Solid electrolyte, electrode, and secondary battery

Abstract

One aspect of the present disclosure provides a halide-based solid electrolyte comprising an alkali metal element, a non-metal element, a halogen element, and oxygen. In the halide-based solid electrolyte, the content of oxygen may be 0.5 mol% or more with respect to the total number of atoms contained in the halide-based solid electrolyte.

Description

Solid electrolyte, electrode, and secondary battery

[0001] The present disclosure relates to a solid electrolyte, an electrode, and a secondary battery.

[0002] In recent years, solid electrolytes have attracted attention as electrolytes used in electrochemical devices such as lithium-ion batteries. Solid electrolytes are considered useful for improving battery performance such as safety, high capacity, rapid charge and discharge, and pack energy density because they are superior to conventional electrolytic solutions in terms of high-temperature durability and high-voltage resistance.

[0003] As solid electrolytes, due to their high ionic conductivity close to that of electrolytic solutions and moderate flexibility, research has been conducted on sulfide-based solid electrolytes such as Li １０ GeP ２ S １２ etc. (for example, Patent Document 1). In addition, as solid electrolytes containing lithium, Li ７ La ３ Zr ２ O １２ (LLZO) and other oxide-based solid electrolytes, and Li ３ InCl ６ 、LiYBr ６ and other halide-based solid electrolytes are also known.

[0004] Halide-based solid electrolytes are highly oxidation-resistant, have low reactivity with moisture in the atmosphere and are stable, have a monovalent charge of halide ions and low activation energy in ion conduction, and are considered useful because of their relatively high stability at high potentials. Patent Document 2 discloses a lithium-ion conductive solid electrolyte composed of Li, La, O, and X, where X is at least one element selected from the group consisting of Cl, Br, and I. Patent Document 3 discloses a solid electrolyte composed of Li, La, O, and I.

[0005] Japanese Patent Application Laid-Open No. 2023-181158 International Publication No. 2020 / 137043 International Publication No. 2020 / 219846

[0006] An object of the present disclosure is to provide a halide-based solid electrolyte having excellent ionic conductivity.

[0007] The present disclosure provides the following [1] to [9].

[0008] [1] A halide-based solid electrolyte composed of an alkali metal element, a non-metal element, a halogen element, and oxygen. [2] The halide-based solid electrolyte according to [1], wherein the non-metal element is at least one selected from the group consisting of N, P, Se, and S. [3] The halide-based solid electrolyte according to [1] or [2], wherein the halogen element contains at least one selected from the group consisting of F, Br, Cl, and I. [4] The halide-based solid electrolyte according to any one of [1] to [3], wherein the oxygen content is 0.5 mol% or more with respect to the total number of atoms contained in the halide-based solid electrolyte. [5] The halide-based solid electrolyte has a composition represented by the general formula (1): A ａ M ｂ O ｃ X ｄ wherein A represents an alkali metal element, M represents a non-metal element excluding O, X represents a halogen element, a is greater than 0 and less than or equal to 0.9, b is greater than 0 and less than or equal to 0.5, c is greater than 0 and less than or equal to 0.25, d is greater than 0 and less than or equal to 0.5, and a + b + c + d = 1. The halide-based solid electrolyte according to any one of [1] to [4]. [6] The halide-based solid electrolyte according to any one of [1] to [5], having a molding density of 1.0 to 5.0 g / cm ３ [7] An electrode containing a solid electrolyte, wherein the solid electrolyte is the halide-based solid electrolyte according to any one of [1] to [6]. [8] A secondary battery including the electrode according to [7]. [9] A secondary battery including a solid electrolyte containing the halide-based solid electrolyte according to any one of [1] to [6].

[0009] According to the present disclosure, a halide-based solid electrolyte excellent in ionic conductivity can be provided.

[0010] Hereinafter, embodiments of the present disclosure will be described. However, the following embodiments are examples for explaining the present disclosure and are not intended to limit the present disclosure to the following contents.

