JP7641719B2 - Internal resistance reducing agent for positive electrode active material layer, and electrode material for secondary battery using the same - Google Patents

Description

The present invention relates to an internal resistance reducing agent for a positive electrode active material layer, and an electrode material for a secondary battery using the same.

In recent years, there has been a strong desire to reduce carbon dioxide emissions in order to combat global warming. In the automotive industry, there is hope that the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs) will help reduce carbon dioxide emissions, and there has been active development of non-aqueous electrolyte secondary batteries, such as motor drive secondary batteries, which hold the key to putting these into practical use.

Secondary batteries for driving motors are required to have extremely high output characteristics and high energy compared to consumer lithium secondary batteries used in mobile phones, laptops, etc. Therefore, lithium secondary batteries, which have the highest theoretical energy of all practical batteries, are attracting attention and are currently being developed at a rapid pace.

The lithium secondary batteries currently in widespread use use a flammable organic electrolyte as the electrolyte. These liquid-based lithium secondary batteries require stricter safety measures against leakage, short circuits, overcharging, etc. than other batteries.

In recent years, research and development of all-solid-state lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively conducted. A solid electrolyte is a material mainly composed of an ion conductor capable of ion conduction in a solid. Therefore, in principle, all-solid-state lithium secondary batteries do not cause various problems caused by flammable organic electrolytes as in conventional liquid-based lithium secondary batteries. In addition, in general, the use of a high-potential, large-capacity positive electrode material and a large-capacity negative electrode material can significantly improve the output density and energy density of a battery. For example, elemental sulfur (S ₈ ), which is a candidate for a positive electrode active material, has an extremely large theoretical capacity of about 1670 mAh/g, and has the advantages of being low cost and abundant in resources.

Technologies have been proposed that aim to fully utilize the large capacity of such a positive electrode active material made of elemental sulfur (S ₈ ) and improve the stability of secondary batteries. For example, Patent Document 1 attempts to achieve the above objective by incorporating metals such as Fe, Ni, Cu, and Al into a sulfur-carbon composite obtained by compounding sulfur and a carbon material through a heat treatment or mechanical treatment.

JP 2017-188301 A

The inventors' investigations revealed that when a secondary battery is produced using the technology described in Patent Document 1 (addition of an elemental metal to the positive electrode active material layer), there is a problem in that sufficient capacity cannot be extracted during charging and discharging at a high charge and discharge rate (i.e., the so-called charge and discharge rate characteristics are insufficient). A secondary battery with such insufficient charge and discharge rate characteristics cannot utilize sufficient capacity in response to rapid charging and discharging.

The present invention aims to provide a means for improving the charge/discharge rate characteristics of secondary batteries that use a positive electrode active material containing sulfur.

The present inventors have conducted extensive research to solve the above problems. As a result, they have found that the above problems can be solved by adding an ionic compound, which has a parameter defined by a predetermined formula within a predetermined range and which makes the band gap energy of sulfur less than a predetermined value when it comes into contact with Li ₂ S ₈ , to a positive electrode active material layer together with a positive electrode active material containing sulfur, and have completed the present invention.

One embodiment of the present invention is an internal resistance reducer for a positive electrode active material layer that is added to a positive electrode active material layer of a secondary battery positive electrode containing a sulfur-containing positive electrode active material to reduce the internal resistance of the positive electrode active material layer, the agent containing an ionic compound consisting of one or more cations and one or more anions as an active ingredient, the parameter X of the compound, which is represented by the following mathematical formula ₁ , satisfies X< _0.1 , and the band gap energy of sulfur in the _Li2S8 when the _Li2S8 comes into contact with the compound is less than 1.4 [eV]:

In the formula, ρ represents the density of the compound [g/cm ³ ], E _fermi represents the Fermi energy of the compound [eV], γ represents the crystal angle of the compound [°], M _total represents the total magnetization of the compound [A/m], L _b represents the length of the crystal lattice b of the compound [Å], and V represents the crystal lattice volume of the compound [Å ³ ].

According to the present invention, it is possible to improve the charge/discharge rate characteristics of a secondary battery using a positive electrode active material containing sulfur.

FIG. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium ion secondary battery which is one embodiment of the lithium ion secondary battery according to the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 shown in FIG. FIG. 3 is an energy diagram of the rate-limiting reactions in both the charge and discharge processes of a lithium ion secondary battery using a positive electrode active material containing sulfur. FIG. 4 is a table showing the results of calculating correlation coefficients between 26 types of physical property parameters of an ionic compound and the band gap energy of sulfur.

The above-mentioned embodiment of the present invention will be described below with reference to the drawings, but the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. The dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios. The present invention will be described below using a stacked type (internal parallel connection type) all-solid-state lithium secondary battery, which is one form of secondary battery. As described above, the solid electrolyte constituting the all-solid-state lithium secondary battery is a material mainly composed of an ion conductor capable of ion conduction in a solid. For this reason, the all-solid-state lithium secondary battery has the advantage that, in principle, various problems caused by flammable organic electrolytes do not occur as in conventional liquid-based lithium secondary batteries. In addition, in general, the use of a high-potential, large-capacity positive electrode material and a large-capacity negative electrode material has the advantage that the output density and energy density of the battery can be significantly improved.

One embodiment of the present invention is an internal resistance reducer for a positive electrode active material layer, which is added to a positive electrode active material layer of a positive electrode for a secondary battery containing a sulfur-containing positive electrode active material to reduce the internal resistance of the positive electrode active material layer, the agent containing an ionic compound consisting of one or more types of cations and one or more types of anions as an active ingredient, the parameter X represented by the above mathematical formula 1 for the compound satisfying X<0.1, and the band gap energy of sulfur in the Li ₂ S ₈ when the Li ₂ S ₈ comes into contact with the compound being less than 1.4 [eV].

The internal resistance reducing agent for the positive electrode active material layer according to the present embodiment (containing a specific ionic compound as an active ingredient) is added to the positive electrode active material layer of a secondary battery positive electrode containing a sulfur-containing positive electrode active material, so that the valence band of the additive (internal resistance reducing agent) overlaps with the sulfur conductor of Li ₂ S ₈ , and more electrons are supplied to the band gap of sulfur, thereby lowering the band gap energy of sulfur contained in the positive electrode active material layer. As a result, the internal resistance of the positive electrode active material layer can be reduced. As a result, it is possible to improve the charge and discharge rate characteristics in a secondary battery using a sulfur-containing positive electrode active material. The estimated mechanism by which the specific ionic compound, which is the active ingredient of the internal resistance reducing agent according to the present embodiment, reduces the internal resistance of the positive electrode active material layer of a secondary battery using a sulfur-containing positive electrode active material will be described later.

