JP7680429B2 - Lithium-ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

In recent years, with the development of portable devices such as personal computers and mobile phones, the demand for batteries as their power source has expanded significantly. In batteries used for such purposes, a liquid electrolyte (electrolytic solution) using a flammable organic solvent as a diluting solvent has been used as a medium for moving ions. In batteries using such electrolytes, problems such as electrolyte leakage, fire, and explosion may occur. In order to solve such problems, in order to ensure essential safety, development of all-solid-state batteries is being promoted in which a solid electrolyte is used instead of a liquid electrolyte and all other elements are composed 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 cause problems such as deterioration of battery performance due to corrosion.

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

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

Furthermore, Patent Document 3 (WO2015/151566A1) discloses an all-solid-state lithium battery including 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 a Li-La-Zr-O based ceramic material and/or a lithium oxynitride phosphate (LiPON) based ceramic material, and a negative electrode layer.

JP 2009-193940 A WO2019/093222A1 WO2015/151566A1

The present inventors have found that, among the above-mentioned low melting point solid electrolytes, LiOH.Li ₂ SO ₄ -based solid electrolytes, such as 3LiOH.Li ₂ SO ₄ , exhibit high lithium ion conductivity. However, when a cell was constructed by combining a positive electrode containing a positive electrode active material composed of an oxide having a layered rock salt structure containing Li, Ni, Co, and Mn with a LiOH.Li ₂ SO ₄ -based solid electrolyte, such as 3LiOH.Li ₂ SO ₄ , and operating the cell, it was found that the discharge amount was lower than the theoretical capacity expected from the amount of active material.

The present inventors have now discovered that by further incorporating Ti into a positive electrode active material composed of an oxide having a layered rock salt structure containing Li, Ni, Co, and Mn, the element diffusion between the positive electrode active material and a _{LiOH.Li2SO4 -} based solid electrolyte is suppressed, thereby improving the discharge capacity.

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

According to one aspect of the present invention, a positive electrode including a positive electrode active material that is composed of an oxide having a layered rock salt structure containing Li, Ni, Co, and Mn and further contains Ti;
a negative electrode including a negative electrode active material;
A _LiOH.Li2SO4 - _based solid electrolyte interposed between the positive electrode and the negative electrode;
A lithium ion secondary battery is provided.

Electron microscope photographs and EPMA mapping images of the cross section of the positive electrode active material (NCM)/solid electrolyte of the all-solid-state battery produced in Example 2. The image located at the far left is the electron microscope photograph (white parts correspond to the NCM, black parts correspond to the solid electrolyte), and EPMA mapping images of Ti, Mn, Co, and Ni are shown in order from there to the right. 1 is an XRD profile of the positive electrode sintered plate (NCM) produced in Example 2. Electron microscope photographs and EPMA mapping images of the cross section of the positive electrode active material (NCM)/solid electrolyte of the all-solid-state battery produced in Example 6. The image located at the far left is the electron microscope photograph (white parts correspond to the NCM, black parts correspond to the solid electrolyte), and EPMA mapping images of Mn, Co, and Ni are shown in order from there to the right.

Lithium ion secondary battery The lithium ion secondary battery of the present invention comprises a positive electrode, a negative electrode, and a LiOH.Li ₂ SO ₄ solid electrolyte. The positive electrode includes a positive electrode active material. The positive electrode active material is composed of an oxide having a layered rock salt structure containing Li, Ni, Co, and Mn, and further includes Ti. The negative electrode includes a negative electrode active material. The LiOH.Li ₂ SO ₄ solid electrolyte is interposed between the positive electrode and the negative electrode, but may be inserted into the voids of the positive electrode or the inside of the negative electrode. In any case, by further including Ti in the positive electrode active material composed of an oxide having a layered rock salt structure containing Li, Ni, Co, and Mn (i.e., lithium cobalt nickel manganese oxide (hereinafter referred to as NCM)), element diffusion between the positive electrode active material and the LiOH.Li ₂ SO ₄ solid electrolyte is suppressed, thereby improving the discharge capacity, i.e., improving the rate characteristics.

