Lithium-ion secondary battery

Abstract

Provided is a lithium ion secondary battery using an LiOH•Li2SO4-based solid electrolyte, and having improved discharge capacity. This lithium ion secondary battery comprises: a positive electrode that includes a positive electrode active material constituted by an oxide having a layered rock salt structure that contains Li, Ni, Co, and Mn; a negative electrode that includes a negative electrode active material; and an LiOH•Li2SO4-based solid electrolyte that is interposed between the positive electrode and the negative electrode, and that also enters the voids of the positive electrode. This positive electrode active material further contains Ti.

Description

[Technical field]

[0001] The present invention relates to a lithium ion secondary battery. [Background technology]

[0002] In recent years, with the development of portable devices such as personal computers and mobile phones, the demand for batteries as their power sources has increased 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 electrolyte solutions, problems such as electrolyte leakage, fire, explosion, etc. may occur. In order to solve such problems, and 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.

[0003] Various all-solid-state batteries have been proposed. For example, Patent Document 1 (JP Patent Publication No. 2009-193940) 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.

[0004] In response to this, all-solid-state batteries using sintered electrodes instead of compact electrodes have also been proposed. Such batteries have the advantage of having high energy density because the sintered electrodes do not contain conductive additives. For example, Patent Document 2 (WO2019 / 093222A1) describes an oriented positive electrode plate that is a lithium composite oxide sintered plate with a porosity of 10 to 50%, and a lithium composite oxide sintered plate that contains Ti and has a voltage of 0.4 V (vs. Li / Li +)A all-solid-state lithium battery is disclosed that includes a negative electrode plate capable of inserting and extracting lithium ions as described above, and a solid electrolyte having a melting point lower than the melting point or 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, Li 3 OCl, xLiOH·yLi 2 SO 4 (where x + y = 1, 0.6 ≦ x ≦ 0.95) (for example, 3LiOH·Li 2 SO 4 ) and various other materials are disclosed. Such a solid electrolyte can penetrate into the voids of the electrode plate as a melt, enabling strong interfacial contact. As a result, it is said that significant improvements in battery resistance and rate performance during charge and discharge, as well as a substantial improvement in battery manufacturing yield, can be achieved.

[0005] Also, 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 (where 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 phosphorous oxynitride (LiPON)-based ceramic material, and a negative electrode layer.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

[0007] Among the above-mentioned low melting point solid electrolytes, the present inventors have particularly focused on 3LiOH·Li 2 SO 4 LiOH·Li etc. 2 SO 4 It has been reported that 3LiOH·Li-based solid electrolytes exhibit high lithium ion conductivity. However, it has been reported that 3LiOH·Li-based solid electrolytes exhibit high lithium ion conductivity. 2 SO 4 LiOH·Li etc. 2 SO 4 When the cells were constructed by combining these solid electrolytes and operated as batteries, it was found that the discharge capacity was lower than the theoretical capacity expected from the amount of active material.

[0008] The present inventors have now discovered that by further incorporating Ti into a positive electrode active material composed of an oxide with a layered rock salt structure containing Li, Ni, Co, and Mn, it is possible to obtain a positive electrode active material and LiOH·Li 2 SO 4 The findings showed that diffusion of elements between the solid electrolyte and the cation-based material is suppressed, thereby improving the discharge capacity.

[0009] Therefore, the object of the present invention is to provide a method for the preparation of LiOH·Li 2 SO 4 The present invention aims to improve the discharge capacity of a lithium ion secondary battery using a solid electrolyte based on this compound.

[0010] 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; LiOH·Li interposed between the positive electrode and the negative electrode 2 SO 4 a solid electrolyte based on ZnO; A lithium ion secondary battery is provided. [Brief description of the drawings]

[0011] [Figure 1]These are 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 on the far left is the 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, the EPMA mapping images of Ti, Mn, Co, and Ni are shown in order. [Diagram 2] 1 shows an XRD profile of the positive electrode sintered plate (NCM) produced in Example 2. [Diagram 3] 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 on the far left is the electron microscope photograph (white parts correspond to NCM, black parts to solid electrolyte), and EPMA mapping images of Mn, Co, and Ni are shown in order from there to the right. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Lithium-ion secondary battery The lithium ion secondary battery of the present invention comprises a positive electrode, a negative electrode, and LiOH·Li 2 SO 4 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 contains Ti. The negative electrode includes a negative electrode active material. LiOH·Li 2 SO 4 The solid electrolyte is interposed between the positive and negative electrodes, but it may also be embedded in the gaps of the positive electrode or inside the negative electrode. In either case, the positive electrode active material is composed of a layered rock-salt oxide containing Li, Ni, Co, and Mn (i.e., lithium cobalt-nickel-manganese oxide (hereinafter referred to as NCM)), and by further incorporating Ti, the positive electrode active material and LiOH·Li 2 SO 4 Element diffusion between the solid electrolyte and the base electrolyte is suppressed, thereby improving the discharge capacity, that is, the rate characteristics.