Figure 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium-ion secondary battery, which is one embodiment of the lithium-ion secondary battery according to the present invention. Figure 2 is a cross-sectional view taken along line 2-2 shown in Figure 1. The layered structure allows the battery to be compact and have a high capacity. In this specification, the flat-layered non-bipolar lithium-ion secondary battery shown in Figures 1 and 2 (hereinafter also simply referred to as a "layered battery") will be described in detail as an example. However, when viewed from the perspective of the internal electrical connection form (electrode structure) of the lithium-ion secondary battery according to this embodiment, it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

As shown in FIG. 1, the stacked battery 10a has a flat rectangular shape, with a negative electrode current collector 25 and a positive electrode current collector 27 extending from both sides for extracting power. The power generating element 21 is wrapped in the battery exterior material (laminate film 29) of the stacked battery 10a, and its periphery is heat-sealed, with the power generating element 21 sealed with the negative electrode current collector 25 and the positive electrode current collector 27 extended to the outside.

The lithium ion secondary battery according to this embodiment is not limited to a laminated flat shape. A wound lithium ion secondary battery may be cylindrical, or may be a cylindrical shape that has been deformed to have a rectangular flat shape, and is not particularly limited. The cylindrical shape may be a laminate film or a conventional cylindrical can (metal can) as the exterior material, and is not particularly limited. Preferably, the power generating element is housed inside a laminate film containing aluminum. This form can achieve weight reduction.

There are also no particular limitations on the way the current collectors (25, 27) shown in FIG. 1 are removed. The negative and positive current collectors 25 and 27 may be pulled out from the same side, or the negative and positive current collectors 25 and 27 may each be divided into multiple pieces and removed from each side; they are not limited to what is shown in FIG. 1. In addition, in a wound lithium-ion battery, the terminals may be formed using, for example, a cylindrical can (metal can) instead of tabs.

As shown in Figure 2, the stacked battery 10a of this embodiment has a structure in which a flat, approximately rectangular power generating element 21, where the charge and discharge reactions actually proceed, is sealed inside a laminate film 29, which is the battery exterior material. Here, the power generating element 21 has a structure in which a positive electrode, a solid electrolyte layer 17, and a negative electrode are stacked. The positive electrode has a structure in which a positive electrode active material layer 15 containing a positive electrode active material is disposed on both sides of a positive electrode collector 11". The negative electrode has a structure in which a negative electrode active material layer 13 containing a negative electrode active material is disposed on both sides of a negative electrode collector 11'. Specifically, the positive electrode, solid electrolyte layer, and negative electrode are stacked in this order so that one positive electrode active material layer 15 and the adjacent negative electrode active material layer 13 face each other through a solid electrolyte layer 17. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode form one single cell layer 19. Therefore, the stacked battery 10a shown in FIG. 1 can be said to have a structure in which a plurality of single cell layers 19 are stacked and electrically connected in parallel.

As shown in FIG. 2, the outermost positive electrode collectors located on both outermost layers of the power generating element 21 each have a positive electrode active material layer 15 disposed on only one side, but active material layers may be provided on both sides. In other words, instead of using a collector dedicated to the outermost layer with an active material layer provided on only one side, a collector with active material layers on both sides may be used as the outermost layer collector as is. In some cases, the negative electrode active material layer 13 and the positive electrode active material layer 15 may be used as the negative electrode and positive electrode, respectively, without using the collectors (11', 11").

The negative electrode collector 11' and the positive electrode collector 11" are respectively attached with a negative electrode collector plate (tab) 25 and a positive electrode collector plate (tab) 27 that are electrically connected to each electrode (positive electrode and negative electrode), and are structured so as to be sandwiched between the ends of the laminate film 29, which is the battery exterior material, and are led out of the laminate film 29. The positive electrode collector plate 27 and the negative electrode collector plate 25 may be attached to the positive electrode collector 11" and the negative electrode collector 11' of each electrode by ultrasonic welding, resistance welding, or the like, respectively, via a positive electrode lead and a negative electrode lead (not shown) as necessary.

The main components of the lithium-ion secondary battery according to this embodiment are described below.

[Current collector]
The current collector has a function of mediating the movement of electrons from the electrode active material layer. There are no particular limitations on the material constituting the current collector. For example, metals and conductive resins can be used as the material constituting the current collector.

Specific examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition, a clad material of nickel and aluminum, or a clad material of copper and aluminum may be used. A foil in which a metal surface is coated with aluminum may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

The latter conductive resin includes a resin in which a conductive filler is added as necessary to a non-conductive polymeric material.

Examples of non-conductive polymeric materials include polyethylene (PE; high density polyethylene (HDPE), low density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), or polystyrene (PS). Such non-conductive polymeric materials can have excellent electric potential resistance or solvent resistance.

A conductive filler can be added to the conductive polymer material or non-conductive polymer material as necessary. In particular, when the resin that serves as the base material of the collector is made of only a non-conductive polymer, a conductive filler is necessarily required to impart conductivity to the resin.

The conductive filler can be used without any particular restriction as long as it is a material having electrical conductivity. For example, metals and conductive carbon can be mentioned as materials having excellent electrical conductivity, potential resistance, or lithium ion blocking properties. There are no particular restrictions on the metal, but it is preferable that it contains at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb, or an alloy or metal oxide containing these metals. There are also no particular restrictions on the conductive carbon. It is preferable that it contains at least one selected from the group consisting of acetylene black, Vulcan (registered trademark), Black Pearl (registered trademark), carbon nanofiber, Ketjen Black (registered trademark), carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene.

There are no particular restrictions on the amount of conductive filler added, so long as it is an amount that can impart sufficient conductivity to the current collector, and it is generally 5 to 80% by mass relative to the total mass of the current collector (100% by mass).

The current collector may be a single layer structure made of a single material, or may be a laminate structure made of a suitable combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector contains at least a conductive resin layer made of a resin having conductivity. In addition, from the viewpoint of blocking the movement of lithium ions between the single cell layers, a metal layer may be provided on a part of the current collector. Furthermore, if the negative electrode active material layer and the positive electrode active material layer described later are conductive and can perform a current collecting function by themselves, it is not necessary to use a current collector as a member separate from these electrode active material layers. In such a form, the negative electrode active material layer described later constitutes the negative electrode as it is, and the positive electrode active material layer described later constitutes the positive electrode as it is.

[Negative electrode (negative electrode active material layer)]
In the secondary battery according to this embodiment, the negative electrode active material layer 13 includes a negative electrode active material. The type of the negative electrode active material is not particularly limited, but includes carbon materials, metal oxides, and metal active materials. Examples of the carbon material include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, and soft carbon. Examples of the metal oxide include Nb ₂ O ₅ , Li ₄ Ti ₅ O ₁₂ , and the like. Furthermore, silicon-based negative electrode active materials and tin-based negative electrode active materials may be used. Here, silicon and tin belong to the 14th group of elements, and are known to be negative electrode active materials that can greatly improve the capacity of non-aqueous electrolyte secondary batteries. These simple substances can absorb and release a large number of charge carriers (lithium ions, etc.) per unit volume (mass), and therefore become high-capacity negative electrode active materials. Here, it is preferable to use Si simple substance as the silicon-based negative electrode active material. Similarly, it is also preferable to use silicon oxide such as SiO _x (0.3≦x≦1.6) disproportionated into two phases of Si phase and silicon oxide phase. In this case, the range of x is more preferably 0.5≦x≦1.5, and even more preferably 0.7≦x≦1.2. Furthermore, an alloy containing silicon (silicon-containing alloy-based negative electrode active material) may be used. On the other hand, negative electrode active materials containing tin element (tin-based negative electrode active materials) include simple Sn, tin alloys (Cu—Sn alloys, Co—Sn alloys), amorphous tin oxides, tin silicon oxides, and the like. Among these, an example of amorphous tin oxide is SnB _0.4 P _0.6 O _3.1 . An example of tin silicon oxide is SnSiO _3. Furthermore, a metal containing lithium may be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is an active material containing lithium, and examples thereof include metallic lithium as well as lithium-containing alloys. The lithium-containing alloy may be, for example, an alloy of Li and at least one of In, Al, Si and Sn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than the above may be used. The negative electrode active material preferably contains metallic lithium, a silicon-based negative electrode active material or a tin-based negative electrode active material, and particularly preferably contains metallic lithium.

