Lithium titanate powder, a negative electrode active material composition using the same, and all-solid-state secondary battery

Abstract

To provide a lithium titanate powder that is used as a negative electrode material for an all-solid-state secondary battery and has excellent initial discharge capacity and charge rate characteristics, a negative electrode active material composition using the same, and an all-solid-state secondary battery.SOLUTION: In a lithium titanate powder whose main component is lithium titanate represented by Li4Ti5O12, a Sn-based solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table is present on the surface of the primary particles of lithium titanate.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a lithium titanate powder, a negative electrode active material composition using the same, and an all-solid-state secondary battery.

Background Art

[0002] In recent years, power storage devices, particularly lithium batteries, have been widely used for small electronic devices such as mobile phones and laptop computers, electric vehicles, and power storage. In the present specification, the term "lithium battery" is used as a concept that also includes so-called lithium ion secondary batteries.

[0003] Currently commercially available lithium batteries are mainly composed of a positive electrode and a negative electrode containing a material capable of intercalating and deintercalating lithium, and a non-aqueous electrolyte consisting of a lithium salt and a non-aqueous solvent. Cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), and chain carbonates such as dimethyl carbonate (DMC) and diethyl carbonate (DEC) are used as the non-aqueous solvent. Since lithium batteries use an electrolyte containing such a flammable organic solvent, liquid leakage is likely to occur, and there is a risk of ignition during a short circuit. Therefore, it is necessary to install a safety device to suppress temperature rise during a short circuit and to have a structure for preventing short circuits. Under such circumstances, all-solid-state secondary batteries using an inorganic solid electrolyte in place of an organic electrolyte have attracted attention. In an all-solid-state secondary battery, all of the positive electrode, negative electrode, and electrolyte are composed of solids, so there is a possibility that the safety and reliability, which are problems of batteries using an organic electrolyte, can be greatly improved. In addition, since simplification of safety devices can be achieved, high energy density can be achieved, so application to electric vehicles, large storage batteries, and the like is expected.

[0004] Unlike conventional lithium-ion secondary batteries that use an electrolyte, in all-solid-state secondary batteries, it is extremely important to form and continuously maintain a good solid-solid interface in order to achieve excellent ionic conductivity and long-term cycle characteristics. Lithium titanate is attracting attention as a way to maintain a good interface between the active material and the solid electrolyte. Because lithium titanate undergoes very little volume change during charging and discharging, it is expected that the interface between the active material and the solid electrolyte will be maintained for a long period of time during charging and discharging. In addition, lithium titanate is attracting attention because of its high safety due to its high reaction potential and the absence of lithium electrodeposition concerns. Patent document 1 discloses an electrode using lithium titanate with a specific BET specific surface area and solid electrolyte particles smaller than the average particle size of lithium titanate, and reports that the contact between lithium titanate and solid electrolyte particles is better than conventional methods. Non-patent document 1 discloses a positive electrode active material LiCoO2 coated with the sulfide solid electrolyte Li4SnS4, and reports that the discharge rate characteristics of the positive electrode are improved in all-solid-state batteries using this active material. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2012-243644 [Non-patent literature]

[0006] [Non-Patent Document 1] Young Eun Choi et al., ChemSusChem, 2017, 10, 2605-2611. [Disclosure of the Invention] [Problems that the invention aims to solve]

[0007] Although using the electrode described in Patent Document 1 resulted in good contact between the lithium titanate powder and the solid electrolyte powder, improving the battery characteristics of the all-solid-state secondary battery, further improvement in charge rate characteristics was needed. In particular, when lithium titanate particles with a relatively small average particle size and a large specific surface area were used, a decrease in battery characteristics was observed even with the configuration of Patent Document 1. This is thought to be because the lithium titanate aggregated, preventing satisfactory contact between the lithium titanate and the solid electrolyte. In addition, even at the point where the solid electrolyte and lithium titanate are in contact, a side reaction between the active site on the surface of the lithium titanate and the solid electrolyte creates a high-resistance layer, which is thought to reduce battery characteristics. To address the above problems, the present invention provides lithium titanate powder, a negative electrode active material composition using the same, and an all-solid-state secondary battery, which can form a negative electrode layer with excellent battery characteristics, particularly charge rate characteristics, by forming a good solid-solid interface with the solid electrolyte regardless of the particle size of the lithium titanate, maintaining that interface over a long period of time, forming a denser negative electrode layer with fewer voids than conventional methods, and effectively suppressing the reaction with the solid electrolyte by pre-treating the active site on the surface of the lithium titanate active material. [Means for solving the problem]

[0008] The inventors of the present invention conducted extensive research to further increase the contact area between lithium titanate and the solid electrolyte, and to suppress side reactions between the active sites on the lithium titanate surface and the solid electrolyte, when using lithium titanate powder, which is prone to aggregation, highly reactive, and has a relatively large specific surface area. As a result, they found that by placing a Sn-based solid electrolyte with conductivity of metal ions belonging to Group 1 of the periodic table on the surface of the primary particles of lithium titanate, the generation of interfaces and voids between lithium titanate particles caused by the aggregation of lithium titanate is significantly suppressed, resulting in a denser negative electrode layer with fewer voids than conventional materials. Furthermore, they found that the Sn-based solid electrolyte with conductivity of metal ions belonging to Group 1 of the periodic table can deactivate the active sites on the surface of lithium titanate, effectively suppressing reactions with the solid electrolyte, thus completing the present invention. By using the negative electrode active material composition containing the lithium titanate powder and solid electrolyte in an all-solid-state secondary battery, the initial discharge capacity can be increased and the charge rate characteristics can be improved. Furthermore, while Non-Patent Document 1 discloses coating the active material with Li4SnS4, a sulfide solid electrolyte, it does not mention or suggest anything about deactivating the active sites on the lithium titanate surface or obtaining a dense negative electrode layer with few voids.

[0009] The present invention relates to lithium titanate powder suitable as a negative electrode material for all-solid-state secondary batteries, a negative electrode active material composition using the same, and an all-solid-state secondary battery.

[0010] In other words, the present invention provides the following (1) to (11).

[0011] (1) Li4Ti5O 12 A lithium titanate powder mainly composed of lithium titanate represented by [formula], characterized in that a Sn-based solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table is present on the surface of the primary particles of the lithium titanate.

[0012] (2) The lithium titanate powder according to (1), wherein the D50 of primary particles corresponding to a cumulative volume frequency of 50% in the volume-based particle size distribution of the lithium titanate powder by laser diffraction scattering method is 0.5 μm or larger.

[0013] (3) The specific surface area of ​​the lithium titanate powder is 1 m² 2 / g or more 10m 2 Lithium titanate powder as described in (1) or (2), which is less than or equal to / g.

[0014] (4) Lithium titanate powder for an all-solid-state battery according to any one of (1) to (3), wherein the Sn-based solid electrolyte present on the surface of the primary particles contains Li4SnS4.

[0015] (5) The lithium titanate powder according to any one of (1) to (4), wherein the content of the Sn-based solid electrolyte in the lithium titanate powder is 0.01% by mass or more and 10% by mass or less.

[0016] (6) A negative electrode active material composition comprising lithium titanate powder as described in any one of items (1) to (5) and an inorganic solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table.

[0017] (7) The negative electrode active material composition according to (6), wherein the inorganic solid electrolyte is a sulfide inorganic solid electrolyte.

[0018] (8) The negative electrode active material composition according to (6) or (7), wherein the content of the inorganic solid electrolyte is 1% by mass or more and 50% by mass or less in the active material composition.

[0019] (9) An all-solid-state secondary battery comprising a positive electrode layer, a negative electrode layer and a solid electrolyte layer, wherein the negative electrode layer is a layer comprising the negative electrode active material composition described in any one of (6) to (8).

[0020] (10) A method for producing Li4SnS4, which is a Sn-based solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table, characterized in that the Li4SnS4 is produced by a liquid-phase method.

[0021] (11) The method for producing Li4SnS4 according to (10), characterized in that Sn or SnS is used as a starting material in the production method. [Effects of the Invention]

[0022] According to the present invention, regardless of the particle size of lithium titanate, the generation of interfaces and voids between lithium titanate due to the aggregation of lithium titanate is significantly suppressed, resulting in a denser negative electrode layer with fewer voids than conventional materials. Furthermore, side reactions between lithium titanate and the solid electrolyte can be effectively suppressed, making it possible to create a negative electrode active material composition and an all-solid-state secondary battery with excellent initial discharge capacity and charge rate characteristics. [Modes for carrying out the invention]

[0023] The present invention relates to lithium titanate powder suitable as a negative electrode material for all-solid-state secondary batteries, a negative electrode active material composition using the same, and an all-solid-state secondary battery.

