Negative electrode active material and all-solid-state battery

Abstract

To provide a negative electrode active material that can operate at a sufficiently low potential and achieve both a high volumetric capacitance and an excellent cycle characteristic and that can be fired with a solid electrolyte at the same time in a wide firing temperature range, being favorable to an all-solid battery use application including an oxide-based solid electrolyte, and to provide an all-solid battery including the negative electrode active material.SOLUTION: A negative electrode active material is represented by a composition formula of TiTa2-xMxO7. Here, x is less than 0.20, and Ti / (Ta+M) is equal to or more than 0.50 and equal to or less than 0.56.SELECTED DRAWING: Figure 12

Description

Technical Field

[0001] The present invention relates to a negative electrode active material and an all-solid-state battery.

Background Art

[0002] In recent years, all-solid-state batteries have been utilized as secondary batteries having a high energy density. Development of electrode active materials for use in all-solid-state batteries has been carried out (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In recent years, secondary batteries have been used in various fields. Secondary batteries using an electrolytic solution have problems such as leakage of the electrolytic solution. Therefore, development of all-solid-state batteries provided with a solid electrolyte and having other components also made of a solid has been carried out. The solid electrolyte has a wider potential window (stability at a wide range of potentials) than the electrolytic solution. In particular, oxide-based solid electrolytes that exhibit high ionic conductivity by sintering have advantages such as a wider potential window and relatively high stability in the atmosphere compared to electrolytic solution-based and other solid electrolyte-based materials.

[0006] The characteristics required for an electrode active material applied to an all-solid-state battery using an oxide-based solid electrolyte include, in addition to basic battery characteristics such as Coulomb efficiency, cycle characteristics, and capacity, that the interdiffusion reaction hardly occurs when co-sintered with the solid electrolyte, and that the volume change accompanying charge and discharge is small. Furthermore, in the case of a negative electrode active material, it should be an oxide-based active material with a small volume change accompanying charge and discharge, and it should cause charge and discharge operations at a sufficiently low operating potential. In particular, in an all-solid-state battery using an ultra-small oxide-based solid electrolyte, the negative electrode active material is required to have a high volume specific capacity, high stability in batch firing, and good cycle characteristics.

[0007] In Non-Patent Document 1, research results of a TiTa2O7 compound, which is an oxide-based negative electrode active material with a high volume specific capacity and capable of operating at a low potential of 1 V vs Li / Li + have been reported. However, the Coulomb efficiency is as low as 50% or less, and the cycle characteristics are such that the capacity retention rate drops below 50% within 10 cycles. A Ti-Ta-O compound that operates at a low potential, has a high volume specific capacity, and is excellent in Coulomb efficiency and cycle characteristics is desired.

[0008] The present invention has been made in view of the above problems, and is capable of operating at a sufficiently low potential, achieving both a high volume specific capacity and good cycle characteristics, and being batch-firable with a solid electrolyte in a wide firing temperature range. An object of the present invention is to provide a negative electrode active material suitable for use in an all-solid-state battery using an oxide-based solid electrolyte, and an all-solid-state battery using the negative electrode active material.

Means for Solving the Problems

[0009] The negative electrode active material according to the present invention is represented by the composition formula of TiTa 2-x M x O7, where x is less than 0.20, and the atomic concentration ratio of Ti / (Ta + M) is 0.50 or more and 0.56 or less.

[0010] The above negative electrode active material may have a monoclinic crystal lattice structure belonging to the space group I2 / m.

[0011] The above negative electrode active material may have a capacitance component in two potential ranges of 1.1 V or more and 1.2 V or less vs. Li / Li+ and 1.6 V or more and 1.8 V or less vs. Li / Li+ in the dQ / dV curve.

[0012] In the above negative electrode active material, x may be 0.

[0013] The all-solid-state battery according to the present invention includes an oxide-based solid electrolyte layer, a first electrode layer provided on a first main surface of the oxide-based solid electrolyte layer and containing a positive electrode active material, and a second electrode layer provided on a second main surface of the oxide-based solid electrolyte layer and containing any one of the above negative electrode active materials.

[0014] In the above all-solid-state battery, the average particle size of the negative electrode active material in the second electrode layer may be 0.5 μm or more and 5 μm or less.

