JP7569328B2 - All-solid-state secondary battery - Google Patents

Description

The present invention relates to an all-solid-state secondary battery, in particular an all-solid-state lithium secondary battery.

In recent years, with the development of portable devices such as personal computers and mobile phones, the demand for batteries as their power source has expanded significantly. In batteries used for such purposes, a liquid electrolyte (electrolytic solution) using a flammable organic solvent as a diluting solvent has been used as a medium for moving ions. In batteries using such electrolytes, problems such as electrolyte leakage, fire, and explosion may occur. In order to solve such problems, in order to ensure essential safety, development of all-solid-state batteries is underway that use solid electrolytes instead of liquid electrolytes and are composed of all other elements made of solids. Since such all-solid-state batteries use solid electrolytes, there is no risk of fire, they do not leak, and they are less likely to have problems such as deterioration of battery performance due to corrosion.

Various all-solid-state batteries have been proposed. For example, Patent Document 1 (JP 2009-193940 A) discloses that in a pressed powder all-solid-state battery of a sulfide-based solid electrolyte and lithium cobalt oxide, the surface of the lithium cobalt oxide is coated with lithium niobate to reduce the interface resistance. Reducing the interface resistance leads to improved charge/discharge characteristics. The battery disclosed in Patent Document 1 is an all-solid-state battery using a pressed powder, and the energy density of the electrode decreases if pores remain between the particles or if a conductive additive that ensures electronic conduction between active materials is added.

In contrast, all-solid-state batteries using sintered electrodes instead of compact electrodes have also been proposed. Such batteries have the advantage that the energy density is high because the sintered electrodes do not contain conductive additives. For example, Patent Document 2 (WO2019/093222A1) discloses an all-solid-state lithium battery comprising an oriented positive electrode plate that is a lithium composite oxide sintered body plate with a porosity of 10 to 50%, a negative electrode plate that contains Ti and is capable of inserting and desorbing lithium ions at 0.4 V (vs. Li/Li ⁺ ) or more, and a solid electrolyte having a melting point lower than the melting point or decomposition temperature of the oriented positive electrode plate or negative electrode plate. This document discloses various materials such as Li ₃ OCl, xLiOH.yLi ₂ SO ₄ (wherein x+y=1, 0.6≦x≦0.95) (e.g., 3LiOH.Li ₂ SO ₄ ) as solid electrolytes having such low melting points. Such solid electrolytes can be infiltrated into the gaps between the electrodes as a melt, resulting in strong interfacial contact, which is believed to result in significant improvements in battery resistance and rate performance during charging and discharging, as well as a significant improvement in battery manufacturing yields.

JP 2009-193940 A WO2019/093222A1

The present inventors have found that, among the above-mentioned low melting point solid electrolytes, _LiOH.Li2SO4 - _based solid electrolytes such as _3LiOH.Li2SO4 exhibit high lithium ion conductivity. However, when a cell was constructed using a _LiOH.Li2SO4 -based solid _electrolyte such as _3LiOH.Li2SO4 in a sintered electrode as disclosed in Reference ₂ and operated as a battery, it was found that the discharge amount was lower than the theoretical capacity expected from the _amount of active material.

The present inventors have now discovered that in an all-solid-state secondary battery using a _{LiOH.Li2SO4 -} based solid electrolyte, the discharge capacity can be improved by providing an intermediate layer of a specific composition at the interface between an electrode active material and the _LiOH.Li2SO4 -based solid _electrolyte .

Therefore, an object of the present invention is to improve the discharge capacity of an all-solid-state secondary battery that employs a LiOH.Li ₂ SO ₄ based solid electrolyte.

According to one aspect of the present invention,
a positive electrode including a positive electrode active material;
a negative electrode including a negative electrode active material;
A _{LiOH.Li2SO4 -} based solid electrolyte interposed between the positive electrode and the negative electrode and also penetrating into the voids of at least one of the positive electrode and the negative electrode;
Including,
In at least one of the positive electrode and the negative electrode into which the solid electrolyte has been permeated, an intermediate layer is further provided at an interface between the solid electrolyte and at least one of the positive electrode active material and the negative electrode active material, the intermediate layer being composed of at least one oxide selected from the group consisting of Y, Nb, Ta, Al, Ti, La, Zr, W, Sn, Ce, and Mn, and/or a lithium composite oxide containing Li and at least one selected from the group consisting of Y, Nb, Ta, Al, Ti, La, Zr, W, Sn, Ce, and Mn.

1 is a SEM image of the positive electrode plate/solid electrolyte interface produced in Example 6. 1 is a TEM image of the positive electrode plate/solid electrolyte interface produced in Example 13.

All-solid-state secondary battery The all-solid-state secondary battery of the present invention includes a positive electrode, a negative electrode, and a LiOH.Li ₂ SO ₄ -based solid electrolyte. The positive electrode includes a positive electrode active material. The negative electrode includes a negative electrode active material. The LiOH.Li ₂ SO ₄ -based solid electrolyte is interposed between the positive electrode and the negative electrode, and also enters the voids of at least one of the positive electrode and the negative electrode. This all-solid-state secondary battery further includes an intermediate layer at the interface between at least one of the positive electrode active material and the negative electrode active material and the solid electrolyte in at least one of the positive electrode and the negative electrode into which the solid electrolyte has entered. This intermediate layer is composed of at least one oxide selected from the group consisting of Y, Nb, Ta, Al, Ti, La, Zr, W, Sn, Ce, and Mn, and/or a lithium composite oxide containing at least one oxide selected from the group consisting of Y, Nb, Ta, Al, Ti, La, Zr, W, Sn, Ce, and Mn and Li. In this way, in an all-solid-state secondary battery using a _{LiOH.Li2SO4 -} based solid electrolyte, the discharge capacity can be improved by providing an intermediate layer of a specific composition at the interface between the electrode active material and the _LiOH.Li2SO4 -based solid _electrolyte .

As mentioned above, all-solid-state lithium batteries that employ low-melting-point solid electrolytes such as _{LiOH.Li2SO4 -} based solid electrolytes are known (see, for example, Patent Document 2), and the solid electrolyte can be infiltrated as a melt into the gaps of the electrode plates to achieve interface contact. As a result, it is possible to improve the battery resistance and rate performance during charging and discharging, as well as the yield of battery production. In particular, _{LiOH.Li2SO4 -} based solid electrolytes such as _3LiOH.Li2SO4 exhibit high lithium ion conductivity, but when a cell was constructed using _{LiOH.Li2SO4 -} based solid electrolytes in a sintered _electrode and operated as a battery, it was found that the discharge amount was lower than the theoretical capacity expected from the amount of active material. The details of the cause are unknown, but it is presumed to be degradation of the solid electrolyte due to a reaction between the solid electrolyte and the active material. In other words, it is presumed that when the solid electrolyte is melted and penetrated into the gaps of the electrode plate, the _LiOH.Li2SO4 -based solid electrolyte, which is a strong _alkaline material, becomes in a molten state at high temperature, causing the components of the electrode plate to dissolve into the solid electrolyte, deteriorating the surface of the electrode plate and decreasing its activity. In this regard, according to the present invention, the above problem is solved by providing the intermediate layer at the interface between the electrode active material and the solid electrolyte, and the discharge capacity can be improved (compared to that without the intermediate layer).

