JP7752951B2 - Positive electrode active material layer and all-solid-state battery - Google Patents

Description

The present invention relates to a positive electrode active material layer and an all-solid-state battery.

In recent years, electronics technology has made remarkable advances, with efforts being made to make portable electronic devices smaller, lighter, thinner, and more multifunctional. Accordingly, there is a strong demand for batteries, which serve as the power source for these electronic devices, to be smaller, lighter, thinner, and more reliable, and all-solid-state batteries, which use a solid electrolyte, have attracted attention.

All-solid-state batteries include a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and a solid electrolyte sandwiched between them. It has been reported that coating the surface of the positive electrode active material with a compound can improve the performance of all-solid-state batteries (e.g., Patent Documents 1 to 3).

For example, Patent Document 1 describes coating the surface of lithium-cobalt composite oxide powder with alumina. Patent Document 2 describes coating the surface of a positive electrode active material with a mixed ionic conductor composed of _{Li3VO4 and Li3PO4 .} Patent Document 3 describes coating with a Li-containing compound.

Japanese Patent Application Laid-Open No. 2005-276454 JP 2010-251256 A JP 2013-26003 A

However, coating materials containing these oxides have poor conformability to the surface of active material particles during molding. As a result, when molding the positive electrode active material layer, voids may form between the active material particles, resulting in insufficient rate characteristics. The C rate indicates the speed of charge and discharge. In constant current charge/discharge measurements, 1 C is defined as the current that fully charges (or discharges) the theoretical capacity of a battery in one hour. The higher the ratio of the capacity when the C rate is increased to the capacity when discharged at a specified C rate, the better the rate characteristics. A battery with excellent discharge rate characteristics is capable of rapid discharge.

The present invention was made in consideration of the above problems, and aims to provide a positive electrode active material, a positive electrode active material layer, and an all-solid-state battery that have excellent rate characteristics.

To solve the above problems, we provide the following solutions.

(1) A positive electrode active material according to a first aspect is a positive electrode active material contained in the positive electrode active material layer of an all-solid-state battery including a positive electrode active material layer, a negative electrode active material layer, and a solid electrolyte layer sandwiched between the positive electrode active material layer and the negative electrode active material layer, the positive electrode active material comprising active material particles and a coating material that coats the active material particles, the coating material having a composition or material different from that of the solid electrolyte contained in the solid _electrolyte layer, the coating material being expressed by a chemical formula _{of LiαAβBγ} , in which A is one or more elements selected from alkali metals, alkaline earth metals, and transition metals, and B is a halogen, or a combination of a halogen and O, _OH , _BO2 , _BO3 , _B4O7 , _CO3 , _NO3 , _AlO2 , _{SiO3 , SiO4} , _PO3 , _PO4 , _{P2O7 , P3O10} , SO ₃ , SO ₄ , S ₂ O ₃ , S ₂ O ₄ , S ₂ O ₇ , BF ₄ , PF ₆ , and BOB, and α satisfies 1≦α≦3, β satisfies 0.5≦β≦1.5, and γ satisfies 4≦γ≦6.

(2) In the positive electrode active material according to the above aspect, in the chemical formula, A may be one or more elements selected from Ti, Zr, Y, Hf, Sc, Mg, Ca, Sr, Ba, Na, K, Al, Ga, and In; B may be a halogen or a halogen and one or more groups selected from O, _CO3 , _SO4 , and _PO4 ; and α may be 1.5≦α≦2.5.

(3) In the positive electrode active material according to the above aspect, the coverage of the coating material with respect to the active material particles may be 85% or more.

(4) The positive electrode active material layer according to the second aspect contains the positive electrode active material according to the above aspect.

(5) The positive electrode active material layer according to the above embodiment may further contain the solid electrolyte.

(6) The all-solid-state battery according to the above aspect comprises the positive electrode active material layer according to the above aspect, a negative electrode active material layer, and a solid electrolyte layer sandwiched between the positive electrode active material layer and the negative electrode active material layer.

The positive electrode active material, positive electrode active material layer, and all-solid-state battery according to the above aspects have excellent rate characteristics.

1 is a cross-sectional view of an all-solid-state battery according to an embodiment of the present invention; 1 is a cross-sectional view of a positive electrode active material according to an embodiment of the present invention;

The present embodiment will now be described in detail, with appropriate reference to the drawings. The drawings used in the following description may show enlarged portions of the characteristic features of the present invention for ease of understanding, and the dimensional proportions of each component may differ from the actual proportions. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them. Appropriate modifications may be made within the scope of the present invention.