Advantages of the Invention

[0015] According to the present invention, it is possible to provide a negative electrode active material suitable for all-solid-state battery applications using an oxide-based solid electrolyte, which can operate at a sufficiently low potential, achieve both a high volume specific capacity and good cycle characteristics, and can be batch-fired with the solid electrolyte in a wide firing temperature range, and an all-solid-state battery using the negative electrode active material.

Brief Description of the Drawings

[0016]

Figure 1

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Mode for Carrying Out the Invention

[0017] Hereinafter, embodiments will be described with reference to the drawings.

[0018] (Embodiment) FIG. 1 is a schematic cross-sectional view showing the basic structure of all-solid-state battery 100. As illustrated in FIG. 1, all-solid-state battery 100 has a structure in which solid electrolyte layer 30 is sandwiched between first internal electrode 10 (first electrode layer) and second internal electrode 20 (second electrode layer). First internal electrode 10 is formed on the first main surface of solid electrolyte layer 30. Second internal electrode 20 is formed on the second main surface of solid electrolyte layer 30.

[0019] When all-solid-state battery 100 is used as a secondary battery, one of first internal electrode 10 and second internal electrode 20 is used as the positive electrode, and the other is used as the negative electrode. In the present embodiment, as an example, it is assumed that first internal electrode 10 is used as the positive electrode and second internal electrode 20 is used as the negative electrode.

[0020] The solid electrolyte layer 30 is mainly composed of a solid electrolyte having ion conductivity. The solid electrolyte of the solid electrolyte layer 30 is, for example, an oxide-based solid electrolyte having lithium ion conductivity. The solid electrolyte is, for example, a phosphate-based solid electrolyte having a NASICON structure. The phosphate-based solid electrolyte having a NASICON structure has high conductivity and is stable in the atmosphere. The phosphate-based solid electrolyte is, for example, a phosphate containing lithium. The phosphate is not particularly limited, and examples thereof include a complex lithium phosphate salt with Ti (for example, LiTi2(PO4)3). Alternatively, Ti can be partially or entirely replaced with a tetravalent transition metal such as Ge, Sn, Hf, Zr, etc. Also, in order to increase the Li content, it may be partially replaced with a trivalent transition metal such as Al, Ga, In, Y, La, etc. More specifically, for example, Li 1+x Al x Ge 2-x (PO4)3, and Li 1+x Al x Zr 2-x (PO4)3, Li 1+x Al x Ti 2-x (PO4)3, etc. can be mentioned.

[0021] The first internal electrode 10 used as the positive electrode contains a substance having an olivine-type crystal structure as the electrode active material. Examples of such an electrode active material include phosphates containing a transition metal and lithium. The olivine-type crystal structure is a crystal possessed by natural olivine and can be discriminated by X-ray diffraction.

[0022] As a typical example of the electrode active material having an olivine-type crystal structure, LiCoPO4 containing Co can be used. Phosphates in which Co, a transition metal, is replaced in this chemical formula can also be used. Here, the ratios of Li and PO4 can vary depending on the valence. It is preferable to use Co, Mn, Fe, Ni, etc. as the transition metal.

[0023] The second internal electrode 20 contains a negative electrode active material.

[0024] In the production of the first internal electrode 10 and the second internal electrode 20, in addition to these electrode active materials, a solid electrolyte having ion conductivity, a conductive material (conductive auxiliary agent), and the like are added. For these members, an internal electrode paste can be obtained by uniformly dispersing a binder and a plasticizer in water or an organic solvent. As the conductive auxiliary agent, a carbon material or the like may be included. As the conductive auxiliary agent, a metal may be included. Examples of the metal of the conductive auxiliary agent include Pd, Ni, Cu, Fe, and alloys containing these. The solid electrolyte contained in the first internal electrode 10 and the second internal electrode 20 can be the same as, for example, the main component solid electrolyte of the solid electrolyte layer 30.

[0025] In this embodiment, as the negative electrode active material, a TiM2O7-based oxide having a monoclinic crystal structure is used. The TiM2O7-based oxide has a low negative electrode operating potential, a small volume change accompanying charge and discharge, exhibits good cycle characteristics, and although the weight specific capacity is low, the volume specific capacity is relatively high. Therefore, it is a negative electrode active material suitable for a small all-solid-state battery in which the battery weight is not a major concern. Generally, TiNb2O7 using Nb as M is widely known. However, when TiNb2O7 is used, the cycle stability becomes low.