(1) Positive electrode The positive electrode (typically a positive electrode plate) contains a positive electrode active material. The positive electrode active material may be a positive electrode active material generally used in lithium secondary batteries, but preferably contains a lithium composite oxide. The lithium composite oxide is an oxide represented by Li _x MO ₂ (0.05<x<1.10, M is at least one transition metal, and M typically contains one or more of Co, Ni, Mn, and Al). The lithium composite oxide preferably has a layered rock salt structure or a spinel structure. A more preferred positive electrode active material is a lithium composite oxide having a layered rock salt structure. Examples of lithium composite oxides having a layered rock salt structure include Li _x CoO ₂ (lithium cobalt oxide), Li _x NiO ₂ (lithium nickel oxide), Li _x MnO ₂ (lithium manganese oxide), Li _x NiMnO ₂ (lithium nickel manganese oxide), Li _x NiCoO ₂ (lithium nickel cobalt oxide), Li _x CoNiMnO ₂ (lithium cobalt nickel manganese oxide), Li _x CoMnO ₂ (lithium cobalt manganese oxide), Li ₂ MnO ₃ , and solid solutions of the above compounds. Particularly preferred are Li _x CoNiMnO ₂ (lithium cobalt nickel manganese oxide) and Li _x CoO ₂ (lithium cobalt oxide, typically LiCoO ₂ ). Particularly preferred lithium composite oxides having a layered rock salt structure are lithium cobalt nickel _manganese oxide (e.g., Li( _{Ni0.5Co0.2Mn0.3} ) _O2 ) or lithium cobalt oxide (typically _LiCoO2 ). On the other hand, examples _of lithium composite oxides having a spinel structure include _LiMn2O4 - _based materials and _{LiNi0.5Mn1.5O4 - based} materials.

The lithium composite oxide may contain one or more elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, Te, Ba, Bi, and W. In addition, LiMPO ₄ (wherein M is at least one selected from Fe, Co, Mn, and Ni) having an olivine structure can also be suitably used.

The positive electrode may be in the form of a molded mixture of a positive electrode active material, an electronic conductive agent, a lithium ion conductive material, a binder, etc., generally called a composite electrode, but is preferably in the form of a sintered plate obtained by sintering a positive electrode raw material powder. In other words, the positive electrode or the positive electrode active material is preferably in the form of a sintered plate. Since a sintered plate does not need to contain an electronic conductive agent or a binder, the energy density of the positive electrode can be increased. The sintered plate may be a dense body or a porous body, and the pores of the porous body may contain a solid electrolyte.

The positive electrode active material or its sintered plate preferably has a density of 50 to 80% by volume, more preferably 55 to 80% by volume, even more preferably 60 to 80% by volume, and particularly preferably 65 to 75% by volume. With a density within this range, the voids in the positive electrode active material can be sufficiently filled with solid electrolyte via an intermediate layer, and the proportion of positive electrode active material in the positive electrode increases, making it possible to achieve a high energy density as a battery.

From the viewpoint of improving the energy density of the battery, the thickness of the positive electrode active material or its sintered plate is preferably 50 to 350 μm, more preferably 75 to 350 μm, even more preferably 75 to 325 μm, even more preferably 100 to 325 μm, particularly preferably 100 to 300 μm, especially more preferably 125 to 300 μm, especially even more preferably 150 to 275 μm, and most preferably 100 to 275 μm.

(2) Negative electrode The negative electrode (typically a negative plate) contains a negative electrode active material. As the negative electrode active material, a negative electrode active material generally used in lithium secondary batteries can be used. Examples of such general negative electrode active materials include carbon-based materials, metals or semimetals such as Li, In, Al, Sn, Sb, Bi, and Si, or alloys containing any of these. In addition, oxide-based negative electrode active materials may be used.

Particularly preferred negative electrode active materials include materials capable of inserting and desorbing lithium ions at 0.4 V (vs. Li/Li ⁺ ) or more, and preferably contain Ti. A negative electrode active material that satisfies such conditions is preferably an oxide containing at least Ti. Preferred examples of such negative electrode active materials include lithium titanate Li ₄ Ti ₅ O ₁₂ (hereinafter, LTO), niobium titanium composite oxide Nb ₂ TiO ₇ , and titanium oxide TiO ₂ , more preferably LTO and Nb ₂ TiO ₇ , and even more preferably LTO. Note that LTO is typically known to have a spinel structure, but other structures can also be adopted during charging and discharging. For example, the reaction of LTO proceeds in the coexistence of two phases of Li ₄ Ti ₅ O ₁₂ (spinel structure) and Li ₇ Ti ₅ O ₁₂ (rock salt structure) during charging and discharging. Therefore, LTO is not limited to a spinel structure.

The negative electrode may be in the form of a molded mixture of a negative electrode active material, an electronic conductive assistant, a lithium ion conductive material, a binder, etc., generally called a composite electrode, but is preferably in the form of a sintered plate obtained by sintering a negative electrode raw material powder. In other words, the negative electrode or the negative electrode active material is preferably in the form of a sintered plate. Since a sintered plate does not need to contain an electronic conductive assistant or a binder, the energy density of the negative electrode can be increased. The sintered plate may be a dense body or a porous body, and the pores of the porous body may contain a solid electrolyte.

The negative electrode active material or its sintered plate preferably has a density of 55 to 80% by volume, more preferably 60 to 80%, and even more preferably 65 to 75%. With a density within this range, the voids in the negative electrode active material can be sufficiently filled with solid electrolyte via an intermediate layer, and the proportion of negative electrode active material in the negative electrode increases, making it possible to achieve a high energy density as a battery.

From the viewpoint of improving the energy density of the battery, the thickness of the negative electrode active material or its sintered plate is preferably 75 to 350 μm, more preferably 100 to 325 μm, even more preferably 125 to 300 μm, and particularly preferably 150 to 275 μm.

(3) Solid electrolyte The solid electrolyte is a _LiOH.Li2SO4 -based solid electrolyte. The _{LiOH.Li2SO4 -} based solid electrolyte is a composite compound of LiOH and _Li2SO4 , and has _a typical composition of the general formula: _{xLiOH.yLi2SO4} (wherein x+ _y =1, _0.6 ≦x≦0.95 ₎ , and a representative example is _3LiOH.Li2SO4 (wherein x=0.75, y=0.25 in the general formula ₎ . Preferably, the _LiOH.Li2SO4 -based solid electrolyte includes a solid _electrolyte identified as _3LiOH.Li2SO4 by X-ray diffraction. This preferred solid electrolyte contains _3LiOH.Li2SO4 as the _main phase. Whether or not 3LiOH.Li ₂ SO ₄ is contained in the solid electrolyte can be confirmed by identifying the X-ray diffraction pattern using 032-0598 of the ICDD database. Here, "3LiOH.Li ₂ SO ₄ " refers to a substance whose crystal structure can be regarded as being the same as 3LiOH.Li ₂ SO ₄ , and the crystal composition does not necessarily have to be the same as 3LiOH.Li ₂ SO _4. In other words, as long as it has a crystal structure equivalent to 3LiOH.Li ₂ SO ₄ , those whose composition is outside of LiOH:Li ₂ SO ₄ = 3:1 are also included in the solid electrolyte of the present invention. Therefore, even if the solid electrolyte contains a dopant such as boron (for example, 3LiOH.Li ₂ SO ₄ in which boron is dissolved and the X-ray diffraction peak is shifted to the higher angle side), it will be referred to as 3LiOH.Li ₂ SO ₄ in this specification as long as the crystal structure can be considered to be the same as 3LiOH.Li ₂ SO _4. Similarly, the solid electrolyte used in the present invention is also allowed to contain inevitable impurities.

