present disclosure. 1 is a cross-sectional schematic diagram of a structured conversion cathode according to an exemplary embodiment of the present disclosure.

1 is a flowchart of a method for forming a structured conversion cathode according to an exemplary embodiment of the present disclosure.

1 is a cross-sectional view of an electrochemical cell according to an exemplary embodiment of the present disclosure.

1 is a cross-sectional view of an electrochemical cell according to an exemplary embodiment of the present disclosure.

1 is a pictorial representation of a cylindrical battery according to an exemplary embodiment of the present disclosure.

FIG. 1 is a perspective view of a coin cell battery assembly according to one or more embodiments of the present disclosure.

1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure. 1 is a SEM micrograph of an example structured conversion cathode construction according to an exemplary embodiment of the present disclosure.

Figures and tables are not necessarily drawn to scale.

Described herein are, among other things, structured cathodes for use in electrochemical cells (e.g., batteries, such as secondary batteries), electrochemical cells (e.g., batteries) including such cathodes, and methods of their formation and use. The structured cathode includes at least one electrochemically active material. The electrochemically active material may be an electrochemically active conversion material. An example of an electrochemically active conversion material is a sulfur-based material in a lithium-sulfur battery. The electrochemically active conversion material may be included in a patterned film. A patterned film including one or more electrochemically active materials can be disposed on a substrate (e.g., a current collector) (e.g., the substrate on which the film is formed) or can be disposed independently. A patterned film is a film having at least one patterned surface, e.g., having recesses extending within the film (e.g., in a direction substantially perpendicular to the surface of the film). The patterned film may be porous. The patterned film may be made from a collection of particles including an electrochemically active material (e.g., a conversion material). A method of using a structured cathode can include expanding a film during electrochemical cycling of an electrochemical cell (e.g., battery) such that a portion of the electrolyte in the cell migrates from the bulk of the film to the recesses extending into the film. A method of fabricating a structured cathode can include providing (e.g., forming) a film including an electrochemically active conversion material and forming recesses in a surface of the film that extend into the film. Forming the recesses can be accomplished by removing material from the film, for example, by laser ablation.

Structured Cathode In some embodiments, a cathode includes a film including an electrochemically active material and a substrate (e.g., a current collector). The electrochemically active material can be a conversion material. The film has a first surface in contact with the substrate (e.g., a current collector) and a second surface on the side of the film opposite the first surface (e.g., the side not in contact with the substrate). The second surface is patterned to include recesses that extend into the film toward the substrate. The recesses can extend completely to the substrate (throughout the entire film), only a portion of the distance from the surface to the substrate, or a combination thereof. The recesses can include holes, grooves (e.g., wells, troughs, channels, or a combination thereof), or both. The holes can be substantially circular in cross section (as is typically the case when formed by laser ablation, since the laser beam is typically circular). The grooves can have a substantially rectangular cross section or a "U"-shaped cross section. The grooves can have a length across the patterned surface that is much longer than the groove width, the groove depth, or both. The recesses may be at least partially filled with an electrolyte, such as a liquid, gel, or polymer electrolyte. In some embodiments, the recesses are filled with a solid electrolyte. The substrate may be electrically conductive, as in the case of a current collector.

Figure 1 is an SEM micrograph of a top view of the surface of a cathode 100 according to an exemplary embodiment of the present invention. The cathode 100 includes a patterned film, a cathode film 102. The cathode film 102 has a patterned second surface (shown) and an unpatterned first surface (not shown), the first surface being opposite the second surface and in contact with a current collector (not shown). The patterned second surface of the film 102 includes recesses, which are grooves, including grooves 106a-d. The grooves 106a-d are interconnected; for example, groove 106a is directly interconnected with groove 106b, and groove 106c is interconnected with groove 106a through groove 106b. The grooves 106a-d do not penetrate completely through the film 102 (they penetrate only a portion of the film 102). The film 102 also includes a top portion 104. The film 102 is a collection of particles, such as core-shell particles and yolk-shell particles, each containing an electrochemically active conversion material (e.g., sulfur). The film 102 may also include additional components, such as a binder and one or more conductive additives.

FIG. 2A is a cross-sectional view illustrating an embodiment of the present disclosure. FIG. 2A shows a cathode 200 in which a patterned film 202 comprising an electrochemically active conversion material is disposed (e.g., formed) on a current collector 210. The patterned film 202 includes an unpatterned first surface 204a disposed on the current collector 210 and a patterned second surface 204b. The patterned second surface 204b includes a top portion and recesses 206a-d extending into the film 202. The recesses 206a-d have at least two substantially uniform dimensions (in the case of grooves, the inner and outer dimensions of the paper may differ), although in general the recesses need not be substantially uniform in size. Specifically, the recesses 206a-d have at least substantially uniform widths 208a and depths 208b. (In other embodiments, the width and/or depth of different recesses may not be uniform.) If recesses 206a-d include one or more holes, the one or more holes may be substantially circular in cross section and characterized by a diameter corresponding to width 208a. If recesses 206a-d include one or more grooves, the one or more grooves may have substantially the same length or different lengths (e.g., as shown in FIG. 1, a generally left-to-right groove, such as groove 106c, is longer than a generally top-to-bottom groove, such as groove 106b). Recesses 206a-d are not uniformly distributed across second surface 204b, as evidenced by their uneven spacing. In some embodiments, recesses 206a-d may be uniformly arranged (e.g., in one or two dimensions). Film 200 may (or may not) be formed from a collection of particles (e.g., core-shell particles or yolk-shell particles), each containing an electrochemically active material. Film 202 may be porous (e.g., in the case of a collection of particles).

When incorporated into an electrochemical cell (e.g., a battery) containing an electrolyte, recesses 206a-d can serve as localized reservoirs for electrolyte that flows in and out of the bulk of cathode film 202 during electrochemical cycling, as indicated by arrow 205. Film 202 has a maximum thickness, as indicated by 208c. Recesses 206a-d do not extend through the entire (maximum) thickness of film 202. Due to the presence of recesses 206a-d, the average (or maximum) mass transport path in may be shorter than that of an unpatterned film, as approximately represented by arrows 207a-b pointing to circular region 207c of film 202, with arrow 207b being shorter than arrow 207a due to recess 206b.

FIG. 2B shows an embodiment similar to FIG. 2A in which an electrolyte 212 is present. The electrolyte 212 at least partially (in this case, completely) fills the recesses 206a-d. The electrolyte 212 may be liquid, gel, polymer, or solid. A separator and an anode (e.g., a lithium anode) or an anode-free configuration (e.g., a current collector with in-situ lithium deposition) can be added (e.g., along with one or more other components) to form a complete electrochemical cell. FIG. 2C shows a mixed electrolyte embodiment in which a solid electrolyte 214 is in (e.g., directly) contact with (e.g., directly on) the patterned second surface 204b (its top), and the recesses 206a-d are at least partially filled with a liquid, gel, or polymer electrolyte 212. FIG. 2D shows a separator 216 disposed on (e.g., in contact with) the patterned second surface 204b (its top), with the recesses 206a-d at least partially filled with the electrolyte 212. In some embodiments, an intermediate layer of insoluble product is formed where the separator 216 contacts the film 202.

The recesses may be arranged in a regular or irregular pattern across the cathode film. For example, the recesses may be arranged in a regular one- or two-dimensional array, or in a random pattern. The patterned surface of the film may have a repeating geometric pattern of recesses that corresponds to, for example, a hexagonal grid (e.g., hexagonal close-packed), a regular grid, or a square grid. Some of the recesses may be interconnected across the patterned surface of the film; for example, grooves may intersect with each other, grooves may intersect with holes, or both. The interconnected recesses may form a network across the extent of the patterned surface. The recesses may extend across the entire cathode film (e.g., all the way to a substrate such as a current collector) or may extend across only a portion of the cathode film.

The recesses can have a length, width, and depth. When the recesses are holes, the length and width dimensions of each hole can be substantially the same, each representing the diameter of the hole. In some embodiments, the diameter of the holes extending into the film corresponds to the resolution limit of the pulsed laser (e.g., at least about 20 nm). In certain embodiments, the diameter of the holes extending into the film is within a range of about 20 nm to about 500 μm (e.g., 20 nm to 50 μm, 20 nm to 100 μm, 20 nm to 200 μm, 20 nm to 300 μm, 100 nm to 100 μm, 100 nm to 200 μm, 1 μm to 100 μm, 1 μm to 200 μm, 10 μm to 100 μm, 10 μm to 200 μm, 50 μm to 100 μm, 50 μm to 200 μm, or 100 μm to 200 μm). If the recess is a groove, it may have a different length and width, for example, a width of 20 nm to 500 μm (e.g., 20 nm to 50 μm, 20 nm to 100 μm, 20 nm to 200 μm, 20 nm to 300 μm, 100 nm to 100 μm, 100 nm to 200 μm, 1 μm to 100 μm, 1 μm to 200 μm, 10 μm to 100 μm, 10 μm to 200 μm, 50 μm to 100 μm, 50 μm to 200 μm, or 100 μm to 200 μm) and a different length (e.g., at least 100 nm, at least 1 μm, at least 10 μm, at least 50 μm, at least 100 μm, at least 250 μm, at least 500 μm, at least 750 μm, or at least 1 mm). In some embodiments, at least some of the recesses have at least one dimension (e.g., width, depth, or width and depth) in the range of 20 nm to 500 μm (e.g., 100 nm to 200 μm or 50 nm to 100 μm). In some embodiments, the length of at least some of the recesses is at least 100 μm (e.g., at least 200 μm, at least 500 μm, or at least 1 mm) (e.g., other recesses have lengths of at least 50 μm, at least 100 μm, at least 200 μm, or at least 500 μm). The spacing between at least some (e.g., all) pairs of adjacent recesses (e.g., when arranged in a regular one- or two-dimensional array across the patterned surface) may be at least 20 μm (e.g., at least 50 μm, at least 75 μm, at least 100 μm, at least 150 μm, at least 200 μm, or at least 250 μm). The recesses may be microstructures (each a microstructure).