[0026] Therefore, in this embodiment, as the negative electrode active material, TiM1 2―x M2 x O7-based oxide in which M1 is Ta is used. Specifically, as the negative electrode active material, an oxide represented by the composition formula TiTa 2-x M2 x O7 is used. M2 may contain at least one or more of Nb, V, W, and Mo, and x may be 0.

[0027] However, if the value of x is too large, there is a risk of deterioration of cycle characteristics. Therefore, in this embodiment, x is set to less than 0.20. From the viewpoint of suppressing the deterioration of cycle characteristics, the smaller x is, the more preferable it is. For example, x is preferably 0.15 or less, more preferably 0.10 or less, and even more preferably 0.

[0028] Here, generally, Nb is likely to cause a two-electron reaction (Nb 5+ →Nb 4+ →Nb 3+ ), so it is considered that the volume change associated with Li insertion / desorption becomes large, and the cycle characteristics are likely to deteriorate. However, Ta is considered to be less likely to cause a two-electron reaction than Nb. Therefore, by using a negative electrode active material that can be represented by the composition formula TiTa 2-x M x O7 (x < 0.20), the volume change associated with Li insertion / desorption can be suppressed to a small level, and as a result, the cycle characteristics become good.

[0029] The ratio Ti / (Ta + M) of Ti to (Ta + M) does not have to be 0.5. However, when Ti / (Ta + M) is low, a secondary phase is likely to appear and the capacity may decrease. Therefore, a lower limit is set for Ti / (Ta + M). In this embodiment, Ti / (Ta + M) is set to 0.5 or more. From the viewpoint of suppressing the appearance of the secondary phase, Ti / (Ta + M) is preferably greater than 0.5, more preferably 0.505 or more, and even more preferably 0.510 or more. On the other hand, when Ti / (Ta + M) is high, a secondary phase is likely to appear and the cycle characteristics may deteriorate. Therefore, an upper limit is set for Ti / (Ta + M). In this embodiment, Ti / (Ta + M) is set to 0.56 or less. From the viewpoint of suppressing the deterioration of cycle characteristics, Ti / (Ta + M) is preferably 0.55 or less, more preferably 0.54 or less. Note that Ti / (Ta + M) can be defined as, for example, the atomic number ratio, and in LA-ICP-MS (laser ablation ICP mass spectrometry), it is defined as the count number ratio. Ti / (Ta + M) can be verified from the product after sintering (after densification).

[0030] In addition, by using a negative electrode active material that can be represented by the compositional formula TiTa 2-x M x O7 (x < 0.20), the mutual diffusion reaction during the co-sintering of the solid electrolyte layer 30 and the second internal electrode 20 can be suppressed. This is because oxides containing Ta as a main element are relatively stable, and it is difficult for element diffusion to occur between solid electrolytes during co-firing.

[0031] In the second internal electrode 20, if the average particle size of the negative electrode active material is too large, the internal resistance of the electrode will increase, and fast charge and discharge may become difficult. If the average particle size is too small, in addition to the increased reactivity during heat treatment, it may inhibit the sintering densification of the solid electrolyte. Therefore, the average particle size of the negative electrode active material in the second internal electrode 20 is preferably 0.5 μm or more and 5 μm or less, more preferably 0.7 μm or more and 3.0 μm or less, and even more preferably 1 μm or more and 2 μm or less.

[0032] In the dQ / dV curve of the negative electrode active material in the second internal electrode 20, it is preferable that there are capacity components in two potential ranges of 1.1 V or more and 1.2 V or more vs. Li / Li+, and 1.6 V or more and 1.8 V or less vs. Li / Li+. Such a negative electrode active material having a two-stage capacity component becomes voltage-selective when used as the negative electrode of the battery. Also, when used only in the low potential region, since Ti always exists in a reduced state, the electronic conductivity of the active material is improved, and charge and discharge at a higher rate become possible.

