Composite solid electrolyte and solid-state battery

Abstract

To provide a composite solid electrolyte that suppresses increase in battery resistance, and a solid-state battery.SOLUTION: The present invention provides a composite solid electrolyte including: a sulfide solid electrolyte; and a polymer electrolyte including a lithium imide salt and an alkyl side chain with 4 or more carbon atoms. The present invention also provides a solid-state battery.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to composite solid electrolytes and solid-state batteries. [Background technology]

[0002] In recent years, secondary batteries such as lithium-ion secondary batteries have been suitably used as portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles such as electric vehicles (EVs), hybrid vehicles (HVs), and plug-in hybrid vehicles (PHVs).

[0003] Secondary batteries use electrolytes containing flammable organic solvents, so they require the installation of safety devices to suppress temperature rises in the event of a short circuit, as well as improvements to the structure and materials to prevent short circuits.In contrast, solid-state batteries, which replace the electrolyte with a solid electrolyte layer and solidify the material, do not use flammable organic solvents inside the battery, so safety devices can be simplified and they are thought to be superior in terms of manufacturing costs and productivity. In the field of solid state batteries, a method is known in which the resistance of a solid state battery is kept low by using a composite solid electrolyte containing a sulfide solid electrolyte and a polymer electrolyte (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2013 / 001623 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in conventional composite solid electrolytes, during storage, the sulfide solid electrolyte and the polymer contained in the polymer electrolyte react with each other at the contact surface, generating reaction products that inhibit ion conduction paths and tend to increase battery resistance. This phenomenon becomes more pronounced when stored at high temperatures for long periods (e.g., 60°C for 3 days). Patent Document 1 attempts to suppress the increase in resistance by combining a sulfide solid electrolyte that is substantially free of bridging sulfur with a solid electrolyte that contains a branched polymer. As described above, various studies have been conducted to suppress the increase in resistance when using the composite solid electrolyte layer, but further technological development is desired. In view of the above circumstances, the present disclosure aims to provide a composite solid electrolyte and a solid battery that suppress the increase in battery resistance. [Means for solving the problem]

[0006] The means for solving the above problems include the following means. <1> a sulfide solid electrolyte; a polymer electrolyte containing a lithium imide salt and a polymer having an alkyl side chain having 4 or more carbon atoms; A composite solid electrolyte comprising: <2> The polymer has an alkyl side chain having 6 to 8 carbon atoms. <1> The composite solid electrolyte according to claim 1. <3> the molar ratio of the polymer to the lithium imide salt (polymer / lithium imide salt) is 4 or more and 7 or less; <1> or <2> The composite solid electrolyte according to claim 1. <4> The polymer contains a compound represented by the following general formula (1): <1> ~ <3> 10. The composite solid electrolyte according to claim 9, wherein the first and second electrodes are electrically connected to each other. [ka] (In the above general formula (1), m represents an integer of 4 or more, and n represents an integer of 1 or more.) <5> The aforementioned <1> ~ <4> 10. A solid-state battery comprising the composite solid electrolyte according to any one of claims 1 to 9. [Effects of the Invention]

[0007] According to the present disclosure, a composite solid electrolyte and a solid-state battery in which an increase in battery resistance is suppressed are provided. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, an embodiment of the present disclosure will be described. These descriptions and examples are intended to illustrate the embodiment and are not intended to limit the scope of the invention.

[0009] <Composite solid electrolyte> The composite solid electrolyte according to the present disclosure is a composite solid electrolyte containing a sulfide solid electrolyte and a polymer electrolyte containing a lithium imide salt and a polymer having an alkyl side chain having four or more carbon atoms. According to the present disclosure, the polymer contained in the polymer electrolyte has an alkyl side chain having four or more carbon atoms. Therefore, the alkyl side chain of this polymer suppresses the interaction between the sulfide solid electrolyte and the main chain of the polymer in the polymer electrolyte. As a result, even during storage (e.g., at 60°C for 3 days), the reaction between the sulfide solid electrolyte and the polymer at the contact surface between the sulfide solid electrolyte and the polymer and the generation of reaction products is suppressed. As a result, the ion conduction path is not blocked by the reaction products, and the resistance is maintained low. In other words, an increase in resistance is suppressed.

[0010] [Polymer electrolyte] The polymer electrolyte includes a polymer and a lithium imide salt. The polymer is not particularly limited as long as it has an alkyl side chain having 4 or more carbon atoms and is capable of dissociating a lithium imide salt. The polymer electrolyte may be used alone or in combination of two or more types. The polymer is preferably a polymer having a polar group in the main chain, and examples thereof include nitrile polymers such as polyacrylonitrile, ether polymers, ester polymers, carbonate polymers such as polycarbonate, amide polymers, sugar chains, etc. Among the above, the type of polymer preferably includes an ether polymer from the viewpoint of further suppressing an increase in resistance.