That is, as mentioned above, it has been found that LiOH.Li ₂ SO ₄ -based solid electrolytes such as 3LiOH.Li ₂ SO ₄ exhibit high lithium ion conductivity. However, when a cell was constructed by combining a positive electrode containing a positive electrode active material composed of NCM with a LiOH.Li ₂ SO ₄ -based solid electrolyte such as 3LiOH.Li ₂ SO ₄ and operating the cell as a battery, it was found that the discharge amount was lower than the theoretical capacity expected from the amount of active material. This is thought to be due to the deterioration of the solid electrolyte (decrease in conductivity) due to the reaction between the positive electrode active material and the solid electrolyte and the formation of a high resistance layer at the interface affecting the charge and discharge characteristics. Therefore, in order to suppress the reaction between NCM and LiOH.Li ₂ SO ₄ -based solid electrolyte, Ti was added to NCM as a heterometal, and an attempt was made to stabilize the structure and reduce the reactivity with the solid electrolyte, and it was found that the above problem was conveniently solved. This is thought to be because the addition of Ti to the NCM suppresses element diffusion between _{the LiOH.Li2SO4} solid electrolyte and the NCM, mitigating the decline in lithium ion conductivity due to the degradation of the solid electrolyte, and as a result, leading to improved rate characteristics. In fact, it was confirmed that the discharge capacity at the same rate was improved in the all-solid-state battery using the NCM with Ti added, compared to the all-solid-state battery using the NCM without the addition of a different element. In addition, it was confirmed from the cross-sectional analysis of the all-solid-state battery before and after charging and discharging that the diffusion of the positive electrode constituent metal elements into the solid electrolyte was suppressed in the NCM with Ti added, compared to the NCM without the addition.

(1) Positive electrode The positive electrode (typically a positive electrode plate) contains a positive electrode active material. The positive electrode active material is composed of an oxide (NCM) with a layered rock salt structure containing Li, Ni, Co, and Mn. 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 sandwiched between them (typically an α- _NaFeO2 type structure: a structure in which transition metals and lithium are regularly arranged in the [111] axis direction of a cubic rock salt structure). A typical NCM has a 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, preferably 0.95≦p≦1.1, 0.1≦x<0.7, 0.1≦y<0.9, 0≦z≦0.6, x+y+z=1), e.g., Li(Ni _0.5 Co _0.2 Mn _0.3 )O ₂ and Li(Ni _0.3 Co _0.6 Mn _0.1 )O _2. Therefore, the molar ratio of Li/(Ni+Co+Mn) in the positive electrode active material (NCM) is preferably 0.90 to 1.30, more preferably 0.95 to 1.10. This molar ratio can be determined by inductively coupled plasma atomic emission spectrometry (ICP-AES).

The positive electrode active material (NCM) further contains Ti. The inclusion of Ti suppresses element diffusion between the positive electrode active material and the LiOH.Li ₂ SO ₄ -based solid electrolyte, thereby improving the discharge capacity. In order to enhance this effect, the molar ratio of Ti/(Ni+Co+Mn) in the positive electrode active material is preferably 0.01 to 0.10, more preferably 0.01 to 0.07, even more preferably 0.02 to 0.07, and particularly preferably 0.02 to 0.05. The molar ratio of Ti/(Ni+Co+Mn) can be determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The form of Ti in the positive electrode active material is not particularly limited, but it is preferable that Ti is solid-dissolved in a layered rock salt structure. By dissolving Ti in the layered rock salt structure, element diffusion between the positive electrode active material (NCM) and the LiOH.Li ₂ SO ₄ based solid electrolyte is more effectively suppressed.

The addition of Ti to the positive electrode active material (NCM) may be performed by any method. For example, an additive-free NCM can be produced by wet-grinding a raw material powder to form a slurry (or paste), tape-molding the slurry (or paste), and drying, degreasing, and sintering the resulting molded body according to a known method (see, for example, _Patent Document ₃ ). In this case, Ti can be preferably added by adding _TiO2 powder to the raw material powder (for example, a mixed powder of ( _{Ni0.5Co0.2Mn0.3} )(OH) ₂ powder and _{Li2CO3 powder} ) before wet grinding.

The positive electrode active material (NCM) preferably further contains B. The inclusion of B is believed to suppress side reactions at the interface between the positive electrode active material and the _LiOH.Li2SO4 - _based solid electrolyte, and to alleviate stress caused by expansion and contraction of the positive electrode active material during charging and discharging, thereby resulting in further improvement of the discharge capacity.

The addition of B to the positive electrode active material (NCM) may be performed by any method. For example, an additive-free NCM can be produced by wet-grinding a raw material powder to form a slurry (or paste), tape-molding the slurry (or paste), and drying, degreasing, and sintering the resulting molded body according to a known method (see, for example, Patent Document 3). In this case, B can be preferably added by adding Li ₃ BO ₃ powder to the raw material powder (for example, a mixed powder of (Ni _0.5 Co _0.2 Mn _0.3 ) (OH) ₂ powder and Li ₂ CO ₃ powder) before wet grinding.

