JP7802008B2 - All-solid-state secondary battery - Google Patents

Description

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

In recent years, with the development of portable devices such as personal computers and mobile phones, demand for batteries as their power source has expanded significantly. Batteries used in these applications have traditionally used liquid electrolytes (electrolyte solutions) that use flammable organic solvents as diluents to transport ions. Batteries using such electrolyte solutions can encounter problems such as electrolyte leakage, fire, and explosion. To address these issues and ensure essential safety, development is underway on all-solid-state batteries that use solid electrolytes instead of liquid electrolytes and are constructed with all other elements made of solids. Because such all-solid-state batteries use solid electrolytes, they are free from the risk of fire, do not leak, and are less susceptible to 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 made of a sulfide-based solid electrolyte and lithium cobalt oxide, the surface of the lithium cobalt oxide is coated with lithium niobate to reduce interfacial resistance. Reducing interfacial resistance leads to improved charge-discharge characteristics. The battery disclosed in Patent Document 1 is an all-solid-state battery that uses pressed powder, and the energy density of the electrode decreases if pores remain between the particles or if a conductive additive is added to ensure electronic conduction between the active materials.

In contrast, all-solid-state batteries using sintered electrodes instead of compacted electrodes have also been proposed. Such batteries have the advantage of high energy density because the sintered electrodes do not contain conductive additives. For example, Patent Document 2 (WO 2019/093222 A1) 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 extracting lithium ions at 0.4 V (vs. Li/Li ⁺ ) or higher, 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 as solid electrolytes with such low melting points, such as Li ₃ OCl and xLiOH·yLi ₂ SO ₄ (where x + y = 1, 0.6≦x≦0.95) (e.g., 3LiOH·Li ₂ SO ₄ ). Such solid electrolytes can be infiltrated into the gaps between the electrodes as a melt, achieving strong interfacial contact, which is believed to result in significant improvements in battery resistance and rate performance during charge and discharge, as well as a significant improvement in battery manufacturing yield.

Furthermore, Patent Document 3 (WO2020/194822A1) discloses that by further incorporating boron into a solid electrolyte identified as 3LiOH·Li ₂ SO ₄ by X-ray diffraction, it is possible to significantly suppress the decrease in lithium ion conductivity even after being kept at high temperatures for a long period of time.

JP 2009-193940 A WO2019/093222A1 WO2020/194822A1

The present inventors have found that, among the above-mentioned low-melting point solid electrolytes, LiOH.Li2SO4- _based solid electrolytes such as _3LiOH.Li2SO4 as shown in Patent Documents 2 and ₃ exhibit high lithium ion conductivity. However, when a cell was constructed using a sintered electrode as disclosed in Patent Document 2 and _{a LiOH.Li2SO4 -} based solid electrolyte such as _3LiOH.Li2SO4 and the cell was operated, it _was found that the discharge amount was lower than the theoretical capacity expected from the amount of active material.

The same problem also applies to a pressed powder electrode (composite electrode) constituting a pressed powder all-solid-state battery such as that disclosed in Patent Document 1. In particular, Patent Document 1 is based on _the premise of an all-solid-state battery using a sulfide-based solid electrolyte such as _{Li7P3S11 ,} which is completely different from _{a LiOH.Li2SO4 - based solid electrolyte, and does not provide any suggestion or solution to the problem of output reduction in an all-solid-state battery using a LiOH.Li2SO4 -} based solid electrolyte.

The present inventors have now discovered that in an all-solid-state secondary battery that employs 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.

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 incorporated into at least one of the positive electrode and the negative electrode;
Including,
In at least one of the positive electrode and the negative electrode incorporating the solid electrolyte, 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 a lithium composite oxide containing Li and at least one selected from the group consisting of Ti, La, Zr, Al, W, Nb, Sn, Ce, Mn, Y, and Ta, and/or an oxide of Y and/or an oxide of Al.

1 is a SEM image of the positive electrode plate/solid electrolyte interface produced in Example A6. 1 is a TEM image of the positive electrode plate/solid electrolyte interface produced in Example A13.

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.Li2SO4} -based solid electrolyte. The positive electrode includes a positive electrode active material. The negative electrode includes a negative electrode active material. The _{LiOH.Li2SO4 -} based solid electrolyte is interposed between the positive electrode and the negative electrode and is also incorporated into 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 the solid electrolyte and at least one of the positive electrode active material and the negative electrode active material in at least one of the positive electrode and the negative electrode in which the solid electrolyte is incorporated. This intermediate layer is composed of a lithium composite oxide containing Li and at least one element selected from the group consisting of Ti, La, Zr, Al, W, Nb, Sn, Ce, Mn, Y, and Ta, and/or an oxide of Y and/or an oxide of Al. In this way, in an all-solid-state secondary battery that uses 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 employing low-melting-point solid electrolytes such as LiOH· _{Li2SO4 -} based solid electrolytes are known (see, for example, Patent Document 2). The solid electrolyte penetrates the voids of the electrode plates as a melt, achieving interfacial contact. As a result, improvements in battery resistance and rate performance during charge and discharge, as well as battery manufacturing yield, can be achieved. In particular, LiOH· _{Li2SO4 -} based solid electrolytes such as _3LiOH · _Li2SO4 exhibit high lithium ion conductivity. However, when cells were constructed using LiOH· _{Li2SO4 -} based solid electrolytes in sintered electrodes and operated, it was found that the discharge capacity was lower than the theoretical capacity expected from the amount of active material. Similar problems also apply to the compacted powder electrodes (composite electrodes) that constitute compacted powder all-solid-state batteries such as those disclosed in Patent Document 1. In this regard, the present invention solves the above problem by providing the above-mentioned intermediate layer at the interface between the electrode active material and the solid electrolyte, thereby improving discharge capacity (compared to those without the intermediate layer). Although the detailed mechanism by which the discharge capacity is improved is unclear, it is speculated that the presence of the intermediate layer may suppress degradation of the solid electrolyte due to a reaction between the solid electrolyte and the active material. More specifically, in all-solid-state batteries of the type in which a sintered electrode is impregnated with a LiOH·Li ₂ SO ₄ -based solid electrolyte (hereinafter referred to as the "impregnated sintered type"), the intermediate layer is thought to suppress reaction promotion due to heat during melting and impregnation. Furthermore, in both impregnated sintered type all-solid-state batteries and all-solid-state batteries of the type using a composite electrode containing active material particles and LiOH·Li ₂ SO ₄ -based solid electrolyte particles (hereinafter referred to as the "composite type"), the intermediate layer is thought to suppress reaction promotion due to heat during high-temperature charge and discharge, and reactions due to electrochemical effects during charge and discharge. Therefore, the all-solid-state battery of the present invention can preferably be of either the impregnated sintered type or the composite type.

(1) Positive Electrode The positive electrode includes a positive electrode active material. The positive electrode active material may be a positive electrode active material commonly used in lithium secondary batteries, but preferably includes 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 includes 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 LixCoO2 (} lithium cobalt oxide ₎ , _LixNiO2 (lithium nickel oxide), _LixMnO2 (lithium manganese oxide), _LixNiMnO2 (lithium _nickel manganese oxide), _LixNiCoO2 (lithium _nickel cobalt oxide), _LixCoNiMnO2 (lithium _{cobalt nickel} manganese oxide), _LixCoMnO2 (lithium cobalt manganese oxide _{) , Li2MnO3 ,} and solid solutions of the above compounds. Particularly preferred are _LixCoNiMnO2 (lithium cobalt nickel manganese oxide) and _LixCoO2 (lithium cobalt oxide, typically _{LiCoO2 )} . 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 used as the positive electrode active material 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. Also suitable for use is LiMPO ₄ (wherein M is at least one element selected from Fe, Co, Mn, and Ni) having an olivine structure.