Therefore, the LiOH.Li ₂ SO ₄ solid electrolyte may contain a heterogeneous phase in addition to the main phase 3LiOH.Li ₂ SO _4. The heterogeneous phase may contain a plurality of elements selected from Li, O, H, S, and B, or may be composed of only a plurality of elements selected from Li, O, H, S, and B. Examples of the heterogeneous phase include LiOH, Li ₂ SO ₄ , and/or Li ₃ BO ₃ derived from the raw material. These heterogeneous phases are considered to be unreacted raw materials remaining when 3LiOH.Li ₂ SO ₄ is formed, but since they do not contribute to lithium ion conduction, it is desirable that the amount of other than Li ₃ BO ₃ is small. For example, the LiOH.Li ₂ SO ₄ solid electrolyte may contain LiOH and/or Li ₂ SO ₄ as a heterogeneous phase such that the molar ratio of LiOH/Li ₂ SO ₄ in the overall composition including 3LiOH.Li ₂ SO ₄ is typically in the range of 1.8 to 3.0, more typically 2.0 to 2.6. However, a heterogeneous phase containing boron such as Li ₃ BO ₃ may be contained in a desired amount since it can contribute to improving the lithium ion conductivity retention after long-term storage at high temperature. However, the solid electrolyte may be composed of a single phase of 3LiOH.Li ₂ SO ₄ in which boron is dissolved.

It is preferable that the LiOH.Li ₂ SO ₄ solid electrolyte (particularly 3LiOH.Li ₂ SO ₄ ) further contains boron. By further containing boron in the solid electrolyte identified as 3LiOH.Li ₂ SO ₄ , the decrease in lithium ion conductivity can be significantly suppressed even after long-term storage at high temperatures. It is presumed that boron is incorporated into one of the sites of the crystal structure of 3LiOH.Li ₂ SO ₄ , improving the stability of the crystal structure against temperature. The molar ratio (B/S) of boron B to sulfur S contained in the solid electrolyte is preferably more than 0.002 and less than 1.0, more preferably 0.003 or more and 0.9 or less, and even more preferably 0.005 or more and 0.8 or less. If B/S is within the above range, it is possible to improve the maintenance rate of lithium ion conductivity. In addition, if B/S is within the above range, the content of unreacted heterogeneous phases containing boron is low, so that the absolute value of lithium ion conductivity can be increased.

The LiOH.Li ₂ SO ₄ based solid electrolyte may be a compact of a powder obtained by pulverizing a molten solid, but is preferably a molten solid (i.e., solidified after being heated and melted).

The LiOH.Li ₂ SO ₄ solid electrolyte is melted and enters the voids in the positive electrode (positive electrode active material) and/or negative electrode (negative electrode active material), but the remaining part is preferably interposed between the positive electrode and the negative electrode as a solid electrolyte layer. The thickness of the solid electrolyte layer (excluding the part that has entered the voids in the positive electrode and negative electrode) is preferably 1 to 500 μm, more preferably 3 to 50 μm, and even more preferably 5 to 40 μm, from the viewpoint of charge/discharge rate characteristics and insulating properties of the solid electrolyte.

(4) Intermediate layer The intermediate layer is provided at the interface between at least one of the positive electrode active material and the negative electrode active material and the solid electrolyte. The intermediate layer is preferably present at the interface between the positive electrode active material and the solid electrolyte, but may be present at the interface between the negative electrode active material and the solid electrolyte. The intermediate layer may be present at both the interface between the positive electrode active material and the solid electrolyte and the interface between the negative electrode active material and the solid electrolyte. The thickness of the intermediate layer is not particularly limited as long as the desired discharge capacity improvement effect is obtained, but is preferably 0.001 to 1 μm, more preferably 0.005 to 0.2 μm, and even more preferably 0.01 to 0.1 μm.

The intermediate layer is composed of at least one oxide selected from the group consisting of Y, Nb, Ta, Al, Ti, La, Zr, W, Sn, Ce, and Mn, and/or a lithium composite oxide containing Li and at least one selected from the group consisting of Y, Nb, Ta, Al, Ti, La, Zr, W, Sn, Ce, and Mn. Preferred examples of such oxides and/or lithium composite oxides include Y oxide (typically Y ₂ O ₃ ), Li and Nb oxide (typically LiNbO ₃ or LiNb ₃ O ₈ ), Li and Ta oxide (typically LiTaO ₃ ), Li and Al oxide (typically LiAlO ₂ ), and Li and Y oxide (typically LiYO ₂ ), Li and Ti oxide (typically Li ₂ TiO ₃ ), Li, La and Zr or Li, La, Zr and Al oxide (typically Li _7-3x Al _x La ₃ Zr ₂ O ₁₂ (0≦x<0.4, more typically 0.02<x<0.4), Li, La and Ti oxide (typically Li _0.33 La _0.55 TiO ₃ ), Li and W oxide (typically Li ₂ Examples of the oxides include oxides of Li and Sn (typically LiSnO ₃ ), oxides of Li and Ce (typically Li ₈ CeO ₆ ), oxides of Li, La and Nb (typically Li ₅ La ₃ Nb ₂ O ₁₂ ), and oxides of Li and Mn (typically LiMnO ₂ ), and any combination thereof, and more preferably oxides of _Li and Nb (typically LiNbO ₃ ), oxides of Li and Al (typically LiAlO ₂ ), and oxides of Li and Y (typically LiYO ₂ ), oxides of Li and Ti (typically Li ₂ TiO ₃ ), oxides of Li, La, Zr and Al (typically Li _6.7 Al _0.1 La ₃ Zr ₂ O ₁₂ ), oxides of Li, La and Ti (typically Li _0.33 La _0.55 TiO ₃ ).

The intermediate layer can be formed by mixing metal salts such as metal alkoxides or nitrates of one or more metal elements constituting the intermediate layer with alcohol such as ethanol or water in a predetermined molar ratio to prepare a solution, immersing the electrode active material (preferably a sintered plate) in this solution and allowing it to penetrate into the inside under reduced pressure, then removing it, wiping it off, and leaving it to stand in the air to hydrolyze the alkoxide or dry the solvent. It is preferable to repeat the above-mentioned process from immersion to leaving it to stand in the air multiple times (for example, 1 to 20 times). It is preferable to heat treat the electrode active material (preferably a sintered plate) on which the intermediate layer has been formed in this way at 400 to 700 ° C for 5 to 60 minutes. When a metal alkoxide is used, the process from preparing the solution to wiping off the solution is preferably performed in a glove box in an Ar atmosphere with a dew point of -50 ° C or less so that the solution does not deteriorate due to hydrolysis, etc.

