JP7624017B2 - All-solid-state secondary battery - Google Patents

Description

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

As a positive electrode active material layer for lithium ion secondary batteries, a powder dispersion type positive electrode obtained by kneading and molding a powder of lithium composite oxide (typically lithium transition metal oxide) with additives such as a binder and a conductive agent is widely known. Such a powder dispersion type positive electrode contains a relatively large amount (for example, about 10% by weight) of a binder that does not contribute to the capacity, so the packing density of the lithium composite oxide as the positive electrode active material is low. For this reason, there is a lot of room for improvement in terms of capacity and charge/discharge efficiency of the powder dispersion type positive electrode. Therefore, attempts have been made to improve the capacity and charge/discharge efficiency by forming the positive electrode or the positive electrode active material layer from a lithium composite oxide sintered plate. In this case, since the positive electrode or the positive electrode active material layer does not contain a binder, it is expected that a high capacity and good charge/discharge efficiency can be obtained by increasing the packing density of the lithium composite oxide.

In addition, in lithium ion secondary batteries, a liquid electrolyte (electrolyte) using a flammable organic solvent as a diluting solvent has been used as a medium for moving ions. In batteries using such electrolytes, problems such as electrolyte leakage, fire, and explosion may occur. In order to solve such problems, and to ensure essential safety, development of all-solid-state batteries is underway that use solid electrolytes instead of liquid electrolytes and are composed of all other elements made of solids. Since such all-solid-state batteries use a solid electrolyte, there is no risk of fire, they do not leak, and they are less likely to have problems such as deterioration of battery performance due to corrosion.

Various all-solid-state batteries using sintered electrodes and solid electrolytes have been proposed. For example, Patent Document 1 (WO2019/093222A1) discloses an all-solid-state lithium battery comprising an oriented positive electrode plate that is a lithium composite oxide sintered plate with a porosity of 10 to 50%, a negative electrode plate that contains Ti and is capable of inserting and desorbing lithium ions at 0.4 V (vs. Li/Li ⁺ ) or 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 such as Li ₃ OCl, xLiOH.yLi ₂ SO ₄ (wherein x+y=1, 0.6≦x≦0.95) (e.g., 3LiOH.Li ₂ SO ₄ ) as solid electrolytes having such low melting points. Such solid electrolytes can be permeated into the voids of the electrode plate as a melt, and strong interface contact can be achieved. As a result, it is said that the battery resistance and the rate performance during charging and discharging can be significantly improved, and the yield of battery production can also be significantly improved. In addition, Patent Document 2 (WO2015/151566A1) discloses an all-solid-state lithium _battery comprising an oriented positive electrode plate having a layered rock salt structure with a basic composition represented by Li p (Ni _x , Co _y , Mn _z )O ₂ (wherein 0.9≦p≦1.3, 0<x<0.8, 0<y<1, 0≦z≦0.7, x+y+z=1), a solid electrolyte layer composed of Li-La-Zr-O-based ceramic material and/or lithium oxynitride phosphate (LiPON)-based ceramic material, and a negative electrode layer. The positive electrode composition actually evaluated in Patent Document 2 has a molar ratio of Ni:Co:Mn of 5:2:3 or 1:1:1.

WO2019/093222A1 WO2015/151566A1

The present inventors have found that, among the above-mentioned low melting point solid electrolytes, _LiOH.Li2SO4 -based solid electrolytes such as _3LiOH.Li2SO4 exhibit high lithium ion conductivity. However, when a conventionally used lithium composite oxide sintered plate having a molar ratio of Ni:Co:Mn of 5:2: ₃ was used as a positive electrode and _{a LiOH.Li2SO4 -} based _solid electrolyte such as _3LiOH.Li2SO4 was used to form a cell, and the cell was operated, it was found that the discharge capacity was lower than the theoretical capacity expected from the amount of active material.

The inventors have now discovered that by constructing an all-solid-state secondary battery using a porous sintered plate of a lithium composite oxide containing Ni, Co and Mn in a molar ratio within a specified range as the positive electrode layer, it is possible to significantly improve charge-discharge characteristics such as discharge capacity.

Therefore, the object of the present invention is to provide an all-solid-state secondary battery having significantly improved charge/discharge characteristics such as discharge capacity.

According to one aspect of the present invention, Ni, Co and Mn are
0.19≦Ni/(Ni+Co+Mn)≦0.41,
0.49≦Co/(Ni+Co+Mn)≦0.71, and 0.09≦Mn/(Ni+Co+Mn)≦0.11
A positive electrode layer including a porous sintered plate composed of a lithium composite oxide having a layered rock salt structure containing the lithium composite oxide in a molar ratio satisfying the following:
a negative electrode layer including a porous sintered plate made of a negative electrode active material;
a solid electrolyte interposed between the positive electrode layer and the negative electrode layer as a separator layer and also filled in the pores of the porous sintered plate of the positive electrode layer and the negative electrode layer;
An all-solid-state secondary battery comprising:

All-solid-state secondary battery The all-solid-state secondary battery of the present invention includes a positive electrode layer, a negative electrode layer, and a solid electrolyte. The positive electrode layer includes a porous sintered plate made of a lithium composite oxide having a layered rock salt structure. The negative electrode layer includes a porous sintered plate made of a negative electrode active material. The solid electrolyte is interposed between the positive electrode layer and the negative electrode layer as a separator layer, and is also filled in the holes of the porous sintered plates of the positive electrode layer and the negative electrode layer. The lithium composite oxide includes Ni, Co, and Mn in molar ratios that satisfy 0.19≦Ni/(Ni+Co+Mn)≦0.41, 0.49≦Co/(Ni+Co+Mn)≦0.71, and 0.09≦Mn/(Ni+Co+Mn)≦0.11. In this way, by constructing an all-solid-state secondary battery using a porous sintered plate of a lithium composite oxide containing Ni, Co, and Mn in a molar ratio within a predetermined range as a positive electrode layer, it is possible to significantly improve charge and discharge characteristics such as discharge capacity.