The positive electrode or positive electrode active material is preferably in the form of a sintered plate. That is, the positive electrode can be in the form of a sintered plate made by sintering positive electrode raw material powder. Since a sintered plate does not need to contain an electronic conductive additive or 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.

Alternatively, the positive electrode may contain particles of a positive electrode active material, particles of a LiOH·Li ₂ SO ₄ -based solid electrolyte, and an electron-conducting additive in the form of a composite. That is, the positive electrode may be in the form of a molded mixture, commonly referred to as a composite electrode. The particle size of the positive electrode active material particles is preferably 0.05 to 50 μm, more preferably 0.1 to 30 μm, and even more preferably 0.5 to 20 μm. The particle size of the LiOH·Li ₂ SO ₄ -based solid electrolyte particles is preferably 0.01 to 50 μm, more preferably 0.05 to 30 μm, and even more preferably 0.1 to 20 μm. The electron-conducting additive may be any electron-conducting material commonly used in electrodes, but is preferably a carbon material. Preferred examples of carbon materials include carbon black, graphite, carbon nanotubes, graphene, reduced graphene oxide, and any combination thereof, but are not limited thereto. Various other carbon materials may also be used. A particularly preferred carbon material is carbon nanotubes.

The filling rate of the positive electrode active material in the positive electrode is preferably 30 to 80% by volume, regardless of the form of the positive electrode (sintered plate or composite). For impregnated sintered compact types, the filling rate of the positive electrode active material is more preferably 50 to 80% by volume, even more preferably 55 to 80% by volume, particularly preferably 60 to 80% by volume, and most preferably 65 to 75% by volume. On the other hand, for composite types, the filling rate of the positive electrode active material is more preferably 30 to 75% by volume, even more preferably 35 to 75% by volume. A filling rate within the above range allows the solid electrolyte to be sufficiently incorporated into the interior of the positive electrode (e.g., the voids in the active material sintered plate or between the constituent particles of the composite) via an intermediate layer. In particular, for impregnated sintered compact types, the increased proportion of positive electrode active material in the positive electrode allows for a high energy density of the battery. The filling rate of the positive electrode active material in the positive electrode is determined as the percentage (%) of the area of the positive electrode active material relative to the total area of the positive electrode based on a binarized image obtained from a cross-sectional SEM image of the positive electrode. The total area of the positive electrode refers to the sum of the areas occupied by the positive electrode active material and other positive electrode components (including not only the solid electrolyte and the electron conduction additive but also voids). Specific procedures for measuring the filling rate of the positive electrode active material in the positive electrode are described later in the Examples. Therefore, the filling rate of the positive electrode active material in a positive electrode in the form of a sintered plate is synonymous with the density of the positive electrode active material sintered plate.

Regardless of the form of the positive electrode (sintered plate or composite), the thickness of the positive electrode is preferably 50 to 350 μm, more preferably 75 to 325 μm, even more preferably 100 to 300 μm, and particularly preferably 100 to 275 μm, from the perspective of improving the energy density of the battery.

(2) Negative Electrode The negative electrode (typically a negative electrode plate) contains a negative electrode active material. A negative electrode active material commonly used in lithium secondary batteries can be used as the negative electrode active material. Examples of such common negative electrode active materials include carbon-based materials such as graphite, metals or semimetals such as Li, In, Al, Sn, Sb, Bi, and Si, or alloys containing any of these. Alternatively, 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 or higher (vs. Li/Li ⁺ ), and preferably contain Ti. A negative electrode active material that satisfies these 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 referred to as LTO), niobium titanium composite oxide Nb ₂ TiO ₇ , and titanium oxide TiO _{2 .} LTO and Nb ₂ TiO ₇ are more preferred, and LTO is even more preferred. While LTO is typically known to have a spinel structure, other structures can also be adopted during charge and discharge. For example, during charge and discharge, LTO undergoes a reaction in the coexistence of two phases: Li ₄ Ti ₅ O ₁₂ (spinel structure) and Li ₇ Ti ₅ O ₁₂ (rock salt structure). Therefore, LTO is not limited to a spinel structure.

The negative electrode or negative electrode active material is preferably in the form of a sintered plate. That is, the negative electrode can be in the form of a sintered plate made by sintering negative electrode raw material powder. Since a sintered plate does not need to contain an electronic conductive additive or 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.

Alternatively, the negative electrode may contain particles of anode active material, particles of LiOH·Li ₂ SO ₄ -based solid electrolyte, and an electron conduction aid in the form of a composite. That is, the negative electrode is preferably in the form of a press-molded mixture containing these components, commonly referred to as a composite electrode. The particle size of the anode active material particles is preferably 0.05 to 50 μm, more preferably 0.1 to 30 μm, and even more preferably 0.5 to 20 μm. The particle size of the LiOH·Li ₂ SO ₄ -based solid electrolyte particles is preferably 0.01 to 50 μm, more preferably 0.05 to 30 μm, and even more preferably 0.1 to 20 μm. The electron conduction aid may be any electron-conductive material commonly used in electrodes, but is preferably a carbon material. Preferred examples of carbon materials include carbon black, graphite, carbon nanotubes, graphene, reduced graphene oxide, and any combination thereof, but are not limited thereto. Various other carbon materials may also be used. A particularly preferred carbon material is carbon nanotubes.

The negative electrode active material filling rate is preferably 30 to 80% by volume, regardless of the negative electrode form (sintered plate or composite). For impregnated sintered compact types, the preferred filling rate is 55 to 80% by volume, even more preferably 60 to 80% by volume, and especially preferably 65 to 75% by volume. On the other hand, for composite types, the preferred filling rate is 30 to 75% by volume, even more preferably 35 to 75% by volume. A filling rate within the above range allows the solid electrolyte to be sufficiently incorporated into the interior of the negative electrode (e.g., the voids in the active material sintered plate or between the constituent particles of the composite) via an intermediate layer. In particular, for impregnated sintered compact types, the increased proportion of negative electrode active material in the negative electrode allows for the realization of a high energy density battery. The negative electrode active material filling rate in the negative electrode is determined as the percentage of the area of the negative electrode active material relative to the total area of the negative electrode, based on a binarized image obtained from a cross-sectional SEM image of the negative electrode. The total area of the negative electrode refers to the sum of the areas occupied by the negative electrode active material and other negative electrode components (including not only the solid electrolyte and the electron conduction additive but also voids). Specific procedures for measuring the packing rate of the negative electrode active material in the negative electrode will be described later in the Examples. Therefore, the packing rate of the negative electrode active material in a sintered plate-shaped negative electrode is synonymous with the density of the negative electrode active material sintered plate.

Regardless of the form of the negative electrode (sintered plate or composite), the thickness of the negative electrode 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, from the perspective of improving the energy density of the battery.

(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 represented by the general formula: _{xLiOH.yLi2SO4} (where x + y = ₁ , and _0.6 ≦x≦0.95). A representative example is _3LiOH.Li2SO4 (where x = 0.75 and 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 a solid electrolyte contains _3LiOH.Li2SO4 can be confirmed by identifying the X-ray diffraction pattern using ICDD database 032-0598. Here, " _{3LiOH.Li2SO4 "} refers to a substance whose crystal structure is considered to be the same as _{that of} 3LiOH.Li2SO4, and the crystal composition does not necessarily have to be the same as _that of _3LiOH.Li2SO4 . In other words, as long as it has a crystal structure equivalent to _that of _3LiOH.Li2SO4 , a substance whose composition deviates from LiOH: _Li2SO4 = ₃ :1 is 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 a higher angle), it will be referred to in this specification as 3LiOH.Li _{2 SO 4 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 allowed to contain inevitable impurities.