The shape of the negative electrode active material may be, for example, particulate (spherical, fibrous), thin film, etc. When the negative electrode active material is particulate, the average particle size (D ₅₀ ) is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. In this specification, the average particle size (D ₅₀ ) of the active material can be measured by a laser diffraction scattering method.

The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably within the range of 40 to 99% by mass, and more preferably within the range of 50 to 90% by mass.

It is preferable that the negative electrode active material layer further contains a solid electrolyte. By containing the solid electrolyte in the negative electrode active material layer, the ionic conductivity of the negative electrode active material layer can be improved. Examples of the solid electrolyte include sulfide solid electrolytes and oxide solid electrolytes, and a sulfide solid electrolyte is preferable.

Examples of the sulfide solid electrolyte include LiI-Li ₂ S-SiS ₂ , LiI-Li ₂ SP ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ , Li ₂ S-P ₂ S ₅ , LiI-Li ₃ PS ₄ , LiI-LiBr-Li ₃ PS _4, Li ₃ PS _4, Li ₂ S-P ₂ S ₅ , Li ₂ S-P ₂ S ₅ -LiI, Li ₂ S-P ₂ S ₅ -Li ₂ O, Li ₂ S-P ₂ S ₅ -Li ₂ O-LiI, Li ₂ S-SiS ₂ , Li ₂ S-SiS ₂ -LiI, Li ₂ S-SiS ₂ -LiBr, Li ₂ S-SiS ₂ -LiCl, Li ₂ S-SiS ₂ -B ₂ S ₃ -LiI, Li ₂ S-SiS ₂ -P ₂ S ₅ -LiI, Li ₂ S-B ₂ S ₃ , Li ₂ S-P ₂ S ₅ -Z _m S _n (where m and n are positive numbers, and Z is Ge, Zn, or Ga), Li ₂ S-GeS ₂ , Li ₂ S-SiS ₂ -Li ₃ PO ₄ , Li ₂ S-SiS ₂ -Li _x MO _y (wherein x and y are positive numbers, and M is any one of P, Si, Ge, B, Al, Ga, and In). Note that the description "Li ₂ S-P ₂ S ₅ " means a sulfide solid electrolyte obtained by using a raw material composition containing Li ₂ S and P ₂ S ₅ , and the same applies to other descriptions.

The sulfide solid electrolyte may have, for example, a Li ₃ PS ₄ skeleton, a Li ₄ P ₂ S ₇ skeleton, or a Li ₄ P ₂ S ₆ skeleton. Examples of sulfide solid electrolytes having a Li ₃ PS ₄ skeleton include LiI-Li ₃ PS ₄ , LiI-LiBr-Li ₃ PS _{4, and} Li ₃ PS _4. Examples of sulfide solid electrolytes having a Li ₄ P ₂ S ₇ skeleton include Li-P-S solid electrolytes called LPS (for example, Li ₇ P ₃ S ₁₁ ). Examples of sulfide solid electrolytes include LGPS represented by Li _(4-x) Ge _(1-x) P _x S ₄ (x satisfies 0<x<1). In particular, the sulfide solid electrolyte contained in the active material layer is preferably a sulfide solid electrolyte containing P element, and more preferably a material containing Li ₂ S-P ₂ S ₅ as a main component. Furthermore, the sulfide solid electrolyte may contain a halogen (F, Cl, Br, I). In a preferred embodiment, the sulfide solid electrolyte contains Li ₆ PS ₅ X (wherein X is Cl, Br or I, preferably Cl).

Furthermore, when the sulfide solid electrolyte is a Li ₂ S-P ₂ S ₅ system, the ratio of Li ₂ S and P ₂ S ₅ , in terms of molar ratio, is preferably within the range of Li ₂ S:P ₂ S ₅ =50:50 to 100:0, and in particular, Li ₂ S:P ₂ S ₅ =70:30 to 80:20.

The sulfide solid electrolyte may be a sulfide glass, a crystallized sulfide glass, or a crystalline material obtained by a solid phase method. The sulfide glass can be obtained, for example, by performing mechanical milling (ball mill, etc.) on the raw material composition. The crystallized sulfide glass can be obtained, for example, by heat treating the sulfide glass at a temperature equal to or higher than the crystallization temperature. The ion conductivity (for example, Li ion conductivity) of the sulfide solid electrolyte at room temperature (25° C.) is preferably, for example, 1×10 ⁻⁵ S/cm or more, and more preferably 1×10 ⁻⁴ S/cm or more. The value of the ion conductivity of the solid electrolyte can be measured by an AC impedance method.

Examples of oxide solid electrolytes include compounds having a NASICON structure. Examples of compounds having a NASICON structure include compounds (LAGP) represented by the general formula Li _1+x Al _x Ge _2-x (PO ₄ ) ₃ (0≦x≦2), and compounds (LATP) represented by the general formula Li _1+x Al _x Ti _2-x (PO ₄ ) ₃ (0≦x≦2). Other examples of oxide solid electrolytes include LiLaTiO (e.g., Li _0.34 La _0.51 TiO ₃ ), LiPON (e.g., Li _2.9 PO _3.3 N _0.46 ), and LiLaZrO (e.g., Li ₇ La ₃ Zr ₂ O ₁₂ ).

The shape of the solid electrolyte may be, for example, a particulate shape such as a true sphere or an oval sphere, a thin film shape, etc. When the solid electrolyte is particulate, its average particle size ( _D50 ) is not particularly limited, but is preferably 40 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. On the other hand, the average particle size ( _D50 ) is preferably 0.01 μm or more, and more preferably 0.1 μm or more.

The content of the solid electrolyte in the negative electrode active material layer is preferably within the range of 1 to 60 mass %, and more preferably within the range of 10 to 50 mass %.

The negative electrode active material layer may further contain at least one of a conductive additive and a binder in addition to the above-mentioned negative electrode active material and solid electrolyte.

