JP7788264B2 - Positive electrode material and secondary battery using the same - Google Patents

Description

The present invention relates to a positive electrode material and 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. The automotive industry is pinning its hopes on reducing carbon dioxide emissions through the introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs), and there has been active development of non-aqueous electrolyte secondary batteries, such as motor drive secondary batteries, which hold the key to commercializing these vehicles.

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 rapidly developed.

Currently widely used lithium secondary batteries use a flammable organic electrolyte. These liquid-based lithium secondary batteries require stricter safety measures against leakage, short circuits, overcharging, and other issues than other batteries.

Therefore, in recent years, research and development on all-solid-state lithium secondary batteries using oxide-based or sulfide-based solid electrolytes has been actively conducted. Solid electrolytes are materials primarily composed of ion conductors capable of ion conduction in solids. Therefore, all-solid-state lithium secondary batteries, unlike conventional liquid-based lithium secondary batteries, do not inherently encounter various problems caused by flammable organic electrolytes. In general, the use of high-potential, large-capacity positive electrode materials and large-capacity negative electrode materials can significantly improve the output density and energy density of batteries. For example, elemental sulfur (S ₈ ) has an extremely large theoretical capacity of approximately 1670 mAh/g and has the advantages of being low cost and abundant in resources.

However, due to factors such as the low electronic conductivity of sulfur, the high capacity properties of sulfur-containing positive electrode active materials have not yet been fully utilized.

For the purpose of improving the discharge capacity and rate characteristics of positive electrode active materials such as elemental sulfur, for example, Patent Document 1 discloses a method for producing a thin film sulfur-coated activated carbon, which is coated with sulfur to a film thickness of 0.2 to 1.4 nm, by immersing activated carbon having a BET specific surface area of 1800 m ² /g or more in a sulfur solution and then separating it from the sulfur solution. According to this document, the obtained thin film sulfur-coated activated carbon is prone to diffusion of electrons and lithium ions within the sulfur, and therefore, by using this as a positive electrode mixture, it is possible to provide an all-solid-state lithium-sulfur battery with excellent discharge capacity and rate characteristics.

JP 2015-88232 A

However, the inventors' investigations have revealed that when the thin-film sulfur-coated activated carbon described in Patent Document 1 is applied to secondary batteries, it may not be possible to sufficiently reduce internal resistance.

The present invention therefore aims to provide a means for further reducing the internal resistance of secondary batteries that use a positive electrode active material containing sulfur.

The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that the above-mentioned problems can be solved by filling a cathode material in which the pores of porous carbon particles are filled with a sulfur-containing cathode active material and a phosphorus-containing sulfide solid electrolyte, and by filling the particles with a higher concentration of sulfide solid electrolyte on the surface side than inside, which led to the completion of the present invention.

One aspect of the present invention is a cathode material in which the pores of porous carbon particles are filled with a cathode active material containing sulfur and a sulfide solid electrolyte containing phosphorus, and the cathode material is characterized in that, in an image of a cross section of the porous carbon particle observed using a scanning electron microscope, the concentration of phosphorus atoms is higher in region B, which is obtained by excluding region A from the cross section and has a similarity ratio of 1/2 with the center of the cross section as a fixed point, than in region A.

According to the present invention, the internal resistance of a secondary battery using a positive electrode active material containing sulfur can be further reduced.

FIG. 1 is a perspective view showing the appearance of a flat laminated type all-solid-state lithium secondary battery according to one embodiment of the present invention. FIG. 2 is a cross-sectional view taken along line 2-2 shown in FIG. FIG. 3 is a schematic cross-sectional view of a positive electrode material 100' in Patent Document 1. FIG. 4 is a cross-sectional schematic diagram of a positive electrode material according to one embodiment of the present invention. FIG. 5 is a cross-sectional schematic diagram of a porous carbon particle for explaining region A and region B in a positive electrode material according to one embodiment of the present invention.

The above-mentioned embodiments of the present invention will be described below with reference to the drawings. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following embodiments. Note that the dimensional proportions in the drawings have been exaggerated for the convenience of explanation and may differ from the actual proportions. The present invention will be described below using as an example a stacked-type (internal parallel connection) all-solid-state lithium secondary battery, which is one form of secondary battery.

As mentioned above, the solid electrolyte that makes up all-solid-state lithium secondary batteries is a material composed primarily of ion conductors that are capable of conducting ions in a solid state. Therefore, all-solid-state lithium secondary batteries have the advantage that, in principle, they do not encounter the various problems that arise from flammable organic electrolytes, as occurs in conventional liquid-based lithium secondary batteries. Another advantage is that, in general, the use of high-potential, high-capacity positive electrode materials and high-capacity negative electrode materials can significantly improve the battery's output density and energy density.

One aspect of the present invention is a cathode material in which the pores of porous carbon particles are filled with a cathode active material containing sulfur and a sulfide solid electrolyte containing phosphorus, and the cathode material is characterized in that, in an image of a cross section of the porous carbon particle observed using a scanning electron microscope, the concentration of phosphorus atoms is higher in region B of the cross section excluding region A, which has a similarity ratio of 1/2 and is set at the center of the cross section as a fixed point. The cathode material of this aspect can further reduce the internal resistance of a secondary battery that uses a cathode active material containing sulfur.

Figure 1 is a perspective view showing the appearance of a flat-layered all-solid-state lithium secondary battery according to one embodiment of the present invention. Figure 2 is a cross-sectional view taken along line 2-2 in Figure 1. The layered structure allows the battery to be compact and have a high capacity. This specification will be described in detail using the flat-layered, non-bipolar lithium secondary battery shown in Figures 1 and 2 (hereinafter simply referred to as a "layered battery"). However, when viewed from the perspective of the internal electrical connection configuration (electrode structure) of the lithium 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 Figure 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, sealing the power generating element 21 with the negative electrode current collector 25 and positive electrode current collector 27 extending to the outside.