Therefore, the _{LiOH.Li2SO4 -} based solid electrolyte may contain a heterogeneous phase in addition to the main _{phase 3LiOH.Li2SO4} . The heterogeneous phase may contain a plurality of elements selected from Li, O, H, S, and B, or may consist only of a plurality of elements selected from Li, O, _H , _S , and B. Examples of the heterogeneous phase include LiOH, _Li2SO4 , and _/ or _Li3BO3 derived from the raw materials. These heterogeneous phases are thought to be unreacted raw materials remaining when _3LiOH.Li2SO4 was formed, but since they do not contribute to lithium ion conduction, it is desirable that the amount of _phases other than _Li3BO3 is small. For example, a _{LiOH.Li2SO4 -} based solid electrolyte may contain LiOH and/ _{or Li2SO4} as a heterophase such that the molar ratio of LiOH/ _Li2SO4 in the overall _composition including _3LiOH.Li2SO4 is typically in the range of 1.8 to _3.0 , more typically 2.0 to 2.6. However, a heterophase containing boron such as _Li3BO3 may be contained in a desired amount because _it can contribute to improving the retention of lithium ion conductivity after long-term storage at high temperatures. However, the solid electrolyte may be composed of _a single phase of _3LiOH.Li2SO4 with boron dissolved _therein .

It is preferable that the LiOH·Li ₂ SO ₄ -based solid electrolyte (particularly 3LiOH·Li ₂ SO ₄ ) further contains boron. By further incorporating boron into a solid electrolyte identified as 3LiOH·Li ₂ SO ₄ , it is possible to significantly suppress a decrease in lithium ion conductivity even after long-term storage at high temperatures. It is presumed that boron is incorporated into one of the sites of the 3LiOH·Li ₂ SO ₄ crystal structure, improving the stability of the crystal structure against temperature. The molar ratio of boron (B) to sulfur (S) contained in the solid electrolyte (B/S) is preferably greater than 0.002 and less than 1.0, more preferably 0.003 or greater and 0.9 or less, and even more preferably 0.005 or greater and 0.8 or less. A B/S ratio within the above range can improve the retention rate of lithium ion conductivity. Furthermore, a B/S ratio within the above range reduces the content of unreacted heterogeneous phases containing boron, thereby increasing the absolute value of lithium ion conductivity.

The _{LiOH.Li2SO4 -} based solid electrolyte may be a compact of powder obtained by pulverizing a molten solid, or a molten solid (i.e., solidified after heating and melting). The former is suitable for the composite-type all-solid-state battery described above, while the latter is suitable for the impregnated sintered-type all-solid-state battery described above. The particle size of the _{LiOH.Li2SO4 -} based solid electrolyte particles that make up the compact is preferably 0.01 to 50 μm, more preferably 0.05 to 30 μm, and even more preferably 0.1 to 20 μm.

The _{LiOH.Li2SO4 -} based solid electrolyte is incorporated into the positive electrode and/or negative electrode (for example, in the voids of the active material sintered plate or between the constituent particles of the composite), and the remaining portion is preferably interposed between the positive electrode and the negative electrode as a solid electrolyte layer. From the viewpoints of charge/discharge rate characteristics and the insulating properties of the solid electrolyte, the thickness of the solid electrolyte layer (excluding the portion incorporated into the positive electrode and the negative electrode) is preferably 1 to 500 μm, more preferably 3 to 50 μm, and even more preferably 5 to 40 μm.

(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 also be present at the interface between the negative electrode active material and the solid electrolyte. The intermediate layer may also 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 a lithium composite oxide containing Li and at least one element selected from the group consisting of Ti, La, Zr, Al, W, Nb, Sn, Ce, Mn, Y, and Ta, and/or an oxide of Y (typically Y ₂ O ₃ ), and/or an oxide of Al (typically Al ₂ O ₃ ). Preferred examples of such lithium composite oxides include an oxide of Li and Ti (typically Li ₂ TiO ₃ ), an oxide of Li, La and Zr or Li, La, Zr and Al (typically Li _7-3x Al _x La ₃ Zr ₂ O ₁₂ (0≦x<0.4, more typically 0.02<x<0.4)), an oxide of Li, La and Ti (typically Li _0.33 La _0.55 TiO ₃ ), an oxide of Li and W (typically Li ₂ WO ₄ ), an oxide of Li and Al (typically LiAlO ₂ ), an oxide of Li and Nb (typically LiNbO ₃ or LiNb ₃ O ₈ ), an oxide of Li and Sn (typically LiSnO ₃ ), an oxide of Li and Ce (typically Li ₈ CeO ₆ ), oxides of Li, La and _Nb (typically _Li5La3Nb2O12 ), oxides of Li and Mn ₍ typically _LiMnO2 ), oxides of Li and Y (typically _LiYO2 ), oxides of Li and Ta (typically _LiTaO3 ), and any combination thereof, more preferably oxides of Li and Ti (typically _Li2TiO3 ), oxides of Li, La, _Zr and _Al ( _{typically Li6.7Al0.1La3Zr2O12 )} , _and oxides of _Li , La _and Ti (typically _{Li0.33La0.55TiO3 )} .

The intermediate layer can be formed by mixing metal salts, such as metal alkoxides or nitrates, of one or more metal elements that will make up the intermediate layer with alcohol, such as ethanol, or water in a predetermined molar ratio to prepare a solution. The electrode active material (preferably a sintered plate or particles) is then immersed in this solution, removed, and allowed to stand in the air to hydrolyze the alkoxides or dry the electrode active material. In the case of a sintered plate, immersion in the solution is preferably performed under reduced pressure to allow penetration, and the process from immersion to standing in the air is preferably repeated multiple times (e.g., 1 to 20 times). Alternatively, the intermediate layer can be formed by mixing metal salts, such as metal alkoxides or nitrates, of one or more metal elements that will make up the intermediate layer with alcohol, such as 2-propanol, or water in a predetermined molar ratio to prepare a solution. This solution is then sprayed onto the electrode active material (preferably particles), followed by drying the electrode active material. The solution is preferably sprayed onto individual particles using a tumbling fluidized bed granulation coating device. In either method, the electrode active material (preferably a sintered plate or particles) on which the intermediate layer has been formed is preferably heat-treated for 5 to 60 minutes at 400 to 700° C. When a metal alkoxide is used, the steps from preparation of the solution to immersion are preferably carried out in an atmosphere with a dew point of −30° C. or lower so as to prevent deterioration of the solution due to hydrolysis or the like.

(5) Manufacturing of All-Solid-State Secondary Batteries In the case of an impregnated sintered body type, all-solid-state secondary batteries can be manufactured by: i) preparing a positive electrode (with an intermediate layer or current collector formed as necessary) and a negative electrode (with an intermediate layer or 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 also be bonded by other methods. In this case, examples of methods for forming the solid electrolyte between the positive electrode and the negative electrode include placing a solid electrolyte compact or powder on one electrode, applying a solid electrolyte powder paste on the electrode by screen printing, using the electrode as a substrate to solidify the solid electrolyte powder by impact deposition or the like, and depositing the solid electrolyte powder on the electrode by electrophoresis to form a film. On the other hand, in the case of a composite type all-solid-state secondary battery, for example, a positive electrode composite powder (including positive electrode active material particles (with an intermediate layer formed as needed), solid electrolyte particles, and an electron conduction aid), a solid electrolyte powder, and a negative electrode composite powder (including negative electrode active material particles (with an intermediate layer formed as needed), solid electrolyte particles, and an electron conduction aid) are each placed in a press mold and pressed. In this case, the various powders may be placed and pressed in any order so that the final product is a positive electrode layer, a solid electrolyte layer, and a negative electrode layer in that order.