The positive electrode may be in the form of a mixture of a positive electrode active material, an electronic conduction assistant, a lithium ion conductive material, a binder, etc., generally called a composite electrode, or a mixture of a positive electrode active material, a LiOH.Li ₂ SO ₄ solid electrolyte, an electronic conduction assistant, etc., formed (composite form). That is, the positive electrode may contain particles of a positive electrode active material, particles of a LiOH.Li ₂ SO ₄ solid electrolyte, and an electronic conduction assistant in the form of a composite. However, the positive electrode is preferably in the form of a sintered plate obtained by sintering a positive electrode raw material powder. That is, the positive electrode or the positive electrode active material is preferably in the form of a sintered plate. Since the sintered plate does not need to contain an electronic conduction assistant or a binder, the energy density of the positive electrode can be increased. The sintered plate may be a dense body or a porous body, and the pores of the porous body may contain a solid electrolyte. In the form of a composite material, the positive electrode active material particles have a preferred particle size of 0.05 to 50 μm, more preferably 0.1 to 30 μm, and even more preferably 0.5 to 20 μm. The LiOH.Li ₂ SO ₄ solid electrolyte particles have a preferred particle size of 0.01 to 50 μm, more preferably 0.05 to 30 μm, and even more preferably 0.1 to 20 μm. The electron conductive assistant is not particularly limited as long as it is an electron conductive material commonly used in electrodes, but a carbon material is preferred. Preferred examples of carbon materials include carbon black, graphite, carbon nanotubes, graphene, reduced graphene oxide, and any combination thereof, but are not limited thereto, and various other carbon materials can also be used.

The density (filling rate) of the positive electrode active material in the positive electrode is preferably 50 to 80% by volume, more preferably 55 to 80% by volume, even more preferably 60 to 80% by volume, and particularly preferably 65 to 75% by volume, regardless of the form of the positive electrode (sintered plate or composite). With a density within this range, the solid electrolyte can be sufficiently filled into the voids in the positive electrode active material, and the proportion of positive electrode active material in the positive electrode increases, making it possible to achieve a high energy density as a battery.

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

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

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

The negative electrode may be in the form of a mixture of a negative electrode active material, an electronic conduction assistant, a lithium ion conductive material, a binder, etc., generally called a composite electrode, or a mixture of a negative electrode active material, a LiOH.Li ₂ SO ₄ solid electrolyte, an electronic conduction assistant, etc., formed. That is, the negative electrode may contain particles of a negative electrode active material, particles of a LiOH.Li ₂ SO ₄ solid electrolyte, and an electronic conduction assistant in the form of a composite material. However, the negative electrode is preferably in the 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. Since the sintered plate does not need to contain an electronic conduction assistant or a binder, the energy density of the negative electrode can be increased. The sintered plate may be a dense body or a porous body, and the pores of the porous body may contain a solid electrolyte. In the form of a composite material, the negative electrode active material particles have a preferred particle size of 0.05 to 50 μm, more preferably 0.1 to 30 μm, and even more preferably 0.5 to 20 μm. The LiOH.Li ₂ SO ₄ solid electrolyte particles have a preferred particle size of 0.01 to 50 μm, more preferably 0.05 to 30 μm, and even more preferably 0.1 to 20 μm. The electron conductive assistant is not particularly limited as long as it is an electron conductive material commonly used in electrodes, but a carbon material is preferred. Preferred examples of carbon materials include carbon black, graphite, carbon nanotubes, graphene, reduced graphene oxide, and any combination thereof, but are not limited thereto, and various other carbon materials can also be used.

The density (filling rate) of the negative electrode active material in the negative electrode is preferably 55 to 80% by volume, more preferably 60 to 80%, and even more preferably 65 to 75%, regardless of the form of the negative electrode (sintered plate or composite). With a density within this range, the solid electrolyte can be sufficiently filled into the voids in the negative electrode active material, and the proportion of the negative electrode active material in the negative electrode increases, making it possible to achieve a high energy density as a battery.

Regardless of the form of the negative electrode (sintered plate or composite), from the viewpoint of improving the energy density of the battery, the thickness of the negative electrode is preferably 75 to 350 μm, more preferably 100 to 325 μm, even more preferably 125 to 300 μm, and particularly preferably 130 to 275 μm.

