Lithium complex oxide sintered plate and all-solid secondary battery

Abstract

The present application provides a lithium complex oxide sintered plate capable of greatly increasing discharge capacity when assembled as a positive electrode in a lithium ion secondary battery. The lithium complex oxide sintered plate is composed of a lithium complex oxide having a layered rock salt structure containing Li, Ni, Co, and Mn. The lithium complex oxide sintered plate has a porosity of 20 to 40%, an average pore diameter of 3.5 μm or more, and an interface length per 1 μm 2 unit cross-sectional area of 0.45 μm or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a lithium composite oxide sintered plate for a positive electrode of a lithium ion secondary battery and a full solid secondary battery. BACKGROUND

[0002] As a positive electrode active material layer for a lithium ion secondary battery, a powder dispersion type positive electrode, which is obtained by kneading a powder of a lithium composite oxide (typically, a lithium transition metal oxide) and an additive such as a binder and a conductive agent, and molding, is known. Since the powder dispersion type positive electrode contains a large amount (for example, about 10% by weight) of a binder that does not contribute to capacity, the packing density of the lithium composite oxide as a positive electrode active material decreases. Therefore, the powder dispersion type positive electrode has a large room for improvement in capacity and charge / discharge efficiency. Thus, attempts have been made to improve the capacity and charge / discharge efficiency by using a lithium composite oxide sintered plate to constitute a positive electrode or a positive electrode active material layer. In this case, since the positive electrode or the positive electrode active material layer does not contain a binder, the packing density of the lithium composite oxide increases, and thus high capacity and good charge / discharge efficiency can be expected.

[0003] In addition, in a lithium ion secondary battery, a liquid electrolyte (electrolyte) using a flammable organic solvent has been used as a medium for moving ions. In a battery using such an electrolyte, problems such as electrolyte leakage, fire, and explosion can occur. In order to eliminate these problems, a full solid battery in which a solid electrolyte is used instead of a liquid electrolyte and all other elements are constituted as solids is being developed in order to ensure intrinsic safety. The electrolyte of this full solid battery is solid, and thus there is no risk of fire, no leakage, and also, problems such as degradation of battery performance due to corrosion are less likely to occur.

[0004] Various full solid batteries using a sintered body electrode and a solid electrolyte have been proposed. For example, Patent Literature 1 (WO2019 / 093222A1) discloses a full solid lithium battery including an oriented positive electrode plate that is a lithium composite oxide sintered plate having a void ratio of 10 to 50%, and a negative electrode plate containing Ti and capable of supplying lithium ions at 0.4 V (vs. Li / Li +) above is inserted and detached; and a solid electrolyte having a lower melting point than the melting point or the decomposition temperature of the oriented positive electrode plate or the negative electrode plate. In this document, as such a solid electrolyte having a low melting point, various materials such as Li3OCl, xLiOH yLi2SO4 (in the formula, x + y = 1, 0.6 ≤ x ≤ 0.95) (for example, 3LiOH Li2SO4), and the like are disclosed. Such a solid electrolyte can penetrate into the voids of the electrode plate in the form of a molten liquid, and thus firm interface contact can be achieved. As a result, significant improvement in battery resistance and rate performance at the time of charge and discharge, and a large improvement in battery manufacturing yield can be achieved. In addition, Patent Document 2 (WO2015 / 151566A1) discloses an all-solid lithium battery having: an oriented positive electrode plate whose basic composition is represented by Li p (Ni x , Co y , Mn z )O2 (in the formula, 0.9 ≤ p ≤ 1.3, 0 < x < 0.8, 0 < y < 1, 0 ≤ z ≤ 0.7, x + y + z = 1) and having a layered rock salt structure; a solid electrolyte layer composed of a Li-La-Zr-O-based ceramic material and / or a lithium phosphorus oxynitride (LiPON)-based ceramic material; and a negative electrode layer.

[0005] Prior art documents

[0006] Patent Documents

[0007] Patent Document 1: WO2019 / 093222A1

[0008] Patent Document 2: WO2015 / 151566A1 SUMMARY

[0009] The inventors of the present application have obtained the insight that, among the above-mentioned low-melting solid electrolytes, LiOH Li2SO4-based solid electrolytes such as 3LiOH Li2SO4 exhibit high lithium ion conductivity. However, it was ascertained that a single cell composed of a sintered body electrode disclosed in Patent Document 1 using a LiOH Li2SO4-based solid electrolyte such as 3LiOH Li2SO4 has a lower discharge capacity than the theoretical capacity assumed from the amount of active material when the battery is operated.

[0010] The inventors of the present application have recently obtained the insight that, in a lithium composite oxide sintered plate for a positive electrode of a lithium ion secondary battery, the discharge capacity can be significantly improved by controlling the microstructure (particularly, the pores) of the lithium composite oxide sintered plate.

[0011] Therefore, an object of the present application is to provide a lithium composite oxide sintered plate that can significantly improve the discharge capacity when assembled as a positive electrode in a lithium ion secondary battery.

[0012] According to an aspect of the present application, there is provided a lithium composite oxide sintered plate, which is a lithium composite oxide sintered plate for a positive electrode of a lithium ion secondary battery, wherein the lithium composite oxide sintered plate is composed of a lithium composite oxide having a layered rock salt structure containing Li, Ni, Co, and Mn,

[0013] a porosity of 20 to 40%,

[0014] an average pore diameter of 3.5 μm or more,

[0015] per 1 μm 2 an interface length per unit cross-sectional area of 0.45 μm or less.

[0016] According to another aspect of the present application, there is provided an all-solid secondary battery, comprising:

[0017] a positive electrode layer containing a lithium composite oxide sintered plate;

[0018] a negative electrode layer containing a negative electrode active material; and

[0019] a LiOH·Li2SO4-based solid electrolyte, which is interposed between the positive electrode layer and the negative electrode layer as a separator layer, and also fills pores of the lithium composite oxide sintered plate. DETAILED DESCRIPTION

[0020] Definitions

[0021] Hereinafter, definitions for determining parameters of the present application are given.