[0024] [Lithium titanate powder of the present invention] Li4Ti5O 12 Lithium titanate powder having Li4Ti5O as its main component, 12 This lithium titanate powder is characterized by the presence of a Sn-based solid electrolyte with conductivity to metal ions belonging to Group 1 of the periodic table on the surface of primary lithium titanate particles, which are mainly composed of [a specific component].

[0025] [Li4Ti5O 12 [Lithium titanate powder, which is the main component of this substance] The lithium titanate powder of the present invention is Li4Ti5O 12 The main component is Li4Ti5O, within the range in which the effects of the present invention can be obtained. 12may contain other crystalline components and / or amorphous components. The main component means that, among the diffraction peaks measured by X-ray diffraction, the ratio of the intensity of the main peak of Li4Ti5O 12 is 90% or more. In the lithium titanate powder of the present invention, among the diffraction peaks measured by X-ray diffraction, the intensity ratio of the main peak of Li4Ti5O 12 is preferably 92% or more, and more preferably 95% or more. As for components other than Li4Ti5O 12 , it is the sum of the intensity of the main peak derived from the crystalline component and the maximum intensity of the halo pattern derived from the amorphous component. In particular, the lithium titanate powder of the present invention, due to raw materials and synthesis conditions during its synthesis, may include anatase titanium dioxide, rutile titanium dioxide, and lithium titanate with a different chemical formula, Li2TiO 3、 Li 0.6 Ti 3.4 O 8、 and the like as said crystalline components. In the lithium titanate powder of the present invention, the lower the generation ratio of crystalline components other than Li4Ti5O 12 , especially Li 0.6 Ti 3.4 O8, the more the charging characteristics and charge / discharge capacity of an electricity storage device can be improved. Among the diffraction peaks measured by X-ray diffraction, when the intensity of the main peak of Li4Ti5O 12 is taken as 100, it is particularly preferable that the sum of the intensity of the main peak of anatase titanium dioxide, the intensity of the main peak of rutile titanium dioxide, and the intensity corresponding to the main peak of Li2TiO3 calculated by multiplying the peak intensity corresponding to the (-133) plane of Li2TiO3 by 100 / 80 is 5 or less. Here, the main peak of Li4Ti5O 12 refers to the main peak of Li4Ti5O 12This peak corresponds to the diffraction peak attributed to the (111) plane (2θ=18.33). The main peak of anatase-type titanium dioxide corresponds to the diffraction peak attributed to the (101) plane (2θ=25.42) in PDF card 01-070-6826. The main peak of rutile-type titanium dioxide corresponds to the diffraction peak attributed to the (110) plane (2θ=27.44) in PDF card 01-070-7347. The peak corresponding to the (-133) plane of Li2TiO3 corresponds to the diffraction peak attributed to the (-133) plane (2θ=43.58) of Li2TiO3 in PDF card 00-033-0831. Li 0.6 Ti 3.4 The main peak of O8 corresponds to the diffraction peak attributed to the (101) plane (2θ=19.98) in PDF card 01-070-2732. "ICDD" stands for International Centre for Diffraction Data, and "PDF" stands for Powder Diffraction File.

[0026] <d50> In the present invention, D50 of the lithium titanate powder is an indicator of the median volume particle size. It refers to the particle size of the primary particles that correspond to 50% of the cumulative volume frequency in the volume-based particle size distribution obtained by laser diffraction scattering, when the cumulative volume frequency is calculated from the smallest particle size. The measurement method will be explained in the examples described later.

[0027] In the lithium titanate powder of the present invention, the primary particle D50 is 0.5 μm or larger, preferably 0.55 μm or larger, and more preferably 0.6 μm or larger, from the viewpoint of initial discharge capacity, charge rate characteristics, and improvement of the density of the negative electrode layer. It is also 5 μm or smaller, preferably 4.5 μm or smaller, more preferably 4 μm or smaller, and even more preferably 2 μm or smaller. Furthermore, the lithium titanate powder may contain a cumulative volume frequency of primary particles with a primary particle diameter of less than 0.5 μm in the range of 10% to 50%, a cumulative volume frequency of primary particles with a primary particle diameter of less than 0.55 μm in the range of 10% to 55%, and a cumulative volume frequency of primary particles with a primary particle diameter of less than 0.6 μm in the range of 10% to 60%. Furthermore, the cumulative volume frequency of primary particles larger than 5 μm may be included in the range of 50% to 90%, the cumulative volume frequency of primary particles larger than 4.5 μm may be included in the range of 45% to 90%, and the cumulative volume frequency of primary particles larger than 4 μm may be included in the range of 40% to 90%.

[0028] <Specific surface area> The specific surface area of ​​the lithium titanate powder of the present invention refers to the adsorption area per unit mass when nitrogen is used as the adsorption gas. The measurement method will be explained in the examples described later.

[0029] The lithium titanate powder of the present invention has a specific surface area of ​​1 m². 2 / g or more 10m 2 If the concentration is less than / g, lithium titanate powder with excellent initial discharge capacity and charge rate characteristics can be obtained. Preferably, 2m 2 / g or more 9m 2 / g or less, more preferably 4m 2 / g or more 8.5m 2 / g or less, and more preferably 6m 2 / g or more 8m 2 It is less than / g.

[0030] <Presence of Sn-based solid electrolytes with conductive properties of metal ions belonging to Group 1 of the periodic table> The lithium titanate powder of the present invention has a Sn-based solid electrolyte having conductivity for metal ions belonging to Group 1 of the periodic table on the surface of the primary particles of lithium titanate. From the viewpoint of improving the conductivity for metal ions belonging to Group 1 of the periodic table, the Sn-containing Sn-based solid electrolyte is preferably one that contains metal atoms belonging to Group 1 of the periodic table, more preferably one that contains Li, Na, or K, and particularly preferably one that contains Li. Furthermore, the Sn-based solid electrolyte is preferably one that contains sulfur atoms (S), that is, it is preferably a sulfide containing Sn and having conductivity for metal ions belonging to Group 1 of the periodic table. Such a Sn-containing sulfide inorganic solid electrolyte can be produced by reacting a metal sulfide belonging to Group 1 of the periodic table with at least one sulfide represented by the following general formula (I), and two or more sulfides represented by general formula (I) may be used in combination. Furthermore, Sn-based solid electrolytes may be described as solid electrolytes containing Sn and possessing conductivity of metal ions belonging to Group 1 of the periodic table, or as sulfide inorganic solid electrolytes containing Sn.

[0031] Sn x S y (I) (x and y represent numbers that give the stoichiometric ratio, depending on the type and valency of Sn.)

[0032] The metal sulfide belonging to Group 1 of the periodic table is any of lithium sulfide, sodium sulfide, or potassium sulfide, with lithium sulfide or sodium sulfide being more preferred, and lithium sulfide being even more preferred.

[0033] The sulfide represented by general formula (I) is preferably either SnS or SnS2.

[0034] The composition ratio of each element in the sulfide inorganic solid electrolyte containing Sn produced as described above can be controlled by adjusting the amounts of metal sulfides belonging to Group 1 of the periodic table, sulfides represented by the general formula (I), and elemental sulfur.

[0035] The aforementioned Sn-based solid electrolyte may contain at least one sulfide represented by the following general formula (II) for the purpose of improving ionic conductivity.

[0036] M x S y (II) (M represents one of P, Si, Ge, B, Al, Ga, or Sb, and x and y represent the numbers that give the stoichiometric ratio, depending on the type of M.)

[0037] Examples of sulfides represented by general formula (II) include P2S5, SiS2, GeS2, B2S3, Al2S3, Ga2S3, and Sb2S 3、 It is preferable that it be either Sb2S5 or Sb2S5.

[0038] The Sn-containing sulfide inorganic solid electrolyte of the present invention may be amorphous glass, crystallized glass, or a crystalline material.

[0039] The following combinations are preferred as sulfide inorganic solid electrolytes containing Sn, but are not particularly limited. Li2S-SnS2, Li2S-SnS2-P2S5, Li2S-SnS2-SiS2, Li2S-SnS2-GeS2, Li2S-SnS2-Al2S3, Li2S-SnS2-Ga2S3, Li2S-SnS2-S b2S3, Li2S-SnS2-Sb2S5, Li2S-SnS2-P2S5-GeS2, Li2S-SnS2-P2S5-SiS2, Li2S-SnS2-P2S5-Sb2S3, Li2S-SnS2-Sb2S5.

[0040] Among the aforementioned combinations, amorphous glass, crystallized glass, and crystalline materials produced by combining Li2S-SnS2, Li2S-SnS2-P2S5, Li2S-SnS2-Sb2S3, and Li2S-SnS2-P2S5-SiS2 are preferred.