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

Therefore, the LiOH.Li ₂ SO ₄ solid electrolyte may contain a heterogeneous phase in addition to the main phase 3LiOH.Li ₂ SO _4. The heterogeneous phase may contain a plurality of elements selected from Li, O, H, S, and B, or may be composed of only a plurality of elements selected from Li, O, H, S, and B. Examples of the heterogeneous phase include LiOH, Li ₂ SO ₄ , and/or Li ₃ BO ₃ derived from the raw material. These heterogeneous phases are considered to be unreacted raw materials remaining when 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.Li2SO4 - _based solid electrolyte may be a compact of powder obtained by pulverizing a molten solid, but is preferably a molten solid (i.e., solidified after heating and melting). The pulverization method of the molten solid is not particularly limited, but a method using a general mortar, ball mill, jet mill, roller mill, cutter mill, ring mill, etc. can be adopted, and may be a wet or dry method.

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

(4) Manufacturing of Lithium Ion Secondary Battery The manufacturing of a lithium ion secondary battery can be performed, for example, by i) preparing a positive electrode (with a current collector formed as necessary) and a negative electrode (with 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 the 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 is abbreviated as "_{NCM","NCM523" means Li(Ni0.5Co0.2Mn0.3 ) O2 ,} and _{"NCM361" means Li(Ni0.3Co0.6Mn0.1)O2 . Li4Ti5O12 is abbreviated as} "LTO".

[Examples 1 to 15]
The example described below is an example relating to an impregnated sintered type all-solid-state secondary battery.

Example 1
(1) Preparation of positive electrode plate (1a) Preparation of NCM green sheet Commercially available (Ni0.5Co0.2Mn0.3)(OH ₎₂ powder (average particle size 9 μm) and _Li2CO3 powder (average particle size ₃ μm) were mixed so that the molar ratio of Li/(Ni ₊ Co _{+ Mn} ) was 1.15, and then held at 750 ° C for 10 hours to obtain a powder consisting of NCM particles. _TiO2 powder weighed so that the molar ratio of Ti/(Ni+Co+Mn) was 0.025 was added to this powder, and the average particle size was adjusted to about 5 μm by wet grinding in a ball mill. The mixed powder was then mixed with a solvent for tape molding, a binder, a plasticizer, and a dispersant. After adjusting the viscosity of the obtained paste, an NCM green sheet was produced by forming it into a sheet on a PET film. 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 measured by SEM observation and found to be about 100 μm. A Au film (thickness 100 nm) was formed as a current collecting layer on one side of this NCM sintered plate by sputtering.

(2) Preparation of negative electrode plate (2a) Preparation of LTO green sheet Commercially available _TiO2 powder (average particle size 1 μm or less) and _Li2CO3 powder (average particle size ₃ μm) were mixed so that the molar ratio of Li/Ti was 0.84, and then held at 1000 ° C for 2 hours to obtain a powder consisting of LTO particles. This powder was 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, an LTO green sheet was produced by forming it into a sheet on a PET film. 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 firing sheath. The temperature was raised to 850°C at a heating rate of 200°C/h and held for 2 hours to perform firing. The thickness of the obtained sintered plate was measured by SEM observation and found to be about 130 μm. A Au film (thickness 100 nm) was formed as a current collecting layer on one side of this LTO sintered plate by sputtering.

(3) Preparation of solid electrolyte (3a) Preparation of raw material powder _{Li2SO4 powder} (commercial product, purity 99% or more), LiOH powder (commercial product, purity 98% or more), and _Li3BO3 (commercial product, purity ₉₉ % or more) were mixed to obtain a raw material mixed powder with a molar ratio _{of Li2SO4} :LiOH: _Li3BO3 = 1: _2.6 :0.05. These powders were handled in a glove box in an Ar atmosphere, 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 an average particle size D50 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) Analysis of Positive Electrode Plate/Solid Electrolyte Interface The battery prepared in (4) above was disassembled in a glove box, and the interface between the positive electrode plate and the solid electrolyte was subjected to SEM observation and elemental mapping by an electron probe microanalyzer (EPMA).

(5b) Measurement of thickness and compactness The thickness and compactness (volume%) of the positive electrode plate (NCM sintered plate without solid electrolyte) prepared in (1b) above and the negative electrode plate (LTO sintered plate without solid electrolyte) prepared in (2b) 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 of the positive electrode plate (or negative electrode plate) was calculated from this SEM image. The SEM image for compactness measurement was an image with a magnification of 1000 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 positive electrode active material (or negative electrode active material) to the total area of the positive electrode active material (or negative electrode active material) and the resin-filled portion (part that was originally a void) in the positive electrode plate (or negative electrode plate) was calculated to determine the density of the positive electrode active material (or negative electrode active material). The threshold value for binarization was set using Otsu's binarization as a discriminant analysis method.