[0022] In the present specification, "porosity" is a volume ratio of pores in a sintered plate. The porosity can be determined by image analysis of a cross-sectional SEM image of the sintered plate. For example, after resin-embedding the sintered plate, cross-sectional polishing using ion milling, and observation of the polished cross-section with an SEM (scanning electron microscope) to obtain a cross-sectional SEM image (e.g., magnification of 500 to 1000 times), the obtained SEM image is analyzed to calculate the proportion (%) of the area of the resin-filled portion in the total area of the electrode active material portion and the resin-filled portion (the portion originally a pore), thereby calculating the porosity (%) of the sintered plate. If the determination can be performed with the desired accuracy, the porosity can be determined without resin-embedding the sintered plate. For example, a sintered plate in which a solid electrolyte is filled in the pores (a positive electrode plate taken out from an all-solid secondary battery) can be subjected to the determination of the porosity in the state where the solid electrolyte is filled.

[0023] In the present specification, the "average pore diameter" is the average value of the diameters of the pores contained in the sintered plate of the electrode. The above "diameter" is typically the length of the line segment bisecting the projected area of the pore (Martin diameter). In the present invention, it is appropriate to calculate the "average value" on a number basis. The 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 in the above pore rate measurement is analyzed, the portion of the electrode active material in the sintered plate and the portion filled with resin (the portion that was originally a pore) are divided, and then, in the region of the portion filled with resin, the maximum Martin diameter of each region is found, and the average value thereof is set as the average pore diameter of the sintered plate. If it can be measured with the desired accuracy, the average pore diameter can be measured without resin-embedding the sintered plate. For example, the sintered plate in which the pores are filled with solid electrolyte (the positive electrode plate taken out from the all-solid secondary battery) can be measured for the average pore diameter in the state filled with solid electrolyte.

[0024] In the present specification, "per 1 μm 2 The "interface length per unit cross-sectional area" is the total length of the interface of the entire pore / active material contained in the unit cross-sectional area per 1 μm 2 The "interface length per unit cross-sectional area" is the total length of the interface of the entire pore / active material contained in the unit cross-sectional area per 1 μm 2 The "interface length per unit cross-sectional area" is the total length of the interface of the entire pore / active material contained in the unit cross-sectional area per 1 μm

[0025] Lithium complex oxide sintered plate

[0026] The lithium complex oxide sintered plate of the present invention is used for the positive electrode of a lithium ion secondary battery. The lithium complex oxide sintered plate is composed of a lithium complex oxide having a layered rock salt structure containing Li, Ni, Co, and Mn. Furthermore, the lithium complex oxide sintered plate has a porosity of 20 to 40%, an average pore diameter of 3.5 μm or more, and an interface length per unit cross-sectional area of 0.5 to 1.5 μm 2The interface length per unit cross-sectional area is 0.45 μm or less. In this way, in the lithium composite oxide sintered plate for the positive electrode of the lithium ion secondary battery, by controlling the microstructure of the lithium composite oxide sintered plate (particularly, the pores), the discharge capacity can be greatly improved.

[0027] As described above, the all-solid lithium battery using a low-melting solid electrolyte such as LiOH-Li2SO4-based solid electrolyte is known (for example, see Patent Document 1), and by the solid electrolyte penetrating into the voids of the electrode plate in the form of a melt, interface contact can be achieved. As a result, improvement in the battery resistance and the rate capability at the time of charge and discharge and improvement in the battery manufacturing yield can be achieved. However, the sintered body electrode is configured with the LiOH-Li2SO4-based solid electrolyte to form a single cell, and when the battery is operated, the discharge capacity is lower than the theoretical capacity assumed from the amount of active material. It is considered that the reason is that a high-resistance layer that causes deterioration (decrease in conductivity) of the solid electrolyte or hinders Li ion conduction at the interface is formed by the reaction of the positive electrode active material and the solid electrolyte, and adversely affects the charge and discharge characteristics. In this regard, it is considered that according to the present application, by controlling the microstructure of the lithium composite oxide sintered plate (particularly, the pores), the above problems are eliminated or mitigated, and as a result, the discharge capacity is greatly improved. In particular, the unique microstructure in which the average pore diameter is large and the interface length per unit cross-sectional area is small greatly contributes to the improvement in the discharge capacity. It is considered that this is because if the above unique microstructure, the element diffusion between the solid electrolyte and the sintered plate is suppressed, and the decrease in Li ion conductivity due to the deterioration of the solid electrolyte is mitigated.

[0028] The lithium composite oxide sintered plate is composed of a lithium composite oxide having a layered rock salt structure containing Li, Ni, Co, and Mn. In other words, the lithium composite oxide sintered plate has a structure obtained by combining a plurality of primary particles composed of a lithium composite oxide having a layered rock salt structure containing Li, Ni, Co, and Mn. The lithium composite oxide is also referred to as lithium cobalt nickel manganese oxide, and is abbreviated as NCM. The layered rock salt structure refers to a crystal structure in which lithium layers and transition metal layers other than lithium are alternately stacked with oxygen layers (typically, an α-NaFeO2-type structure, that is, a structure in which transition metals and lithium are regularly arranged in the

[111] axial direction of a cubic rock salt type structure). A typical NCM has a composition represented by Li p (Ni x , Co y , Mn z )O2 (in the formula, 0.9 ≤ p ≤ 1.3, 0 < x < 0.8, 0 < y < 1, 0 ≤ z ≤ 0.7, x + y + z = 1, preferably 0.95 ≤ p ≤ 1.10, 0.1 ≤ x < 0.7, 0.1 ≤ y < 0.9, 0 ≤ z ≤ 0.6, x + y + z = 1), for example, Li(Ni 0.5 Co0.2 Mn 0.3 )O2 and Li(Ni 0.3 Co 0.6 Mn 0.1 )O2. Therefore, the molar ratio of Li / (Ni+Co+Mn) in the lithium complex oxide is preferably 0.95 to 1.10, more preferably 0.97 to 1.08, and further preferably 0.98 to 1.05.

