NO20221351A1

NO20221351A1 - An energy storage device for providing a power to energy ratio of at least 25

Info

Publication number: NO20221351A1
Application number: NO20221351A
Authority: NO
Inventors: Fengliu Lou; Rex Miles Philcox; Xiaoyong Xia; Yong Li
Original assignee: Beyonder As
Priority date: 2022-12-16
Filing date: 2022-12-16
Publication date: 2024-06-17
Also published as: WO2024128922A1

Description

AN ENERGY STORAGE DEVICE FOR PROVIDING A POWER TO ENERGY RATIO OF AT LEAST 25

The invention relates to an energy storage device for providing a power to energy ratio of at least 25.

An energy storage device is a device that can store electrical energy, for example batteries, supercapacitors, and metal-ion capacitors. An energy storage device may refer to a single energy storage cell or a plurality of electrically connected energy storage cells. Each energy storage cell comprises at least one negative electrode that is typically also referred to as the anode, and at least one positive electrode that is typically also referred to as the cathode, an electrolyte to allow diffusion of charge carrier ions, and one or more separators to prevent the electrodes from contacting each other while still allowing diffusion of ions. The anode and cathode typically comprise a layer of active material on each side of a current collector, and each cell may comprise a plurality of anodes and cathodes stacked on top of each other, or alternatively an anode and cathode rolled into a jelly roll. The current collectors of the anodes are typically connected to each other at an anode tab, while the current collectors of the cathodes are connected to each other at a cathode tab. The anode tab and cathode tab are often positioned at the same or opposite side of the energy storage cell.

Generally, within known technology, there is a trade-off between obtaining an energy storage device with a high energy density and an energy storage device with a high power density. Energy storage devices may therefore be classified by their ratio of power density to energy density, also referred to as P/E ratio. The P/E ratio is defined herein as the maximum power that the energy storage device can provide for 1 second (in W) divided by the energy provided by the energy storage device (in Wh) when being discharged at a rate of 1C. Different applications may require energy storage devices with different P/E ratios depending on their use.

Metal-ion batteries such as lithium-ion batteries generally have insertion-type materials with faradaic charge-storage mechanism. During charging, the metal ions will be extracted from the cathode and diffuse through the electrolyte to intercalate or alloy in the anode, while the reverse reaction will occur during discharging. The anode material for metal-ion batteries may for example comprise intercalation materials such as graphite, hard carbon, or soft carbon, but also alloying materials such as silicon. The cathode materials may comprise materials with a high concentration of metal ions and a high electrode potential. Metal-ion batteries are characterized by a high energy density, but a relatively low power density and cyclability. The market for metal-ion batteries is dominated by lithium-ion batteries (LiBs) designed to have a high energy density, herein referred to as high energy lithium-ion batteries (HE-LiBs), which typically have a P/E ratio below 10.

Supercapacitors have a different charge-storage mechanism, where metal ions and anions from the electrolyte upon charging will adsorb onto the surface of each electrode, respectively, and be released back into the electrolyte upon discharging. Since supercapacitors rely on the non-faradaic charge storage mechanism of surface adsorption, their electrodes generally comprise materials with a large surface area such as activated carbon. Supercapacitors are characterized by a high power density and cyclability, but a relatively low energy density. The P/E ratio of supercapacitors is therefore very high, typically above 100.

Metal-ion capacitors are hybrid energy storage devices which integrate a metal-ion battery anode, for example graphite or hard carbon, and a supercapacitor cathode, typically activated carbon, together. Therefore, they exhibit a high specific power, a good cyclic stability, and a moderate specific energy, so they have a wide range of potential applications. The P/E ratio of metal-ion capacitors may typically be above 50. However, since neither the anode nor the cathode contains inherent metal ions, it is necessary to pre-dope the metal-ion capacitor with metal ions as charge carriers to run it properly. Metal-ion predoping may also lower the electrode potential of the anode to further increase the energy density.