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

Furthermore, there are no particular limitations on how the current collectors (25, 27) shown in Figure 1 are removed. The negative current collector 25 and the positive current collector 27 may be pulled out from the same side, or the negative current collector 25 and the positive current collector 27 may each be divided into multiple pieces and removed from each side; they are not limited to what is shown in Figure 1. Furthermore, in wound-type lithium batteries, 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 actual charge/discharge reaction takes place, 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 positive electrode active material layers 15 containing a positive electrode material according to one embodiment of the present invention are disposed on both sides of a positive electrode current collector 11". This reduces the internal resistance of the stacked battery 10a. The negative electrode has a structure in which negative electrode active material layers 13 containing a negative electrode active material are disposed on both sides of a negative electrode current 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 faces the adjacent negative electrode active material layer 13 with the solid electrolyte layer 17 interposed therebetween. As a result, the adjacent positive electrode, solid electrolyte layer, and negative electrode constitute a single cell layer 19. Therefore, the stacked battery 10a shown in FIG. 2 can be said to have a structure in which a plurality of cell layers 19 are stacked and electrically connected in parallel.

As shown in FIG. 2, the outermost negative electrode current collectors located on both outermost layers of the power generating element 21 each have a negative electrode active material layer 13 disposed on only one side, but active material layers may be provided on both sides. In other words, instead of using a current collector exclusively for the outermost layer with an active material layer provided on only one side, a current collector with active material layers on both sides may be used as the outermost current collector. 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 current collectors (11', 11").

The negative electrode current collector 11' and the positive electrode current collector 11" are respectively fitted with a negative electrode current collector (tab) 25 and a positive electrode current collector (tab) 27 that are electrically connected to the respective electrodes (positive and negative electrodes), and are structured so as to be sandwiched between the edges of the laminate film 29, which is the battery outer casing, and extend to the outside of the laminate film 29. The positive electrode current collector 27 and the negative electrode current collector 25 may be attached to the positive electrode current collector 11" and the negative electrode current collector 11' of each electrode via a positive electrode lead and a negative electrode lead (not shown) by ultrasonic welding, resistance welding, or the like, as necessary.

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

[Current collector]
The current collector has a function of mediating the transfer 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. Other materials that may be used include clad materials of nickel and aluminum, and clad materials of copper and aluminum. Foils in which the metal surface is coated with aluminum may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the standpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

Another example of a conductive resin is a resin in which a conductive filler is added to a non-conductive polymer material.

The current collector may have a single layer structure made of a single material, or a laminate structure made of an appropriate combination of layers made of these materials. From the perspective of reducing the weight of the current collector, it is preferable for it to contain at least a conductive resin layer made of a conductive resin. Furthermore, from the perspective of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on part of the current collector. Furthermore, if the negative electrode active material layer and positive electrode active material layer described below are themselves conductive and can perform a current collecting function, it is not necessary to use a current collector as a separate component from these electrode active material layers. In such a configuration, the negative electrode active material layer described below directly constitutes the negative electrode, and the positive electrode active material layer described below directly constitutes the positive electrode.

[Negative electrode (negative electrode active material layer)]
In the stacked battery according to the embodiment shown in FIGS. 1 and 2 , the negative electrode active material layer 13 contains a negative electrode active material. The type of negative electrode active material is not particularly limited, but examples include carbon materials, metal oxides, and metal active materials. Furthermore, a lithium-containing metal may also be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is a lithium-containing active material, and examples thereof include metallic lithium and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with 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 those described above may also be used. The negative electrode active material preferably contains metallic lithium or a lithium-containing alloy, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and particularly preferably contains metallic lithium or a lithium-containing alloy. When metallic lithium or a lithium-containing alloy is used as the negative electrode active material, the lithium secondary battery as an electrical device may be a so-called lithium deposition type in which lithium metal as the negative electrode active material is deposited on the negative electrode current collector during charging. Therefore, in this configuration, the thickness of the negative electrode active material layer increases as the charging process progresses and decreases as the discharging process progresses. The negative electrode active material layer does not need to be present during full discharge, but in some cases, a negative electrode active material layer made of a certain amount of lithium metal may be present during full discharge.

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

The negative electrode active material layer preferably further contains a solid electrolyte. By including a solid electrolyte in the negative electrode active material layer, the ionic conductivity of the negative electrode active material layer can be improved. Examples of solid electrolytes include sulfide solid electrolytes and oxide solid electrolytes, but a sulfide solid electrolyte is preferred.

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 -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 (where x and y are positive numbers and M is P, Si, Ge, B, Al, Ga or In), etc. The term "Li ₂ S-P ₂ S ₅ " means a sulfide solid electrolyte obtained using a raw material composition containing Li ₂ S and P ₂ S ₅ , and the same applies to other terms.

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 known as LPS (for example, Li ₇ P ₃ S ₁₁ ). Examples of sulfide solid electrolytes that may be used include LGPS, which is represented by Li _(4-x) Ge _(1-x) P _x S ₄ (where x satisfies 0<x<1). Among these, a sulfide solid electrolyte containing P element is preferred, and a material containing Li ₂ S—P ₂ S ₅ as a main component is more preferred. 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 (where 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 molar ratio of Li ₂ S and P ₂ S ₅ is preferably within the range of Li ₂ S:P ₂ S ₅ = 50:50 to 100:0, and particularly preferably Li ₂ S:P ₂ S ₅ = 70:30 to 80:20.

The sulfide solid electrolyte may be sulfide glass, crystallized sulfide glass, or a crystalline material obtained by a solid-phase method. Sulfide glass can be obtained, for example, by mechanical milling (ball mill, etc.) a raw material composition. Crystallized sulfide glass can be obtained, for example, by heat treating sulfide glass at a temperature equal to or higher than the crystallization temperature. The ionic conductivity (e.g., 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 ionic conductivity value 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 Li1 _{+ xAlxGe2 -x} ( _PO4 ) ₃ (0≦x≦2) and compounds (LATP) represented by the general formula Li1 _{+ xAlxTi2 -x} ( _PO4 ) _{3 (} 0≦ _{x ≦2). Other examples of oxide solid electrolytes include LiLaTiO (e.g., Li0.34La0.51TiO3), LiPON (e.g., Li2.9PO3.3N0.46 ) , and LiLaZrO (} e.g., _{Li7La3Zr2O12 )} .

The solid electrolyte content 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%.

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

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

[Solid electrolyte layer]
In the stacked battery according to the embodiment shown in FIGS. 1 and 2 , the solid electrolyte layer is interposed between the above-described 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 restrictions 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 some cases, solid electrolytes other than those mentioned above may be used in combination.