[0011] The term "nitrile polymer" refers to a polymer containing a structural unit derived from a nitrile group. The term "ether polymer" refers to a polymer containing a structural unit having an ether bond. The term "ester polymer" refers to a polymer containing a structural unit derived from an ester group. The term "carbonate polymer" refers to a polymer containing a structural unit derived from a carbonate group. The term "amide polymer" refers to a polymer containing a structural unit derived from an amide group.

[0012] The polymer has an alkyl side chain having 4 or more carbon atoms, and from the viewpoint of further suppressing an increase in resistance, the alkyl side chain preferably has 4 or more and 12 or less carbon atoms, more preferably 5 or more and 10 or less carbon atoms, and even more preferably has 6 or more and 8 or less carbon atoms.

[0013] The polymer preferably contains a compound represented by the following general formula: When the polymer contains a compound represented by the following general formula (1), it is more likely to suppress an increase in resistance due to the generation of a reaction product between the polymer and the sulfide solid electrolyte at the contact surface between the two in the polymer electrolyte, which inhibits the ion conduction path. [ka] In the general formula (1), n ​​represents an integer of 1 or more. In general formula (1), m represents an integer of 4 or more, and from the viewpoint of further suppressing an increase in resistance, m is preferably 4 or more and 12 or less, more preferably 5 or more and 10 or less, and even more preferably 6 or more and 8 or less.

[0014] Examples of compounds represented by general formula (1) are shown below, but the present disclosure is not limited thereto.

[0015] [ka]

[0016] The weight average molecular weight (Mw) of the polymer in terms of polystyrene determined by gel permeation chromatography (GPC) is not particularly limited, and may be, for example, 10,000 to 300,000, or 15,000 to 250,000.

[0017] The molar ratio of the polymer to the lithium imide salt (polymer / lithium imide salt) is preferably 2 or more, more preferably 3 or more and 8 or less, and more preferably 4 or more and 7 or less, from the viewpoint of further suppressing an increase in resistance.

[0018] Examples of lithium imide salts include LiN(Rf1SO2)2, LiN(FSO2)2, LiN(Rf1SO2)(Rf2SO2), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Among these, from the viewpoint of battery performance, it is preferable that the lithium imide salt contains LiTFSI. The lithium imide salt may be used alone or in combination of two or more.

[0019] [Sulfide solid electrolyte] The sulfide solid electrolyte preferably contains sulfur (S) as the main anion element, and more preferably contains, for example, Li element, A element, and S element. The element A is at least one selected from the group consisting of P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In. The sulfide solid electrolyte may further contain at least one of O and a halogen element. Examples of the halogen element (X) include F, Cl, Br, I, etc. The composition of the sulfide solid electrolyte is not particularly limited, and examples include xLi2S·(100 - x)P2S5 (70 ≤ x ≤ 80), yLiI·zLiBr·(100 - y - z)(xLi2S·(1 - x)P2S5) (0.7 ≤ x ≤ 0.8, 0 ≤ y ≤ 30, 0 ≤ z ≤ 30). The sulfide solid electrolyte may have a composition represented by the following general formula (1). Li 4-x Ge 1-x P x S4(0 < x < 1) ··· Formula (1) In formula (1), at least a part of Ge may be substituted with at least one selected from the group consisting of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. Also, at least a part of P may be substituted with at least one selected from the group consisting of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. A part of Li may be substituted with at least one selected from the group consisting of Na, K, Mg, Ca, and Zn. A part of S may be substituted with a halogen. The halogen is at least one of F, Cl, Br, and I. The sulfide solid electrolyte may be used alone or in combination of two or more.

[0020] From the perspective of battery performance, for example, the sulfide solid electrolyte is preferably a Li2S - P2S5 - based sulfide solid electrolyte. When it is a Li2S - P2S5 - based sulfide solid electrolyte, reaction products of the polymer and the sulfide solid electrolyte are generated at the contact surface between the polymer and the sulfide solid electrolyte in the polymer electrolyte, which inhibits the ion conduction path and easily increases the resistance. However, with the configuration of the composite solid electrolyte of the present disclosure, an increase in resistance is also suppressed in this case.

[0021] 〔Other components〕 The composite solid electrolyte according to the present disclosure may further contain other components in addition to the sulfide solid electrolyte, polymer, and polymer electrolyte containing lithium imide salt, as needed, within the scope of the effects of the present disclosure. Examples of the other components include oxide solid electrolytes, halide solid electrolytes, binders (e.g., rubber-based binders, fluoride-based binders, etc.), and conductive additives (e.g., fibrous carbon materials, etc.).

[0022] [Method for producing composite solid electrolyte] The method for producing the composite solid electrolyte of the present disclosure is not particularly limited, and known methods for producing solid electrolytes can be applied. Examples of the method for producing the composite solid electrolyte of the present disclosure include a method in which a polymer electrolyte and a solid electrolyte material are mixed in a solvent to prepare a slurry, the slurry is applied to a substrate, and the solvent is then dried to form a composite solid electrolyte layer; and a method in which a polymer electrolyte layer and a sulfide solid electrolyte layer that have been separately prepared are joined to form a composite solid electrolyte layer.

