Solid electrolyte, secondary battery, and method for manufacturing the same

Abstract

To provide a solid electrolyte having high lithium ion conductivity and easily restoring lithium ion conductivity after reaction with moisture and to provide a secondary battery using the solid electrolyte and a method for manufacturing the same.SOLUTION: There is provided a solid electrolyte having a composition represented by the composition formula of Li3-2x-αIn1-xNbxL6-α (provided that L is one or more halogens of F, Cl, Br and I, x satisfies the expression 0<x≤0.35 and α satisfies the expression 0≤α<1). The secondary battery has a positive electrode layer containing a positive electrode active material, a negative electrode layer containing a negative electrode active material and a separator layer interposed between the positive electrode layer and the negative electrode layer. At least one of the cathode layer, the anode layer and the separator layer contains the solid electrolyte.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a solid electrolyte, a secondary battery, and a method for manufacturing the same. [Background technology]

[0002] Lithium-ion rechargeable batteries possess excellent characteristics such as being small, lightweight, yet possessing high electromotive force and high energy density. Leveraging these superior characteristics, lithium-ion rechargeable batteries are used in a wide range of applications, including small electronic devices such as mobile phones and laptop computers, as well as large electric drive systems such as electric vehicles and hybrid vehicles.

[0003] In recent years, secondary batteries that use lithium-ion conductive solids as electrolytes have been attracting attention with the aim of further improving the safety of secondary batteries. Among these secondary batteries, those in which all the materials constituting the battery are solid are sometimes called all-solid-state secondary batteries.

[0004] As a solid electrolyte used in secondary batteries, sulfide solid electrolytes, which consist of sulfur-containing compounds, are known. Sulfide solid electrolytes have desirable properties as solid electrolytes, such as high lithium ion conductivity and high plasticity, which allows for a large contact area with active materials, but they have the problem of readily reacting with moisture to generate hydrogen sulfide gas.

[0005] To address this problem, a halide solid electrolyte has been proposed that does not generate hydrogen sulfide gas even when reacting with water. For example, Patent Document 1 describes a solid electrolyte comprising Li, M, and X, wherein M contains at least yttrium and X is at least one selected from the group consisting of Cl, Br, and I. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2019 / 135346

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, the solid electrolyte described in Patent Document 1 has a problem that its lithium ion conductivity significantly decreases due to the reaction with moisture. Therefore, when manufacturing a secondary battery using the solid electrolyte described in Patent Document 1, it is necessary to strictly control the moisture in the atmosphere during the manufacturing process and the sealing state by the exterior material, etc., which leads to an increase in manufacturing cost and an increase in the environmental load during manufacturing. Also, from the viewpoint of improving the characteristics of the secondary battery, it is desired to further improve the ion conductivity compared to the solid electrolyte described in Patent Document 1.

[0008] The present invention has been made in view of such a background, and provides a solid electrolyte having high lithium ion conductivity and capable of easily recovering lithium ion conductivity after reaction with moisture, a secondary battery using this solid electrolyte, and a method for manufacturing the same.

Means for Solving the Problems

[0009] One aspect of the present invention is a composition represented by the compositional formula of Li 3-2x-α In 1-x Nb x L 6-α (where L is one or more halogens selected from F, Cl, Br, and I, x satisfies 0 < x ≦ 0.35, and α satisfies 0 ≦ α < 1), which is in a solid electrolyte. and, It has a crystal structure that can be assigned to the space group C2 / m. in a solid electrolyte.

[0010] Another aspect of the present invention is a secondary battery having a positive electrode layer containing a positive electrode active material, a negative electrode layer containing a negative electrode active material, and a separator layer interposed between the positive electrode layer and the negative electrode layer, and the solid electrolyte of the above aspect is included in at least one of the positive electrode layer, the negative electrode layer, and the separator layer.

[0011] Still another aspect of the present invention is a method for manufacturing a secondary battery according to the above aspect, comprising: manufacturing a single cell having the positive electrode layer, the negative electrode layer, and the separator layer; and then heating the single cell in an inert gas atmosphere.

Advantages of the Invention

[0012] The solid electrolyte has the specific composition. A solid electrolyte having such a composition has high lithium ion conductivity. Further, since the solid electrolyte does not contain sulfur atoms, no harmful gas such as hydrogen sulfide is generated even when it reacts with moisture. Furthermore, even when the solid electrolyte once reacts with moisture, its lithium ion conductivity can be easily restored by heating in an inert gas atmosphere.

