Anode active material composition, and all-solid-state secondary battery containing the same

Abstract

To provide a negative electrode active material composition and an all-solid secondary battery that contain lithium titanate powder suitable as a negative electrode material of the all-solid secondary battery.SOLUTION: Provided is a negative electrode active material composition that contains: lithium titanate powder that contains Li4Ti5O12 as a main component; and an inorganic solid electrolyte that has the conductivity of metal ions belonging to Group I of the periodic table. Sulfur elements or compounds containing sulfur elements exist on a surface of lithium titanate particles configuring the lithium titanate powder.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material composition using lithium titanate powder, which is suitable as a negative electrode material for an all-solid-state secondary battery, and to an all-solid-state secondary battery. [Background technology]

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

[0003] Currently available lithium batteries mainly consist of a positive electrode and a negative electrode containing materials capable of intercalating and deintercalating lithium, and a non-aqueous electrolyte composed of a lithium salt and a non-aqueous solvent. As non-aqueous solvents, 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. Because lithium batteries use an electrolyte containing flammable organic solvents, they are prone to leakage and pose a risk of ignition in the event of a short circuit. Therefore, safety devices to suppress temperature rise during short circuits and structures to prevent short circuits are necessary. In this context, all-solid-state secondary batteries, which use inorganic solid electrolytes instead of organic electrolytes, are attracting attention. Because all-solid-state secondary batteries consist entirely of solid positive and negative electrodes and electrolytes, they have the potential to significantly improve the safety and reliability issues that plague batteries using organic electrolytes. Furthermore, the simplification of safety devices allows for higher energy density, making them promising for applications in electric vehicles and large-scale storage batteries.

[0004] Unlike conventional lithium-ion secondary batteries that use electrolytic solutions, in all-solid-state secondary batteries, it is very important to form a good solid-solid interface and continuously maintain that interface from the perspective of realizing excellent ionic conductivity and long-term cycle characteristics. Lithium titanate has attracted attention in order to maintain a good interface between the active material and the solid electrolyte. Since lithium titanate has a very small volume change associated with charge and discharge, it is expected that the interface between the active material and the solid electrolyte will be maintained over a long period during charge and discharge. Patent Document 1 discloses an electrode using lithium titanate having a specific BET specific surface area and solid electrolyte particles smaller than the average particle size of lithium titanate, and it has been reported that the contact between lithium titanate and the solid electrolyte particles has become better than before. Patent Document 2 also discloses lithium titanium composite oxide particles in which at least one element selected from the group consisting of phosphorus and sulfur or a compound of this element is coated on at least a part of the surface. According to Patent Document 2, it has been reported that when applied as an electrode material for an electricity storage device, gas generation associated with the decomposition of a non-aqueous electrolyte (for example, a non-aqueous electrolytic solution) is suppressed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] By using the electrode of Patent Document 1, although the contact between the lithium titanate powder and the solid electrolyte powder was improved and the battery characteristics of the all-solid-state secondary battery were improved, further improvement was required. In particular, when using lithium titanate particles with a relatively small average particle size, a decrease in battery characteristics was also observed even in the configuration of Patent Document 1. This is considered to be because the lithium titanate particles aggregated with each other, and satisfactory contact between the lithium titanate powder and the solid electrolyte powder could not be obtained. The present invention provides a negative electrode active material composition that can form a good solid-solid interface with a solid electrolyte regardless of the particle size of the lithium titanate powder, and can further form a dense negative electrode layer with fewer voids and a higher filling rate than conventional ones, and an all-solid-state secondary battery.

Means for Solving the Problems

[0007] As a result of repeated studies to further increase the contact area between lithium titanate particles and a solid electrolyte even when using lithium titanate powder with a relatively small average particle size, the inventors of the present invention found that by allowing a sulfur element or a sulfur element-containing compound to be present on the surface of the primary particles of lithium titanate, a good solid-solid interface is formed between the lithium titanate powder and the solid electrolyte, and a dense negative electrode layer with fewer voids and a higher filling rate than conventional ones can be obtained, thus completing the present invention. By using the negative electrode active material composition containing the lithium titanate powder and the solid electrolyte in an all-solid-state secondary battery, the DC resistance during charging (charging DC resistance) can be reduced. Incidentally, Patent Document 2 does not describe or suggest at all the effect of enhancing the density of the negative electrode layer containing the negative electrode active material and the solid electrolyte in an all-solid-state secondary battery.