Pre-doping of metal-ion batteries with metal-ions may also be advantageous to reduce the negative effects of the loss of metal ions consumed during formation of the solidelectrolyte interphase or due to undesired side reactions during the lifetime of the battery.

Even though metal-ion capacitors are being developed to provide an energy storage device with performance between supercapacitors and metal-ion batteries, there still exist a range of applications where metal-ion capacitors will have too low energy density while most available metal-ion batteries such as HE-LiBs will have too low power density.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow. The invention is defined by the independent patent claims, while the dependent claims define advantageous embodiments of the invention.

The invention relates more specifically to an energy storage device for providing a power to energy ratio of at least 25, wherein the energy storage device comprises:

− an anode comprising an anode current collector covered at least partly by an anode active material layer comprising hard carbon, the anode active material layer having a layer thickness of maximum 100 µm;

− a cathode comprising a cathode current collector covered at least partly by a cathode active material layer comprising lithium iron phosphate, the cathode active material layer having a layer thickness of maximum 100 µm;

− a separator for preventing electrical connection between the anode and the cathode while allowing lithium ions to diffuse though the separator, and

− an electrolyte.

This energy storage device will thereby be suitable for applications which require more power than typical HE-LiBs, while simultaneously having a higher energy density than supercapacitors and metal-ion capacitors. Different features in combination are used to obtain the high P/E ratio. For example, the anode comprises hard carbon, since hard carbon has a high rate capability due to a structure which is less crystalline than the graphite anodes typically used in HE-LiBs. The cathode comprises lithium iron phosphate (LFP), because LFP has a high safety compared to other available cathode materials. The energy storage device according to the invention will thereby be safer even when providing a high power output. The thin active material layers on the electrodes decrease the required diffusion distance of the lithium ions within the layers, as well as increases the electrodes’ electrical conductivity, whereby the internal resistance of the device is decreased, and the maximum power output is increased. The current collectors may typically have the form of thin sheets, for example with a of thickness 5-20 µm, to obtain a high surface-to-volume ratio, thereby obtaining large amount of active material relative to the weight of current collector even with a thin active material layer. Thinner active material layers will experience less stress and strain due to volume change of the active material upon charging and discharging, which will increase the cycle life of the device. The active material layer thicknesses may be even smaller, for example, the layer thickness of the anode active material may be less than 75 or less than 50 µm, and the layer thickness of the cathode active material layer may be less than 75 or less than 50 µm. The concentrations of the active materials, LFP and hard carbon, in the active material layers may preferably be high, for example at least 85%, at least 90%, or at least 95%, to have a high energy density. However, the anode and cathode active material layers may typically also comprise further additives such as binders for improving the strength and/or stability of the active material layers, and/or conductive additives for improving the electrical conductivity. For example, the anode active material layer may comprise carboxymethyl cellulose (CMC) as a binder, typically together with styrene-butadiene rubber (SBR) for additional flexibility, and carbon black for improved electrical conductivity. The cathode active material layer may for example comprise polyvinylidene fluoride (PVDF) as a binder and carbon black for improved electrical conductivity. The concentrations of each additive may typically be in the region of 1-5% by weight. The energy storage device may be designed to provide a P/E ratio of at least 35, or even at least 50 depending on the P/E requirement of an application.

In one embodiment, the cathode active material layer may further comprise activated carbon. This will have the effect of increasing the rate capability of the cathode further, since lithium ions may be adsorbed at least temporarily on the activated carbon during discharge. The higher the percentage of activated carbon is in the cathode, the more the energy storage device will resemble a metal-ion capacitor, and the higher the P/E ratio may be.