[0041] The mixing ratio of the metal sulfide belonging to Group 1 of the periodic table and the sulfide represented by the general formula (I) is not particularly limited as long as it can be used as a Sn-based solid electrolyte, but is preferably in a molar ratio of 50:50 to 90:10. When the molar ratio of the metal sulfide is 50 or more and 90 or less, sufficient ionic conductivity can be enhanced. The mixing ratio (molar ratio) is more preferably 60:40 to 90:10, and still more preferably 60:40 to 80:20.

[0042] In order to increase ionic conductivity, the Sn-containing sulfide inorganic solid electrolyte may contain at least one lithium halide selected from LiI, LiBr, LiCl, and LiF, or a Li salt such as lithium oxide and lithium phosphate, in addition to the metal sulfide belonging to Group 1 of the periodic table and the sulfide represented by general formula (I), and it is more preferred that the electrolyte contains LiI. However, the mixing ratio of the sulfide inorganic solid electrolyte and these Li salts is preferably 50:50 to 95:5 (molar ratio), and more preferably 55:45 to 90:10.

[0043] Among the Sn-containing sulfide inorganic solid electrolytes, Li4SnS 4、 Li2SnS 3、 Li 3.85 Sn 0.85 Sb 0.15 S 4、 and xLi4SnS4·(1-x)LiI (0<x<1) are more preferred, with Li4SnS4 being particularly preferred.

[0044] Preferable examples of the method for producing the Sn-containing sulfide inorganic solid electrolyte include, but are not particularly limited to, a solid-phase method, a sol-gel method, a mechanochemical method, a liquid-phase method, and a melt-quenching method. For example, a method for producing Li4SnS4 is described below.

[0045] <Method for synthesizing Li4SnS4 using a mechanochemical method> The starting materials for synthesizing Li4SnS4 are not particularly limited, but commercially available materials can be used. In particular, it is preferable to use high-purity materials. Examples of starting materials include lithium sulfide (Li2S), tin(IV) sulfide (SnS2), tin(II) sulfide (SnS), tin (Sn), and sulfur (S), and these can be combined in any way.

[0046] In the mechanochemical method, the processing apparatus and processing conditions are not particularly limited, as long as the components can be mixed uniformly. A ball mill can typically be used as the processing device. Ball mills are preferred because they can generate a large amount of mechanical energy. Among ball mills, planetary ball mills are preferred because, while the pods rotate on their own axis, the base plate revolves around the pods in the opposite direction to the pods' rotation, allowing for the efficient generation of high impact energy.

[0047] The processing conditions can be appropriately set depending on the processing equipment used. For example, when using a ball mill, the higher the rotational speed and the longer the processing time, the more uniformly the raw material mixture can be mixed. When using a planetary ball mill, the rotational speed can be set to 50-800 revolutions per minute, preferably 100-700 revolutions per minute, and more preferably 200-600 revolutions per minute. The processing time can be set to 10-100 hours, preferably 10-80 hours, and more preferably 20-60 hours. Under these processing conditions, Li4SnS4 can be obtained without any residual starting material.

[0048] <Method for producing Li4SnS4 using the liquid-phase method> In the liquid-phase method, the starting materials used in the mechanochemical method can also be used. Li4SnS4 can be obtained by a method that sequentially includes the steps of reacting the starting materials in a solvent and removing the solvent from the resulting mixed solution. The reaction process involves Li2S and SnS 2、 Starting materials selected from SnS, Sn, and S can be appropriately chosen to achieve a stoichiometric ratio of Li4SnS4. These starting materials are added to water or an alcohol solvent and mixed to prepare a mixed solution. It is preferable to use Sn or SnS as the starting material because they become more soluble in the solvent after reacting in the mixed solution. As for the alcohol solvent, methanol, ethanol, isopropyl alcohol, etc., which have a boiling point of 100°C or less are preferred because they are easy to remove. Furthermore, from an industrial standpoint, water is preferred because it is easy to recover and dispose of.

[0049] The specific method of the reaction process is not particularly limited, but typically, a method of dynamic mixing of the above-mentioned mixed solution contained in a container is applied using a stirring blade, stirring bar, beads, ball, homogenizer, etc. The reaction process is preferably carried out while heating in order to dissolve the starting materials in the solvent and to allow the reaction to proceed efficiently. The temperature of the reaction system in the reaction process is preferably 50°C or higher, more preferably 60°C or higher, and even more preferably 70°C or higher.

[0050] The atmosphere of the reaction system in the reaction process is not particularly limited and can be set as appropriate, such as air, dry air, or an inert gas such as nitrogen or argon. The reaction time in the reaction step is appropriately determined depending on the amount of raw materials used, the dynamic mixing method, etc., and is preferably 1 to 24 hours, more preferably 2 to 12 hours. This reaction step yields a solution in which the reaction product is dissolved. This reaction solution can be used as a processing agent when producing the lithium titanate powder of the present invention.

[0051] The solvent removal step is a step of removing the solvent from the reaction liquid to obtain a solid composition. The method of this solvent removal step is not particularly limited, and conventionally known methods such as decantation, filtration, and evaporation to dryness can be applied. In the solvent removal step, drying is preferably performed at 80°C or higher, more preferably 100°C or higher, and particularly preferably 110°C or higher. Further, the solvent may be removed under either atmospheric pressure or reduced pressure. The atmosphere when removing the solvent under atmospheric pressure is not particularly limited, and an inert gas is preferable, which may be appropriately selected from air, dry air, or inert gases such as nitrogen gas and argon gas. It is preferable that the solvent is completely removed. Through this solvent removal step, Li4SnS4 is obtained.

[0052] The Sn-containing solid electrolyte such as Li4SnS4 obtained by the above synthesis method may be subjected to heat treatment for the purpose of improving ion conductivity. As for the heat treatment temperature, the upper limit is preferably 500°C or lower, more preferably 400°C or lower, and still more preferably 350°C or lower. The lower limit of the heat treatment temperature is preferably 150°C or higher, more preferably 200°C or higher, and still more preferably 250°C or higher. Hexagonal Li4SnS4 can be obtained up to around 200°C, while orthorhombic Li4SnS4 can be obtained when the temperature is 250°C or higher and 350°C or lower. Any of these crystal forms can be used as a high ion conductive solid electrolyte. The heat treatment time is preferably 0.1 hour to 8 hours, more preferably 1 hour to 5 hours. It should be noted that since the time varies depending on the solid electrolyte, it is preferable to set it appropriately.

[0053] The phrase "containing a Sn-based solid electrolyte such as the Sn-containing sulfide inorganic solid electrolyte (more specifically, the Sn-containing solid electrolyte is present on the surface of primary particles of lithium titanate)" means that a Raman spectrum corresponding to the Sn-based solid electrolyte is detected in a known analytical device such as Raman spectroscopic analysis for the lithium titanate powder of the present invention.

[0054] <Content of Sn-based Solid Electrolyte> The content of the Sn-based solid electrolyte in the lithium titanate powder is calculated using the detected metal element Sn in inductively coupled plasma emission spectrometry (ICP-AES) or X-ray fluorescence analysis (XRF) of the lithium titanate powder of the present invention. The lower limit of the amount detected by inductively coupled plasma emission spectrometry is usually 0.001 mass%. When the Sn-based solid electrolyte is present on the surface of the lithium titanate particles, the content of the Sn-based solid electrolyte is the content in terms of a solid electrolyte containing Sn, calculated from the content of the element Sn in the lithium titanate powder determined by X-ray fluorescence analysis (XRF) in the lithium titanate powder, and is between 0.01 mass% and 10 mass%. If the content of the Sn-based solid electrolyte is within this range, lithium titanate powder for the negative electrode of an all-solid-state secondary battery with excellent charge rate characteristics can be obtained. More preferably, it is between 0.01 mass% and 8 mass%, even more preferably between 0.1 mass% and 6 mass%, and even more preferably between 0.1 mass% and 4 mass%. The content rate refers to the proportion of the mass of the Sn-based solid electrolyte to the total mass of the lithium titanate powder.

[0055] [Li4Ti5O of ​​the present invention] 12 [Method for producing lithium titanate powder with as the main component] Below, an example of a method for producing lithium titanate powder according to the present invention will be described, divided into a raw material preparation step, a calcination step, and a surface treatment step. However, the method for producing lithium titanate powder according to the present invention is not limited thereto.

[0056] <Preparation process of raw materials> The lithium titanate powder of the present invention consists of a titanium raw material and a lithium raw material. As the titanium raw material, titanium compounds such as anatase-type titanium dioxide and rutile-type titanium dioxide are used. It is preferable that the raw material reacts easily with the lithium raw material in a short time, and from this viewpoint, anatase-type titanium dioxide is preferred. In order to sufficiently react the raw materials with short firing time, the D50 of the titanium raw material is preferably 5 μm or less.