In some embodiments, the recesses have one or more vertical walls (holes or grooves). In some embodiments, the recesses are tapered such that at least one dimension of the recess narrows with the distance the recess extends into the cathode film. In some embodiments, the recesses each have a depth of at least 25% of the maximum thickness of the cathode film. For example, some of the recesses may have a depth that extends into the cathode film that is at least 25% of the maximum thickness of the film (e.g., at least 50%, at least 75%, at least 80%, or at least 90% of the thickness). Some of the recesses can extend completely through the patterned surface (e.g., to the current collector). In some embodiments, the film is discontinuous (e.g., the recesses define one or more islands each containing electrochemically active material that is part of a collection of individual structures). In some embodiments, the cathode film is continuous (e.g., a portion of the film is disposed beneath each recess). In some embodiments, at least some of the recesses extend completely through the film (e.g., to a substrate, such as a current collector, on which the film is disposed). In some embodiments, the recesses do not extend entirely through the film. The cathode film may have a linear density of recesses across at least one direction of at least 2/mm (e.g., at least 4/mm, at least 5/mm, at least 6/mm, at least 8/mm, or at least 10/mm). The cathode film may have an areal density of ^recesses (e.g., ^measured in a plane across the film where the recesses are maximally present, e.g., coincident with the top of the patterned ^second surface) of at least ⁵ /mm (e.g., at least 6/ ^mm , at least 8/mm, at least ¹⁰ /mm, at least 15/mm, or at least 20/mm). The cathode film can have both such a linear density and such an areal density.

The cathode film may be porous (e.g., before and/or after calendering). In some embodiments, the film is heavily calendered to leave little or no porosity, such as 40% or less (e.g., 30% or less, 20% or less, or 10% or less) of the initial porosity before calendering. The change in porosity can also be measured by comparing the initial film thickness before calendering to the film thickness after calendering (and, e.g., before patterning); for example, the thickness may be reduced to 40% or less (e.g., 30% or less, 20% or less, or 10% or less) of the initial thickness. The initial thickness may be, for example, 1 mm or less (e.g., 500 μm or less), and the final thickness after calendering may be 200 μm or less (e.g., 100 μm or less, 50 μm or less, 25 μm or less, or 20 μm or less). In some embodiments, the film is a porous collection of individual structures (e.g., nanostructures) containing an electrochemically active material (e.g., a conversion material). The individual structures can be or include particles (e.g., nanoparticles), fibers (e.g., nanofibers), rods (e.g., nanorods), or combinations thereof. In some embodiments, the individual structures have at least one dimension (e.g., diameter, length, width, height, or a combination thereof) that is 500 nm or less (e.g., 250 nm or less, 100 nm or less, or 50 nm or less). In some embodiments, the structures include core-shell particles (e.g., nanoparticles) having a core of electrochemically active material or yolk-shell particles (e.g., nanoparticles) having a yoke of electrochemically active material. The shell of the core-shell or yolk-shell particles may be selectively permeable. A film that is a porous assembly of individual structures may further include a conductive additive and/or a binder (e.g., a polymeric binder) dispersed within the assembly (e.g., to facilitate electron transport through the assembly and/or to bind the individual structures together). The cathode may be substantially free of carbon (e.g., carbon content of 10% by weight or less, carbon content of 5% by weight or less, carbon content of 2% by weight or less, carbon content of 1% by weight or less).

The electrochemically active material in the cathode can be or include a chalcogenide (e.g., S, Se, and/or Te). The electrochemically active material can be an electrochemically active conversion material (e.g., _S8 ). The electrochemically active conversion material can be or include (i) sulfur in the form of an _S8 cyclic octaatomic molecule, (ii) sulfur in the form of lithium _{sulfide (e.g., Li2S2 and} /or _Li2S ), (iii) sulfur in the form of an electroactive organosulfur compound, and (iv) sulfur in the form of an electroactive sulfur-containing polymer [e.g., carbon-sulfur polymer (( _{C2Sx ) n} , where x = 2.5 to 50 and n > 2)], or (v) any two or more of (i)-(iv). In some embodiments, the electrochemically active material includes a metal sulfide. In some embodiments, the electrochemically active material is an intercalation material. In some embodiments, the cathode film includes an intercalation material, such as an electrochemically active intercalation material. In some embodiments, the cathode film further includes one or more metal sulfides in addition to the first electrochemically active material. The film can include a conductive additive (e.g., conductive carbon), a binder (e.g., a polymeric binder), or both. In certain embodiments, the electrochemically active intercalation material includes one or more components selected from the group consisting of metal oxides, metal sulfides, metal phosphates, metal selenides, mixtures, and/or combinations thereof. In certain embodiments, the electrochemically active intercalation material (e.g., lithium ions) includes one or more metal sulfides. In certain embodiments, the metal sulfides are selected from the group consisting of vanadium sulfide (e.g., _VS2 ), molybdenum sulfide (e.g., MoS2 and _/ or _Mo6S8 ), and titanium sulfide (e.g., _{TiS2 )} . In certain embodiments, the metal sulfide is selected from the group consisting of _{VS2 , MoS2} , _Mo6S8 , and _{TiS2 .} In certain embodiments, the metal sulfide is _TiS2 . In certain embodiments, the metal sulfide is _Mo6S8 .

The recesses may be dispersed throughout the patterned surface of the film such that any point within the film (e.g., in the bulk of the film) is no more than 500 μm (e.g., no more than 200 μm, no more than 100 μm, no more than 50 μm, no more than 25 μm, or no more than 20 μm) from at least one edge of at least one of the recesses. In some embodiments, the recesses are dispersed throughout the patterned surface of the film such that any point within the film is within a distance of no more than 3 times (e.g., no more than 2 times, no more than 1.5 times, or no more than) the maximum thickness of the film from the nearest recess. The maximum thickness may be the smallest linear distance between the top of the patterned surface of the film and the opposite surface of the film (e.g., the surface that contacts the substrate) (e.g., as indicated by arrow 208c in FIG. 2A). The recesses can represent at least 5% (e.g., at least 10%, at least 15%, at least 20%, at least 25%, at least 33%, or at least 50%) of the total volume contained between a plane coincident with the top of the patterned surface of the film and the opposing (e.g., unpatterned) surface (e.g., the surface in contact with the current collector).

In some embodiments, the total volume of the recesses corresponds to (e.g., coincides with) the expected volume of electrolyte displaced from the bulk of the cathode during electrochemical cycling (e.g., when the electroactive material contained in the cathode is fully discharged). The expected volume of displaced electrolyte can be estimated empirically or calculated numerically. In this way, regardless of the benefits that may be achieved in terms of reducing the average mass transport path to the cathode (e.g., the bulk of the cathode), the recesses can function as local reservoirs to prevent permanent loss of electrolyte (e.g., loss to the periphery of the electrochemical cell) and/or the adverse effects of pressure buildup and/or electrochemical cell expansion that may be caused by electrochemical cycling. Thus, in some embodiments, the recesses are filled with more electrolyte during one phase of the electrochemical cycle than during other phases. For example, in a lithium-sulfur battery in which sulfur is the electrochemically active conversion material in the cathode film, sulfur is converted to lithium sulfide during discharge, causing volume expansion and allowing electrolyte to displace. If the surface of the cathode film is patterned, electrolyte can migrate into the recesses of the patterned surface.

In some embodiments, the surfaces of recesses extending into the film (e.g., the portions of the patterned surface defined by the recesses) are coated with a solid material having a composition different from the composition of the bulk cathode in the film. The surfaces of the recesses may also be coated with a solid epitaxial material (e.g., formed by atomic layer deposition after the formation of the recesses).

The patterned films may be disposed (e.g., formed) on a substrate (e.g., a current collector). In some embodiments, each film is disposed on two opposite sides of the substrate (e.g., a current collector), and optionally, one or both of the films may have a patterned surface with recesses extending into the film. The substrate on which one or more films are disposed may be porous. The recesses of a patterned film disposed on a porous substrate may extend entirely through the film such that they intersect with pores in the substrate (e.g., pores that extend entirely through the substrate). In some embodiments, the recesses of each patterned film disposed on opposite sides of a porous substrate may extend entirely through the film such that they intersect with pores in the substrate, thereby defining pores that extend entirely through the cathode (e.g., for free flow of liquid electrolyte).