(5c) Molar Ratio of Metal Elements The molar ratio of the Ti content in the positive electrode plate to the total content of Ni, Co, and Mn (Ti/(Ni+Co+Mn)) and the molar ratio of the Li content in the positive electrode plate to the total content of Ni, Co, and Mn (Li/(Ni+Co+Mn)) were calculated from the measurement results of metal element analysis by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

(5c) Charge/Discharge Evaluation The discharge capacity of the battery prepared in (4) above at an operating temperature of 150° C. was measured in a voltage range of 2.5 V to 1.5 V. This measurement was performed by charging the battery at a constant current until the battery voltage reached the upper limit of the voltage range, and then discharging the battery until it reached the lower limit of the voltage range.

Example 2
In the above (1a) NCM pulverization, _TiO2 powder was added so that the molar ratio of Ti/(Ni+Co+Mn) was 0.05. The battery was produced and evaluated in the same manner as in Example 1. FIG. 1 shows an electron microscope photograph and an EPMA mapping image of the cross section of the positive electrode active material (NCM)/solid electrolyte of the all-solid-state battery produced in this example. The image located at the leftmost position in FIG. 1 is an electron microscope photograph (the white part corresponds to the NCM, and the black part corresponds to the solid electrolyte), and from there to the right, EPMA mapping images of Ti, Mn, Co, and Ni are shown in order. FIG. 2 also shows the XRD profile of the positive electrode sintered plate (NCM) produced in this example.

Example 3
In the preparation of the mixed powder _of raw materials for solid electrolyte (3a) above, _Li2SO4 powder, LiOH powder and _Li3BO3 powder were mixed in _{a molar} ratio of _Li2SO4 :LiOH: _Li3BO3 = ₁ :3.0:0.05. Except for this, a battery was prepared and evaluated in the same manner as in Example 2.

Example 4
A battery was produced and evaluated in the same manner as in Example 1, except that in the NCM pulverization in (1a) above, _TiO2 powder was added so that the molar ratio of Ti/(Ni+Co+Mn) was 0.07.

Example 5
In the above (1a) NCM grinding, _TiO2 powder was added so that the molar ratio of Ti/(Ni+Co+Mn) was 0.10. Except for this, a battery was produced and evaluated in the same manner as in Example 1.

Example 6 (Comparison)
A battery was produced and evaluated in the same manner as in Example 1, except that _TiO2 powder was not added in the NCM pulverization in (1a) above. Figure 3 shows an electron microscope photograph and an EPMA mapping image of the cross section of the positive electrode active material (NCM)/solid electrolyte of the all-solid-state battery produced in this example. The image located at the far left in Figure 3 is an electron microscope photograph (the white part corresponds to the NCM, and the black part corresponds to the solid electrolyte), and EPMA mapping images of Mn, Co, and Ni are shown in order from there to the right.

Example 7 (Comparison)
In the NCM pulverization in (1a) above, _a battery was produced and evaluated in the same manner as in Example 1, except that instead of adding _TiO2 powder, _Al2O3 powder was added so that the molar ratio of Al/(Ni+Co+Mn) was 0.05. Note that instead of the Ti content in the positive electrode plate, the molar ratio of the Al content to the total content of Ni, Co and Mn (Al/(Ni+Co+Mn)) was calculated.

Example 8 (Comparative)
In the NCM pulverization in (1a) above, _a battery was produced and evaluated in the same manner as in Example 1, except that instead of adding _TiO2 powder, _Nb2O5 powder was added so that the molar ratio of Nb/(Ni+Co+Mn) was 0.025. Note that instead of the Ti content in the positive electrode plate, the molar ratio of the Nb content to the total content of Ni, Co and Mn (Nb/(Ni+Co+Mn)) was calculated.

Example 9
A _battery was produced and evaluated in the same manner as in Example 1, except that (i) in the preparation of the NCM green sheet in (1a) above, _{TiO2 powder} weighed out so that the molar ratio of Ti/(Ni+Co+ _Mn ) was 0.05 and _Li3BO3 powder were added in an amount of 0.5 wt% (based on the total amount of the powder consisting of NCM particles and Li3BO3 powder), and (ii) in the preparation of the NCM sintered plate in (1b) above, the NCM green sheet was sintered at 940°C.

Example 10
(i) In the preparation of the NCM green sheet in (1a) above, _TiO2 powder weighed out so that the molar ratio of Ti/(Ni+Co+Mn) was 0.05 and 1.0% by weight of _{Li3BO3 powder} (relative to the total amount of the powder consisting of NCM particles and _{Li3BO3 powder} ) were added, and the mixed powder was adjusted to an average particle size of about 6 μm by wet grinding in a ball mill, and (ii) in the preparation of the NCM sintered plate in (1b) above, the NCM green sheet was sintered at 950 ° C. The batteries were prepared and evaluated in the same manner as in Example 1.