[0013] In addition, the secondary battery contains the solid electrolyte according to the above aspect in at least one of the positive electrode layer, the negative electrode layer, and the separator layer. Therefore, even when it reacts with moisture due to exposure to air during the manufacturing process of the secondary battery or entry of air into the exterior material due to leakage, etc., the lithium ion conductivity of the solid electrolyte can be easily restored by applying heat, and thus the battery characteristics can be restored.

[0014] In the method for manufacturing the secondary battery, the lithium ion conductivity of the solid electrolyte can be restored by a simple method of heating the single cell containing the solid electrolyte in an inert gas atmosphere after manufacturing the single cell. Also, in the manufacturing method, even if moisture in the atmosphere is not strictly controlled during the manufacturing process of the secondary battery, the battery characteristics can be easily restored by heating the single cell at the final stage of manufacturing the secondary battery. Therefore, according to the manufacturing method, an increase in manufacturing cost and an increase in environmental load during manufacturing can be easily avoided.

[0015] As described above, according to the above embodiment, it is possible to provide a solid electrolyte that has high lithium-ion conductivity and can recover its lithium-ion conductivity after reaction with water, a secondary battery using this solid electrolyte, and a method for manufacturing the same. [Brief explanation of the drawing]

[0016] [Figure 1] Figure 1 is an explanatory diagram showing the X-ray diffraction patterns of solid electrolytes S2-S4 and R1-R3 in Example 1. [Figure 2] Figure 2 is an explanatory diagram showing the X-ray diffraction patterns of solid electrolytes S2, S6-S8 and solid electrolyte R1 in Example 1. [Figure 3] Figure 3 is an explanatory diagram showing the X-ray diffraction patterns of solid electrolytes S4, S9-S11 and solid electrolyte R1 in Example 1. [Figure 4] Figure 4 is a cross-sectional view showing the main parts of the pressure molding jig used in Example 1. [Figure 5] Figure 5 is an explanatory diagram showing the relationship between the value of x and ionic conductivity in solid electrolytes S1-S5 and R1-R3. [Figure 6] Figure 6 is a cross-sectional view showing the main components of the secondary battery in Example 2. [Figure 7] Figure 7 is an explanatory diagram showing the charge and discharge curve of test cell C1 in Example 2. [Figure 8] Figure 8 is an explanatory diagram showing the charge-discharge curve of test cell C2 in Example 2. [Figure 9] Figure 9 is an explanatory diagram showing the charge-discharge curve of test cell C3 in Example 2. [Figure 10] Figure 10 is an explanatory diagram showing the charge-discharge curve of test cell C4 in Example 2. [Modes for carrying out the invention]

[0017] (solid electrolyte) The solid electrolyte is Li 3-2x-α In 1-x Nbx L 6-α (However, L is one or more halogens selected from F, Cl, Br, and I, x satisfies 0 < x ≦ 0.35, and α satisfies 0 ≦ α < 1) and has a composition represented by the following compositional formula.

[0018] The value of x in the above compositional formula, that is, the molar ratio of the content of Nb to the total content of Li, In, Nb, and halogen exceeds 0 and is 0.35 or less. The solid electrolyte having such a composition has a crystal structure that can be attributed to the space group C2 / m. That is, the solid electrolyte has a crystal structure similar to that of a solid electrolyte having a composition of Li3InL6 (where L is one or more halogens selected from F, Cl, Br, and I, and α satisfies 0 ≦ α < 1), and a part of In (indium) in the crystal structure of Li3InL6 is substituted with Nb (niobium).

[0019] Since a part of In in the crystal structure of the solid electrolyte is substituted with Nb, the lithium ion conductivity can be improved. Further, even when the solid electrolyte having the above-described composition and crystal structure reacts with moisture, the lithium ion conductivity can be easily restored by heating the solid electrolyte in an inert gas atmosphere. From the viewpoint of further improving the lithium ion conductivity of the solid electrolyte, the value of x in the above compositional formula is preferably 0.07 ≦ x ≦ 0.35, more preferably 0.15 ≦ x ≦ 0.35, and particularly preferably 0.20 ≦ x ≦ 0.35.

[0020] When the value of x in the above compositional formula is greater than 0.35, the substitution amount of In becomes excessively large, and it may be difficult to maintain the desired crystal structure. As a result, there is a risk of causing a decrease in lithium ion conductivity.

[0021] The solid electrolyte contains one or more halogens L selected from the group consisting of F (fluorine), Cl (chlorine), Br (bromine), and I (iodine). For example, the solid electrolyte may contain at least Cl as halogen L. From the viewpoint of further improving the lithium ion conductivity of the solid electrolyte, it is more preferable that the solid electrolyte contains Cl as halogen L and at least one of Br and I halogens.