The solid electrolyte layer may further contain a binder in addition to the specified solid electrolyte described above.

The thickness of the solid electrolyte layer varies depending on the configuration of the desired lithium secondary battery, but from the perspective 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 lower limit on the thickness of the solid electrolyte layer, but it is preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more.

[Cathode active material layer]
1 and 2 , the positive electrode active material layer contains the positive electrode material according to one embodiment of the present invention, which is formed by filling the pores of porous carbon particles with a positive electrode active material containing sulfur and a sulfide solid electrolyte containing phosphorus.

(Positive electrode active material containing sulfur)
The type of sulfur-containing positive electrode active material is not particularly limited, but includes elemental sulfur (S) and lithium sulfide (Li ₂ S), as well as particles or thin films of organic sulfur compounds or inorganic sulfur compounds, and can 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 organic sulfur compounds include disulfide compounds, sulfur-modified polyacrylonitrile, sulfur-modified polyisoprene, rubeanic acid (dithiooxamide), polycarbonate, etc., as typified by the compounds described in International Publication No. 2010/044437. On the other hand, inorganic sulfur compounds are preferred because of their excellent stability, and specific examples include elemental sulfur (S), lithium sulfide (Li ₂ S), S-carbon composite, TiS ₂ , TiS ₃ , TiS ₄ , NiS, NiS ₂ , CuS, FeS ₂ , Li ₂ S, MoS ₂ , MoS ₃ , MnS, MnS ₂ , CoS, CoS _2, etc. Among these, S, lithium sulfide (Li ₂ S), S-carbon composite, TiS ₂ , TiS ₃ , TiS ₄ , FeS ₂ and MoS ₂ are preferred, elemental sulfur (S), lithium sulfide (Li ₂ S), TiS ₂ and FeS ₂ are more preferred, and elemental sulfur (S) and lithium sulfide (Li ₂ S) are particularly preferred from the viewpoint of high capacity. As the elemental sulfur (S), α-sulfur, β-sulfur, or γ-sulfur having an _S8 structure can be used.

The positive electrode material according to this embodiment may further include a sulfur-free positive electrode active material in addition to the sulfur-containing positive electrode active material. Examples of sulfur-free positive electrode active materials include layered rock salt active materials such as LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiVO ₂ , and Li(Ni—Mn—Co)O ₂ , spinel active materials such as LiMn ₂ O ₄ and LiNi _0.5 Mn _1.5 O ₄ , olivine 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 those mentioned 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 listed above may also be used. However, the proportion of the content of the sulfur-containing positive electrode active material in the total amount (100% by mass) of the positive electrode active material 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.

(phosphorus-containing sulfide solid electrolyte)
The positive electrode material according to this embodiment essentially contains a sulfide solid electrolyte containing phosphorus. In this specification, the "sulfide solid electrolyte containing phosphorus" is also simply referred to as a "phosphorus-containing sulfide solid electrolyte." Preferably, the phosphorus-containing sulfide solid electrolyte is a sulfide solid electrolyte containing at least lithium atoms, phosphorus atoms, and sulfur atoms. Examples of sulfide solid electrolytes containing lithium atoms, phosphorus atoms, and sulfur atoms include LiI-Li ₂ S-P 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 -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 -P ₂ S ₅ -LiI, 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—SiS ₂ —Li ₃ PO ₄ , etc. Examples of sulfide solid electrolytes having a Li ₄ P ₂ S ₇ skeleton include Li-P—S-based solid electrolytes known as LPS (e.g., Li ₇ P ₃ S ₁₁ ). LGPS, for example, represented by Li _(4-x) Ge _(1-x) P _x S ₄ (x satisfies 0<x<1), may also be used. Of these, it is more preferable that the phosphorus-containing sulfide solid electrolyte contained in the active material layer is a material containing Li ₂ S—P ₂ S ₅ as a main component. Furthermore, the phosphorus-containing sulfide solid electrolyte may contain a halogen (F, Cl, Br, I). In a preferred embodiment, the phosphorus-containing sulfide solid electrolyte contains Li ₆ PS ₅ X (wherein X is Cl, Br, or I, and preferably Cl). These sulfide solid electrolytes have high ionic conductivity and can therefore effectively contribute to reducing internal resistance.

(Porous carbon particles)
The positive electrode material according to this embodiment essentially contains porous carbon particles. In this specification, porous carbon particles refer to a particulate carbon material having pores and consisting primarily of carbon. Here, "consisting primarily of carbon" refers to containing carbon atoms as the main component, and is a concept that encompasses both "consisting solely of carbon atoms" and "consisting essentially of carbon atoms.""Consisting essentially of carbon atoms" means that impurities of about 2 to 3 mass % or less can be tolerated.

Examples of porous carbon particles include activated carbon, Ketjen Black® (highly conductive carbon black), (oil) furnace black, channel black, acetylene black, thermal black, lamp black, and other carbon blacks; carbon particles (carbon supports) made from coke, natural graphite, artificial graphite, and the like. Alternatively, a ceramic or other mold may be mixed with a carbon raw material such as resin, fired in an inert atmosphere, and then the mold dissolved in acid to synthesize a carbon material with a porous structure in which the shape of the mold has been transferred. This carbon material can be used as porous carbon particles. In this case, the pore size and pore volume of the resulting carbon material can be altered by appropriately adjusting the particle size of the mold and the blending ratio of the carbon raw materials.

The BET specific surface area of the porous carbon particles is preferably 200 m ² /g or more, more preferably 500 m ² /g or more, even more preferably 800 m ² /g or more, particularly preferably 1200 m ² /g or more, and most preferably 1500 m ² /g or more. The pore volume of the porous carbon particles is preferably 1.0 mL/g or more, more preferably 1.3 mL/g or more, and even more preferably 1.5 mL/g or more. If the BET specific surface area and pore volume of the porous carbon particles are within these ranges, a sufficient amount of pores can be retained, and ultimately a sufficient amount of positive electrode active material can be retained. The BET specific surface area and pore volume of the porous carbon particles can be measured by nitrogen adsorption/desorption measurement. This nitrogen adsorption/desorption measurement is performed using a BELSORP mini manufactured by Microtrac-Bell Corporation, at a temperature of -196°C using a multipoint method. The BET specific surface area is determined from the adsorption isotherm in the relative pressure range of 0.01<P/P ₀ <0.05. The pore volume is determined from the volume of N ₂ adsorbed at a relative pressure of 0.96.