The specifications and evaluation results of the batteries prepared in each example are shown in Table 1. The charge/discharge characteristics were compared under the same rate conditions, and the discharge capacity measured in Example 6 (comparison) was set to 100, and the relative values were calculated and shown in Table 1.

Example 11
An NCM green sheet was prepared as follows, and a battery was prepared and evaluated in the same manner as in Example 1, except that in the preparation of the NCM sintered plate in (1b) above, the NCM green sheet was sintered at 950°C.

(1a') Preparation of NCM green sheet Commercially available ( _{Ni0.3Co0.6Mn0.1} )(OH) ₂ powder (average particle size _7-8μm ) and _Li2CO3 powder (average particle size _3μm ) were mixed so that the molar ratio of Li/(Ni+Co+Mn) was 1.15, and then held at 850°C for 10 hours to obtain a powder consisting of NCM particles. _TiO2 powder weighed so that the molar ratio of Ti/(Ni+Co+Mn) was 0.02 was added to _this powder, and _Li3BO3 powder was further added at 1.0 _wt % (relative to the total amount of the powder consisting of NCM particles and _{Li3BO3 powder} ), and the average particle size was adjusted to about 5μm by wet grinding in a ball mill. The mixed powder was then mixed with a solvent, binder, plasticizer, and dispersant for tape casting. The obtained paste was adjusted in viscosity and then molded into a sheet on a PET film to produce an NCM green sheet. The thickness of the NCM green sheet was adjusted so that the thickness after firing would be 100 μm.

Example 12
In the preparation of the NCM green sheet (1a') above, _TiO2 powder was added so that the molar ratio of Ti/(Ni+Co+Mn) was 0.05. The same procedure as in Example 11 was repeated to prepare and evaluate the battery.

Example 13
In the preparation of the NCM green sheet (1a') above, _TiO2 powder was added so that the molar ratio of Ti/(Ni+Co+Mn) was 0.05, _Li3BO3 powder was not added, and the average _particle size was adjusted to about 4 μm by wet grinding in a ball mill. Except for this, a battery was prepared and evaluated in the same manner as in Example 11.

Example 14 (Comparative)
A battery was produced and evaluated in the same manner as in Example 11, except that in the production of the NCM green sheet (1a') above, _TiO2 powder and _{Li3BO3 powder} were not added.

Example 15 (Comparative)
In the preparation of the NCM green sheet in (1a') above, _TiO2 powder was not added, and in the preparation of the NCM sintered plate in (1b) above, the NCM green sheet was sintered at 920 ° C. The same procedures as in Example 13 were followed to prepare and evaluate the battery.

The specifications and evaluation results of the batteries prepared in Examples 11 to 15 are shown in Table 2. The charge/discharge characteristics are shown in Table 2 as relative values calculated with the discharge capacity measured in Example 14 (comparison) taken as 100.

[Examples 16 to 20]
The example described below is an example relating to a composite type all-solid-state secondary battery.

Example 16
(1) Preparation of Positive Electrode Active Material Powder Commercially available ( _{Ni0.3Co0.6Mn0.1} )(OH) ₂ powder (average particle size _7-8 μm) and _Li2CO3 powder (average particle size 3 μm) were weighed out so that the molar ratio of Li/(Ni+Co+Mn) _{was 1.02} , and _TiO2 powder weighed out so that the molar ratio of Ti/(Ni+Co+Mn) was 0.02 was added and mixed. The mixture was then kept at 850° C. for ₁₀ hours to obtain a powder made of NCM particles with an average particle size of about 6.5 μm.

(2) Preparation of Negative Electrode Active Material Powder Commercially available carbon powder (average particle size: 10 to 14 μm) was prepared.

(3) Preparation of solid electrolyte (3a) Preparation of raw material powder _{Li2SO4 powder} (commercial product, purity 99% or more), LiOH powder (commercial product, purity 98% or more), and _Li3BO3 (commercial product, purity ₉₉ % or more) were mixed to obtain a raw material mixed powder with a molar ratio _{of Li2SO4} :LiOH: _Li3BO3 = 1: _2.2 :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) Pulverization The obtained solidified product was pulverized in a mortar in a glove box in an Ar atmosphere, and further pulverized using balls to obtain a solid electrolyte powder having an average particle size D50 of 1 to 20 μm.