Examples of conductive assistants include, but are not limited to, metals such as aluminum, stainless steel (SUS), silver, gold, copper, and titanium, alloys or metal oxides containing these metals, carbon fibers (specifically, vapor-grown carbon fibers (VGCF), polyacrylonitrile-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, activated carbon fibers, etc.), carbon nanotubes (CNT), and carbon black (specifically, acetylene black, Ketjen Black (registered trademark), furnace black, channel black, thermal lamp black, etc.). In addition, particulate ceramic materials and resin materials coated with the above-mentioned metal materials by plating or the like can also be used as conductive assistants. Among these conductive assistants, from the viewpoint of electrical stability, it is preferable to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, copper, titanium, and carbon, more preferably to include at least one selected from the group consisting of aluminum, stainless steel, silver, gold, and carbon, and even more preferably to include at least one carbon. These conductive assistants may be used alone or in combination of two or more.

The conductive assistant is preferably particulate or fibrous. When the conductive assistant is particulate, the shape of the particles is not particularly limited, and may be any shape, such as powder, sphere, rod, needle, plate, column, irregular shape, scale, spindle shape, etc.

When the conductive assistant is particulate, the average particle diameter (primary particle diameter) is not particularly limited, but is preferably 0.01 to 10 μm from the viewpoint of the electrical properties of the battery. In this specification, the "particle diameter of the conductive assistant" means the maximum distance L between any two points on the contour line of the conductive assistant. The value of the "average particle diameter of the conductive assistant" is calculated as the average particle diameter of particles observed in several to several tens of fields of view using an observation means such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

When the negative electrode active material layer contains a conductive assistant, the content of the conductive assistant in the negative electrode active material layer is not particularly limited, but is preferably 0 to 10 mass %, more preferably 2 to 8 mass %, and even more preferably 4 to 7 mass %, relative to the total mass of the negative electrode active material layer. Within such a range, it is possible to form a stronger electron conduction path in the negative electrode active material layer, which can effectively contribute to improving the battery characteristics.

On the other hand, binders are not particularly limited, but examples include the following materials:

Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are replaced by other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydrogenated product, styrene-isoprene-styrene block copolymer and its hydrogenated product, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene Fluorine resins such as ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), etc., vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), etc. Examples of the fluororubber include vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene fluororubber (VDF-PFMVE-TFE fluororubber), vinylidene fluoride-chlorotrifluoroethylene fluororubber (VDF-CTFE fluororubber), and other vinylidene fluoride-based fluororubbers, epoxy resins, etc. Among these, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable.

The thickness of the negative electrode active material layer varies depending on the configuration of the intended secondary battery, but is preferably within the range of 0.1 to 1000 μm, for example.

[Solid electrolyte layer]
In the secondary battery according to this embodiment, the solid electrolyte layer is interposed between the above-mentioned positive electrode active material layer and negative electrode active material layer, and is a layer that essentially contains a solid electrolyte.

There are no particular limitations on the specific form of the solid electrolyte contained in the solid electrolyte layer, and the solid electrolytes and their preferred forms exemplified in the section on the negative electrode active material layer can be used in the same way. In some cases, a solid electrolyte other than the above-mentioned solid electrolyte may be used in combination. However, the content of the sulfide solid electrolyte in the total amount of the solid electrolyte (100% by mass) is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

The solid electrolyte layer may further contain a binder in addition to the solid electrolyte described above. The examples and preferred forms described in the section on the negative electrode active material layer may also be used for the binder that may be contained in the solid electrolyte layer.

The thickness of the solid electrolyte layer varies depending on the configuration of the intended lithium ion secondary battery, but from the viewpoint of improving the volumetric energy density of the battery, it is preferably 600 μm or less, more preferably 500 μm or less, and even more preferably 400 μm or less. On the other hand, there is no particular limit on the lower limit of the thickness of the solid electrolyte layer, but it is preferably 10 μm or more, more preferably 50 μm or more, and even more preferably 100 μm or more.

[Positive electrode active material layer]
In the secondary battery according to this embodiment, the positive electrode active material layer includes a positive electrode active material containing sulfur. The type of the positive electrode active material containing sulfur is not particularly limited, but may include, in addition to sulfur element (S), particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and may be any material that can release lithium ions during charging and absorb lithium ions during discharging by utilizing the oxidation-reduction reaction of sulfur. Examples of the organic sulfur compound include disulfide compounds, sulfur-modified polyacrylonitrile, sulfur-modified polyisoprene, rubeanic acid (dithiooxamide), polycarbonate sulfide, and the like, which are typified by compounds described in the pamphlet of International Publication No. 2010/044437. Among them, disulfide compounds, sulfur-modified polyacrylonitrile, and rubeanic acid are preferred, and sulfur-modified polyacrylonitrile is particularly preferred. As the disulfide compound, those having a dithiobiurea derivative, a thiourea group, a thioisocyanate, or a thioamide group are more preferred. Here, the sulfur-modified polyacrylonitrile is a modified polyacrylonitrile containing sulfur atoms, obtained by mixing sulfur powder and polyacrylonitrile and heating under an inert gas or under reduced pressure. Its estimated structure is a structure in which polyacrylonitrile is ring-closed to form a polycyclic ring, and at least a part of S is bonded to C, as shown in, for example, Chem. Mater. 2011, 23, 5024-5028. The compound described in this document has strong peak signals near 1330 cm ^-1 and 1560 cm ^-1 in the Raman spectrum, and further has peaks near 307 cm ^-1 , 379 cm ^-1 , 472 cm ^-1 , and 929 cm ^-1 . On the other hand, inorganic sulfur compounds are preferred because of their excellent stability, specifically, sulfur element (S), TiS ₂ , TiS ₃ , TiS ₄ , NiS, NiS ₂ , CuS, FeS ₂ , Li ₂ S, MoS ₂ , MoS ₃ and the like. Among them, S, S-carbon composite, TiS ₂ , TiS ₃ , TiS ₄ , FeS ₂ and MoS ₂ are preferred, sulfur element (S), TiS ₂ and FeS ₂ are more preferred, and sulfur element (S) is particularly preferred from the viewpoint of high capacity. In addition, as the sulfur element (S), α sulfur, β sulfur, or γ sulfur having an S ₈ structure can be used. These sulfur elements absorb lithium ions during discharge and exist in the positive electrode active material layer in the form of lithium (poly)sulfide.

Here, when elemental sulfur (S) is used as the positive electrode active material, it is preferable that the elemental sulfur (S) as the positive electrode active material is contained in the positive electrode active material layer in the form of a composite material obtained by compounding with a conductive material. This composite material is in a state in which a mixture of elemental sulfur and a conductive material is compounded by subjecting it to a heat treatment or mechanical mixing. More specifically, it is in a state in which sulfur is distributed on the surface or in the pores of the conductive material, in a state in which sulfur and the conductive material are uniformly dispersed at the nano level and aggregated to form particles, in a state in which the conductive material is distributed on the surface or inside of fine sulfur particles, or in a state in which a combination of two or more of these states is present.

Carbon materials are preferably used as the conductive material constituting the composite material. Examples of carbon materials include, but are not limited to, activated carbon (e.g., MSC-30 manufactured by Kansai Thermochemical Co., Ltd.), carbon black, acetylene black, Ketjen Black (registered trademark), carbon fiber, graphene, etc. These materials can also be used in combination. For example, when activated carbon is used in combination with Ketjen Black (registered trademark), the plateau region during charging and discharging is expanded, and there is an advantage in that the retention rate of the charge and discharge capacity is increased even after repeated charge and discharge cycles. In particular, it is preferable for the carbon material to have pores, and activated carbon is more preferable. In addition, when the conductive material has pores, it is particularly preferable that the positive electrode active material and the internal resistance reducing agent are arranged inside the pores.