[0011] Unless otherwise specified, the materials exemplified in this specification can be used singly or in combination of two or more. The content of each component in the composition means the total amount of the plurality of substances present in the composition when there are a plurality of substances corresponding to each component in the composition, unless otherwise specified.

[0012] One embodiment of the halide-based solid electrolyte consists of an alkali metal element, a non-metal element, a halogen element, and oxygen. It can also be said that the above halide-based solid electrolyte is a compound composed of an alkali metal element, a non-metal element, a halogen element, and oxygen. The above halide-based solid electrolyte substantially does not have a metal element other than the alkali metal element.

[0013] The inventors have found through research that by introducing oxygen into a halide-based solid electrolyte composed of an alkali metal element, a non-metal element, and a halogen element and weakening the bond between the non-metal element or the halogen element and the alkali metal element, the halide-based solid electrolyte according to the present disclosure has excellent ionic conductivity compared to those without oxygen. The present disclosure is made based on the above findings.

[0014] The alkali metal element in the above halide-based solid electrolyte may be, for example, at least one selected from the group consisting of lithium (Li), sodium (Na), and potassium (K), preferably includes at least one of Li and Na, preferably includes Li, and may be Li.

[0015] The above non-metal element is a non-metal element other than a halogen element and oxygen. The above non-metal element may be, for example, at least one selected from the group consisting of nitrogen (N), phosphorus (P), selenium (Se), and sulfur (S), may contain N, and may be only N.

[0016] When the above non-metal element M contains two or more elements, it preferably contains N, and may be, for example, two or more elements including N and at least one selected from the group consisting of P, Se, and S.

[0017] The halogen element in the above-mentioned halide-based solid electrolyte may be at least one selected from the group consisting of, for example, fluorine (F), bromine (Br), chlorine (Cl), and iodine (I). From the viewpoint of achieving a higher level of both ionic conductivity and oxidation resistance, it is preferable to include Cl, and may be Cl.

[0018] The above halide-based solid electrolyte contains oxygen as a constituent element. The lower limit of the oxygen content may be, for example, 0.5 mol% or more, 1 mol% or more, 3 mol% or more, or 5 mol% or more, relative to the total number of atoms contained in the halide-based solid electrolyte. It is believed that by having the lower limit of the oxygen content within the above range, the interaction between nonmetallic elements and O is strengthened, and the ionic conductivity of the resulting halide-based solid electrolyte can be further improved. The upper limit of the oxygen content may be, for example, 25 mol% or less, 23 mol% or less, 20 mol% or less, or 17 mol% or less, relative to the total number of atoms contained in the halide-based solid electrolyte. It is believed that by having the upper limit of the oxygen content within the above range, the interaction between alkali metal elements and oxygen does not become too strong, and the ionic conductivity of the halide-based solid electrolyte can be further improved. The oxygen content may be adjusted within the above range, and may be, for example, 0.5 to 25 mol% relative to the total number of atoms contained in the halide-based solid electrolyte.

[0019] The above halide-based solid electrolyte is, for example, a general formula (1): A ａ M ｂ O ｃ X ｄ The compound may have a composition represented by the above general formula (1). In the above general formula (1), A represents an alkali metal element, M represents a nonmetal element other than O, O represents oxygen, and X represents a halogen element. The alkali metal element may be any of the elements listed as alkali metal elements in the above description of halide solid electrolytes. The halogen element may be any of the elements listed as halogen elements in the above description of halide solid electrolytes.

[0020] In the above general formula (1), a is greater than 0 and less than or equal to 0.9, b is greater than 0 and less than or equal to 0.5, c is greater than 0 and less than or equal to 0.25, and d is 0 or greater and less than or equal to 0.5, satisfying a + b + c + d = 1.

[0021] The above value a may be, for example, 0.1 to 0.85, 0.2 to 0.8, or 0.3 to less than 0.75. It is believed that when the upper limit of the value of a is within the above range, cation defects are more likely to occur in the crystal structure, and the ionic conductivity is more likely to improve. It is believed that when the lower limit of the value of a is within the above range, the carrier concentration of cations increases, and the ionic conductivity is more likely to improve.

