Anode active material and all-solid-state battery

Abstract

A negative electrode active material is characterized by being represented by compositional formula of TiTa2-xMxO7, wherein 0.2≤x≤1.0 and M contains at least Nb.　

Description

[Technical Field]

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

[0002] In recent years, all-solid-state batteries have been utilized as secondary batteries with high energy density, and electrode active materials for use in all-solid-state batteries have been developed (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2014 / 038311 [Patent Document 2] Patent No. 4707950 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, secondary batteries have been used in a variety of fields. Secondary batteries that use liquid electrolytes have problems such as electrolyte leakage. Therefore, development is underway for all-solid-state batteries that include a solid electrolyte and other solid components. Solid electrolytes have a wider potential window (stability over a wide range of potentials) than liquid electrolytes. In particular, oxide-based solid electrolytes, which exhibit high ionic conductivity through sintering, have the advantage of having a wider potential window than liquid electrolytes and other solid electrolytes, and are relatively stable in the atmosphere.

[0005] The properties required for electrode active materials used in all-solid-state batteries using oxide-based solid electrolytes include not only basic battery properties such as Coulomb efficiency, cycle characteristics, and capacity, but also low interdiffusion reactions when co-sintered with the solid electrolyte, small volume changes during charge and discharge, and, in the case of negative electrode active materials, charge and discharge operations at a sufficiently low operating potential.

[0006] Furthermore, to operate devices such as integrated circuits (ICs), it is necessary to control the cell voltage so that it does not drop below 1.8V. Therefore, many batteries have an end-point voltage (lower limit voltage) set, and are designed to charge when the monitored cell voltage drops to the end-point voltage. The discharge curve of a typical battery changes to a steeper slope compared to the slope of the voltage drop in the potential plateau at the end of discharge when the battery's remaining capacity becomes low. This voltage drop in a short period of time is one of the factors that makes it difficult to detect the end-point.

[0007] The present invention has been made in view of the above-mentioned problems, and aims to provide an anode active material suitable for use in all-solid-state batteries, which is an oxide-based anode active material rather than a carbon-based or silicon-based anode, which undergoes a large volume change upon charge and discharge, and which has battery characteristics such as high coulombic efficiency, good cycle characteristics, and high capacity, is less likely to react with an electrolyte material during heat treatment, has a linear discharge curve that makes it easy to grasp the remaining battery capacity, and has a shape that also makes it easy to detect the end-point voltage, and an all-solid-state battery using the anode active material. [Means for solving the problem]

[0008] The negative electrode active material according to the present invention is TiTa 2-x M x O7, where 0.2≦x≦1.0, and M contains at least Nb.

[0009] In the above negative electrode active material, 1.0V vs. Li / Li + After charging to 3.0V vs. Li / Li + The discharge capacity when the battery is discharged to 90% is taken as 100%, and the minimum absolute value of the slope of the discharge curve |dV / dSOC| in the range of remaining battery capacity 90% to 10% may be 3.5 [mV / %] or more, and the difference between the maximum and minimum values ​​may be less than 8.5 [mV / %].

[0010] In the negative electrode active material, the TiTa 2-x M x O7 is TiTa1.5 Nb 0.5 It could be O7.

[0011] The all-solid-state battery according to the present invention is characterized by comprising: 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-mentioned negative electrode active materials.

[0012] In the all-solid-state battery, the average particle size of the negative electrode active material in the second electrode layer may be 1 μm or more and 10 μm or less. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a negative electrode active material and an all-solid-state battery that can easily detect the remaining battery charge and the end-point voltage. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic cross-sectional view showing the basic structure of an all-solid-state battery. [Figure 2] FIG. 1 is a diagram illustrating a discharge curve of a positive electrode active material having an olivine-type crystal structure. [Figure 3] FIG. 2 is a diagram illustrating a charge / discharge curve of the negative electrode active material of the present embodiment. [Figure 4] FIG. 1 is a schematic cross-sectional view of an all-solid-state battery according to an embodiment. [Figure 5] FIG. 1 is a schematic cross-sectional view of another all-solid-state battery. [Figure 6] FIG. 1 is a diagram illustrating a flow of a method for manufacturing an all-solid-state battery. [Figure 7] 1(a) and 1(b) are diagrams illustrating the lamination process. [Figure 8] FIG. 10 is a diagram showing charge / discharge curves of Example 3. [Figure 9] (a) shows the discharge curve of Comparative Example 1, (b) shows the discharge curve of Example 3, and (c) shows the discharge curve of Comparative Example 4. [Figure 10](a) is a plot of |dV / dSOC| in the range of SOC 90% to 10% for Comparative Example 1, (b) is a plot of |dV / dSOC| in the range of SOC 90% to 10% for Example 3, and (c) is a plot of |dV / dSOC| in the range of SOC 90% to 10% for Comparative Example 4. [Figure 11] FIG. 1(a) is a plot diagram of the maximum and minimum values ​​of |dV / dSOC| versus x in Examples 1 to 5 and Comparative Examples 1 to 4, and FIG. 1(b) is a plot diagram of the difference between the maximum and minimum values ​​of |dV / dSOC| versus x in Examples 1 to 5 and Comparative Examples 1 to 4. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments will be described with reference to the drawings.