[0013] That is, as mentioned above, 3LiOH·Li 2 SO 4LiOH·Li etc. 2 SO 4 It has been reported that NCM-based solid electrolytes exhibit high lithium ion conductivity. However, it has been reported that 3LiOH·Li is not suitable for use in positive electrodes that contain positive electrode active materials composed of NCM. 2 SO 4 LiOH·Li etc. 2 SO 4 When a cell was constructed by combining NCM and LiOH·Li and the cell was operated, it was found that the discharge amount was lower than the theoretical capacity expected from the amount of active material. This is thought to be due to the deterioration of the solid electrolyte (decrease in conductivity) caused by the reaction between the positive electrode active material and the solid electrolyte, and the formation of a high resistance layer at the interface, which may affect the charge and discharge characteristics. 2 SO 4 In order to suppress the reaction between the solid electrolytes, we tried to stabilize the structure and reduce the reactivity with the solid electrolyte by adding Ti as a dissimilar metal to the NCM. It was found that the above problem was solved favorably. This is because the addition of Ti to the NCM resulted in the formation of LiOH·Li 2 SO 4 It is believed that the diffusion of elements between the Ti-based solid electrolyte and the NCM is suppressed, which alleviates the decrease in lithium ion conductivity due to the degradation of the solid electrolyte, resulting in improved rate characteristics. In fact, it was confirmed that the all-solid-state battery using the Ti-added NCM has a higher discharge capacity at the same rate than the all-solid-state battery using the NCM without the addition of foreign elements. In addition, cross-sectional analysis of the all-solid-state battery before and after charging and discharging confirmed that the diffusion of the positive electrode constituent metal elements into the solid electrolyte is suppressed in the Ti-added NCM compared to the non-added NCM.

[0014] (1) Positive electrode The positive electrode (typically a positive plate) contains a positive electrode active material. The positive electrode active material is composed of a layered rock-salt oxide (NCM) containing Li, Ni, Co, and Mn. The layered rock-salt structure is a crystal structure in which lithium layers and layers of transition metals other than lithium are alternately stacked with oxygen layers in between (typically α-NaFeO 2 NCM is a cubic rock-salt structure in which transition metals and lithium are regularly arranged along the

[0111] axis of the cubic rock-salt structure. p (Nix , Co y , Mn z )O 2 (wherein 0.9 ≦ p ≦ 1.3, 0 < x < 0.8, 0 < y < 1, 0 ≦ z ≦ 0.7, and x + y + z = 1, preferably 0.95 ≦ p ≦ 1.1, 0.1 ≦ x < 0.7, 0.1 ≦ y < 0.9, 0 ≦ z ≦ 0.6, and x + y + z = 1), and has a composition represented by, for example, 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).

[0015] The positive electrode active material (NCM) further contains Ti. The inclusion of Ti suppresses the elemental diffusion between the positive electrode active material and the LiOH·Li 2 SO 4 -based solid electrolyte, and thereby improves the discharge capacity. 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, still 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 existence form of Ti in the positive electrode active material is not particularly limited, but it is preferable that Ti is dissolved in the layered rock salt structure. By dissolving Ti in the layered rock salt structure, the elemental diffusion between the positive electrode active material (NCM) and the LiOH·Li 2 SO 4 -based solid electrolyte is more effectively suppressed.

[0016] 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-milling 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, the raw material powder (e.g., (Ni 0.5 Co 0.2 Mn 0.3 )(OH) 2 Powder and Li 2 CO 3 Powder mixture with TiO 2 The addition of Ti can be preferably carried out by adding the powder.

[0017] The positive electrode active material (NCM) preferably further contains B. By containing B, the positive electrode active material and LiOH·Li 2 SO 4 It is believed that this can suppress side reactions at the interface with the solid electrolyte and ease the stress caused by the expansion and contraction of the positive electrode active material during charging and discharging, thereby resulting in a further improvement in discharge capacity.