[0008] The present invention relates to a negative electrode active material composition using lithium titanate powder suitable as a negative electrode material for an all-solid-state secondary battery, and an all-solid-state secondary battery.

[0009] That is, the present invention provides the following (1) to (8).

[0010] (1) Li4Ti5O 12A negative electrode active material composition comprising lithium titanate powder having as its main component and an inorganic solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table, wherein a sulfur element or a sulfur element-containing compound exists on the surface of the lithium titanate particles constituting the lithium titanate powder.

[0011] (2) The negative electrode active material composition according to (1), wherein the inorganic solid electrolyte is a sulfide inorganic solid electrolyte.

[0012] (3) The negative electrode active material composition according to (1) or (2), wherein the content of the sulfur element or sulfur element-containing compound in the lithium titanate powder is 0.01% by mass or more and 1% by mass or less in terms of sulfur element.

[0013] (4) The negative electrode active material composition according to any one of (1) to (3), wherein the sulfur-containing compound is lithium sulfate.

[0014] (5) The negative electrode active material composition according to any one of (1) to (4), wherein the D50 of primary particles corresponding to 50% of the volume accumulation in the volume-based particle size distribution of the lithium titanate powder by laser diffraction scattering method is 0.5 μm or more.

[0015] (6) The negative electrode active material composition according to any one of (1) to (5), wherein the D50 of primary particles corresponding to 50% of the volume accumulation in the volume-based particle size distribution of the lithium titanate powder by laser diffraction scattering method is 10 μm or less.

[0016] (7) The negative electrode active material composition according to any one of (1) to (6), 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.

[0017] (8) 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 any one of (1) to (7) above. [Effects of the Invention]

[0018] According to the present invention, a negative electrode active material composition and an all-solid-state secondary battery can be obtained in which a good solid-solid interface is formed with the solid electrolyte regardless of the particle size of the lithium titanate powder, and a dense negative electrode layer with fewer voids and a higher packing density than conventional methods is formed, thereby reducing the DC resistance of charging. [Modes for carrying out the invention]

[0019] The present invention relates to a negative electrode active material composition using lithium titanate powder, which is suitable as a negative electrode material for an all-solid-state secondary battery, and to an all-solid-state secondary battery.

[0020] [Negative electrode active material composition] The negative electrode active material composition of the present invention is Li4Ti5O 12 A negative electrode active material composition comprising lithium titanate powder having as its main component and an inorganic solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table, wherein a sulfur element or a sulfur element-containing compound is present on the surface of the lithium titanate particles constituting the lithium titanate powder.