In some embodiments, the film is patterned after application to a substrate (e.g., a current collector). In some embodiments, the film is fabricated by applying a wet slurry to a substrate and then drying the slurry before patterning. In some embodiments, the film is calendered before patterning its surface. In some embodiments, the film's surface is patterned by laser ablation. A pulsed laser may be used for laser ablation patterning. The pulsed laser may emit pulses with durations of 1000 femtoseconds or less (e.g., 500, 400, 300, 200, 150, 100, 50, 25, 15, 10, 5, 4, 3, 2, or 1 femtosecond or less). Laser ablation is an attractive process for patterning a film's surface because experiments have shown that morphology and porosity can be substantially maintained near recesses formed in the film during patterning (see Examples below).

In some embodiments, the average mass transport path to the electrochemically active material in a structured cathode (e.g., a cathode comprising a patterned film) is shorter than the average mass transport path to the electrochemically active material in a comparable cathode without recesses (e.g., a cathode comprising an unpatterned or smooth film). Similarly, in some embodiments, alternatively or additionally, the tortuosity of the structured cathode is reduced compared to the tortuosity of an otherwise comparable cathode without recesses. In some embodiments, the capacity of the structured cathode is greater than the capacity of a comparable cathode without recesses at the same current density, e.g., due to greater utilization of bulk electrochemically active material during electrochemical cycling. For example, the capacity of the structured cathode can be at least 5% greater (e.g., at least 10% greater, at least 20% greater, at least 30% greater, or more) than a comparable cathode at the same cycling rate (current density). In some embodiments, the cathode has a high volumetric capacity. Actual volumetric capacity may vary depending on cycling rate, electrode thickness, temperature, electrolyte chemical composition, or a combination thereof.

A film may include a single layer of material or multiple layers. The individual structures assembled into the porous film may be of only one type (e.g., core-shell particles or yolk-shell particles each containing a core or yolk of electrochemically active material, such as a conversion material) or multiple types (e.g., a mixture of one or more electrochemically active intercalation materials and one or more electrochemically active conversion materials). A multilayer structure may include individual layers of different electrochemically active materials, e.g., an electrochemically active intercalation material layer may be positioned above an electrochemically active conversion material layer, or vice versa. The recesses in the patterned surface of a multilayer film may extend through only one of the multiple layers, or through two or more of the multiple layers. Thus, the advantage of significantly shortening the length of the material transport path to one or more layers in the multilayer structure that are not the surface layer(s) (e.g., an electrochemically active intercalation material layer covered by an electrochemically active conversion material layer) may be realized. One or more layers (e.g., each layer) of the multilayer cathode film may be porous.

In certain embodiments, the substrate (e.g., current collector) comprises a component selected from a metal foil, a metallized polymer film, and a carbon composition. In some embodiments, the current collector comprises aluminum foil, copper foil, nickel foil, stainless steel foil, titanium foil, zirconium foil, molybdenum foil, nickel foam, copper foam, carbon paper or fiber sheet, a polymer substrate coated with a conductive metal, and/or combinations thereof. In certain embodiments, the current collector comprises a metal foil. In certain embodiments, the current collector comprises a metallized polymer film. In certain embodiments, the current collector comprises a carbon composition. In certain embodiments, the cathode comprises a conductive carbon coating between the current collector and a second active layer comprising a lithium ion intercalation active material.

In certain embodiments, the cathode film includes a conductive additive that facilitates electron movement within the cathode. For example, in certain embodiments, the conductive additive is selected from the group consisting of a carbon-based material, a graphite-based material, a conductive polymer, a metal salt, an oxide, a sulfide, or a chalcogenide, and combinations thereof. In certain embodiments, the conductive additive includes a carbon-based material. In certain embodiments, the conductive additive includes a carbon-based material. For example, in certain embodiments, the conductive additive is selected from the group consisting of conductive carbon powders such as carbon black, SuperP®, C-NERGY™ Super C65, Ensaco® black, Ketjenblack®, acetylene black, synthetic graphite such as Timrex® SFG-6, Timrex® SFG-15, Timrex® SFG-44, Timrex® KS-6, Timrex® KS-15, Timrex® KS-44, natural flake graphite, graphene, graphene oxide, carbon nanotubes, fullerenes, hard carbon, mesocarbon microbeads, and the like. In certain embodiments, the conductive additive comprises one or more conductive polymers. For example, in certain embodiments, the conductive polymer is selected from the group consisting of polyaniline, polythiophene, polyacetylene, polypyrrole, and the like. In certain embodiments, a single conductive additive is used. In some embodiments, multiple conductive additives are used together.

In certain embodiments, the cathode includes a binder (e.g., a material that binds individual structures (e.g., particles) together and/or adheres the individual structures to a substrate such as a current collector). Exemplary binders include polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropene) (PVDF/HFP), polytetrafluoroethylene (PTFE), KynarFlex® 2801, Kynar® Powerflex® LBG, Kynar® HSV 900, Teflon®, carboxymethylcellulose, styrene-butadiene rubber (SBR), polyethylene oxide, polypropylene oxide, polyethylene, polypropylene, polyacrylate, polyvinylpyrrolidone, poly(methyl methacrylate), polyethyl acrylate, polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polycaprolactam, polyethylene terephthalate, polybutadiene, polyisoprene, or polyacrylic acid, or any derivative, mixture, or copolymer thereof. In some embodiments, the binder is a water-soluble binder such as sodium alginate or carboxymethylcellulose. Generally, the binder holds the active material together and in contact with the current collector (e.g., aluminum foil or copper foil). In certain embodiments, the binder is selected from the group consisting of poly(vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, alkylated polyethylene oxide, cross-linked polyethylene oxide, polyvinyl ether, poly(methyl methacrylate), polyvinylidene fluoride, copolymers of polyhexafluoropropylene and polyvinylidene fluoride, polyethyl acrylate, polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polystyrene, and derivatives, mixtures, and copolymers thereof.

In certain embodiments, the cathode further comprises a coating layer. For example, in certain embodiments, the coating layer comprises a polymer, an inorganic material, or a mixture thereof. In certain such embodiments, the polymer is selected from the group consisting of polyvinylidene fluoride, copolymers of polyvinylidene fluoride and hexafluoropropylene, poly(vinyl acetate), poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate), poly(methyl methacrylate-co-ethyl acrylate), polyacrylonitrile, polyvinyl chloride-co-vinyl acetate, polyvinyl alcohol, poly(1-vinylpyrrolidone-co-vinyl acetate), cellulose acetate, polyvinylpyrrolidone, polyacrylates, polymethacrylates, polyolefins, polyurethanes, polyvinyl ethers, acrylonitrile-butadiene rubber, styrene butadiene rubber, acrylonitrile-butadiene styrene, sulfonated styrene/ethylene-butylene/styrene triblock copolymers, polyethylene oxide, and derivatives, mixtures, and copolymers thereof. In certain such embodiments, the inorganic material includes, for example, colloidal silica, amorphous silica, surface-treated silica, colloidal alumina, amorphous alumina, tin oxide, titanium oxide, titanium sulfide ( _TiS2 ), vanadium oxide, zirconium oxide ( _ZrO2 ), iron oxide, iron sulfide (FeS), iron titanate ( _FeTiO3 ), barium titanate ( _BaTiO3 ), and combinations thereof. In certain embodiments, the organic material includes conductive carbon. In certain embodiments, the organic material includes, alternatively, graphene, graphene nitride, or graphene oxide.

In certain embodiments, the provided structured cathode precursors can be formulated without a binder, which can be added during the manufacture of the cathode film (e.g., dissolved in a solvent used to form a slurry from the provided mixture). In embodiments in which a binder is included in the provided cathode film, the binder can be activated when the mixture is slurried to manufacture the cathode film.

Materials suitable for use in the cathode mixture are described in "Cathode Materials for Lithium Sulfur Batteries: Design, Synthesis, and Electrochemical Performance," Lianfeng, et al., Interchopen.com, published June 1, 2016, and "The Strategies of Advanced Cathode Composites for Lithium-Sulfur Batteries," Zhou et al. , SCIENCE CHINA Technological Sciences, Volume 60, Issue 2: 175-185 (2017), the entire disclosures of each of which are incorporated herein by reference.

Methods for Fabricating Structured Cathodes Structured cathodes can be formed using conventional processes appropriately adapted for mass production of cathodes. For example, many cathodes are fabricated using roll-to-roll processing. One or more additional steps can be added to the roll-to-roll process or another process to form a structured cathode. As an example, slot die coating using an appropriately shaped die can form specific recesses, such as parallel grooves of varying depth, in the cathode film during coating. In general, methods for fabricating structured cathodes include providing (e.g., forming) a film containing an electrochemically active material (e.g., a conversion material) and forming recesses in the surface of the film that extend into the film (e.g., through only a portion of the film or the entire film). The recesses and film can be formed simultaneously (e.g., in the case of slot die coating), or the recesses can be formed after the film is fabricated. An initial film can be formed (e.g., by slot die coating) (e.g., on a substrate such as a current collector) and then calendered. In some embodiments, the recesses are formed in the film only after calendering, forming the patterned surface of the film. The recesses can be formed by removing material from the film, or by rearranging material within the film (e.g., by debossing a portion of the film, e.g., by compressing, stamping, and/or imprinting), by depositing material in a pattern (e.g., using a special slot or die), or any combination thereof. Laser ablation is a preferred method for forming the recesses.