[0018] 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-milling 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, the raw material powder (e.g., (Ni 0.5 Co 0.2 Mn 0.3 )(OH) 2 Powder and Li 2 CO 3 Powder mixture with Li 3 BO 3 B addition can be preferably carried out by adding powder.

[0019] The positive electrode is generally a composite electrode, which is a mixture of a positive electrode active material, an electronic conductive agent, a lithium ion conductive material, and a binder, or a positive electrode active material, LiOH·Li 2 SO 4 The positive electrode may be in the form of a mixture of a LiOH-based solid electrolyte, an electron conductive assistant, etc. (a composite material form). 2 SO 4 The positive electrode may contain a Li-based solid electrolyte particle and an electronic conductive assistant in the form of a composite material. 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 conductive 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 preferred particle size of the positive electrode active material particles is 0.05 to 50 μm, more preferably 0.1 to 30 μm, and even more preferably 0.5 to 20 μm. LiOH·Li 2 SO 4 The particle diameter of the solid electrolyte particles is preferably 0.01 to 50 μm, more preferably 0.05 to 30 μm, and even more preferably 0.1 to 20 μm. The electron conductive assistant is not particularly limited as long as it is an electron conductive material generally used in electrodes, but is preferably a carbon material. Preferred examples of the carbon material 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.

[0020] The density (filling rate) of the positive electrode active material in the positive electrode is preferably 50 to 80 volume %, more preferably 55 to 80 volume %, further preferably 60 to 80 volume %, and particularly preferably 65 to 75 volume % regardless of the form of the positive electrode (sintered plate or composite material). With a density within such a range, the solid electrolyte can be sufficiently filled into the voids in the positive electrode active material, and the proportion of the positive electrode active material in the positive electrode increases, so that a high energy density as a battery can be realized.

[0021] Regardless of the form of the positive electrode (sintered plate or composite material), the thickness of the positive electrode is preferably 75 to 350 μm, more preferably 100 to 325 μm, further preferably 100 to 300 μm, and particularly preferably 100 to 275 μm, from the viewpoint of improving the energy density of the battery.

[0022] (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.

[0023] Particularly preferred negative electrode active materials are those with a voltage of 0.4V (vs. Li / Li + ) and above, and preferably contains Ti, which is capable of inserting and extracting lithium ions. A negative electrode active material that satisfies these conditions is preferably made of a titanium-containing oxide. A preferred example of such a negative electrode active material is lithium titanate Li 4 Ti 5 O 12 (hereinafter referred to as LTO), niobium titanium composite oxide Nb 2 TiO 7 , titanium oxide TiO 2 More preferably, LTO and Nb 2 TiO 7 More preferably, LTO is used. Although LTO is typically known to have a spinel structure, it can adopt other structures during charging and discharging. For example, LTO has a Li 4 Ti 5 O 12 (spinel structure) and Li 7 Ti 5 O 12 The reaction proceeds in the coexistence of two phases, i.e., spinel and tetrahydrofuran (rock salt structure). Therefore, LTO is not limited to a spinel structure.

[0024] The negative electrode is generally a composite electrode, which is a mixture of a negative electrode active material, an electronic conductive agent, a lithium ion conductive material, and a binder, or a negative electrode active material, LiOH·Li 2 SO 4 The negative electrode may be in the form of a mixture of a LiOH-based solid electrolyte, an electron conductive assistant, etc. 2 SO 4 The negative electrode may contain particles of a Li-based solid electrolyte and an electron conductive 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 electron conductive assistant or a binder, the energy density of the negative electrode can be increased. The sintered plate may be a dense body or a porous body, and the pores of the porous body may contain a solid electrolyte. In the form of a composite material, the negative electrode active material particles preferably have a particle size of 0.05 to 50 μm, more preferably 0.1 to 30 μm, and even more preferably 0.5 to 20 μm. LiOH·Li 2 SO 4 The particle diameter of the solid electrolyte particles is preferably 0.01 to 50 μm, more preferably 0.05 to 30 μm, and even more preferably 0.1 to 20 μm. The electron conductive assistant is not particularly limited as long as it is an electron conductive material generally used in electrodes, but is preferably a carbon material. Preferred examples of the carbon material 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.

[0025] 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, so that a high energy density as a battery can be realized.

[0026] Regardless of the form of the negative electrode (sintered plate or composite material), the thickness of the negative electrode is preferably 75 to 350 μm, more preferably 100 to 325 μm, further preferably 125 to 300 μm, and particularly preferably 130 to 275 μm, from the viewpoint of improving the energy density of the battery.