[0016] (Embodiment) Fig. 1 is a schematic cross-sectional view showing the basic structure of an all-solid-state battery 100. As illustrated in Fig. 1, the all-solid-state battery 100 has a structure in which a solid electrolyte layer 30 is sandwiched between a first internal electrode 10 (first electrode layer) and a second internal electrode 20 (second electrode layer). The first internal electrode 10 is formed on a first main surface of the solid electrolyte layer 30. The second internal electrode 20 is formed on a second main surface of the solid electrolyte layer 30.

[0017] When the all-solid-state battery 100 is used as a secondary battery, one of the first internal electrode 10 and the second internal electrode 20 is used as a positive electrode, and the other is used as a negative electrode. In this embodiment, as an example, the first internal electrode 10 is used as a positive electrode, and the second internal electrode 20 is used as a negative electrode.

[0018] 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 air. The phosphate-based solid electrolyte is, for example, a phosphate containing lithium. The phosphate is not particularly limited, but examples include composite lithium phosphate salts with Ti (e.g., LiTi2(PO4)3). Alternatively, Ti can be partially or completely substituted with a tetravalent transition metal such as Ge, Sn, Hf, or Zr. Furthermore, to increase the Li content, it may be partially substituted with a trivalent transition metal such as Al, Ga, In, Y, or La. 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.

[0019] The first internal electrode 10 used as a positive electrode contains a substance having an olivine crystal structure as an electrode active material. Examples of such electrode active materials include phosphates containing a transition metal and lithium. The olivine crystal structure is a crystal possessed by natural olivine, and can be identified by X-ray diffraction.

[0020] A typical example of an electrode active material with an olivine crystal structure is LiCoPO4, which contains Co. Phosphates in which the transition metal Co is substituted in this chemical formula can also be used. The ratio of Li and PO4 can vary depending on the valence. It is preferable to use Co, Mn, Fe, Ni, etc. as the transition metal.

[0021] The second internal electrode 20 contains a negative electrode active material.

[0022] 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 additive), etc. are added. For these components, an internal electrode paste can be obtained by uniformly dispersing a binder and a plasticizer in water or an organic solvent. A carbon material or the like may be included as the conductive additive. A metal may be included as the conductive additive. Examples of the metal for the conductive additive include Pd, Ni, Cu, Fe, and alloys containing these. The solid electrolyte included in the first internal electrode 10 and the second internal electrode 20 can be, for example, the same as the main solid electrolyte of the solid electrolyte layer 30.

[0023] Here, we will explain the voltage change during cell charging and discharging of an all-solid-state battery. All-solid-state batteries have the characteristic that their cell voltage decreases as discharging progresses and the remaining battery power becomes low. To drive devices such as IC chips, it is necessary to control the cell voltage of an all-solid-state battery so that it does not fall below a specified voltage (e.g., 1.8 V). Therefore, many batteries have an end-point voltage (lower limit voltage) set, and are designed to charge when the monitored cell voltage drops to the end-point voltage. Therefore, it is desirable to be able to easily detect the end-point voltage.

[0024] However, the discharge curve of a typical all-solid-state battery has a potential plateau (also called a plateau) based on the specific redox potential of the positive and negative electrodes, making it difficult to estimate the remaining battery capacity during the discharge process. Furthermore, the voltage drop at the end of the potential plateau becomes steeper toward the end of discharge, when the remaining battery capacity becomes low. For example, as illustrated in Figure 2, the cathode active material with the olivine-type crystal structure described above experiences a rapid voltage drop when discharging from a fully charged state. Thereafter, a potential plateau appears where the cell voltage remains nearly constant (or barely drops) even as the discharge progresses. From the end of the potential plateau, the voltage rapidly drops as the discharge progresses. Therefore, there is a risk that the voltage may fall significantly below the end-point voltage during the period between the detection of the end-point voltage and the start of charging. In Figure 2, the horizontal axis represents the discharge amount, and the vertical axis represents the cell voltage.