The present invention will be described in more detail _{with reference} to the following examples. In the following description, unless otherwise specified, Li( _{Ni0.5Co0.2Mn0.3} ) _O2 will be abbreviated as "NCM", _Li4Ti5O12 will be abbreviated as " _LTO ", _{and LiCoO2} will be abbreviated as "LCO".

[Examples A1 to A26]
The example described below relates to an impregnated sintered body type all-solid-state secondary battery.

Example A1
(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 Li/(Ni ₊ Co _{+ Mn} ) molar ratio was 1.30, and then the mixture was heated to 750 ° C for 15 hours to obtain a powder consisting of NCM particles. This powder was then pulverized to an average particle size of approximately 5 μm, and then mixed with a solvent, binder, plasticizer, and dispersant for tape casting. The viscosity of the resulting paste was adjusted, and the paste was then tape-cast onto 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 there for 10 hours to sinter it. The NCM sintered plate was thus obtained as a positive electrode plate. A 100 nm thick Au film was formed as a current collecting layer on one side of the obtained NCM sintered plate by sputtering.

(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 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 the intermediate layer. The above series of operations was then repeated once more (i.e., a total of two 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.

(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 Li/Ti molar ratio was 0.84, and then heated to 1000 °C for 2 hours to obtain a powder consisting of LTO particles. This powder was pulverized to an average particle size of approximately 2 μm and then mixed with a solvent, binder, plasticizer, and dispersant for tape casting. The viscosity of the resulting paste was adjusted, and the paste was tape-cast onto a film to produce an LTO green sheet. The thickness of the LTO green sheet was adjusted so that the thickness after firing would be 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 there for 2 hours to sinter it. Thus, an LTO sintered plate was obtained as a negative electrode plate. A gold film (100 nm thick) was formed as a current collecting layer on one side of the obtained LTO sintered plate by sputtering.

(3) Preparation of solid electrolyte (3a) Preparation of raw material powders _{Li2SO4 powder} (commercially available, purity 99% or higher), LiOH powder (commercially available, purity 98% or higher), and _Li3BO3 (commercially available, purity ₉₉ % or higher) were mixed in a molar ratio _{of Li2SO4} :LiOH: _Li3BO3 = 1: ₃ :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 lower, with great care taken to prevent deterioration such as moisture absorption.

(3b) Melt synthesis: The raw material powder mixture was placed in a crucible in an Ar atmosphere. The crucible was placed 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 a rate of 100°C/h to form a solid.

(3c) Mortar Pulverization 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) Fabrication of an all-solid-state battery. Solid electrolyte powder containing 5 wt% 30 μm diameter _ZrO2 beads was placed on a positive electrode plate, and a negative electrode plate was placed on top of that. A weight was then placed on the negative electrode plate, and the mixture was heated in an electric furnace at 400 °C for 45 minutes. During this process, the solid electrolyte powder melted and subsequently solidified, forming a solid electrolyte layer between the electrode plates. A battery was fabricated using the resulting cell consisting of a positive electrode plate, solid electrolyte, and negative electrode plate.

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

(5b) Measurement of Compactness (Active Material Filling Rate) The compactness (active material filling rate in the electrode) (volume %) of the positive electrode plate (NCM sintered plate not containing an intermediate layer or solid electrolyte) prepared in (1b) above and the negative electrode plate (LTO sintered plate not containing an intermediate layer or solid electrolyte) prepared in (2b) above was measured as follows. First, the positive electrode plate (or negative electrode plate) was embedded in resin, and the cross section was polished by ion milling. The polished cross section was then observed with an SEM to obtain a cross-sectional SEM image. The SEM image was taken at a magnification of 1000 times. The obtained image was first subjected to a 100% blurring process using image analysis software (Image-Pro Premier, manufactured by Media Cybernetics, Inc.), followed by binarization. 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) and resin-filled portions (portions that were originally voids) in the positive electrode plate (or negative electrode plate) was calculated to determine the density (active material filling rate) of the positive electrode active material (or negative electrode active material). The threshold 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, assuming that the discharge capacity of a battery (comparative example) having the same configuration except that no intermediate layer was formed was 100.

Example A2
A battery was fabricated and evaluated in the same manner as in Example A1, except that the number of times of film formation in the above (1c) intermediate layer formation was set to five in total.

Example A3
A battery was produced and evaluated in the same manner as in Example A1, 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. An NCM sintered plate was immersed in this solution, the pressure was reduced, and the NCM sintered plate was then removed from the solution. The NCM sintered plate was then heat-treated at 400 °C for 30 minutes to form an intermediate layer. This process was then repeated four more times (i.e., a total of five film formations). 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 A4
A battery was produced and evaluated in the same manner as in Example A1, 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) at a molar ratio of 0.015:1. An NCM sintered plate was immersed in this solution, the pressure was reduced, and the NCM sintered plate was then removed from the solution. The NCM sintered plate was then heat-treated at 400°C for 30 minutes to form an intermediate layer. This process was then repeated four more times (i.e., a total of five film formations). 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 A5
A battery was produced and evaluated in the same manner as in Example A1, 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, and ethanol (solvent) in a molar ratio of 0.015:0.015:1. An NCM sintered plate was immersed in this solution, the pressure was reduced, and the NCM sintered plate was then removed from the solution. The above-mentioned procedure 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 procedures was then repeated nine more times (i.e., a total of 10 film formations). 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 A6
A battery was produced and evaluated in the same manner as in Example A1, 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. An NCM sintered plate was immersed in this solution, and after depressurization, the NCM sintered plate was removed from the solution. The above-mentioned operation 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 four more times (i.e., a total of five film formations). 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 A7 (comparison)
A battery was fabricated and evaluated in the same manner as in Example A1, except that the intermediate layer was not formed as described in (1c) above. The discharge capacity measured in this example was used as a reference value of 100 for calculating the relative values of the discharge capacities in Examples A1 to A6.

Example A8
A battery was produced and evaluated in the same manner as in Example A1, 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 Li/(Ni+Co ₊ Mn) molar ratio was 1.15, and then heated to 750°C for 10 hours to obtain a powder consisting of NCM particles. This powder was then pulverized to an average particle size of approximately 5 μm and mixed with a solvent, binder, plasticizer, and dispersant for tape casting. The viscosity of the resulting paste was adjusted, and the paste was tape-cast onto a film to produce an NCM green sheet. The thickness of the NCM green sheet was adjusted so that the thickness after firing would be 100 μm.

(Production 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 rate of 200°C/h and held there for 10 hours to sinter it. An NCM sintered plate was thus obtained as a positive electrode plate. A 100 nm thick Au film was formed as a current collecting layer on one side of the obtained NCM sintered plate by sputtering.

Example A9 (comparison)
A battery was fabricated and evaluated in the same manner as in Example A8, except that the intermediate layer was not formed as described in (1c) above. The discharge capacity measured in this example was used as a reference value of 100 for calculating the relative values of the discharge capacities in Examples A8 and A10.

Example A10 (Comparative)
A battery was produced and evaluated in the same manner as in Example A8, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming the intermediate layer was prepared by mixing lithium ethoxide, tetraethoxysilane, and ethanol (solvent) in a molar ratio of 0.030:0.015:1. An NCM sintered plate was immersed in this solution, the pressure was reduced, and the NCM sintered plate was then removed from the solution. The process from preparing 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 allowed to stand in the air for 10 minutes to form the intermediate layer. The above series of steps was then repeated three more times (i.e., a total of four film formations). 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 A11
A battery was fabricated and evaluated in the same manner as in Example A8, except that the raw material powder in (3a) above was prepared as follows.