[0027] (3) Solid electrolyte The solid electrolyte is LiOH·Li 2 SO 4 LiOH Li 2 SO 4 The solid electrolyte is LiOH and Li 2 SO 4 It is a complex compound with a typical composition of the general formula: xLiOH yLi 2 SO 4 (wherein x+y=1, 0.6≦x≦0.95). A representative example is 3LiOH·Li 2 SO 4 (In the above general formula, x=0.75, y=0.25). Preferably, LiOH·Li 2 SO 4 The solid electrolyte was identified as 3LiOH·Li by X-ray diffraction. 2 SO 4 The preferred solid electrolyte is 3LiOH Li 2 SO 4 The main phase of the solid electrolyte is 3LiOH·Li. 2 SO 4 Whether or not it is contained can be confirmed by identifying it in the X-ray diffraction pattern using ICDD database 032-0598. 2 SO 4 " means that the crystal structure is 3LiOH·Li 2 SO 4 The crystal composition is 3LiOH Li. 2 SO 4 It is not necessarily the same as 3LiOH·Li. 2 SO 4 As long as the composition is LiOH:Li, 2 SO 4Therefore, a solid electrolyte containing a dopant such as boron (for example, 3LiOH·Li in which boron is dissolved in solid and the X-ray diffraction peak is shifted to the high angle side) is also included in the solid electrolyte of the present invention. 2 SO 4 ), the crystal structure is 3LiOH·Li 2 SO 4 As long as it can be considered the same as 3LiOH·Li 2 SO 4 In the present specification, the solid electrolyte used in the present invention is also allowed to contain unavoidable impurities.

[0028] Therefore, LiOH Li 2 SO 4 The main phase of the solid electrolyte is 3LiOH·Li 2 SO 4 In addition, a heterogeneous phase may be included. The heterogeneous phase may contain a plurality of elements selected from Li, O, H, S, and B, or may consist 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 These heterogeneous phases are described in detail in the literature. 2 SO 4 It is believed that unreacted raw materials remain when forming the Li 3 BO 3 Other than these, it is preferable to have a small amount. 3 BO 3 The boron-containing heterogeneous phase may be contained in a desired amount because it can contribute to improving the retention of lithium ion conductivity after long-term storage at high temperatures. However, the solid electrolyte is a 3LiOH·Li in which boron is dissolved. 2 SO 4 It may be configured as a single phase.

[0029] LiOH Li 2 SO 4Solid electrolytes (especially 3LiOH·Li 2 SO 4 ) preferably further contains boron. 2 SO 4 By further incorporating boron into the solid electrolyte identified as 3LiOH·Li, the decrease in lithium ion conductivity can be significantly suppressed even after long-term storage at high temperatures. 2 SO 4 It is presumed that boron B is incorporated into one of the sites of the crystal structure, 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 further preferably 0.005 or more and 0.8 or less. If the B / S is within the above range, it is possible to improve the retention rate of lithium ion conductivity. In addition, if the B / S is within the above range, the content of unreacted heterogeneous phases containing boron is low, and therefore the absolute value of lithium ion conductivity can be increased.

[0030] LiOH Li 2 SO 4 The 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 method or a dry method.

[0031] LiOH Li 2 SO 4 The solid electrolyte is melted and enters the voids in the positive electrode (positive electrode active material) and / or negative electrode (negative electrode active material), and the remaining part is preferably interposed between the positive electrode and the negative electrode as a solid electrolyte layer. From the viewpoint of charge / discharge rate characteristics and insulating properties of the solid electrolyte, the thickness of the solid electrolyte layer (excluding the part that enters the voids in the positive electrode and the negative electrode) is preferably 1 to 500 μm, more preferably 3 to 50 μm, and further preferably 5 to 40 μm.

[0032] (4) Manufacture of lithium-ion secondary batteries The lithium ion secondary battery can be manufactured, 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. EXAMPLES

[0033] 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", and "NCM523" is Li(Ni 0.5 Co 0.2 Mn 0.3 )O 2 "NCM361" means Li(Ni 0.3 Co 0.6 Mn 0.1 )O 2 Also, Li 4 Ti 5 O 12 shall be abbreviated as "LTO."

[0034] [Examples 1-15] The example described below is an example relating to an impregnated sintered body type all-solid-state secondary battery.