[0033] When fabricating the all-solid-state battery 100, a stacked capacitor type structure in which the first internal electrode 10 and the second internal electrode 20 are alternately stacked in parallel via the solid electrolyte layer 30 is suitable for increasing the capacitance density while reducing the size. At this time, it is preferable that the thickness of the first internal electrode 10 and the thickness of the second internal electrode 20 are approximately the same. However, since the capacity per unit volume of the negative electrode active material provided in this embodiment is higher than that of a general positive electrode active material, it is preferable to achieve a capacity balance by incorporating more positive electrode active material than the volume of the negative electrode active material. Therefore, a balance can be achieved by incorporating more active material with high electronic conductivity into the first internal electrode 10 than the volume of the negative electrode active material to reduce the conductive assistant, or by incorporating more active material with high ionic conductivity into the first internal electrode 10 than the volume of the negative electrode active material to reduce the ionic conduction assistant. It is preferable to achieve a balance between the capacity balance and the electronic conduction balance by incorporating more LiCoPO4, which has high electronic conduction after charging, than the volume of the negative electrode active material and making the conductive assistant less than the volume of the negative electrode conductive assistant. When the first internal electrode 10 and the second internal electrode 20 have approximately the same thickness, it is necessary to make the volume ratio of the negative electrode active material smaller than the volume ratio of the positive electrode active material in order to achieve a capacity balance. Therefore, the volume ratio of the negative electrode active material in the second internal electrode 20 is preferably about 20 to 60 vol.%.

[0034] FIG. 2 is a schematic cross-sectional view of a stacked all-solid-state battery 100a in which a plurality of battery units are stacked. The all-solid-state battery 100a includes a stacked chip 60 having a substantially rectangular parallelepiped shape. In the stacked chip 60, a first external electrode 40a and a second external electrode 40b are provided so as to be in contact with two side surfaces out of the four side surfaces other than the upper and lower surfaces at the ends in the stacking direction. The two side surfaces may be two adjacent side surfaces or two opposite side surfaces. In the present embodiment, it is assumed that the first external electrode 40a and the second external electrode 40b are provided so as to be in contact with two opposite side surfaces (hereinafter referred to as two end surfaces).

[0035] In the following description, those having the same composition range, the same thickness range, and the same particle size distribution range as the all-solid-state battery 100 are denoted by the same reference numerals, and detailed description thereof is omitted.

[0036] In the all-solid-state battery 100a, a plurality of first internal electrodes 10 and a plurality of second internal electrodes 20 are alternately stacked via a solid electrolyte layer 30. The edges of the plurality of first internal electrodes 10 are exposed on the first end face of the stacked chip 60 and not exposed on the second end face. The edges of the plurality of second internal electrodes 20 are exposed on the second end face of the stacked chip 60 and not exposed on the first end face. Thereby, the first internal electrode 10 and the second internal electrode 20 are alternately electrically connected to the first external electrode 40a and the second external electrode 40b. Note that the solid electrolyte layer 30 extends from the first external electrode 40a to the second external electrode 40b. Thus, the all-solid-state battery 100a has a structure in which a plurality of battery units are stacked.

[0037] A cover layer 50 is stacked on the upper surface of the stacked structure of the first internal electrode 10, the solid electrolyte layer 30, and the second internal electrode 20 (in the example of FIG. 2, the upper surface of the uppermost first internal electrode 10). Also, a cover layer 50 is stacked on the lower surface of the stacked structure (in the example of FIG. 2, the lower surface of the lowermost first internal electrode 10). The cover layer 50 is mainly composed of an inorganic material containing, for example, Al, Zr, Ti, etc. (for example, Al2O3, ZrO2, TiO2, etc.). The cover layer 50 may contain the main component of the solid electrolyte layer 30 as the main component.

[0038] The first internal electrode 10 and the second internal electrode 20 may include a current collector layer. For example, as illustrated in FIG. 3, a first current collector layer 11 may be provided in the first internal electrode 10. Also, a second current collector layer 21 may be provided in the second internal electrode 20. The first current collector layer 11 and the second current collector layer 21 are mainly composed of a conductive material. For example, metals, carbon, etc. can be used as the conductive material of the first current collector layer 11 and the second current collector layer 21. By connecting the first current collector layer 11 to the first external electrode 40a and the second current collector layer 21 to the second external electrode 40b, the current collection efficiency is improved.

[0039] Subsequently, a method for manufacturing the all-solid-state battery 100a illustrated in FIG. 2 will be described. FIG. 4 is a diagram illustrating the flow of the method for manufacturing the all-solid-state battery 100a.

[0040] (Production process of the negative electrode active material powder) Raw materials such as TiO2 and Ta2O5 are weighed so that they are TiTa 2-x M x O7 (x < 0.20), and Ti / Ta is 0.50 or more and 0.56 or less, and then they are kneaded and mixed. After mixing, they are calcined at 1100 °C in the air, and the obtained calcined powder is kneaded again. Then, by heat-treating at 1300 °C in the air, the target TiTa 2-x M x O7 (x < 0.20) synthetic powder is obtained. After the synthetic powder is kneaded again, it is sieved through a #150 stainless steel mesh to obtain the negative electrode active material powder.