[0029] The lithium complex oxide sintered plate has a porosity of 20 to 40%, preferably 20 to 38%, more preferably 20 to 36%, and further preferably 20 to 33%. If within the above range, in the case of producing a battery, the solid electrolyte can be sufficiently filled in the pores, and, since the proportion of the positive electrode active material in the positive electrode increases, a high energy density as a battery can be achieved.

[0030] The lithium complex oxide sintered plate has an average pore diameter of 3.5 μm or more, preferably 3.5 to 15.0 μm, more preferably 3.5 to 10.0 μm, and further preferably 3.5 to 8.0 μm. If within the above range, the solid electrolyte portion (solid electrolyte portion at a distance from the interface) that is not easily deteriorated by a side reaction between the solid electrolyte and the sintered plate increases. It is considered that, as a result, the diffusion of elements between the solid electrolyte and the sintered plate is inhibited, the decrease in Li ion conductivity due to the deterioration of the solid electrolyte is mitigated, and the discharge capacity is more effectively improved.

[0031] The lithium complex oxide sintered plate has a solid electrolyte content of 0.5 to 5.0 μm per 1 μm 2 The interface length per unit cross-sectional area is 0.45 μm or less, preferably 0.10 to 0.40 μm, more preferably 0.10 to 0.35 μm, and further preferably 0.10 to 0.30 μm. If within the above range, the area where the sintered plate and the solid electrolyte undergo a side reaction decreases. It is considered that, as a result, the diffusion of elements between the solid electrolyte and the sintered plate is inhibited, the decrease in Li ion conductivity due to the deterioration of the solid electrolyte is mitigated, and the discharge capacity is more effectively improved.

[0032] From the viewpoint of improving the energy density of a battery and the like, the thickness of the lithium complex oxide sintered plate is preferably 30 to 300 μm, more preferably 50 to 300 μm, and further preferably 80 to 300 μm.

[0033] Method for manufacturing lithium complex oxide sintered plate

[0034] The lithium complex oxide sintered plate of the present application can be produced by any method, but is preferably produced by the following (a) to (c), that is, (a) production of NCM raw material powder, (b) production of NCM green sheet, and (c) firing of the NCM green sheet.

[0035] (a) Preparation of NCM raw material powder

[0036] First, NCM raw material powder is prepared. The preferred NCM raw material powder is Li(Ni 0.5 Co 0.2 Mn 0.3 )O2 powder or Li(Ni 0.3 Co 0.6 Mn 0.1 )O2 powder. The Li(Ni 0.5 Co 0.2 Mn 0.3 )O2 powder can be prepared by mixing (Ni 0.5 Co 0.2 Mn 0.3 )(OH)2 powder and Li2CO3 powder weighed in a molar ratio of Li / (Ni+Co+Mn) of 1.00 to 1.30, and firing at 700 to 1200°C (preferably 750 to 1000°C) for 1 to 24 hours (preferably 2 to 15 hours). Alternatively, the Li(Ni 0.3 Co 0.6 Mn 0.1 )O2 powder can be preferably prepared by mixing (Ni 0.3 Co 0.6 Mn 0.1 )(OH)2 powder and Li2CO3 powder weighed in a molar ratio of Li / (Ni+Co+Mn) of 1.00 to 1.30, and firing at 700 to 1200°C (preferably 750 to 1000°C) for 1 to 24 hours (preferably 2 to 15 hours).

[0037] To realize the unique microstructure (particularly, pores) in the lithium complex oxide sintered plate of the present application, it is preferable to produce slightly large NCM raw material powder having a volume-based D50 particle diameter of 3 to 20 μm (preferably, 5 to 15 μm) and slightly small NCM raw material powder having a volume-based D50 particle diameter of 0.05 to 1 μm (preferably, 0.1 to 0.6 μm), and use the mixed powder obtained by mixing them. The proportion of the slightly large NCM raw material powder in the mixed powder of the above two sizes is preferably 50 to 99% by weight, and more preferably 70 to 95% by weight. The slightly small NCM raw material powder can be produced by pulverizing the slightly large NCM raw material powder by a known method using a ball mill or the like. At this time, for the slightly small NCM raw material powder, it is preferable to add lithium borate (Li3BO3or the like) or lithium sulfate (Li2SO4) for the purpose of promoting sintering. By adding a sintering aid like this, it is easy to realize the desired microstructure (interface length, pore diameter, etc.), and it is also possible to lower the firing temperature compared to the case where no addition is made. The amount of Li3BO3or the like added to the slightly small NCM raw material powder is preferably 0.3 to 69% by weight, and more preferably 1.5 to 51% by weight, with respect to the total amount of the mixed powder after the addition of Li3BO3.

[0038] (b) Production of NCM green sheet

[0039] The NCM raw material powder (preferably, the above-mentioned NCM mixed powder), a solvent, a binder, a plasticizer, and a dispersant are mixed to produce a paste. After adjusting the viscosity of the obtained paste, it is molded into a sheet shape, whereby an NCM green sheet is produced.

[0040] (c) Production of NCM sintered plate

[0041] The NCM green sheet thus produced is cut into a desired size and shape, and is loaded into a sintering pot to be fired. It is preferable to perform the firing by raising the temperature at a rate of 50 to 600°C / h (preferably, 100 to 300°C / h) to 800 to 1000°C (preferably, 850 to 970°C) and maintaining it for 1 to 24 hours (preferably, 2 to 12 hours). A lithium complex oxide sintered plate (NCM sintered plate) is thus obtained.