(5) Manufacturing of all-solid-state secondary battery The manufacturing of an all-solid-state secondary battery can be performed, for example, by i) preparing a positive electrode (with an intermediate layer or a current collector formed as necessary) and a negative electrode (with an intermediate layer or a current collector formed as necessary), and ii) sandwiching a solid electrolyte between the positive electrode and the negative electrode and applying pressure, heat, or the like to integrate the positive electrode, the solid electrolyte, and the negative electrode. The positive electrode, the solid electrolyte, and the negative electrode may be bonded by other methods. In this case, examples of the method for forming a solid electrolyte between the positive electrode and the negative electrode include a method of placing a molded body or powder of the solid electrolyte on one electrode, a method of applying a paste of the solid electrolyte powder on the electrode by screen printing, a method of impacting and solidifying the powder of the solid electrolyte by an aerosol deposition method or the like using the electrode as a substrate, and a method of depositing the solid electrolyte powder on the electrode by electrophoresis to form a film.

The present invention will be described more specifically with reference to the following examples. In the following description, Li _{( Ni0.5Co0.2Mn0.3} ) _O2 is abbreviated as "NCM", _Li4Ti5O12 is abbreviated as "LTO", and _LiCoO2 is abbreviated as _{" LCO "} .

Example 1
(1) Preparation of Positive Electrode Plate (1a) Preparation of NCM Green Sheet Commercially available ( _{Ni0.5Co0.2Mn0.3} )(OH) ₂ powder (average particle size 9 μm) and _Li2CO3 powder (average particle size 3 μm) were mixed so that the molar ratio of Li/(Ni ₊ Co _{+ Mn} ) was 1.30, and then held at 750 ° C for 15 hours to obtain a powder consisting of NCM particles. This powder was pulverized to adjust the average particle size to about 5 μm, and then this powder was mixed with a solvent for tape casting, a binder, a plasticizer, and a dispersant. After adjusting the viscosity of the obtained paste, the paste was tape-cast on a film to produce an NCM green sheet. The thickness of the NCM green sheet was adjusted so that the thickness after firing was 100 μm.

(1b) Preparation of NCM sintered plate The NCM green sheet was degreased by holding it at 450°C for 6 hours, and then heated to 870°C at a heating rate of 200°C/h and held for 10 hours to perform sintering. In this way, the NCM sintered plate was obtained as a positive electrode plate. A Au film (thickness 100 nm) was formed as a current collecting layer by sputtering on one side of the obtained NCM sintered plate.

(1c) Formation of intermediate layer Niobium ethoxide: lithium ethoxide: ethanol were mixed in a molar ratio of 0.03:0.03:1 to prepare a solution for forming the intermediate layer. The NCM sintered plate prepared in (1b) above was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. The above-mentioned operation was performed in a glove box in an Ar atmosphere with a dew point of -50 ° C or less. Then, the NCM sintered plate was taken out of the glove box and left to stand in the air for 10 minutes to form the intermediate layer. Then, the above series of operations was repeated once more (i.e., a total of two films were formed). Finally, the NCM sintered plate was heat-treated at 400 ° C for 30 minutes to obtain a positive electrode plate with an intermediate layer formed thereon.

(2) Preparation of negative electrode plate (2a) Preparation of LTO green sheet Commercially available _TiO2 powder (average particle size 1 μm or less) and _Li2CO3 powder (average particle size ₃ μm) were mixed so that the molar ratio of Li/Ti was 0.84, and then held at 1000 ° C for 2 hours to obtain a powder consisting of LTO particles. This powder was pulverized to adjust the average particle size to about 2 μm, and then mixed with a solvent for tape casting, a binder, a plasticizer, and a dispersant. After adjusting the viscosity of the obtained paste, the paste was tape-cast on a film to produce an LTO green sheet. The thickness of the LTO green sheet was adjusted so that the thickness after firing was 100 μm.

(2b) Preparation of LTO sintered plate The LTO green sheet was degreased by holding it at 450 ° C for 6 hours, and then heated to 1000 ° C at a heating rate of 200 ° C / h and held for 2 hours to perform sintering. In this way, the LTO sintered plate was obtained as a negative electrode plate. A Au film (thickness 100 nm) was formed as a current collecting layer by sputtering on one side of the obtained LTO sintered plate.

(3) Preparation of solid electrolyte (3a) Preparation of raw material _{powder Li2SO4} powder (commercial product, purity 99% or more), LiOH powder (commercial product, purity 98% or more), and _Li3BO3 (commercial product, purity 99% or more ₎ were mixed to obtain _a raw material mixed powder in a molar ratio _{of Li2SO4} :LiOH: _Li3BO3 = 1:3:0.05. These powders were handled in a glove box in an Ar atmosphere with a dew point of -50°C or less, and sufficient care was taken to prevent deterioration such as moisture absorption.

(3b) Melt synthesis The raw material mixed powder was put into a crucible in an Ar atmosphere. The crucible was set in an electric furnace and heat-treated at 430° C. for 2 hours to prepare a melt. The melt was then cooled in the electric furnace at 100° C./h to form a solidified product.

(3c) Mortar Crushing The obtained coagulated product was pulverized in a mortar in an Ar atmosphere to obtain a solid electrolyte powder having an average particle size D50 of 5 to 50 μm.

(4) Preparation of all-solid-state battery A solid electrolyte powder containing 5% by weight of 30 μm diameter _ZrO2 beads was placed on the positive electrode plate, and a negative electrode plate was placed on top of the solid electrolyte powder. A weight was placed on the negative electrode plate, and the plate was heated at 400 ° C for 45 minutes in an electric furnace. At this time, the solid electrolyte powder melted, and after subsequent solidification, a solid electrolyte layer was formed between the electrode plates. A battery was prepared using the cell consisting of the obtained positive electrode plate / solid electrolyte / negative electrode plate.

(5) Evaluation (5a) Analysis of Electrode Plate/Solid Electrolyte Interface The battery prepared in (4) above was disassembled in a glove box, and EDX analysis was performed using a SEM and a TEM on the interface between the electrode plate on which the intermediate layer was formed and the solid electrolyte.

(5b) Measurement of Compactness The compactness (volume %) of the positive electrode plate (NCM sintered plate not including intermediate layer or solid electrolyte) prepared in (1b) above and the negative electrode plate (LTO sintered plate not including intermediate layer or solid electrolyte) prepared in (2b) above was measured as follows. First, the positive electrode plate (or negative electrode plate) was filled with resin, the cross section was polished by ion milling, and the polished cross section was observed with a SEM to obtain a cross-sectional SEM image. The SEM image was an image with a magnification of 1000 times. The obtained image was first subjected to a 100% blurring process with a 2D filter using image analysis software (Image-Pro Premier manufactured by Media Cybernetics), and then subjected to binarization processing. The ratio (%) of the area of the positive electrode active material (or negative electrode active material) to the total area of the positive electrode active material (or negative electrode active material) part and the resin-filled part (part that was originally a void) in the positive electrode plate (or negative electrode plate) was calculated to determine the density of the positive electrode active material (or negative electrode active material). The threshold value for binarization was set using Otsu's binarization as a discriminant analysis method.

(5c) Charge/Discharge Evaluation The discharge capacity of the battery prepared in (4) above at an operating temperature of 150° C. was measured in the voltage range of 2.7 V to 1.5 V by the following procedure. The battery was charged at a constant current until the battery voltage reached the upper limit of the voltage range, and then charged at a constant voltage until the current value reached a 0.01 C rate, and then discharged to the lower limit of the voltage range. The discharge capacity was calculated as a relative value when the discharge capacity of a battery (comparative example) having the same configuration except that no intermediate layer was formed was taken as 100.