The positive electrode active material layer may further contain a positive electrode active material that does not contain sulfur in addition to the positive electrode active material that contains sulfur. Examples of the positive electrode active material that does not contain sulfur include layered rock salt type active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , and Li(Ni-Mn-Co)O ₂ , spinel type active materials such as LiMn ₂ O ₄ and LiNi _0.5 Mn _1.5 O ₄ , olivine type active materials such as LiFePO ₄ and LiMnPO ₄ , and Si-containing active materials such as Li ₂ FeSiO ₄ and Li ₂ MnSiO _4. Examples of oxide active materials other than the above include Li ₄ Ti ₅ O ₁₂ .

In some cases, two or more types of positive electrode active materials may be used in combination. Of course, positive electrode active materials other than those mentioned above may also be used. However, the content of the positive electrode active material containing sulfur in the total amount of the positive electrode active material (100% by mass) is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

The shape of the positive electrode active material may be, for example, particulate (spherical, fibrous), thin film, etc. When the positive electrode active material is particulate, the average particle size (D ₅₀ ) is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, even more preferably in the range of 100 nm to 20 μm, and particularly preferably in the range of 1 to 20 μm. In this specification, the average particle size (D ₅₀ ) of the active material can be measured by a laser diffraction scattering method.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably within the range of 35 to 99 mass %, and more preferably within the range of 40 to 90 mass %. When the positive electrode active material is in the form of the composite material, the content is calculated based on the mass of only the positive electrode active material (sulfur element) excluding the conductive material.

The positive electrode active material layer may further include a conductive additive and/or a binder, and specific and preferred forms thereof may be similarly adopted to those described in the section on the negative electrode active material layer above. Similarly, the positive electrode active material layer preferably further includes a solid electrolyte, and particularly preferably further includes a sulfide solid electrolyte. Specific and preferred forms of the solid electrolyte, such as the sulfide solid electrolyte, may be similarly adopted to those described in the section on the negative electrode active material layer above.

Therefore, according to a preferred embodiment of the present invention, a positive electrode material for a secondary battery is provided, which includes a positive electrode active material containing sulfur, a conductive material, a sulfide solid electrolyte, and the internal resistance reducing agent according to this embodiment. In addition, according to another preferred embodiment of the present invention, a positive electrode material for a secondary battery is also provided, which includes a positive electrode active material containing sulfur, a conductive material having pores, a sulfide solid electrolyte, and the internal resistance reducing agent according to this embodiment, and the positive electrode active material and the internal resistance reducing agent are disposed inside the pores. According to the positive electrode material for a secondary battery according to these embodiments, it is possible to make a large amount of the internal resistance reducing agent present in the reaction field where the electrode reaction of sulfur progresses, and also to sufficiently secure an electron conduction path to sulfur, which has the advantage that the effect of the present invention can be further expressed.

In the internal resistance reducing agent for the positive electrode active material layer according to this embodiment, the active ingredient is an ionic compound consisting of one or more cations and one or more anions. The ionic compound that is the active ingredient of the internal resistance reducing agent is also characterized in that the parameter X for the compound, which is expressed by the following mathematical formula 1, satisfies X<0.1.

In Equation 1, ρ represents the density of the compound [g/cm ³ ], E _fermi represents the Fermi energy of the compound [eV], γ represents the crystal angle of the compound [°], M _total represents the total magnetization of the compound [A/m], L _b represents the length of the crystal lattice b of the compound [Å], and V represents the crystal lattice volume of the compound [Å ³ ]. Hereinafter, the process of deriving the relational equation (X<0.1) using the parameter X will be described.

The inventors analyzed the elementary reactions that make up the charge/discharge reaction in a lithium-ion secondary battery that uses a sulfur-containing cathode active material using density functional theory (DFT), which is an electronic state calculation method based on density functional theory (DFT), in order to identify the rate-determining step in the charge/discharge reaction. Here, density functional theory (DFT) (density functional theory) is a type of first-principles calculation that predicts physical properties and chemical reactions from the Schrödinger equation (Dirac equation) without using experimental data or empirical parameters, and is a method of calculating the formation energy Eg and the energy level of the molecular orbital (the orbit in which the electrons that make up the molecule can exist) for the molecular structure that is the subject of the calculation.

Here, since elemental sulfur (S ₈ ) does not contain lithium, when viewed as a positive electrode active material, it can be said that a lithium ion secondary battery containing this positive electrode active material is in the most charged state. On the other hand, as the discharge of this lithium secondary battery progresses, elemental sulfur (S ₈ ), which is the positive electrode active material, gradually combines with lithium to go through various reduction products and finally become Li ₂ S. In other words, it can be said that a lithium ion secondary battery containing only Li ₂ S as a positive electrode active material is in the most discharged state.

Here, the following can be assumed as reduction products (intermediates) having different molar ratios of sulfur atoms and lithium atoms between elemental sulfur (S ₈ ) and Li ₂ S: (S ₈ ) → Li ₂ S ₈ → Li ₂ S ₇ → Li ₂ S ₆ → Li ₂ S ₅ → Li ₂ S ₄ → Li ₂ S ₃ → Li ₂ S ₂ → (Li ₂ S). Therefore, the inventors performed structural optimization (calculation to change the positions of atoms constituting the molecule and obtain the structure with the lowest formation energy) based on DFT for elemental sulfur ( _{S 8} ) and Li ₂ S and each of these assumed reduction products (intermediates), and calculated the formation energy in a stable atomic configuration. For all possible combinations of reactants and products, the Gibbs free energy (ΔG) for each reaction was calculated by subtracting the energy of formation of the reactants (E _reactants ) from the energy of formation of the products (E _products ). The detailed calculation conditions are as follows:
Simulation program: Vienna ab-initio simulation package (VASP)
Correlation functional: B3LYP
Cutoff energy: 400 [eV]
Pseudopotential: PAW GGA
Electronic relaxation algorithm: preconditioned residuum-minimization
Ion relaxation algorithm: RMM-DIIS.

Here, in the charging reaction, ΔG in each reaction is a positive value, and the product is energetically more unstable than the reactant. Therefore, in the charging reaction, it was determined that the reaction with the smallest ΔG among the reactions assumed when the reactant is fixed is actually proceeding. The results of actual calculations are shown in Table 1 below (values rounded off to the third decimal place) assuming that the charging reaction of a lithium secondary battery containing Li ₂ S as a positive electrode active material, which is the most discharged state, proceeds.

As shown in Table 1, for Li ₂ S, for example, a reaction (2Li ₂ S → Li ₂ S ₂ + 2Li) is assumed in which lithium is released to generate Li ₂ S _2. The formation energies of the reactants (E _reactants ) and the formation energies of the reactants (E _products ) in this case are 14.41 [eV] and 15.44 [eV], respectively, and the Gibbs free energy (ΔG = E _products - E _reactants ) calculated as the difference between these is calculated to be 1.02 [eV].