[0057] Lithium compounds such as lithium hydroxide monohydrate, lithium oxide, lithium bicarbonate, and lithium carbonate are used as lithium raw materials.

[0058] Furthermore, the charging ratio of titanium and lithium raw materials should be such that the atomic ratio of Li to Ti (Li / Ti) is 0.81 or higher, and preferably 0.83 or higher. This is because a low charging ratio may promote the generation of specific impurity phases in the lithium titanate powder obtained after firing, which could adversely affect battery characteristics.

[0059] In the present invention, when firing a mixture consisting of the above raw materials in a short time, it is preferable to prepare the mixed powder constituting the mixture before firing so that the D95 in the volume-based particle size distribution measured by laser diffraction scattering is 5 μm or less. Here, D95 is the particle size of the primary particle at which the cumulative volume frequency calculated by volume fraction, when accumulated from the smallest particle size, accounts for 95%.

[0060] The following methods can be used to prepare the mixture. The first method involves mixing the raw materials and then grinding them simultaneously. The second method involves grinding each raw material until the D95 is 5 μm or less, and then mixing them, or mixing them while lightly grinding them. The third method involves producing powders consisting of fine particles from each raw material by methods such as crystallization, classifying them as needed, and then mixing them, or mixing them while lightly grinding them. Among these, the first method, in which the raw materials are mixed and ground simultaneously, is industrially advantageous because it involves fewer steps. A conductive agent may also be added at the same time.

[0061] In any of the first to third methods, there are no particular restrictions on the method of mixing the raw materials; either wet mixing or dry mixing is acceptable. For example, a Henschel mixer, ultrasonic dispersion device, homomixer, mortar and pestle, ball mill, centrifugal ball mill, planetary ball mill, vibrating ball mill, Attritor-type high-speed ball mill, bead mill, roll mill, etc., can be used.

[0062] If the mixture obtained by any of the first to third methods is a mixed powder, it can be used as is for the next firing step. If the mixture is a mixed slurry consisting of mixed powder, the mixed slurry can be dried using a rotary evaporator or the like before being used for the next firing step. If the firing is performed using a rotary kiln, the mixed slurry can be placed directly into the furnace.

[0063] <Firing Process> Next, the resulting mixture is calcined. From the viewpoint of reducing the proportion of specific impurity phases, increasing the crystallinity of lithium titanate, and increasing the crystallite size and primary particle size of the powder, the maximum calcination temperature is 800°C or higher, preferably 810°C or higher. From the viewpoint of increasing the specific surface area of ​​the powder obtained by calcination and reducing the amount of impurities originating from the furnace tube, the maximum calcination temperature is 1100°C or lower, preferably 1000°C or lower, and more preferably 960°C or lower. Similarly, from the above two viewpoints, the holding time at the maximum calcination temperature is 2 minutes to 60 minutes, preferably 5 minutes to 45 minutes, and more preferably 5 minutes to 35 minutes. When the maximum calcination temperature is high, it is preferable to select a shorter holding time. During the heating process of calcination, from the viewpoint of increasing the crystallite size obtained by calcination, it is good practice to shorten the residence time at 700°C to 800°C, preferably within 15 minutes.

[0064] The firing method is not particularly limited as long as it can be fired under the aforementioned conditions. Usable firing methods include fixed-bed furnaces, roller hearth furnaces, mesh belt furnaces, fluidized-bed furnaces, and rotary kilns. However, for efficient firing in a short time, roller hearth furnaces, mesh belt furnaces, and rotary kilns are preferred. When using a roller hearth furnace or a mesh belt furnace in which the mixture is placed in a sagger for firing, it is preferable to place a small amount of mixture in the sagger in order to ensure uniformity of the temperature distribution of the mixture during firing and to maintain consistent quality of the resulting lithium titanate powder.

[0065] A rotary kiln-type firing furnace is particularly preferred for producing the lithium titanate powder of the present invention because it does not require a container for holding the mixture, allows for continuous loading of the mixture during firing, and provides a uniform thermal history to the fired material, resulting in a homogeneous lithium titanate powder.

[0066] The atmosphere during firing is not particularly limited, regardless of the type of firing furnace, as long as it is an atmosphere that can remove the detached moisture and carbon dioxide. Usually, an air atmosphere using compressed air is used, but an oxygen, nitrogen, or hydrogen atmosphere is also acceptable.

[0067] Although the lithium titanate powder after calcination shows slight aggregation, it does not require grinding that would destroy the particles. Therefore, after calcination, it is sufficient to perform crushing or classification to break down the aggregation as needed. If only crushing to break down the aggregation is performed without grinding, the high crystallinity of the lithium titanate powder after calcination is maintained.

[0068] The lithium titanate powder obtained through the above process, before surface treatment (hereinafter sometimes referred to as the base lithium titanate powder; also hereinafter sometimes referred to as the base lithium titanate particles), is mixed with a treatment agent and preferably heat-treated.

[0069] <Surface treatment process> The lithium titanate powder of the present invention is a lithium titanate powder containing a Sn-based solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table, and when applied as a negative electrode material for an all-solid-state secondary battery, it can form a dense negative electrode layer and impart excellent charge rate characteristics. The lithium titanate powder of the present invention can be produced by adding the Sn-based solid electrolyte that has been synthesized in advance in the firing process, or by adding a solution obtained by reacting the starting materials in a liquid-phase method (without undergoing a solvent removal process), but more preferably, the lithium titanate powder of the present invention can be produced by a surface treatment process such as the following.

[0070] The lithium titanate powder obtained through the above process, before surface treatment (hereinafter sometimes referred to as the base lithium titanate powder; also hereinafter sometimes referred to as the base lithium titanate particles constituting the base lithium titanate powder), is mixed with a treatment agent (the Sn-based solid electrolyte or reaction solution) and preferably heat-treated.

[0071] Suitable Sn-based solid electrolytes (treatment agents) that have conductivity to metal ions belonging to Group 1 of the periodic table and are used in surface treatment processes include the solid electrolytes mentioned above.

[0072] The amount of Sn-based solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table can be any amount as long as the amount of Sn-based solid electrolyte in the lithium titanate powder falls within the desired range, but it is preferable to add it in a proportion of 0.1% by mass or more relative to the base lithium titanate powder. Furthermore, it is preferable to add it in a proportion of 20% by mass or less relative to the base lithium titanate powder, more preferably 10% by mass or less, and particularly preferably 8% by mass or less. When using a reaction solution in the liquid phase method, the mass of the generated Sn-based solid electrolyte can be estimated and adjusted as appropriate.

[0073] There are no particular restrictions on the method of mixing the lithium titanate powder of the base material with the Sn-based solid electrolyte or the reaction solution. Either wet mixing or dry mixing can be used. However, it is preferable to uniformly disperse the Sn-based solid electrolyte on the surface of the lithium titanate particles of the base material, and in this respect, wet mixing is preferred.

[0074] For dry mixing, for example, a paint mixer, Henschel mixer, ultrasonic dispersion device, homomixer, mortar and pestle, ball mill, centrifugal ball mill, planetary ball mill, vibrating ball mill, Attritor-type high-speed ball mill, bead mill, roll mill, etc. can be used.

[0075] For wet mixing, the treatment agent and the lithium titanate powder base material are added to water or an alcohol solvent and mixed in a slurry state. As for the alcohol solvent, methanol, ethanol, isopropyl alcohol, etc., which have a boiling point of 100°C or less are preferred because they are easy to remove. Furthermore, from an industrial standpoint, water is preferred because it is easy to recover and dispose of.

[0076] Regarding the amount of solvent, any amount that sufficiently wets the treatment agent and the lithium titanate particles of the substrate is acceptable. However, it is preferable that the treatment agent and the lithium titanate particles of the substrate be uniformly dispersed in the solvent. For this reason, it is preferable that the amount of solvent dissolved in the solvent be 50% or more of the total amount of treatment agent added to the solvent. Since the amount of treatment agent dissolved in the solvent increases with temperature, it is preferable to mix the lithium titanate powder of the substrate and the treatment agent in the solvent while heating. Furthermore, heating also reduces the amount of solvent, so mixing while heating is an industrially suitable method. The mixing temperature is preferably 40°C to 100°C, and more preferably 60°C to 100°C.