Example 2
A battery was produced and evaluated in the same manner as in Example 1, except that the number of times of film formation in the above (1c) intermediate layer formation was set to 5 in total.

Example 3
A battery was produced and evaluated in the same manner as in Example 1, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium nitrate, yttrium nitrate, and water (solvent) in a molar ratio of 0.015:0.015:1. The NCM sintered plate was immersed in this solution, and after decompression, the NCM sintered plate was removed from the solution. Then, the plate was heat-treated at 400°C for 30 minutes to form an intermediate layer. The above process was then repeated four more times (i.e., a total of five film formations were performed). Finally, the NCM sintered plate was heat-treated at 700°C for 30 minutes to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 4
A battery was produced and evaluated in the same manner as in Example 1, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing yttrium nitrate and water (solvent) in a molar ratio of 0.015:1. The NCM sintered plate was immersed in this solution, and after decompression, the NCM sintered plate was removed from the solution. Then, the NCM sintered plate was heat-treated at 400°C for 30 minutes to form an intermediate layer. Then, the above operation was repeated four more times (i.e., a total of five film formations were performed). Finally, the NCM sintered plate was heat-treated at 700°C for 30 minutes to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 5
A battery was produced and evaluated in the same manner as in Example 1, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium ethoxide: aluminum butoxide: ethanol (solvent) in a molar ratio of 0.015:0.015:1. The NCM sintered plate was immersed in this solution, and after decompression, the NCM sintered plate was removed from the solution. The above-mentioned operation was performed in a glove box in an Ar atmosphere with a dew point of -50°C or less. The NCM sintered plate was then removed from the glove box and left to stand in the air for 10 minutes to form an intermediate layer. The above series of operations was then repeated nine more times (i.e., a total of 10 film formations were performed). Finally, the NCM sintered plate was heat-treated at 700°C for 30 minutes to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 6
A battery was produced and evaluated in the same manner as in Example 1, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium ethoxide, tantalum ethoxide, and ethanol (solvent) in a molar ratio of 0.03:0.03:1. The NCM sintered plate was immersed in this solution, and after decompression, the NCM sintered plate was removed from the solution. The above-mentioned operation was performed in a glove box in an Ar atmosphere with a dew point of -50°C or less. The NCM sintered plate was then removed from the glove box and left to stand in the air for 10 minutes to form an intermediate layer. The above series of operations was then repeated four more times (i.e., a total of five film formations were performed). Finally, the NCM sintered plate was heat-treated at 500°C for 30 minutes to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 7 (Comparison)
Except for not forming the intermediate layer in (1c) above, a battery was produced and evaluated in the same manner as in Example 1. The discharge capacity measured in this example was set as the reference value 100 for calculating the relative values of the discharge capacities in Examples 1 to 6.

Example 8
A battery was produced and evaluated in the same manner as in Example 1, except that the preparation of the NCM green sheet in (1a) above and the preparation of the NCM sintered plate in (1b) above were carried out as follows.

(Preparation of NCM Green Sheet)
Commercially available ( _{Ni0.5Co0.2Mn0.3} )(OH) ₂ powder (average particle size 9 μm) and _Li2CO3 powder (average particle size ₃ μm) were mixed so that the molar ratio of Li/(Ni+Co ₊ Mn) was _1.15 , and then held at 750 ° C for 10 hours to obtain a powder consisting of NCM particles. This powder was pulverized to adjust the average particle size to about 5 μm, and then mixed with a solvent for tape casting, a binder, a plasticizer, and a dispersant. After adjusting the viscosity of the obtained paste, the paste was tape-cast on a film to produce an NCM green sheet. The thickness of the NCM green sheet was adjusted so that the thickness after firing was 100 μm.

(Preparation of NCM sintered plate)
The NCM green sheet was degreased by holding it at 450°C for 6 hours, and then heated to 920°C at a heating rate of 200°C/h and held for 10 hours to sinter it. In this way, an NCM sintered plate was obtained as a positive electrode plate. A Au film (thickness 100 nm) was formed as a current collecting layer on one side of the obtained NCM sintered plate by sputtering.

Example 9 (Comparison)
Except for not forming the intermediate layer in the above (1c), a battery was produced and evaluated in the same manner as in Example 8. The discharge capacity measured in this example was set as the reference value 100 for calculating the relative values of the discharge capacities in Examples 8 and 10.

Example 10 (Comparative)
A battery was produced and evaluated in the same manner as in Example 8, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium ethoxide: tetraethoxysilane: ethanol (solvent) in a molar ratio of 0.030: 0.015: 1. The NCM sintered plate was immersed in this solution, and after decompression, the NCM sintered plate was removed from the solution. The work from preparation of the solution to wiping off the solution was carried out in a glove box in an Ar atmosphere with a dew point of -50 ° C or less. The NCM sintered plate was then removed from the glove box and left to stand in the air for 10 minutes to form an intermediate layer. The above series of operations was then repeated three more times (i.e., a total of four film formations were performed). Finally, the NCM sintered plate was heat-treated at 400 ° C for 30 minutes to obtain a positive electrode plate on which an intermediate layer was formed.

Example 11
A battery was produced and evaluated in the same manner as in Example 8, except that the raw material powder in (3a) above was prepared as follows.

(Preparation of raw powder)
_{Li2SO4 powder} (commercial product, purity 99% or more), LiOH powder (commercial product, purity 98% or more), and _Li3BO3 (commercial product, purity 99% or more) were mixed in _a molar ratio of _Li2SO4 : _{LiOH : Li3BO3} = 1:1.8:0.05 to obtain a raw material mixed powder. These powders were handled in a glove box in an Ar atmosphere with a dew point of -50°C or less, with great care taken to prevent deterioration such as moisture absorption.

Example 12 (Comparative)
Except for not forming the intermediate layer in the above (1c), a battery was produced and evaluated in the same manner as in Example 11. The discharge capacity measured in this example was set as the reference value 100 for calculating the relative value of the discharge capacity in Example 11.

Example 13
(1) Preparation of Positive Electrode Plate (1a) Preparation of LCO Green Sheet Commercially available _Co3O4 powder (average particle size 0.9 μm) and commercially available _{Li2CO3 powder} (average particle size 3 μm) were mixed so that the molar ratio of Li _/ Co was 1.02, and then held at 750 ° C for 5 hours. The obtained powder was pulverized in a pot mill so that the average particle size was 0.4 μm to obtain LCO powder. The obtained LCO powder was mixed with a dispersion medium, a binder, a plasticizer, and a dispersant. The viscosity of the obtained mixture was adjusted to prepare an LCO slurry. The slurry thus prepared was tape-cast on a film to form an LCO green sheet. The thickness of the LCO green sheet was set to a value such that the thickness after firing was 60 μm.

(1b) Preparation of _{Li2CO3 Green} Sheet Commercially available _Li2CO3 raw material powder (average particle size _{3 μm} ), a dispersion medium, a binder, a plasticizer, and a dispersant were mixed. The resulting mixture was adjusted in viscosity to prepare a _Li2CO3 slurry. The _Li2CO3 slurry thus prepared was tape-cast onto a film to form a _{Li2CO3 green} sheet. The thickness of _{the Li2CO3 green} sheet after drying was set so that the Li/Co ratio, which is the molar ratio of the Li content in the _Li2CO3 green sheet to the Co content in the LCO _green sheet, could be 0.2.