[0022] The above value b may be, for example, 0.05 to 0.45, 0.1 to 0.4, or 0.15 to 0.35. It is thought that when the upper limit of the value of b is within the above range, the interaction between the nonmetal element M and the alkali metal element is weakened, and the ionic conductivity is more likely to improve. It is thought that when the lower limit of the value of b is within the above range, the bond between the nonmetal element M and O in the crystal structure is more likely to form, the interaction between the nonmetal element M and the alkali metal element is weakened, and the ionic conductivity is more likely to improve.

[0023] The above value c may be, for example, 0.01 to 0.20, 0.02 to 0.18, or 0.03 to 0.15. It is thought that if the upper limit of the value of c is within the above range, the interaction between the alkali metal element and O will not become too strong, and the ionic conductivity will be more easily improved. It is thought that if the lower limit of the value of c is within the above range, the interaction between the nonmetal element and O will be strengthened, and the ionic conductivity of the resulting halide-based solid electrolyte will be more easily improved.

[0024] The above value d may be, for example, 0.05 to 0.45, 0.07 to 0.4, or 0.1 to 0.35. It is believed that when the upper limit of the value of d is within the above range, strong bonds between halogen element X and alkali metal ions are less likely to form in the crystal structure, and ionic conductivity is more likely to improve. It is believed that when the lower limit of the value of d is within the above range, oxidation resistance is more likely to improve.

[0025] In this specification, the elemental composition ratios of halide-based solid electrolytes refer to values ​​measured by the following methods: The proportion of alkali metal element A refers to a value determined by inductively coupled plasma atomic emission spectroscopy (ICP), the proportion of halogen element X refers to a value determined by ion chromatography, the proportion of oxygen O refers to a value determined by non-dispersive infrared absorption spectroscopy, and the proportion of non-metallic element M refers to a value determined by inductively coupled plasma atomic emission spectroscopy (ICP) or non-dispersive infrared absorption spectroscopy. These values ​​are used to calculate the above composition ratios. Note that when preparing solid electrolytes oneself, the composition ratios can be calculated from the elemental composition ratios of the raw materials.

[0026] The above solid electrolyte is, for example, Li １３．５ N ３．５ ClO, Li ９ N ２ The compound may have a composition represented by ClO, etc.

[0027] The ionic conductivity of the above halide-based solid electrolyte is, for example, 2.0 × 10⁻⁶. －３ mS / cm or more, 3.0×10 －３ mS / cm or more, 4.0×10 －３ mS / cm or more, 5.0×10 －３ mS / cm or higher, or 6.0 × 10⁻⁶ －３ It can be set to mS / cm or higher.

[0028] In this specification, ionic conductivity refers to a value calculated from the results of AC impedance measurements taken under the conditions of 25°C, a frequency range of 0.1 Hz to 1 MHz, and an applied voltage of 100 mV (vs. open-circuit voltage), and more specifically, it shall be determined by the method described in the embodiments of this application.

[0029] The shape of the above-mentioned halide-based solid electrolyte is not particularly limited, but for example, it may be in the form of a powder.

[0030] The molding density of the above halide-based solid electrolyte is, for example, 1.0 to 5.0 g / cm³. ３ , 1.05-4.9g / cm ３ , 1.1-4.7g / cm ３ , or 1.15 to 4.5 g / cm³３ That's fine.

[0031] In this specification, the molding density of a halide-based solid electrolyte refers to the value calculated by preparing a sample by filling an insulating cylinder with an inner diameter of 10 mm with 100 mg of the halide-based solid electrolyte and applying a pressure of 370 MPa, then measuring the volume of the sample. Specifically, it shall be determined by the method described in the examples of this application.

[0032] One example of a method for producing a halide-based solid electrolyte involves altering a raw material composition containing an alkali metal element, a nonmetal element, and a halogen element by a mechanochemical method. In the above raw material composition, at least one of the alkali metal element, nonmetal element, and halogen element is an oxide. For example, lithium halide is a compound that contains both an alkali metal element and a halogen element.