As mentioned above, all-solid-state lithium batteries impregnated with a low-melting point solid electrolyte such as a _LiOH.Li2SO4 -based solid electrolyte are known (see, for example, Patent Document ₁ ), and the solid electrolyte penetrates the gaps of the electrode plate as a melt to realize interface contact. As a result, it is possible to improve the battery resistance and rate performance during charging and discharging, as well as the yield of battery production. However, when a conventionally used lithium composite oxide sintered plate with a molar ratio of Ni:Co:Mn of 5:2: ₃ was used as the positive electrode, and a cell was constructed together with a _{LiOH.Li2SO4 -} based solid electrolyte such as _3LiOH.Li2SO4 , and the battery was operated, it was found that the discharge capacity was lower than the theoretical capacity expected from the amount of active material. This problem is conveniently solved by using a porous sintered plate of lithium composite oxide containing Ni, Co and Mn in a molar ratio within the above range as the positive electrode layer. This is believed to be because the lithium composite oxide containing Ni, Co and Mn in a molar ratio within the above range has small expansion/contraction or volume change associated with charging/discharging. In fact, when an all-solid-state battery with deteriorated performance was analyzed after charging/discharging, a gap was generated between the solid electrolyte impregnated in the positive electrode sintered plate of Ni:Co:Mn=5:2:3 and the positive electrode active material particles constituting the positive electrode sintered plate. That is, since the lithium composite oxide particles of Ni:Co:Mn=5:2:3 that have been used conventionally have the property of shrinking during charging, it is believed that the stress associated with particle shrinkage caused peeling between the positive electrode plate and the solid electrolyte, suppressing the diffusion of Li ions, thereby reducing the discharge capacity of the all-solid-state battery. In this regard, the porous sintered plate composed of the lithium composite oxide in the above composition range of the present invention has small expansion/contraction or volume change during charging/discharging, so that the above-mentioned problems are unlikely to occur, and as a result, it is believed that the charge/discharge characteristics such as discharge capacity are significantly improved.

(1) Positive electrode layer The positive electrode layer includes a porous sintered plate composed of a lithium composite oxide having a layered rock salt structure as a positive electrode active material. In other words, the porous sintered plate has a structure in which a plurality of primary particles composed of a lithium composite oxide having a layered rock salt structure are bonded together. This lithium composite oxide contains Ni, Co, and Mn in a molar ratio that satisfies 0.19≦Ni/(Ni+Co+Mn)≦0.41, 0.49≦Co/(Ni+Co+Mn)≦0.71, and 0.09≦Mn/(Ni+Co+Mn)≦0.11, more preferably in a molar ratio that satisfies 0.29≦Ni/(Ni+Co+Mn)≦0.31, 0.59≦Co/(Ni+Co+Mn)≦0.61, and 0.09≦Mn/(Ni+Co+Mn)≦0.11. As mentioned above, a composition (molar ratio) within the above range brings about a significant improvement in charge/discharge characteristics such as discharge capacity. In this regard, the behavior of the volume change during charging/discharging of a lithium composite oxide containing Ni, Co, and Mn (hereinafter abbreviated as NCM) differs depending on the composition, particularly the molar ratio of Ni, Co, and Mn, but if the composition (molar ratio) is within the above range, the expansion/contraction or volume change of the NCM porous sintered plate accompanying charging/discharging is small, and some compositions have almost no volume change during charging/discharging (for example, Ni:Co:Mn=3:6:1). Therefore, it is considered that peeling between the NCM porous sintered plate and the solid electrolyte is suppressed, and a significant improvement in charge/discharge characteristics such as discharge capacity is realized. Specific examples of the NCM composition (molar ratio) within the above range include Ni:Co:Mn=2:7:1, 3:6:1, and 4:5:1, and particularly preferably Ni:Co:Mn=3:6:1. In this way, when the molar ratio of Co is increased, the volume of the NCM porous sintered plate is less likely to change during charging and discharging. On the other hand, when the molar ratio of Co is small, such as Ni:Co:Mn=5:2:3 or 8:1:1, the volume of the NCM porous sintered plate changes significantly during charging and discharging.

The degree of orientation I _[003 ] / I [104], defined as the ratio of the diffraction intensity I [003] due to the ( _{003) plane to the diffraction intensity I [104]} due to the (104) plane in the XRD profile measured by X-ray diffraction _{(XRD )} , is 1.2 to 3.6, preferably 1.2 to 3.5, more preferably 1.2 to 3.0, and even more preferably 1.2 to 2.6. Within such a range, the volume change of the NCM porous sintered plate during charging and discharging is even less likely to occur, peeling between the sintered plate and the solid electrolyte is suppressed, and the charge and discharge characteristics such as discharge capacity can be more effectively improved. Here, in lithium composite oxides having a layered rock salt type crystal structure such as NCM, there are crystal faces (faces other than the (003) face, for example, the (101) face and the (104) face) in which lithium ions enter and exit well, and the (003) face in which they do not. In this specification, the diffraction intensities of the (003) and (104) planes by XRD are used as indices for calculating the degree of orientation for the sake of convenience.