[0035] Example 1 (1) Preparation of the positive electrode plate (1a) Preparation of NCM green sheet Commercially available (Ni 0.5 Co0.2 Mn 0.3 )(OH) 2 Powder (average particle size 9 μm) and Li 2 CO 3 The powder (average particle size 3 μm) was mixed and then held at 750 °C for 10 hours to obtain a powder consisting of NCM particles. TiO weighed out so that the molar ratio of Ti / (Ni + Co + Mn) was 0.025 was added to this powder. 2 The powder was added and adjusted to an average particle size of about 5 μm by wet grinding in a ball mill, and then this mixed powder was mixed with a solvent for tape casting, a binder, a plasticizer, and a dispersant. After adjusting the viscosity of the resulting paste, the 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 it would be 100 μm after firing.

[0036] (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 rate of 200°C / h and held for 10 hours for sintering. The thickness of the resulting sintered plate was measured by SEM observation and found to be approximately 100 μm. A Au film (thickness 100 nm) was formed as a current collecting layer on one side of the NCM sintered plate by sputtering.

[0037] (2) Preparation of negative electrode plate (2a) Preparation of LTO green sheet Commercially available TiO weighed out to give a Li / Ti molar ratio of 0.84 2 Powder (average particle size 1 μm or less) and Li 2 CO 3 After mixing the powders (average particle size 3μm), the mixture was kept at 1000℃ for 2 hours to obtain a powder consisting of LTO particles. This powder was adjusted to an average particle size of about 2μm by wet grinding in a ball mill, and then mixed with a solvent, binder, plasticizer, and dispersant for tape casting. After adjusting the viscosity of the resulting paste, it was molded into a sheet on a PET film to produce an LTO green sheet. The thickness of the LTO green sheet was adjusted so that it would be 130μm after firing.

[0038] (2b) Preparation of LTO sintered plate The LTO green sheet peeled off from the PET film was punched out into a circle with a diameter of 11 mm and placed in a sintering sheath. The temperature was raised to 850°C at a rate of 200°C / h and held for 2 hours for sintering. The thickness of the resulting sintered plate was measured by SEM observation and found to be approximately 130 μm. A Au film (thickness 100 nm) was formed as a current collecting layer on one side of the LTO sintered plate by sputtering.

[0039] (3) Preparation of solid electrolyte (3a) Preparation of raw powder Li 2 SO 4 Powder (commercially available, purity 99% or higher), LiOH powder (commercially available, purity 98% or higher), and Li 3 BO 3 (Commercially available, purity 99% or higher) 2 SO 4 :LiOH:Li 3 BO 3 The raw material powder mixture was obtained by mixing the two powders in a molar ratio of 1:2.6:0.05. These powders were handled in a glove box in an Ar atmosphere, and care was taken to prevent deterioration such as moisture absorption.

[0040] (3b) Melt synthesis The raw material powder mixture was placed in a high-purity alumina crucible in an Ar atmosphere. The crucible was then placed 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 solid.

[0041] (3c) Mortar grinding The obtained solidified 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.

[0042] (4) Fabrication of all-solid-state batteries The solid electrolyte powder was placed on the positive plate, and the negative plate was placed on top of that. A weight was then placed on the negative plate, and the plate was heated at 400°C for 45 minutes in an electric furnace. During this process, the solid electrolyte powder melted, and after subsequent solidification, a solid electrolyte layer was formed between the electrode plates. A battery was fabricated using the resulting cell consisting of a positive plate / solid electrolyte / negative plate.

[0043] (5) Evaluation (5a) Analysis of the positive electrode / solid electrolyte interface The battery fabricated 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).

[0044] (5b) Measurement of thickness and density The thickness and compactness (volume %) of the positive electrode plate (NCM sintered plate not containing a solid electrolyte) prepared in (1b) above and the negative electrode plate (LTO sintered plate not containing a 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 an 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) part and the part filled with resin (part that was originally a gap) in the positive electrode plate (or negative electrode plate) was calculated to obtain the compactness 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.

[0045] (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).

[0046] (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.

[0047] Example 2 In the NCM grinding in (1a) above, TiO was weighed out so that the molar ratio of Ti / (Ni+Co+Mn) was 0.05. 2 Except for adding the powder, 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 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 EPMA mapping images of Ti, Mn, Co, and Ni are shown in order from there to the right. FIG. 2 shows the XRD profile of the positive electrode sintered plate (NCM) produced in this example.

[0048] Example 3 In the preparation of the solid electrolyte raw material mixed powder in the above (3a), Li 2 SO 4 Powder, LiOH powder and Li 3 BO 3 Li powder 2 SO 4 :LiOH:Li 3 BO 3 A battery was produced and evaluated in the same manner as in Example 2, except that the components were mixed so as to obtain a molar ratio of 1:3.0:0.05.