In addition to the above, Li ₂ S may react with Li ₂ S ₂ , Li ₂ S ₃ , Li 2 S ₄ , Li ₂ S ₅ , Li ₂ S ₆ , and Li _{2 S 7} to produce Li ₂ S ₃ , Li ₂ S ₄ , Li ₂ S ₅ , Li ₂ S ₆ , Li ₂ S ₇ , and Li ₂ S ₈ . The ΔG of each of these reactions is shown in Table 1 above, but all of them are larger than the ΔG in the reaction that produces Li ₂ S _2. Therefore, it was concluded that the reaction that produces Li ₂ S ₂ , which has the smallest ΔG, actually proceeds with respect to Li ₂ S. In addition, the analysis was continued in the same manner, and it was concluded that the reaction with the smallest ΔG (shown in bold in Table 1) among the reactions assumed for each reactant actually proceeds. As a result, it was considered that in the charging reaction, Li ₂ S produces Li ₂ S ₂ , and then Li ₂ S ₃ , Li ₂ S ₄ , Li ₂ S ₅ , Li ₂ S ₆ , Li ₂ S ₇ , Li _{2 S 8 and} S ₈ in sequence. Here, the reaction with the largest ΔG among the eight reactions from Li ₂ S to S ₈ is the reaction of producing S ₈ from Li ₂ S in the final stage (Li ₂ S ₈ → S ₈ + 2Li). From this, it was considered that the reaction of producing S ₈ is the reaction with the slowest reaction rate (rate-determining reaction) in the charging process.

On the other hand, for the discharge reaction, the results of actual calculations were shown in Table 2 below, assuming that the discharge reaction of a lithium secondary battery containing S ₈ , which is the most charged state, as the positive electrode active material proceeds. In addition, ΔG in each discharge reaction is a negative value, and the product is more energetically stable than the reactant. For this reason, it was determined that the reaction with the largest absolute value of ΔG is actually proceeding among the reactions assumed when the reactant is fixed in the discharge reaction.

As shown in Table 2, only the reaction ( _{S 8 +} 2Li → Li ₂ S ₈ ) that produces Li ₂ S ₈ is assumed for S 8. Then, Li 2 S 8 produces Li 2 S 4 (Li _{2 S 8 + 2Li → 2Li 2 S 4 ), Li 2} S ₄ produces Li ₂ S ₂ (Li ₂ S ₄ + 2Li → 2Li ₂ S ₂ ), and Li _{2 S 2} produces Li _{2 S} (Li ₂ S ₂ + 2Li → 2Li _{2 S} ) so that the _absolute value _{of ΔG is} maximized (shown _in bold in Table ₂ ). Here, as described above, in the discharge reaction, the product is more energetically stable than the reactant. However, in order for the reaction to actually proceed, it is necessary to exceed the excitation energy of the reactant of each reaction. Therefore, for each of the four reactions from _S8 to _Li2S described above, the excitation energy of the reactants ( _S8 , _Li2S8 , _Li2S4 , _Li2S2 ) was calculated. Specifically, the _difference between the energy level (ELUMO) of the LUMO (lowest unoccupied molecular orbital; the lowest energy level among _the molecular orbitals that do not contain electrons) of each reactant _and the _Fermi level (energy level at which the probability of an electron existing is 50%; _Efermi ) was considered to correspond to the excitation energy, and the excitation energy E° was calculated according to the formula E°= _ELUMO - _Efermi . As a result, the excitation energy E° of each of the four reactions was calculated as the value shown in Table 2 above. Here, among the four reactions from _S8 to _Li2S , the reaction with the largest excitation energy E° is the reaction ( _S8 + _2Li → _Li2S8 ) in the initial stage of producing _Li2S8 from _{S8 .} From this, it was thought that the reaction of _{producing Li2S8} from _S8 was the reaction with the slowest reaction rate (rate-determining reaction) in the discharge process.

From the calculation based on DFT as described above, the rate-limiting reaction in the charging process of a lithium ion secondary battery using a positive electrode active material containing sulfur was identified as the production reaction of S ₈ from Li ₂ S (Li ₂ S ₈ → S ₈ + 2Li). Similarly, the rate-limiting reaction in the discharging process of a lithium ion secondary battery using a positive electrode active material containing sulfur was identified as the production reaction of Li ₂ S ₈ from S ₈ (S ₈ + 2Li → Li ₂ S ₈ ). Here, the energy state diagram of the above two rate-limiting reactions is shown in FIG. 3. As shown in FIG. 3, the production reaction of Li ₂ S ₈ from S ₈ , which is the rate-limiting reaction in the discharging reaction (S ₈ + 2Li → Li ₂ S ₈ ), is an exothermic reaction in which ΔG shows a negative value. However, although this reaction is an exothermic reaction, it does not proceed unless energy exceeding the band gap energy (activation energy) E _b is given. In contrast, the rate-determining reaction in the charging reaction, the production reaction of _S8 from _Li2S ( _Li2S8 → _S8 + 2Li), is an endothermic reaction with a positive ΔG value. This reaction does _not proceed unless energy is provided that exceeds the energy barrier ( _Ea ), which is the sum of the Gibbs free energy (ΔG) and the above-mentioned band gap energy (activation energy) _Eb .

Based on these findings, the present inventors have searched for additives that can reduce _Ea , which is an index of the energy barrier in each of the two rate-determining reactions. Specifically, the present inventors have conducted research on various ionic compounds. In the process, for 26 types of physical parameters possessed by various ionic compounds, physical parameters that have a large positive or negative effect on the band gap energy of sulfur in Li ₂ S ₈ (i.e., the absolute value of the correlation coefficient with the band gap energy of sulfur is large) were examined by statistical analysis. FIG. 4 is a table showing the results of calculating the correlation coefficient between 26 types of physical parameters possessed by ionic compounds and the band gap energy of sulfur. In this case, the values of the 26 types of physical parameters were adopted from the values listed on the Materials Project website (https://materialsproject.org/). Meanwhile, the value of the band gap energy of sulfur in Li ₂ S ₈ when in contact with each ionic compound was calculated using the same simulation program as above. The detailed calculation conditions are as follows:
Simulation program: Vienna ab-initio simulation package (VASP)
Correlation functional: B3LYP
Cutoff energy: 500 [eV]
Pseudopotential: PAW GGA
Electronic relaxation algorithm: preconditioned residuum-minimization
Ion relaxation algorithm: RMM-DIIS.

Various software can be used for statistical analysis, but JUSE-StatWorks (JUSE Institute of Science and Technology, Inc.) was used here. As a result, the following six types of physical property parameters were identified as having large positive and negative correlation coefficients with the band gap energy of sulfur:
(Physical property parameters with large negative correlation coefficients)
Density of ionic compounds (ρ [g/cm ³ ]): correlation coefficient is -0.544
Fermi energy of ionic compounds (E _fermi [eV]): correlation coefficient is -0.511
Crystal angle of ionic compounds (γ [°]): correlation coefficient is -0.325
(Physical property parameters with large positive correlation coefficients)
Magnetization of ionic compounds (M _total [A/m]): correlation coefficient is 0.438
Crystal lattice b length of ionic compound (L _b [Å]): correlation coefficient is 0.436
Crystal lattice volume of ionic compounds (V [Å ³ ]): correlation coefficient is 0.396.