[0021] [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. 12 It may contain crystalline and / or amorphous components other than Li4Ti5O. The main component is the diffraction peak measured by X-ray diffraction, which is Li4Ti5O. 12 This means that the proportion of the main peak intensity is 90% or more. The lithium titanate powder of the present invention is Li4Ti5O 12 The intensity ratio of the main peak is preferably 92% or higher, and more preferably 95% or higher. Li4Ti5O 12As other components, it is the sum of the intensity of the main peak due to the crystalline component and the highest intensity of the halo pattern due to the amorphous component. In particular, the lithium titanate powder of the present invention may contain anatase-type titanium dioxide, rutile-type titanium dioxide, and lithium titanate having different chemical formulas, such as Li2TiO 3、 Li 0.6 Ti 3.4 O 8、 etc. as the crystalline components. The lithium titanate powder of the present invention has less generation ratio of crystalline components other than these Li4Ti5O 12 , particularly Li 0.6 Ti 3.4 O8, the charging characteristics and charge-discharge capacity of the power storage device can be improved. Among the diffraction peaks measured by the X-ray diffraction method, when the intensity of the main peak of Li4Ti5O 12 is set to 100, the intensity of the main peak of anatase-type titanium dioxide, the intensity of the main peak of rutile-type 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, and the sum of these intensities is particularly preferably 5 or less. Here, the main peak of Li4Ti5O 12 is a peak corresponding to the diffraction peak attributed to the (111) plane (2θ = 18.33) of Li4Ti5O 12 in the PDF card 00-049-0207 of ICDD (PDF2010). The main peak of anatase-type titanium dioxide is a peak corresponding to the diffraction peak attributed to the (101) plane (2θ = 25.42) in the PDF card 01-070-6826. The main peak of rutile-type titanium dioxide is a peak corresponding to the diffraction peak attributed to the (110) plane (2θ = 27.44) in the PDF card 01-070-7347. The peak corresponding to the (-133) plane of Li2TiO3 is a peak corresponding to the diffraction peak attributed to the (-133) plane (2θ = 43.58) of Li2TiO3 in the PDF card 00-033-0831. Li 0.6 Ti 3.4The 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.

[0022] <Presence of sulfur element or sulfur-containing compound> The lithium titanate powder of the present invention has sulfur or a sulfur-containing compound present on the surface of the lithium titanate particles constituting the lithium titanate powder. The presence of sulfur or a sulfur-containing compound means that sulfur is detected in known analytical instruments such as X-ray fluorescence analysis (XRF) or inductively coupled plasma atomic emission spectrometry (ICP-AES) of the lithium titanate powder of the present invention. The lower limit of the amount detected by inductively coupled plasma atomic emission spectrometry is usually 0.001% by mass in terms of sulfur.

[0023] <Content of sulfur element or sulfur-containing compound> The content of sulfur element or sulfur element-containing compound in the lithium titanate powder of the present invention, as determined by X-ray fluorescence analysis (XRF), is 0.01% by mass or more and 1% by mass or less, in terms of sulfur element. When the content of sulfur element or sulfur element-containing compound is within this range, a dense negative electrode layer with few voids and high packing efficiency can be obtained, and an all-solid-state secondary battery with reduced DC charging resistance can be obtained. The content of sulfur element or sulfur element-containing compound is preferably 0.01% by mass or more and 0.9% by mass or less, more preferably 0.01% by mass or more and 0.85% by mass or less, even more preferably 0.01% by mass or more and 0.8% by mass or less, especially preferably 0.06% by mass or more and 0.75% by mass or less, even more preferably 0.1% by mass or more and 0.75% by mass or less, particularly preferably 0.1% by mass or more and 0.7% by mass or less, and most preferably 0.13% by mass or more and 0.4% by mass or less, in terms of sulfur element. The content rate refers to the proportion of the mass of the sulfur element or sulfur-containing compound relative to the total mass of lithium titanate powder, expressed in terms of sulfur element equivalents.

[0024] Furthermore, in the lithium titanate powder of the present invention, it is sufficient that sulfur elements or sulfur-containing compounds are present on the surface of the lithium titanate particles constituting the lithium titanate powder, and it is preferable that there are more sulfur elements or sulfur-containing compounds on the surface than inside the primary lithium titanate particles contained in the lithium titanate powder. Specifically, in cross-sectional analysis of the primary lithium titanate particles using a scanning transmission electron microscope, if the atomic concentration of sulfur elements contained in the sulfur elements or sulfur-containing compounds at a depth of 1 nm from the surface of the primary lithium titanate particles, measured by energy-dispersive X-ray spectroscopy, is C1 (atm%) and the atomic concentration of sulfur elements at a depth of 100 nm from the surface of the lithium titanate particles is D2 (atm%), then it is preferable that the following formula (I) is satisfied, and it is more preferable that the following formula (II) is satisfied. C1>C2 (I) C1 / C2≧5 (II)