(Preparation of raw powder)
A raw material _mixed powder was obtained by mixing _Li2SO4 powder (commercially available, purity 99% or higher), LiOH powder (commercially available, purity 98% or higher ₎ , and _Li3BO3 (commercially available, purity 99% or higher) in a _molar ratio of _Li2SO4 : _LiOH : _Li3BO3 = 1:1.8:0.05. These powders were handled in a glove box in an Ar atmosphere with a dew point of -50°C or lower, with great care taken to prevent deterioration such as moisture absorption.

Example A12 (Comparative)
A battery was fabricated and evaluated in the same manner as in Example A11, except that the intermediate layer was not formed as described in (1c) above. The discharge capacity measured in this example was used as a reference value of 100 for calculating the relative value of the discharge capacity in Example A11.

Example A13
(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 Li _/ Co molar ratio was 1.02, and then the mixture was heated to 750°C for 5 hours. The resulting powder was pulverized to an average particle size of 0.4 μm to obtain LCO powder. The resulting LCO powder was mixed with a dispersion medium, a binder, a plasticizer, and a dispersant. The viscosity of the resulting mixture was adjusted to prepare an LCO slurry. The slurry thus prepared was tape-cast onto 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 would be 60 μm.

(1b) Preparation of _{Li2CO3 Green} Sheet Commercially available _Li2CO3 raw powder (average particle size 3 μm) was mixed with a dispersion medium, a binder, a plasticizer, and a dispersant. The viscosity of the resulting mixture was adjusted 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 dried _{Li2CO3 green} sheet 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, would be 0.2.

(1c) Preparation of LCO sintered plate The LCO green sheet peeled from the 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 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 resulting calcined _LCO plate, was 0.1. The cut _{Li2CO3 green} sheet piece was placed on the calcined LCO plate. The sintered plate and 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. The temperature was then raised 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. The LCO sintered plate was thus obtained as a positive electrode plate. An Au film (thickness: 100 nm) was formed as a current collecting layer on the obtained LCO sintered body plate 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 the 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. The LCO sintered plate was then removed from the glove box and left to stand in the air for 10 minutes to form the intermediate layer. The above series of operations was then repeated two more times (i.e., a total of three film formations). Finally, the LCO 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 LTO powder (average particle size 0.7 μm), a dispersion medium, a binder, a plasticizer, and a dispersant were mixed. The viscosity of the resulting 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 would be 60 μm.

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

(3) Preparation of solid electrolyte (3a) Preparation of raw material powders _{Li2SO4 powder} (commercially available, purity 99% or higher) and LiOH powder (commercially available, purity 98% or higher) 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 lower, with great care 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 placed in an electric furnace and heat-treated at 430° C. for 2 hours to prepare a melt. Subsequently, the melt was cooled in the electric furnace at 100° C./h to form a solid.

(3c) Mortar Pulverization 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 A1(4).

(5) Evaluation Analysis of the electrode plate/solid electrolyte interface and charge/discharge evaluation were carried out in the same manner as in Example A1(5).

Example A14
After the LTO sintered plate was produced in (2b) above, an intermediate layer was formed on the LTO sintered plate in the following manner, but a battery was produced and evaluated in the same manner as in Example A13.

(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 Example A13 (2b) was immersed in this solution, and after depressurization, the LTO sintered plate was removed from the solution. The above-mentioned operation was carried out 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). Finally, the LTO sintered plate was heat-treated at 400 ° C for 30 minutes to obtain a negative electrode plate with an intermediate layer formed thereon.

Example A15 (Comparative)
A battery was fabricated and evaluated in the same manner as in Example A13, except that the intermediate layer was not formed as described in (1c) above. The discharge capacity measured in this example was used as a reference value of 100 for calculating the relative values of the discharge capacities in Examples A13 and A14.

Example A16
(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 Li/(Ni ₊ Co+ _Mn ) molar ratio was 1.15, and then the mixture was heated to 750 ° C for 10 hours to obtain a powder consisting of NCM particles. This powder was then pulverized to an average particle size of approximately 5 μm, and then mixed with a solvent for tape casting, a binder, a plasticizer, and a dispersant. The viscosity of the resulting paste was adjusted, and the paste was then tape-cast onto 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 there for 10 hours to sinter it. The NCM sintered plate was thus obtained as a positive electrode plate. A 100 nm thick Au film was formed as a current collecting layer on one side of the obtained NCM sintered plate by sputtering.

(1c) Formation of Intermediate Layer Titanium tetraisopropoxide: lithium ethoxide: ethanol were mixed in a molar ratio of 0.0225:0.005:1 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 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 8 film formations). 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 Li/Ti molar ratio was 0.84, and then heated to 1000 °C for 2 hours to obtain a powder consisting of LTO particles. This powder was pulverized to an average particle size of approximately 2 μm and then mixed with a solvent, binder, plasticizer, and dispersant for tape casting. The viscosity of the resulting paste was adjusted, and the paste was tape-cast onto a film to produce an LTO green sheet. The thickness of the LTO green sheet was adjusted so that the thickness after firing would be 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 there for 2 hours to sinter it. In this way, an LTO sintered plate was obtained as a negative electrode plate. A gold film (thickness 100 nm) was formed as a current collecting layer on one side of the obtained LTO sintered plate by sputtering.

(3) Preparation of solid electrolyte (3a) Preparation of raw material powders _{Li2SO4 powder} (commercially available, purity 99% or higher), LiOH powder (commercially available, purity 98% or higher), and _Li3BO3 (commercially available, purity ₉₉ % or higher) were mixed in a molar ratio _{of Li2SO4} :LiOH: _Li3BO3 = 1: _2.6 :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 lower, with great care taken to prevent deterioration such as moisture absorption.

(3b) Melt synthesis: The raw material powder mixture was placed in a crucible in an Ar atmosphere. The crucible was placed 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 a rate of 100°C/h to form a solid.

(3c) Mortar Pulverization 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 A1(4).

(5) Evaluation Analysis of the electrode plate/solid electrolyte interface and charge/discharge evaluation were carried out in the same manner as in Example A1(5).

Example A17
A battery was produced and evaluated in the same manner as in Example A16, 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, lanthanum nitrate (anhydrous), zirconium tetra-n-butoxide, and 2-ethoxyethanol in a molar ratio of 0.000335:0.000005:0.00015:0.0001:1. An NCM sintered plate was immersed in this solution, and the pressure was reduced to allow the solution to penetrate into the pores of the positive electrode plate. This process was carried out in an atmosphere with a dew point of -30°C or lower. The plate was then heat-treated at 700°C for 30 minutes to form an intermediate layer. This process was then repeated once more (i.e., a total of two film formations). A positive electrode plate with an intermediate layer formed thereon was obtained.

Example A18
A battery was produced and evaluated in the same manner as in Example A16, 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. The plate was then heat-treated at 700°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) to obtain a positive electrode plate with an intermediate layer formed thereon.

Example A19
A battery was produced and evaluated in the same manner as in Example A16, 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. An NCM sintered plate was immersed in this solution, and the pressure was reduced to allow the solution to penetrate into the pores of the positive electrode plate. The plate was then heat-treated at 800°C for 30 minutes to form an intermediate layer. This 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 A20
A battery was produced and evaluated in the same manner as in Example A16, 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. 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 process was carried out 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 processes was then repeated seven more times (i.e., a total of eight film formations). 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 A21
A battery was produced and evaluated in the same manner as in Example A16, 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, and ethanol (solvent) in a molar ratio of 0.0225:0.0225:1. An NCM sintered plate was immersed in this solution, and the pressure was reduced to allow the solution to penetrate into the pores of the positive electrode plate. The above-mentioned process was carried out in an atmosphere with a dew point of -30°C or lower. The NCM sintered plate was then left to stand in the air for 5 minutes to form an intermediate layer. The above series of processes was then repeated seven more times (i.e., a total of eight film formations). 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 A22
A battery was produced and evaluated in the same manner as in Example A16, except that the intermediate layer in (1c) above was formed as follows.