Figure 3 is a flowchart of method 300 according to an exemplary embodiment of the present disclosure. In step 302, the slurry is deposited (e.g., by slot die coating) onto a current collector to form an initial cathode film. In step 304, the initial cathode film is calendered. The calendering can be "aggressive" and significantly reduce the initial porosity of the film. For example, if the slurry contains individual structures (e.g., nanostructures) such as particles, rods, fibers, or combinations thereof, the calendering can significantly compress the structures compared to the initial deposition, reducing porosity. In step 306, portions of the calendered film are removed, forming recesses in the surface of the film. Laser ablation can be used to perform the removal.

Laser ablation can be used to form recesses in a film (e.g., a film that has already been calendered). Laser ablation generally acts to remove material from a film. One reason laser ablation is desirable is its high controllability. Pulsed lasers can be used to precisely control the ablation. In some embodiments, the pulsed laser irradiates the film (e.g., a film that has already been calendered) with pulses of less than 1000 femtoseconds (e.g., less than 500, 400, 300, 200, 150, 100, 50, 25, 15, 10, 5, 4, 3, 2, or 1 femtosecond) in duration.

Forming the recesses, such as by laser ablation, can be performed in-line (e.g., during battery manufacturing). That is, in certain embodiments, conventional cathode manufacturing lines can be modified to pattern recesses in the cathode film surface without the need for significant retooling. For example, in some embodiments, recesses can be formed in the cathode film surface during battery manufacturing by simply adding a laser ablation device at the appropriate location within the roll-to-roll manufacturing process.

In some embodiments, the provided (e.g., formed) film is calendered. The film may be calendered on a substrate (e.g., a current collector). The substrate may be porous. Different films may be calendered on different sides of the substrate (e.g., one on each of two opposite sides). When two such films are present, each film may have a patterned surface, such as by laser ablation on both sides. Forming the recesses may be performed only after the initial film has been calendered. In this way, adverse changes to the shape or size of the recesses that may be caused by calendering the cathode film can be avoided. Calendering may be applied to an extent that 40% or less (e.g., 30% or less, 20% or less, or 10% or less) of the initial porosity present in the cathode film before calendering remains. Alternatively or additionally, calendering may reduce the maximum thickness of the cathode film to 40% or less (e.g., 30% or less, 20% or less, or 10% or less) of the initial thickness before calendering.

In some embodiments, forming the recesses in the cathode film includes removing material (e.g., electrochemically active material, and, if present, binder and/or conductive additives). Removing material may include laser ablation of the material. Removing material may include scraping, cutting, and/or scratching (e.g., with one or more blades). In some embodiments, forming the recesses in the cathode film includes debossing (e.g., compressing, stamping, and/or imprinting) the film.

In some embodiments, providing the cathode film includes forming a film. Forming the cathode film can include assembling individual structures (e.g., particles such as nanoparticles) including an electrochemically active material, such as a conversion material. Such an assembly can be cast from a slurry. The assembly can include one or more of slurry coating, slot die coating, spin coating, spray drying, drawdown coating, doctor blade coating, inkjet printing, comma coating, and inverted comma coating. The initial assembly can be porous (e.g., highly porous) and can maintain some porosity after calendaring.

Electrochemical Cells In some embodiments, the structured cathodes disclosed herein are included in an electrochemical cell. The electrochemical cell may be a battery, such as a secondary battery. The cathode included in the battery may be a conversion cathode including an electrochemically active conversion material, such as a lithium-sulfur battery or a sodium-sulfur battery. In some embodiments, the electrochemical cell includes a structured cathode disclosed herein, an electrolyte, an anode, and optionally a separator. The structured cathode may be porous, and the electrolyte may be disposed within the bulk of the cathode film, with recesses in the patterned surface of the film providing a local reservoir for a portion of the electrolyte transferred from the bulk of the film during electrochemical cycling. The electrolyte may at least partially fill the recesses in the film of the structured cathode. The electrolyte may be liquid, gel, polymer, or solid. The electrolyte may also be in direct contact with the patterned surface of the film, for example, without recesses. The battery may include a solid, polymer, or gel electrolyte (e.g., a polymer gel electrolyte) that at least partially fills the recesses in the patterned surface of the structured cathode. The battery can include a liquid electrolyte that at least partially fills the recesses in the patterned surface of the structured cathode. The battery can include a mixed electrolyte, such as a solid electrolyte and a liquid electrolyte. For example, a liquid electrolyte can at least partially fill the recesses in the patterned surface of the cathode film, and a solid electrolyte can contact the patterned surface (e.g., at least the non-recessed portions of the patterned surface). Such a battery can simultaneously utilize the advantages of both solid and liquid electrolytes.

In some embodiments, the battery includes a non-conductive separator in contact with the patterned surface of the cathode film (e.g., portions of the film that are not recessed). Such contact may define a separator-cathode interface. One or more insoluble products (e.g., non-equilibrium insoluble products) may be disposed (e.g., precipitated) in a higher concentration on the patterned surface of the film at the separator-cathode interface than in the recesses of the patterned surface. The recesses not in contact with the non-conductive separator may be substantially free of insoluble products.

In some embodiments, the battery includes a protected lithium metal anode in contact with a structured cathode disclosed herein. In some embodiments, the battery has an anode-free configuration (e.g., lithium deposits on the current collector during the first electrochemical cycle).

The structured cathodes disclosed herein can be used with electrolytes that do not contain sulfonamide salts (e.g., LiTFSI), thereby realizing cost savings by avoiding expensive electrolytes that may be required to achieve high performance. In some embodiments, the batteries have a low (e.g., very low) electrolyte-to-sulfur (E/S) ratio, e.g., 10 or less, 7 or less, 5 or less, 3 or less, or less than 3.

The present disclosure provides secondary sulfur batteries comprising the cathodes and compositions described herein. In certain embodiments, such batteries comprise a lithium-containing anode composition coupled to a cathode composition provided by a lithium-conducting electrolyte. In some embodiments, such batteries also comprise additional components, such as a separator between the anode and cathode, anode and cathode current collectors, terminals capable of coupling the cell to an external load, and packaging such as a flexible pouch or rigid metal container. In some embodiments, the present disclosure is directed to a lithium-sulfur battery comprising a sulfur-containing cathode, a lithium-containing anode, and an electrolyte ionically bonding the anode and cathode. It is further contemplated that the present disclosure regarding secondary sulfur batteries can be adapted for use in sodium-sulfur batteries, and such batteries are also considered within the scope of certain embodiments of the present disclosure.

FIG. 4 shows a cross-section of an electrochemical cell 500 according to an exemplary embodiment of the present disclosure. The electrochemical cell 500 includes a negative electrode 502, a positive electrode 504, a separator 506 interposed between the negative electrode 502 and the positive electrode 504, a container 510, and a fluid electrolyte 512 in contact with the negative electrode 502 and the positive electrode 504, respectively. Such a cell optionally includes additional layers of electrodes and separators 502a, 502b, 504a, 504b, 506a, and 506b. FIG. 5 shows another view of a cross-section through a representative cell stack showing the negative electrode 502, the positive electrode 504, and the separator 506 inserted between the negative electrode 502 and the positive electrode 804. Also shown in FIG. 5 are the layers including the electrode 504. Specifically, the layers include a current collector 504-1, a cathode layer 504-2 including a lithium intercalation active material, and a cathode layer 504-3 including a conversion active material. As shown, the lithium intercalation active material 504-2 is interposed between the current collector 504-1 and the cathode layer 504-3.

The negative electrode 502 (sometimes referred to herein as the anode) includes an active negative electrode material capable of accepting cations. Non-limiting examples of active negative electrode materials for lithium-based electrochemical cells include Li metal, Li alloys (e.g., Li alloys such as Si, Sn, _Bi , In, and/or Al alloys), _Li4Ti5O12 , hard carbon, _graphitic carbon, metal chalcogenides, and/or amorphous carbon. According to some embodiments of the present disclosure, most (e.g., greater than 90% by weight) of the active negative electrode material may be initially contained in the discharged positive electrode 504 (sometimes referred to herein as the cathode) when the electrochemical cell 500 is first fabricated; thus, the active electrode material forms part of the first electrode 502 during the initial charging of the electrochemical cell 500.

Techniques for depositing an electroactive material on a portion of the negative electrode 502 are described in U.S. Patent Application Publication No. 2016/0172660, and similarly U.S. Patent Application Publication No. 2016/0172661, the contents of each of which are incorporated herein by reference to the extent such contents are not inconsistent with this disclosure.