The average pore diameter of the porous carbon particles is not particularly limited, but is preferably 1 to 50 nm, and particularly preferably 1 to 30 nm. If the average pore diameter of the porous carbon particles is within this range, electrons can be sufficiently supplied to the sulfur-containing positive electrode active material disposed inside the pores, even to the active material located far from the pore walls. The average pore diameter of the conductive material can be calculated by nitrogen adsorption/desorption measurements, similar to the calculation of the BET specific surface area and pore volume.

The average particle diameter (primary particle diameter) of porous carbon particles is not particularly limited, but is preferably 0.05 to 50 μm, more preferably 0.1 to 20 μm, and even more preferably 0.5 to 10 μm. In this specification, "particle diameter of porous carbon particles" refers to the longest distance L between any two points on the contour line of a porous carbon particle. The value of "average particle diameter of porous carbon particles" is calculated as the arithmetic mean value of particle diameters of particles observed within several to several tens of fields of view using an observation tool such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

As described above, the positive electrode material according to this embodiment is obtained by filling the pores of porous carbon particles with a sulfur-containing positive electrode active material and a phosphorus-containing sulfide solid electrolyte. Furthermore, this positive electrode material is characterized in that, in an image of a cross section of a porous carbon particle observed using a scanning electron microscope, the concentration of phosphorus atoms is higher in region B, which is obtained by excluding region A from the cross section and has a similarity ratio of 1/2 with the center of the cross section as a fixed point, than in region A.

Figure 3 is a cross-sectional schematic diagram of a cathode material 100' in Patent Document 1, a prior art. Figure 4 is a cross-sectional schematic diagram of a cathode material 100 according to one embodiment of the present invention. In Figures 3 and 4, porous carbon particles 110 have numerous pores 110a, and the pores 110a are filled with a cathode active material 120. The cathode material 100' in Figure 3 is produced by mechanically mixing porous carbon particles 110 filled with the cathode active material 120 and a sulfide solid electrolyte 130. Because the pore diameter of the pores 110a is nanoscale, the sulfide solid electrolyte 130 does not enter the pores 110a due to mechanical mixing, and instead adheres to the surface of the porous carbon particles 110. On the other hand, the cathode material 100 in Figure 4 is produced by impregnating porous carbon particles 110 filled with the cathode active material 120 into a solution containing the sulfide solid electrolyte 130. The solution containing the sulfide solid electrolyte 130 can easily enter the nanoscale pores 110a. Therefore, as shown in FIG. 4, the sulfide solid electrolyte 130 fills the portions of the pores 110a that are not filled with the positive electrode active material 120. Therefore, in the positive electrode material 100 of FIG. 4, a continuous phase consisting of the positive electrode active material 120 fills the interior of the pores 110a and is also present on the surfaces of the porous carbon particles 110, with the sulfide solid electrolyte 130 disposed as a dispersed phase within the continuous phase. As a result, at least a portion of the sulfide solid electrolyte disposed on the surfaces of the porous carbon particles is in contact with at least a portion of the positive electrode active material (sulfur) similarly disposed on the surfaces of the porous carbon particles. This configuration makes it possible to form a particularly large number of reaction sites (three-phase interfaces). Furthermore, in the positive electrode material 100 of FIG. 4, the surface portions of the porous carbon particles 110 are filled with more sulfide solid electrolyte 130 than the interior portions. Furthermore, a coating layer consisting of the sulfide solid electrolyte 130 is formed on the surfaces of the porous carbon particles 110. These arrangements can improve the ionic conductivity of the positive electrode material 100. Furthermore, in the positive electrode material 100 of FIG. 4, more positive electrode active material 120 is packed into the interior of the porous carbon particles 110 than in the surface portions. This arrangement can improve the electronic conductivity of the positive electrode material 100. Therefore, by applying the positive electrode material 100 having these arrangements to a secondary battery, it is possible to further reduce the internal resistance of the secondary battery.

As described above, the positive electrode material according to this embodiment is characterized in that the surface portion of the porous carbon particles 110 is filled with more sulfide solid electrolyte 130 than the interior thereof. That is, the positive electrode material according to this embodiment is characterized in that, in an image of a cross section of a porous carbon particle observed using a scanning electron microscope, the concentration of phosphorus atoms is higher in region B, which is obtained by excluding region A from the cross section, than in region A, which has a similarity ratio of 1/2 and is set at the center of the cross section. FIG. 5 is a diagram illustrating region A and region B in a positive electrode material according to one embodiment of the present invention. As shown in FIG. 5 , in this specification, region A is defined as a region surrounded by a figure with a similarity ratio of 1/2 and set at the center C of the cross section of the porous carbon particle 110 as a fixed point. That is, the cross-sectional shape of the porous carbon particle and the shape of region A are similar, and the similarity ratio between them is cross section:region A = 1:1/2. The center C of the cross section is the point on the line connecting any point X on the cross-sectional shape of the porous carbon particle and point Y on the shape of region A corresponding to point X, where line segment XY = line segment YC. Depending on the cross-sectional shape of the porous carbon particle, the center C may coincide with the center of gravity of the cross-section of the porous carbon particle. In this case, the center C of the cross-section is the intersection of any two lines that bisect the cross-sectional area of the porous carbon particle. When the cross-sectional shape of the porous carbon particle is d, at least a portion of region A may be located outside the cross-section of the porous carbon particle. In this specification, the cross-section of the porous carbon particle in this case is excluded from the measurement target. In addition, in this specification, the region excluding region A from the cross-section is referred to as region B. As described above, since the ratio of cross-section:region A is 1:1/2, the relationship of cross-sectional area of the porous carbon particle: area of region A: area of region B is 1:1/4:3/4. Whether the concentration of phosphorus atoms is higher in region B than in region A can be determined by performing elemental mapping of phosphorus using energy dispersive X-ray spectroscopy (EDX) in an image of the cross-section of the porous carbon particle observed with a scanning electron microscope (SEM), and using the count number of phosphorus elements as an index. A specific determination method will be described in the Examples below. When the phosphorus atom concentration in region A is P _A and the phosphorus atom concentration in region B is P _B , P _B /P _A must be greater than 1, preferably 1.3 or more and 2 or less, and more preferably 1.5 or more and 2 or less. When the value of _{P B} /P _A is in the above range, the ionic conductivity of the positive electrode material 100 can be improved. As a result, it is possible to further reduce the internal resistance of a secondary battery using the positive electrode material 100. The value of P _B /P _A can be controlled by the concentration of the phosphorus-containing sulfide solid electrolyte solution when impregnating the sulfur-filled porous carbon particles with the phosphorus-containing sulfide solid electrolyte solution in the production of the positive electrode material. By increasing the concentration of the phosphorus-containing sulfide solid electrolyte solution, the value of P _B /P _A increases.