[0025] In the lithium titanate powder of the present invention, it is preferable that, in cross-sectional analysis of the primary lithium titanate particles constituting the lithium titanate powder using a scanning transmission electron microscope, the sulfur element or sulfur-containing compound is not detected at a depth of 100 nm from the surface of the primary lithium titanate particles, as measured by energy-dispersive X-ray spectroscopy. It is preferable that the sulfur element or sulfur-containing compound is fixed to the surface of the primary particles in a chemically bonded state. When the sulfur element or sulfur-containing compound exists in such a state, a dense negative electrode layer with few voids and a high packing efficiency can be obtained, resulting in an all-solid-state secondary battery with excellent DC charging resistance. The lower limit of the detection amount in measurement by energy-dispersive X-ray spectroscopy varies depending on the element and state being measured, but is usually 0.5 atm%. Therefore, the sulfur element may be detected at a depth of about 100 nm in a range of 0.5 atm% or less.

[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 at which the cumulative volume frequency calculated from the volume fraction obtained by laser diffraction-scattering particle size distribution measurement accumulates to 50% when 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 preferably 0.5 μm or larger, more preferably 0.55 μm or larger, and more preferably 0.6 μm or larger, from the viewpoint of obtaining a dense negative electrode layer with few voids and high packing efficiency. It is also preferably 12 μm or smaller, more preferably 10 μm or smaller, more preferably 7 μm or smaller, even more preferably 5 μm or smaller, and particularly preferably 4.8 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%. It may also 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 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%. On the other hand, the lithium titanate powder may contain a cumulative volume frequency of primary particles with a primary particle diameter of 5 μm or more in the range of 50% to 90%, and a cumulative volume frequency of primary particles with a primary particle diameter of 4.5 μm or more in the range of 45% to 90%. Furthermore, it may contain a cumulative volume frequency of primary particles with a primary particle diameter of 4 μm or more in the range of 40% to 90%, a cumulative volume frequency of primary particles with a primary particle diameter of 2 μm or more in the range of 15% to 90%, and a cumulative volume frequency of primary particles with a primary particle diameter of 1.8 μm or more in the range of 10% to 90%.

[0028] [Method for producing lithium titanate powder of the present invention] 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.

[0029] <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.

[0030] Lithium compounds such as lithium hydroxide monohydrate, lithium oxide, lithium bicarbonate, and lithium carbonate are used as lithium raw materials.

[0031] 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.

[0032] 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 D95 in the particle size distribution curve measured by a laser diffraction / scattering particle size distribution analyzer is 5 μm or less. Here, D95 is the particle size at which the cumulative volume frequency calculated by volume fraction, when accumulated from the smallest particle size, accounts for 95%.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] <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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] <Surface treatment process> The lithium titanate powder of the present invention is a lithium titanate powder containing a sulfur element or a sulfur element-containing compound, and when applied as a negative electrode material for an all-solid-state secondary battery, it can form a dense negative electrode layer with a high packing efficiency and reduce the DC resistance of charging. The lithium titanate powder of the present invention can be produced by adding the sulfur element or a sulfur element-containing compound (hereinafter sometimes referred to as a treatment agent) in the firing process, but more preferably, the lithium titanate powder of the present invention can be produced by the following surface treatment process.

[0042] 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 that constitute the base lithium titanate powder), is mixed with a treatment agent and preferably heat-treated.

[0043] The sulfur element or the sulfur element-containing compound (treatment agent) is not particularly limited, but examples include sulfur (e.g., powdered sulfur), sulfates such as lithium sulfate, sodium sulfate, titanium sulfate, lithium methyl sulfate, and lithium ethyl sulfate, sulfonates such as lithium methanesulfonate and lithium ethanesulfonate, and S=O group-containing imide salts such as LiN(SO2F)2[LiFSI], LiN(SO2CF3)2[LiTFSI], and LiN(SO2C2F5)2. Among these, compounds containing an S=O group are preferred, lithium sulfate and its hydrate, and lithium methyl sulfate, lithium methanesulfonate, and LiTFSI are more preferred, and lithium sulfate and its hydrate are even more preferred. In other words, if the lithium titanate particles constituting the lithium titanate powder of the present invention have a sulfur-containing compound on their surface, the sulfur-containing compound is preferably one of the sulfur-containing compounds listed as the treatment agent above, more preferably a compound containing an S=O group, even more preferably lithium sulfate and its hydrate, and further preferably lithium methyl sulfate, lithium methanesulfonate, and LiTFSI, and even more preferably lithium sulfate and its hydrate. These may be further oxidized by calcination or the like.