[0077] In the case of wet mixing, depending on the heat treatment method, it is preferable to remove the solvent before the heat treatment performed after the mixing process. The solvent is preferably removed by evaporation to dryness. Methods for evaporation to dryness include heating and evaporating the slurry while stirring with a stirring blade, using a drying device that allows drying while stirring, such as a conical dryer, and using a spray dryer. If the heat treatment is performed using a rotary kiln, the mixed raw materials can be supplied to the furnace as a slurry.

[0078] It is preferable to perform a heat treatment after mixing the lithium titanate powder of the base material with the treatment agent. The heat treatment temperature should be such that the Sn-based solid electrolyte diffuses to at least the surface region of the lithium titanate particles of the base material, without causing a significant reduction in the specific surface area due to sintering of the lithium titanate of the base material. The upper limit of the heat treatment temperature is preferably 500°C or less, and more preferably 400°C or less. The lower limit of the heat treatment temperature is preferably 150°C or higher, and more preferably 200°C or higher. The heat treatment time is preferably 0.1 hours to 8 hours, and more preferably 0.5 hours to 5 hours. The temperature and time at which the Sn-based solid electrolyte diffuses to at least the surface region of the lithium titanate particles of the base material vary depending on the Sn-based solid electrolyte, so it is best to set them appropriately.

[0079] The heating method used in heat treatment is not particularly limited. Suitable heat treatment furnaces include fixed-bed furnaces, roller hearth furnaces, mesh belt furnaces, fluidized-bed furnaces, and rotary kilns. The atmosphere during heat treatment can be either air or an inert atmosphere such as nitrogen.

[0080] The lithium titanate powder obtained after heat treatment as described above shows slight aggregation, but does not require grinding that would destroy the particles. Therefore, after heat treatment, it is sufficient to perform crushing or classification to break down the aggregation as needed.

[0081] The lithium titanate powder of the present invention may be mixed with a treatment agent in a surface treatment process, then granulated and heat-treated to produce a powder containing secondary particles formed by the aggregation of primary particles. Granulation can be performed by any method that produces secondary particles, but a spray dryer is preferred because it can process large quantities.

[0082] To reduce the moisture content of the lithium titanate powder of the present invention, dew point control may be performed during the heat treatment process. Since the moisture content of the powder increases if exposed to the atmosphere after heat treatment, it is preferable to handle the powder in a dew point-controlled environment during cooling in the heat treatment furnace and after heat treatment. The powder after heat treatment may be classified as needed to bring the particles within the desired maximum particle size range. When dew point control is performed during the heat treatment process, it is preferable to seal the lithium titanate powder of the invention in an aluminum laminate bag or the like before exposing it to an environment outside of dew point control. Even under dew point control, grinding the lithium titanate powder after heat treatment makes it easier for moisture to be absorbed from the crushed surface, increasing the moisture content of the powder; therefore, it is preferable not to grind the powder after heat treatment. Regarding heat treatment conditions, temperature and holding time being within a specific range greatly affect the secondary particle morphology and surface treatment process. The heat treatment temperature is preferably 250°C or higher, and preferably below 400°C. This is because if the heat treatment temperature exceeds 400°C, the specific surface area decreases significantly, drastically reducing battery performance, especially rate characteristics. Furthermore, a holding time of one hour or more is preferable, as a shorter heating time is presumed to increase the moisture content of the powder and also affect the surface condition of the particles.

[0083] [Negative electrode active material composition] The negative electrode active material composition for all-solid-state secondary batteries of the present invention is Li4Ti5O 12 Lithium titanate powder having Li4Ti5O as its main component, 12 The negative electrode active material composition is characterized by comprising lithium titanate powder, which has a Sn-based solid electrolyte having conductivity for metal ions belonging to Group 1 of the periodic table on the surface of primary particles of lithium titanate, which has a Sn-based solid electrolyte having conductivity for metal ions belonging to Group 1 of the periodic table, and an inorganic solid electrolyte having conductivity for metal ions belonging to Group 1 of the periodic table.

[0084] <Periodic table> The periodic table of this invention refers to the long-period periodic table of elements as defined by IUPAC (International Union of Pure and Applied Chemistry).

[0085] [Inorganic solid electrolyte] Inorganic solid electrolytes are solid electrolytes that are inorganic, and a solid electrolyte is a solid electrolyte that can move ions within itself. Since inorganic solid electrolytes are solid in a steady state, they do not usually dissociate or become liberated into cations and anions. Inorganic solid electrolytes are not particularly limited as long as they have conductivity to metal ions belonging to Group 1 of the periodic table, and generally have little to no electronic conductivity.

[0086] In the present invention, the inorganic solid electrolyte has conductivity of metal ions belonging to Group 1 of the periodic table. Typical examples of inorganic solid electrolytes include (A) sulfide inorganic solid electrolytes and (B) oxide inorganic solid electrolytes. In the present invention, sulfide inorganic solid electrolytes are preferably used because they have high ionic conductivity and a dense molded body with few grain boundaries can be formed by pressurization at room temperature alone.

[0087] (A) Sulfide inorganic solid electrolyte The sulfide inorganic solid electrolyte is preferably one that contains a sulfur atom (S), has conductivity of a metal ion belonging to Group 1 of the periodic table, and is also an electronic insulator. The sulfide inorganic solid electrolyte can be produced by reacting a metal sulfide belonging to Group 1 of the periodic table with at least one sulfide represented by the following general formula (III), and two or more sulfides represented by general formula (III) may be used in combination.

[0088] M x S y (III) (M represents one of P, Si, Ge, B, Al, Ga, or Sb, and x and y represent the numbers that give the stoichiometric ratio, depending on the type of M.)

[0089] The metal sulfide belonging to Group 1 of the periodic table is any of lithium sulfide, sodium sulfide, or potassium sulfide, with lithium sulfide or sodium sulfide being more preferred, and lithium sulfide being even more preferred.

[0090] The sulfide represented by general formula (III) is preferably one of P2S5, SiS2, GeS2, B2S3, Al2S3, Ga2S3, or Sb2S5, with P2S5 being particularly preferred.

[0091] The composition ratio of each element in the sulfide inorganic solid electrolyte produced as described above can be controlled by adjusting the blending amounts of the metal sulfides belonging to Group 1 of the periodic table, the sulfides represented by the general formula (III), and elemental sulfur.

[0092] The sulfide inorganic solid electrolyte of the present invention may be amorphous glass, crystallized glass, or a crystalline material.

[0093] The following combinations are preferred as sulfide inorganic solid electrolytes, but are not particularly limited. Li2S-P2S5, Li2S-P2S5-Al2S3, Li2S-GeS2, Li2S-Ga2S3, Li2S-GeS2-Ga2S3, Li2S-GeS2-P2S5, Li2S -GeS2-Sb2S5, Li2S-GeS2-Al2S3, Li2S-SiS2, Li2S-Al2S3, Li2S-SiS2-Al2S3, Li2S-SiS2-P2S5, Li 10 GeP2S 12 .

[0094] Among the aforementioned combinations, LPS glass and LPS glass ceramics manufactured using a combination of Li2S-P2S5 are preferred.

[0095] The mixing ratio of the metal sulfide belonging to Group 1 of the periodic table and the sulfide represented by the general formula (III) is not particularly limited as long as it can be used as a solid electrolyte, but it is preferably 50:50 to 90:10 (molar ratio). If the molar ratio of the metal sulfide is 50 or more and 90 or less, the ionic conductivity can be sufficiently increased. The mixing ratio (molar ratio) is more preferably 60:40 to 80:20, and even more preferably 70:30 to 80:20.

[0096] The sulfide inorganic solid electrolyte may contain, in addition to metal sulfides belonging to Group 1 of the periodic table and sulfides represented by the general formula (III), at least one lithium halide selected from LiI, LiBr, LiCl, and LiF, or a Li salt such as lithium oxide or lithium phosphate, in order to increase ionic conductivity. However, the mixing ratio of the sulfide inorganic solid electrolyte and these Li salts is preferably 60:40 to 95:5 (molar ratio), and more preferably 80:20 to 95:5.

[0097] In addition to the above, other suitable sulfide inorganic solid electrolytes include algerodite-type solid electrolytes such as Li6PS5Cl and Li6PS5Br.

[0098] The methods for producing the sulfide inorganic solid electrolyte mentioned above include, but are not limited to, solid-phase methods, sol-gel methods, mechanical milling methods, liquid-phase methods, and molten-quenching methods.

[0099] (B) Oxide inorganic solid electrolyte

[0100] The oxide inorganic solid electrolyte is preferably one that contains oxygen atoms, has the conductivity of metal ions belonging to Group 1 of the periodic table, and also has electronic insulating properties.