[All-solid-state battery]
1 is a cross-sectional schematic diagram of an all-solid-state battery 10 according to this embodiment. The all-solid-state battery 10 has a laminate 4 and terminal electrodes 5 and 6. The terminal electrodes 5 and 6 are in contact with opposing surfaces of the laminate 4. The terminal electrodes 5 and 6 extend in a direction intersecting (orthogonal to) the laminate surface of the laminate 4.

The laminate 4 has a positive electrode 1, a negative electrode 2, and a solid electrolyte layer 3. The number of positive electrode 1 and negative electrode 2 layers is not important. The solid electrolyte layer 3 is located between the positive electrode 1 and the negative electrode 2. One end of the positive electrode 1 is connected to a terminal electrode 5. One end of the negative electrode 2 is connected to a terminal electrode 6. Between the positive electrode 1 and the terminal electrode 6 and between the negative electrode 2 and the terminal electrode 5, for example, a solid electrolyte layer 3 is located.

The all-solid-state battery 10 is charged or discharged by the exchange of ions between the positive electrode 1 and the negative electrode 2 via the solid electrolyte layer 3. While a stacked-type battery is shown in Figure 1, a wound-type battery may also be used. The all-solid-state battery 10 is used, for example, in laminated batteries, prismatic batteries, cylindrical batteries, coin batteries, button batteries, etc. The all-solid-state battery 10 may also be an injection-type battery in which the solid electrolyte layer 3 is dissolved or dispersed in a solvent.

"Positive electrode"
As shown in FIG. 1, a positive electrode 1 includes, for example, a positive electrode current collector 1A and a positive electrode active material layer 1B containing a positive electrode active material.

(Positive electrode current collector)
The positive electrode current collector 1A preferably has high electrical conductivity. For example, metals such as silver, palladium, gold, platinum, aluminum, copper, nickel, titanium, and stainless steel, alloys thereof, or conductive resins can be used. The positive electrode current collector 1A may be in the form of powder, foil, punched, or expanded.

(Positive electrode active material layer)
The positive electrode active material layer 1B is formed on one or both surfaces of the positive electrode current collector 1A. The positive electrode active material layer 1B contains a positive electrode active material, and may also contain a conductive additive, a binder, and a solid electrolyte described later, as necessary.

(Cathode active material)
2 is a cross-sectional view of a cathode active material 20 included in the cathode active material layer 1B. In the cathode active material layer 1B, a plurality of cathode active materials 20 are in close contact with each other. The cathode active material 20 includes active material particles 21 and a coating material 22.

The active material particles 21 are, for example, lithium-containing transition metal oxides, transition metal fluorides, polyanions, transition metal sulfides, transition metal oxyfluorides, transition metal oxysulfides, or transition metal oxynitrides.

The active material particles 21 are not particularly limited as long as they are capable of reversibly absorbing and releasing lithium ions, and inserting and desorbing lithium ions. For example, positive electrode active materials used in known lithium ion secondary batteries can be used.

The active material particles 21 are, for example, composite metal oxides, such as lithium cobalt oxide ( _LiCoO2 ), lithium nickel oxide ( _LiNiO2 ), lithium manganese spinel ( _LiMn2O4 ), composite metal oxides represented by the general _formula : LiNixCoyMnzMaO2 ( _x + _y +z+a ₌ 1 _, 0≦x≦1, 0≦y≦1, 0≦z≦1 _, 0≦a≦1, M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr), lithium _vanadium compounds ( _LiV2O5 , _Li3V2 ( _PO4 ) ₃ , _LiVOPO4 ), olivine-type LiMPO4, and _{the like.} (wherein M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, V, Nb, _Ti , Al, and _Zr ), lithium titanate ( _{Li4Ti5O12 )} , _{LiNixCoyAlzO2} (0.9< _x +y+ _z <1.1), etc.

Lithium-free active material particles 21 can also be used as the active material particles 21. These active material particles 21 can be used by disposing metallic lithium or a negative electrode active material doped with lithium ions in the negative electrode in advance and starting the battery by discharging. Examples of these positive electrode active materials include lithium-free metal oxides ( _MnO2 , _V2O5 , _etc. ), lithium-free metal sulfides ( _MoS2, etc.), and lithium-free fluorides ( _FeF3 , _VF3 , etc.).

The coating material 22 coats the surfaces of the active material particles 21. The coating material 22 does not have to coat the entire surfaces of the active material particles 21; it is sufficient that the coating material 22 coats only a portion of the surfaces of the active material particles 21. It is preferable that the coverage rate of the coating material 22 with respect to the active material particles 21 is 85% or more.