[0033] Examples of alkali metal element sources include halides, nitrides, and oxides. Examples of compounds containing nonmetallic elements include alkali metal nitrides, phosphoric acid compounds, and nonmetallic element halides. Examples of compounds containing halogen elements include alkali metal halides and nonmetallic element halides.

[0034] In the mechanochemical method, for example, a ball mill can be used to supply mechanical energy such as collision, shearing, and friction between the raw material solids. The ball milling process may be performed dry. Before the ball milling process, the raw material composition may be mixed (pre-mixed) in a mortar and pestle for one minute or more. By performing such pre-mixing, the raw material composition introduced into the container during the ball milling process becomes more uniform, resulting in a more uniform halide-based solid electrolyte obtained by the ball milling process, and thus improving oxidation resistance.

[0035] Zirconia balls can be used for the ball mill. The diameter of the balls may be, for example, 1 to 15 mm, 2 to 10 mm, or 2 to 5 mm.

[0036] The ball filling rate in the container during the ball milling process may be adjusted by the amount of raw material composition filled. The ratio of the mass of the raw material composition to the mass of the balls (100 [total mass of raw material composition] / [total mass of balls]) may be, for example, 5.0% by mass or less, 4.5% by mass or less, 3.0% by mass or less, 2.8% by mass or less, 2.6% by mass or less, or 2.5% by mass or less. When the above ratio is within the above range, the impact of the balls can be supplied more sufficiently to the raw material composition. The ratio of the mass of the raw material composition to the mass of the balls may be, for example, 0.3% by mass or more, 0.5% by mass or more, 1.0% by mass or more, 1.5% by mass or more, or 1.8% by mass or more. When the lower limit of the above ratio is within the above range, the contamination of the halide-based solid electrolyte with impurities due to collisions between balls can be further reduced, the occurrence of crystal defects due to impurity contamination can be suppressed, and oxidation resistance can be further improved. The ratio of the mass of the raw material composition to the mass of the ball may be adjusted within the range described above, for example, 0.3 to 5.0% by mass.

[0037] The rotational speed of the ball mill may be, for example, 200 to 700 rpm, 200 to 500 rpm, or 250 to 350 rpm.

[0038] The processing time using the ball mill may be, for example, 24 hours or more, 24 to 72 hours, or 36 to 60 hours. By having the ball mill processing time within the above range, each raw material solid is more thoroughly mixed, elemental substitution by the mechanochemical method is more sufficient, and the oxidation resistance of the resulting halide-based solid electrolyte can be further improved.

[0039] The above-described method for producing a halide-based solid electrolyte may further include a step of heat-treating the solid obtained by the mechanochemical method at 150 to 500°C (annealing step).

[0040] The heating time in the annealing process may be, for example, 1 to 24 hours, 2 to 20 hours, or 5 to 15 hours.

[0041] The above-mentioned halide-based solid electrolytes have high ionic conductivity and are therefore useful as materials for forming solid electrolyte layers and electrodes in batteries. Furthermore, because the above-mentioned halide-based solid electrolytes can exhibit excellent ionic conductivity, they are also useful as constituent materials for secondary batteries such as lithium-ion secondary batteries.

[0042] One embodiment of an electrode is an electrode comprising a solid electrolyte. The solid electrolyte is composed of a halide-based solid electrolyte according to this disclosure. The electrode comprises an electrode active material (positive electrode active material or negative electrode active material). One embodiment of a secondary battery comprises the above electrode. Another embodiment of a secondary battery comprises a solid electrolyte including a halide-based solid electrolyte according to this disclosure. The following description will use a lithium-ion secondary battery as an example.

[0043] One embodiment of a lithium-ion secondary battery includes the above-mentioned electrodes. The electrodes may be either a positive electrode or a negative electrode.