[0049] Example 4 In the NCM grinding in (1a) above, TiO was weighed out so that the molar ratio of Ti / (Ni+Co+Mn) was 0.07. 2 A battery was produced and evaluated in the same manner as in Example 1, except that the powder was added.

[0050] Example 5 In the NCM grinding in (1a) above, TiO was weighed out so that the molar ratio of Ti / (Ni+Co+Mn) was 0.10. 2 A battery was produced and evaluated in the same manner as in Example 1, except that the powder was added.

[0051] Example 6 (comparison) In the NCM grinding of (1a) above, TiO 2 Except for not adding the powder, the battery was fabricated and evaluated in the same manner as in Example 1. 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 fabricated 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 from there to the right, EPMA mapping images of Mn, Co, and Ni are shown in order.

[0052] Example 7 (comparison) In the NCM grinding of (1a) above, TiO 2 Instead of adding powder, weigh out Al so that the molar ratio of Al / (Ni+Co+Mn) is 0.05. 2 O 3 Except for adding the powder, a battery was produced and evaluated in the same manner as in Example 1. Note that the molar ratio (Al / (Ni+Co+Mn)) of the Al content to the total content of Ni, Co, and Mn was calculated instead of the Ti content in the positive electrode plate.

[0053] Example 8 (comparison) In the NCM grinding of (1a) above, TiO 2Instead of adding powder, weigh out Nb to give a molar ratio of Nb / (Ni+Co+Mn) of 0.025. 2 O 5 Except for adding the powder, a battery was produced and evaluated in the same manner as in Example 1. Note that the molar ratio (Nb / (Ni+Co+Mn)) of the Nb content to the total content of Ni, Co and Mn was calculated instead of the Ti content in the positive electrode plate.

[0054] Example 9 (i) In the preparation of the NCM green sheet in (1a) above, TiO was weighed out so that the molar ratio of Ti / (Ni+Co+Mn) was 0.05. 2 Powder and Li 3 BO 3 The powders were (NCM particles and Li 3 BO 3 A battery was fabricated and evaluated in the same manner as in Example 1, except that (i) 0.5 wt. % of the powder was added (based on the total amount of the powder), and (ii) in the preparation of the NCM sintered plate in (1b) above, the NCM green sheet was sintered at 940°C.

[0055] Example 10 (i) In the preparation of the NCM green sheet in (1a) above, TiO was weighed out so that the molar ratio of Ti / (Ni+Co+Mn) was 0.05. 2 Powder and Li 3 BO 3 The powders were (powder consisting of NCM particles and Li 3 BO 3 A battery was fabricated and evaluated in the same manner as in Example 1, except that (i) 1.0 wt. % of the powder was added (based on the total amount of the powder) and the mixed powder was adjusted to an average particle size of approximately 6 μm by wet pulverization 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.

[0056] result 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.

[0057] [Table 1]

[0058] 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 an NCM sintered plate in (1b) above, the NCM green sheet was sintered at 950°C.

[0059] (1a') Preparation of NCM green sheet Commercially available (Ni 0.3 Co 0.6 Mn 0.1 )(OH) 2 Powder (average particle size 7-8μm) and Li 2 CO 3 The powder (average particle size 3 μm) was mixed and then held at 850 °C for 10 hours to obtain a powder consisting of NCM particles. TiO weighed out so that the molar ratio of Ti / (Ni + Co + Mn) was 0.02 was added to this powder. 2 Add powder and Li 3 BO 3 The powders were (powder consisting of NCM particles and Li 3 BO 3 The powder was added at 1.0% by weight (based on the total powder amount) and adjusted to an average particle size of about 5 μm by wet grinding in a ball mill, and then this mixed powder was mixed with a solvent for tape casting, a binder, a plasticizer, and a dispersant. The viscosity of the resulting paste was adjusted, and the 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 it would be 100 μm after firing.

[0060] Example 12 In the preparation of the NCM green sheet in (1a') above, TiO was weighed out so that the molar ratio of Ti / (Ni+Co+Mn) was 0.05. 2 A battery was fabricated and evaluated in the same manner as in Example 11, except that the powder was added.

[0061] Example 13 In the preparation of the NCM green sheet in (1a') above, TiO was weighed out so that the molar ratio of Ti / (Ni+Co+Mn) was 0.05. 2 Powder was added, Li 3 BO 3 A battery was produced and evaluated in the same manner as in Example 11, except that no powder was added and the average particle size was adjusted to about 4 μm by wet pulverization in a ball mill.