[0042] All-solid-state secondary battery

[0043] The lithium complex oxide sintered plate of the present application is used for a positive electrode of a lithium ion secondary battery (typically, a full solid battery). Therefore, according to a preferred embodiment of the present application, a full solid secondary battery is provided, which has a positive electrode layer containing the lithium complex oxide sintered plate of the present application, a negative electrode layer, and a LiOH-Li2SO4-based solid electrolyte. The negative electrode layer contains a negative electrode active material. The LiOH-Li2SO4-based solid electrolyte is interposed between the positive electrode layer and the negative electrode layer as a separator layer, and also fills the pores of the lithium complex oxide sintered plate. In this way, the full solid secondary battery containing the lithium complex oxide sintered plate, the microstructure (particularly, the pores) of which is controlled, as the positive electrode layer can exhibit a higher discharge capacity than the conventional full solid secondary battery employing the LiOH-Li2SO4-based solid electrolyte.

[0044] The negative electrode layer (typically, a negative electrode plate) contains a negative electrode active material. As the negative electrode active material, a negative electrode active material generally used in a lithium ion secondary battery can be employed. As examples of such a general negative electrode active material, a carbon-based material, a metal or semimetal such as Li, In, Al, Sn, Sb, Bi, Si, or an alloy containing any of them, or an oxide-based negative electrode active material can be given.

[0045] A particularly preferred negative electrode active material contains a material capable of intercalating and deintercalating lithium ions at 0.4 V (vs. Li / Li + ) or more, and preferably contains Ti. The negative electrode active material satisfying the above condition is preferably an oxide containing at least Ti. As a preferred example of such a negative electrode active material, lithium titanate Li4Ti5O 12 (hereinafter sometimes referred to as LTO), a niobium-titanium composite oxide Nb2TiO7, and titanium oxide TiO2, more preferably LTO and Nb2TiO7, and further preferably LTO can be given. Note that LTO is known to typically have a spinel structure, but other structures can also be employed at the time of charge and discharge. For example, LTO reacts at the time of charge and discharge in a two-phase coexistence of Li4Ti5O 12 (spinel structure) and Li7Ti5O 12 (rock salt structure). Therefore, LTO is not limited to the spinel structure.

[0046] The negative electrode can be a form in which a mixture of a negative electrode active material, an electron-conducting aid, a lithium ion-conducting material, a binder, and the like is molded, which is generally referred to as a mixed electrode, and is preferably a form of a sintered plate obtained by sintering a negative electrode raw material powder. That is, the negative electrode or the negative electrode active material is preferably in the form of a sintered plate. The sintered plate does not contain an electron-conducting aid or a binder, and thus the energy density of the negative electrode can be increased. The sintered plate can be a dense body or a porous body, and a solid electrolyte can be contained in the pores of the porous body.

[0047] The porosity of the negative electrode active material or the sintered sheet thereof is preferably 20 to 45%, more preferably 20 to 40%, and further preferably 25 to 35%. If the porosity is within such a range, the solid electrolyte can be sufficiently filled in the pores of the negative electrode active material, and the proportion of the negative electrode active material in the negative electrode increases, and thus, a high energy density as a battery can be achieved.

[0048] The thickness of the negative electrode active material or the sintered sheet thereof is preferably 40 to 410 μm, more preferably 65 to 410 μm, further preferably 100 to 410 μm, and particularly preferably 107 to 270 μm, from the viewpoint of improving the energy density of the battery and the like.

[0049] The solid electrolyte is a LiOH-Li2SO4-based solid electrolyte. The LiOH-Li2SO4-based solid electrolyte is a complex compound of LiOH and Li2SO4, and a typical composition is represented by the general formula: xLiOH-yLi2SO4 (in the formula, x+y = 1, 0.6≤x≤0.95), and as a representative example, 3LiOH-Li2SO4 (composition of x = 0.75, y = 0.25 in the above general formula) can be given. Preferably, the LiOH-Li2SO4-based solid electrolyte contains a solid electrolyte identified as 3LiOH-Li2SO4 by X-ray diffraction. The preferred solid electrolyte contains 3LiOH-Li2SO4 as a main phase. Whether or not 3LiOH-Li2SO4 is contained in the solid electrolyte can be confirmed by identifying using the ICDD database No. 032-0598 in the X-ray diffraction pattern. Here, "3LiOH-Li2SO4" means a substance whose crystal structure is considered to be the same as that of 3LiOH-Li2SO4, and the crystal composition does not necessarily need to be the same as that of 3LiOH-Li2SO4. That is, as long as the crystal structure is equivalent to that of 3LiOH-Li2SO4, a substance whose composition deviates from LiOH:Li2SO4 = 3: 1 is also included in "3LiOH-Li2SO4". Therefore, even if the solid electrolyte contains a dopant such as boron (for example, 3LiOH-Li2SO4 in which boron is solid-solved and the X-ray diffraction peak shifts to the high angle side), as long as the crystal structure is considered to be the same as that of 3LiOH-Li2SO4, it is 3LiOH-Li2SO4 referred to in the present specification. Similarly, the solid electrolyte used in the present application also allows the inclusion of unavoidable impurities.