(Deposition of intermediate layer)
A solution for forming the 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, and the pressure was reduced, allowing the solution to penetrate into the pores of the positive electrode plate. The above-mentioned process was carried out in an atmosphere with a dew point of -30°C or lower. The NCM sintered plate was then left to stand in the air for 5 minutes to form the intermediate layer. The above series of processes was then repeated seven more times (i.e., a total of eight film formations). 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 A23
A battery was produced and evaluated in the same manner as in Example A16, 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. 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 plate was then heat-treated at 800 °C for 30 minutes to form an intermediate layer. This 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 A24
A battery was produced and evaluated in the same manner as in Example A16, 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, and 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. The plate was then 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) to obtain a positive electrode plate with an intermediate layer formed thereon.

Example A25
A battery was produced and evaluated in the same manner as in Example A16, 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. The plate was then heat-treated at 400 °C for 30 minutes to form an intermediate layer. This 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 A26 (comparison)
A battery was fabricated and evaluated in the same manner as in Example A16, except that the intermediate layer was not formed as described in (1c) above. The discharge capacity measured in this example was used as a reference value of 100 for calculating the relative values of the discharge capacities in Examples A16 to A25.

The evaluation results of Results Examples A1 to A26 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 and solid electrolyte on which the intermediate layer was formed in each of the cells in Examples A1 to A26, EDX analysis detected metals other than Li used as the intermediate layer and oxygen. Although Li cannot be detected by EDX, XRD evaluation confirmed that when the intermediate layer solution was evaporated to dryness and heat-treated at the same temperature as the intermediate layer formation, oxides of the respective metal elements (Example A4) or lithium composite oxides (other than Example A4) were formed. From this, it is inferred that an intermediate layer consisting of an oxide of the element used in the intermediate layer (Example A4) or its lithium composite oxide (other than Example A4) was formed at the interface between the electrode plate and solid electrolyte in Examples A1 to A6, A8, A11, A13, A14, and A16 to A25. SEM images of the positive electrode plate/solid electrolyte interface actually produced in Examples A6 and A13 are shown in Figures 1 and 2, respectively. In Figure 1 (Example A6), a bright area was present at the interface between the NCM particle and the solid electrolyte, and Ta was detected in this area, revealing the formation of an intermediate layer of oxides of Li and Ta with a thickness of 0.1 to ₁ μm. Also, in Figure 2 (Example A13), a layer (between the arrows) was present at the interface between the LCO particle and the solid electrolyte (in the figure, 3LHS means _3LiOH.Li2SO4 ), and Nb was detected in this area, revealing the formation of an intermediate layer of oxides of Li and Nb with a thickness of 20 to 30 nm.

(Charge/discharge evaluation)
Table 1 shows the cell configurations and discharge capacities of Examples A1 to A7. The results in Table 1 indicate that Examples A1 to A6, in which an intermediate layer was formed on the positive electrode plate, exhibited improved discharge capacity compared to Example A7 (Comparative Example), in which no intermediate layer was formed. The mechanism by which the intermediate layer improves discharge capacity is unclear; however, possible reasons include suppressing degradation of the solid electrolyte due to the reaction between the NCM and the solid electrolyte (reduced conductivity) and suppressing the formation of a high-resistance layer at the interface. The aforementioned phenomenon depends on the type of solid electrolyte and the type of electrode active material. It was found that, in combinations of LiOH- _{Li2SO4 -} based electrolytes and positive electrode active materials, oxides of Li and Nb, oxides of Li and Y, Y oxide, oxides of Li and Al, and oxides of Li and Ta were effective.

Table 2 shows the cell configuration and discharge capacity of Examples A8 to A10. In Examples A8 to A10, the density of the active material (active material filling rate in the positive electrode) (volume %) was changed by changing the manufacturing method of the positive electrode plate. Table 2 also shows that Example A8, in which an intermediate layer was formed using oxides of Li and Nb, had an improved discharge capacity compared to Example A9 (Comparative Example), in which no intermediate layer was formed. In other words, it was found that the intermediate layer was effective even when the microstructure of the positive electrode plate changed. Furthermore, Example A10 (Comparative Example), in which an intermediate layer was formed using oxides of Li and Si, had a lower discharge capacity compared to Example A8, indicating the existence of suitable materials for the intermediate layer, as described above.

Table 3 shows the cell configurations and discharge capacities of Examples A11 and A12. The molar ratio of LiOH: _Li2SO4 in the _{LiOH.Li2SO4 -} based solid electrolyte was changed in Examples A11 and _A12 . Table 3 also shows that Example A11, in which an oxide of Li and Nb was formed as an intermediate layer, had an improved discharge capacity compared to Example A12 (comparative example), in which no intermediate layer was formed, demonstrating that the intermediate layer is effective even when the composition of the _{LiOH.Li2SO4 -} based solid electrolyte is changed.

Table 4 shows the cell configurations and discharge capacities of Examples A13 to A15. In Examples A13 to A15, an LCO sintered plate was used as the positive electrode, and the _{LiOH.LiSO4 -} based solid electrolyte was changed. Table 4 also shows that Example A13, in which an intermediate layer of Li and Nb oxides was formed, had an improved discharge capacity compared to Example A15 (Comparative Example), in which no intermediate layer was formed. This demonstrates the effectiveness of the intermediate layer even when using LCO, which has a layered rock salt structure different from NCM, as the positive electrode plate. Example A14, in which an intermediate layer of Li and Nb oxides was formed on the negative electrode plate, also showed an increased discharge capacity compared to Example A15 (Comparative Example), demonstrating the effectiveness of the intermediate layer. 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 a 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.

The cell configurations and discharge capacities of Examples A16 to A26 are shown in Table 5. In Examples A16 to A26, an NCM sintered plate was used as the positive electrode, and the molar ratio of LiOH: _LiSO4 in the _{LiOH.LiSO4 -} based solid electrolyte was changed. Table 5 shows that Examples A16 to _A25 , in which various lithium composite oxides were formed as intermediate layers, had improved discharge capacities compared to Example A26 (comparative example), in which no intermediate layer was formed. From the above, it was found that discharge characteristics can be 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.

[Examples B1 to B14]
The example described below is an example relating to a composite-type all-solid-state secondary battery.

Example B1 (Comparative)
(1) Preparation of Positive Electrode Active Material Powder Commercially available (Ni0.5Co0.2Mn0.3)(OH) ₂ powder (average particle size _D50 : 9 μm) and _Li2CO3 powder (average particle size D50: 3 μm) were weighed so that the molar ratio of Li/(Ni ₊ Co _{+ Mn} ) was 1.0, and then mixed. The mixture was kept at 850°C for 10 hours to obtain a powder consisting of NCM particles with an average particle size D50 of approximately 8 μm (hereinafter referred to as NCM powder).

(2) Preparation of negative electrode active material powder Commercially available _TiO2 powder (average particle size D50: 1 μm or less) and _Li2CO3 powder (average particle size D50: ₃ μm) were mixed so that the Li/Ti molar ratio was 0.84, and then the mixture was heated to 1000°C for 2 hours to obtain a powder of LTO particles. This powder was then pulverized to an average particle size D50 of approximately 3 μm to obtain an LTO powder.