The reason why the charge/discharge characteristics such as discharge capacity are improved when the degree of orientation I _[003] /I _[104] is within the above range can be considered as follows. First, as a premise, orientation/non-orientation will be explained. When NCM is in the form of a powder (i.e., a form other than a sintered body), the particle arrangement is random, so there is no bias in the orientation of the crystal plane. This is called non-orientation. On the other hand, in a form such as a sintered plate, the particle arrangement is fixed, so the orientation of the crystal plane is likely to be biased, and such a state is called orientation. In this regard, if the degree of orientation I _[003] /I _[104] of the NCM porous sintered plate is within the above-mentioned range, the degree of orientation I _[003] /I _[104] of the NCM powder, which is a guide for non-orientation, is equal to or close to 1.4 (composition of Ni:Co:Mn=5:2:3) or 2.3 (composition of Ni:Co:Mn=3:6:1), so it can be said to be close to non-orientation (random), that is, essentially not oriented (or not very oriented). The NCM primary particles have anisotropy in their expansion and contraction behavior during charging and discharging (this tendency is particularly prominent in NCM with Ni:Co:Mn=3:6:1), but if the NCM porous sintered plate is close to non-orientation (random), the anisotropy of the expansion and contraction of the NCM primary particles that constitute it is offset or mitigated by the randomness of the crystal orientation, so that the expansion and contraction of the NCM porous sintered plate as a whole is small. As a result, volumetric changes in the NCM porous sintered plate during charging and discharging are less likely to occur, peeling between the sintered plate and the solid electrolyte is suppressed, and charging and discharging characteristics such as discharge capacity are more effectively improved.

The porosity of the NCM porous sintered plate is 10 to 40%, preferably 15 to 38%, more preferably 18 to 36%, and even more preferably 20 to 33%. Within such a range, when a battery is produced, the pores can be sufficiently filled with solid electrolyte, and the proportion of the positive electrode active material in the positive electrode increases, so that a high energy density as a battery can be realized. In this specification, "porosity" refers to the volume ratio of the pores in the sintered plate. This porosity can be measured by image analysis of a cross-sectional SEM image of the sintered plate. For example, after filling the sintered plate with resin and polishing the cross-section by ion milling, the polished cross-section is observed with an SEM (scanning electron microscope) to obtain a cross-sectional SEM image (for example, 500 to 1000 times magnification), and the obtained SEM image is analyzed to calculate the ratio (%) of the area of the resin-filled portion to the total area of the electrode active material portion and the resin-filled portion (the portion that was originally a pore), and the porosity (%) of the sintered plate can be calculated. If the measurement can be performed with the desired accuracy, the porosity may be measured without filling the sintered plate with resin. For example, the porosity of a sintered plate (a positive electrode plate removed from an all-solid-state secondary battery) whose pores are filled with a solid electrolyte can be measured while the pores are still filled with the solid electrolyte.

The average primary particle diameter of the NCM porous sintered plate is preferably 0.4 to 5.0 μm, more preferably 0.5 to 4.0 μm, even more preferably 0.6 to 3.0 μm, particularly preferably 0.8 to 2.5 μm, and most preferably 1.0 to 2.2 μm. Within such a range, improvement of charge/discharge characteristics such as discharge capacity can be more effectively realized. This is thought to be because the anisotropy of the expansion and contraction of the lithium composite oxide primary particles accompanying charge/discharge is reduced due to the primary particles being small as described above. In this specification, the "average primary particle diameter" is the average value of the diameter of the primary particles contained in the sintered plate of the electrode. This average primary particle diameter can be measured by image analysis of a cross-sectional SEM image of the sintered plate. Specifically, the cross-sectional SEM image (for example, 5000 times magnification) obtained in the same manner as in the above-mentioned porosity measurement can be analyzed, and the average primary particle diameter can be calculated by the intercept method as follows. First, a straight line (segment) with a total length L is drawn randomly in an SEM image with a magnification of 5000 times, the number of intersections _nL between the segment and the grain boundaries of the primary particles is calculated, and the average primary particle diameter _D1 is calculated by the formula _D1 = 1.5 x L / _nL . The same operation as above is performed twice, changing the position each time, to calculate the average primary particle diameters _D2 and _D3 . The average value of the obtained average primary particle diameters _D1 , _D2 , and _D3 is calculated to be the average primary particle diameter D of the porous sintered plate. If the measurement can be performed with the desired accuracy, the average primary particle diameter may be measured without filling the sintered plate with resin. For example, the average primary particle diameter of a sintered plate (a positive electrode plate taken out of an all-solid-state secondary battery) whose pores are filled with a solid electrolyte can be measured while the solid electrolyte is still filled.