[0050] Therefore, in the LiOH-Li2SO4-based solid electrolyte, in addition to the 3LiOH-Li2SO4as the main phase, a heterogeneous phase can be included. The heterogeneous phase can include a plurality of elements selected from Li, O, H, S, and B, or can be composed of only a plurality of elements selected from Li, O, H, S, and B. As examples of the heterogeneous phase, LiOH, Li2SO4, and / or Li3BO3, and the like derived from the raw materials can be given. It is considered that these heterogeneous phases are the raw materials that have not reacted and remained when the 3LiOH-Li2SO4is formed, and since they do not contribute to the conduction of lithium ions, it is preferable that the amount of the heterogeneous phase other than Li3BO3be small. However, the heterogeneous phase containing boron such as Li3BO3can contribute to the improvement of the degree of maintenance of the lithium ion conductivity after the high-temperature long-time retention, and thus can be contained in the desired amount. However, the solid electrolyte can be composed of a single phase of 3LiOH-Li2SO4in which boron is solid-solved.

[0051] The LiOH-Li2SO4-based solid electrolyte (particularly 3LiOH-Li2SO4) preferably further includes boron. By further including boron in the solid electrolyte identified as 3LiOH-Li2SO4, it is possible to significantly suppress the decrease in the lithium ion conductivity after the high-temperature long-time retention. It is presumed that boron enters any of the sites in the crystal structure of 3LiOH-Li2SO4, and the stability of the crystal structure to temperature is improved. The molar ratio of boron B to sulfur S (B / S) included 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 further preferably 0.005 or more and 0.8 or less. If the B / S is within the above range, the maintenance rate of the lithium ion conductivity can be improved. In addition, if the B / S is within the above range, the content of the unreacted heterogeneous phase including boron is reduced, and thus the absolute value of the lithium ion conductivity can be improved.

[0052] The LiOH-Li2SO4-based solid electrolyte can be a compact of a powder obtained by pulverizing a melt-solidified body, and is preferably a melt-solidified body (i.e., a solid obtained by solidifying after being heated and melted).

[0053] The LiOH-Li2SO4-based solid electrolyte is preferably further filled in the pores of the negative electrode layer. From the viewpoint of the charge-discharge rate characteristics and the insulating property of the solid electrolyte, the thickness of the solid electrolyte layer (excluding the portion that enters the pores in the positive electrode layer and the negative electrode layer) is preferably 1 to 500 μm, more preferably 3 to 50 μm, and further preferably 5 to 40 μm.

[0054] Manufacture of all-solid-state secondary battery

[0055] The manufacturing of a lithium ion secondary battery (typically, a full solid battery) can be performed as follows, for example: i) preparing a positive electrode (i.e., the lithium composite oxide sintered plate of the present application) and a negative electrode; ii) sandwiching a solid electrolyte between the positive electrode and the negative electrode, and performing pressurization, heating, etc., to integrate the positive electrode, the solid electrolyte, and the negative electrode. The positive electrode, the solid electrolyte, and the negative electrode can be combined using other methods. In this case, as examples of methods for forming a solid electrolyte between a positive electrode and a negative electrode, there can be mentioned a method of loading a molded body or a powder of a solid electrolyte on one electrode, a method of applying a paste of a solid electrolyte powder on an electrode using screen printing, a method of impinging a powder of a solid electrolyte using an aerosol deposition method, etc., a method of depositing a solid electrolyte powder on an electrode using an electrophoretic method, etc.

[0056] Example

[0057] The present application is further concretely described by the following examples. Note that in the following description, Li(Ni 0.5 Co 0.2 Mn 0.3 )O2, Li(Ni 0.3 Co 0.6 Mn 0.1 )O2, etc., lithium composite oxides having a layered rock salt structure containing Li, Ni, Co, and Mn are simply referred to as "NCM", and Li4Ti5O 12 is simply referred to as "LTO".

[0058] First, NCM raw material powders 1 to 10 for making a positive electrode plate were prepared as shown below. In addition, the characteristics of these raw material powders are summarized in Table 1.

[0059] [Preparation of NCM raw material powder 1]

[0060] A commercially available (Ni 0.5 Co 0.2 Mn 0.3 )(OH)2 powder (average particle diameter 9 to 10 μm) and a Li2CO3 powder (average particle diameter 3 μm) weighed in a molar ratio of Li / (Ni+Co+Mn) = 1.15 were mixed, and then heat-treated at 750°C for 10 hours to obtain NCM raw material powder 1. The volume-based D50 particle diameter of this powder was 8 μm.

[0061] [Preparation of NCM raw material powder 2]

[0062] Li3BO3 (2.45% by weight relative to the total amount of NCM raw material powder 1 and Li3BO3) was added to NCM raw material powder 1, and after the volume-based D50 particle diameter was adjusted to about 0.4 μm by wet pulverization using a ball mill, drying was performed to obtain NCM raw material powder 2.

[0063] [Production of NCM raw material powder 3]

[0064] Li3BO3 (9.2% by weight relative to the total amount of NCM raw material powder 1 and Li3BO3) was added to NCM raw material powder 1, and after the volume-based D50 particle diameter was adjusted to about 0.4 μm by wet pulverization using a ball mill, drying was performed to obtain NCM raw material powder 3.

[0065] [Production of NCM raw material powder 4]

[0066] NCM raw material powder 4 was obtained by wet pulverization using a ball mill to adjust the volume-based D50 particle diameter to about 5.5 μm, followed by drying, with respect to NCM raw material powder 1.

[0067] [Production of NCM raw material powder 5]

[0068] Commercially available (Ni 0.3 Co 0.6 Mn 0.1 ) (OH)2 powder (average particle diameter 7 to 8 μm) and Li2CO3 powder (average particle diameter 3 μm) were mixed in a manner such that the molar ratio of Li / (Ni+Co+Mn) was 1.15, and then heat treatment was performed at 850°C for 10 hours to obtain NCM raw material powder 5. The volume-based D50 particle diameter of this powder was 6.5 μm.

[0069] [Production of NCM raw material powder 6]

[0070] Li3BO3 (9.2% by weight relative to the total amount of NCM raw material powder 5 and Li3BO3) was added to NCM raw material powder 5, and after the volume-based D50 particle diameter was adjusted to about 0.4 μm by wet pulverization using a ball mill, drying was performed to obtain NCM raw material powder 6.