[0044] The amount of sulfur element or sulfur element-containing compound (treatment agent) added can be any amount as long as the content of the sulfur element or sulfur element-containing compound in the lithium titanate powder falls within the range of the present invention, but it is sufficient to add it in a proportion of 0.1% by mass or more relative to the base lithium titanate powder. Alternatively, it is sufficient to add it in a proportion of 12% by mass or less relative to the base lithium titanate powder, preferably 10% by mass or less, and more preferably 8% by mass or less. Two or more treatment agents may be used in combination.

[0045] There are no particular restrictions on the method of mixing the lithium titanate powder base material with the sulfur element or the compound containing the sulfur element. Either wet mixing or dry mixing can be used. However, it is preferable to uniformly disperse the sulfur element or the compound containing the sulfur element on the surface of the lithium titanate particles of the base material, and in this respect, wet mixing is preferred.

[0046] 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.

[0047] 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.

[0048] Regarding the amount of solvent, any amount that sufficiently wets the treatment agent and the lithium titanate powder substrate is acceptable. However, it is sufficient that the treatment agent and the lithium titanate powder substrate are uniformly dispersed in the solvent. For this reason, it is preferable that the amount of solvent dissolved in the solvent is 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 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.

[0049] 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.

[0050] It is preferable to perform a heat treatment after mixing the lithium titanate powder base material with the treatment agent. The heat treatment temperature should be such that the sulfur element or sulfur-containing compound 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 particles of the base material. The upper limit of the heat treatment temperature should be 700°C or less, preferably 600°C or less. The lower limit of the heat treatment temperature should be 300°C or higher, preferably 400°C or higher. The heat treatment time should be 0.1 hours to 8 hours, preferably 0.5 hours to 5 hours. The temperature and time at which the sulfur element or sulfur-containing compound diffuses to at least the surface region of the lithium titanate particles of the base material should be set appropriately, as the reactivity differs depending on the sulfur element or sulfur-containing compound.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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 it is 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. The heat treatment conditions, specifically the temperature and holding time, greatly affect the secondary particle morphology and surface treatment process. The heat treatment temperature should be 450°C or higher and 550°C or lower. This is because exceeding 550°C significantly reduces the specific surface area, drastically increasing battery performance, particularly the DC charging resistance. Furthermore, a holding time of one hour or more is preferable, as a shorter holding time is presumed to increase the moisture content of the powder and also affect the surface condition of the particles.

[0055] <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).

[0056] [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 (an electrolyte that is solid at 25°C). 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.

[0057] 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.

[0058] (A) Sulfide inorganic solid electrolyte The sulfide inorganic solid electrolyte is preferably one that contains the element sulfur (S), has conductivity of metal ions 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.

[0059] M x S y (III) (M represents one of P, Si, Ge, B, Al, Ga, and Sb, and x and y are numbers that give the stoichiometric ratio depending on the type of M.)

[0060] The metal sulfide belonging to Group 1 of the periodic table is any of lithium sulfide, sodium sulfide, and potassium sulfide, with lithium sulfide and sodium sulfide being more preferred, and lithium sulfide being even more preferred.

[0061] The sulfide represented by general formula (III) is preferably one of P2S5, SiS2, GeS2, B2S3, Al2S3, Ga2S3, and Sb2S5, with P2S5 being more preferred.

[0062] 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 sulfide belonging to Group 1 of the periodic table, the sulfide represented by the general formula (III), and elemental sulfur.

[0063] The sulfide inorganic solid electrolyte of the present invention may be amorphous glass, crystallized glass, or a crystalline material.