The six physical parameters identified in this way have different unit systems, digits, and positive and negative values. For this reason, the six physical parameters were standardized and then added together to obtain the above formula 1. The values of the physical parameters of the 24 ionic compounds were then substituted into the formula 1 derived in this way to obtain parameter X, which was then used to investigate its relationship with the band gap energy value calculated above. The results are shown in Table 3 below.

As shown in Table 3, among various ionic compounds, if the parameter X is less than 0.1 (X<0.1), the band gap energy of sulfur in Li ₂ S ₈ when the compound comes into contact with Li ₂ S ₈ is small. When graphite (black lead) is added as an additive to the positive electrode active material layer, the band gap energy of sulfur in Li ₂ S ₈ when Li ₂ S ₈ comes into contact with graphite is 1.4 [eV]. Therefore, in this example, a compound having a higher additive effect than graphite (i.e., the band gap energy is less than 1.4 [eV]) is used.

From the above, the range of the parameter X is set to less than 0.1 (X<0.1) as the ionic compound as an additive in which the band gap energy of sulfur is less than 1.4 [eV]. From the viewpoint of excellent effect of reducing the band gap energy of sulfur, the parameter X preferably satisfies X≦0.021, more preferably satisfies ≦-0.343, even more preferably satisfies X≦-0.996, particularly preferably satisfies X≦-1.0, and most preferably satisfies X≦-1.232. There is no particular restriction on the specific ionic compound that satisfies the provision regarding such parameter X, and it can be used without particular restriction as long as the range of the parameter X calculated by the above-mentioned calculation process is within the above range. An example of an ionic compound that can be used as an active ingredient of the internal resistance reducing agent according to this embodiment is preferably one or more selected from the group consisting of GaN, SrN, AgO, AlP, NiP ₂ and TaN, more preferably NiP ₂ and/or TaN, and particularly preferably TaN. When these preferred compounds are used as additives, the band gap energy (E _b ) of sulfur becomes particularly small, thereby reducing the internal resistance of the positive electrode active material layer and significantly improving the charge/discharge rate characteristics of the secondary battery.

As described above, the internal resistance reducing agent for the positive electrode active material layer according to this embodiment can be added to the positive electrode active material layer of a secondary battery positive electrode containing a sulfur-containing positive electrode active material to reduce the internal resistance of the positive electrode active material layer. As a result, the charge/discharge rate characteristics can be improved in a secondary battery using a sulfur-containing positive electrode active material. Here, the estimated mechanism by which the internal resistance reducing agent according to this embodiment reduces the internal resistance in the positive electrode active material layer of a secondary battery using a sulfur-containing positive electrode active material is speculated to be as follows.

That is, as a result of the above-mentioned statistical analysis, the above six types of physical parameters have been identified as physical parameters with large positive and negative correlation coefficients with the band gap energy of sulfur. Considering the influence that these six types of physical parameters may have on the band gap energy of sulfur, first, among the physical parameters with a negative correlation coefficient and a large absolute value, the density (ρ) of the ionic compound means that the larger this value is, the more elements exist per unit volume of the ionic compound, or the more elements with larger atomic weights exist. Therefore, it is considered that when an ionic compound with a large density (ρ) is added, more electrons interact with sulfur, thereby reducing the band gap energy. Regarding the Fermi energy (E _fermi ) of the ionic compound, the larger this value is, the greater the overlap of the valence band of the additive with respect to the band gap of sulfur. Therefore, it is considered that when an ionic compound with a large Fermi energy (E _fermi ) is added, more electrons are supplied to the band gap of sulfur, thereby reducing the band gap energy. The crystal angle (γ) of an ionic compound means the angle between the crystal lattice a-b, and is an index of the crystal distortion of the ionic compound. Here, the less the distortion, the more stable the crystal is and the less likely it is to cause an interaction. Therefore, an ionic compound with a large crystal angle (γ) has a large crystal distortion and is more likely to cause an interaction with sulfur, and as a result, it is considered to contribute to the reduction of the band gap energy. Next, among the physical parameters with a positive correlation coefficient and a large absolute value, the magnetization (M _total ) of an ionic compound means that the larger this value is, the more unpaired electrons there are and the higher the tendency for the magnetic moments to align with each other (ferromagnetic). Therefore, the higher the magnetization (M _total ) of an ionic compound, the more difficult it is to supply electrons to sulfur. For this reason, when an ionic compound with a low magnetization ( _M total ) is added, it is considered that more electrons are supplied to the band gap of sulfur, thereby reducing the band gap energy. As for the length (L _b ) of the crystal lattice b of an ionic compound, the smaller this value is, the more elements exist per unit volume of the ionic compound. Therefore, it is believed that when an ionic compound with a small crystal lattice b length (L _b ) is added, more electrons interact with sulfur, reducing the band gap energy. Finally, the smaller the crystal lattice volume (V) of the ionic compound, the more elements there are in the unit volume of the ionic compound. Therefore, it is believed that when an ionic compound with a small crystal lattice volume (V) is added, more electrons interact with sulfur, reducing the band gap energy.

In the positive electrode material for secondary batteries according to this embodiment, the content of the internal resistance reducing agent is not particularly limited, but is preferably 2% by mass or less, more preferably 1% by mass or less, relative to 100% by mass of the positive electrode material. By configuring in this manner, the content of the positive electrode active material contained in the positive electrode material is prevented from becoming too small more than necessary, which can effectively contribute to improving the energy density of the battery. In addition, the blending balance of each component in the positive electrode active material layer is appropriate, and the paths of electronic conduction and ion conduction can be sufficiently secured. There is no particular limit on the lower limit of the content of the internal resistance reducing agent, but from the viewpoint of expressing the function as an internal resistance reducing agent, it is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and particularly preferably 0.5% by mass or more. In addition, when the positive electrode material for secondary batteries according to this embodiment contains a conductive material together with a sulfide solid electrolyte, the mass ratio of the internal resistance reducing agent to the conductive material is not particularly limited. However, from the viewpoint of ensuring a sufficient electron conduction path to the sulfur, the ratio of the mass of the conductive material to the mass of the internal resistance reducing agent (conductive material/internal resistance reducing agent) is preferably 8 or more, more preferably 10 or more, even more preferably 13 or more, and particularly preferably 17 or more. There is no particular upper limit to this value, but as an example, it is 200 or less, preferably 150 or less, more preferably 100 or less, and particularly preferably 50 or less.

[Positive and negative current collector plates]
The material constituting the current collector plate (25, 27) is not particularly limited, and a known highly conductive material conventionally used as a current collector plate for a secondary battery can be used. As the material constituting the current collector plate, for example, a metal material such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof is preferable. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable. In addition, the same material may be used for the positive electrode current collector plate 27 and the negative electrode current collector plate 25, or different materials may be used.