[0071] [Production of NCM raw material powder 7]

[0072] Li3BO3 (16.8% by weight relative to the total amount of NCM raw material powder 5 and Li3BO3) was added to NCM raw material powder 5, and after the volume-based D50 particle diameter was adjusted to about 0.4 μm by wet pulverization using a ball mill, drying was performed to obtain NCM raw material powder 7.

[0073] [Production of NCM raw material powder 8]

[0074] For the NCM raw material powder 5, after adjusting the volume-based D50 particle diameter to about 0.4 μm by wet pulverization using a ball mill, drying was performed to obtain the NCM raw material powder 8.

[0075] [Production of NCM raw material powder 9]

[0076] For the NCM raw material powder 5, after adjusting the volume-based D50 particle diameter to about 4.3 μm by wet pulverization using a ball mill, drying was performed to obtain the NCM raw material powder 9.

[0077] [Production of NCM raw material powder 10]

[0078] After mixing a commercially available (Ni 0.3 Co 0.6 Mn 0.1 ) (OH)2 powder (average particle diameter 7 to 8 μm) and a Li2CO3 powder (average particle diameter 3 μm) in a manner such that the molar ratio of Li / (Ni+Co+Mn) was 1.15, the mixture was maintained at 950°C for 10 hours. For the obtained powder, after adjusting the volume-based D50 particle diameter to about 1.9 μm by wet pulverization using a ball mill, drying was performed to obtain the NCM raw material powder 10.

[0079] Using the above-described raw material powders 1 to 10, positive electrode sheets and batteries were produced as shown below, and various evaluations were performed.

[0080] Example 1

[0081] (1) Production of positive electrode sheet

[0082] (1a) Production of NCM green sheet

[0083] First, the NCM raw material powders 1 and 2 were uniformly mixed in a blending ratio (weight ratio) of 80:20, and an NCM mixed powder A was prepared as shown in Table 1. The mixed powder A, a solvent for casting, a binder, a plasticizer, and a dispersant were mixed. After adjusting the viscosity of the obtained paste, a sheet was formed on a PET (polyethylene terephthalate) film, and thus an NCM green sheet was produced. The thickness of the NCM green sheet was adjusted so that the thickness after firing was 100 μm.

[0084] (1b) Production of NCM sintered sheet

[0085] The NCM green sheet peeled off from the PET film was punched into a circle of 11 mm in diameter using a punch, and was loaded into a sintering boat. The temperature was raised to 940°C at a rate of 200°C / h and held for 10 hours, whereby sintering was performed. The thickness of the sintered plate obtained was about 100 μm thick as observed by SEM. An Au film (thickness 100 nm) was formed on one side of the NCM sintered plate as a current collector layer by sputtering. In this way, a positive electrode plate was obtained.

[0086] (2) Production of a negative electrode plate

[0087] (2a) Production of an LTO green sheet

[0088] A commercially available Ti02 powder (average particle diameter 1 μm or less) and a Li2CO3 powder (average particle diameter 3 μm) weighed in a molar ratio of Li / Ti = 0.84 were mixed, and then heat-treated at 1000°C for 2 hours to obtain a powder composed of LTO particles. The powder was adjusted to an average particle diameter of about 2 μm by wet pulverization using a ball mill, and was mixed with a solvent, a binder, a plasticizer, and a dispersant for casting. The viscosity of the paste obtained was adjusted, and the paste was formed into a sheet shape on a PET film, whereby an LTO green sheet was produced. The thickness of the LTO green sheet was adjusted so that the thickness after sintering was 130 μm.

[0089] (2b) Production of an LTO sintered plate

[0090] The LTO green sheet peeled off from the PET film was punched into a circle of 11 mm in diameter using a punch, and was loaded into a sintering boat. The temperature was raised to 850°C at a rate of 200°C / h and held for 2 hours, whereby sintering was performed. The thickness of the sintered plate obtained was about 130 μm as observed by SEM. An Au film (thickness 100 nm) was formed on one side of the LTO sintered plate as a current collector layer by sputtering. In this way, a negative electrode plate was obtained.

[0091] (3) Production of a solid electrolyte

[0092] (3a) Preparation of a raw material mixed powder

[0093] A Li2SO4 powder (commercial product, purity 99% or more), a LiOH powder (commercial product, purity 98% or more), and a Li3BO3 powder (commercial product, purity 99% or more) were mixed in a molar ratio of Li2SO4:LiOH:Li3BO3 = 1:2.6:0.05, to obtain a raw material mixed powder. The powders were handled in a glove box under an Ar atmosphere, with care taken so that they would not be deteriorated by moisture absorption or the like.

[0094] (3b) Melt synthesis

[0095] The raw material mixed powder was put in a crucible made of high-purity alumina under an Ar atmosphere. The crucible was fixed to an electric furnace, and a melt was produced by heat treatment at 430°C for 2 hours under an Ar atmosphere. Next, the melt was cooled at 100°C / h in the electric furnace to form a solidification.

[0096] (3c) Mortar pulverization

[0097] The obtained solidification was pulverized using a mortar in a glove box under an Ar atmosphere, whereby a solid electrolyte powder having a volume-based D50 particle size of 5 to 50 μm was obtained.

[0098] (4) Production of all-solid-state battery

[0099] The solid electrolyte powder was loaded on the positive electrode plate, and the negative electrode plate was loaded on the solid electrolyte powder. Further, a weight was loaded on the negative electrode plate, and the assembly was heated at 400°C for 45 minutes in an electric furnace. At this time, the solid electrolyte powder was melted and formed a solid electrolyte layer between the electrode plates after solidification. A battery was produced using the obtained single cell composed of a positive electrode plate / solid electrolyte / negative electrode plate.