[0025] Therefore, in this embodiment, a TiTa2O7-based oxide with a monoclinic structure is used as the negative electrode active material. TiTa2O7, in which Ta is not replaced with another metal element, has a low negative electrode operating potential, small volume change during charge and discharge, and exhibits good cycle characteristics. Its low gravimetric capacity but relatively high volumetric capacity makes it an ideal negative electrode active material for small all-solid-state batteries where battery weight is not a major concern. However, it is difficult to detect the end-point voltage in the charge and discharge curve of TiTa2O7.

[0026] In this embodiment, a part of Ta is replaced with another metal element, TiTa 2-x M x O7 (where 0.2≦x≦1.0, M contains at least Nb and may further contain a pentavalent metal element) is used as the negative electrode active material. When Ta in TiTa2O7 is substituted with Nb, Nb generally has a higher two-electron reactivity (Nb 5+ →Nb 4+ →Nb 3+ ) because it is easy to 2-x M xIn O7, it is thought that (3 + x) electrons react per mole, and the larger x, the greater the capacity. On the other hand, the higher the number of reaction electrons, the greater the volume change associated with Li insertion and desorption, which is thought to lead to a deterioration in cycle characteristics.

[0027] TiTa 2-x M x As shown in Figure 3, the negative electrode potential of O7 does not remain constant with respect to the amount of discharge, but rather rises gradually at a moderate rate, making it easier to grasp the remaining battery capacity at any stage of discharge and to detect the end-point voltage.

[0028] TiTa 2-x M x In O7, if the value of x is too small, there is a risk of a decrease in actual capacity and a decrease in detectability of remaining battery power. Therefore, x is preferably 0.2 or more. For example, in terms of actual capacity, x is more preferably 0.5 or more. Furthermore, in terms of detectability of remaining battery power, x is preferably 0.2 or more, and more preferably 0.3 or more.

[0029] If the value of x is too large, there is a risk of deterioration in coulombic efficiency and cycle characteristics, a decrease in the detectability of the remaining battery capacity and the end-point voltage, and degradation due to reaction with the solid electrolyte during heat treatment. Therefore, x is preferably 1.0 or less. For example, in terms of battery characteristics such as coulombic efficiency and cycle characteristics, x is preferably 1.0 or less, and more preferably 0.7 or less. Furthermore, in terms of the detectability of the remaining battery capacity and the end-point voltage, x is preferably 1.0 or less, and more preferably 0.7 or less. Furthermore, in terms of reactivity with the solid electrolyte during heat treatment, x is preferably 1.0 or less, and more preferably 0.5 or less.

[0030] All of M may be Nb. For example, TiTa 1.5 Nb 0.5 It is preferable to use O7.

[0031] Negative electrode characteristics: 1.0V vs. Li / Li +After charging (Li inserted) to 3.0V vs. Li / Li + The discharge capacity when discharging (Li desorption) to 100% is taken as 100%, and we focus on the absolute value |dV / dSOC| of the slope of the discharge curve (i.e., the ratio of voltage change to change in remaining battery capacity) in the range of 90% to 10% (SOC 90% to 10%) of the remaining battery capacity (defined as SOC in this embodiment). A higher value is preferable because it reduces the proportion of the potential plateau that reduces the accuracy of detecting the remaining battery capacity. It is preferable that this value is always 3.5 [mV / %] or more within the above range. Furthermore, the smaller the difference between the maximum and minimum values ​​of |dV / dSOC|, the more linear the discharge curve becomes, making it easier to detect the remaining battery capacity and the end-point voltage. Therefore, the difference is preferably 8.5 [mV / %] or less, more preferably 7.5 [mV / %] or less, and even more preferably 6.5 [mV / %] or less. When |dV / dSOC| is within the above preferred range, the battery can be discharged with a constant voltage change, and the remaining charge can be determined with high accuracy, making it easy to use.

[0032] If the average particle size of the negative electrode active material in the second internal electrode 20 is too large, the resistance within the electrode increases, making high-speed charge / discharge difficult, which is undesirable. If the average particle size is too small, the reactivity during heat treatment increases, and the linearity of the charge / discharge curve (battery remaining capacity detection ability) decreases due to a decrease in composition uniformity within the negative electrode active material particles, which is undesirable. The average particle size of the negative electrode active material in the second internal electrode 20 is preferably 1 μm to 10 μm, more preferably 1.5 μm to 8 μm, and even more preferably 2 μm to 6 μm.