(4) Preparation of all-solid-state battery (4a) Preparation of positive electrode composite powder and negative electrode composite powder The positive electrode active material powder obtained in (1) above, the solid electrolyte powder obtained in (3) above, and the electron conductive assistant (acetylene black (commercially available)) were weighed out to a volume ratio of 60:40:2, and these were mixed in a mortar to prepare a positive electrode composite powder. Similarly, the negative electrode active material powder obtained in (2) above, the solid electrolyte powder obtained in (3) above, and the electron conductive assistant (acetylene black (commercially available)) were weighed out to a volume ratio of 60:40:2, and these were mixed in a mortar to prepare a negative electrode composite powder.

(4b) Press molding In a press mold having a hole diameter of 10 mm, the positive electrode layer, the solid electrolyte layer, and the negative electrode layer were charged with powder and pressed at 100 MPa in that order so that the thicknesses of the layers were 110 μm, 500 μm, and 200 μm, respectively. After stacking the three layers in this way, the laminate was pressed at 150 MPa to obtain a press molded body.

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

(5) Evaluation (5a) Charge/Discharge Evaluation For the batteries prepared in (4) above, the discharge capacity of the batteries at an operating temperature of 150° C. was measured in a voltage range of 3.95 V to 2.0 V. This measurement was performed by charging at a constant current until the battery voltage reached the upper limit of the voltage range, and then discharging until the battery voltage reached the lower limit of the voltage range.

(5b) Measurement of filling rate The filling rate (volume %) of the active material in each of the positive electrode and the negative electrode of the all-solid-state battery prepared in (4) above was measured as follows. First, the cross section of the all-solid-state battery was polished by ion milling, and then the cross section of the polished positive electrode (or negative electrode) was observed with a SEM to obtain a cross-sectional SEM image. The SEM image was an image with a magnification of 1000 times. The obtained image was subjected to binarization processing using image analysis software (Image-Pro Premier, manufactured by Media Cybernetics). The threshold value for binarization was set using Otsu's binarization as a discriminant analysis method. Based on the obtained binarized image, the filling rate F (%) of the positive electrode active material (or negative electrode active material) in the positive electrode (or) negative electrode was calculated using the following formula:
Filling rate F = [ _SA / ( _SA + _SB )] x 100
(In the formula, S _A is the area of the portion in the binarized image occupied by the positive electrode active material (or the negative electrode active material), and S _B is the area of the portion in the binarized image other than the positive electrode active material (or the negative electrode active material), including the areas occupied by the solid electrolyte, the electronic conductive assistant, and the voids.)
The calculation was made as follows.

Example 17
In the preparation of the positive electrode active material in (1) above, 1.0 wt% of Li ₃ BO ₃ powder (relative to the total amount of the powder made of NCM particles and the Li ₃ BO ₃ powder) was added to synthesize the powder made of NCM particles with an average particle size of 7 μm. Except for this, a battery was prepared and evaluated in the same manner as in Example 16.

Example 18
A battery was produced and evaluated in the same manner as in Example 17, except that a negative electrode active material powder was produced as follows, and that in the charge/discharge evaluation in (5a) above, the discharge capacity of the battery at an operating temperature of 150° C. was measured in a voltage range of 2.5 V to 1.5 V.

(Preparation of negative electrode active material powder)
Commercially available _TiO2 powder (average particle size 1 μm or less) and _Li2CO3 powder (average particle size 3 μm) were mixed so that the molar ratio of Li/ _Ti was 0.84, and then held at 1000°C for 2 hours to obtain a powder consisting of LTO particles with an average particle size of approximately 3.5 μm.

Example 19
A battery was produced and evaluated in the same manner as in Example 16, except that the positive electrode active material powder was produced as follows.

(Preparation of Positive Electrode Active Material Powder)
A commercially available ( _{Ni0.5Co0.2Mn0.3} )(OH) ₂ powder (average particle size ₉ μm) and _Li2CO3 powder (average particle size ₃ μm) were weighed out so that the molar ratio of Li/(Ni+Co ₊ Mn) was 1.02, and _TiO2 powder weighed out so that the molar ratio of Ti/(Ni+Co+Mn) was 0.05 was added to the powder. 1.0 wt% of _{Li3BO3 powder} (relative to the total amount of the powder consisting of NCM particles and _{Li3BO3 powder} ) was then added and mixed. The mixture was then kept at 850° C. for 10 hours to obtain a powder consisting of NCM particles with an average particle size of about 8 μm.

Example 20 (Comparative)
A battery was produced and evaluated in the same manner as in Example 16, except that in the preparation of the positive electrode active material powder in (1) above, _TiO2 powder was not added.

The specifications and evaluation results of the batteries prepared in each example are shown in Table 3. The charge/discharge characteristics were compared under the same rate conditions, and the discharge capacity measured in Example 20 (comparison) was set to 100, and the relative values were calculated and shown in Table 3.