[0064] 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 .

[0065] Among the aforementioned combinations, LPS glass and LPS glass ceramics manufactured using a combination of Li2S-P2S5 are preferred.

[0066] 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.

[0067] 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 lithium salts such as lithium oxide and lithium phosphate, in order to increase ionic conductivity. However, the mixing ratio of the sulfide inorganic solid electrolyte and these lithium salts is preferably 60:40 to 95:5 (molar ratio), and more preferably 80:20 to 95:5.

[0068] In addition to the above, other suitable sulfide inorganic solid electrolytes include algerodite-type solid electrolytes such as Li6PS5Cl and Li6PS5Br.

[0069] The methods for producing the sulfide inorganic solid electrolyte mentioned above include, but are not particularly limited to, solid-phase methods, sol-gel methods, mechanical milling methods, solution methods, and melt-quenching methods.

[0070] (B) Oxide inorganic solid electrolyte

[0071] The oxide inorganic solid electrolyte is preferably one that contains an oxygen element, has metal ion conductivity belonging to Group 1 of the periodic table, and also has electronic insulating properties.

[0072] 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.

[0073] 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 using a laser diffraction / scattering particle size distribution analyzer.

[0074] The content of the inorganic solid electrolyte is not particularly limited, but it should 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 negative electrode 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 should 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. The content of lithium titanate powder and inorganic solid electrolyte in the negative electrode active material composition is preferably 99:1 to 30:70, more preferably 95:5 to 40:60, even more preferably 80:20 to 50:50, and particularly preferably 75:25 to 50:50, in terms of the mass ratio of "lithium titanate powder:inorganic solid electrolyte".

[0075] [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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] [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.

[0081] The reason why the negative electrode active material composition of the present invention yields a dense negative electrode layer with fewer voids and a higher packing density than conventional materials in all-solid-state secondary batteries, and why the DC resistance of charging is reduced, is not entirely clear, but it is thought to be as follows. The negative electrode active material composition of the present invention comprises an inorganic solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table and lithium titanate powder in which a sulfur element or a sulfur element-containing compound is present on the surface of lithium titanate particles. Normally, when lithium titanate and an inorganic solid electrolyte are mixed, especially when the particle size of the lithium titanate powder is small, the lithium titanate particles aggregate with each other, and the lithium titanate powder and solid electrolyte do not mix uniformly in the negative electrode active material composition, resulting in a negative electrode active material composition with many voids and low packing density. On the other hand, the presence of a sulfur element or a sulfur element-containing compound on the surface of the lithium titanate particles of the present invention suppresses aggregation between the lithium titanate particles, and further increases the affinity with the inorganic solid electrolyte, especially sulfide inorganic solid electrolytes, allowing for uniform mixing in the negative electrode active material composition. As a result, the solid electrolyte and the lithium titanate powder of the present invention form a good solid-solid interface in the negative electrode active material composition, and it is possible to form a dense negative electrode layer with fewer voids and a higher packing density than conventional methods, which is expected to improve the characteristics of all-solid-state secondary batteries. In lithium-ion secondary batteries using organic electrolytes, even if aggregation of lithium titanate particles occurs, the organic electrolyte, which acts as a carrier for metal ions such as lithium ions, easily penetrates these aggregated areas. Therefore, a solid-liquid interface is easily formed, and even in these aggregated areas, intercalation and release reactions of metal ions such as lithium ions are possible via the organic electrolyte. For this reason, aggregation of lithium titanate particles has rarely been a problem in secondary batteries using organic electrolytes. In contrast, in all-solid-state secondary batteries, the inorganic solid electrolyte, which acts as a carrier for metal ions such as lithium ions, cannot penetrate these aggregated areas, preventing the formation of a solid-solid interface. As a result, intercalation and release reactions of metal ions such as lithium ions do not occur, and these ions cannot contribute to the battery reaction. In other words, the problem of aggregation of lithium titanate particles is unique to all-solid-state secondary batteries using inorganic solid electrolytes, and this problem becomes particularly pronounced as the particle size of the lithium titanate particles decreases. In contrast, according to the present invention, the presence of sulfur elements or sulfur-containing compounds on the surface of lithium titanate particles suppresses aggregation between lithium titanate particles, and further increases the affinity with inorganic solid electrolytes, particularly sulfide inorganic solid electrolytes. As a result, a denser negative electrode layer with fewer voids and a higher packing density than conventional materials can be obtained, effectively solving the problems caused by the occurrence of aggregated portions as described above.