The negative electrode 502 and the positive electrode 504 may further include one or more conductive additives, as described above. According to some embodiments of the present disclosure, the negative electrode 502 and/or the positive electrode 504 further include one or more polymer binders, as described below.

Figure 6 shows an example of a battery according to various embodiments described below. While a cylindrical battery is shown here for illustrative purposes, other types of configurations, including prismatic or pouch (laminated-type) batteries, may also be used if desired. The exemplary Li battery 600 includes a negative electrode 602, a positive electrode 604, a separator 606 interposed between the negative electrode 602 and the positive electrode 604, an electrolyte (not shown) impregnated in the separator 606, a battery case 605, and a sealing member 608 that seals the battery case 605. Of course, the exemplary battery 600 may simultaneously embody multiple aspects of the present disclosure in various designs.

In some embodiments, the lithium-sulfur battery of the present disclosure includes a lithium anode, a sulfur-based cathode, and an electrolyte that enables ion transport between the anode and the cathode. In certain embodiments described herein, the anode portion of the battery includes the anode and a portion of the electrolyte in contact therewith. Similarly, in certain embodiments described herein, the cathode portion of the battery includes the cathode and a portion of the electrolyte in contact therewith. In certain embodiments, the battery includes a lithium-ion permeable separator that defines a boundary between the anode portion and the cathode portion. In certain embodiments, the battery includes a case that encapsulates both the anode portion and the cathode portion. In certain embodiments, the battery case includes a conductive anode end cover in electrical communication with the anode and a conductive cathode end cover in electrical communication with the cathode to facilitate charging and discharging via an external circuit.

Anode In certain embodiments, a lithium battery (e.g., a lithium-sulfur battery) includes a lithium anode. Any lithium anode suitable for use in a lithium-sulfur cell can be used. In certain embodiments, the anode of a lithium-sulfur battery includes a negative electrode active material selected from a material in which lithium intercalation occurs reversibly, a material that reacts with lithium ions to form a lithium-containing compound, metallic lithium, a lithium alloy, and combinations thereof. In certain embodiments, the anode includes metallic lithium. In certain embodiments, the lithium-containing anode composition includes a carbon-based compound. In certain embodiments, the carbon-based compound is selected from the group consisting of crystalline carbon, amorphous carbon, graphite, and mixtures thereof. In certain embodiments, the material that reacts with lithium ions to form a lithium-containing compound is selected from the group consisting of tin oxide (SnO ₂ ), titanium nitrate, and silicon. In certain embodiments, the lithium alloy includes an alloy of lithium and another alkali metal (e.g., sodium, potassium, rubidium, or cesium). In certain embodiments, the lithium alloy includes an alloy of lithium and a transition metal. In certain embodiments, the lithium alloy comprises an alloy of lithium with a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, Sn, and combinations thereof. In certain embodiments, the lithium alloy comprises an alloy of lithium with indium. In certain embodiments, the anode comprises a lithium-silicon alloy. Examples of suitable lithium-silicon alloys include Li ₁₅ Si ₄ , Li ₁₂ Si ₇ , Li ₇ Si ₃ , Li ₁₃ Si ₄ , and Li ₂₁ Si ₅ /Li ₂₂ Si ₅ . In certain embodiments, the lithium metal or lithium alloy exists as a composite with another material. In certain embodiments, such a composite comprises a material such as graphite, graphene, a metal sulfide or oxide, or a conductive polymer.

The anode can be protected from redox shuttling reactions and detrimental runaway reactions by any method reported in the art, such as by chemical passivation or polymerization, to create a protective layer on the surface of the anode. For example, in certain embodiments, the anode comprises an inorganic protective layer, an organic protective layer, or a mixture thereof, on the surface of lithium metal. In certain embodiments, the inorganic protective layer comprises Mg, Al, B, Sn, Pb, Cd, Si, In, Ga, lithium silicate, lithium borate, lithium phosphate, lithium phosphate nitride, lithium silicate sulfide, lithium borosulfide, lithium aluminosulfide, lithium phosphosulfide, lithium fluoride, or a combination thereof. In certain embodiments, the organic protective layer comprises a conductive monomer, oligomer, or polymer selected from poly(p-phenylene), polyacetylene, poly(p-phenylenevinylene), polyaniline, polypyrrole, polythiophene, poly(2,5-ethylenevinylene), acetylene, poly(perinaphthalene), polyacene, and poly(naphthalene-2,6-diyl), or a combination thereof.

Furthermore, in certain embodiments, during charging and discharging of a lithium-sulfur battery, inert sulfur material, generated from the electroactive sulfur material of the cathode, is deposited on the anode surface. As used herein, the term "inert sulfur" refers to sulfur that is inactive in repeated electrochemical and chemical reactions and cannot participate in the electrochemical reactions of the cathode. In certain embodiments, the inert sulfur on the anode surface acts as a protective layer for such an electrode. In certain embodiments, the inert sulfur is lithium sulfide.

Anode-free (e.g., anodeless) configurations are also contemplated. In these configurations, a current collector is provided in place of the anode, and electrochemically active species, such as lithium in a lithium-sulfur battery, are deposited on the surface of the current collector during the first electrochemical cycle (or the first few electrochemical cycles). Such lithium may be obtained from the electrolyte and/or one or more additives within the electrochemical cell. The surface of the current collector then serves as a lithium source during further electrochemical cycles.

Furthermore, it is contemplated that the present disclosure may be adapted for use in sodium-sulfur batteries. Such sodium-sulfur batteries include sodium-based anodes and are within the scope of the present disclosure.

Electrode Preparation There are various methods for manufacturing electrodes for use in lithium batteries (e.g., lithium-sulfur batteries). One process, known as the "wet process," involves adding the cathode active material, binder, and conductive material (i.e., cathode mixture) to a liquid to prepare a slurry composition. These slurries are typically in the form of viscous liquids formulated to facilitate downstream coating operations. Thorough mixing of the slurry can be important for coating and drying operations, which affect the performance and quality of the electrode. Suitable mixing devices include ball mills, magnetic stirrers, ultrasonicators, planetary mixers, high-speed mixers, homogenizers, universal mixers, and static mixers. The liquid used to make the slurry can be one that uniformly disperses the cathode active material, binder, conductive material, and any additives and is easily evaporated. Suitable slurry liquids include, for example, N-methylpyrrolidone, acetonitrile, methanol, ethanol, propanol, butanol, tetrahydrofuran, water, isopropyl alcohol, dimethylpyrrolidone, and the like.

In some embodiments, the prepared composition is coated onto a current collector and dried to form an electrode. Specifically, the slurry is used to coat a conductor and form an electrode by evenly spreading the slurry over the conductor, which in certain embodiments is then roll-pressed (e.g., calendered) and heated as known in the art. Generally, the matrix of positive electrode active material and conductive material is held together in the conductor by a binder. In certain embodiments, the matrix comprises a lithium-conducting polymer binder, such as polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropene) (PVDF/HFP), polytetrafluoroethylene (PTFE), Kynar Flex® 2801, Kynar Powerflex LBG, Kynar HSV 900, Teflon®, styrene butadiene rubber (SBR), polyethylene oxide (PEO), or polytetrafluoroethylene (PTFE). In certain embodiments, additional carbon particles, carbon nanofibers, or carbon nanotubes are dispersed in the matrix to improve electrical conductivity. Alternatively or additionally, in certain embodiments, lithium ions are dispersed in the matrix to improve lithium conductivity.

In certain embodiments, the current collector is selected from the group consisting of aluminum foil, copper foil, nickel foil, stainless steel foil, titanium foil, zirconium foil, molybdenum foil, nickel foam, copper foam, carbon paper or fiber sheet, a polymer substrate coated with a conductive metal, and/or combinations thereof.

PCT Publication Nos. WO2015/003184, WO2014/074150, and WO2013/040067, the entire disclosures of which are incorporated herein by reference, describe various methods of manufacturing electrodes and electrochemical cells.

Separator In certain embodiments, an electrochemical cell (e.g., a lithium-sulfur battery) includes a separator that physically separates the anode and cathode. In certain embodiments, the separator is a material that is substantially or completely impermeable to the electrolyte. In certain embodiments, the separator is impermeable to polysulfide ions dissolved in the electrolyte. In certain embodiments, the separator is impermeable to the electrolyte as a whole, thereby preventing the passage of electrolyte-soluble sulfides. In some embodiments, ionic conductivity throughout the separator is provided, for example, via apertures in such a separator. In certain such embodiments, the separator as a whole inhibits or limits the passage of electrolyte-soluble sulfides between the anode and cathode portions of the battery as a result of its impermeability. In certain embodiments, the impermeable material separator is configured to allow lithium ion transport between the anode and cathode of the battery during charging and discharging of the cell. In some such embodiments, the separator does not completely separate the anode and cathode from each other. To allow sufficient lithium ion flow between the anode and cathode portions of the battery, one or more electrolyte permeable channels must be provided that bypass or penetrate the impermeable surface of the separator. In some embodiments, if the separator itself is completely impermeable, the channels are provided through an annulus between the periphery of the separator and the wall of the battery case.