[0100] (5) Evaluation

[0101] (5a) Measurement of thickness and porosity

[0102] The thickness and porosity (%) of each of the positive electrode plate (NCM sintered plate in a state not containing a solid electrolyte) produced in the above (1) and the negative electrode plate (LTO sintered plate in a state not containing a solid electrolyte) produced in the above (2) were measured as follows. First, after resin embedding of the positive electrode plate (or the negative electrode plate), cross-section polishing was performed using an ion mill, and then the polished cross-section was observed using an SEM to obtain a cross-section SEM image. The thickness was calculated from the SEM image. The SEM images for porosity measurement were images at a magnification of 1000x and 500x. With respect to the obtained images, using an image analysis software (manufactured by Media Cybernetics, Inc., Image-Pro Premier), a 2-value processing was performed, and the proportion (%) of the area of the portion filled with resin in the positive electrode active material (or the negative electrode active material) portion and the total area of the portion filled with resin (the portion originally a pore) was calculated, and was set as the porosity (%) of the positive electrode plate (or the negative electrode plate). With respect to the threshold value at the time of 2-value processing, Otsu's 2-value processing was set as a discriminant analysis method. The porosity of the positive electrode plate is shown in Table 2, and the porosity of the negative electrode plate was 38%.

[0103] (5b) Measurement of average pore diameter

[0104] Using the SEM image for porosity measurement described above, the average pore diameter was determined as follows. Using an image analysis software (manufactured by Media Cybernetics, Inc., Image-Pro Premier), a 2-value processing was performed, and the portion of the positive electrode active material in the positive electrode plate and the portion filled with the resin (the portion originally as a pore) were divided. Thereafter, in the region of the portion filled with the resin, the maximum Feret diameter of each region was calculated, and the average value thereof was set as the average pore diameter (pm) of the positive electrode plate (or the negative electrode plate). The average pore diameter of the positive electrode plate is shown in Table 2, and the average pore diameter of the negative electrode plate was 2.1 pm.

[0105] (5c) The molar ratio of each 1 pm 2 Measurement of interface length per unit cross-sectional area

[0106] Using the SEM image for porosity measurement described above, the interface length per 1 pm 2 unit cross-sectional area was determined as follows. Using an image analysis software (manufactured by Media Cybernetics, Inc., Image-Pro Premier), a 2-value processing was performed, and the portion of the positive electrode active material in the positive electrode plate and the portion filled with the resin (the portion originally as a pore) were divided. Thereafter, in the region of the portion filled with the resin, the circumference of the entire region (i.e., the total length of the interface between the portion of the positive electrode active material and the portion filled with the resin) and the area of the entire region analyzed (i.e., the region including both the portion of the positive electrode active material and the portion filled with the resin) were calculated. The circumference was divided by the area of the entire region analyzed, and was set as the interface length per 1 pm 2 unit cross-sectional area (pm). The results are shown in Table 2.

[0107] (5d) Measurement of molar ratio of metal elements in the positive electrode plate

[0108] The molar ratio of the content of Li relative to the total content of Ni, Co, and Mn, Li / (Ni+Co+Mn), in the positive electrode plate produced in the above (1) was calculated from the measurement results of the metal element analysis by inductively coupled plasma emission spectrometry (ICP-AES method). The results are shown in Table 2.

[0109] (5e) Identification of solid electrolyte by XRD

[0110] The LiOH-Li2SO4-based solid electrolyte obtained in the above (3c) was analyzed by X-ray diffraction (XRD), and as a result, it was identified as 3LiOH-Li2SO4.

[0111] (5f) Charge-discharge evaluation

[0112] The discharge capacity of the battery produced in the above (4) was measured at an operating temperature of 150°C in a voltage range of 2.5 V to 1.5 V. The measurement was performed by performing constant current constant voltage charging until the battery voltage reached the upper limit of the above voltage range, and then performing discharging until the lower limit of the above voltage range was reached. The results are shown in Table 2 as relative values to other examples described later.

[0113] Example 2

[0114] In the production of the positive electrode plate of the above (1), 1) NCM mixed powder B containing NCM raw material powders 1 and 3 shown in Table 1 at a blending ratio (weight ratio) of 90:10 was used instead of mixed powder A; 2) the firing temperature was set to 950°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 1, and various evaluations were performed.

[0115] Example 3

[0116] In the production of the positive electrode plate of the above (1), 1) NCM mixed powder C containing NCM raw material powders 5 and 6 shown in Table 1 at a blending ratio (weight ratio) of 90:10 was used instead of mixed powder A; 2) the firing temperature was set to 920°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 1, and various evaluations were performed.

[0117] Example 4

[0118] In the production of the positive electrode plate of the above (1), the firing temperature was set to 950°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 3, and various evaluations were performed.

[0119] Example 5

[0120] In the production of the positive electrode plate of the above (1), 1) NCM mixed powder D containing NCM raw material powders 5 and 6 shown in Table 1 at a blending ratio (weight ratio) of 95:5 was used instead of mixed powder A; 2) the firing temperature was set to 920°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 1, and various evaluations were performed.

[0121] Example 6

[0122] In the production of the positive electrode plate of the above (1), the firing temperature was set to 950°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 5, and various evaluations were performed.

[0123] Example 7

[0124] In the production of the positive electrode plate of the above (1), 1) NCM mixed powder E containing NCM raw material powders 5 and 7 shown in Table 1 at a blending ratio (weight ratio) of 95:5 was used instead of mixed powder A; 2) the firing temperature was set to 920°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 1, and various evaluations were performed.

[0125] Example 8

[0126] In the production of the positive electrode plate of the above (1), the firing temperature was set to 950°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 7, and various evaluations were performed.