In one embodiment, the device may have a device length, a device width, and a device thickness, and the aspect ratio between the device length and the device width may be between 2-5, and the device may comprise an anode tab connected to the anode current collector and a cathode tab connected to the cathode current collector, and the anode tab and the cathode tab may be positioned opposite each other on each end of the device. By having a relatively long cell, i.e a cell with a high aspect ratio between the device length and width, and positioning the tabs opposite each other on each end, the tabs will be far away from each other, and the heat generated in one tab area will not influence the heat generated in the other tab area. Heat in the tab areas may arise from resistance in the connection between the current collectors and the tabs and from heat generated in the center of the cell and conducted to the tabs area via the current collectors, as the current collectors typically have a high thermal conductivity. The heat generated in the cell will thereby be distributed over a longer distance and surface area which will decrease the risk of having a too high temperature at specific sites within the cell. This may also make the required cooling more predictable and the cell safer. In one embodiment, the aspect ratio between the device length and the device width may be between 3-4.

In one embodiment, the device length may be at least 300 mm, and the device width may be at least 90 mm. By having a relatively large surface area, the total energy and power of the device may be relatively large even if the thickness is small. A larger cell has a smaller fraction of components which does not contribute to the energy storage capacity such as casing or pouch, whereby the energy and/or power density or specific energy and/or power of the cell may be larger.

In one embodiment, a tab width of the anode tab and cathode tab may be at least 50% of the device width. Wide anode and cathode tabs provide a larger area for connection with the anode and cathode current collectors. Additionally, wider tabs will have a higher electrical conductance than narrower tabs with the same thickness due to a larger crosssectional area, which is beneficial for high charge and discharge rate. In some embodiments, the tab widths of the anode and cathode tabs may be at least 60% or at least 70% of the device width. For typical sizes, the tabs may be at least 50 mm, 60 mm, or 70 mm wide. The tab thickness may be chosen in consideration of the tab width and the power requirement of the energy storage device and may for example be 0.1-0.7 mm, typically 0.3-0.5 mm. The anode and cathode tabs may typically comprise different materials due to the different stability of different materials in different potential windows, and the different materials may have different electrical conductivity. The anode tabs and cathode tabs may therefore require different tab thicknesses, for example 0.3 mm for a nickel-plated copper anode tab and 0.5 mm for an aluminum cathode tab.

In one embodiment, the device thickness may be less than 10 mm. A thinner cell may result in less heat generated within the cell. If a thicker cell is charged or discharged at high rate, it may be more difficult to remove enough of the heat generated in the center of the cell by surface cooling to keep the center at an accepted temperature. When many cells are packed closely together in a battery module, thinner cells will provide more frequent spaces between the cells and thereby make it easier to remove the generated heat with passive or active cooling. Thinner cells may be counterintuitive to the skilled person working with HE-LiBs, since they will have a lower energy density due to the additional mass of the tabs and container. However, when working with cells with a high P/E ratio, some energy density may preferably be sacrificed for the increased temperature control and thereby improved safety and cycle life. The device thickness may in one embodiment be less than 8 mm depending on the cell design and required cooling. Thinner cells may also have fewer electrodes in the stack, and thereby fewer electrical connections between the current collectors and the tabs. This is important as a significant amount of heat is generated in this region due to a high electrical resistance in the connection and a high flow of current. The number of electrodes in the cell may for example be maximum 40, maximum 35, or maximum 31. The device thickness may be chosen so that the temperature difference within the cell is less than 10 °C when charged or discharged at rated maximum current.

In one embodiment, the anode may be pre-doped with lithium ions. This may have the advantage of decreasing the potential of the anode to increase the cell voltage, and also increase the cell cyclability by providing additional lithium ions to make up for those being consumed by formation and growth of the solid-electrolyte interphase.

In one embodiment, the separator may have a porosity of at least 45 %. This allows for easy diffusion of ions through the separator and may result in good power performance of the energy storage device. The separator may have a thickness of less than 20 µm, since it is easier for ions to diffuse through a thin separator relative to a thicker separator with a similar porosity. A thin separator may therefore improve the power performance of the energy storage device, as well as increasing the energy density due to a lower percentage of auxiliary components.