The average pore diameter of the NCM porous sintered plate is preferably 0.5 μm or more, more preferably 0.5 to 15.0 μm, even more preferably 0.7 to 15.0 μm, particularly preferably 0.8 to 10.0 μm, and most preferably 0.9 to 8.0 μm. Within such a range, the solid electrolyte portion (solid electrolyte portion at a distance from the interface) that is less susceptible to deterioration due to side reactions between the solid electrolyte and the sintered plate increases. This is thought to be because element diffusion between the solid electrolyte and the sintered plate is suppressed, and peeling between the sintered plate and the solid electrolyte due to deterioration of the solid electrolyte is suppressed. In this specification, the "average pore diameter" is the average value of the diameters of the pores contained in the sintered plate of the electrode. Such a "diameter" is typically the length of a line segment that divides the projected area of the pores in half (Martin diameter). In the present invention, the "average value" is suitably calculated based on the number of pieces. This average pore diameter can be measured by image analysis of a cross-sectional SEM image of the sintered plate. For example, the SEM image obtained by the above-mentioned porosity measurement is analyzed, and the electrode active material portion and the resin-filled portion (the portion that was originally pores) in the sintered plate are separated. The maximum Martin diameter of each region in the resin-filled portion is then calculated, and the average value of these is taken as the average pore diameter of the sintered plate. If the measurement can be performed with the desired accuracy, the average pore diameter may be measured without filling the sintered plate with resin. For example, the average pore diameter of a sintered plate (a positive electrode plate taken out of an all-solid-state secondary battery) whose pores are filled with a solid electrolyte can be measured while the solid electrolyte is still filled.

From the viewpoint of improving the energy density of the battery, the thickness of the NCM porous sintered plate is preferably 30 to 200 μm, more preferably 50 to 200 μm, and even more preferably 80 to 200 μm.

The NCM porous sintered plate may be manufactured by any method, and may be appropriately manufactured by referring to a known manufacturing method of a porous sintered lithium composite oxide. For example, an NCM green sheet may be manufactured using an NCM raw material powder containing Ni, Co, and Mn in a molar ratio satisfying 0.19≦Ni/(Ni+Co+Mn)≦0.41, 0.49≦Co/(Ni+Co+Mn)≦0.71, and 0.09≦Mn/(Ni+Co+Mn)≦0.11, and the NCM green sheet may be sintered. The various properties of the NCM porous sintered plate, such as the porosity, average primary particle size, and average pore size, can be appropriately controlled by controlling the particle size of the NCM raw material powder, using a mixed powder of two or more NCM raw material powders having different particle size distributions, or adjusting the sintering conditions.

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

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

A porous sintered plate made of negative electrode active material does not need to contain electronic conductive additives or binders, which increases the energy density of the negative electrode. The pores in the porous sintered plate are filled with a solid electrolyte.

The porosity of the negative electrode active material or its sintered plate is preferably 20 to 45%, more preferably 20 to 40%, and even more preferably 25 to 35%. With a porosity within this range, the pores in the negative electrode active material can be sufficiently filled with solid electrolyte, and the proportion of negative electrode active material in the negative electrode increases, making it possible to achieve a high energy density as a battery.

From the viewpoint of improving the energy density of the battery, the thickness of the negative electrode active material or its sintered plate is preferably 40 to 270 μm, more preferably 65 to 270 μm, even more preferably 100 to 270 μm, and particularly preferably 107 to 270 μm.

(3) Solid electrolyte The solid electrolyte is not particularly limited as long as it can be applied to all-solid-state secondary batteries, particularly all-solid-state lithium secondary batteries. For example, garnet-based ceramic materials, nitride-based ceramic materials, perovskite-based ceramic materials, phosphate-based ceramic materials, sulfide-based ceramic materials, borosilicate-based ceramic materials, lithium-halide-based materials, and polymer-based materials can be mentioned. Examples of garnet-based ceramic materials include Li-La-Zr-O-based materials (specifically, Li ₇ La ₃ Zr ₂ O _12, etc.), and Li-La-Ta-O-based materials (specifically, Li ₇ La ₃ Ta ₂ O ₁₂ , etc.). Examples of nitride-based ceramic materials include Li ₃ N. Examples of perovskite ceramic materials include Li-La-Zr-O materials (specifically, _{LiLa1 -xTixO3 (} 0.04≦x≦0.14) and the like). Examples of phosphate ceramic materials include lithium phosphate, nitrogen-substituted lithium phosphate (LiPON), Li-Al-Ti-P-O, Li-Al-Ge-P-O, and Li-Al _- Ti-Si-P-O (specifically, Li1 _{+x+ yAlxTi2-xSiyP3 - yO12 (} 0≦x≦0.4 _, 0<y≦0.6) and the like). Examples of sulfide ceramic _materials include LiOH- _Li2SO4 and _{Li3BO3 - Li2SO4 - Li2CO3} . Examples of borosilicate ceramic materials include Li ₂ O—B ₂ O ₃ —SiO _2. Examples of lithium-halide based materials include Li ₃ OX (wherein X is Cl and/or Br), Li ₂ (OH) _1-a F _a Cl (wherein 0≦a≦0.3), and Li ₂ OHX (wherein X is Cl and/or Br).