[0033] When fabricating an all-solid-state battery 100, a multilayer capacitor-type structure in which first internal electrodes 10 and second internal electrodes 20 are alternately stacked in parallel with solid electrolyte layers 30 interposed therebetween is suitable for increasing capacity density while miniaturizing the battery. In this case, it is preferable to make the thickness of the first internal electrode 10 and the thickness of the second internal electrode 20 approximately the same. However, because the negative electrode active material provided by this embodiment has a higher capacity per volume than a typical positive electrode active material, it is preferable to balance the capacity by adding a larger volume of positive electrode active material than the negative electrode active material. Therefore, this balance can be achieved by adding a higher volume of active material with high electronic conductivity to the first internal electrode 10 than the volume of the negative electrode active material to reduce the amount of conductive additive, or by adding a higher volume of active material with high ionic conductivity to the negative electrode active material to reduce the amount of ionic conductive additive. Adding a larger volume of LiCoPO4, which has high electronic conductivity after charging, than the volume of the negative electrode active material and reducing the volume of conductive additive to achieve a balance between capacity and electronic conductivity is preferable. 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, and therefore the volume ratio of the negative electrode active material in the second internal electrode 20 is preferably approximately 15 to 50 vol.%. Note that making the first internal electrode 10 and the second internal electrode 20 have approximately the same thickness means that the thickness of one of the first internal electrode 10 and the second internal electrode 20 is within ±20% of the other.

[0034] FIG. 4 is a schematic cross-sectional view of a stacked-type 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 contact two side surfaces, which are two of the four surfaces other than the top and bottom surfaces at the ends in the stacking direction. The two side surfaces may be two adjacent side surfaces or two side surfaces facing each other. In this embodiment, the first external electrode 40a and the second external electrode 40b are provided so as to contact two side surfaces facing each other (hereinafter referred to as two end surfaces).

[0035] In the following description, components having the same composition range, thickness range, and particle size distribution range as those of the all-solid-state battery 100 are denoted by the same reference numerals, and detailed description thereof will be 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 with solid electrolyte layers 30 interposed therebetween. The edges of the plurality of first internal electrodes 10 are exposed at the first end face of the laminated chip 60 but are not exposed at the second end face. The edges of the plurality of second internal electrodes 20 are exposed at the second end face of the laminated chip 60 but are not exposed at the first end face. As a result, the first internal electrodes 10 and the second internal electrodes 20 are alternately electrically connected to the first external electrode 40a and the second external electrode 40b. The solid electrolyte layer 30 extends from the first external electrode 40a to the second external electrode 40b. In this way, the all-solid-state battery 100a has a structure in which a plurality of battery units are stacked.

[0037] A cover layer 50 is laminated on the upper surface of the laminated structure of the first internal electrode 10, the solid electrolyte layer 30, and the second internal electrode 20 (in the example of FIG. 4, on the upper surface of the first internal electrode 10, which is the uppermost layer). In addition, a cover layer 50 is laminated on the lower surface of the laminated structure (in the example of FIG. 4, on the lower surface of the first internal electrode 10, which is the lowermost layer). 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 a main component.

[0038] The first internal electrode 10 and the second internal electrode 20 may each include a current collector layer. For example, as illustrated in FIG. 5, a first current collector layer 11 may be provided within the first internal electrode 10. Furthermore, a second current collector layer 21 may be provided within 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, metal, carbon, or the like can be used as the conductive material for the first current collector layer 11 and the second current collector layer 21. Connecting the first current collector layer 11 to the first external electrode 40a and connecting the second current collector layer 21 to the second external electrode 40b improves current collection efficiency.

[0039] Next, a description will be given of a method for manufacturing the all-solid-state battery 100a illustrated in Fig. 4. Fig. 6 is a diagram illustrating a flow of the method for manufacturing the all-solid-state battery 100a.

[0040] (Process for producing negative electrode active material powder) TiTa 2-x Nb x The raw materials for O7, TiO2, Ta2O5, and Nb2O5, are weighed out to a molar ratio of 2:(2-x):x, and then crushed and mixed. After mixing, the mixture is calcined at 1100°C in air, and the calcined powder obtained is crushed again. After that, it is heat-treated at 1300°C in air to obtain the desired TiTa 2-x Nb x The O7 synthetic powder is obtained. The synthetic powder is crushed again and then sieved through a #150 stainless steel mesh to obtain the negative electrode active material powder.