[0041] If the firing temperature during synthesis is too high, the adhesion of particles becomes intense and it becomes difficult to handle, which is not preferable. If it is too low, the uniformity of each metal atom decreases, which is not preferable. The firing temperature is preferably 1150 °C or more and 1450 °C or less, more preferably 1200 °C or more and 1400 °C or less, and even more preferably 1250 °C or more and 1350 °C or less.

[0042] (Production process of the raw material powder for the solid electrolyte layer) First, the raw material powder for the solid electrolyte layer constituting the above-mentioned solid electrolyte layer 30 is produced. For example, by mixing raw materials, additives, etc. and using a solid-phase synthesis method or the like, the raw material powder for the solid electrolyte layer can be produced. The obtained raw material powder can be adjusted to a desired average particle size by dry grinding. For example, it is adjusted to a desired average particle size using a planetary ball mill with 5 mmφ ZrO2 balls.

[0043] (Production process of the raw material powder for the cover layer) First, the raw material powder of the ceramics constituting the above-mentioned cover layer 50 is produced. For example, by mixing raw materials, additives, etc. and using a solid-phase synthesis method or the like, the raw material powder for the cover layer can be produced. The obtained raw material powder can be adjusted to a desired average particle size by dry grinding. For example, it is adjusted to a desired average particle size using a planetary ball mill with 5 mmφ ZrO2 balls.

[0044] (Process for manufacturing paste for internal electrodes) Next, a paste for internal electrodes for manufacturing the above-described first internal electrode 10 and second internal electrode 20 is manufactured. For example, a paste for internal electrodes can be obtained by uniformly dispersing a conductive auxiliary agent, an electrode active material, a solid electrolyte material, a sintering auxiliary agent, a binder, a plasticizer, etc. in water or an organic solvent. As the solid electrolyte material, the above-described solid electrolyte paste may be used. As the conductive auxiliary agent, a carbon material or the like is used. A metal may be used as the conductive auxiliary agent. Examples of the metal of the conductive auxiliary agent include Pd, Ni, Cu, Fe, alloys containing these, etc. Pd, Ni, Cu, Fe, alloys containing these, various carbon materials, etc. may be further used. When the compositions of the first internal electrode 10 and the second internal electrode 20 are different, the respective pastes for internal electrodes may be manufactured individually.

[0045] As the sintering auxiliary agent of the paste for internal electrodes, for example, any one or a plurality of glass components such as Li-B-O-based compounds, Li-Si-O-based compounds, Li-C-O-based compounds, Li-S-O-based compounds, Li-P-O-based compounds, etc. are included.

[0046] (Process for manufacturing paste for external electrodes) Next, a paste for external electrodes for manufacturing the above-described first external electrode 40a and second external electrode 40b is manufactured. For example, a paste for external electrodes can be obtained by uniformly dispersing a conductive material, glass frit, a binder, a plasticizer, etc. in water or an organic solvent.

[0047] (Process for manufacturing solid electrolyte green sheet) The raw material powder for the solid electrolyte layer is uniformly dispersed in an aqueous solvent or an organic solvent together with a binder, a dispersant, a plasticizer, etc., and wet pulverization is performed to obtain a solid electrolyte slurry having a desired average particle size. At this time, a bead mill, a wet jet mill, various kneaders, a high-pressure homogenizer, etc. can be used, and it is preferable to use a bead mill from the viewpoint that the adjustment of the particle size distribution and the dispersion can be performed simultaneously. A binder is added to the obtained solid electrolyte slurry to obtain a solid electrolyte paste. By coating the obtained solid electrolyte paste, a solid electrolyte green sheet 51 can be produced. The coating method is not particularly limited, and a slot die method, a reverse coat method, a gravure coat method, a bar coat method, a doctor blade method, etc. can be used. The particle size distribution after wet pulverization can be measured using, for example, a laser diffraction measuring apparatus using the laser diffraction scattering method.