The coverage rate is determined as follows. First, a cross section of the positive electrode active material layer 1B is cut out and measured using a scanning electron microscope (SEM). Multiple positive electrode active materials 20 are visible in the cross-sectional SEM image. Some of the multiple positive electrode active materials 20 are in close contact with each other, making the interfaces difficult to see. Therefore, 30 positive electrode active materials 20 whose grain boundaries are clearly visible are randomly selected from the cross-sectional SEM image. Next, the perimeter of the cross section of each positive electrode active material 20 is determined. Furthermore, within the perimeter of each positive electrode active material 20, the length of the portion of the active material particle 21 coated with the coating material 22 is measured. Whether the surface of the active material particle 21 is coated with the coating material 22 can be determined from the contrast of the image. The coverage rate of each positive electrode active material 20 is then determined by dividing the "length of the portion of the active material particle 21 coated with the coating material 22" by the "perimeter of the positive electrode active material 20." Finally, the coverage rate of the coating material 22 on the active material particles 21 is determined by calculating the average coverage rate of each positive electrode active material 20.

The coating material 22 is a compound represented by _the chemical _{formula LiαAβBγ} . The coating material 22 has a different composition or material from the solid electrolyte that constitutes the solid electrolyte layer 3, which will be described later.

In the chemical formula, A is one or more elements selected from alkali metals, alkaline earth metals, and transition metals. For example, A is one or more elements selected from Ti, Zr, Y, Hf, Sc, Mg, Ca, Sr, Ba, Na, K, Al, Ga, and In, and is preferably Zr or Y.

In the chemical formula, B includes halogens such as F, Cl, Br, and I. B may consist of only halogens, or may consist of halogens and polyvalent polyanions. The polyanions _are , for example, at least one group selected from _the group consisting _of O, OH, _BO2 , _BO3 , _B4O7 , _CO3 , _NO3 , _AlO2 , _{SiO3, SiO4 , PO3} , _PO4 , _{P2O7 , P3O10} , _{SO3 , SO4} , _S2O3 , _S2O4 , _S2O7 , _BF4 , _PF6 , and BOB. It is particularly preferred that B consists of only halogens, _or halogens and one or more groups selected from O, _CO3 , _SO4 , and _PO4 .

In the chemical formula, α satisfies 1≦α≦3, β satisfies 0.5≦β≦1.5, and γ satisfies 4≦γ≦6. It is also preferable that α satisfies 1.5≦x≦2.5.

Examples of the coating material 22 include _{Li2ZrCl6 , Li3YCl6} , _Li2ZrSO4Cl4 , _Li2Zr ( _PO4 ) _{1 / 3Cl5 , and Li2Zr} ( _CO3 ) _{1/ 2Cl5} .

(Conductive additive)
The conductive additive is not particularly limited as long as it improves the electronic conductivity in the positive electrode active material layer 1B, and known conductive additives can be used. Examples of the conductive additive include carbon-based materials such as graphite, carbon black, graphene, and carbon nanotubes; metals such as gold, platinum, silver, palladium, aluminum, copper, nickel, stainless steel, and iron; conductive oxides such as ITO; and mixtures thereof. The conductive additive may be in the form of powder or fiber.

(Binder)
The binder bonds the positive electrode current collector 1A and the positive electrode active material layer 1B, the positive electrode active material layer 1B and the solid electrolyte layer 3, and various materials constituting the positive electrode active material layer 1B.

The binder can be used to the extent that it does not impair the functionality of the positive electrode active material layer 1B. If a binder is not required, it need not be included. The binder content in the positive electrode active material layer 1B is, for example, 0.5 to 30 volume % of the positive electrode active material layer. If the binder content is within this range, the resistance of the positive electrode active material layer 1B will be sufficiently low.

The binder may be any material capable of bonding as described above, such as fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). In addition to the above, binders may also be used, such as cellulose, styrene-butadiene rubber, ethylene-propylene rubber, polyimide resin, and polyamide-imide resin. Alternatively, a conductive polymer with electronic conductivity or an ionic conductive polymer with ionic conductivity may be used as the binder. An example of an electronic conductive polymer is polyacetylene. In this case, the binder also functions as a conductive additive particle, so no conductive additive is required. Examples of ionic conductive polymers that can conduct lithium ions include those that are composites of monomers of polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide, polyphosphazene, etc.) with lithium salts or lithium-based alkali metal salts such as LiClO ₄ , LiBF ₄ , and LiPF ₆ . The polymerization initiator used for the composite is, for example, a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-mentioned monomers. The properties required for the binder include oxidation/reduction resistance and good adhesiveness.