(3) Preparation of Solid Electrolyte A solid electrolyte was prepared in a glove box with _a dew point of -30° _C or less by the following procedure. First, _Li2SO4 powder (commercially available, purity 99% or more), LiOH powder (commercially available, purity 98% _or more), and _Li3BO3 (commercially available, purity 99% or more) were mixed in a molar ratio of _Li2SO4 : _LiOH : _Li3BO3 = 1:2.2:0.05 to obtain a raw material mixed powder. The raw material mixed powder was placed in a crucible. This crucible was placed in an electric furnace and heat-treated at 430°C for 2 hours to prepare a melt. Subsequently, the melt was cooled in the electric furnace at 100°C/h to form a solid. The obtained solid was pulverized in a mortar to obtain a solid electrolyte powder having an average particle size D50 of 5 to 50 μm.

(4) Fabrication of All-Solid-State Battery An all-solid-state battery was fabricated in an environment with a dew point of −30° C. or less by the following procedure.

(4a) Preparation of Positive Electrode Composite Powder and Negative Electrode Composite Powder The NCM powder obtained in (1) above, the solid electrolyte powder obtained in (3) above, and an electron conduction assistant (carbon nanotubes (commercially available)) were weighed out to a volume ratio of 40:60:2, and these were mixed in a mortar to prepare a positive electrode composite powder. Similarly, the LTO powder obtained in (2) above, the solid electrolyte powder obtained in (3) above, and an electron conduction assistant (carbon nanotubes (commercially available)) were weighed out to a volume ratio of 40:60:2, and these were mixed in a mortar to prepare a negative electrode composite powder.

(4b) Press molding A powder of each layer was poured into a press mold having a hole diameter of 10 mm, and a pressure of 100 MPa was applied to form a cathode layer, a solid electrolyte layer, and an anode layer in this order so that the thicknesses of the layers were 110 μm, 500 μm, and 200 μm, respectively. After the three layers were stacked in this manner, the laminate was pressed at 150 MPa to obtain a press-molded body.

(4c) Attachment of Pressurizing Jig The press-molded body was sandwiched between a pair of stainless steel plates so as to have a layer structure of stainless steel plate/cathode layer/solid electrolyte layer/negative electrode layer/stainless steel plate, and the press-molded body together with the stainless steel plates was held at 150 MPa to obtain an all-solid-state battery as an evaluation cell.

(5) Evaluation (5a) Charge/Discharge Evaluation The battery prepared in (4) above was placed in a thermostatic chamber at 150°C. The discharge capacity of the battery at an operating temperature of 150°C was measured in a voltage range of 2.7 V to 1.5 V using the following procedure. The battery was charged at a constant current until the battery voltage reached the upper limit of the voltage range, followed by constant voltage charging until the current value reached 30 μA, and then discharged at a constant current of 3 mA until the current reached the lower limit of the voltage range, and the discharge capacity was measured. The discharge capacity measured in this example was used as the reference value 100 for calculating the relative values of the discharge capacities in Examples B2 to B12.

(5b) Measurement of Filling Rate The filling rate (volume %) of the active material in each of the positive electrode and negative electrode of the all-solid-state battery prepared in (4) above was measured as follows. First, the cross section of the all-solid-state battery was polished by ion milling, and then the cross section of the polished positive electrode (or negative electrode) was observed with an SEM to obtain a cross-sectional SEM image. The SEM image was an image at 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 a binarization process. The threshold value for binarization was set using Otsu's binarization as a discriminant analysis method. Based on the obtained binarized image, the filling rate F (%) of the positive electrode active material (or negative electrode active material) in the positive electrode (or) negative electrode was calculated using the following formula:
Filling rate F = [ _SA / ( _SA + _SB )] x 100
(wherein _SA is the area of the portion in the binarized image occupied by the positive electrode active material (or negative electrode active material), and _SB is the area of the portion in the binarized image other than the positive electrode active material (or negative electrode active material), including the areas occupied by the solid electrolyte, the electron-conducting additive, and voids.)
It was calculated by:

Example B2
A battery was produced and evaluated in the same manner as in Example B1, except that 1) an intermediate layer was formed on the NCM powder produced in (1) above as follows, and 2) the electrode active material/solid electrolyte interface was analyzed as follows.

(Formation of intermediate layer)
Titanium tetraisopropoxide:lithium ethoxide:ethanol were mixed in a molar ratio of 0.006:0.012:1 to prepare a solution for forming an intermediate layer. NCM powder was immersed in this solution and stirred, and the mixed solution was filtered to thoroughly dry the NCM powder. These operations were carried out in an atmosphere with a dew point of -30°C or lower. The NCM powder was then left to stand in the air for 30 minutes. Finally, the NCM powder was heat-treated at 400°C for 30 minutes to obtain NCM powder with an intermediate layer formed thereon.

(EDX analysis of electrode active material particle surface)
The surfaces of the electrode active material particles on which the intermediate layer was formed were subjected to EDX analysis using an SEM.

Example B3
A battery was fabricated and evaluated in the same manner as in Example B2, except that in forming the intermediate layer, 1) a solution obtained by mixing lithium ethoxide: aluminum tri-sec-butoxide: lanthanum nitrate (anhydrous): zirconium tetra-n-butoxide: 2-ethoxyethanol in a molar ratio of 0.000335:0.000005:0.00015:0.0001:1 was used, and 2) the heat treatment temperature of the NCM powder was set to 700°C.

Example B4
A battery was fabricated and evaluated in the same manner as in Example B2, except that in forming the intermediate layer, 1) a solution in which lithium ethoxide: lanthanum nitrate (anhydrous): titanium tetraisopropoxide: ethanol were mixed at a molar ratio of 0.00099:0.00165:0.003:1 was used, and 2) the heat treatment temperature of the NCM powder was set to 700°C.

Example B5
A battery was fabricated and evaluated in the same manner as in Example B2, except that in forming the intermediate layer, 1) a solution in which lithium hydroxide: tungsten (IV) oxide: water was mixed at a molar ratio of 0.048:0.024:1 was used, 2) the heat treatment temperature of the NCM powder was set to 800°C, and 3) the series of operations was performed in an air atmosphere.

Example B6
A battery was fabricated and evaluated in the same manner as in Example B2, except that in forming the intermediate layer, 1) a solution in which aluminum tri-sec-butoxide: lithium ethoxide: 2-ethoxyethanol was mixed at a molar ratio of 0.0075:0.0075:1 was used, and 2) the heat treatment temperature of the NCM powder was set to 400°C.

Example B7
A battery was fabricated and evaluated in the same manner as in Example B2, except that in forming the intermediate layer, 1) a solution in which niobium ethoxide:lithium ethoxide:ethanol was mixed at a molar ratio of 0.0075:0.0075:1 was used, and 2) the heat treatment temperature of the NCM powder was set to 400°C.

Example B8
A battery was fabricated and evaluated in the same manner as in Example B2, except that in forming the intermediate layer, 1) a solution in which niobium ethoxide:lithium ethoxide:ethanol was mixed at a molar ratio of 0.018:0.006:1 was used, and 2) the heat treatment temperature of the NCM powder was set to 600°C.

Example B9
A battery was fabricated and evaluated in the same manner as in Example B2, except that 1) a solution in which lithium ethoxide: tin isopropoxide: ethanol was mixed at a molar ratio of 0.015:0.015:1 was used in forming the intermediate layer, and 2) the heat treatment temperature of the NCM powder was set to 600°C.

Example B10
A battery was fabricated and evaluated in the same manner as in Example B2, except that in forming the intermediate layer, 1) an aqueous solution in which lithium nitrate: cerium nitrate: water was mixed at a molar ratio of 0.008:0.001:1 was used, 2) the heat treatment temperature of the NCM powder was set to 800°C, and 3) the series of operations was performed in an air atmosphere.

Example B11
A battery was fabricated and evaluated in the same manner as in Example B2, except that in forming the intermediate layer, 1) a solution in which lithium ethoxide: lanthanum nitrate (anhydrous): niobium ethoxide: ethanol were mixed at a molar ratio of 0.00025:0.00015:0.0001:1 was used, and 2) the heat treatment temperature of the NCM powder was set to 800°C.