A preferred solid electrolyte is a _LiOH.Li2SO4 -based solid electrolyte. The _{LiOH.Li2SO4 -} based solid electrolyte is a composite compound of LiOH and _Li2SO4 , and has a typical composition of the general formula: _{xLiOH.yLi2SO4} (wherein x+ _y = ₁ , 0.6≦x≦0.95), and a representative example is _3LiOH.Li2SO4 (wherein x=0.75, y=0.25 in the general formula ₎ . Preferably, _{the LiOH.Li2SO4 -} based solid electrolyte includes a solid _electrolyte identified as _3LiOH.Li2SO4 by X-ray diffraction. This preferred solid electrolyte contains _3LiOH.Li2SO4 as the _main phase. Whether or not 3LiOH.Li ₂ SO ₄ is contained in a solid electrolyte can be confirmed by identifying the X-ray diffraction pattern using 032-0598 of the ICDD database. Here, "3LiOH.Li ₂ SO ₄ " refers to a substance whose crystal structure can be regarded as being the same as 3LiOH.Li ₂ SO ₄ , and the crystal composition does not necessarily have to be the same as 3LiOH.Li ₂ SO _4. In other words, as long as it has a crystal structure equivalent to 3LiOH.Li ₂ SO ₄ , those whose composition is outside of LiOH:Li ₂ SO ₄ = 3:1 are also included in "3LiOH.Li ₂ SO ₄ ". Therefore, even if the solid electrolyte contains a dopant such as boron (for example, 3LiOH.Li ₂ SO ₄ in which boron is dissolved and the X-ray diffraction peak is shifted to the higher angle side), it will be referred to as 3LiOH.Li ₂ SO ₄ in this specification as long as the crystal structure can be considered to be the same as 3LiOH.Li ₂ SO _4. Similarly, the solid electrolyte used in the present invention is also allowed to contain inevitable impurities.

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

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

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

The _{LiOH.Li2SO4 -} based solid electrolyte is filled into the holes of the porous sintered plates of the positive and negative electrode layers by melting, and the remaining part is interposed between the positive and negative electrode layers as a separator layer (solid electrolyte layer). The thickness of the separator layer (excluding the part that has entered the holes of the positive and negative electrode layers) is preferably 1 to 500 μm, more preferably 3 to 50 μm, and even more preferably 5 to 40 μm, from the viewpoint of charge/discharge rate characteristics and insulating properties of the solid electrolyte.

(4) Intermediate layer It is preferable that 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. By providing the intermediate layer, the discharge capacity can be further improved (compared to one without the intermediate layer). Although the detailed mechanism by which the discharge capacity is improved is not clear, it is speculated that the presence of the intermediate layer may suppress the deterioration of the solid electrolyte due to the reaction between the solid electrolyte and the active material. It is more preferable that the intermediate layer is present at the interface between the positive electrode active material and the solid electrolyte, but the intermediate layer may be present at the interface between the negative electrode active material and the solid electrolyte. The intermediate layer may be present at both the interface between the positive electrode active material and the solid electrolyte and the interface between the negative electrode active material and the solid electrolyte. The thickness of the intermediate layer is not particularly limited as long as the desired discharge capacity improvement effect is obtained, but is preferably 0.001 to 1 μm, more preferably 0.005 to 0.2 μm, and even more preferably 0.01 to 0.1 μm.

The intermediate layer is preferably 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 (typically _Y2O3 ). 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 an oxide of 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 ), an oxide of Li, La, and _Nb (typically _Li5La3Nb2O12 ), an oxide of Li and Mn (typically _LiMnO2 ), an oxide of Li and Y (typically _LiYO2 ), an oxide of Li and _Ta (typically _LiTaO3 ), _and any combination thereof, more preferably an oxide of Li and Ti (typically _Li2TiO3 ), an oxide of _Li , La _{, Zr} , and _Al (typically _{Li6.7Al0.1La3Zr2O12} ), and an oxide 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 constituting the intermediate layer with alcohol such as ethanol or water in a predetermined molar ratio to prepare a solution, immersing the electrode active material (preferably a sintered plate or particles) in this solution, removing it, and leaving it in the air to hydrolyze the alkoxide or dry the solvent. In the case of a sintered plate, it is preferable to immerse it in the solution under reduced pressure to allow it to penetrate inside, and it is also preferable to repeat the above-mentioned immersion and leaving it in the air several times (for example, 1 to 20 times). It is preferable to heat treat the electrode active material (preferably a sintered plate or particles) on which the intermediate layer has been formed in this way at 400 to 700 ° C for 5 to 60 minutes. When a metal alkoxide is used, it is preferable to perform the preparation of the solution and the immersion process in an atmosphere with a dew point of -30 ° C or less so that the solution does not deteriorate due to hydrolysis, etc.

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

The present invention will be described in more detail with reference to the following examples. In the following description _{, a lithium composite oxide having a layered rock salt structure containing Li, Ni, Co and Mn, such as (Ni0.3Co0.6Mn0.1)O2, is abbreviated as "NCM", and Li4Ti5O12 is abbreviated as " LTO} ".

First, NCM raw material powders 1 to 6 were prepared for making the positive electrode plate as shown below. The preparation methods for these raw material powders are summarized in Table 1.

[Preparation of NCM raw material powder 1]
Commercially available ( _{Ni0.5Co0.2Mn0.3} )(OH) ₂ powder (average particle size 9-10 μm) and _{Li2CO3 powder} (average particle size 3 μm) were mixed, weighed so that the molar ratio of Li/(Ni+Co ₊ Mn) was _1.15 . The resulting mixed powder was held at 750°C for 10 hours, and the volumetric D50 particle size was adjusted to about 5.5 μm by wet pulverization in a ball mill, and then dried to obtain NCM raw material powder 1.