(solid electrolyte)
The solid electrolyte contained in the positive electrode active material layer 1B improves ion conduction within the positive electrode active material layer 1B. The solid electrolyte is different from the above-described coating material 22. Details of the solid electrolyte will be described later.

"Negative electrode"
As shown in FIG. 1, the negative electrode 2 includes, for example, a negative electrode current collector 2A and a negative electrode active material layer 2B containing a negative electrode active material.

(Negative electrode current collector)
The negative electrode current collector 2A preferably has high electrical conductivity. For example, it is preferable to use a metal such as silver, palladium, gold, platinum, aluminum, copper, nickel, stainless steel, or iron, or an alloy thereof, or a conductive resin. The negative electrode current collector 2A may be in the form of a powder, foil, punched, or expanded.

(Negative electrode active material layer)
The negative electrode active material layer 2B is formed on one or both surfaces of the negative electrode current collector 2A. The negative electrode active material layer 2B contains a negative electrode active material and a predetermined compound, and may also contain a conductive additive, a binder, and the above-mentioned solid electrolyte, as necessary.

(Negative electrode active material)
The negative electrode active material contained in the negative electrode active material layer 2B is a compound capable of absorbing and releasing mobile ions. The negative electrode active material can be any negative electrode active material used in known lithium-ion secondary batteries. Examples of the negative electrode active material include carbon materials such as alkali metals, alkali metal alloys, graphite (natural graphite, artificial graphite), carbon nanotubes, non-graphitizable carbon, easily graphitizable carbon, and low-temperature fired carbon; metals that can combine with alkali metals such as aluminum, silicon, tin, germanium, and alloys thereof; oxides such as SiO _x (0<x<2), iron oxide, titanium oxide, and tin dioxide; and lithium metal oxides such as lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

(Conductive additive)
The conductive additive improves the electronic conductivity of the negative electrode active material layer 2 B. As the conductive additive, the same materials as those used for the positive electrode active material layer 1 B can be used.

(Binder)
The binder bonds the negative electrode current collector 2A and the negative electrode active material layer 2B, the negative electrode active material layer 2B and the solid electrolyte layer 3, and the various materials constituting the negative electrode active material layer 2B. The binder may be the same material as that used for the positive electrode active material layer 1B. The binder content may also be the same as that used for the positive electrode active material layer 1B. If the binder is not necessary, it need not be contained.

"Solid electrolyte layer"
The solid electrolyte layer 3 includes a solid electrolyte that can move ions by an externally applied electric field, etc. For example, the solid electrolyte conducts lithium ions and inhibits the movement of electrons.

The solid electrolyte contains, for example, lithium. The solid electrolyte may also be, for example, an oxide-based material, a sulfide-based material, or a halide-based material.

Examples of oxide-based solid electrolytes include perovskite-type compounds, lysicron-type compounds, garnet-type compounds, nasicron-type compounds, glass compounds, and phosphate compounds.

_{La0.5Li0.5TiO3} is an example of a perovskite-type compound _{, Li14Zn ( GeO4} ) ₄ is an example of a lithicon-type compound, _{and Li7La3Zr2O12} is an example _of a garnet _- type _compound . LiZr ₂ (PO ₄ ) ₃ , Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) _3, Li _1.5 Al _0.5 Ge _1.5 (PO ₄ ) ₃ , Li _1.55 Al _0.2 Zr _1.7 Si _0.25 P _9.75 O ₁₂ , Li _1.4 Na _0.1 Zr _1.5 Al _0.5 (PO ₄ ) ₃ , Li _1.4 Ca _0.25 Er _0.3 Zr _1.7 (PO ₄ ) _3.2 , Li _1.4 Ca _0.25 Yb _0.3 Zr _1.7 ( _PO4 ) _3.2 is an example of a Nasicon-type compound. _{Li 2} O—V ₂ O ₅ —SiO ₂ is an example of a glass compound. _{Li 3} PO ₄ , Li _3.5 Si _0.5 P _0.5 O ₄ , and Li _2.9 PO _3.3 N _0.46 are examples of phosphate compounds.

Examples of sulfide-based solid electrolytes include Li _3.25 Ge _0.25 P _0.75 S ₄ (crystal), Li ₁₀ GeP ₂ S ₁₂ (crystal, LGPS), Li ₆ PS ₅ Cl (crystal, argyrodite type), Li _9.54 Si _1.74 P _1.44 S _11.7 Cl _0.3 (crystal), Li _3.25 P _0.95 S ₄ (glass ceramics), Li ₇ P ₃ S ₁₁ (glass ceramics), 70Li ₂ S·30P ₂ S ₅ (glass), 30Li ₂ S·26B ₂ S ₃ ·44LiI (glass), and 50Li ₂ S·17P ₂ S ₅ ·33LiBH ₄ (glass), 63Li ₂ S·36SiS ₂ ·Li ₃ PO ₄ (glass), and 57Li ₂ S·38SiS ₂ ·5Li ₄ SiO ₄ (glass).