In one embodiment, the electrolyte may have a lithium salt concentration above 1 M. A high concentration of salt may increase the power performance of the energy storage device due to the presence of more charge carriers in the electrolyte. The electrolyte may comprise 1-3 % vinylene carbonate to stabilize the SEI and thereby improve the cyclic stability of the device.

In one embodiment, the active material layers may comprise active material particles having a D50 particle size of less the 10 µm. A small particle size of the active materials decreases the diffusion length of the lithium ions in the particles, which may increase the rate capability of the energy storage device. In embodiments, the D50 particle size of the active materials may be less than 8 µm, or even less than 6 µm.

In the following is described examples of preferred embodiments illustrated in the accompanying drawings, wherein:

Fig. 1 shows an energy storage device according to the invention;

Fig. 2 shows an expanded view of a bi-layer electrode stack;

Fig. 3 shows a collapsed view of the bi-layer electrode stack from figure 2, and

Fig. 4 shows a multilayer electrode stack with four bi-layer electrode stacks as shown in figure 3.

In the following are described examples of embodiments of the invention. In the drawings, the reference numeral 1 indicates an energy storage device according to the invention. The drawings are illustrated in a schematic manner, and the features therein are not necessarily drawn to scale.

Figure 1 shows an energy storage device 1 according to the invention as a single cell. The energy storage device 1 has a cathode tab 3 and an anode tab 5 at opposite ends. The cathode tab 3 is welded together with the cathode current collectors (not visible in figure 1) at the cathode tab area 7, and the anode tab 5 is welded together with the anode current collectors (not visible in figure 1) at the anode tab area 9. The anode and cathode tab areas 7, 9 are covered by a pouch. The energy storage device 1 has a device length 11 (defined as the length covered by the pouch), a device width 13, and device thickness 15.

Figure 2 shows an exploded view of a bi-layer electrode stack 31 comprising an anode 17, and cathode 19, and a separator 21 positioned between the anode 17 and cathode 19 to prevent these from being in direct contact. The anode 17 comprises an anode current collector 23, which is coated on both sides with an anode active material layer 25 except on an anode current collector protrusion 24 for being connected to an anode tab 5 (not shown in figure 2). Similarly, the cathode current collector 27 is coated on both sides with a cathode active material layer 29, except on a cathode current collector protrusion 28 for being connected to the cathode tab 3 (not shown in figure 2). The anode active material layer 25 has a layer thickness of 47,5 µm, and the cathode active material layer 29 has a layer thickness of 38 µm.

Figure 3 is a collapsed view of the bi-layer electrode stack 31 shown in figure 2 with the anode 17, cathode 19, and separator 21. As in figure 2, the anode 17 comprises and anode current collector 23 coated on both sides with an anode active material layer 25, and the cathode 19 comprises a cathode current collector 27 coated on both sides with a cathode active material layer 29. If soaked in electrolyte (except for the anode and cathode current collector protrusions 24, 28), the bi-layer electrode stack 31 would fully function as an energy storage device 1 by electrically connecting the anode and cathode current collector protrusions 24, 28 to each other through an external electrical system (not shown). Upon charging, positively charged lithium ions move from the cathode active material layer 29 through the separator 21 to the anode active material 25 via the electrolyte, while electrons simultaneously move from the cathode active material layer 29 to the anode active material layer 25 via the cathode current collector 27, the external system, and the anode current collector 23. When discharging, the reaction will be reversed.

Figure 4 shows a multi-layer cell stack 33 comprising four of the bi-layer electrode stacks 31 shown in figure 3. The different bilayer electrode stacks 31 are separated from each other by separators 21. The anode current collector protrusions 24 on one side will be connected to each other and to an anode tab 5 (not shown in figure 4), while the cathode current collector protrusions 28 will be connected to each other and to a cathode tab 3 (not shown in figure 4). The multi-layer cell stack 33 with welded anode tab 5 and cathode tab 3 may be enclosed with a pouch to obtain an energy storage device 1 as shown in figure 1.