[0101] Examples of oxide inorganic solid electrolytes include Li, which has a LISICON (Lithium superionic conductor) type crystal structure. 3.5 Zn 0.25 La, which has a perovskite crystal structure, is GeO4. 0.55 Li 0.35 LiTi2P3O has a TiO3, NASICON (Natrium superionic conductor) type crystal structure. 12 Li7La3Zr2O has a garnet-type crystal structure. 12 (LLZ), lithium phosphate (Li3PO4), LiPON (Lithium Phosphate with some of the oxygen replaced by nitrogen), Li3BO3-Li2SO4, Li2O-B2O3-P2O5, Li2O-SiO2, and Li6BaLa2Ta2O 12 These are some examples of preferred materials.

[0102] The volume-average particle size of the inorganic solid electrolyte is not particularly limited, but it should be 0.01 μm or larger, and preferably 0.1 μm or larger. The upper limit should be 100 μm or less, and preferably 50 μm or less. The volume-average particle size of the inorganic solid electrolyte can be measured by laser diffraction scattering.

[0103] The content of the inorganic solid electrolyte is not particularly limited, but it may be 1% by mass or more, preferably 5% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more in the active material composition. A higher content of the inorganic solid electrolyte is preferable because it makes it easier to obtain contact between the lithium titanate powder and the solid electrolyte. However, if the content of the inorganic solid electrolyte is too high, the battery capacity of the all-solid-state secondary battery will decrease, so it may be 70% by mass or less, and preferably 50% by mass or less. Normally, a lower content of the inorganic solid electrolyte is preferable to increase the battery capacity of the all-solid-state secondary battery, but if the content is low, it becomes difficult to obtain contact between the lithium titanate powder and the solid electrolyte. By using the lithium titanate powder used in the negative electrode active material composition of the present invention, satisfactory contact between the lithium titanate powder and the solid electrolyte can be obtained even when the inorganic solid electrolyte content is low.

[0104] [Other contents] The negative electrode active material composition of the present invention may also contain a conductive agent and a binder, in addition to the lithium titanate powder and the inorganic solid electrolyte.

[0105] The conductive agent for the negative electrode is not particularly limited as long as it is an electron-conducting material that does not undergo chemical changes. Examples include graphites such as natural graphite (flaky graphite, etc.) and artificial graphite; carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; single-phase carbon nanotubes, multi-walled carbon nanotubes (multi-walled concentric cylindrical graphite layer) (non-fishbone-shaped); cup-type carbon nanotubes (fishbone-shaped); segmented carbon nanofibers (non-fishbone structure); platelet-type carbon nanofibers (playing card-shaped); and other carbon nanotubes. Furthermore, graphites, carbon blacks, and carbon nanotubes may be mixed as appropriate. While not particularly limited, the specific surface area of ​​the carbon black is preferably 30 m². 2 / g~3000m 2 / g, and more preferably 50m 2 / g~2000m 2 The specific surface area of ​​graphites is preferably 30 m². 2 / g~600m 2 / g, and more preferably 50m 2 / g~500m 2 The ratio is / g. The aspect ratio of the carbon nanotubes is 2 to 150, preferably 2 to 100, and more preferably 2 to 50.

[0106] The amount of conductive agent to be added should be optimized as it varies depending on the specific surface area of ​​the active material and the type and combination of conductive agents. However, it is sufficient for the negative electrode active material composition to contain 0.1% to 10% by mass, preferably 0.5% to 5% by mass. By setting the amount in the range of 0.1% to 10% by mass, the active material ratio can be made sufficient, thereby increasing the conductivity of the negative electrode layer while ensuring sufficient initial discharge capacity of the energy storage device per unit mass and unit volume of the negative electrode layer.

[0107] Examples of binders for the negative electrode include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), and carboxymethylcellulose (CMC). While not particularly limited, the molecular weight of polyvinylidene fluoride is between 20,000 and 1,000,000. From the viewpoint of further improving the binding properties of the negative electrode layer, it is preferable that the molecular weight be 25,000 or more, more preferably 30,000 or more, and even more preferably 50,000 or more. From the viewpoint of further improving conductivity without hindering contact between the active material and the conductive agent, it is preferable that the molecular weight be 500,000 or less. In particular, the specific surface area of ​​the active material is 10 m². 2 If the amount is greater than or equal to 1g, the molecular weight is preferably 100,000 or more.

[0108] The amount of binder added should be optimized as it varies depending on the specific surface area of ​​the active material and the type and combination of conductive agents, but it is sufficient if it is included in the negative electrode active material composition at a concentration of 0.2% to 15% by mass. From the viewpoint of improving binding properties and ensuring the strength of the negative electrode layer, it is preferable to have 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more. From the viewpoint of not reducing the active material ratio and thus reducing the initial discharge capacity of the energy storage device per unit mass and unit volume of the negative electrode layer, it is preferable to have 10% by mass or less, and even more preferably 5% by mass or less.

[0109] [Method for preparing a negative electrode active material composition] The method for producing the negative electrode active material composition of the present invention is not particularly limited, but preferred methods include adding a specific proportion of the inorganic solid electrolyte powder to the lithium titanate powder and mixing them using a mixer, stirrer, disperser, etc., or adding the lithium titanate powder to a slurry containing a solid electrolyte.

[0110] The negative electrode active material composition of the present invention can be used as the negative electrode of an all-solid-state secondary battery. In this case, it is preferable to form a press-molded body by press molding of the negative electrode active material composition of the present invention. The conditions for press molding are not particularly limited, but the molding temperature may be 15°C to 200°C, preferably 25°C to 150°C, and the molding pressure may be 180 MPa to 1080 MPa, preferably 300 MPa to 800 MPa. The negative electrode active material composition of the present invention can form a dense molded body with few voids, and therefore a dense negative electrode layer with few voids can be formed. The molded body obtained using the negative electrode active material composition of the present invention has a filling rate of 67% to 100%, preferably 73.5% to 100%. The method for measuring the filling rate will be explained in the examples described later.

[0111] [All-solid-state secondary battery] The all-solid-state secondary battery of the present invention is composed of a positive electrode layer, a negative electrode layer, and a solid electrolyte layer located between the positive electrode layer and the negative electrode layer, but the Li4Ti5O 12 A negative electrode active material composition containing lithium titanate powder, mainly composed of lithium titanate represented by [formula], and an inorganic solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table, is used in the negative electrode layer. The method for producing the negative electrode layer is not particularly limited, and suitable examples include a method of pressurizing the negative electrode active material composition, or a method of adding the negative electrode active material composition to a solvent to make a slurry, then applying this negative electrode active material composition to a current collector, drying, and pressurizing it.

[0112] Examples of the negative electrode current collector include aluminum, stainless steel, nickel, copper, titanium, calcined carbon, or materials coated with carbon, nickel, titanium, or silver on their surfaces. The surfaces of these materials may also be oxidized, and surface treatment may be used to create irregularities on the surface of the negative electrode current collector. Examples of the negative electrode current collector form include sheets, nets, foils, films, punched materials, laths, porous materials, foams, fiber bundles, and molded nonwoven fabrics. Porous aluminum is preferred as the form of the negative electrode current collector. The porosity of the porous aluminum is 80% or more and 95% or less, preferably 85% or more and 90% or less.

[0113] As long as the negative electrode layer contains the negative electrode active material composition of the present invention, the components such as the positive electrode layer and the solid electrolyte layer can be used without any particular limitations. For example, as a positive electrode active material used in the positive electrode layer of an all-solid-state secondary battery, a composite metal oxide containing one or more elements selected from the group consisting of cobalt, manganese, and nickel, along with lithium, is used. These positive electrode active materials can be used individually or in combination of two or more elements. Examples of such lithium composite metal oxides include LiCoO2 and LiCo 1-x M x O2 (where M is one or more elements selected from Sn, Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, and Cu, 0.001 ≤ x ≤ 0.05), LiMn2O4, LiNiO2, LiCo 1-x Ni x O2(0.01 <x<1)、LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, LiLiLi 0.5 Mn 0.3 Co 0.2 O2, LiLiLi 0.8 Mn 0.1 Co 0.1 O2, LiLiLi 0.8 Co 0.15 Al 0.05 Solid solutions of O2, Li2MnO3 and LiMO2 (where M is a transition metal such as Co, Ni, Mn, or Fe), and LiNi 1 / 2 Mn 3 / 2 One or more types selected from O4 are preferred, and two or more types are more preferred. Furthermore, combinations such as LiCoO2 and LiMn2O4, LiCoO2 and LiNiO2, and LiMn2O4 and LiNiO2 may also be used.