Examples of solid electrolytes made of halide materials _{include Li2ZrCl6} , _Li3YCl6 , _Li2ZrSO4Cl4 , _Li2Zr ( _PO4 ) _{1 / 3Cl5} , _Li2Zr ( _CO3 ) _{1/ 2Cl5 ,} and _Li3ZrOCl5 . In addition, Cl in these solid _electrolytes may be replaced _with F, Br, or I.

The solid electrolyte layer 3 may contain substances other than the solid electrolyte material. For example, the solid electrolyte layer 3 may contain an oxide or halide of an alkali metal element, an oxide or halide of a transition metal element, or the like. The solid electrolyte layer 3 may also contain a binder. The binder may be the same as those described above.

At least one of the positive electrode active material layer 1B, negative electrode active material layer 2B, and solid electrolyte layer 3 may contain a nonaqueous electrolyte solution, an ionic liquid, or a gel electrolyte. The inclusion of any of these substances improves the rate characteristics, which are one of the battery characteristics.

[Method of manufacturing all-solid-state battery]
All-solid-state batteries can be fabricated, for example, using a powder molding method. First, a resin holder with a through hole in the center, a lower punch, and an upper punch are prepared. The diameter of the through hole in the resin holder is, for example, 10 mm, and the diameters of the lower punch and upper punch are, for example, 9.99 mm. The lower punch is inserted from below the through hole in the resin holder, and powdered solid electrolyte is poured from the opening side of the resin holder. Next, the upper punch is inserted on top of the poured powdered solid electrolyte, and the resin holder is placed in a press and pressed. The pressing pressure is, for example, 373 MPa. The powdered solid electrolyte is pressed between the upper punch and the lower punch within the resin holder to form the solid electrolyte layer 3.

The powdered positive electrode active material layer material contains active material particles and a coating material. The active material particles are obtained by mixing and reacting raw material powders in a predetermined molar ratio to achieve the desired composition. The reaction method is not critical, and mechanochemical milling, sintering, melting, liquid-phase, solid-phase, and other methods can be used. The active material particles and coating material are then mixed in a predetermined molar ratio and processed using mechanochemical milling. Through the mechanochemical process, the coating material adheres to the surfaces of the active material particles, coating the surfaces of the active material particles with the coating material.

Next, the upper punch is temporarily removed, and the material for the positive electrode active material layer is placed on the upper punch side of the solid electrolyte layer 3. The upper punch is then reinserted and pressed. The pressing pressure is, for example, 373 MPa. The material for the positive electrode active material layer becomes the positive electrode active material layer 1B through pressing.

Next, the lower punch is temporarily removed, and the material for the negative electrode active material layer is placed on the lower punch side of the solid electrolyte layer 3. For example, the sample is turned upside down, and the material for the negative electrode active material layer is placed on top of the solid electrolyte layer 3.

Then, the lower punch is inserted again and pressed. The pressing pressure is, for example, 373 MPa. The material for the negative electrode active material layer becomes the negative electrode active material layer 2B through pressing. Through the above procedure, the all-solid-state battery 10 of this embodiment is obtained.

If necessary, the all-solid-state battery 10 may be mounted on a stainless steel disk and a Teflon (registered trademark) disk with four screw holes in the following order: stainless steel disk / Teflon (registered trademark) disk / all-solid-state battery 10 / Teflon (registered trademark) disk / stainless steel disk, and the four screws may be tightened. The all-solid-state battery 10 may also be a similar mechanism with a shape-retaining function.

If necessary, the battery may be inserted into an exterior body (aluminum laminate bag) to which an externally drawn positive electrode terminal and an externally drawn negative electrode terminal have been attached, and lead wires may be connected between the screw on the side of the upper punch and the externally drawn positive electrode terminal inside the exterior body, and between the screw on the side of the lower punch and the externally drawn negative electrode terminal inside the exterior body, and finally the opening of the exterior body may be heat-sealed. The exterior body improves weather resistance.

The above-mentioned manufacturing method for the all-solid-state battery 10 has been described using a powder molding method as an example, but it may also be manufactured using a sheet molding method containing resin.