[0127] Example 9

[0128] In the production of the positive electrode plate of the above (1), 1) NCM mixed powder F containing NCM raw material powders 5 and 8 shown in Table 1 at a blending ratio (weight ratio) of 90:10 was used instead of mixed powder A; 2) the firing temperature was set to 950°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 1, and various evaluations were performed.

[0129] Example 10

[0130] In the production of the positive electrode plate of the above (1), 1) NCM mixed powder G containing NCM raw material powders 5 and 8 shown in Table 1 at a blending ratio (weight ratio) of 95:5 was used instead of mixed powder A; 2) the firing temperature was set to 950°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 1, and various evaluations were performed.

[0131] Example 11

[0132] In the production of the positive electrode plate of the above (1), the firing temperature was set to 970°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 10, and various evaluations were performed.

[0133] Example 12 (Comparative)

[0134] In the production of the positive electrode plate of the above (1), 1) only NCM raw material powder 4 shown in Table 1 was used instead of mixed powder A; 2) the firing temperature was set to 920°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 1, and various evaluations were performed.

[0135] Example 13 (Comparative)

[0136] In the production of the positive electrode plate of the above (1), 1) only NCM raw material powder 9 shown in Table 1 was used instead of mixed powder A; 2) the firing temperature was set to 920°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 1, and various evaluations were performed.

[0137] Example 14 (Comparative)

[0138] In the production of the positive electrode plate of the above (1), 1) only the NCM raw material powder 10 shown in Table 1 was used instead of the mixed powder A; 2) the firing temperature was set to 890°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 1, and various evaluations were performed.

[0139] Example 15 (Comparative)

[0140] In the production of the positive electrode plate of the above (1), the firing temperature was set to 920°C, and otherwise, the positive electrode plate and the battery were produced in the same manner as in Example 14, and various evaluations were performed.

[0141] Results

[0142] The specifications of the positive electrode plates produced in each example and the evaluation results of the single cells are shown in Table 2. Note that the charge-discharge characteristics were compared under the same rate conditions, the discharge capacity measured in Example 13 (Comparative) was set to 100, and the relative values with respect to this discharge capacity were calculated and shown in Table 2.

[0143] [Table 1]

[0144]

[0145] [Table 2]

[0146]

[0147] The batteries of Examples 1 to 11 in which the lithium complex oxide sintered plate satisfying the requirements of the present application were used exhibited significantly higher discharge capacities than the batteries of Examples 12 to 15 (comparative examples) in which the lithium complex oxide sintered plate not satisfying the requirements of the present application. This is considered to be because, by making the interface length small, the area where the positive electrode layer and the solid electrolyte undergo a side reaction is reduced, and in addition, by making the average pore diameter large, the solid electrolyte portion (solid electrolyte portion at a distance from the interface) that is not easily deteriorated by a side reaction is increased. It is considered that as a result of this, the decrease in Li ion conductivity due to the deterioration of the solid electrolyte is mitigated, and thus the rate characteristics and the discharge capacity are greatly improved.

Claims

1. A lithium complex oxide sintered plate for use in a positive electrode of a lithium ion secondary battery comprising a solid electrolyte, the lithium complex oxide sintered plate is characterized in that, the lithium complex oxide sintered plate is composed of a lithium complex oxide having a layered rock salt structure comprising Li, Ni, Co, and Mn, the porosity is 20 to 40%, the average pore diameter is 3.5 μm or more, per 1 μm 2 The interface length per unit cross-sectional area is 0.45 μm or less, per 1μm 2 The interface length per unit cross-sectional area is: per 1μm of the sintered plate. 2 The total length of the interfaces of all pores / active materials contained within a unit cross-sectional area is determined as follows: By analyzing a cross-sectional SEM image of the sintered plate, the portion containing the electrode active material and the portion filled with resin (i.e., the original pore portion) are separated. Then, within the resin-filled region, the perimeter of the entire region (i.e., the total length of the interfaces between the positive electrode active material portion and the resin-filled portion) and the area of ​​the analyzed entire region (i.e., the region including both the positive electrode active material portion and the resin-filled portion) are calculated. The perimeter is then divided by the area of ​​the analyzed entire region, and this is set as the length per 1 μm. 2 Interface length per unit cross-sectional area.

2. The lithium complex oxide sintered plate according to claim 1, characterized in that, The interface length per 1 μm 2 The interface length per unit cross-sectional area is 0.10 to 0.40 μm.

3. The lithium complex oxide sintered plate according to claim 1 or 2, characterized in that, the porosity is 20 to 36%.

4. The lithium complex oxide sintered plate according to claim 1 or 2, characterized in that, the average pore diameter is 3.5 to 15.0 μm.

5. The lithium complex oxide sintered plate according to claim 1 or 2, characterized in that, the thickness of the lithium complex oxide sintered plate is 30 to 300 μm.

6. The lithium complex oxide sintered plate according to claim 1 or 2, characterized in that, the molar ratio of Li / (Ni+Co+Mn) in the lithium complex oxide is 0.95 to 1.

10.

7. A full-solid secondary battery characterized by comprising: comprising: a positive electrode layer comprising the lithium complex oxide sintered plate according to any one of claims 1 to 6; a negative electrode layer comprising a negative electrode active material; and a LiOH-Li2SO4-based solid electrolyte interposed as a separator layer between the positive electrode layer and the negative electrode layer, and also filled in pores of the lithium complex oxide sintered plate.

8. The all-solid secondary battery according to claim 7, characterized in that, The negative active material is Li4Ti5O 12 .

9. The all-solid secondary battery according to claim 7 or 8, characterized in that, the LiOH-Li2SO4-based solid electrolyte is also filled in pores of the negative electrode layer.

10. The all-solid secondary battery according to claim 7 or 8, characterized in that, the LiOH-Li2SO4-based solid electrolyte comprises a solid electrolyte identified as 3LiOH-Li2SO4 by X-ray diffraction.

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