[0044] Examples of positive electrode active materials include lithium-containing composite metal oxides comprising lithium (Li) and at least one transition metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, and Cu. Examples of such lithium composite metal oxides include LiCoO ２ LiNiO ２ LiMn ２ O ４ Li ２ MnO ３ LiNi ｘ Mn ｙ Co １－ｘ－ｙ O ２ [0<x+y<1], LiNi ｘ Co ｙ Al １－ｘ－ｙ O ２ [0<x+y<1], LiCr ０．５ Mn ０．５ O ２ LiFePO ４ Li ２ FeP ２ O ７ LiMnPO ４ LiFeBO ３ Li ３ V２ (PO ４ ) ３ Li ２ CuO ２ Li ２ FeSiO ４ , and Li ２ MnSiO ４ These are some examples.

[0045] Examples of negative electrode active materials include metals such as Li, Si, Sn, Si-Mn, Si-Co, Si-Ni, In, and Au, as well as alloys containing these metals, carbon materials such as graphite, and materials in which lithium ions are inserted between layers of the carbon material.

[0046] The above electrode may contain other components in addition to the halide-based solid electrolyte and electrode active material. Examples of other components include solid electrolyte compounds other than the halide-based solid electrolyte, conductive additives, and binders.

[0047] The conductive additive may be, for example, a carbon material. Examples of carbon materials include graphene, graphite, carbon black, fullerene, carbon nanotubes, and carbon fibers. Examples of graphite include natural graphite (such as flaky graphite) and artificial graphite. Examples of carbon black include acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black. Examples of carbon fibers include carbon fibers such as vapor-grown carbon fibers (VGCF).

[0048] Examples of binders include fluororesins and synthetic rubbers. Fluorine resins are preferably those having a carbon chain as their main chain. The carbon chain may be formed by radical polymerization of a compound having an ethylenically unsaturated group. Examples of fluororesins include poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and polyvinylidene fluoride (PVDF). Examples of synthetic rubbers include SBR (styrene-butadiene rubber).

[0049] The electrode may have a configuration in which a layer of the composition containing the above-mentioned solid electrolyte and electrode active material is provided on the current collector. The material of the current collector is not particularly limited and may be a single metal or an alloy of metals such as Cu, Mg, Ti, Fe, Co, Ni, Zn, Al, Ge, In, Au, Pt, Ag, and Pd.

[0050] Another embodiment of the lithium-ion secondary battery comprises a solid electrolyte including the halide-based solid electrolyte described above. The lithium-ion secondary battery may have a stacked structure comprising a positive electrode layer, a solid electrolyte layer, and a negative electrode layer in that order.

[0051] Although several embodiments have been described above, this disclosure is not limited in any way to the embodiments described above. Furthermore, the descriptions of the embodiments described above are applicable to each other.

[0052] The contents of this disclosure will be described in more detail below with reference to examples and comparative examples. However, this disclosure is not limited to the following examples.

[0053] (Example 1) In an argon atmosphere having a dew point of -70°C or lower (hereinafter referred to as a dry argon atmosphere), LiCl, Li ３ N, and Li ２ O was weighed to the proportions shown in Table 1 to prepare the raw material composition. The raw material composition was mixed for 1 minute using a mortar and pestle in a dry argon atmosphere. 1.6 g of the mixed raw material composition was placed in a zirconia pot, and then 80 g of zirconia balls with a diameter of 4 mm were added to the pot. Subsequently, a solid electrolyte was obtained by ball milling using a planetary ball mill at 300 rpm for 24 hours. The planetary ball mill used was the "PM 400" (product name) manufactured by Verder Scientific Co., Ltd.

[0054] (Example 2) LiCl, Li ３ N, and Li ２ A solid electrolyte was obtained in the same manner as in Example 1, except that O was weighed to the amount shown in Table 1.

[0055] (Comparative Example 1) LiCl and Li ３A solid electrolyte was obtained in the same manner as in Example 1, except that N was weighed to the amount shown in Table 1.

[0056]

[0057] <Evaluation of Solid Electrolyte Properties> For each of the solid electrolytes prepared in the examples and comparative examples, the composition formula, O content, and molding density were evaluated using the method described below. The results are shown in Table 2.