The positive electrode material according to this embodiment preferably has more positive electrode active material 120 packed into the interior of the porous carbon particles 110 than into the surface portion thereof. That is, in a preferred embodiment of the positive electrode material, the concentration of sulfur atoms is higher in region A than in region B. Whether the concentration of sulfur atoms is higher in region A than in region B can be determined by performing elemental mapping of sulfur using energy dispersive X-ray spectroscopy (EDX) in an image of the cross section of the porous carbon particles observed with a scanning electron microscope (SEM), and using the count number of sulfur elements as an indicator. Specific determination methods will be described in the Examples below. When the sulfur atom concentration in region A is S _A and the sulfur atom concentration in region B is S _B , S _A /S _B must exceed 1, preferably 1.3 or more and 2 or less, and more preferably 1.5 or more and 2 or less. When the value of _{S A} /S _B is within the above range, the electronic conductivity of the positive electrode material 100 can be improved. As a result, the internal resistance of a secondary battery using the positive electrode material 100 can be further reduced. The value of S _A /S _B can be controlled by the viscosity of the liquid when porous carbon particles are impregnated with a liquid containing a sulfur-containing cathode active material in the production of a cathode material. If the viscosity of the liquid is low, the liquid will penetrate deep into the pores of the porous carbon particles, and the sulfur atom concentration in region B near the center of the porous carbon particles will be relatively high. When using a molten liquid obtained by heating and melting sulfur as the liquid containing a sulfur-containing cathode active material, for example, the viscosity of the molten liquid can be reduced by increasing the temperature of the molten liquid. When using a lithium sulfide solution obtained by dissolving lithium sulfide in a solvent as the liquid containing a sulfur-containing cathode active material, for example, the viscosity of the solution can be reduced by reducing the concentration of lithium sulfide in the solution.

As shown in FIG. 4, the cathode material of this embodiment preferably has a coating layer containing a sulfide solid electrolyte 130 on at least a portion of the surface of the porous carbon particle 110. The presence of such a coating layer can improve ionic conductivity between adjacent porous carbon particles. As a result, the internal resistance of a secondary battery incorporating the cathode material can be further reduced. In this case, the thickness of the coating layer is preferably 300 nm or less, more preferably 10 nm to 200 nm, even more preferably 10 nm to 100 nm, and preferably 10 nm to 50 nm. A coating layer thickness of 300 nm or less can suppress a decrease in electronic conductivity between adjacent porous carbon particles. A coating layer thickness of 10 nm or more can further improve ionic conductivity between adjacent porous carbon particles. The thickness of the coating layer can be determined by performing elemental mapping of carbon and phosphorus using energy dispersive X-ray spectroscopy (EDX) on a cross-sectional image of the porous carbon particle observed with a transmission electron microscope (TEM) and measuring the distance from the particle boundary to the outermost surface of the coating layer. Specific methods for determining this are described in the Examples section below. The thickness of the coating layer can be controlled by the concentration of the phosphorus-containing sulfide solid electrolyte solution used when impregnating porous carbon particles filled with a sulfur-containing cathode active material in the production of the cathode material. A low concentration of the solution results in a smaller coating layer thickness.

The method for producing the cathode material according to this embodiment having the above-described configuration is not particularly limited, but is preferably a production method including a step of impregnating porous carbon particles with a liquid containing a sulfur-containing cathode active material to obtain a composite material (hereinafter also referred to as step (1)), and a step of impregnating the composite material with a phosphorus-containing sulfide solid electrolyte solution (hereinafter also referred to as step (2)). This makes it possible to fill the porous carbon particles with the phosphorus-containing sulfide solid electrolyte so that its concentration is higher on the surface side than in the interior. An example of such a production method is briefly described below.

In step (1), a composite material is obtained by impregnating porous carbon particles with a liquid containing a sulfur-containing cathode active material. Examples of the liquid containing the sulfur-containing cathode active material include a molten liquid obtained by heating and melting the cathode active material itself, and a cathode active material solution obtained by dissolving the cathode active material in a solvent. For example, when the sulfur-containing cathode active material is elemental sulfur, the liquid containing the sulfur-containing cathode active material can be a molten liquid obtained by heating and melting sulfur. That is, in a preferred embodiment of the method for producing a cathode material, the liquid is a molten liquid obtained by heating and melting sulfur. As another example, when the sulfur-containing cathode active material is lithium sulfide (Li ₂ S), the liquid containing the sulfur-containing cathode active material can be a lithium sulfide solution obtained by dissolving lithium sulfide in a solvent (preferably, an alcohol having 1 to 4 carbon atoms). That is, in a preferred embodiment of the method for producing a cathode material, the liquid is a lithium sulfide solution obtained by dissolving lithium sulfide in a solvent (preferably, an alcohol having 1 to 4 carbon atoms).

In step (1), when a composite is obtained by impregnating porous carbon particles with a molten liquid obtained by heating and melting a positive electrode active material (e.g., sulfur), it is preferable to first mix the sulfur-containing positive electrode active material and porous carbon particles by mixing using a mixing means such as a mortar or milling using a grinding means such as a planetary ball mill, and then melt the sulfur-containing positive electrode active material (e.g., sulfur) by heat treatment. This heat treatment results in a composite in which the sulfur-containing positive electrode active material is filled even inside the pores of the porous carbon particles.