As will be apparent to those skilled in the art, the optimal separator dimensions require balancing the competing challenges of maximizing impedance to polysulfide migration while allowing sufficient lithium ion flow. Aside from this consideration, the shape and orientation of the separator are not particularly limited and will depend in part on the battery configuration. For example, the separator may be substantially circular for coin-type cells or substantially rectangular for pouch-type cells. As described herein, the separator surface may lack openings, such that lithium ion flux occurs exclusively around the edges of the impermeable sheet. However, certain embodiments are contemplated in which some or all of the required lithium ion flux is provided through openings in the separator. In some embodiments, the separator is substantially flat. However, it is not excluded that curved or other non-planar configurations may be used.

The separator can be of any suitable thickness. To maximize the battery's energy density, it is generally preferred that the separator be as thin and lightweight as possible. However, the separator should be thick enough to provide sufficient mechanical robustness and ensure adequate impermeability. In certain embodiments, the separator has a thickness of about 1 micron to about 200 microns, preferably about 5 microns to about 100 microns, and more preferably about 10 microns to about 30 microns.

Electrolyte In certain embodiments, the lithium-sulfur battery includes an electrolyte comprising an electrolyte salt. Examples of the electrolyte salt include, for example, lithium trifluoromethanesulfonimide, lithium triflate, lithium perchlorate, LiPF ₆ , LiBF ₄ , tetraalkylammonium salts (e.g., tetrabutylammonium tetrafluoroborate, TBABF ₄ ), and salts that are liquid at room temperature (e.g., imidazolium salts such as 1-ethyl-3-methylimidazolium bis-(perfluoroethylsulfonyl)imide, EMIBeti).

In certain embodiments, the electrolyte comprises one or more alkali metal salts. In certain embodiments, such salts include lithium salts, such as LiCF ₃ SO ₃ , LiClO ₄ , LiNO ₃ , LiPF ₆ , and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or combinations thereof. In certain embodiments, the electrolyte comprises an ionic liquid, such as 1-ethyl-3-methylimidazolium-TFSI, N-butyl-N-methyl-piperidinium-TFSI, N-methyl-n-butylpyrrolidinium-TFSI, and N-methyl-N-propylpiperidinium TFSI, or combinations thereof. In certain embodiments, the electrolyte comprises a superionic conductor, such as a sulfide, oxide, and phosphate (e.g., phosphorus pentasulfide), or combinations thereof.

In certain embodiments, the electrolyte is a liquid. For example, in certain embodiments, the electrolyte comprises an organic solvent. In certain embodiments, the electrolyte comprises only one organic solvent. In some embodiments, the electrolyte comprises a mixture of two or more organic solvents. In certain embodiments, the mixture of organic solvents comprises organic solvents from at least two groups selected from the group consisting of weakly polar solvents, strongly polar solvents, and lithium-protective solvents.

As used herein, the term "weakly polar solvent" is defined as a solvent capable of dissolving elemental sulfur and having a dielectric constant less than 15. In some embodiments, the weakly polar solvent is selected from aryl compounds, bicyclic ethers, and acyclic carbonate compounds. Non-limiting examples of weakly polar solvents include xylene, dimethoxyethane, 2-methyltetrahydrofuran, diethyl carbonate, dimethyl carbonate, toluene, dimethyl ether, diethyl ether, diglyme, tetraglyme, and the like. As used herein, the term "strongly polar solvent" is defined as a solvent capable of dissolving lithium polysulfide and having a dielectric constant greater than 15. In some embodiments, the strong polar solvent is selected from bicyclic carbonate compounds, sulfoxide compounds, lactone compounds, ketone compounds, ester compounds, sulfate compounds, and sulfite compounds. Non-limiting examples of strong polar solvents include hexamethylphosphoric triamide, γ-butyrolactone, acetonitrile, ethylene carbonate, propylene carbonate, N-methylpyrrolidone, 3-methyl-2-oxazolidone, dimethylformamide, sulfolane, dimethylacetamide, dimethylsulfoxide, dimethyl sulfate, ethylene glycol diacetate, dimethyl sulfite, ethylene glycol sulfite, etc. As used herein, the term "lithium protecting solvent" is defined as a solvent that forms a good protective layer on the lithium surface, i.e., a stable solid electrolyte interface (SEI) layer, and exhibits a cycling efficiency of at least 50%. In some embodiments, the lithium protecting solvent is selected from saturated ether compounds, unsaturated ether compounds, and heterocyclic compounds containing one or more heteroatoms selected from the group consisting of N, O, and/or S. Non-limiting examples of lithium-protecting solvents include tetrahydrofuran, 1,3-dioxolane, 3,5-dimethylisoxazole, 2,5-dimethylfuran, furan, 2-methylfuran, 1,4-oxane, 4-methyldioxolane, and the like.

In certain embodiments, the electrolyte is a liquid (e.g., an organic solvent). In some embodiments, the liquid is selected from the group consisting of an organic carbonate, an ether, a sulfone, water, an alcohol, a fluorocarbon, or any combination thereof. In certain embodiments, the electrolyte comprises an ether solvent.

In certain embodiments, the organic solvent comprises an ether. In certain embodiments, the organic solvent is selected from the group consisting of 1,3-dioxolane, dimethoxyethane, diglyme, triglyme, gamma-butyrolactone, gamma-valerolactone, and combinations thereof. In certain embodiments, the organic solvent comprises a mixture of 1,3-dioxolane and dimethoxyethane. In certain embodiments, the organic solvent comprises a 1:1 (v/v) mixture of 1,3-dioxolane and dimethoxyethane. In certain embodiments, the organic solvent is selected from the group consisting of diglyme, triglyme, gamma-butyrolactone, gamma-valerolactone, and combinations thereof. In certain embodiments, the electrolyte comprises sulfolane, sulfolene, dimethyl sulfone, or methyl ethyl sulfone. In some embodiments, the electrolyte comprises ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate.

In certain embodiments, the electrolyte comprises a liquid (e.g., an organic solvent). In some embodiments, the liquid is selected from the group consisting of an organic carbonate, an ether, a sulfone, water, an alcohol, a fluorocarbon, or any combination thereof. In certain embodiments, the electrolyte comprises an ether solvent. In certain embodiments, the electrolyte comprises a liquid selected from the group consisting of sulfolane, sulfolene, dimethyl sulfone, and methyl ethyl sulfone. In certain embodiments, the electrolyte comprises a liquid selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

In certain embodiments, the electrolyte is solid. In certain embodiments, the solid electrolyte comprises a polymer. In certain embodiments, the solid electrolyte comprises a glass, a ceramic, an inorganic composite, or a combination thereof. In certain embodiments, the solid electrolyte comprises a polymer composite comprising a glass, a ceramic, an inorganic composite, or a combination thereof. In certain embodiments, such solid electrolytes include one or more liquid components as plasticizers or to form a "gel electrolyte."

In order that the present disclosure may be more fully understood, the following examples are set forth. It should be understood that these examples are for illustrative purposes only and should not be construed as limiting in any manner.

Structured conversion cathodes suitable for use in lithium-sulfur batteries were constructed, and their microstructures were characterized by SEM. Figures 8A-8B show examples of structured cathodes, including cathode films containing electrochemically active materials, with grooves recessed into the film's patterned surface at regular widths and/or intervals across the patterned surface. The films had a ratio of electrochemically active conversion material to conductive carbon and binder in individual structures of approximately 10:5:4, e.g., approximately 55% sulfur, approximately 25% conductive carbon, and approximately 20% binder. Overall, the electrochemically active sulfur content of the tested cathodes was approximately 3-3.5 mg/ ^cm² . Using laser ablation, the groove width and spacing can be tailored as needed. For example, grooves can be formed using a laser with a power to create a 35 μm circle for approximately 400-600 μs. For example, Figure 8A shows a structured cathode containing grooves approximately 80 μm wide and spaced approximately 165 μm apart. Alternatively, Figure 8B shows a structured cathode containing grooves approximately 70 μm wide and spaced approximately 185 μm apart. Wider or narrower grooves and/or greater or smaller spacing between grooves can be used.

The 3D structured cathode samples (including the patterned surface) were tested against a control cathode (unpatterned) of similar composition. A comparison between the specific sample shown in FIG. 8B ("3D structured cathode") and the control ("control") at the third and fifth cycles is shown in Table 1.

Figure 9 shows a structured conversion cathode suitable for use in lithium-sulfur batteries, with a consistent morphology within grooves that extend to the surface of the cathode film as well as the top of the cathode surface. The grooves were formed by laser ablation. The left panel shows a minimally magnified view of the grooves. The area within the red box is magnified to form the view in the middle panel, and the area within that red box is further magnified to form the view in the far right panel. As can be seen by close inspection of the incremental magnifications in this example, the surface of the grooves exhibits similar (e.g., identical) morphology and porosity to the top of the unablated cathode (and thus, e.g., presumably the bulk of the cathode film).

Figures 10-13 show additional example configurations of structured cathodes. The structured cathodes include a film containing an electrochemically active conversion material, in which grooves, or recesses, are formed in the surface of the film using laser ablation. The recesses extend into the film but do not extend entirely through the film (e.g., no current collector is exposed below the film). These structured cathodes are suitable for use in lithium-sulfur batteries.