[0022] The value of α in the above compositional formula is 0 or greater and less than 1. By setting the value of α in the above compositional formula to the above specific range, the crystal structure of the solid electrolyte can be made into a desired structure. If the value of α in the above compositional formula is 1 or greater, it may become difficult to maintain the desired crystal structure. As a result, this may lead to a decrease in lithium ion conductivity. From the viewpoint of more reliably obtaining a solid electrolyte with the desired crystal structure, the value of α in the above compositional formula is preferably 0.95 or less, and more preferably 0.90 or less.

[0023] (Method for manufacturing solid electrolytes) The method for producing the solid electrolyte is as follows, for example. First, halogenated Li, halogenated In, and halogenated Nb are prepared as raw materials, and these halides are weighed to obtain the desired composition ratio of the solid electrolyte. Next, these raw material compounds are mixed in an inert gas atmosphere while being subjected to impact using a planetary ball mill or the like. The raw material compounds mixed while being subjected to mechanical energy in this way become the solid electrolyte through a mechanochemical reaction.

[0024] Alternatively, the solid electrolyte can be obtained by sintering instead of the mechanochemical method described above. In this case, halogenated Li, halogenated In, and halogenated Nb are prepared as raw materials, and these halides are weighed to obtain the desired composition ratio of the solid electrolyte. After mixing the raw material compounds, the mixture is sintered in a vacuum or in an inert gas atmosphere.

[0025] When storing the solid electrolyte obtained by the above method, strict control of the moisture content in the atmosphere during storage is not necessary. As described above, in the case of sulfide solid electrolytes, it is necessary to strictly control the moisture content in the atmosphere during storage from the viewpoint of avoiding a decrease in lithium ion conductivity due to the reaction with moisture. In contrast, even when the solid electrolyte reacts with moisture, the influence of moisture can be reduced by heating the solid electrolyte, and the lithium ion conductivity can be restored. Therefore, for example, even when the solid electrolyte is stored in an atmosphere containing moisture such as air, the lithium ion conductivity of the solid electrolyte can be restored by heating the solid electrolyte before the assembly of the secondary battery is completed. As a result, a secondary battery equipped with a solid electrolyte having high lithium ion conductivity can be obtained.

[0026] (Secondary battery) The secondary battery provided with the solid electrolyte has a positive electrode layer containing a positive electrode active material, a negative electrode layer containing a negative electrode active material, and a separator layer interposed between the positive electrode layer and the negative electrode layer. The solid electrolyte may be contained in any of the positive electrode layer, the negative electrode layer, and the separator layer. For example, the solid electrolyte may be contained only in the separator layer, or may be contained in all of the positive electrode layer, the negative electrode layer, and the separator layer. From the viewpoint of taking advantage of the above-described advantage of restoring lithium ion conductivity by heating, it is preferable that the solid electrolyte is contained in all of the positive electrode layer, the negative electrode layer, and the separator layer.

[0027] In addition, in the secondary battery, the specific solid electrolyte and a solid electrolyte other than the solid electrolyte may be used in combination. As the solid electrolyte other than the solid electrolyte, a sulfide solid electrolyte, an oxide solid electrolyte, an organic polymer solid electrolyte, or the like can be used. Examples of the sulfide solid electrolyte include Li2S-P2S5, Li2S-SiS2, Li 10 GeP2S 12 and the like. Examples of the oxide solid electrolyte include, for example, Li7LaZr2O 12Examples of garnet-type solid electrolytes include (LaLi)TiO3 and other perovskite-type solid electrolytes, as well as Li3PO4 and its N-substituted derivatives. Examples of organic polymer solid electrolytes include compounds of polymer compounds and lithium salts such as LiPF6 and LiBF6.

[0028] The positive electrode layer of a secondary battery contains at least a positive electrode active material. As the positive electrode active material, for example, compounds known as positive electrode active materials for lithium-ion secondary batteries can be used. Examples of such compounds include oxides having a layered rock salt type structure such as lithium cobalt oxide (LCO), lithium nickel oxide (LNO), lithium nickel cobalt manganese oxide (NCM), and lithium nickel cobalt aluminate (NCA); oxides having a spinel structure such as lithium manganese oxide (LMO); and phosphorylated compounds having an olivine structure such as lithium iron phosphate. By increasing the content of the positive electrode active material in the positive electrode layer, the energy density of the secondary battery can be further increased. From this viewpoint, the content of the positive electrode active material in the positive electrode layer is preferably 60 to 99% by mass, and more preferably 70 to 95% by mass.