[0048] (Lamination process) As illustrated in FIG. 5(a), an internal electrode paste 52 is printed on one surface of the solid electrolyte green sheet 51. A reverse pattern 53 is printed in a region on the solid electrolyte green sheet 51 where the internal electrode paste 52 is not printed. As the reverse pattern 53, the same one as the solid electrolyte green sheet 51 can be used. A plurality of printed solid electrolyte green sheets 51 are laminated with alternating shifts. As illustrated in FIG. 5(b), a cover sheet 54 is pressure-bonded from above and below in the lamination direction to obtain a laminate. In this case, in the laminate, a substantially rectangular parallelepiped laminate is obtained such that the internal electrode paste 52 is alternately exposed at both end faces. The cover sheet 54 can be formed by coating a raw material powder for the cover layer by the same method as in the solid electrolyte green sheet production process. The cover sheet 54 is formed thicker than the solid electrolyte green sheet 51. It may be made thick at the time of coating, or may be made thick by stacking a plurality of coated sheets.

[0049] Next, an external electrode paste 55 is applied to each of the two end faces by a dipping method or the like and dried. Thereby, a molded body for forming the all-solid-state battery 100a is obtained.

[0050] (Firing process) Next, the obtained laminate is fired. The firing conditions can be, without particular limitation, in an oxidizing atmosphere or a non-oxidizing atmosphere, and the maximum temperature is preferably 400°C to 1000°C, more preferably 500°C to 900°C, etc. A step of holding at a temperature lower than the maximum temperature in an oxidizing atmosphere may be provided to sufficiently remove the binder before reaching the maximum temperature. In order to reduce the process cost, it is desirable to fire at as low a temperature as possible. After firing, a re-oxidation treatment may be performed. Through the above steps, the all-solid-state battery 100a is produced.

[0051] Note that by sequentially laminating the internal electrode paste, the current collector paste containing the conductive material, and the internal electrode paste, a current collector layer can be formed in the first internal electrode 10 and the second internal electrode 20.

Example

[0052] Hereinafter, an all-solid-state battery was fabricated according to the embodiment, and its characteristics were examined.

[0053] (Example 1) TiO2 and Ta2O5 as raw materials were weighed in a molar ratio of 1:1 so as to have a composition ratio of TiTa2O7, and were crushed and mixed. After mixing, they were calcined at 1100°C in the air, the obtained calcined powder was crushed again, and the target TiTa2O7 synthetic powder was obtained by heat treatment at 1300°C in the air. After the synthetic powder was crushed again, it was sieved through a #150 stainless steel mesh to obtain the negative electrode active material powder. From the XRD measurement, the same diffraction peaks as those of TiTa2O7 were observed, and no other secondary phase peaks were observed.

[0054] A coating slurry composed of the negative electrode active material powder, PVdF binder, acetylene black, and NMP was prepared, a coating film was formed on a copper foil, and a negative electrode half cell with a metallic lithium foil disposed as the counter electrode was sealed in a 2032 coin cell. A charge-discharge test was performed in the range of 3 to 1V at a charge-discharge rate of 25°C and 0.1C. Fig. 6 shows the results of the charge-discharge test.

[0055] The initial discharge capacity at 1.0V cut-off was 120 mAh / g. The discharge capacity after 20 cycles relative to the initial discharge capacity was 97.5%. When an experiment was conducted in which this negative electrode active material was mixed with the solid electrolyte LAGP at a volume ratio of 50:50 and heat-treated in air, no secondary phase was observed up to 730°C.

[0056] (Example 2) Ti 1.06 Ta2O 7.12 Except that TiO2 and Ta2O5 as raw materials were weighed at a molar ratio of 1.06:1 so that the composition ratio (Ti / Ta = 0.530) was obtained, the negative electrode active material powder was prepared and evaluated in the same manner as in Example 1. Diffraction peaks the same as those of TiTa2O7 were observed from the XRD measurement, and no other secondary phase peaks were observed.

[0057] A coating slurry composed of the negative electrode active material powder, PVdF binder, acetylene black, and NMP was prepared, a coating film was formed on a copper foil, and a negative electrode half cell with a metal lithium foil disposed as the counter electrode was sealed in a 2032 coin cell. A charge-discharge test was conducted in the range of 3 to 1V at a charge-discharge rate of 25°C and 0.1C. Fig. 7 shows the results of the charge-discharge test.

[0058] The initial discharge capacity at 1.0V cut-off was 121 mAh / g. The discharge capacity after 20 cycles relative to the initial discharge capacity was 99.7%. When an experiment was conducted in which this negative electrode active material was mixed with the solid electrolyte LAGP at a volume ratio of 50:50 and heat-treated in air, no secondary phase was observed up to 730°C.