[0058] [Determination of Compositional Formula] The compositional formulas of the solid electrolytes prepared in the examples and comparative examples were determined from the elemental composition ratios in the raw material compositions.

[0059] [Determination of O content] The O content was determined for the solid electrolytes prepared in the examples and comparative examples based on the elemental composition ratios in the raw material compositions.

[0060] [Measurement of Molding Density of Solid Electrolytes] For each of the solid electrolytes prepared in the examples and comparative examples, 100 mg of the solid electrolyte was filled into an insulating cylinder with an inner diameter of 10 mm, and a pressure of 370 MPa was applied to prepare a measurement sample. The volume of the measurement sample was measured, and the density was calculated.

[0061] <Evaluation of Solid Electrolytes> For each of the solid electrolytes prepared in the examples and comparative examples, AC impedance measurements and cyclic voltammetry measurements were performed using the methods described below, and the ionic conductivity and oxidation resistance were evaluated from the results.

[0062] [Evaluation of Ionic Conductivity: AC Impedance Measurement] (Preparation of Evaluation Cell X) The following steps for preparing evaluation cell X were all carried out in a glove box purged with an inert gas. First, 100 mg of the above solid electrolyte material was filled into an insulating cylinder with an inner diameter of 10 mm, and a solid electrolyte layer was formed by applying a pressure of 370 MPa. Next, current collectors made of stainless steel were attached to both sides of the solid electrolyte layer, and lead wires were attached to each current collector to prepare evaluation cell X.

[0063] (AC Impedance Measurement) An impedance measuring device is used to measure under the following conditions: 25°C, frequency range 0.1 Hz to 1 MHz, applied voltage 100 mV (vs. open circuit voltage). The ionic conductivity σ can be calculated using the following formula: σ (mS・cm) －１ )=1000×t(cm) / (R(Ω)×A(cm ２ In the formula, R represents the impedance value, A represents the sample area, and t represents the sample thickness. The results are shown in Table 2.

[0064]

[0065] As shown in Table 2, it was confirmed that the halide-based solid electrolyte of the example that satisfies the requirements of the halide-based solid electrolyte according to this disclosure has superior ionic conductivity compared to the solid electrolyte of the comparative example. In particular, it was confirmed that the ionic conductivity of the halide-based solid electrolyte of the example can be improved by being a compound having oxygen as a constituent element.

[0066] According to this disclosure, a halide-based solid electrolyte with excellent ionic conductivity can be provided.

Claims

1. A halide-type solid electrolyte consisting of alkali metal elements, nonmetal elements, halogen elements, and oxygen.

2. The halide-based solid electrolyte according to claim 1, wherein the nonmetallic element is at least one selected from the group consisting of N, P, Se, and S.

3. The halide-based solid electrolyte according to claim 1 or 2, wherein the halogen element comprises at least one selected from the group consisting of F, Br, Cl, and I.

4. The halide-based solid electrolyte according to claim 1 or 2, wherein the oxygen content is 0.5 mol% or more relative to the total number of atoms contained in the halide-based solid electrolyte.

5. The halide-based solid electrolyte is general formula (1): A ａ M ｂ O ｃ X ｄ A halide-based solid electrolyte according to claim 1 or 2, having a composition represented by the formula, wherein A represents an alkali metal element, M represents a nonmetal element other than O, X represents a halogen element, a is greater than 0 and 0.9 or less, b is greater than 0 and 0.5 or less, c is greater than 0 and 0.25 or less, d is greater than 0 and 0.5 or less, and a + b + c + d = 1.

6. Molding density is 1.0 to 5.0 g / cm³ ３ The halide-based solid electrolyte according to claim 1 or 2.

7. An electrode comprising a solid electrolyte, wherein the solid electrolyte is a halide-based solid electrolyte according to claim 1 or 2.

8. A secondary battery comprising the electrode described in claim 7.

9. A secondary battery comprising a solid electrolyte containing the halide-based solid electrolyte described in claim 1 or 2.

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