[0029] Furthermore, the positive electrode layer of the secondary battery may contain the solid electrolyte in addition to the positive electrode active material. The solid electrolyte content in the positive electrode layer is preferably 1 to 40% by mass, and more preferably 5 to 30% by mass.

[0030] The positive electrode layer may contain a binder interposed between the particles constituting the positive electrode layer. Examples of binders include fluorine-based binders such as polyvinylidene fluoride and polytetrafluoroethylene, rubber-based binders such as styrene-butadiene rubber, olefin-based binders such as polypropylene and polyethylene, and cellulose-based binders such as carboxymethylcellulose.

[0031] Furthermore, the positive electrode layer may contain conductive agents or conductive additives to enhance electrical conductivity. Examples of conductive agents include graphite, carbon black, acetylene black, and coke. The content of conductive agents and conductive additives in the positive electrode layer can be appropriately set according to the electrical conductivity, the lithium-ion conductivity of the positive electrode layer, and the desired energy density of the secondary battery.

[0032] The negative electrode layer of a secondary battery contains at least a negative electrode active material. As the negative electrode active material, for example, compounds known as negative electrode active materials for lithium-ion secondary batteries can be used. Such compounds include, for example, silicon-based active materials such as Si, Si alloys and silicon oxide; metals that can react with lithium such as Sn, Sn alloys, Al and Al alloys; oxides such as SnO2; carbon materials having a graphite structure such as graphite and hard carbon; and lithium titanate (Li4Ti5O2). 12 Examples include lithium oxides, metallic lithium, and lithium alloys. By increasing the content of the negative electrode active material in the negative electrode layer, the energy density of the secondary battery can be further increased. From this viewpoint, the content of the negative electrode active material in the negative electrode layer is preferably 60 to 99% by mass, and more preferably 70 to 95% by mass.

[0033] Furthermore, the negative electrode layer of the secondary battery may contain the solid electrolyte in addition to the negative electrode active material. The solid electrolyte content in the negative electrode layer is preferably 1 to 40% by mass, and more preferably 5 to 30% by mass.

[0034] The negative electrode layer may contain binders interposed between the particles constituting the negative electrode layer, as well as conductive agents and conductive additives to enhance electrical conductivity. The binders, conductive agents, and conductive additives that can be used in the negative electrode layer are the same as those used in the positive electrode layer.

[0035] The separator layer of the secondary battery may contain the solid electrolyte. In addition to the solid electrolyte, the separator layer may also contain a binder to bind the particles of the solid electrolyte together, if necessary. The binder that can be used in the separator layer is the same as that used in the positive electrode layer.

[0036] (Manufacturing method for secondary batteries) A method for manufacturing a secondary battery using the solid electrolyte described above is as follows: First, a positive electrode composite material to form the positive electrode layer, a separator composite material to form the separator layer, and a negative electrode composite material to form the negative electrode layer are prepared. Next, the positive electrode composite material is placed into a press mold. Then, the positive electrode layer is formed inside the press mold by compressing the positive electrode composite material inside the press mold.

[0037] Next, a separator composite material is placed on the positive electrode layer in the press mold, and the separator composite material is compressed together with the positive electrode layer. This forms a separator layer on the positive electrode layer. Subsequently, a negative electrode composite material is placed on the separator layer in the press mold, and the negative electrode composite material is compressed together with the positive electrode layer and the separator layer. As a result, a single cell consisting of a stacked positive electrode layer, a separator layer, and a negative electrode layer can be obtained. Note that the order in which the positive electrode layer, negative electrode layer, and separator layer are formed in the above method can be changed as appropriate. Furthermore, in the above method, it is possible to stack multiple single cells by repeatedly forming the positive electrode layer, the separator layer, and the negative electrode layer.

[0038] After forming a single cell as described above, a secondary battery can be obtained by enclosing the single cell in an outer casing equipped with terminals for extracting power from the outside.

[0039] When the solid electrolyte is used in a secondary battery, the atmosphere during the process from the preparation of each composite material to the preparation of a single cell is not particularly limited.

[0040] In the method for manufacturing a secondary battery described above, it is preferable to manufacture a single cell having the positive electrode layer, the negative electrode layer, and the separator layer, and then heat the single cell in an inert gas atmosphere. In this case, even if moisture was present in the atmosphere used to manufacture the single cell, the lithium ion conductivity of the solid electrolyte can be restored by heating the single cell in an inert gas atmosphere. After restoring the lithium ion conductivity of the solid electrolyte by heating, a secondary battery with excellent properties can be easily obtained by sealing the single cell in an outer casing material.