[Preparation of NCM raw material powder 2]
Commercially available ( _{Ni0.3Co0.6Mn0.1} )(OH) ₂ powder (average particle size 7-8 μm) and _{Li2CO3 powder} (average particle size 3 μm) were mixed so that the molar ratio of Li/(Ni+Co+Mn) was _1.15 . The resulting mixed powder was held at 850°C for 10 hours to obtain NCM raw material powder 2. The _volumetric D50 particle size of this powder was approximately 6.5 μm.

[Preparation of NCM raw material powder 3]
The NCM raw material powder 2 was adjusted to a volumetric D50 particle size of about 4.3 μm by wet pulverization in a ball mill, and then dried to obtain the NCM raw material powder 3.

[Preparation of NCM raw material powder 4]
Commercially available ( _{Ni0.2Co0.7Mn0.1} )(OH) ₂ powder (average particle size 7-8 μm) and _{Li2CO3 powder} (average particle size 3 μm) were mixed so that the molar ratio of Li/(Ni+Co ₊ Mn) was _1.15 . The resulting mixed powder was held at 850°C for 10 hours, and the volumetric D50 particle size was adjusted to about 4.5 μm by wet pulverization in a ball mill, and then dried to obtain NCM raw material powder 4.

[Preparation of NCM raw material powder 5]
Commercially available ( _{Ni0.4Co0.5Mn0.1} )(OH) ₂ powder (average particle size 8-9 μm) and _{Li2CO3 powder} (average particle size 3 μm) were mixed, weighed so that the molar ratio of Li/(Ni+Co ₊ Mn) was _1.15 . The resulting mixed powder was held at 850°C for 10 hours, and the volumetric D50 particle size was adjusted to about 4.6 μm by wet pulverization in a ball mill, and then dried to obtain NCM raw material powder 5.

[Preparation of NCM raw material powder 6]
Commercially available ( _{Ni0.3Co0.6Mn0.1} )(OH) ₂ powder (average particle size 7-8 μm) and _{Li2CO3 powder} (average particle size ₃ μm) were mixed, weighed so that the molar ratio of Li/(Ni+Co ₊ Mn) was 1.15. The resulting mixed powder was held at 750°C for 10 hours, and the volumetric D50 particle size was adjusted to about 0.4 μm by wet pulverization in a ball mill, and then dried to obtain NCM raw material powder 6.

Positive electrodes and batteries were produced using the above raw material powders 1 to 6 as shown below, and various evaluations were performed.

Example 1 (Comparison)
(1) Preparation of Positive Electrode Plate (1a) Preparation of NCM Green Sheet First, NCM raw material powder 1 shown in Table 1 was prepared. This NCM raw material powder 1 was mixed with a solvent for tape casting, a binder, a plasticizer, and a dispersant. After adjusting the viscosity of the obtained paste, the paste was formed into a sheet on a PET (polyethylene terephthalate) film to prepare an NCM green sheet. The thickness of the NCM green sheet was adjusted so that the thickness after firing would be 100 μm.

(1b) Preparation of NCM sintered plate The NCM green sheet peeled off from the PET film was punched out into a circle with a diameter of 11 mm and placed in a sintering sheath. The temperature was raised to 920°C at a heating rate of 200°C/h and held for 10 hours to perform sintering. The thickness of the obtained sintered plate was about 100 μm thick by SEM observation. A Au film (thickness 100 nm) was formed as a current collecting layer on one side of this NCM sintered plate by sputtering. In this way, a positive electrode plate was obtained.

(1c) Formation of intermediate layer Tetra-i-propoxytitanium: lithium ethoxide: ethanol were mixed in a molar ratio of 0.0225:0.045:1 to prepare a solution for forming an intermediate layer. The NCM sintered plate prepared in (1b) above was immersed in this solution, the pressure was reduced, and the solution was impregnated into the pores of the positive electrode plate. The above-mentioned operation was performed in a glove box in an Ar atmosphere with a dew point of -50°C or less. Then, the NCM sintered plate was taken out of the glove box and left to stand in the air for 10 minutes to form an intermediate layer. Then, the above series of operations was repeated seven more times (i.e., a total of eight times were formed). Finally, the NCM sintered plate was heat-treated at 400°C for 30 minutes to obtain a positive electrode plate with an intermediate layer formed thereon.

(2) Preparation of negative electrode plate (2a) Preparation of LTO green sheet Commercially available _TiO2 powder (average particle size 1 μm or less) and _Li2CO3 powder (average particle size ₃ μm) weighed so that the molar ratio of Li/Ti is 0.84 were mixed, and then held at 1000 ° C for 2 hours to obtain a powder consisting of LTO particles. This powder was adjusted to an average particle size of about 2 μm by wet grinding in a ball mill, and then mixed with a solvent for tape casting, a binder, a plasticizer, and a dispersant. After adjusting the viscosity of the obtained paste, the paste was molded into a sheet on a PET film to produce an LTO green sheet. The thickness of the LTO green sheet was adjusted so that the thickness after firing was 130 μm.