From elemental mapping by SEM and EPMA (Figures 1 and 3), it was confirmed that the diffusion of transition metals into the solid electrolyte portion in the voids of the positive plate was suppressed in NCM523 with added Ti (Example 2; see Figure 1) compared to pure NCM523 without added elements such as Ti (Example 6 (comparison); see Figure 3). Similarly, for NCM361, it is believed that the diffusion of transition metals into the solid electrolyte portion in the voids of the positive plate was suppressed in NCM361 with added Ti (Example 13) compared to NCM361 without added Ti (Example 14). From this, it is believed that the decrease in Li ion conductivity due to degradation of the solid electrolyte was mitigated in positive plates using NCM with added Ti (Examples 1 to 5 and 13), leading to improved rate characteristics. In addition, since the NCM to which Al was added (Example 7 (comparison)) and the NCM to which Nb was added (Example 8 (comparison)) had low discharge capacity, it is understood that selective addition of Ti to the NCM contributes to a remarkable improvement in discharge capacity.

In addition, it was found that the discharge capacity was further improved in the positive electrode plates (Examples 9 to 12) using NCMs to which not only Ti but also B was added. This is thought to be because the addition of B contributes to suppressing side reactions at the interface between the positive electrode active material and the _LiOH.Li2SO4 - _based solid electrolyte, and to alleviating stress caused by the expansion and contraction of the positive electrode active material during charging and discharging, thereby further improving the discharge capacity.

From the EPMA mapping of Example 2 shown in Figure 1, it was confirmed that Ti was uniformly present in the positive electrode active material portion. In addition, from the XRD profile of Example 2 shown in Figure 2, only the peak of NCM, which has a layered rock salt structure, was detected. From this, it is considered that Ti is dissolved in the NCM, which is the positive electrode.

In the composite type batteries of Examples 16 to 19, the addition of Ti (and B as necessary) to the NCM improved the rate characteristics compared to a pure NCM (Example 20) to which these were not added. The addition of Ti to suppress the diffusion of transition metals into the solid electrolyte, the addition of B to suppress side reactions at the interface with the solid electrolyte, and the relaxation of stress caused by the expansion and contraction of the positive electrode active material during charging and discharging, which were confirmed in the impregnated sintered body type batteries, also occurred in the composite type batteries, which is thought to have led to the improvement in the rate characteristics.

Claims (11)

a positive electrode including a positive electrode active material that is composed of an oxide having a layered rock salt structure containing Li, Ni, Co, and Mn, and further contains Ti;
a negative electrode including a negative electrode active material;
A _LiOH.Li2SO4 - _based solid electrolyte interposed between the positive electrode and the negative electrode;
Equipped with
The molar ratio of Ti/(Ni+Co+Mn) in the positive electrode active material is 0.01 to 0.10;
The LiOH.Li2SO4 _{- based} solid electrolyte was identified as _3LiOH.Li2SO4 by X - ray _diffraction ,
A _lithium ion _secondary battery, wherein the LiOH.Li2SO4 -based solid electrolyte further contains boron, and a molar ratio (B/S) of boron B to sulfur S contained in the LiOH.Li2SO4 - based solid electrolyte is greater than 0.002 and less than _{1.0 .} The lithium ion secondary battery according to claim 1, wherein the positive electrode active material further contains B. The lithium-ion secondary battery according to claim 1 or 2, wherein the positive electrode active material is in the form of a sintered plate. The lithium ion secondary battery according to claim 1 or 2, wherein the positive electrode comprises particles of the positive electrode active material, particles of the _{LiOH.Li2SO4 -} based solid electrolyte, and an electronic conduction assistant in the form of a mixture. The lithium ion secondary battery according to any one of claims 1 to 4, wherein the positive electrode active material has a density of 50 to 80 volume %. 6. The lithium ion secondary battery according to claim 1, wherein the molar ratio of Ti/(Ni+Co+Mn) in the positive electrode active material is 0.01 to 0.07 . The lithium ion secondary battery according to any one of claims 1 to 6, wherein Ti is dissolved in the layered rock salt structure in the positive electrode active material. The lithium ion secondary battery according to any one of claims 1 to 7, wherein the molar ratio of Li/(Ni+Co+Mn) in the positive electrode active material is 0.95 to 1.10. The lithium ion secondary battery according to any one of claims 1 to 8, wherein the negative electrode active material is in the form of a sintered plate. The lithium ion secondary battery according to any one of claims 1 to 9, wherein the negative electrode active material is composed of a titanium-containing oxide. The lithium ion secondary battery according to claim 10, wherein the titanium-containing oxide is lithium titanate.

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