Figures 14-23C show additional example configurations of structured cathodes. Examples include cathode films containing an electrochemically active material with recessed holes of consistent diameter and/or consistent spacing across the patterned surface of the film. The film composition was (i) 80 wt% of a mixture of sulfur and metal sulfide additives, (ii) 10 wt% carbon (e.g., C65 and Ketjen Black), and (iii) 10 wt% binder (e.g., Na-PAA). Figures 14-19 show top views of calendered (Figures 14-16) and uncalendered (Figures 17-19) films. Figures 20-23C show cross-sectional views of calendered (Figures 20-22) and uncalendered (Figures 23A-23C) films. The hole pattern corresponded to a hexagonal grid resembling the hexagonal close-packed (HCP) arrangement characteristic of the (111) planes of a face-centered cubic (FCC) structure. Figures 14-19 show structured cathodes containing holes approximately 50 μm wide and spaced approximately 100 μm apart (e.g., holes 1602, 1604, and 1606). Wider or narrower holes and/or greater or closer spacing between holes can be used.

High-magnification SEM images, such as those shown in Figure 18, show that well-defined holes were created by laser ablation. Cross-sectional images of both the calendered (Figures 20-22) and uncalendered (Figures 23A-23C) films show that laser ablation created consistent holes in the films, e.g., holes 2002 and 2004 in Figure 20, hole 2102 in Figure 21, and hole 2202 in Figure 22. Figures 23A-23C also show that in some cases, holes (e.g., holes 2304, 2306, 2308) did not extend all the way to substrate 2302. Additionally, holes were observed intersecting with the pore structure within the cathode film.

A comparison between the specific samples shown in Figures 17-19 and Figures 23A-23C ("3D structured cathode" with holes, no calendering) at the third and fifth cycles is shown in Table 2.

The structured cathodes were also tested against a control cathode at the same current density (mA/ ^cm2 ). The experimental structured cathodes exhibited an increased material rate (mA/g activity). Without intending to be bound by any particular theory, the increased material rate can be explained, at least in part, by the material removed to form recesses in the patterned surface of the experimentally structured cathode.

The systems, devices, methods, and processes of the present disclosure are intended to encompass variations and modifications developed using information from the embodiments described herein. Modifications and/or variations of the systems, devices, methods, and processes described herein may be made by those skilled in the art.

Throughout the description, when articles, devices, and systems are described as having, including, or comprising particular components, or when processes and methods are described as having, including, or comprising particular steps, it is believed that there are additionally articles, devices, and systems according to certain embodiments of the disclosure that consist essentially of or consist of the recited components, and that there are processes and methods according to certain embodiments of the disclosure that consist essentially of or consist of the recited processing steps.

It should be understood that the order of steps or the order for performing certain actions is immaterial so long as operability is not lost. Additionally, two or more steps or actions may be performed simultaneously. As will be understood by those skilled in the art, the terms "on," "under," "upward," "underneath," "below," and "on" are relative terms and may be interchanged to refer to different orientations of layers, elements, and substrates included in this disclosure. For example, in some embodiments, a first layer on a second layer means that the first layer is directly on and in contact with the second layer. In other embodiments, a first layer on a second layer can include another layer therebetween.

Headings are provided for the convenience of the reader and are not intended to be limiting with respect to the claimed subject matter. Statements in the Background section should not be construed as admissions of prior art.

Specific embodiments of the present disclosure have been described above. However, it should be expressly noted that the present disclosure is not limited to these embodiments, but rather additions and modifications to those expressly described in this disclosure are also intended to be within the scope of the present disclosure. Furthermore, it should be understood that the features of the various embodiments described in this disclosure are not mutually exclusive and may exist in various combinations and permutations, even if such combinations or permutations are not expressly stated, without departing from the spirit and scope of the present disclosure. While the present disclosure has been described in detail with particular reference to specific embodiments thereof, it will be understood that variations and modifications may occur within the spirit and scope of the claimed invention.

Claims (80)