[0041] Furthermore, in the above manufacturing method, by controlling the moisture content in the atmosphere from the heating of the single cell to its sealing in the outer casing, the influence of moisture on the lithium ion conductivity of the solid electrolyte in the final secondary battery can be reduced. Therefore, it becomes unnecessary to strictly control the amount of moisture in the atmosphere at each stage from the preparation of each composite material to the formation of the single cell, making it easy to avoid increased manufacturing costs and increased environmental impact during manufacturing.

[0042] The heating temperature and heating time of a single cell may be set appropriately according to the composition of the solid electrolyte, etc. However, if the heating temperature of a single cell is excessively low, or if the heating time is excessively short, the recovery of the lithium-ion conductivity of the solid electrolyte may be insufficient. On the other hand, if the heating temperature of a single cell is excessively high, or if the heating time is excessively long, the substances contained in the single cell may deteriorate, potentially degrading the characteristics of the secondary battery. [Examples]

[0043] (Example 1) In this example, an example of the solid electrolyte will be described with reference to Figures 1 to 5. The solid electrolyte in this example (Table 1, solid electrolytes S1 to S14) is Li 3-2x-α In 1-x Nb x L 6-αIt has a composition represented by the compositional formula ((where L is one or more halogens selected from F, Cl, Br, and I, x satisfies 0 < x ≤ 0.35, and α satisfies 0 ≤ α < 1)).

[0044] The method for producing the solid electrolyte in this example is as follows. In the production of the solid electrolyte, all operations were carried out in an argon atmosphere with a dew point of -70°C or lower and an oxygen concentration of 1 volume ppm or lower. First, InCl3, NbCl5, LiF, LiCl, LiBr, and LiI as raw material compounds were weighed so as to have a desired molar ratio. These raw material compounds were sealed in a zirconia pod together with ceramic balls, and then the zirconia pod was sealed in a sealed container. Thereafter, the sealed container was attached to a planetary ball mill and rotated at a rotational speed of 500 rpm for 30 hours. By this operation, the raw material compounds in the zirconia pod were reacted by the mechanochemical method.

[0045] After the treatment by the planetary ball mill was completed, the sealed container was opened and the powder in the zirconia pod was recovered. This powder was held at a temperature of 300°C for 3 hours. Thus, the solid electrolytes S1 to S14 shown in Table 1 were obtained.

[0046] In this example, for comparison with the solid electrolytes S1 to S14, solid electrolytes R1 to R8 having the compositions shown in Table 1 were prepared. The production method of the solid electrolytes R1 to R5 is the same as that of the solid electrolytes S1 to S14 except that the mixing ratio of the raw material compounds is different. Also, the solid electrolyte R6 is Li3PS4 which is common as a sulfide solid electrolyte. The solid electrolytes R7 and the solid electrolyte R8 each have the composition shown in Table 1. The production methods of the solid electrolytes R seven and the solid electrolyte R8 conform to the method described in Patent Document 1.

[0047] In this example, the following method was used to evaluate the crystal structure of the solid electrolytes shown in Table 1, measure the ionic conductivity, and evaluate the recovery of the ionic conductivity after exposure to air.

[0048] <Evaluation of crystal structure> The crystal structure of each solid electrolyte was evaluated by powder X-ray diffraction. A Rigaku Corporation "SmartLab®" X-ray diffractometer was used, with CuKα characteristic X-rays, an X-ray tube voltage of 40kV, and an irradiation current of 20mA. Figures 1-3 show the X-ray diffraction patterns of solid electrolytes S2-S4, S6-S11, and R1-R3 as examples. In Figures 1-3, the vertical axis represents diffraction intensity (relative intensity), and the horizontal axis represents the diffraction angle 2θ (unit: °).

[0049] <Measurement of ionic conductivity> For measuring ionic conductivity, solid electrolyte pellets P were first prepared using the pressure molding jig 1 shown in Figure 4. As shown in Figure 4, the pressure molding jig 1 has a cylindrical mold portion 11 with a through hole 111, an upper punch 12 inserted into one end of the through hole 111 of the mold portion 11, and a lower punch 13 inserted into the other end of the through hole 111. The mold portion 11 is made of electrically insulating ceramic. The upper punch 12 and the lower punch 13 are made of electrically conductive stainless steel. As shown in Figure 4, pellets P can be formed by sandwiching the solid electrolyte between the upper punch 12 and the lower punch 13 and compressing it.