(2b) Preparation of LTO sintered plate The LTO green sheet peeled off from the PET film was punched out into a circle with a diameter of 11 mm and placed in a sintering sheath. The temperature was raised to 850°C at a heating rate of 200°C/h and held for 2 hours to perform sintering. The thickness of the obtained sintered plate was about 130 μm by SEM observation. A Au film (thickness 100 nm) was formed as a current collecting layer on one side of this LTO sintered plate by sputtering. In this way, a negative electrode plate was obtained.

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

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

(3c) Mortar Grinding The obtained coagulated product was pulverized in a mortar in a glove box in an Ar atmosphere to obtain a solid electrolyte powder having a volume-based D50 particle size of 5 to 50 μm.

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

(5) Evaluation (5a) Degree of Orientation The positive electrode plate prepared in (1) above was subjected to XRD (X-ray diffraction) measurement. This measurement was performed by measuring the XRD profile when X-rays were irradiated onto the plate surface of the positive electrode plate using an XRD device (Rigaku Corporation, RINT-TTR III). From this XRD profile, the ratio of the diffraction intensity (peak height) I _[003] due to the (003) plane to the diffraction intensity (peak height) I _[104] due to the (104) plane of NCM, I _[003] / I _[104] , was calculated, and this was taken as the degree of orientation.

(5b) Measurement of thickness and porosity The thickness and porosity (volume%) of the positive electrode plate (NCM sintered plate not containing a solid electrolyte) prepared in (1) above and the negative electrode plate (LTO sintered plate not containing a solid electrolyte) prepared in (2) above were measured as follows. First, the positive electrode plate (or negative electrode plate) was filled with resin, and then the cross section was polished by ion milling, and the polished cross section was observed with a SEM to obtain a cross-sectional SEM image. The thickness was calculated from this SEM image. The SEM images for porosity measurement were images with magnifications of 1000 times and 500 times. The obtained image was binarized using image analysis software (Image-Pro Premier, manufactured by Media Cybernetics), and the ratio (%) of the area of the resin-filled portion to the total area of the positive electrode plate (or negative electrode plate) of the positive electrode active material (or negative electrode active material) portion and the resin-filled portion (portion that was originally a pore) in the positive electrode plate (or negative electrode plate) was calculated to obtain the porosity (%) of the positive electrode plate (or negative electrode plate). The threshold value for binarization was set using Otsu's binarization as a discriminant analysis method. The porosity of the positive electrode plate was as shown in Table 2, and the porosity of the negative electrode plate was 38%.

(5c) Measurement of average pore size The average pore size was measured using the SEM image used in the above porosity measurement as follows. Using image analysis software (Image-Pro Premier, manufactured by Media Cybernetics), binarization processing was performed, and the positive electrode plate (or negative electrode plate) was separated into the positive electrode active material (or negative electrode active material) portion and the resin-filled portion (the portion that was originally a pore). Then, in the region of the portion filled with resin, the maximum Martin diameter of each region was obtained, and the average value thereof was taken as the average pore diameter (μm) of the positive electrode plate (or negative electrode plate). The average pore diameter of the positive electrode plate was as shown in Table 2, and the average pore diameter of the negative electrode plate was 2.1 μm.

(5d) Average primary particle diameter Using SEM images (magnification 5000x) obtained in the same manner as in the above porosity measurement, the average primary particle diameter was calculated by the intercept method as follows. First, straight lines (line segments) with a total length L were randomly drawn in the SEM image at a magnification of 5000x, and the number of intersections _nL between the line segments and the grain boundaries of the primary particles was calculated. The line segment length L and the number of intersections _nL between the line segments and the grain boundaries of the primary particles were calculated using the following formula:
D ₁ =1.5×L/n _L
The average primary particle diameter _D1 was obtained by the above. The same operation as above was performed twice, changing the position each time, to calculate the average primary particle diameters _D2 and _D3 . The average value of the obtained average primary particle diameters _D1 , _D2, and _D3 was calculated to be the average primary particle diameter D of the positive electrode plate. The average primary particle diameter D of the positive electrode plate was as shown in Table 2.

(5e) Measurement of molar ratio of metal elements in positive electrode plate The molar ratios Ni/(Ni+Co+Mn), Co/(Ni+Co+Mn), and Mn/(Ni+Co+Mn) of each element relative to the total content of Ni, Co, and Mn in the positive electrode plate prepared in (1) above were calculated from the measurement results of metal element analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES method). The results were within ±0.01 of the molar ratios shown in Table 2.

(5f) Identification of solid electrolyte by XRD When the LiOH.Li ₂ SO ₄ -based solid electrolyte obtained in (3c) above was analyzed by X-ray diffraction (XRD), it was identified as 3LiOH.Li ₂ SO ₄ .

(5g) Charge/Discharge Evaluation The discharge capacity of the battery prepared in (4) above was measured in a voltage range of 2.5 V to 1.5 V at an operating temperature of 150° C. This measurement was performed by charging at a constant current and constant voltage until the battery voltage reached the upper limit of the voltage range, and then discharging to the lower limit of the voltage range. At this time, the discharge capacity of Example 1 was regarded as 100, and was used as a standard for showing the discharge capacities of Examples 2 to 9 described later in relative values.

Example 2
In the preparation of the positive electrode plate in (1) above, except that NCM raw material powder 3 shown in Table 1 was used instead of NCM raw material powder 1, a positive electrode plate and a battery were prepared in the same manner as in Example 1, and various evaluations were performed.