In step (1), when porous carbon particles are impregnated with a positive electrode active material solution prepared by dissolving a positive electrode active material in a solvent to obtain a composite material, a positive electrode active material solution is prepared by dissolving the positive electrode active material in a solvent in advance. The solvent used in this step is not particularly limited, as long as it can dissolve the positive electrode active material (as long as the solubility at normal pressure and 25°C is 1 g/L or more). For example, when lithium sulfide is used as the positive electrode active material, a lower alcohol is suitable as the solvent from the viewpoints of solubility, operability, safety, etc., and an alcohol having 1 to 4 carbon atoms is preferable. Examples of alcohols having 1 to 4 carbon atoms include methanol, ethanol, 1-propanol, 2-propanol, n-butanol, 2-butanol, and tert-butanol. Among these, from the viewpoints of solubility of lithium sulfide, operability, safety, etc., methanol, ethanol, 1-propanol, and 2-propanol are preferred, with methanol and ethanol being more preferred, and ethanol being particularly preferred.

The above-mentioned solvent preferably has a low water content, specifically, less than 0.2% by mass. A water content of less than 0.2% by mass can prevent the positive electrode active material (lithium sulfide) from being decomposed by the water in the solvent. More preferably, the water content in the solvent is 0.1% by mass or less, even more preferably 0.05% by mass or less, even more preferably 0.02% by mass or less, even more preferably 0.01% by mass or less, even more preferably 0.005% by mass or less, and particularly preferably 0.002% by mass or less. The water content in the solvent can be measured, for example, by Karl Fischer coulometric titration.

Next, porous carbon particles are impregnated with the resulting positive electrode active material solution to prepare a dispersion, and the solvent is then removed from the dispersion while stirring. The solvent is removed by heat treatment, preferably under reduced pressure, at a temperature of, for example, 60 to 250°C, preferably 150 to 180°C, for, for example, 1 to 120 hours, preferably 1 to 10 hours. This results in a composite material in which the sulfur-containing positive electrode active material is filled inside the pores of the porous carbon particles.

In step (2), the composite material is impregnated with a phosphorus-containing sulfide solid electrolyte solution. The phosphorus-containing sulfide solid electrolyte solution can be prepared by dissolving the phosphorus-containing sulfide solid electrolyte in a solvent. The solvent is not particularly limited as long as it can dissolve the phosphorus-containing sulfide solid electrolyte (as long as the solubility is 1 g/L or more at normal pressure and 25°C). From the viewpoints of solubility, operability, safety, and the like, a lower alcohol is suitable as such a solvent, and an alcohol having 1 to 4 carbon atoms is preferred. That is, in a preferred embodiment of the method for producing a positive electrode material, the phosphorus-containing sulfide solid electrolyte solution is obtained by dissolving the phosphorus-containing sulfide solid electrolyte in an alcohol having 1 to 4 carbon atoms. The alcohol having 1 to 4 carbon atoms described above in step (1) can be used as the alcohol having 1 to 4 carbon atoms.

Next, the composite is impregnated with the resulting phosphorus-containing sulfide solid electrolyte solution, and the solvent is then removed while stirring the dispersion. The solvent is preferably removed under reduced pressure at a temperature of 70°C or less. This prevents decomposition of the solid electrolyte and further reduces internal resistance.

The mass ratio of the sulfur-containing cathode active material to the phosphorus-containing sulfide solid electrolyte in the cathode material of this embodiment is not particularly limited, but may be, for example, 1:99 to 99:1 (sulfur-containing cathode active material:phosphorus-containing sulfide solid electrolyte). The mass ratio is preferably 50:50 to 1:99 (positive electrode active material:phosphorus-containing sulfide solid electrolyte), more preferably 1:1 to 1:20. This range ensures a sufficient amount of cathode active material, resulting in a high-performance battery. This is also preferable because it ensures the ionic conductivity of the cathode material and ensures a sufficient ionic conduction network within the pores. Furthermore, it is also preferable because it is less likely to decompose the solid electrolyte due to an excessive amount of solid electrolyte. The mass ratio of the sulfur-containing cathode active material to the porous carbon particles is also not particularly limited, but for example, the ratio (sulfur-containing cathode active material/porous carbon particles) is 0.5 to 5, preferably 1 to 4. This range further reduces the internal resistance of the secondary battery. The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 35 to 99 mass%, and more preferably in the range of 40 to 90 mass%. Note that this content value is calculated based on the mass of only the positive electrode active material, excluding the porous carbon particles and phosphorus-containing sulfide solid electrolyte.

The positive electrode active material layer may further contain a conductive additive (one that does not hold the positive electrode active material or solid electrolyte inside the pores) and/or a binder.

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

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

[Positive electrode lead and negative electrode lead]
Although not shown, the current collector and the current collecting plate may be electrically connected via a positive electrode lead or a negative electrode lead. Materials used in known lithium secondary batteries may be used as the constituent materials of the positive electrode and negative electrode lead. It is preferable that the portion removed from the outer casing be covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, resulting in electrical leakage and affecting the product (e.g., automotive parts, particularly electronic devices).

[Battery exterior material]
As the battery exterior material, a known metal can case can be used, or a bag-shaped case using an aluminum-containing laminate film 29 that 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 desirable from the viewpoint of its high output and excellent cooling performance, making it suitable for use in batteries for large equipment such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable for the exterior body because it allows for easy adjustment of the collective pressure applied to the power generating element from the outside.

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

The above describes one embodiment of a lithium secondary battery, but 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 lithium secondary battery according to this embodiment can be used in a bipolar battery, which 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.

Furthermore, 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 are no particular restrictions on the amount of liquid electrolyte (electrolytic solution) that can be contained in the solid electrolyte layer, but it is preferable that the amount be 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 present invention will be described in more detail below using examples. However, the technical scope of the present invention is not limited to the following examples.

<<Example of test cell production>>
[Example 1]
(Preparation of positive electrode material)
(1) In a glove box in an argon atmosphere with a dew point of −68° C. or lower, 2.571 g of sulfur (manufactured by Aldrich Chemical Co., Ltd.) as a positive electrode active material and 0.429 g of porous carbon powder (manufactured by Toyo Tanso Co., Ltd., Knobel (registered trademark) P(3)010) as porous carbon particles were thoroughly mixed in an agate mortar, and the mixed powder was then placed in a sealed pressure-resistant autoclave and heated at 185° C. for 8 hours to melt the sulfur, thereby preparing a sulfur-impregnated porous carbon composite.