[0050] The method for producing pellet P is more specifically as follows: First, the pressure molding jig 1 is placed in an argon atmosphere with a dew point of -70°C or lower and an oxygen concentration of 1 ppm by volume or lower, and the lower punch 13 is inserted into the through hole 111 of the mold part 11. In this state, the solid electrolyte stored in the aforementioned argon atmosphere is placed into the through hole 111. Then, the upper punch 12 is inserted into the through hole 111, and the solid electrolyte is sandwiched between the upper punch 12 and the lower punch 13. Then, 6000 kg / cm is applied between the upper punch 12 and the lower punch 13. 2 By applying pressure to compress the solid electrolyte, a pellet P of the solid electrolyte was formed.

[0051] Next, while maintaining the aforementioned argon atmosphere, the electrochemical AC impedance of the solid electrolyte was measured. Specifically, first, 4000 kg / cm² was applied between the upper punch 12 and the lower punch 13.2 While holding the pellet P under pressure, a potentiostat equipped with a frequency response analyzer was electrically connected to the pellet P via the upper punch 12 and the lower punch 13. Then, electrochemical AC impedance measurements were performed to determine the complex impedance of the solid electrolyte at room temperature.

[0052] The complex impedance obtained in this way was analyzed using a Cole-Cole plot, and the magnitude of the real part of the complex impedance at the measurement point with the smallest absolute value of phase was taken as the resistance R for ionic conduction of the solid electrolyte. The ionic conductivity σ (unit: mS / cm) of the solid electrolyte was given by the aforementioned resistance R (unit: Ω) and the cross-sectional area S (unit: cm) of the pellet P. 2 Using ) and the thickness L of the pellet P (unit: cm), it is expressed by the following formula (1). σ = 1000L / (R·S) ···(1)

[0053] The cross-sectional area S of pellet P is specifically the area of ​​pellet P in a plane perpendicular to the compression direction, and the thickness of pellet P is the outer dimension of pellet P in the compression direction. The ionic conductivity of each solid electrolyte was as shown in Table 1.

[0054] <Evaluation of the recovery of ionic conductivity after exposure to air> The recovery of ionic conductivity after exposure to air was evaluated using solid electrolytes S2-S4 and R6-R8. First, solid electrolytes stored in an argon atmosphere with a dew point of -70°C or lower and an oxygen concentration of 1 ppm by volume or lower were exposed to air at a temperature of 24-25°C and a relative humidity of 20-22% for 10 minutes to allow the solid electrolytes to react with the moisture in the air. After 10 minutes, the solid electrolytes were moved back into the argon atmosphere.

[0055] Using the solid electrolytes after reaction with water, electrochemical AC impedance measurements were performed in the same manner as the ionic conductivity measurements described above. Ionic conductivity was then calculated based on the obtained complex impedance. Table 1 shows the ionic conductivity of each solid electrolyte after exposure to air.

[0056] Furthermore, the solid electrolytes exposed to air were heated in an argon atmosphere at 300°C for 3 hours. Electrochemical AC impedance measurements were performed using the heated solid electrolytes in the same manner as the ionic conductivity measurements described above. Ionic conductivity was then calculated based on the obtained complex impedance. Table 1 shows the ionic conductivity of each solid electrolyte after heating. For solid electrolytes where the recovery of ionic conductivity after air exposure and heating was not evaluated, the symbol "-" is indicated in Table 1.

[0057] [Table 1]

[0058] As shown in Table 1, solid electrolytes S1 to S14 all have the aforementioned specific composition. Furthermore, as shown in Figures 1 to 3, the crystal structure of these solid electrolytes is the same as that of solid electrolyte R1, which has the composition Li3InCl6. Therefore, these solid electrolytes have higher ionic conductivity than solid electrolytes R1 to R5, whose compositions fall outside the aforementioned specific range.

[0059] Figure 5 shows a graph with the value of x in the compositional formula on the horizontal axis and the value of ionic conductivity (unit: mS / cm) on the vertical axis for solid electrolytes S1-S5 and R1-R3, where the value of α in the compositional formula is 0 and the halogen is only Cl. As shown in Figure 5, it can be seen that in solid electrolytes having only Cl as the halogen, the ionic conductivity of the solid electrolyte is improved by setting the value of x in the compositional formula to be greater than 0 and 0.35 or less, preferably 0.07 or more and 0.35 or less, more preferably 0.15 or more and 0.35 or less, and even more preferably 0.20 or more and 0.35 or less.