[Positive and negative electrode leads]
Although not shown in the figures, the current collectors (11', 11") and the current collector plates (25, 27) may be electrically connected via positive and negative electrode leads. As the constituent materials of the positive and negative electrode leads, materials used in known lithium ion secondary batteries may be similarly adopted. Note that the portion removed from the exterior is preferably covered with a heat-resistant insulating heat-shrink tube or the like so as to prevent contact with peripheral devices or wiring, causing electric leakage and affecting products (e.g., automobile parts, particularly electronic devices, etc.).

[Battery exterior material]
As the battery exterior material, a known metal can case can be used, and a bag-shaped case using a laminate film 29 containing aluminum, which can cover the power generating element as shown in Figures 1 and 2, can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is preferable from the viewpoint of being excellent in high output and cooling performance and being suitable for use in batteries for large equipment for EVs and HEVs. In addition, since the group pressure applied to the power generating element from the outside can be easily adjusted, a laminate film containing aluminum is more preferable as the exterior body.

The stacked battery according to the present embodiment has a configuration in which multiple single cell layers are connected in parallel, and thus has high capacity and excellent cycle durability. Therefore, the stacked battery according to the present embodiment is suitable for use as a power source for driving EVs and HEVs.

Although one embodiment of a lithium-ion secondary battery has been described above, the present invention is not limited to the configuration described in the above embodiment, and can be modified as appropriate based on the claims.

For example, the type of battery to which the lithium ion secondary battery of the present invention can be applied includes a bipolar battery that includes a bipolar electrode having a positive electrode active material layer electrically bonded to one surface of a current collector and a negative electrode active material layer electrically bonded to the opposite surface of the current collector.

The secondary battery according to this embodiment does not have to be an all-solid-state type. That is, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolytic solution). There is no particular limit to the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferable that the amount is such that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and leakage of the liquid electrolyte (electrolytic solution) does not occur.

The liquid electrolyte (electrolyte) that can be used has a form in which a lithium salt is dissolved in an organic solvent. Examples of organic solvents that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL). Among these, from the viewpoint of further improving the rapid charging characteristics and output characteristics, the organic solvent is preferably a chain carbonate, more preferably at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), and more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

Examples of the lithium salt include Li( _FSO2 ) _2N (lithium bis(fluorosulfonyl)imide; _LiFSI ), _Li ( _C2F5SO2 ) _2N , _LiPF6 , _LiBF4 , _LiClO4 , _LiAsF6 , and _LiCF3SO3 . Among _these , the lithium salt is preferably Li( _FSO2 ) _2N (LiFSI) from the viewpoints of battery output and charge/discharge cycle characteristics.

The liquid electrolyte (electrolyte) may further contain additives other than the above-mentioned components. Specific examples of such compounds include, for example, ethylene carbonate, vinylene carbonate, methylvinylene carbonate, dimethylvinylene carbonate, phenylvinylene carbonate, diphenylvinylene carbonate, ethylvinylene carbonate, diethylvinylene carbonate, vinylethylene carbonate, 1,2-divinylethylene carbonate, 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, and 1-ethyl-2-vinylethylene carbonate. Examples of the additives include ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacryloxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used alone or in combination of two or more. The amount of additive used in the electrolyte can be adjusted as appropriate.

[Battery pack]
A battery pack is a device that consists of multiple batteries connected together. More specifically, it is a device that uses at least two batteries and is configured in series or parallel or both. By connecting them in series or parallel, it is possible to freely adjust the capacity and voltage.

A small, removable battery pack can be formed by connecting multiple batteries in series or in parallel. Then, multiple such small, removable battery packs can be further connected in series or in parallel to form a large-capacity, high-output battery pack (battery module, battery pack, etc.) suitable for vehicle drive power sources and auxiliary power sources that require high volumetric energy density and high volumetric output density. How many batteries are connected to form a battery pack, and how many stages of small battery packs are stacked to form a large-capacity battery pack, can be determined based on the battery capacity and output of the vehicle (electric vehicle) in which it is to be installed.

[vehicle]
The battery or a battery pack formed by combining a plurality of batteries can be mounted on a vehicle. In the present invention, a battery with excellent long-term reliability and long life can be configured, so that by mounting such a battery, a plug-in hybrid electric vehicle with a long EV driving distance or an electric vehicle with a long driving distance per charge can be configured. For example, if the battery or a battery pack formed by combining a plurality of batteries is used in a hybrid vehicle, a fuel cell vehicle, or an electric vehicle (all of which include four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, and buses, and light vehicles, etc.), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles), it becomes a vehicle with a long life and high reliability. However, the use is not limited to automobiles, and it can be applied to various power sources of other vehicles, for example, mobile bodies such as trains, and can also be used as a mounted power source for an uninterruptible power supply device, etc.

10a stacked battery,
11' negative electrode current collector,
11" positive electrode current collector,
13 negative electrode active material layer,
15 positive electrode active material layer,
17 solid electrolyte layer,
19 cell layer,
21 power generating element,
25 negative electrode current collector,
27 positive electrode current collector,
29 Laminating film.

Claims (10)

An internal resistance reducer for a positive electrode active material layer, which is added to a positive electrode active material layer of a secondary battery positive electrode containing a positive electrode active material containing elemental sulfur to reduce the internal resistance of the positive electrode active material layer, the agent containing an ionic compound consisting of one or more types of cations and one or more types of anions as an active ingredient, _the parameter X represented by the following mathematical formula 1 for the compound satisfying X< _0.1 , and the band gap energy of sulfur in the _Li2S8 when the _Li2S8 comes into contact with the compound being less than 1.4 [eV] (excluding those in which the ionic compound is TaN) :
In the formula, ρ represents the density [g/cm ³ ] of the compound, E _fermi represents the Fermi energy [eV] of the compound, γ represents the crystal angle [°] of the compound, M _total represents the magnetization [A/m] of the compound, L _b represents the length [Å] of the crystal lattice b of the compound, and V represents the crystal lattice volume [Å ³ ] of the compound. The internal resistance reducing agent according to claim 1, wherein X satisfies X≦0.021. The internal resistance reducing agent according to claim 2, wherein X satisfies X≦-0.343. The internal resistance reducing agent according to claim 3, wherein X satisfies X≦-0.996. The internal resistance reducing agent according to claim 4, wherein X satisfies X≦-1.232. 2. The internal resistance reducing agent according to claim 1, wherein the active ingredient comprises one or more ionic compounds selected from the group consisting of GaN, SrN, AgO, AlP , and _NiP2 . The internal resistance reducing agent according to claim 1 , wherein the active ingredient comprises _NiP2 . A positive electrode material for a secondary battery, comprising: a positive electrode active material containing elemental sulfur; a conductive material; a sulfide solid electrolyte; and the internal resistance reducing agent according to any one of claims 1 to 7 . 9. The positive electrode material for a secondary battery according to claim 8 , wherein the conductive material has pores, and the positive electrode active material and the internal resistance reducing agent are disposed inside the pores. 10. The positive electrode material for a secondary battery according to claim 8 , wherein the content of the internal resistance reducing agent is 2 mass% or less with respect to 100 mass% of the positive electrode material.