[0082] 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 and a high packing rate can be obtained. The molded body obtained using the negative electrode active material composition of the present invention has a packing rate of 72.5% to 100%, preferably 73.5% to 100%. The method for measuring the packing rate will be explained in the examples described later.

[0083] [All-solid-state secondary battery] The all-solid-state secondary battery of the present invention is composed of a positive electrode, a negative electrode, and a solid electrolyte layer located between the positive and negative electrodes, but the Li4Ti5O 12 A negative electrode active material composition containing lithium titanate powder, which is the main component, 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.

[0084] 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.

[0085] 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.

[0086] 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 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] The solid electrolyte layer is located between the positive and negative electrodes, and its thickness 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 differ from the solid electrolyte used in the electrodes. The solid electrolyte layer may also contain a binder such as butadiene rubber or butyl rubber.

[0091] 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]

[0092] 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. [Example 1] <Raw material preparation process> A raw material mixture slurry was prepared by weighing Li2CO3 (average particle size 4.6 μm) and anatase-type TiO2 (average particle size 3.1 μm) so that the atomic ratio of Li to Ti (Li / Ti) was 0.83. Deionized water was added to the raw material powder so that the solid content concentration of the slurry was 41% by mass, and the mixture was stirred. 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.

[0093] <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.

[0094] <Post-processing steps> The calcined material recovered from the calcined material recovery side of the furnace tube was crushed using a hammer mill (Dalton, AIIW-5 model) under the following conditions: screen opening: 0.5 mm, rotation speed: 8,000 rpm, and powder feed rate: 25 kg / hr.

[0095] <Surface treatment process> Deionized water was added to the crushed calcined powder to achieve a slurry solid content of 30% by mass, and the mixture was stirred. 0.16% by mass of lithium sulfate monohydrate (Li2SO4·H2O) was added to the crushed calcined powder as a treatment agent to prepare a mixed slurry. This mixed slurry was mixed in a paint shaker for 3 hours, dried at 100°C, and then heat-treated in a muffle furnace at 500°C for 1 hour to produce lithium titanate powder.

[0096] [Examples 2 and 3] Lithium titanate powders according to Examples 2 and 3 were produced in the same manner as in Example 1, except that the amount of lithium sulfate monohydrate (Li2SO4·H2O) added in the surface treatment process was as shown in Table 1.

[0097] [Comparative Example 1] Lithium titanate powder according to Comparative Example 1 was produced in the same manner as in Example 1, except that lithium sulfate monohydrate (Li2SO4·H2O) was not added in the surface treatment process.

[0098] [Measurement of sulfur element content] The sulfur content in the lithium titanate powder of Examples 1-3 and Comparative Example 1 was measured as follows.

[0099] <X-ray fluorescence analysis (XRF): Identification and measurement of sulfur content> Using an X-ray fluorescence spectrometer (manufactured by SII Technology Co., Ltd., product name "SPS5100"), the sulfur element contained in the lithium titanate powder of each example and comparative example was identified and quantitatively analyzed.

[0100] [Measurement of powder properties] The various physical properties of the lithium titanate powder in each example and comparative example were measured as follows.

[0101] <Calculation of D50 of primary particles: Dry laser diffraction scattering method> The D50 of lithium titanate powder in each example and comparative 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 measurement 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.