[0062] Example 14 (comparison) In the preparation of the NCM green sheet in (1a') above, TiO 2 Powder and Li 3 BO 3 A battery was fabricated and evaluated in the same manner as in Example 11, except that no powder was added.

[0063] Example 15 (comparison) In the preparation of the NCM green sheet in (1a') above, TiO 2 A battery was produced and evaluated in the same manner as in Example 13, except that no powder was added, and that in the production of the NCM sintered plate in (1b) above, the NCM green sheet was sintered at 920°C.

[0064] result Table 2 shows the specifications and evaluation results of the batteries produced in Examples 11 to 15. Note that 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.

[0065] [Table 2]

[0066] [Examples 16-20] The example described below is an example relating to a composite type all-solid-state secondary battery.

[0067] Example 16 (1) Preparation of positive electrode active material powder Commercially available (Ni 0.3 Co 0.6 Mn 0.1 )(OH) 2 Powder (average particle size 7-8μm) and Li 2 CO 3 The powder (average particle size 3 μm) was weighed out with TiO so that the molar ratio of Ti / (Ni+Co+Mn) was 0.02. 2 After adding the powder and mixing, the mixture was kept at 850°C for 10 hours to obtain a powder consisting of NCM particles with an average particle size of approximately 6.5 μm.

[0068] (2) Preparation of negative electrode active material powder Commercially available carbon powder (average particle size: 10 to 14 μm) was prepared.

[0069] (3) Preparation of solid electrolyte (3a) Preparation of raw powder Li 2 SO 4 Powder (commercially available, purity 99% or higher), LiOH powder (commercially available, purity 98% or higher), and Li 3 BO 3 (Commercially available, purity 99% or higher) 2 SO 4 :LiOH:Li 3 BO 3 The raw material powder mixture was obtained by mixing the two powders in a molar ratio of 1:2.2:0.05. These powders were handled in a glove box in an Ar atmosphere, and care was taken to prevent deterioration such as moisture absorption.

[0070] (3b) Melt synthesis The raw material powder mixture was placed in a high-purity alumina crucible in an Ar atmosphere. The crucible was then placed 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 solid.

[0071] (3c) Crushing 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.

[0072] (4) Fabrication of all-solid-state batteries (4a) Preparation of Positive Electrode Mixture Powder and Negative Electrode Mixture Powder The positive electrode active material powder obtained in (1) above, the solid electrolyte powder obtained in (3) above, and an electron conductive assistant (acetylene black (commercially available)) were weighed out to a volume ratio of 60:40:2, and 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 an electron conductive assistant (acetylene black (commercially available)) were weighed out to a volume ratio of 60:40:2, and mixed in a mortar to prepare a negative electrode composite powder.

[0073] (4b) Press molding The powders were placed in a press mold with a hole diameter of 10 mm, and each layer was pressed at 100 MPa so that the thicknesses of the positive electrode layer, solid electrolyte layer, and negative electrode layer were 110 μm, 500 μm, and 200 μm, respectively. After the three layers were stacked in this manner, the laminate was pressed at 150 MPa to obtain a press molded body.

[0074] (4c) Attaching the pressure tool The press-molded body was sandwiched between a pair of stainless steel plates to form a layer structure of stainless steel plate / positive electrode 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.

[0075] (5) Evaluation (5a) 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 3.95 V to 2.0 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.

[0076] (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 by 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=[S A / (S A +S B )] x 100 (In the formula, S A is the area of ​​the positive electrode active material (or negative electrode active material) in the binarized image, and S B is the area of ​​the part other than the positive electrode active material (or negative electrode active material) in the binarized image, and includes the area occupied by the solid electrolyte, the electronic conductive assistant, and the voids. The calculation was made as follows.

[0077] Example 17 In the preparation of the positive electrode active material in (1) above, Li 3 BO 3 The powders were (powder consisting of NCM particles and Li 3 BO 3 A battery was produced and evaluated in the same manner as in Example 16, except that 1.0 wt % of NCM particles (based on the total amount of the powder) was added to synthesize the powder and a powder having an average particle size of 7 μm was obtained.

[0078] Example 18 A battery was prepared and evaluated in the same manner as in Example 17, except that a negative electrode active material powder was prepared 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.

[0079] (Preparation of negative electrode active material powder) Commercially available TiO weighed out to give a Li / Ti molar ratio of 0.84 2 Powder (average particle size 1 μm or less) and Li 2 CO 3 After mixing the powders (average particle size 3 μm), the mixture was kept at 1000° C. for 2 hours to obtain a powder made of LTO particles with an average particle size of about 3.5 μm.