(1c) Preparation of LCO sintered plate The LCO green sheet peeled off from the PET film was heated to 600°C at a heating rate of 200°C/h, degreased for 3 hours, and then calcined at 900°C for 3 hours. The dried _Li2CO3 green sheet was cut out to a size such that the Li/Co ratio, which is the molar ratio of the Li content in the _{Li2CO3 green} sheet to the Co content in the obtained LCO calcined plate, was 0.1. The cut out _{Li2CO3 green} sheet piece was placed on the _LCO calcined plate. The sintered plate and the green sheet piece were stacked, and the stack was heated to 600°C at a heating rate of 200°C/h and degreased for 3 hours, and then heated to 800°C at 200°C/h and held for 5 hours, and then heated to 850°C at 200°C/h and held for 24 hours to perform sintering. In this way, the LCO sintered plate was obtained as a positive electrode plate. On the obtained sintered LCO plate, an Au film (thickness: 100 nm) was formed as a current collecting layer by sputtering.

(1d) Formation of intermediate layer Niobium ethoxide: lithium ethoxide: ethanol were mixed in a molar ratio of 0.015:0.015:1 to prepare a solution for forming an intermediate layer. The LCO sintered plate prepared in (1b) above was immersed in this solution, and after depressurization, the LCO sintered plate was removed from the solution. The above-mentioned operation was performed in a glove box in an Ar atmosphere with a dew point of -50°C or less. Then, the LCO sintered plate was removed from the glove box and left to stand in the air for 10 minutes to form an intermediate layer. Then, the above series of operations was repeated two more times (i.e., a total of three film formations were performed). Finally, the LCO sintered plate was heat-treated at 400°C for 30 minutes to obtain a positive electrode plate on which an intermediate layer was formed.

(2) Preparation of negative electrode plate (2a) Preparation of LTO green sheet Commercially available LTO powder (average particle size 0.7 μm), a dispersion medium, a binder, a plasticizer, and a dispersant were mixed. The viscosity of the obtained negative electrode raw material mixture was adjusted to prepare an LTO slurry. The slurry thus prepared was tape-cast onto a film to form an LTO green sheet. The thickness of the LTO green sheet after drying was set to a value such that the thickness after firing was 60 μm.

(2b) Preparation of LTO sintered plate The obtained green sheet was held at 500° C. for 5 hours, then heated at a heating rate of 200° C./h, and sintered at 800° C. for 5 hours. A Au film (thickness 100 nm) was formed as a current collecting layer on the obtained LTO sintered body by sputtering.

(3) Preparation of solid electrolyte (3a) Preparation of raw material _{powder Li2SO4} powder (commercial product, purity 99% or more) and LiOH powder (commercial product, purity 98% or more) were mixed in a molar ratio of _Li2SO4 : _LiOH = 1:3 to obtain a raw material mixed powder. These powders were handled in a glove box in an Ar atmosphere with a dew point of -50°C or less, and sufficient care was taken to prevent deterioration such as moisture absorption.

(3b) Melt synthesis The raw material mixed powder was placed in a crucible in an Ar atmosphere, and the crucible was set in an electric furnace and heat-treated at 430° C. for 2 hours to produce a melt. The melt was then cooled in the electric furnace at 100° C./h to form a solidified product.

(3c) Mortar Crushing The obtained coagulated product was pulverized in a mortar in an Ar atmosphere to obtain a solid electrolyte powder having an average particle size D50 of 5 to 50 μm.

(4) Preparation of All-Solid-State Battery A battery was prepared in the same manner as in Example 1(4).

(5) Evaluation Analysis of the electrode plate/solid electrolyte interface and evaluation of charge/discharge were performed in the same manner as in Example 1(5).

Example 14
A battery was produced and evaluated in the same manner as in Example 13, except that after the LTO sintered plate was produced in the above (2b), an intermediate layer was formed on the LTO sintered plate in the following manner.

(Deposition of intermediate layer)
Niobium ethoxide: lithium ethoxide: ethanol were mixed in a molar ratio of 0.015:0.015:1 to prepare a solution for forming an intermediate layer. The LTO sintered plate prepared in (2b) of Example 13 was immersed in this solution, and after depressurization, the LTO sintered plate was removed from the solution. The above-mentioned operation was performed in a glove box in an Ar atmosphere with a dew point of -50°C or less. The LTO sintered plate was then removed from the glove box and left to stand in the air for 10 minutes to form an intermediate layer. The above series of operations was then repeated two more times (i.e., a total of three film formations were performed). Finally, the LTO sintered plate was heat-treated at 400°C for 30 minutes to obtain a negative electrode plate on which an intermediate layer was formed.

Example 15 (Comparative)
Except for not forming the intermediate layer in the above (1c), a battery was produced and evaluated in the same manner as in Example 13. The discharge capacity measured in this example was set as the reference value 100 for calculating the relative values of the discharge capacities in Examples 13 and 14.

Example 16
(1) Preparation of positive electrode plate (1a) Preparation of NCM green sheet Commercially available ( _{Ni0.5Co0.2Mn0.3} )(OH) ₂ powder (average particle size 9 μm) and _Li2CO3 powder (average particle size 3 μm) were mixed so that the molar ratio of Li/(Ni ₊ Co _{+ Mn} ) was 1.15, and then held at 750 ° C for 10 hours to obtain a powder consisting of NCM particles. This powder was pulverized to adjust the average particle size to about 5 μm, and then this powder was mixed with a solvent for tape casting, a binder, a plasticizer, and a dispersant. After adjusting the viscosity of the obtained paste, the paste was tape-cast on a film to produce an NCM green sheet. The thickness of the NCM green sheet was adjusted so that the thickness after firing was 100 μm.

(1b) Preparation of NCM sintered plate The NCM green sheet was degreased by holding it at 450°C for 6 hours, and then heated to 920°C at a heating rate of 200°C/h and held for 10 hours to perform sintering. In this way, the NCM sintered plate was obtained as a positive electrode plate. A Au film (thickness 100 nm) was formed as a current collecting layer by sputtering on one side of the obtained NCM sintered plate.

(1c) Formation of intermediate layer Titanium tetraisopropoxide: lithium ethoxide: ethanol were mixed in a molar ratio of 0.0225:0.05:1 to prepare a solution for forming an intermediate layer. The NCM sintered plate prepared in (1b) above was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. The above-mentioned operation was performed in an atmosphere with a dew point of -30°C or less. Then, the NCM sintered plate was left to stand in the air for 5 minutes to form an intermediate layer. Then, the above series of operations was repeated 7 more times (i.e., a total of 8 times were formed). Finally, the NCM sintered plate was heat-treated at 400°C for 30 minutes to obtain a positive electrode plate with an intermediate layer formed thereon.

(2) Preparation of negative electrode plate (2a) Preparation of LTO green sheet Commercially available _TiO2 powder (average particle size 1 μm or less) and _Li2CO3 powder (average particle size ₃ μm) weighed so that the molar ratio of Li/Ti was 0.84 were mixed, and then held at 1000 ° C for 2 hours to obtain a powder consisting of LTO particles. This powder was pulverized to adjust the average particle size to about 2 μm, and then mixed with a solvent for tape casting, a binder, a plasticizer, and a dispersant. After adjusting the viscosity of the obtained paste, the paste was tape-cast on a film to produce an LTO green sheet. The thickness of the LTO green sheet was adjusted so that the thickness after firing was 200 μm.