For example, first, a solid electrolyte paste containing a powdered solid electrolyte is prepared. The prepared solid electrolyte paste is applied to a PET film, a fluorine-based resin film, or the like, dried, and peeled off to prepare the solid electrolyte layer 3. A positive electrode active material paste containing a positive electrode active material is applied to a positive electrode current collector 1A and dried to form a positive electrode active material layer 1B, thereby preparing the positive electrode 1. A paste containing a negative electrode active material and a compound is applied to a negative electrode current collector 2A and dried to form a negative electrode active material layer 2B, thereby preparing the negative electrode 2.

Next, the solid electrolyte layer 3 is sandwiched between the positive electrode 1 and the negative electrode 2, and the entire assembly is pressed and bonded. Through these steps, the all-solid-state battery 10 of this embodiment is obtained.

In the all-solid-state battery 10 according to this embodiment, the surfaces of the active material particles 21 are coated with a predetermined coating material 22. Oxide-based coating materials are hard materials and have poor conformability to the surfaces of the active material particles 21, but the above-described coating material 22 has high conformability to the surfaces of the active material particles 21. As a result, irregularities associated with the coating material 22 are less likely to form on the surface of the positive electrode active material 20, and gaps are less likely to form between other positive electrode active materials 20. In other words, the positive electrode active material layer 1B has a high density. Rate characteristics tend to improve when ion conduction paths between the positive electrode active materials 20 are secured. In the all-solid-state battery 10 according to this embodiment, the positive electrode active material layer 1B has a high density and excellent rate characteristics.

The above describes embodiments of the present invention in detail with reference to the drawings. However, the configurations and combinations thereof in each embodiment are merely examples, and additions, omissions, substitutions, and other modifications to the configurations are possible without departing from the spirit of the present invention.

"Example 1"
(Preparation of positive electrode composite)
First, LCO (LiCoO ₂ ) was prepared as active material particles 21. LiCl and ZrCl ₄ were weighed as raw materials for coating material 22 in a molar ratio of 2:1. These were placed in a ZrO ₂ container together with 5 mm diameter ZrO ₂ balls in a glove box. These were then subjected to mechanochemical milling using a planetary ball mill. The mechanochemical milling was performed at a rotation speed of 500 rpm for 24 hours to obtain coating material 22. Next, active material particles 21 and coating material 22 were placed in a ZrO ₂ container together with 5 mm diameter ZrO ₂ balls in a glove box. These were then subjected to mechanochemical milling using a planetary ball mill. The mechanochemical milling was performed at a rotation speed of 150 rpm for 3 hours. The resulting powder was then sieved through a 100 μm mesh sieve. These materials were weighed out so that the ratio of the positive electrode active material, conductive additive, and solid electrolyte was 65 parts by weight, 3 parts by weight, and 32 parts by weight, and mixed in an agate mortar to prepare a positive _electrode mixture. The conductive additive was carbon black, and the solid electrolyte was _Li6PS5Cl .

(Preparation of negative electrode composite)
The negative electrode active material, conductive additive, and solid electrolyte were weighed out in a ratio of 60 parts by weight, 2 parts by weight, and 38 parts by weight, and mixed in an agate mortar to prepare a negative _electrode mixture. The conductive additive was carbon black, and the solid electrolyte was _Li6PS5Cl .

(Preparation of solid electrolyte)
The solid electrolyte was _Li6PS5Cl . The solid electrolyte was made by weighing out _Li2S , _{P2S5 ,} and LiCl in a molar ratio of 2.5:0.5:1. Inside a glove box, the mixture was placed in a _ZrO2 container along with 5 mm _{diameter ZrO2} balls. Then, the mixture was subjected to mechanochemical milling using a planetary ball mill. The mechanochemical milling was performed at a rotation speed of 600 rpm for 10 hours. The resulting powder was then sieved through a 100 μm mesh sieve.

(Fabrication of Battery Element)
A resin holder, a lower punch (also serving as a negative electrode current collector), and an upper punch (also serving as a positive electrode current collector) were prepared.
A lower punch was inserted from below the resin holder, and 110 mg of solid electrolyte was added from above the resin holder. An upper punch was then inserted on top of the solid electrolyte. This first unit was placed in a press, and a solid electrolyte layer was formed at a pressure of 186 MPa. The first unit was removed from the press, and the upper punch was removed.

Next, 12 mg of positive electrode mixture was placed on top of the solid electrolyte layer (upper punch side) in the resin holder, the upper punch was inserted on top of it, and the second unit was placed in the press and molded at a pressure of 186 MPa. The second unit was then removed, turned upside down, and the lower punch was removed. 8 mg of negative electrode mixture was placed on top of the solid electrolyte layer (lower punch side), the lower punch was inserted on top of it, and the third unit was placed in the press and molded at a pressure of 373 MPa. In this way, a battery element consisting of a positive electrode current collector/positive electrode/solid electrolyte layer/negative electrode/negative electrode current collector was produced.