[0059] (Example 3) Ti 1.12 Ta2O 7.24 Except that TiO2 and Ta2O5 as raw materials were weighed at a molar ratio of 1.12:1 so that the composition ratio (Ti / Ta = 0.560) was obtained, the negative electrode active material powder was prepared and evaluated in the same manner as in Example 1. Diffraction peaks the same as those of TiTa2O7 were observed from the XRD measurement, and other secondary phase peaks were also observed.

[0060] A coating slurry composed of a negative electrode active material powder, PVdF binder, acetylene black, and NMP was prepared, a coating film was formed on a copper foil, and a negative electrode half cell with a metallic lithium foil disposed as a counter electrode was sealed in a 2032 coin cell. A charge-discharge test was conducted in the range of 3 to 1 V at a charge-discharge rate of 25 °C and 0.1 C. The results of the charge-discharge test are shown in FIG. 8.

[0061] The initial discharge capacity at 1.0 V cut-off was 123 mAh / g. The discharge capacity after 20 cycles with respect to the initial discharge capacity was 96.8%. When an experiment was conducted in which this negative electrode active material was mixed with a solid electrolyte LAGP at a volume ratio of 50:50 and heat-treated in the air, no secondary phase was observed up to 730 °C.

[0062] (Comparative Example 1) Ti 0.97 Ta2O 6.94 A negative electrode active material powder was prepared and evaluated in the same manner as in Example 1 except that raw material TiO2 and Ta2O5 were weighed at a molar ratio of 0.97:1 so that the composition ratio (Ti / Ta = 0.485) was obtained. Diffraction peaks identical to those of TiTa2O7 were observed from XRD measurement, and other secondary phase peaks were also observed.

[0063] A coating slurry composed of a negative electrode active material powder, PVdF binder, acetylene black, and NMP was prepared, a coating film was formed on a copper foil, and a negative electrode half cell with a metallic lithium foil disposed as a counter electrode was sealed in a 2032 coin cell. A charge-discharge test was conducted in the range of 3 to 1 V at a charge-discharge rate of 25 °C and 0.1 C. The results of the charge-discharge test are shown in FIG. 9.

[0064] The initial discharge capacity at 1.0 V cut-off was 98 mAh / g. The discharge capacity after 20 cycles with respect to the initial discharge capacity was 86.3%. When an experiment was conducted in which this negative electrode active material was mixed with a solid electrolyte LAGP at a volume ratio of 50:50 and heat-treated in the air, no secondary phase was observed up to 730 °C.

[0065] (Comparative Example 2) Ti 1.18 Ta2O 7.36The negative electrode active material powder was produced and evaluated in the same manner as in Example 1, except that TiO2 and Ta2O5 as raw materials were weighed at a molar ratio of 1.18:1 so as to obtain the composition ratio. The same diffraction peaks as those of TiTa2O7 were observed from the XRD measurement, and other secondary phase peaks were also observed.

[0066] A coating slurry composed of a negative electrode active material powder, a PVdF binder, acetylene black, and NMP was prepared, a coating film was formed on a copper foil, and a negative electrode half cell with a metallic lithium foil disposed as a counter electrode was sealed in a 2032 coin cell. A charge-discharge test was conducted in the range of 3 to 1 V at a charge-discharge rate of 25 °C and 0.1C. Fig. 10 shows the results of the charge-discharge test.

[0067] The initial discharge capacity at 1.0 V cut-off was 129 mAh / g. The discharge capacity after 20 cycles with respect to the initial discharge capacity was 93.3%. When an experiment was conducted in which this negative electrode active material was mixed with a solid electrolyte LAGP at a volume ratio of 50:50 and heat-treated in the air, no secondary phase was observed up to 730 °C.

[0068] The results of Examples 1 to 3 and Comparative Examples 1 and 2 are summarized in Table 1. When a secondary phase was confirmed, it was judged as slightly good "△". When no secondary phase was confirmed, it was judged as good "〇". If the initial discharge capacity was 110 mAh / g or more, it was judged as good "〇". If the initial discharge capacity was 100 mAh / g or less, it was judged as non-conforming "×". If the discharge capacity after 20 cycles with respect to the initial discharge capacity was 95% or more, it was judged as passing "〇", if it was 90% or more and less than 95%, it was judged as slightly good "△", and if it was less than 90%, it was judged as non-conforming "×". If the maximum temperature at which no secondary phase was generated in the heat treatment with the solid electrolyte was 700 °C or more, it was judged as passing "〇", if it was 650 °C or more and less than 700 °C, it was judged as slightly good "△", and if it was less than 650 °C, it was judged as non-conforming "×".