[0060] Furthermore, as shown in Table 1, the solid electrolytes S2 to S4 having the specific composition and crystal structure could recover their ionic conductivity by heating, even after exposure to air and reaction with water. In contrast, the solid electrolytes R6 to R8 did not recover their ionic conductivity even after heating following reaction with water.

[0061] (Example 2) This example shows a secondary battery using the solid electrolyte described above. As shown in Figure 6, the secondary battery 2 in this example has a positive electrode layer 21 containing a positive electrode active material and a solid electrolyte, a separator layer 22 containing a solid electrolyte and laminated on the positive electrode layer 21, and a negative electrode layer 23 containing a negative electrode active material and a solid electrolyte and laminated on the separator layer 22. Specifically in this example, secondary batteries 2 (test cells C1 to C4) were fabricated as follows, and their charge and discharge characteristics were evaluated.

[0062] <Test cell C1> All operations in the fabrication process of test cell C1 were carried out in an argon atmosphere with a dew point of -70°C or lower and an oxygen concentration of 1 ppm by volume. First, a positive electrode composite material containing a positive electrode active material, a conductive additive, and a solid electrolyte, and a negative electrode composite material containing a negative electrode active material, a conductive additive, and a solid electrolyte were prepared. The positive electrode active material used in the positive electrode composite material of test cell C1 was lithium nickel cobalt manganese (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 The conductive additive is O2, and the solid electrolyte is the solid electrolyte S4(Li) in Example 1. 2.4 In 0.7 Nb 0.3 Cl6) is the ratio of positive electrode active material to solid electrolyte in the positive electrode composite material, in terms of mass ratio, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2:Li 2.4 In 0.7 Nb 0.3 The ratio of Cl6 to Cl6 is 75:25. Furthermore, the content of the conductive additive in the positive electrode mixture is 5% by mass relative to the total mass of the positive electrode active material and solid electrolyte (100% by mass).

[0063] The negative electrode composite material contains a negative electrode active material, a conductive additive, and a solid electrolyte. The negative electrode active material used in the negative electrode composite material of test cell C1 is Li4Ti5O 12 The conductive additive is acetylene black, and the solid electrolyte is Li3PS4. The ratio of negative electrode active material to solid electrolyte in the negative electrode composite material is Li4Ti5O by mass ratio. 12 The ratio of Li3PS4 to Li3PS4 is 40:60. Furthermore, the content of the conductive additive in the negative electrode mixture is 5% by mass relative to the total of 100% by mass of the negative electrode active material and solid electrolyte.

[0064] Next, the lower punch 13 was inserted into the through hole 111 of the mold section 11 of the pressure molding jig 1 shown in Figure 4. In this state, 50 mg of Li3PS4 as a solid electrolyte was placed into the through hole 111. Then, the upper punch 12 was inserted into the through hole 111, and 1000 kg / cm was applied between the upper punch 12 and the lower punch 13. 2 A load was applied. This compressed the solid electrolyte and formed the separator layer 22.

[0065] Next, the upper punch 12 was removed from the pressure molding jig 1, and 75 mg of negative electrode composite material was placed into the through hole 111. Then, the upper punch 12 was reinserted into the through hole 111, and 1000 kg / cm² of pressure was applied between the upper punch 12 and the lower punch 13. 2 A load was applied. This compressed the negative electrode composite material, forming a negative electrode layer 23 on one side of the separator layer 22.

[0066] Next, the lower punch 13 was removed from the pressure molding jig 1, and 20 mg of positive electrode composite material was placed into the through hole 111. Then, the lower punch 13 was reinserted into the through hole 111, and 4000 kg / cm² of pressure was applied between the upper punch 12 and the lower punch 13. 2 A load was applied. This compressed the positive electrode composite material, forming a positive electrode layer 21 on the other side of the separator layer 22. Thus, a test cell C1 as a secondary battery 2 was obtained.

[0067] <Test cell C2> For the positive electrode layer of test cell C2, a solid electrolyte S4 was used, which was exposed to air and then heated in an argon atmosphere. The conditions for exposure to air and heating in an argon atmosphere for the solid electrolyte S4 were the same as those for Example 1 described above. The other components and manufacturing method of test cell C2 were the same as those for test cell C1.

[0068] <Test cell C3> In test cell C3, the solid electrolyte used for the positive electrode layer was the same as that used in Example 1, solid electrolyte R6 (Li3PS4). The other components and manufacturing method of test cell C3 were the same as those of test cell C1.

[0069] <Test cell C4> In test cell C4, Li3PS4 was used as the solid electrolyte for the positive electrode layer, after being exposed to air and then heated in an argon atmosphere. The conditions for exposure to air and heating in the argon atmosphere of the Li3PS4 were the same as those in Example 1 described above. The other components and manufacturing method of test cell C4 were the same as those of test cell C1.