(2) In a glove box with an argon atmosphere _and a dew point of -68°C or less, 0.353 g of sulfide solid electrolyte (Ampcera, _Li6PS5Cl ) was added to 18 mL of ultra-dehydrated ethanol (Fujifilm Wako Pure Chemical Industries, Ltd.) and stirred until the solution became clear, dissolving the solid electrolyte in the ethanol. 1.647 g of the sulfur-impregnated porous carbon composite prepared above was added to the resulting solution, and the mixture was thoroughly stirred to fully disperse the sulfur-impregnated porous carbon composite in the solution. The container containing this dispersion was connected to a vacuum device, and while stirring the dispersion in the container with a magnetic stirrer, the container was evacuated to a pressure of 1 Pa or less using an oil-sealed rotary pump. Because the solvent ethanol volatilizes under reduced pressure, the ethanol was removed over time, leaving porous carbon particles with sulfur and solid electrolyte filled in the pores in the container. After removing the ethanol under reduced pressure, the container was heated to 70°C under reduced pressure and heat-treated for 3 hours to prepare a positive electrode material.

(Preparation of positive electrode mixture)
In a glove box in an argon atmosphere with a dew point of −68 ° C. or less, 40 g of 5 mm diameter zirconia balls, 0.130 g of the cathode material prepared above, and 0.070 g of sulfide solid electrolyte (Ampcera, Li ₆ PS ₅ Cl) were placed in a 45 mL zirconia container and milled at 370 rpm for 6 hours using a planetary ball mill (Fritsch, Premium line P-7) to obtain a powder of a cathode mixture. The composition (mass ratio) of the cathode mixture was sulfur: solid electrolyte: porous carbon = 50:40:10.

(Production of test cells (all-solid-state lithium secondary batteries))
The battery was fabricated in a glove box with an argon atmosphere at a dew point of -68 ° C. or less. A SUS cylindrical convex punch (10 mm diameter) was inserted into one side of a Macor cylindrical tube jig (tube inner diameter 10 mm, outer diameter 23 mm, height 20 mm), and 80 mg of sulfide solid electrolyte (Ampcera, Li ₆ PS ₅ Cl) was placed from the top of the cylindrical tube jig. Then, another SUS cylindrical convex punch was inserted to sandwich the solid electrolyte, and a solid electrolyte layer with a diameter of 10 mm and a thickness of approximately 0.6 mm was formed in the cylindrical tube jig by pressing it at a pressure of 75 MPa for 3 minutes using a hydraulic press. Next, the cylindrical convex punch inserted from above was once removed, and 7.5 mg of the positive electrode mixture prepared above was placed on one side of the solid electrolyte layer in the cylindrical tube. A cylindrical convex punch (also serving as a positive electrode current collector) was again inserted from above and pressed at a pressure of 300 MPa for 3 minutes to form a positive electrode active material layer having a diameter of 10 mm and a thickness of approximately 0.06 mm on one side of the solid electrolyte layer. Next, the lower cylindrical convex punch (also serving as a negative electrode current collector) was removed, and a lithium foil (manufactured by Nilaco Corporation, thickness 0.20 mm) punched to a diameter of 8 mm and an indium foil (manufactured by Nilaco Corporation, thickness 0.30 mm) punched to a diameter of 9 mm were stacked as negative electrodes. The cylindrical tube jig was inserted from the bottom so that the indium foil was positioned on the solid electrolyte layer side. The cylindrical convex punch was again inserted and pressed at a pressure of 75 MPa for 3 minutes to form a lithium-indium negative electrode. In this manner, a test cell (all-solid-state lithium secondary battery) was produced in which the negative electrode current collector (punch), lithium-indium negative electrode, solid electrolyte layer, positive electrode active material layer, and positive electrode current collector (punch) were stacked in this order.

[Example 2]
A test cell was prepared in the same manner as in Example 1, except that in step (2) of preparing the positive electrode material, 0.353 g of the solid electrolyte was dissolved in 15 mL of ultra-dehydrated ethanol.

[Comparative Example 1]
A positive electrode material was prepared by the following method, without performing steps (1) and (2) of the preparation of the positive electrode material. In a glove box with an argon atmosphere having a dew point of −68°C or lower, 40 g of 5 mm diameter zirconia balls, 0.100 g of sulfur, 0.080 g of sulfide solid electrolyte, and 0.020 g of porous carbon powder were placed in a 45 mL zirconia container and milled at 370 rpm for 6 hours in a planetary ball mill. A test cell was prepared using the same method as in Example 1, except for this.

[Comparative Example 2]
The step (2) of preparing the positive electrode material was omitted, and the sulfur-impregnated porous carbon composite obtained in step (1) was used as the positive electrode material. A positive electrode mixture was prepared by the following method. In a glove box with an argon atmosphere having a dew point of -68 ° C or less, 40 g of 5 mm diameter zirconia balls, 0.120 g of the positive electrode material (sulfur-impregnated porous carbon composite) obtained in step (1) of preparing the positive electrode material, and 0.080 g of a sulfide solid electrolyte were placed in a 45 mL zirconia container and milled at 370 rpm for 6 hours using a planetary ball mill to obtain a powder of the positive electrode mixture. The composition (mass ratio) of the positive electrode mixture was sulfur: solid electrolyte: porous carbon = 50:40:10. A test cell was prepared using the same method as in Example 1 above.

[Comparative Example 3]
A positive electrode material was prepared using the following method, without performing steps (1) and (2) of the positive electrode material preparation described above. In a glove box with an argon atmosphere and a dew point of −68°C or less, 0.500 g of sulfide solid electrolyte was added to 100 mL of ultra-dehydrated ethanol and stirred until the solution became transparent, dissolving the solid electrolyte in the ethanol. To the resulting solution, 1.00 g of activated carbon (MSC-30, manufactured by Kansai Thermochemical Co., Ltd.) in the form of porous carbon particles was added and stirred thoroughly to thoroughly disperse the activated carbon in the solution. The container containing this dispersion was connected to a vacuum device, and while stirring the dispersion in the container with a magnetic stirrer, the container was reduced in pressure to 1 Pa or less using an oil-sealed rotary pump. Because the solvent ethanol volatilizes under reduced pressure, the ethanol was removed over time, leaving porous carbon particles with the solid electrolyte filled in their pores in the container. After removing the ethanol under reduced pressure in this way, the container was heated to 180°C under reduced pressure and heat-treated for 3 hours to prepare a solid electrolyte-impregnated porous carbon composite.