[0041] TiTa 2-x M x In O7, the crystal structure is composed of randomly arranged TiO6 octahedra, TaO6 octahedra, and MO6 octahedra. Therefore, the more uniform these structures are, the less likely a plateau in the redox potential will be observed compared to a single metal oxide. Therefore, the higher the synthesis temperature, the more likely it is that a charge-discharge curve with a stable shape will be obtained. A firing temperature that is too high during synthesis is undesirable because it causes excessive particle adhesion, making handling difficult, while a temperature that is too low is undesirable because it reduces the uniformity of each metal atom. The firing temperature is preferably between 1150°C and 1450°C, more preferably between 1200°C and 1400°C, and even more preferably between 1250°C and 1350°C.

[0042] (Process for producing raw material powder for solid electrolyte layer) First, raw material powder for the solid electrolyte layer that constitutes the above-described solid electrolyte layer 30 is prepared. For example, raw materials, additives, etc. are mixed and the raw material powder for the solid electrolyte layer can be prepared using a solid-phase synthesis method or the like. The obtained raw material powder can be adjusted to a desired average particle size by dry-milling. For example, the desired average particle size is adjusted using a planetary ball mill with 5 mm diameter ZrO2 balls.

[0043] (Cover layer raw material powder production process) First, a ceramic raw material powder for the cover layer 50 is prepared. For example, raw materials, additives, etc. are mixed and solid-phase synthesis is used to prepare the raw material powder for the cover layer. The obtained raw material powder can be dry-milled to adjust the average particle size to the desired size. For example, the desired average particle size is adjusted using a planetary ball mill with 5 mm diameter ZrO2 balls.

[0044] (Internal electrode paste manufacturing process) Next, an internal electrode paste for producing the first internal electrode 10 and the second internal electrode 20 is prepared. For example, the internal electrode paste can be obtained by uniformly dispersing a conductive additive, an electrode active material, a solid electrolyte material, a sintering additive, a binder, a plasticizer, etc. in water or an organic solvent. The solid electrolyte material may be the above-mentioned solid electrolyte paste. A carbon material or the like may be used as the conductive additive. A metal may be used as the conductive additive. Examples of the metal conductive additive include Pd, Ni, Cu, Fe, and alloys containing these. Pd, Ni, Cu, Fe, alloys containing these, and various carbon materials may also be used. If the first internal electrode 10 and the second internal electrode 20 have different compositions, a paste for each internal electrode may be prepared separately.

[0045] The sintering aid of the internal electrode paste contains one or more glass components such as Li-BO based compounds, Li-Si-O based compounds, Li-CO based compounds, Li-SO based compounds, and Li-PO based compounds.

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

[0047] (Solid electrolyte green sheet manufacturing process) The raw material powder for the solid electrolyte layer is uniformly dispersed in an aqueous or organic solvent along with a binder, dispersant, plasticizer, etc., and then wet-pulverized to obtain a solid electrolyte slurry with a desired average particle size. This process can be performed using a bead mill, wet jet mill, various kneaders, high-pressure homogenizers, etc., with the bead mill being preferred because it allows for simultaneous adjustment of particle size distribution and dispersion. A binder is added to the resulting solid electrolyte slurry to obtain a solid electrolyte paste. The resulting solid electrolyte paste can be coated to produce a solid electrolyte green sheet 51. The coating method is not particularly limited, and can include slot die coating, reverse coating, gravure coating, bar coating, doctor blade coating, etc. The particle size distribution after wet-pulverization can be measured, for example, using a laser diffraction measurement device using laser diffraction scattering.

[0048] (Lamination process) As shown in FIG. 7( a), an internal electrode paste 52 is printed on one side of a solid electrolyte green sheet 51. A reverse pattern 53 is printed on the area of ​​the solid electrolyte green sheet 51 where the internal electrode paste 52 is not printed. The reverse pattern 53 may be the same as the solid electrolyte green sheet 51. After printing, multiple solid electrolyte green sheets 51 are stacked with an alternating offset. As shown in FIG. 7( b), a laminate is obtained by pressing cover sheets 54 from above and below in the stacking direction. In this case, a laminate having a substantially rectangular parallelepiped shape is obtained so that the internal electrode paste 52 is exposed alternately on two end faces of the laminate. The cover sheet 54 can be formed by coating a raw material powder for the cover layer using a method similar to that used in the solid electrolyte green sheet preparation process. The cover sheet 54 is formed thicker than the solid electrolyte green sheet 51. The thickness may be increased during coating, or by stacking multiple coated sheets.