Example 3
In the preparation of the positive electrode plate in (1) above, except that NCM raw material powder 4 shown in Table 1 was used instead of NCM raw material powder 1, a positive electrode plate and a battery were prepared in the same manner as in Example 1, and various evaluations were performed.

Example 4
In the preparation of the positive electrode plate in (1) above, except that NCM raw material powder 5 shown in Table 1 was used instead of NCM raw material powder 1, a positive electrode plate and a battery were prepared in the same manner as in Example 1, and various evaluations were performed.

Example 5
In the preparation of the positive electrode plate in (1) above, except that NCM raw material powder 6 shown in Table 1 was used instead of NCM raw material powder 1, a positive electrode plate and a battery were prepared in the same manner as in Example 1, and various evaluations were performed.

Example 6
A positive electrode plate and a battery were produced in the same manner as in Example 1, except that in the preparation of the positive electrode plate in (1) above, 1) NCM mixed powder A containing NCM raw material powders 2 and 6 shown in Table 1 in a blending ratio (weight ratio) of 90:10 was used instead of NCM raw material powder 1, and 2) the firing temperature was set to 950° C., and various evaluations were performed.

Example 7
In the preparation of the positive electrode plate in (1) above, 1) NCM mixed powder A containing NCM raw material powders 2 and 6 shown in Table 1 in a blending ratio (weight ratio) of 90:10 was used instead of NCM raw material powder 1, and 2) the firing temperature was set to 900° C. A positive electrode plate and a battery were prepared in the same manner as in Example 1, and various evaluations were performed.

Example 8
A positive electrode plate and a battery were produced in the same manner as in Example 1, except that in the preparation of the positive electrode plate in (1) above, 1) NCM mixed powder B containing NCM raw material powders 2 and 6 shown in Table 1 in a blending ratio (weight ratio) of 70:30 was used instead of NCM raw material powder 1, and 2) the firing temperature was set to 950° C., and various evaluations were performed.

Example 9
In the preparation of the positive electrode plate in (1) above, 1) NCM mixed powder C containing NCM raw material powders 2 and 6 shown in Table 1 in a blending ratio (weight ratio) of 95:5 was used instead of NCM raw material powder 1, and 2) the firing temperature was set to 970° C. A positive electrode plate and a battery were prepared in the same manner as in Example 1, and various evaluations were performed.

The specifications of the positive electrode plates and the evaluation results of the cells prepared in each example are shown in Table 2. As described above, the charge/discharge characteristics were compared under the same rate conditions, and the discharge capacity measured in Example 1 (comparison) was regarded as 100, and the relative values were calculated and shown in Table 2.

Claims (11)

Ni, Co and Mn,
0.19≦Ni/(Ni+Co+Mn)≦0.41,
0.49≦Co/(Ni+Co+Mn)≦0.71, and 0.09≦Mn/(Ni+Co+Mn)≦0.11
A positive electrode layer including a porous sintered plate composed of a lithium composite oxide having a layered rock salt structure containing the lithium composite oxide in a molar ratio satisfying the following:
a negative electrode layer including a porous sintered plate made of a negative electrode active material;
a solid electrolyte interposed between the positive electrode layer and the negative electrode layer as a separator layer and also filled in the pores of the porous sintered plate of the positive electrode layer and the negative electrode layer;
An all-solid-state secondary battery equipped with the above. The lithium composite oxide contains Ni, Co and Mn,
0.29≦Ni/(Ni+Co+Mn)≦0.31,
0.59≦Co/(Ni+Co+Mn)≦0.61, and 0.09≦Mn/(Ni+Co+Mn)≦0.11
The all-solid-state secondary battery according to claim 1 , wherein the molar ratio satisfies: The porous sintered plate in the positive electrode layer has an orientation degree I[003]/I _{[104] of 1.2 to 3.6, which is defined as the ratio of the diffraction intensity I[} 003] attributable to the (003) plane to the diffraction intensity I _[ 104] attributable to the ( ₁₀₄ ) plane in an XRD profile measured by X-ray diffraction (XRD ₎ . The all-solid-state secondary battery according to claim 1 or 2. The all-solid-state secondary battery according to any one of claims 1 to 3, wherein the average primary particle diameter of the porous sintered plate in the positive electrode layer is 0.4 to 5.0 μm. The all-solid-state secondary battery according to any one of claims 1 to 4, wherein the average pore diameter of the porous sintered plate in the positive electrode layer is 0.5 to 15 µm. The all-solid-state secondary battery according to any one of claims 1 to 5, wherein the porosity of the porous sintered plate in the positive electrode layer is 10 to 40%. The all-solid-state secondary battery according to any one of claims 1 to 6, wherein the solid electrolyte is a LiOH.Li ₂ SO ₄ based solid electrolyte. The all-solid-state secondary battery according to claim 7 , wherein the LiOH.Li ₂ SO ₄ based solid electrolyte comprises a solid electrolyte identified as 3LiOH.Li ₂ SO ₄ by X-ray diffraction. The all-solid-state secondary battery according to claim 7 or 8, wherein the _{LiOH.Li2SO4 -} based solid electrolyte further contains boron. The all-solid-state secondary battery according to any one of claims 1 to 9, wherein the negative electrode active material is a titanium-containing oxide. The all-solid-state secondary battery according to claim 10, wherein the titanium-containing oxide is lithium titanate.