(Measurement of coverage)
A cross-section of the produced battery element was photographed under an SEM to determine the coverage of the coating material 22 with respect to the active material particles 21. The coverage of the positive electrode active material of Example 1 was 90%.

(Battery evaluation: discharge rate maintenance characteristics)
The discharge rate characteristics of the fabricated lithium ion batteries were evaluated. The discharge rate characteristics were determined as the ratio (%) of the discharge capacity at a discharge rate of 1 C (a current value at which discharge is completed in 1 hour when constant current discharge is performed at 25° C.) to the discharge capacity at a discharge rate of 0.1 C (a current value at which discharge is completed in 10 hours when constant current discharge is performed at 25° C.) taken as 100%, as the discharge rate retention characteristic.

To evaluate the discharge rate retention characteristics, the fabricated cells were initially charged and discharged at a current density of 0.05 mA/ ^cm² , and the actual capacity of the fabricated cells was measured. Based on the actual capacity obtained, the current densities for discharge rates of 0.1 C and 1 C were determined.

After the initial charge/discharge, the battery was charged at a constant current of 0.1 C to 4.1 V, and then charged at a constant voltage of 0.02 C until a current density of 0.02 C was reached. After charging, a 5-minute rest period was allowed, followed by a constant current discharge at 0.1 C to 2.5 V. The discharge capacity at 0.1 C was measured, followed by a 5-minute rest period.

Then, the battery was charged at a constant current of 0.1 C up to 4.1 V, and then charged at a constant voltage of 0.02 C until the current density reached 0.02 C. After charging, a 5-minute rest period was allowed, followed by a constant current discharge at 1 C down to 2.5 V, and the discharge capacity at 1 C was measured.

The discharge rate characteristics were then calculated by dividing the measured discharge capacity at 1 C by the discharge capacity at 0.1 C. The discharge rate retention characteristics of the all-solid-state battery of Example 1 were 86%.

"Examples 2 to 9"
Examples 2 to 9 differ from Example 1 in that the covering material 22 is changed.
In Example 2, the coating material 22 was Li ₂ ZrSO ₄ Cl _4. In Example 2, Li ₂ SO ₄ and ZrCl ₄ were used as raw materials for the coating material 22.
In Example 3, the coating material 22 was Li ₃ YCl _6. In Example 3, LiCl and YCl ₃ were used as raw materials for the coating material 22.
In Example 4, the coating material 22 was Li ₃ InCl _6. In Example 4, LiCl and InCl ₃ were used as raw materials for the coating material 22.
In Example 5, the coating material 22 was Li ₂ Zr(PO ₄ ) _1/3 Cl _5. In Example 5, LiCl, ZrCl ₄ , and Li ₃ PO ₄ were used as raw materials for the coating material 22.
In Example 6, the coating material 22 was Li _2.3 Zr _0.9 Na _0.1 Cl _6. In Example 6, the raw materials for the coating material 22 were LiCl, ZrCl ₄ , and NaCl.
In Example 7, the coating material 22 was _{Li2.1Zr0.9Al0.1Cl6} . In Example ₇ , LiCl, _ZrCl4 , and _AlCl3 were used _as raw materials _for the coating material 22.
In Example 8, the coating material 22 was Li _2.8 Zr _0.6 Ba _0.4 Cl _6. In Example 8, LiCl, ZrCl ₄ and BaCl ₂ were used as raw materials for the coating material 22.
In Example 9, the coating material 22 was Li ₂ Zr(CO ₃ ) _1/2 Cl _5. In Example 9, LiCl, ZrCl ₄ , and Li ₂ CO ₃ were used as raw materials for the coating material 22.

Then, as in Example 1, the coverage and discharge rate retention characteristics of the coating material 22 were determined. The coverage was 90% for both Examples 2 and 3. The discharge rate retention characteristics of Example 2 was 87%, and the discharge rate retention characteristics of Example 3 was 70%. The coverage was 89% for Example 4, 88% for Example 5, 91% for Example 6, 87% for Example 7, 89% for Example 8, and 90% for Example 9. The discharge rate retention characteristics of Example 4 was 71%, 79% for Example 5, 85% for Example 6, 82% for Example 7, 69% for Example 8, and 86% for Example 9.

"Examples 10 to 14"
Examples 10 to 14 differ from Example 1 in that the coverage of the coating material 22 was changed. The coverage of the coating material 22 can be changed by changing the mixing ratio of the active material particles and the raw material of the coating material in the mechanochemical milling process when producing the positive electrode active material.