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

[0050] (Firing process) Next, the obtained laminate is fired. The firing conditions are not particularly limited, and may be in an oxidizing atmosphere or a non-oxidizing atmosphere, with the maximum temperature preferably being 400°C to 1000°C, more preferably 500°C to 900°C. A step of maintaining the temperature in an oxidizing atmosphere at a temperature lower than the maximum temperature may be provided in order to thoroughly remove the binder before the maximum temperature is reached. In order to reduce process costs, 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, an all-solid-state battery 100a is produced.

[0051] In addition, by sequentially stacking the internal electrode paste, the current collector paste containing a conductive material, and the internal electrode paste, a current collector layer can be formed within the first internal electrode 10 and the second internal electrode 20. [Example]

[0052] Hereinafter, all-solid-state batteries were fabricated according to the embodiments, and their characteristics were investigated.

[0053] Example 1 TiTa 1.8 Nb 0.2 The raw materials TiO2, Ta2O5, and Nb2O5 were weighed in a molar ratio of 2:1.8:0.2, and then crushed and mixed to obtain the desired composition of TiO7. After mixing, the mixture was calcined at 1100°C in air, and the resulting calcined powder was crushed again and further heat-treated at 1300°C in air to obtain the desired TiTa 1.8 Nb 0.2 The resulting powder was crushed again and sieved through a #150 stainless steel mesh to obtain the negative electrode active material powder. XRD analysis revealed the same diffraction peaks as those of TiTa2O7, but no other secondary phase peaks were observed.

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

[0055] The initial discharge capacity is 801mAh / cm 3 The discharge capacity after 20 cycles was 96.6% of the initial charge capacity. The slope of the discharge (Li desorption) curve, |dV / dSOC|, was calculated in the range of 1.05 V to 1.70 V. The minimum value across this range was 3.8 mV / %, confirming high non-flatness. Furthermore, the difference between the maximum and minimum values ​​was 7.6 mV / %, confirming relatively high linearity. This negative electrode active material was mixed with the solid electrolyte LAGP in a 50:50 volume ratio and heat-treated in air. No secondary phase was observed up to 730°C.

[0056] Example 2 TiTa 1.7 Nb 0.3 A negative electrode active material powder was prepared and evaluated in the same manner as in Example 1, except that the raw materials TiO2, Ta2O5, and Nb2O5 were weighed out in a molar ratio of 2:1.7:0.3 so as to achieve a composition ratio of O7.

[0057] The initial discharge capacity is 833mAh / cm 3 The capacity retention rate was 96.3%. The minimum value of |dV / dSOC| was 4.2 [mV / %], which was highly non-flat, and the difference between the maximum and minimum values ​​was 6.7 [mV / %], which was highly linear. When mixed with a solid electrolyte and heat-treated, no secondary phase was observed up to 730°C.

[0058] Example 3 TiTa 1.5 Nb 0.5 A negative electrode active material powder was prepared and evaluated in the same manner as in Example 1, except that the raw materials TiO2, Ta2O5, and Nb2O5 were weighed out in a molar ratio of 2:1.5:0.5 so as to achieve a composition ratio of O7.

[0059] The initial discharge capacity is 863mAh / cm 3 The capacity retention rate was 94.9%. The minimum value of |dV / dSOC| was 5.7 [mV / %], which was highly non-flat, and the difference between the maximum and minimum values ​​was 5.2 [mV / %], which was very linear. When mixed with a solid electrolyte and heat-treated, no secondary phase was observed up to 710°C.

[0060] Example 4 TiTa 1.3 Nb 0.7 A negative electrode active material powder was prepared and evaluated in the same manner as in Example 1, except that the raw materials TiO2, Ta2O5, and Nb2O5 were weighed in a molar ratio of 2:1.3:0.7 so as to achieve a composition ratio of O7.

[0061] The initial discharge capacity is 924mAh / cm 3 The capacity retention rate was 94.4%. The minimum value of |dV / dSOC| was 5.5 [mV / %], which was highly non-flat, and the difference between the maximum and minimum values ​​was 6.2 [mV / %], which was very linear. When mixed with a solid electrolyte and heat-treated, no secondary phase was observed up to 690°C.