In a glove box with an argon atmosphere at a dew point of -68°C or below, 0.750 g of the solid electrolyte-impregnated porous carbon composite prepared above and 2.50 g of sulfur were thoroughly mixed in an agate mortar. The mixed powder was then placed in a sealed pressure-resistant autoclave and heated at 170°C for 3 hours to melt the sulfur, thereby impregnating the solid electrolyte-impregnated porous carbon composite with the sulfur to prepare a positive electrode material. A test cell was otherwise prepared using the same method as in Example 1 above.

<<Measurement of the concentration of phosphorus atoms and sulfur atoms in the positive electrode material and the thickness of the coating layer>>
The concentrations of phosphorus atoms and sulfur atoms were measured for each of the positive electrode materials prepared in the above examples and comparative examples. A JEOL Ltd. "JSM-IT800" field emission scanning electron microscope was used as the measurement device. EDS analysis was performed on the elements phosphorus, sulfur, and carbon, and elemental mapping of each element was performed on each porous carbon particle. When observing the porous carbon particles, the cross section of the particle was exposed by FIB (focused ion beam) processing, and carbon elemental mapping was performed on the observed particles to determine the particle boundaries.

Next, in the cross section of the particle observed by SEM, phosphorus elemental mapping was performed on region A (the internal region of the particle) with a similarity ratio of 1/2, with the center as a fixed point, and the value obtained by counting signals derived from phosphorus was designated p1. Furthermore, phosphorus elemental mapping was performed on region B (the surface region of the particle) excluding region A from the cross section, and the value obtained by counting signals derived from phosphorus was designated p2. This operation was performed on 10 particles, and the sum of p1 for the 10 particles was designated P1, and the sum of p2 for the 10 particles was designated P2. Since the area of region A: the area of region B = 1/4:3/4, if the phosphorus atom concentration in region A is designated P _A and the phosphorus atom concentration in region B is designated P _B , then P _B /P _A = P2/(P1×3). When P _B /P _A exceeds 1, it was determined that the phosphorus atom concentration in region B is higher than that in region A.

Similarly, in the cross section of the particle observed by SEM, elemental mapping of sulfur was performed on region A (the internal region of the particle) with a similarity ratio of 1/2 with the center as a fixed point, and the value obtained by counting signals derived from sulfur was designated as s1. Furthermore, elemental mapping of sulfur was performed on region B (the surface region of the particle) excluding region A from the cross section, and the value obtained by counting signals derived from sulfur was designated as s2. This operation was performed on 10 particles, and the sum of s1 for the 10 particles was designated as S1, and the sum of s2 for the 10 particles was designated as S2. Since the area of region A: the area of region B = 1/4:3/4, if the phosphorus atom concentration in region A is designated as S _A and the phosphorus atom concentration in region B is designated as S _B , then S _A /S _B = (S1 × 3) /S2. When S _A /S _B exceeds 1, it was determined that the concentration of sulfur atoms in region A is higher than that in region B.

The thickness of the coating layer on the surface of the porous carbon particles was measured for each of the positive electrode materials prepared in the above examples and comparative examples. A JEOL Ltd. "JEM-ARM200F" cryo-transmission electron microscope was used as the measurement device to observe the coating layer on the particle surface. To observe the porous carbon particles, the particle cross-section was exposed using FIB (focused ion beam) processing, and carbon and phosphorus elemental mapping was performed on the observed particles using EDS analysis. The particle boundary was determined from the carbon elemental mapping, and the outermost surface of the coating layer was determined from the phosphorus elemental mapping. The distance from the particle boundary to the outermost surface of the coating layer was then measured as the thickness of the coating layer for each particle. This procedure was performed on 50 particles, and the arithmetic average value was calculated to determine the thickness of the coating layer.

<<Measurement of the internal resistance of the test cell>>
The internal resistance values of the test cells prepared in the above Examples and Comparative Examples were measured by the following method. All of the following measurements were performed using a charge/discharge tester (HJ-SD8, manufactured by Hokuto Denko Corporation) in a constant temperature bath set at 25°C. The measurement device used was a high-performance electrochemical measurement system manufactured by Bio-Logic SAS (France) under the trade name "SP-200." Measurements were performed in a frequency range of 7 MHz to 0.1 Hz, at room temperature (25°C), and with an applied voltage of 50 mV, and impedance was measured as the electrochemical characteristics. The test cell was placed in the bath, and evaluation was performed after the cell temperature became constant.

These results are shown in Table 1 below.

The results shown in Table 1 show that the present invention can further reduce the internal resistance of secondary batteries that use a positive electrode active material containing sulfur.

Furthermore, Examples 1 and 2 also demonstrated that the internal resistance is reduced more significantly when the thickness of the coating layer containing the sulfide solid electrolyte formed on the surface of the porous carbon particles is smaller.

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 plate,
27 positive electrode current collector plate,
29 Laminating film,
100, 100' positive electrode material,
110 porous carbon particles,
110a pores,
120 positive electrode active material,
130 sulfide solid electrolyte,
A area A,
B area B,
C center.

Claims (4)

A positive electrode material in which a positive electrode active material containing sulfur and a sulfide solid electrolyte containing phosphorus are filled into pores of porous carbon particles,
In an image of a cross section of the porous carbon particle observed using a scanning electron microscope, a region B obtained by excluding the region A from the cross section and having a similarity ratio of 1/2 with the center of the cross section as a fixed point has a higher concentration of phosphorus atoms than a region A,
A positive electrode material, wherein the concentration of sulfur atoms is higher in the region A than in the region B. The porous carbon particles further have a coating layer containing the sulfide solid electrolyte on at least a portion of the surface thereof,
2. The cathode material according to claim 1, wherein the coating layer has a thickness of 300 nm or less. The cathode material according to claim 2 , wherein the coating layer has a thickness of 10 nm or more and 200 nm or less. A secondary battery comprising the positive electrode material described in any one of claims 1 to 3.