[0069] It was comprehensively determined from five indicators. For each item, 〇 was given 2 points, △ was given 1 point, and × was given 0 points. The total score was calculated based on the results of each item. If the total score was 0 - 6, the comprehensive judgment was "×" (failed); if the total score was 7 - 9, the comprehensive judgment was "〇" (passed). The above results are summarized in Table 1.

Table 1

[0070] (Comparative Example 3) Except that TiO2 and Nb2O5 as raw materials were weighed at a molar ratio of 1:1 to obtain the composition ratio of TiNb2O7, the negative electrode active material powder was prepared and evaluated in the same manner as in Example 1.

[0071] Figure 11(a) is a diagram showing the dQ / dV curve when CC charge and discharge were performed at 0.1C (rate) until the lower limit voltage of 0.6V for Example 2. Figure 11(b) is a diagram showing the dQ / dV curve when CC charge and discharge were performed at 0.1C (rate) until the lower limit voltage of 0.6V for Comparative Example 3. The solid line is the charge curve, and the dotted line is the discharge curve. From the results of Figure 11(a), it can be seen that in the dQ / dV curve, there are capacity components in two potential ranges: 1.1V or more and 1.2V or more vs. Li / Li+, and 1.6V or more and 1.8V or less vs. Li / Li+.

[0072] Figure 12(a) is a diagram showing the charge and discharge curves at 1.0V cut-off, 0.8V cut-off, and 0.6V cut-off for Example 2. Figure 12(b) is a diagram showing the charge and discharge curves at 1.0V cut-off, 0.8V cut-off, and 0.6V cut-off for Comparative Example 3. Comparing Figure 12(a) and Figure 12(b), it can be seen that in Comparative Example 3, the voltage at the start of discharge is 1.0V or more, while in Example 2, the voltage at the start of discharge is around 0.7V. Therefore, it can be seen that in Example 2, it is possible to operate even in the low potential region up to 0.7V. This is considered to be due to the use of Ta instead of Nb, so it is considered that it is also possible to operate in the low potential region in Examples 1 and 3.

[0073] As described above in detail with respect to the embodiments of the present invention, the present invention is not limited to such specific embodiments, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims.

Explanation of Reference Numerals

[0074] 10 First internal electrode 11 First current collector layer 20 Second internal electrode 21 Second current collector layer 30 Solid electrolyte layer 40a First external electrode 40b Second external electrode 50 Cover layer 51 Solid electrolyte green sheet 52 Paste for internal electrode 53 Reverse pattern 54 Cover sheet 55 Paste for external electrode 60 Multilayer chip 100, 100a All-solid-state battery

Claims

1. TiTa ２－ｘ M ｘ O ７ which is represented by the compositional formula of 0 ≤ x < 0.20, M is at least one or more of Nb, V, W, and Mo, The negative electrode active material is characterized in that the molar ratio Ti / (Ta + M) of Ti to Ta + M is 0.53 or more and 0.56 or less.

2. The negative electrode active material according to Claim 1, characterized in that it has a monoclinic crystal lattice structure belonging to the space group I2 / m.

3. The negative electrode active material according to Claim 1 or Claim 2, characterized in that in the dQ / dV curve, it has capacitance components in two potential ranges of 1.1 V or more and 1.2 V or less vs. Li / Li⁺, and 1.6 V or more and 1.8 V or less vs. Li / Li⁺.

4. The negative electrode active material according to any one of Claims 1 to 3, characterized in that x is 0.

5. An oxide-based solid electrolyte layer, A first electrode layer provided on the first main surface of the oxide-based solid electrolyte layer and containing a positive electrode active material, A second electrode layer provided on the second main surface of the oxide-based solid electrolyte layer and containing the negative electrode active material according to any one of Claims 1 to 4, and a all-solid-state battery characterized by comprising the same.

6. The all-solid-state battery according to Claim 5, characterized in that the average particle diameter of the negative electrode active material in the second electrode layer is 0.5 μm or more and 5 μm or less.

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