(2b) Preparation of LTO sintered plate The LTO green sheet was degreased by holding it at 450 ° C for 6 hours, and then heated to 850 ° C at a heating rate of 200 ° C / h and held for 2 hours to perform sintering. In this way, the LTO sintered plate was obtained as a negative electrode plate. A Au film (thickness 100 nm) was formed as a current collecting layer by sputtering on one side of the obtained LTO sintered plate.

(3) Preparation of solid electrolyte (3a) Preparation of raw material _{powder Li2SO4} powder (commercial product, purity 99% or more), LiOH powder (commercial product, purity 98% or more), and _Li3BO3 (commercial _product , purity 99% or more ₎ were mixed to obtain a raw material mixed powder with _a molar ratio of _Li2SO4 :LiOH: _Li3BO3 = 1:2.6:0.05. These powders were handled in a glove box in an Ar atmosphere with a dew point of -50°C or less, and sufficient care was taken to prevent deterioration such as moisture absorption.

(3b) Melt synthesis The raw material mixed powder was put into a crucible in an Ar atmosphere. The crucible was set in an electric furnace and heat-treated at 430° C. for 2 hours to prepare a melt. The melt was then cooled in the electric furnace at 100° C./h to form a solidified product.

(3c) Mortar Crushing The obtained coagulated product was pulverized in a mortar in an Ar atmosphere to obtain a solid electrolyte powder having an average particle size D50 of 5 to 50 μm.

(4) Preparation of All-Solid-State Battery A battery was prepared in the same manner as in Example 1(4).

(5) Evaluation Analysis of the electrode plate/solid electrolyte interface and evaluation of charge/discharge were performed in the same manner as in Example 1(5).

Example 17
A battery was produced and evaluated in the same manner as in Example 16, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
Lithium ethoxide: aluminum butoxide: lanthanum nitrate (anhydrous): zirconium tetra-n-butoxide: 2-ethoxyethanol were mixed in a molar ratio of 0.000335: 0.000005: 0.00015: 0.0001: 1 to prepare a solution for forming an intermediate layer. An NCM sintered plate was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. The above-mentioned operation was performed in an atmosphere with a dew point of -30°C or less. Then, the intermediate layer was formed by heat treatment at 700°C for 30 minutes. Then, the above operation was repeated once more (i.e., the film was formed twice in total). A positive electrode plate with an intermediate layer formed thereon was obtained.

Example 18
A battery was produced and evaluated in the same manner as in Example 16, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium ethoxide, lanthanum nitrate (anhydrous), titanium tetraisopropoxide, and ethanol in a molar ratio of 0.00099:0.00165:0.003:1. An NCM sintered plate was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. The above-mentioned operation was performed in an atmosphere with a dew point of -30°C or less. Then, the intermediate layer was formed by heat treatment at 700°C for 30 minutes. The above operation was then repeated once more (i.e., a total of two film formations) to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 19
A battery was produced and evaluated in the same manner as in Example 16, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium hydroxide, tungsten (IV) oxide, and water in a molar ratio of 0.048:0.024:1. The NCM sintered plate was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. Then, the plate was heat-treated at 800°C for 30 minutes to form an intermediate layer. The above process was then repeated once more (i.e., a total of two film formations) to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 20
A battery was produced and evaluated in the same manner as in Example 16, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
Niobium ethoxide: lithium ethoxide: ethanol were mixed in a molar ratio of 0.0225:0.0225:1 to prepare a solution for forming an intermediate layer. The NCM sintered plate was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. The above-mentioned operation was performed in an atmosphere with a dew point of -30°C or less. Then, the NCM sintered plate was left to stand in the air for 5 minutes to form an intermediate layer. Then, the above series of operations was repeated another 7 times (i.e., a total of 8 times of film formation). Finally, the NCM sintered plate was heat-treated at 400°C for 30 minutes to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 21
A battery was produced and evaluated in the same manner as in Example 16, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium ethoxide: aluminum butoxide: ethanol (solvent) in a molar ratio of 0.0225:0.0225:1. The NCM sintered plate was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. The above-mentioned operation was performed in an atmosphere with a dew point of -30°C or less. The NCM sintered plate was then left to stand in the air for 5 minutes to form an intermediate layer. The above series of operations was then repeated seven more times (i.e., a total of eight film formations were performed). Finally, the NCM sintered plate was heat-treated at 400°C for 30 minutes to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 22
A battery was produced and evaluated in the same manner as in Example 16, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium ethoxide, tin isopropoxide, and ethanol (solvent) in a molar ratio of 0.015:0.015:1. The NCM sintered plate was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. The above-mentioned operation was performed in an atmosphere with a dew point of -30°C or less. The NCM sintered plate was then left to stand in the air for 5 minutes to form an intermediate layer. The above series of operations was then repeated seven more times (i.e., a total of eight film formations were performed). Finally, the NCM sintered plate was heat-treated at 600°C for 30 minutes to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 23
A battery was produced and evaluated in the same manner as in Example 16, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium nitrate, cerium nitrate, and water in a molar ratio of 0.008:0.001:1. The NCM sintered plate was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. Then, the plate was heat-treated at 800°C for 30 minutes to form an intermediate layer. The above process was then repeated once more (i.e., a total of two film formations were performed) to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 24
A battery was produced and evaluated in the same manner as in Example 16, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium ethoxide: lanthanum nitrate (anhydrous): niobium ethoxide: ethanol in a molar ratio of 0.00025:0.00015:0.0001:1. An NCM sintered plate was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. The above-mentioned operation was performed in an atmosphere with a dew point of -30°C or less. Then, the plate was heat-treated at 800°C for 30 minutes to form an intermediate layer. The above operation was then repeated once more (i.e., a total of two film formations were performed) to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 25
A battery was produced and evaluated in the same manner as in Example 16, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming an intermediate layer was prepared by mixing lithium nitrate, manganese nitrate, and water in a molar ratio of 0.006:0.006:1. An NCM sintered plate was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. Then, the plate was heat-treated at 400°C for 30 minutes to form an intermediate layer. The above process was then repeated once more (i.e., a total of two film formations were performed) to obtain a positive electrode plate with an intermediate layer formed thereon.

Example 26 (Comparative)
Except for not forming the intermediate layer in (1c) above, a battery was produced and evaluated in the same manner as in Example 16. The discharge capacity measured in this example was set as the reference value 100 for calculating the relative values of the discharge capacities in Examples 16 to 25.

Results The evaluation results of Examples 1 to 26 are explained below with reference to Tables 1 to 5 and FIGS.