[0114] Furthermore, lithium-containing olivine-type phosphates can also be used as the positive electrode active material. Lithium-containing olivine-type phosphates containing at least one selected from iron, cobalt, nickel, and manganese are particularly preferred. Specific examples include LiFePO4, LiCoPO4, LiNiPO4, and LiMnPO4. Some of these lithium-containing olivine-type phosphates may be substituted with other elements, and some of the iron, cobalt, nickel, and manganese can be substituted with one or more elements selected from the group consisting of Co, Mn, Ni, Mg, Al, B, Ti, V, Nb, Cu, Zn, Mo, Ca, Sr, W, and Zr, or they can be coated with compounds or carbon materials containing these other elements. Among these, LiFePO4 or LiMnPO4 is preferred. Furthermore, lithium-containing olivine-type phosphate can also be used in combination with, for example, the aforementioned positive electrode active material.

[0115] The conductive agent for the positive electrode is not particularly limited as long as it is an electron-conducting material that does not undergo chemical changes. Examples include graphite such as natural graphite (flaky graphite, etc.) and artificial graphite, and carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black. Graphite and carbon black may also be used in appropriate mixtures. The amount of conductive agent added to the positive electrode active material composition is preferably 1 to 10% by mass, and particularly preferably 2 to 5% by mass.

[0116] The positive electrode active material composition contains at least the positive electrode active material and a solid electrolyte, and may optionally contain a conductive agent such as acetylene black or carbon black, and a binder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), carboxymethylcellulose (CMC), or ethylene propylene diene polymer. The method for producing the positive electrode is not particularly limited, and suitable examples include a method of pressurizing the powder of the positive electrode active material composition, or a method of adding the powder of the positive electrode active material composition to a solvent to make a slurry, and then applying this positive electrode active material composition to an aluminum foil or stainless steel lath plate of a current collector, followed by drying and pressurizing.

[0117] The surface of the positive electrode active material may be coated with another metal oxide. Examples of surface coating agents include metal oxides containing Ti, Nb, Ta, W, Zr, Al, Si, or Li. Specifically, Li4Ti5O 12 Examples include Li2Ti2O5, LiTaO3, LiNbO3, LiAlO2, Li2ZrO3, Li2WO4, Li2TiO3, Li2B4O7, Li3PO4, Li2MoO4, Li3BO3, LiBO2, Li2CO3, Li2SiO3, SiO2, TiO2, ZrO2, Al2O3, B2O3, etc.

[0118] The thickness of the solid electrolyte layer is not particularly limited, but may be between 1 μm and 100 μm. The constituent material of the solid electrolyte layer can be the aforementioned sulfide inorganic solid electrolyte or oxide inorganic solid electrolyte, and may be different from the solid electrolyte used in the electrode. The solid electrolyte layer may also contain a binder such as butadiene rubber or butyl rubber.

[0119] There are no particular limitations on the structure of all-solid-state rechargeable batteries; coin-type batteries, cylindrical batteries, prismatic batteries, laminated batteries, etc., can be used. [Examples]

[0120] Next, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples and encompasses various combinations that can be easily inferred from the spirit of the invention. [Manufacturing Example 1] <Raw material preparation process> Li₂CO₃ (average particle size 4.6 μm) and TiO₂ (specific surface area 10 m²) are used so that the atomic ratio of Li to Ti (Li / Ti) is 0.83. 2 A raw material mixture slurry was prepared by weighing the raw material powder (by g) and adding deionized water to the mixture so that the solid content concentration of the slurry was 41% by mass, and then stirring. This raw material mixture slurry was then wet-mixed and pulverized using a bead mill (Willi E. Bakkofen, model: Dynomill KD-20BC, agitator material: polyurethane, vessel inner surface material: zirconia). Zirconia beads (outer diameter: 0.65 mm) were packed into the vessel at 80% by volume, and the process was carried out with an agitator peripheral speed of 13 m / s and a slurry feed rate of 55 kg / hr, while controlling the internal pressure of the vessel to 0.02 to 0.03 MPa.

[0121] <Firing Process> The obtained mixed slurry was introduced into the furnace core from the raw material supply side of a rotary kiln-type firing furnace (furnace core length: 4 m, furnace core diameter: 30 cm, external heating type) equipped with an anti-adhesion mechanism, dried in a nitrogen atmosphere, and fired. At this time, the tilt angle of the furnace core from the horizontal direction was 2.5 degrees, the rotation speed of the furnace core was 20 rpm, and the flow rate of nitrogen introduced into the furnace core from the calcined material recovery side was 20 L / min. The heating temperature of the furnace core was set to 600°C on the raw material supply side, 840°C in the center, and 840°C on the calcined material recovery side, and the holding time of the calcined material at 840°C was 30 minutes.

[0122] <Post-processing steps> The calcined material recovered from the calcined material recovery side of the furnace tube was crushed using a bead mill (AIMEX, NVM-1.5 type) to obtain a calcined powder sample. Powder X-ray diffraction measurements were performed on the obtained calcined powder sample, and it was confirmed to be lithium titanate (ICDD (PDF2010) PDF card 00-049-0207).

[0123] <Preparation of Sn-based Solid Electrolyte> Production of Li4SnS4 by mechanochemical method In a glove box under an argon atmosphere, lithium sulfide (Li2S) and tin disulfide (SnS2) were weighed to achieve a molar ratio of Li2S:SnS2 = 2:1, and mixed in an agate mortar to obtain a raw material composition. Next, zirconia balls (3 mm in diameter, 160 g) and 2 g of the obtained raw material composition were put into an 80 mL zirconia pot, and the container was sealed under an argon atmosphere. This pot was set in a planetary ball mill, and mechanical milling was performed at a rotation speed of 510 rpm for 40 hours to obtain Li4SnS4. The ionic conductivity of the obtained Li4SnS4 was 1.6×10 -5 S / cm.

[0124] <Surface Treatment Step> Ion-exchanged water was added to the obtained fired powder sample such that the solid content concentration of the slurry became 30% by mass, and the mixture was crushed by stirring. Li4SnS4 synthesized by a mechanochemical method as a treatment agent was added in an amount of 1% by mass relative to 100 g of the crushed fired powder to prepare a mixed slurry. After this mixed slurry was subjected to mixing treatment with a paint shaker for 10 minutes, dried at a temperature of 100°C, and then heat-treated at 320°C for 1 hour using a muffle furnace, thereby obtaining lithium titanate powder (hereinafter referred to as LTO) according to Production Example 1, in which a Sn-based solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table exists on the particle surface.

[0125] [Production Examples 2 to 5] Lithium titanate powders according to Production Examples 2 to 5 were produced in the same manner as in Production Example 1, except that the production process was performed under the conditions shown in Table 1. Note that in Production Example 5, Li4SnS4 synthesized by the mechanochemical method as a treatment agent was not used.

[0126] [Production Example 6] Production of Li4SnS4 by liquid phase method In a glove box under an argon atmosphere, lithium sulfide (Li₂S), tin sulfide (SnS), and sulfur (S) were weighed out to achieve a molar ratio of Li₂S:SnS:S = 2:1:1, and mixed in an agate mortar to obtain a raw material composition. Next, 2 g of the obtained raw material composition and 18 g of ion-exchanged water were charged into a glass container, and reacted by stirring at 80°C for 5 hours. The obtained reaction solution was vacuum-dried at 150°C for 5 hours to obtain Li₄SnS₄. The ionic conductivity of the obtained Li₄SnS₄ was 1.1×10 -5 S / cm.

[0127] [Production Example 7] Production of Li₄SnS₄ Using Liquid Phase Method Li₄SnS₄ was obtained by production under the same conditions as Production Example 6, except that lithium sulfide (Li₂S), tin (Sn), and sulfur (S) were weighed out to achieve a molar ratio of Li₂S:Sn:S = 2:1:2. The ionic conductivity of the obtained Li₄SnS₄ was 1.6×10 -5 S / cm. The maximum peak intensity of the impurity phase in X-ray diffraction measurement was 1 / 20 or less of the maximum peak intensity of the main phase. Almost single-phase Li₄SnS₄ was obtained. The obtained sample was identified as hexagonal Li₄SnS₄.

[0128] [Measurement of Metal Element Content] The content of metal elements contained in the lithium titanate powders of Production Examples 1 to 5 was measured as follows.

[0129] <X-ray Fluorescence Analysis (XRF): Identification of Metal Elements> Quantitative analysis of elements contained in the lithium titanate powder of each production example was performed using an X-ray fluorescence analyzer (manufactured by SII Technology Co., Ltd., product name "SPS5100").

[0130] [Measurement of Powder Physical Properties] Various physical properties of the lithium titanate powder of each production example were measured as follows.

[0131] <Measurement of Specific Surface Area> The specific surface area (m 2 The specific surface area ( / g) was measured using a fully automated BET specific surface area analyzer (Mountec Co., Ltd., product name "Macsorb HM model-1208"), with nitrogen gas used as the adsorption gas. 0.5g of the sample powder was weighed, placed in a φ12 standard cell (HM1201-031), degassed under vacuum at 100°C for 0.5 hours, and then measured using the BET single-point method.