[0070] Next, the discharge capacity of test cells C1 to C4 was measured using the following method. First, the test cells were charged in constant current-constant voltage mode at a temperature of 25°C. The current density in constant current mode was set to 1 / 10C, ​​and the system was switched to constant voltage mode when the secondary battery voltage reached 2.95V. In constant voltage mode, the charging voltage was set to 2.95V, and charging was carried out until the current density reached 1 / 100C. After the current density in constant voltage mode reached 1 / 100C, charging and discharging was paused for 10 minutes to stabilize the potential of the test cells. After that, the cells were discharged to 1.5V with a constant current at a current density of 1 / 10C. Note that "C," the unit of current density during discharge, theoretically represents the current density at which the charge rate reaches 100% in one hour. That is, theoretically, the secondary battery can be completely discharged by discharging at a current density of 1C for one hour. In this example, the current density corresponding to 1C is specifically 170mA / g.

[0071] Figures 7 to 10 show the charge-discharge curves for each test cell. Table 2 shows the discharge capacities of test cells C1 to C4 calculated based on the charge-discharge curves.

[0072] [Table 2]

[0073] As shown in Figures 7 and 8, the charge-discharge curve of test cell C2 using solid electrolyte S4 after exposure to air and heating showed almost no change compared to the charge-discharge curve of test cell C1 using solid electrolyte S4 before exposure to air. Furthermore, as shown in Table 2, the discharge capacity of test cell C2 was approximately 91% of the discharge capacity of test cell C1 using solid electrolyte S4 before reaction with moisture. From these results, it can be understood that in secondary batteries using solid electrolytes with the aforementioned specific composition and crystal structure, even if the solid electrolyte reacts with moisture during the manufacturing process, the effect of moisture can be mitigated and the charge-discharge characteristics can be restored by heating.

[0074] On the other hand, as shown in Table 2, test cell C3, using solid electrolyte R6 before exposure to air, had roughly the same discharge capacity as test cells C1 and C2. However, the discharge capacity of test cell C4, using solid electrolyte R6 after exposure to air and heating, decreased to about 44% of that of test cell C3. Furthermore, a comparison of Figures 9 and 10 shows that the slope of the charge-discharge curve in test cell C4 was steeper than that of test cell C3, indicating an increase in internal resistance during charge and discharge. These results suggest that, when using conventional sulfide solid electrolytes, once the solid electrolyte reacts with water during the manufacturing process, the charge-discharge characteristics do not recover even after heating.

[0075] Based on the above results, it can be understood that it is preferable for the positive electrode layer of a secondary battery to contain the solid electrolyte.

[0076] Although specific embodiments of the solid electrolyte and secondary battery according to the present invention have been described above based on the examples, the embodiments of the solid electrolyte and secondary battery according to the present invention are not limited to the embodiments described above, and the configuration can be appropriately modified without impairing the spirit of the present invention.

[0077] For example, in the above-described embodiment, an example of a secondary battery in which the solid electrolyte is contained in the positive electrode layer was shown, but the solid electrolyte may also be contained in the negative electrode layer or the separator layer. The above-described effects can be obtained if the solid electrolyte is contained in one or more of the positive electrode layer, negative electrode layer and separator layer. [Explanation of symbols]

[0078] 2 Secondary battery 21 Positive electrode layer 22 Separator layer 23 Negative electrode layer

Claims

1. Li ３－２ｘ－α In １－ｘ Nb ｘ L ６－α (wherein L is one or more halogens from F, Cl, Br and I, x satisfies 0 < x ≤ 0.35, and α satisfies 0 ≤ α < 1) and the composition represented by the empirical formula, A solid electrolyte having a crystal structure that can be assigned to the space group C2 / m.

2. The solid electrolyte according to claim 1, wherein the solid electrolyte contains at least Cl as the halogen.

3. A positive electrode layer containing positive electrode active material, A negative electrode layer containing negative electrode active material, It has a separator layer interposed between the positive electrode layer and the negative electrode layer, A secondary battery comprising at least one of the positive electrode layer, the negative electrode layer, and the separator layer containing the solid electrolyte described in claim 1 or 2.

4. A method for manufacturing a secondary battery according to claim 3, A single cell having the positive electrode layer, the negative electrode layer, and the separator layer is manufactured. A method for manufacturing a secondary battery, comprising subsequently heating the single cell in an inert gas atmosphere.

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