Example B12
A battery was fabricated and evaluated in the same manner as in Example B2, except that in forming the intermediate layer, 1) an aqueous solution in which lithium nitrate:manganese nitrate:water was mixed at a molar ratio of 0.006:0.006:1 was used, 2) the heat treatment temperature of the NCM powder was set to 400°C, and 3) the series of operations was performed in an air atmosphere.

Example B13 (Comparative)
In the above (1), _a battery was fabricated and evaluated in _the same manner as in Example B1, except that Li( _{Ni0.3Co0.6Mn0.1} ) _O2 powder (hereinafter referred to as NCM361 powder) was prepared as the positive electrode active material powder as follows. 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 B14.

(Preparation of Positive Electrode Active Material Powder)
(Ni0.6Co0.2Mn0.2)(OH)2 powder (commercially available, average particle size D50: 10 μm), Co(OH) ₂ powder (average particle size D50: _{0.7 μm), and Li2CO3 powder (commercially available, average particle size D50: 3 μm) were mixed so that the molar ratio of Li/(Ni+Co+ Mn )} was 1.15 _and the Ni _: Mn _: Co = ₃ :6: ₁ (molar ratio). The mixture was then heated to 920 ° C for 10 hours to obtain a powder consisting of Li( _{Ni0.3Co0.6Mn0.1} ) _O2 particles. This powder was then pulverized to obtain NCM361 powder with an average particle size D50 of 2.5 μm.

Example B14
A battery was produced and evaluated in the same manner as in Example B2, except that NCM361 powder was produced as the positive electrode active material powder in the same manner as in Example B13.

Example B15 (comparison)
A battery was fabricated and evaluated in the same manner as in Example B13, except that 1) instead of LTO powder, commercially available graphite powder (average particle size 15 μm) was used as the negative electrode active material powder, which had been dried by holding it in an Ar atmosphere at 300° C. for 2 hours, and 2) charge/discharge evaluation was performed in a voltage range of 4.15 V to 2.0 V. The discharge capacity measured in this example was used as the reference value 100 for calculating the relative value of the discharge capacity in Example B16.

Example B16
A battery was produced and evaluated in the same manner as in Example B15, except that the intermediate layer was formed as follows to form an Al oxide.

(Formation of intermediate layer)
A solution of aluminum tri-sec-butoxide and 2-propanol mixed at a molar ratio of 0.0063:1 was sprayed onto 100 g of NCM361 powder using a tumbling fluidized bed granulation coating apparatus (MP-micro, manufactured by POWEREX Corporation), followed by drying. The operating conditions of the tumbling fluidized bed granulation coating apparatus were: inlet gas: air, inlet air temperature: 95°C, inlet air volume: 0.05 ^m3 /h, rotor rotation speed: 150 rpm, and spray rate: 0.5 g/min. Finally, the NCM361 powder was heat-treated at 400°C for 30 minutes to obtain NCM361 powder with an Al oxide intermediate layer formed thereon.

The evaluation results of Examples B1 to B16 are explained below with reference to Tables 6 to 8. In the following explanation of the results, NCM is used as a term that includes not only Li(Ni _0.5 Co _0.2 Mn _0.3 )O ₂ (NCM523) but also NCM361.

(EDX analysis of electrode active material particle surface)
In Examples B2 to B12 and B14, in which an intermediate layer was formed, EDX analysis detected metals other than Li used in the intermediate layer and oxygen on the surface of the active material particles on which the intermediate layer was formed in each cell. Although Li cannot be detected by EDX, XRD evaluation confirmed that when the solution for forming the intermediate layer was evaporated to dryness and then heat-treated, a lithium composite oxide of each metal element was formed. From this, it is presumed that an intermediate layer made of a lithium composite oxide of the elements used in the intermediate layer was formed on the surface of the active material particles in Examples B2 to B12 and B14. Furthermore, in Example B16, in which an intermediate layer was formed, EDX analysis detected Al used in the intermediate layer and oxygen, so it is presumed that an intermediate layer made of Al oxide was formed on the surface of the active material particles.

(Charge/discharge evaluation)
The cell configurations and discharge capacities of Examples B1 to B16 are shown in Tables 6 to 8. The results shown in Tables 6 to 8 indicate that Examples B2 to B12, B14, and B16, in which an intermediate layer was formed on the active material particles, had improved discharge capacities compared to Examples B1, B13, and B15 (Comparative Examples), in which no intermediate layer was formed. The mechanism by which the intermediate layer improves discharge capacity is unclear, but possible reasons include suppression of degradation of the solid electrolyte due to reaction between the NCM and the solid electrolyte (reduced conductivity), and suppression of the formation of a high-resistance layer at the interface. The above-mentioned phenomenon depends on the type of solid electrolyte and the type of electrode active material. In the combination of a LiOH-Li ₂ SO ₄ -based electrolyte and a positive electrode active material, it has been found that oxides of Li and Ti, 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, oxides of Li and Mn, and oxides of Al are effective as intermediate layers. In particular, oxides of Li and Ti, oxides of Li, La, Zr, and Al, and oxides of Li, La, and Ti have been found to be more effective. This is thought to be because these materials have high lithium ion conductivity, which suppresses the reaction between the NCM and the solid electrolyte while not interfering with the movement of lithium ions at the interface.

Claims (11)

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 incorporated into at least one of the positive electrode and the negative electrode;
Including,
At least one of the positive electrode and the negative electrode incorporating the solid electrolyte 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 a lithium composite oxide containing Li and at least one selected from the group consisting of Ti, La, Zr, Al, W, Nb, Sn, Ce, Mn, Y, and Ta, and/or an oxide of Y and/or an oxide of Al ;
the intermediate layer is present at the interface between the positive electrode active material and the solid electrolyte, and the filling rate of the negative electrode active material in the negative electrode is 30 to 80% by volume . The all-solid-state secondary battery according to claim 1, wherein the lithium composite oxide is at least one selected from the group consisting of oxides of Li and Ti, oxides of Li, La and Zr or 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, oxides of Li and Mn, oxides of Li and Y, and oxides of Li and Ta. 3. The all-solid-state secondary battery according to claim ₁ , wherein the _{LiOH.Li2SO4 -} based solid electrolyte comprises a solid electrolyte identified as _3LiOH.Li2SO4 by X-ray diffraction. The all-solid-state secondary battery according to any one of claims 1 to 3, wherein the LiOH.Li ₂ SO ₄ -based solid electrolyte further contains boron. The all-solid-state secondary battery according to any one of claims 1 to 4 , wherein the positive electrode active material is a lithium composite oxide having a layered rock salt structure. 6. The all-solid-state secondary battery according to claim 5 , wherein the lithium composite oxide having a layered rock salt structure is lithium cobalt-nickel-manganese oxide. The all-solid-state secondary battery according to any one of claims 1 to 6 , wherein a filling rate of the positive electrode active material in the positive electrode is 30 to 80% by volume. The all-solid-state secondary battery according to any one of claims 1 to 7 , 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 8 , wherein the negative electrode active material is in the form of a sintered plate. 8. The all-solid-state secondary battery according to claim 1 , wherein the positive electrode comprises particles of the positive electrode active material, particles of the _{LiOH.Li2SO4 -} based solid electrolyte, and an electron conduction assistant in the form of a mixture. The all-solid-state secondary battery according to any one of claims 1 to 7 and 10 , wherein the negative electrode comprises particles of the negative electrode active material, particles of the _{LiOH.Li2SO4 -} based solid electrolyte, and an electron conduction assistant in the form of a mixture.