[0132] <Calculation of D50 of primary particles: Dry laser diffraction scattering method> The D50 of lithium titanate powder for each manufacturing example was calculated from the particle size distribution curve measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT3300EXII, manufactured by Nikkiso Co., Ltd.). 50 mg of the sample was placed in a container containing 50 ml of deionized water as the measurement solvent. The container was shaken by hand until the powder was visibly and uniformly dispersed in the solvent, and then the container was placed in the measurement cell for measurement. For the crushing process, ultrasound (30 W, 3 s) was applied using an ultrasonic device in the instrument. Further measurement of the particle size distribution was performed by adding more measurement solvent until the slurry transmittance was within the appropriate range (indicated by the green bar on the instrument). The D50 of the crushed mixed powder was calculated from the obtained particle size distribution curve.

[0133] [Table 1]

[0134] [Preparation of the negative electrode active material composition] [Example 1] In a glove box under an argon atmosphere, lithium titanate powder from Production Example 1 and sulfide inorganic solid electrolyte powder having the composition of Li6PS5Cl (volume-average particle size measured using a laser diffraction / scattering particle size distribution analyzer: 6 μm) were weighed in a mass ratio of lithium titanate:Li6PS5Cl = 80:20 and mixed in an agate mortar. Next, zirconia balls (3 mm in diameter, 20 g) were placed in an 80 mL zirconia pot, and the mixed powder was added. Then, this pot was set in a planetary ball mill and stirred at a rotation speed of 200 rpm for 15 minutes to obtain the negative electrode active material composition of Example 1. [Examples 2-5, Comparative Examples 1 and 2] The negative electrode active material compositions described in Tables 2 and 3 below were prepared in the same manner as in Example 1, except that lithium titanate powder produced by the method described in Table 1 was used.

[0135] [Measurement of physical properties of negative electrode active material composition] Each of the above negative electrode active material compositions was weighed out at a dose of 100 mg, and these samples were pressed at room temperature for 10 minutes (360 MPa) to produce pellets (molded bodies) with a diameter of 10 mm and a thickness of approximately 0.7 mm. <Evaluation of filling rate and relative density ratio> The packing density was calculated using the following formula, based on the pellet density of the negative electrode active material composition calculated from the volume and mass of the above pellets, the density of Li6PS5Cl (true density), the density of lithium titanate (true density), and the density calculated from the mixing ratio of lithium titanate powder in the negative electrode active material composition (α; 0 < α < 1) (α is the content ratio of lithium titanate powder when the entire negative electrode active material composition is set to 1). Packing ratio (%) = (Pellet density of negative electrode active material composition / ((1-α)Li6PS5Cl density (true density) + α × lithium titanate density (true density)) × 100 Then, using the obtained packing density values, the relative density ratio of the pellets of the negative electrode active material compositions of Examples 1-5 and Comparative Examples 1-2 was calculated, with the value of Comparative Example 1 set to 100%. The results are shown in Tables 2 and 3.

[0136] [Evaluation of battery characteristics] All-solid-state secondary batteries were fabricated using pellets of the negative electrode active material composition from each example, and their battery characteristics were evaluated. The evaluation results are shown in Tables 2 and 3.

[0137] [Manufacturing of sulfide inorganic solid electrolytes] In a glove box under an argon atmosphere, lithium sulfide (Li2S) and phosphorus pentasulfide (P2S5) were weighed in a molar ratio of Li2S:P2S5 = 75:25, and mixed in an agate mortar to obtain the raw material composition. Next, zirconia balls (3 mm in diameter, 160 g) and 2 g of the obtained raw material composition were placed in an 80 mL zirconia pot, and the container was sealed under an argon atmosphere. This pot was set in a planetary ball mill and mechanical milling was performed at a rotation speed of 510 rpm for 16 hours to obtain yellow powder sulfide inorganic solid electrolyte (LPS glass). 80 mg of the obtained LPS glass was placed over an area of ​​0.785 cm². 2 A pellet-shaped solid electrolyte layer was obtained by pressing it at a pressure of 360 MPa using a pellet molding machine having a molding section.

[0138] [Fabrication of all-solid-state secondary batteries] A solid-state secondary battery was fabricated by laminating pellets of the negative electrode active material composition of each embodiment, the pelletized solid electrolyte layer, and a lithium indium alloy foil as a counter electrode in this order, and then sandwiching the laminate between stainless steel current collectors.

[0139] <Measurement of initial discharge capacity and charge rate characteristics> In a constant temperature bath at 45°C, the coin-type battery prepared using the method described above was charged to 0.5V with a current equivalent to 0.05C of the theoretical capacity of lithium titanate, with the direction in which Li is absorbed into the evaluation electrode considered as charging. Then, constant current constant voltage charging was performed until the charging current at 0.5V was equivalent to 0.01C, followed by constant current discharge to 2V with a current equivalent to 0.05C. The initial discharge capacity (mAh / g) was calculated by dividing the discharge capacity (mAh) by the mass of lithium titanate. Next, the 0.2C charge capacity was determined by charging to 0.5V with a current equivalent to 0.2C of the theoretical capacity of lithium titanate, and then discharging to 2V with a current of 0.05C. The charge rate characteristic (%) was calculated by dividing this 0.2C charge capacity by the initial discharge capacity. For Examples 1-4, the initial discharge capacity and charge rate characteristics were examined relative to the values ​​of Comparative Example 1 (set to 100%), and for Example 5, the values ​​of Comparative Example 2 (set to 100%) were examined relative to each other. The evaluation results are shown in Tables 2 and 3. In 1C, C represents the current value during charging and discharging. For example, 1C refers to the current value that can completely discharge (or fully charge) the theoretical capacity in 1 / 1 hour, and 0.1C refers to the current value that can completely discharge (or fully charge) the theoretical capacity in 1 / 0.1 hour.

[0140] [Table 2]

[0141] [Table 3]

[0142] Tables 2 and 3 above show that Examples 1 to 5 of the all-solid-state secondary battery using the negative electrode active material composition of the present invention exhibited excellent initial discharge capacity and further improved charge rate characteristics. It should be noted that the lithium titanate powder used in Manufacturing Examples 1 to 4 in Examples 1 to 5 was prepared by introducing Li4SnS4, manufactured by a mechanochemical method, as a treatment agent through a surface treatment process. Therefore, Li4SnS4 was localized on the surface of the primary lithium titanate particles. Furthermore, in Production Examples 1 to 4, Li4SnS4 synthesized by a mechanochemical method was used as the treatment agent. However, even when using Li4SnS4 produced by the liquid-phase method obtained in Production Example 6, it is possible to achieve localized Li4SnS4 on the surface of the primary particles of lithium titanate. It is also believed that similar results to those in Examples 1 to 5 can be obtained by using such lithium titanate powder.

[0143] Based on the above results, the negative electrode active material composition of the present invention effectively suppresses side reactions between the active site on the lithium titanate surface and the solid electrolyte, thereby exhibiting excellent battery characteristics.

Claims

1. Li ４ Ti ５ O １２ Lithium titanate powder having lithium titanate represented by as the main component, The specific surface area of ​​the lithium titanate powder is 1 m² / g or more and 10 m² / g or less. On the surface of the primary particles of lithium titanate, there is a Sn-based solid electrolyte having conductivity for metal ions belonging to Group 1 of the periodic table. Lithium titanate powder wherein the content of the Sn-based solid electrolyte in the lithium titanate powder is 0.01% by mass or more and 10% by mass or less.

2. The lithium titanate powder according to claim 1, wherein the D50 of primary particles corresponding to 50% of the cumulative volume frequency in the volume-based particle size distribution of the lithium titanate powder by laser diffraction scattering method is 0.5 μm or larger.

3. The Sn-based solid electrolyte present on the surface of the primary particles is Li ４ SnS ４ Lithium titanate powder according to claim 1, characterized by containing the above.

4. A negative electrode active material composition comprising lithium titanate powder according to any one of claims 1 to 3 and an inorganic solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table.

5. The negative electrode active material composition according to claim 4, wherein the inorganic solid electrolyte is a sulfide inorganic solid electrolyte.

6. The negative electrode active material composition according to claim 4, wherein the content of the inorganic solid electrolyte is 1% by mass or more and 50% by mass or less in the negative electrode active material composition.

7. An all-solid-state secondary battery comprising a positive electrode layer, a negative electrode layer, and a solid electrolyte layer, wherein the negative electrode layer is a layer containing the negative electrode active material composition described in claim 4.

Images