(Analysis of the electrode plate/solid electrolyte interface)
At the interface between the electrode plate on which the intermediate layer was formed and the solid electrolyte in each of the cells in Examples 1 to 26, metals other than Li used as the intermediate layer and oxygen were detected by EDX analysis. Although Li cannot be detected by EDX, when the solution of the intermediate layer is evaporated to dryness and heat-treated at the same temperature as the intermediate layer formation, it can be confirmed by XRD evaluation that an oxide of each metal element (Example 4) or a lithium composite oxide (other than Example 4) is formed. From this, it is presumed that an intermediate layer consisting of an oxide of the element used in the intermediate layer (Example 4) or its lithium composite oxide (other than Example 4) is formed at the interface between the electrode plate and the solid electrolyte in each of Examples 1 to 15. SEM images of the positive electrode plate/solid electrolyte interface actually produced in Examples 6 and 13 are shown in Figures 1 and 2, respectively. In Figure 1 (Example 6), a bright area exists at the interface between the NCM particles and the solid electrolyte, and Ta is detected from this area, and it was found that an intermediate layer of oxides consisting of Li and Ta having a thickness of 0.1 to 1 μm was formed. In addition, in FIG. 2 (Example 13), a layer (between the arrows) exists at the interface between the LCO particle and the solid electrolyte (in the figure, 3LHS means 3LiOH·Li ₂ SO ₄ ), and Nb is detected in this portion, indicating that an intermediate layer of an oxide of Li and Nb with a thickness of 20 to 30 nm has been formed.

(Charge/Discharge Evaluation)
Table 1 shows the cell configurations and discharge capacities of Examples 1 to 7. From the results shown in Table 1, it was found that in Examples 1 to 6 in which an intermediate layer was formed on the positive electrode plate, the discharge capacity was improved compared to Example 7 (comparative example) in which no intermediate layer was formed. The mechanism by which the intermediate layer improves the discharge capacity is unclear, but it is thought that it may be due to the suppression of degradation of the solid electrolyte due to the reaction between the NCM and the solid electrolyte (reduction in conductivity), the suppression of the formation of a high resistance layer at the interface, etc. The above-mentioned phenomenon depends on the type of solid electrolyte and the type of electrode active material, and it was found that in the combination of LiOH.Li ₂ SO ₄ -based electrolyte and positive electrode active material, oxides of Li and Nb, oxides of Li and Y, oxides of Y, oxides of Li and Al, and oxides of Li and Ta are effective.

Table 2 shows the cell configuration and discharge capacity of Examples 8 to 10. In Examples 8 to 10, the density (volume %) of the active material was changed by changing the manufacturing method of the positive electrode plate. Table 2 also shows that Example 8, in which an oxide of Li and Nb was formed as the intermediate layer, had an improved discharge capacity compared to Example 9 (Comparative Example), in which no intermediate layer was formed. In other words, it was found that the intermediate layer has an effect even if the microstructure of the positive electrode plate changes. Furthermore, Example 10 (Comparative Example), in which an oxide of Li and Si was formed as the intermediate layer, had a lower discharge capacity compared to Example 8, indicating the presence of a suitable material as described above for the intermediate layer.

Table 3 shows the cell configuration and discharge capacity of Examples 11 and 12. In Examples 11 and ₁₂ , the molar ratio of LiOH: _Li2SO4 in the _LiOH.Li2SO4 -based solid electrolyte was changed. Table 3 also shows that Example 11, in which an oxide of Li and _Nb was formed as an intermediate layer, had an improved discharge capacity compared to Example 12 (comparative example) in which no intermediate layer was formed, and that the intermediate layer had an effect even if the composition of the _{LiOH.Li2SO4 -} based solid electrolyte was changed.

Table 4 shows the cell configurations and discharge capacities of Examples 13 to 15. In Examples 13 to 15, an LCO sintered plate was used as the positive electrode, and the LiOH.Li ₂ SO ₄ solid electrolyte was changed. Table 4 also shows that Example 13, in which an oxide of Li and Nb was formed as an intermediate layer, had an improved discharge capacity compared to Example 15 (Comparative Example), in which no intermediate layer was formed, and that the intermediate layer was effective even when LCO, which has a layered rock salt structure different from NCM, was used as the positive electrode plate. Example 14, in which an oxide of Li and Nb was formed as an intermediate layer on the negative electrode plate, also had an increased discharge capacity compared to Example 15 (Comparative Example), and that the intermediate layer was effective. From the above, it was found that forming an _intermediate layer of oxides of Li and Nb, oxides of Li and Y, oxides of Y, oxides of Li and Al, or oxides of Li and _Ta at the interface between various electrode active materials and the _LiOH.Li2SO4 -based solid electrolyte improves discharge characteristics, and that this effect is particularly remarkable at the interface between the positive electrode plate and the _LiOH.Li2SO4 -based solid electrolyte.

Table 5 shows the cell configurations and discharge capacities of Examples 16 to 26. In Examples 16 to 26, an NCM sintered plate was used as the positive electrode, and the molar ratio of LiOH: _Li2SO4 in the _LiOH.Li2SO4 -based solid electrolyte was _changed . Table 5 shows that Examples 16 to 25, in which various _lithium composite oxides were formed as intermediate layers, had improved discharge capacities compared to Example 26 (Comparative Example), in which no intermediate layer was formed. From the above, it was found that the discharge characteristics are improved by forming an _intermediate layer of oxides of Li and _Ti , oxides of Li, La and Zr or oxides of Li, La, Zr and Al, oxides of Li, La and Ti, oxides of Li and W, oxides of Li and Al, oxides of Li and Nb, oxides of Li and Sn, oxides of Li and Ce, oxides of Li, La and Nb, or oxides of Li and Mn at the interface between the positive electrode active material and the LiOH.Li2SO4-based solid electrolyte.

Claims (7)

a positive electrode including a positive electrode active material;
a negative electrode including a negative electrode active material;
A _{LiOH.Li2SO4 -} based solid electrolyte interposed between the positive electrode and the negative electrode and also penetrating into the voids of at least one of the positive electrode and the negative electrode;
The LiOH.Li2SO4 -based _solid electrolyte further contains boron _;
an all-solid-state secondary battery, wherein at least one of the positive electrode and the negative electrode into which the solid electrolyte has been permeated further comprises an intermediate layer at an interface between the solid electrolyte and at least one of the positive electrode active material and the negative electrode active material, the intermediate layer being composed of an oxide of Y and/or a lithium composite oxide containing Li and at least one selected from the group consisting of Y, Nb, Ta, Al, Ti, La, Zr, W, Sn, Ce, and Mn, and the intermediate layer is present at the interface between the positive electrode active material and the solid electrolyte . The all-solid-state secondary battery according to claim 1, wherein the oxide and/or lithium composite oxide is at least one selected from the group consisting of Y oxide, Li and Nb oxide, Li and Ta oxide, Li and Al oxide, Li and Y oxide, Li and Ti oxide, Li, La and Zr or Li, La, Zr and Al oxide, Li, La and Ti oxide, Li and W oxide, Li and Nb oxide, Li and Sn oxide, Li and Ce oxide, Li, La and Nb oxide, and Li and Mn oxide. The all-solid-state secondary battery according to claim 1 or 2, wherein the LiOH.Li ₂ SO ₄ -based solid electrolyte comprises a solid electrolyte identified as 3LiOH.Li ₂ SO ₄ by X-ray diffraction. The all-solid-state secondary battery according to any one of claims 1 to 3 , wherein the positive electrode active material is a lithium composite oxide having a layered rock salt structure. 5. The all-solid-state secondary battery according to claim 4 , wherein the lithium composite oxide having a layered rock salt structure is lithium cobalt nickel manganese oxide or lithium cobalt oxide. The all-solid-state secondary battery according to any one of claims 1 to 5 , wherein the positive electrode active material is in the form of a sintered plate. The all-solid-state secondary battery according to any one of claims 1 to 6 , wherein the positive electrode active material has a density of 50 to 80 volume %.