[0062] Example 5 A negative electrode active material powder was produced and evaluated in the same manner as in Example 1, except that the raw materials TiO2, Ta2O5, and Nb2O5 were weighed out in a molar ratio of 2:1:1 to achieve a composition ratio of TiTaNbO7.

[0063] The initial discharge capacity is 894mAh / cm 3 The capacity retention rate was 93.2%. The minimum value of |dV / dSOC| was 5.4 [mV / %], which was highly non-flat, and the difference between the maximum and minimum values ​​was 7.9 [mV / %], which was relatively linear. When mixed with a solid electrolyte and heat-treated, no secondary phase was observed up to 680°C.

[0064] (Comparative Example 1) A negative electrode active material powder was produced and evaluated in the same manner as in Example 1, except that the raw materials TiO2 and Ta2O5 were weighed out in a molar ratio of 2:1 to achieve a composition ratio of TiTa2O7.

[0065] The initial discharge capacity is 721mAh / cm 3 The capacity retention rate was 98.6%. The minimum value of |dV / dSOC| was 0.6 [mV / %], which showed low non-flatness, and the difference between the maximum and minimum values ​​was 20.2 [mV / %], which showed very low linearity. When mixed with a solid electrolyte and heat-treated, no secondary phase was observed up to 730°C.

[0066] (Comparative Example 2) TiTa 1.85 Nb 0.15 A negative electrode active material powder was prepared and evaluated in the same manner as in Example 1, except that the raw materials TiO2, Ta2O5, and Nb2O5 were weighed out in a molar ratio of 2:1.85:0.15 so as to achieve a composition ratio of O7.

[0067] The initial discharge capacity is 754mAh / cm 3 The capacity retention rate was 97.2%. The minimum value of |dV / dSOC| was 3.0 [mV / %], which showed low non-flatness, and the difference between the maximum and minimum values ​​was 8.7 [mV / %], which showed low linearity. When mixed with a solid electrolyte and heat-treated, no secondary phase was observed up to 730°C.

[0068] (Comparative Example 3) TiTa 0.9 Nb 1.1 A negative electrode active material powder was prepared and evaluated in the same manner as in Example 1, except that the raw materials TiO2, Ta2O5, and Nb2O5 were weighed out in a molar ratio of 2:0.9:1.1 so as to achieve a composition ratio of O7.

[0069] The initial discharge capacity is 930mAh / cm 3The capacity retention rate was 91.8%. The minimum value of |dV / dSOC| was 5.1 [mV / %], which was highly non-flat, and the difference between the maximum and minimum values ​​was 8.6 [mV / %], which was low in linearity. When mixed with a solid electrolyte and heat-treated, no secondary phase was observed up to 670°C.

[0070] Comparative Example 4 A negative electrode active material powder was produced and evaluated in the same manner as in Example 1, except that the raw materials TiO2 and Nb2O5 were weighed out in a molar ratio of 2:1 to achieve a composition ratio of TiNb2O7.

[0071] The initial discharge capacity is 1069mAh / cm 3 The capacity retention rate was 90.3%. The minimum value of |dV / dSOC| was 0.9 [mV / %], which showed low non-flatness, and the difference between the maximum and minimum values ​​was 16.4 [mV / %], which showed very low linearity. When mixed with a solid electrolyte and heat-treated, no secondary phase was observed up to 650°C.

[0072] FIG. 8 shows the charge / discharge curves of Example 3. FIG. 9(a) shows the discharge curve of Comparative Example 1. FIG. 9(b) shows the discharge curve of Example 3. FIG. 9(c) shows the discharge curve of Comparative Example 4. FIG. 10(a) is a plot of |dV / dSOC| in the SOC range of 90% to 10% of Comparative Example 1. FIG. 10(b) is a plot of |dV / dSOC| in the SOC range of 90% to 10% of Example 3. FIG. 10(c) is a plot of |dV / dSOC| in the SOC range of 90% to 10% of Comparative Example 4. FIG. 11(a) is a plot of the maximum and minimum values ​​of |dV / dSOC| versus x in Examples 1 to 5 and Comparative Examples 1 to 4. FIG. 11(b) is a plot of the difference between the maximum and minimum values ​​of |dV / dSOC| versus x in Examples 1 to 5 and Comparative Examples 1 to 4.