In one example, the device length 11 of the energy storage device 1 is 352.5 mm, the device width 13 is 100.2 mm, and the device thickness 15 is 7.1 mm. The tab width of both the anode tab 5 and the cathode tab 3 is 70 mm. The cathode tab 3 comprises aluminum and has a tab thickness of 0.5 mm, while the anode tab 5 comprises nickel-plated copper and has a tab thickness of 0.3 mm. The multi-layer electrode stack 33 comprises 31 anodes 17 and 30 cathodes 19. Each anode 17 comprises a copper anode current collector 23 with a thickness of 8 µm coated on both sides with an anode active material layer 25 comprising hard carbon, carbon black, CMC, and SBR in a mass ratio of 94.2:2:1.5:2.3, respectively, with a coating weight of 3.17 mg/cm<2 >and a press density of 0.9 g/cm<3>. Each cathode 19 comprises an aluminum cathode current collector 27 with a thickness of 12 µm coated on both sides with a cathode active material layer 29 comprising LFP, active carbon, PVDF, and carbon black in a mass ratio of 87.5:5:2.5:5, respectively, with a coating weight of 7.4 mg/cm<2 >and press density of 1.7 g/cm<3>. The electrolyte comprises 1.1 M LiPF6 lithium salt and 2.5% vinylene carbonate in a solution of ethylene carbonate, ethyl methyl carbonate, and propylene carbonate in a ratio of 38:57:5. A cellulose separator with a thickness of 18 µm and a porosity of 47% was used. The D50 particle sizes were 7 µm for LFP and 5 µm for hard carbon.

In another example, the device is similar to the first example, except that the cathode active material layer comprises LFP, PVDF, and carbon black in a mass ratio of 92.5:2.5:5, respectively.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

C l a i m s

1. Energy storage device (1) for providing a power to energy ratio of at least 25, wherein the energy storage device (1) comprises:

a. an anode (17) comprising an anode current collector (23) covered at least partly by an anode active material layer (25) comprising hard carbon, the anode active material layer (25) having a layer thickness of maximum 100 µm;

b. a cathode (19) comprising a cathode current collector (27) covered at least partly by a cathode active material layer (27) comprising lithium iron phosphate, the cathode active material layer (27) having a layer thickness of maximum 100 µm;

c. a separator (21) for preventing electrical connection between the anode (17) and the cathode (19) while allowing lithium ions to diffuse though the separator (21), and

d. an electrolyte.

2. The energy storage device (1) according to claim 1, wherein each anode active material layer (25) has a layer thickness of maximum 100 µm and each cathode active material layer (27) has a layer thickness of maximum 100 µm

3. The energy storage device (1) according to claim 1 or 2, wherein the cathode active material layer (27) further comprises activated carbon.

4. The energy storage device (1) according to any of the preceding claim, wherein the device (1) has a device length (11), a device width (13), and a device thickness (15), and wherein the aspect ratio between the device length (11) and the device width (13) is between 2-5, and wherein the device (1) comprises an anode tab (5) connected to the anode current collector (23) and a cathode tab (3) connected to the cathode current collector (27), and wherein the anode tab (5) and the cathode tab (3) are positioned opposite each other on each end of the device (1).

5. The energy storage device (1) according to claim 4, wherein the device length (11) is at least 300 mm, and the device width (13) is at least 90 mm.

6. The energy storage device (1) according to claim 4 or 5, wherein a tab width (13) of the anode tab (5) and cathode tab (3) is at least 50% of the device width (13).

7. The energy storage device (1) according to any of the claims 4-6, wherein the device thickness (15) is less than 10 mm.

8. The energy storage device (1) according to any of the preceding claims, wherein the anode (17) is pre-doped with lithium ions.

9. The energy storage device (1) according to any of the preceding claims, wherein the separator (21) has a thickness of less than 20 µm.

10. The energy storage device (1) according to any of the preceding claims, wherein the active material layers (25, 27) comprise active material particles having a D50 particle size of less the 10 µm.