[0080] Example 19 A battery was produced and evaluated in the same manner as in Example 16, except that a positive electrode active material powder was produced as follows.

[0081] (Preparation of Positive Electrode Active Material Powder) Commercially available (Ni 0.5 Co 0.2 Mn 0.3 )(OH) 2 Powder (average particle size 9 μm) and Li 2 CO 3 The powder (average particle size 3 μm) was weighed out with TiO so that the molar ratio of Ti / (Ni+Co+Mn) was 0.05. 2 Add powder and Li 3 BO 3 The powders were (powder consisting of NCM particles and Li 3 BO 3 After adding 1.0 wt. % of NCM (based on the total amount of powder) and mixing, the mixture was kept at 850°C for 10 hours to obtain a powder consisting of NCM particles with an average particle size of approximately 8 μm.

[0082] Example 20 (comparison) In the preparation of the positive electrode active material powder in (1) above, TiO 2A battery was fabricated and evaluated in the same manner as in Example 16, except that no powder was added.

[0083] result 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.

[0084] [Table 3]

[0085] From element mapping by SEM and EPMA (Figures 1 and 3), it was confirmed that the diffusion of transition metals into the solid electrolyte part in the void of the positive plate is suppressed in NCM523 with Ti added (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 considered that the diffusion of transition metals into the solid electrolyte part in the void of the positive plate is suppressed in the case of NCM361 with Ti added (Example 13) compared to the case of NCM361 without Ti added (Example 14). From this, it is considered that the decrease in Li ion conductivity due to the deterioration of the solid electrolyte is mitigated in the positive plate using NCM with Ti added (Examples 1 to 5 and 13), leading to the improvement of the rate characteristics. In addition, since the discharge capacity of NCM with Al added (Example 7 (Comparison)) and NCM with Nb added (Example 8 (Comparison)) was low, it can be seen that selective addition of Ti to NCM contributes to a remarkable improvement of the discharge capacity.

[0086] In addition, it was found that the discharge capacity was further improved in the positive electrode plate using NCM with not only Ti but also B added (Examples 9 to 12). This is because the addition of B enhances the interaction between the positive electrode active material and LiOH·Li 2 SO 4 This is believed to contribute to suppressing side reactions at the interface with the solid electrolyte and to alleviating stress caused by expansion and contraction of the positive electrode active material during charging and discharging, thereby further improving the discharge capacity.

[0087] 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 is a layered rock salt structure, was detected. From this, it is considered that Ti is dissolved in the NCM, which is the positive electrode.

[0088] 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 pure NCM (Example 20) without these additions. The suppression of the diffusion of transition metals into the solid electrolyte by the addition of Ti, the suppression of side reactions at the interface with the solid electrolyte by the addition of B, 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

1. 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; LiOH.Li interposed between the positive electrode and the negative electrode ２ SO ４ a solid electrolyte based on ZnO; Equipped with The molar ratio of Ti / (Ni+Co+Mn) in the positive electrode active material is 0.01 to 0.10; The LiOH.Li 2 SO 4 -based solid electrolyte was identified as 3LiOH.Li 2 SO 4 by X-ray diffraction, A lithium ion secondary battery, wherein the LiOH.Li 2 SO 4 -based solid electrolyte further contains boron, and a molar ratio (B / S) of boron B to sulfur S contained in the LiOH.Li 2 SO 4 -based solid electrolyte is greater than 0.002 and less than 1.

0.

2. The lithium ion secondary battery according to claim 1 , wherein the positive electrode active material further comprises B.

3. 3. The lithium ion secondary battery according to claim 1, wherein the positive electrode active material is in the form of a sintered plate.

4. The positive electrode comprises particles of the positive electrode active material, the LiOH.Li ２ SO ４ 3. The lithium ion secondary battery according to claim 1, comprising particles of a lithium-based solid electrolyte and an electronic conduction assistant in the form of a mixture.

5. 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 any one of claims 1 to 5, wherein the molar ratio of Ti / (Ni+Co+Mn) in the positive electrode active material is 0.01 to 0.

07.

7. The lithium ion secondary battery according to any one of claims 1 to 6, wherein in the positive electrode active material, Ti is dissolved in the layered rock salt structure.

8. 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.

9. 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.

10. 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.

11. 11. The lithium ion secondary battery according to claim 10, wherein the titanium-containing oxide is lithium titanate.

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