[0073] The results of Examples 1 to 5 and Comparative Examples 1 to 4 are summarized in Table 1. The initial discharge capacity was 550 to 700 mAh / cm 3 Cannot be used (×), 700~850mAh / cm 3 Can be used (△), 850~1000mAh / cm 3Above 1000mAh / cm 3 The cycle characteristics were evaluated as the ratio of discharge capacity after 20 cycles when the initial charge capacity was taken as 100% (i.e., the evaluation criteria also take into account the initial coulomb efficiency), with less than 90% being particularly poor (××), 90-92% being unusable (×), 92-94% being usable (△), 94-96% or more being good (〇), and 96% or more being particularly excellent (◎). The minimum value (threshold for non-flatness) of the discharge curve |dV / dSOC|, which determines the detectability of the remaining battery charge and the end-point voltage, of less than 3.5 [mV / %] was rated as unusable (×), the difference between the maximum and minimum values ​​(linearity) of 10 [mV / %] or more was rated as particularly poor (××), those between 8.5 and 10 [mV / %] were rated as unusable (×), those between 7.5 and 8.5 [mV / %] were rated as usable (△), those between 6.5 and 7.5 [mV / %] were rated as good (〇), and those under 6.5 [mV / %] were rated as particularly excellent (◎). Stability during heat treatment with the solid electrolyte was assessed by the maximum temperature at which a 50:50 weight ratio powder mixture did not produce a secondary phase diffraction peak in XRD. The following criteria were used: unusable below 660°C (×); usable between 660 and 680°C (△); good between 680 and 700°C (○); and exceptionally excellent above 700°C (◎). Evaluation was based on five criteria: ◎ 20 points, ○ 10 points, △ 5 points, × 0 points, and ×× -10 points. A total score of less than 30 points was assigned an ×× rating, 30 to 50 points an × rating, 50 to 65 points an △ rating, 65 to 75 points an ◯ rating, and 75 points or above an ◎ rating. The results are summarized in Table 1. [Table 1]

[0074] In Examples 1 to 5, the overall score was 50 points or more. 2-x M x This is thought to be because a negative electrode active material was used that is represented by the composition formula O7, where 0.2≦x≦1.0 and M contains at least Nb.

[0075] Example 6 TiTa prepared in Example 31.5 Nb 0.5 An all-solid-state battery was fabricated and evaluated using O7 according to the embodiment. The slope of the discharge curve |dV / dSOC| was calculated over the entire range from 3.7 V to 3.0 V, and the minimum value was 5.2 [mV / %], indicating high non-flatness. Furthermore, the difference between the maximum and minimum values ​​was 6.1 [mV / %], indicating relatively high linearity. This confirmed the high detectability of the remaining battery charge and the end point.

[0076] (Comparative Example 5) An all-solid-state battery was fabricated and evaluated according to the embodiment using the TiNb2O7 fabricated in Comparative Example 4. The slope |dV / dSOC| of the discharge curve was calculated over the entire range from 3.7 V to 3.0 V, and it was confirmed that the minimum value was 1.3 [mV / %], indicating low non-flatness. Furthermore, the difference between the maximum and minimum values ​​was 15.8 [mV / %], indicating very low linearity. This confirmed that the detectability of the remaining battery charge and the detectability of the end point were very poor.

[0077] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as defined in the claims. [Explanation of symbols]

[0078] 10 1st internal electrode 11 First current collector layer 20 Second internal electrode 21 Second current collector layer 30 Solid electrolyte layer 40a First outer electrode 40b 2nd external electrode 50 cover layers 51 Solid electrolyte green sheet 52 Internal electrode paste 53 Reverse pattern 54 Cover Sheet 55 External electrode paste 60 stacked chips 100,100a solid state battery

Claims

1. A composition represented by the formula TiTa 2-x Nb x O 7, A negative electrode active material characterized in that 0.2≦x≦1.

0.

2. 1.0V vs. Li / Li ＋ After charging to 3.0V vs. Li / Li ＋ 2. The negative electrode active material according to claim 1, wherein the minimum absolute value of the slope of the discharge curve |dV / dSOC| in the range of the remaining battery capacity of 90% to 10% is 3.5 [mV / %] or more, and the difference between the maximum value and the minimum value is less than 8.5 [mV / %], where the discharge capacity when the battery is discharged to 100% is taken as 100%.

3. The TiTa 2-x Nb x O 7 is TiTa １．５ Nb ０．５ O ７ 3. The negative electrode active material according to claim 1, wherein

4. 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; a second electrode layer provided on a second main surface of the oxide-based solid electrolyte layer, the second electrode layer including the negative electrode active material according to claim 1 .

5. 5. The all-solid-state battery according to claim 4, wherein the average particle size of the negative electrode active material in the second electrode layer is 1 μm or more and 10 μm or less.

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