A conversion cathode for a secondary lithium battery (e.g., a lithium-sulfur battery), the cathode comprising a film including an electrochemically active conversion material, the film having a first surface in contact with a substrate (e.g., a current collector) and a patterned second surface opposite the first surface (e.g., the side not in contact with the substrate), the patterned second surface including recesses extending into the film toward the substrate (e.g., in a direction substantially perpendicular to the first surface). The conversion cathode of claim 1, wherein the patterned second surface of the film has a repeating geometric pattern of the recesses. The conversion cathode of claim 2, wherein the repeating geometric pattern corresponds to a hexagonal grid. The conversion cathode of claim 2, wherein the repeating geometric pattern conforms to an equiangular grid or a square grid. A conversion cathode as described in any one of the preceding claims, wherein at least some (e.g., all) of the recesses are interconnected across the second surface (e.g., forming a network of recesses across the extent of the second surface). A conversion cathode as described in any one of the preceding claims, wherein the recesses in the second surface of the film comprise holes. The conversion cathode of claim 6, wherein the holes are substantially circular in cross section. The conversion cathode of claim 7, wherein the diameter of the holes is at least 20 nm and no greater than 500 μm (e.g., 20 nm to 50 μm, 20 nm to 100 μm, 20 nm to 200 μm, 20 nm to 300 μm, 100 nm to 100 μm, 100 nm to 200 μm, 1 μm to 100 μm, 1 μm to 200 μm, 10 μm to 100 μm, 10 μm to 200 μm, 50 μm to 100 μm, 50 μm to 200 μm, or 100 μm to 200 μm). A conversion cathode as described in any one of claims 6 to 8, wherein each of the holes has a depth of at least 25% of the thickness of the cathode film (e.g., a depth of at least 50%, at least 75%, at least 80%, or at least 90% of the thickness of the cathode film), or the holes extend completely through the film from the second surface to the first surface. A conversion cathode as described in any one of the preceding claims, wherein the recess comprises a groove. The conversion cathode of claim 10, wherein the groove has a width and a length across the second surface, the width being smaller than the length, and the width being at least 20 nm and no greater than 500 μm (e.g., 20 nm to 50 μm, 20 nm to 100 μm, 20 nm to 200 μm, 20 nm to 300 μm, 100 nm to 100 μm, 100 nm to 200 μm, 1 μm to 100 μm, 1 μm to 200 μm, 10 μm to 100 μm, 10 μm to 200 μm, 50 μm to 100 μm, 50 μm to 200 μm, or 100 μm to 200 μm). The conversion cathode of claim 10 or claim 11, wherein each of the grooves has a depth of at least 25% of the thickness of the cathode film (e.g., a depth of at least 50%, at least 75%, at least 80%, or at least 90% of the thickness of the cathode film), or the grooves extend completely through the film from the second surface to the first surface. A conversion cathode as described in any one of the preceding claims, wherein the recess is at least partially filled with electrolyte. The conversion cathode of claim 13, wherein the electrolyte is liquid. The conversion cathode of claim 13, wherein the electrolyte comprises a polymer. The conversion cathode described in claim 13 or claim 15, wherein the electrolyte is solid. A conversion cathode as described in any one of the preceding claims, wherein the recesses are distributed in a regular pattern across the second surface of the film. A conversion cathode as described in any one of the preceding claims, wherein the recesses are dispersed across the second surface such that no point in the film is 500 μm or less (e.g., 200 μm or less, 100 μm or less, 50 μm or less, 25 μm or less, or 20 μm or less) from at least one edge of at least one of the recesses. A conversion cathode as described in any one of the preceding claims, wherein the recesses are dispersed across the second surface such that no point in the film is within a distance from the nearest recess that is no more than 3 times (e.g., no more than 2 times or no more than 1.5 times) the maximum thickness of the film. The conversion cathode of claim 19, wherein the distance is less than or equal to the maximum thickness of the film. A conversion cathode as described in any one of the preceding claims, wherein the recess represents at least 5% (e.g., at least 10%, at least 15%, at least 20%, at least 25%, at least 33%, or at least 50%) of the total volume contained between a plane coinciding with the top of the second surface and the first surface. A conversion cathode as described in any one of the preceding claims, wherein the surface of the recess (e.g., the portion of the second surface defined by the recess) is coated with a solid material having a composition different from the composition of the bulk cathode within the film. The conversion cathode of claim 22, wherein the surface of the recess is coated with a solid epitaxial material (e.g., formed by atomic layer deposition after formation of the recess). A conversion cathode as described in any one of the preceding claims, wherein the film is a first film, and the conversion cathode further includes a second film disposed on a side of the substrate opposite the first film. The conversion cathode of claim 24, wherein the second film has a patterned second surface including recesses extending into the second film and a first surface opposite the second surface, the first surface of the second film being in contact with the substrate. The conversion cathode described in claim 24 or claim 25, wherein the substrate is porous. The conversion cathode of claim 26, wherein the recesses in the first film extend entirely through the first film and intersect with pores in the substrate. A conversion cathode as described in any one of claims 25 to 27, wherein the recesses in the second film extend entirely through the second film and intersect with the pores in the substrate. A conversion cathode as described in claim 27 or claim 28, wherein the pores in the substrate extend entirely through the substrate (e.g., thereby defining pores that extend entirely through the cathode). A conversion cathode as described in any one of the preceding claims, wherein the second surface of the film is patterned after the film is applied to the substrate. The conversion cathode of claim 30, wherein the film is fabricated by applying a wet slurry to the substrate and then drying the slurry before patterning. The conversion cathode of claim 30 or claim 31, wherein the film is calendered before patterning the second surface. A conversion cathode as described in any one of the preceding claims, wherein the second surface is patterned by laser ablation. The conversion cathode of claim 33, wherein the laser ablation uses a pulsed laser to pattern the second surface. The conversion cathode of claim 34, wherein the pulsed laser emits pulses having a duration of less than 1000 femtoseconds (e.g., less than 500, 400, 300, 200, 150, 100, 50, 25, 15, 10, 5, 4, 3, 2, or 1 femtosecond). A conversion cathode as described in any one of the preceding claims, wherein the film is porous (e.g., a porous collection of individual structures (e.g., particles) (e.g., nanoparticles) (e.g., core-shell particles or yolk-shell particles)). 10. The conversion cathode of any one of the preceding claims, wherein the electrochemically active conversion material comprises: (i) elemental sulfur (e.g., in the form of an octacyclic molecular form of _S8 ); (ii) sulfur in the form of lithium _sulfide (e.g., _Li2S2 and/or _Li2S ); (iii) sulfur in the form of an electrochemically active organosulfur compound; (iv) sulfur in the form of an electrochemically active sulfur-containing polymer; or (v) a combination of two or more of (i)-(iv). The conversion cathode of any one of the preceding claims, wherein the cathode film further comprises one or more metal sulfides. The conversion cathode of claim 38, wherein at least one of the one or more metal sulfides is an intercalation electrochemically active material. The conversion cathode of any one of the preceding claims, further comprising a conductive additive (e.g., conductive carbon). The conversion cathode of any one of the preceding claims, further comprising a polymer binder. A conversion cathode according to any one of the preceding claims, wherein the cathode is substantially carbon-free (e.g., having a carbon content of 10% by weight or less, a carbon content of 5% by weight or less, a carbon content of 2% by weight or less, a carbon content of 1% by weight or less). (i) the average mass transport path to the electrochemically active conversion material is shorter than the average mass transport path to the electrochemically active conversion material in an otherwise equivalent cathode without the recess;
(ii) the degree of bending of the cathode is reduced compared to the degree of bending of an otherwise equivalent cathode without the recess; or
(iii) both (i) and (ii);
A conversion cathode according to any one of the preceding claims. (i) the capacity of the cathode is greater than the capacity of an otherwise equivalent cathode without the recess at the same current density;
(ii) the cathode has a high volumetric capacity; or
(iii) both (i) and (ii);
A conversion cathode according to any one of the preceding claims. A conversion cathode as described in any one of the preceding claims, wherein at least some (e.g., all) of the recesses extend completely through the film. A conversion cathode as described in any one of the preceding claims, wherein at least some (e.g., all) of the recesses do not extend completely through the film. A (e.g., lithium-sulfur) secondary battery comprising the conversion cathode described in any one of claims 1 to 46. The battery of claim 47, further comprising an electrolyte disposed within the film, the recess being a local reservoir for a portion of the electrolyte displaced from the bulk of the film during electrochemical cycling of the battery. The battery of claim 46 or claim 48, further comprising a liquid electrolyte that at least partially fills the recess (e.g., the liquid electrolyte also directly contacts the second surface that is not in the recess). The battery of claim 47, further comprising a solid, polymer, or gel electrolyte (e.g., a polymer gel electrolyte) at least partially filling the recess. The battery of claim 50, further comprising a liquid electrolyte in contact with the solid, polymer, or gel electrolyte (e.g., the liquid electrolyte is disposed within the recess and the solid, polymer, or gel electrolyte is in direct contact with the second surface). The battery of any one of claims 47 to 51, further comprising a non-conductive separator, wherein the second surface of the film is in contact with the non-conductive separator (e.g., at a portion of the second surface that is not a recess) [e.g., thereby defining an interface between the separator and the cathode, and wherein the non-equilibrium insolubilized product is disposed (e.g., precipitated) in a higher concentration on the surface of the film at the interface between the separator and the cathode than in the recess]. The battery described in any one of claims 47 to 51, further comprising a solid electrolyte, and the second surface of the film is in contact with the solid electrolyte (e.g., in a portion of the second surface that is not in a recess). The battery described in any one of claims 47 to 51, further comprising a protected lithium anode, wherein the second surface of the film is in contact with the protected lithium anode. The battery of any one of claims 47 to 53, wherein the battery has an anode-free configuration (e.g., the battery includes a current collector, and lithium is deposited on the current collector during the first electrochemical cycle). The battery described in any one of claims 48 to 55, wherein the electrolyte does not contain a sulfonamide salt (e.g., lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)). The battery of any one of claims 47 to 56, wherein the battery has a low electrolyte-to-sulfur (E/S) ratio. A method of operating (i) a battery (e.g., a lithium-sulfur battery) comprising the cathode of any one of claims 1 to 46 and an electrolyte, or (ii) the battery of any one of claims 47 to 57, the method comprising expanding the film (e.g., by expanding individual structures (e.g., particles) assembled within the film) (e.g., by reducing the porosity of the film) during an electrochemical cycle of the battery (e.g., during discharge of the battery) so that a portion of the electrolyte migrates from the bulk of the film into the recesses. The method of claim 58, further comprising returning the portion of the electrolyte to the bulk of the film (e.g., by shrinking the film) during further electrochemical cycling of the battery (e.g., during charging of the battery). the battery further includes a separator in contact with the second surface of the film, thereby defining a separator-cathode interface;
forming non-equilibrium insoluble products during electrochemical cycling;
disposing (e.g., precipitating) the non-equilibrium insoluble product on the surface of the film at the separator-cathode interface;
60. The method of claim 58 or claim 59, comprising: The method of claim 60, wherein the non-equilibrium insoluble product is disposed on the surface of the film at the separator/cathode interface at a concentration higher than the concentration at which the non-equilibrium insoluble product is disposed in the recesses. The method of claim 61, wherein the non-equilibrium insoluble product is not disposed within the recess. The method of any one of claims 60 to 62, comprising reversibly transporting lithium (e.g., in the form of polysulfides) to the cathode through the recesses during electrochemical cycling. The method of claim 63, wherein the transport of lithium to the cathode through the recesses occurs at a rate and/or to a greater extent (e.g., based on the amount of lithium transported) than the rate and/or extent of simultaneous lithium transport through the surface on which the non-equilibrium insoluble product is disposed. The method of any one of claims 58 to 64, wherein said migration into the recess reduces electrolyte migration into one or more voids in the cell stack and/or the battery during electrochemical cycling. 1. A method for making a cathode for a battery (e.g., a lithium-sulfur battery), comprising:
providing (e.g., forming) a film including an electrochemically active conversion material;
forming a recess in a surface of the film that extends into the film;
A method comprising: The method of claim 66, wherein the forming includes removing a portion of the film. The method of claim 67, wherein the removing comprises laser ablating the film. The method of claim 68, wherein the laser ablation comprises irradiating the film with a pulsed laser. The method of claim 69, wherein the pulsed laser emits pulses having a duration of less than 1000 femtoseconds (e.g., less than 500, 400, 300, 200, 150, 100, 50, 25, 15, 10, 5, 4, 3, 2, or 1 femtosecond). The method of any one of claims 68 to 70, wherein the laser ablation is performed in-line (e.g., during battery manufacturing). The method of any one of claims 66 to 71, comprising calendering the film [e.g., on a substrate (e.g., a current collector)]. The method of claim 72, wherein the forming is performed after the calendaring. 74. The method of claim 73, wherein (i) the calendaring process leaves the film with 40% or less (e.g., 30% or less, 20% or less, or 10% or less) of its initial porosity before calendaring, (ii) the maximum thickness of the film is 40% or less (e.g., 30% or less, 20% or less, or 10% or less) of its initial thickness before calendaring, or (iii) both (i) and (ii). The method of any one of claims 66 to 74, wherein the forming comprises scraping, cutting, and/or scratching (e.g., with one or more blades). The method of any one of claims 66 to 75, wherein the forming comprises debossing (e.g., compressing, imprinting, and/or stamping) the film. The method of any one of claims 66 to 76, wherein the providing step includes forming the film, and forming the film includes assembling individual structures (e.g., particles) that include the electrochemically active material. The method of claim 77, wherein the assembling comprises one or more elements selected from the group consisting of slurry coating, slot die coating, spin coating, spray drying, drawdown coating, doctor blade coating, inkjet printing, comma coating, and inverted comma coating. The method of any one of claims 66 to 78, wherein the method is performed as part of a roll-to-roll manufacturing process (e.g., a roll-to-roll cathode manufacturing process or a roll-to-roll battery manufacturing process). The method of any one of claims 66 to 79, wherein the cathode is a cathode according to any one of claims 1 to 46.