[0102] [Preparation of the negative electrode active material composition] In a glove box under an argon atmosphere, lithium titanate powder and Li6PS5Cl powder (volume-average particle size obtained using a laser diffraction / scattering particle size distribution analyzer: 6 μm), which are inorganic solid electrolytes of each example and comparative example, were weighed in a mass ratio of lithium titanate:Li6PS5Cl = 70:30 and mixed in an agate mortar to obtain a negative electrode active material composition.

[0103] [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. <Calculation of the packing density of the negative electrode active material composition, and calculation of the relative density ratio> The packing density of the negative electrode active material composition 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 (true density) of Li6PS5Cl, the density (true density) of lithium titanate, 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 rate (%) = (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 negative electrode active material compositions of Examples 1 to 3 was calculated, with the packing density value of Comparative Example 1 set to 100% as the baseline. The results are shown in Table 1.

[0104] [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 Table 1.

[0105] [Synthesis 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.

[0106] [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.

[0107] <Measurement of DC resistance during charging> In a constant temperature bath at 25°C, coin-type batteries prepared using the method described above were charged to 0.5V with a current equivalent to 0.05C, the theoretical capacity of lithium titanate, with the direction in which Li is absorbed into the evaluation electrode being considered as the charging direction. Then, constant current / constant voltage charging was performed, continuing until the charging current at 0.5V was equivalent to 0.01C. Finally, constant current discharge was performed, discharging to 2V with a current equivalent to 0.05C. Next, the DC resistance during charging was calculated using the current values ​​corresponding to each C rate (0.1C, 0.15C, 0.2C, 0.25C, 0.3C, 0.4C) and the voltage after 10 seconds when charging at that C rate. In addition, the DC resistances of Examples 1-3 were examined relative to the values ​​of Comparative Example 1, which were set to 100%. The evaluation results are shown in Table 1. 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, while 0.1C refers to the current value that can completely discharge (or fully charge) the theoretical capacity in 1 / 0.1 hour.

[0108] [Table 1]

[0109] Table 1 above shows that sulfur elements are present on the surface of the lithium titanate powder used in the present invention (that is, since lithium sulfate monohydrate was used as a treatment agent in the surface treatment process, it can be said that sulfur elements derived from lithium sulfate monohydrate are present on the surface of the lithium titanate powder). Furthermore, in Examples 1 to 3 of the negative electrode active material composition of the present invention, the relative density ratio of the negative electrode active material composition is improved compared to Comparative Example 1, and a negative electrode active material composition with fewer voids and a high packing density is formed, and the DC charging resistance in the all-solid-state secondary battery using this negative electrode active material composition is reduced.

[0110] Based on the above results, by using the negative electrode active material composition of the present invention, the aggregation of lithium titanate particles can be suppressed, thereby making the negative electrode layer more dense. This negative electrode layer allows for the formation of continuous ion and electron paths, resulting in excellent battery characteristics.

Claims

1. Li ４ Ti ５ O １２ A negative electrode active material composition comprising lithium titanate powder having as its main component and an inorganic solid electrolyte having conductivity of metal ions belonging to Group 1 of the periodic table, wherein a sulfur element or a sulfur element-containing compound is present on the surface of the lithium titanate particles constituting the lithium titanate powder. A negative electrode active material composition wherein the content of the sulfur element or sulfur element-containing compound in the lithium titanate powder is 0.01% by mass or more and 1% by mass or less, in terms of sulfur element.

2. The negative electrode active material composition according to claim 1, wherein the inorganic solid electrolyte is a sulfide inorganic solid electrolyte.

3. The negative electrode active material composition according to claim 1, wherein the sulfur element-containing compound is lithium sulfate.

4. The negative electrode active material composition according to claim 1, wherein the volume-based particle size distribution of the lithium titanate powder by laser diffraction scattering method has a D50 of 0.5 μm or more for primary particles corresponding to a volume accumulation of 50%.

5. The negative electrode active material composition according to claim 1, wherein the volume-based particle size distribution of the lithium titanate powder by laser diffraction scattering method has a D50 of 10 μm or less for primary particles corresponding to a volume accumulation of 50%.

6. The negative electrode active material composition according to claim 1, 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 any one of claims 1 to 6.

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