Then, similarly to Example 1, the coverage and discharge rate retention characteristics of the coating material 22 were determined. The coverage of Example 10 was 85%, the coverage of Example 11 was 84%, the coverage of Example 12 was 95%, the coverage of Example 13 was 98%, and the coverage of Example 14 was 70%. The discharge rate retention characteristics of Example 10 was 81%, the discharge rate retention characteristics of Example 11 was 50%, the discharge rate retention characteristics of Example 12 was 93%, the discharge rate retention characteristics of Example 13 was 95%, and the discharge rate retention characteristics of Example 14 was 45%.

"Example 15"
Example 15 differs from Example 1 in that the solid electrolyte was Li ₇ La ₃ Zr ₂ O _12. The solid electrolyte was prepared by treating raw material powders by a solid-phase reaction method.

Then, as in Example 1, the coverage rate and discharge rate retention characteristics of the coating material 22 were determined. The coverage rate of Example 15 was 90%. The discharge rate retention characteristics of Example 15 was 47%.

"Comparative Example 1"
Comparative Example 1 differs from Example 1 in that no coating material 22 was formed. That is, the active material particles 21 were used as they were as the positive electrode active material.

Then, similarly to Example 1, the coverage rate and discharge rate retention characteristics of the coating material 22 were determined. In Comparative Example 1, the coating material 22 was not formed, so the coverage rate was 0%. Furthermore, the discharge rate retention characteristics of Comparative Example 1 was 8%.

"Comparative Example 2, Comparative Example 3"
Comparative Example 2 differs from Example 1 in that the coating material 22 is LiNbO _3. Comparative Example 3 differs from Example 15 in that the coating material 22 is LiNbO ₃ .

Then, as in Example 1, the coverage rate and discharge rate retention characteristics of the coating material 22 were determined. The coverage rates of Comparative Example 2 and Comparative Example 3 were both 90%. Furthermore, the discharge rate retention characteristics of Comparative Example 2 were 12%, and the discharge rate retention characteristics of Comparative Example 3 were 3%.

The results of Examples 1 to 9 and Comparative Examples 1 to 3 are summarized in Table 1.

As shown in Table 1, it was confirmed that the discharge rate retention characteristics improved when the positive electrode active material was coated with a specified coating material. It was also confirmed that the discharge rate retention improved significantly when the coating material coverage rate exceeded 85%.

DESCRIPTION OF SYMBOLS 1... Positive electrode, 1A... Positive electrode current collector, 1B... Positive electrode active material layer, 2... Negative electrode, 2A... Negative electrode current collector, 2B... Negative electrode active material layer, 3... Solid electrolyte layer, 4... Laminated body, 5, 6... Terminal electrode, 10... All solid battery, 20... Positive electrode active material, 21... Active material particles, 22... Covering material

Claims (4)

a positive electrode active material and a solid electrolyte,
the positive electrode active material includes active material particles and a coating material that coats the active material particles,
the coating material has a different composition or material from the solid electrolyte,
The coating material is represented by the chemical formula Li _α A _β B _γ ,
In the above chemical formula:
A is one or more elements selected from alkali metals, alkaline earth metals, and transition metals;
B is a halogen or a halogen and at least one _{group selected from the group consisting of O, OH, BO2, BO3, B4O7 , CO3 , NO3 , AlO2 , SiO3} , _SiO4 , _PO3 , _PO4 , _P2O7 , _P3O10 , _SO3 , _SO4 , _S2O3 , _S2O4 , _{S2O7 , BF4 , PF6 , BOB ;}
α satisfies 1≦α≦3, β satisfies 0.5≦β≦1.5, and γ satisfies 4≦γ≦6,
The positive electrode active material layer, wherein the solid electrolyte is a solid electrolyte containing a halogen, and the halogen is any one of F, Br, and Cl . In the above chemical formula:
A is one or more selected from Ti, Zr, Y, Hf, Sc, Mg, Ca, Sr, Ba, Na, K, Al, Ga, and In;
B is a halogen or a halogen and one or more groups selected from O, CO ₃ , SO ₄ and PO ₄ ;
The positive electrode active material layer according to claim 1 , wherein α satisfies the condition 1.5≦α≦2.5. The positive electrode active material layer according to claim 1 or 2, wherein the coating material has a coverage of 85% or more of the active material particles. 4. An all-solid-state battery comprising: the positive electrode active material layer according to claim 1; a negative electrode active material layer; and a solid electrolyte layer sandwiched between the positive electrode active material layer and the negative electrode active material layer.