[0023] <Solid battery> The solid-state battery of the present disclosure includes the composite solid electrolyte of the present disclosure. The solid-state battery of the present disclosure includes, for example, a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer. The solid electrolyte layer includes the composite solid electrolyte of the present disclosure. The composite solid electrolyte may contain an electrolytic solution in an amount of less than 10 mass% of the total amount of the electrolyte. The composite solid electrolyte of the present disclosure, even in a solid-state battery including an electrode (e.g., a negative electrode, more specifically, a negative electrode containing silicon) containing an active material that easily expands and contracts, prevents an increase in resistance caused by a reaction between the sulfide solid electrolyte and the polymer in the polymer electrolyte due to friction in the solid electrolyte layer caused by the expansion and contraction of the electrode before and after storage.

[0024] The solid-state battery of the present disclosure is a solid-state lithium-ion secondary battery. Examples of solid-state batteries include power sources for vehicles, electronic devices, and electricity storage. Examples of vehicles include electric four-wheeled vehicles, electric two-wheeled vehicles, gasoline-powered automobiles, and diesel-powered automobiles. Examples of electric four-wheeled vehicles include battery-electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), and hybrid electric vehicles (BEVs). Examples of electric two-wheeled vehicles include electric motorcycles and electrically assisted bicycles. Examples of electronic devices include handheld devices (e.g., smartphones, tablet computers, audio players, etc.), portable devices (e.g., notebook computers, CD (Compact Disc) players, and mobile devices (e.g., power tools, commercial video cameras, etc.). Among these, the solid-state battery of the present disclosure is preferably used as a power source for driving hybrid vehicles, plug-in hybrid vehicles, or electric vehicles. [Example]

[0025] Example 1 (Preparation of sulfide solid electrolyte) Li2S-P2S5 glass-ceramics containing LiI were prepared by ball mill mixing and sintering.

[0026] (Preparation of polymer electrolyte solution) Oxetane derivatives bearing a four-carbon alkyl side chain were synthesized by the Williamson ether reaction of 3-ethyl-3-hydroxymethyloxetane with a four-carbon alkyl compound modified with a leaving group using a base. The oxetane derivative was used as a monomer and subjected to cationic ring-opening polymerization using a boron trifluoride diethyl ether complex as a catalyst to obtain a polymer having an alkyl side chain with four carbon atoms (polyoxetane resin: a compound represented by general formula (1), compound m4). The weight-average molecular weight obtained by the above-mentioned measurement method was 47.5 kg / mol. The obtained polymer and LiTFSI were dissolved in acetonitrile to prepare a polymer electrolyte solution with a polymer to LiTFSI molar ratio (polymer:LiTFSI) of 5:1.

[0027] (Fabrication of polymer electrolyte membrane) The polymer electrolyte solution was cast onto an Al foil and coated onto the foil using an applicator by the blade method. The coated electrode was dried on a hot plate at 130°C for 1 hour, and then dried in a vacuum atmosphere at 130°C for 6 hours. This resulted in a polymer electrolyte membrane.

[0028] (Preparation of composite solid electrolyte) A composite solid electrolyte membrane was obtained by laminating a sulfide solid electrolyte membrane and a polymer electrolyte membrane.

[0029] <Examples 2 and 3, Comparative Examples 1 and 2> In preparing the polymer electrolyte solution, the alkyl compound having 4 carbon atoms modified with the leaving group in Example 1 was used, and an alkyl compound having 1, 2, 6, or 8 carbon atoms modified with the leaving group was used, to obtain polymer electrolytes (polyoxetane resins: compounds represented by general formula (1), including compounds m6 and m8 and compounds m1 and m2 shown below) having alkyl side chains with the carbon numbers shown in Table 1. Then, composite solid electrolytes of each example were obtained with the same specifications as in Example 1. The weight-average molecular weight values ​​of each polymer electrolyte were as follows: m1: 147.0 kg / mol m2: 48.4 kg / mol m6: 172.7 kg / mol m8: 135.1 kg / mol

[0030] [ka]

[0031] <Evaluation of conductivity retention rate before and after high-temperature storage> (Calculation of resistance value before high temperature storage) In a glove box with a dew point of -80°C, the composite solid electrolyte membrane of each example was punched out to a diameter of 11.28 mm, mounted on aluminum tabs at both ends, and vacuum-laminated to obtain evaluation cells. Each evaluation cell was constrained at a confining pressure of 1 MPa and soaked at 60°C for 3 hours. After soaking, the resistance was determined using the AC impedance method. Measurements were performed using a Solartron 1260 with an applied voltage of 10 mV, a measurement frequency range of 0.1 Hz to 1 MHz, and a measurement temperature of 25°C. Curve fitting was performed on the arc component of the obtained impedance spectrum, and the high-resistance side of the intersection with the real axis was taken as the reaction resistance.

[0032] (Calculation of resistance value after high temperature storage) The evaluation cell for each example was left stationary in a glove box at 60°C for 3 days as a storage test. After the storage test, the resistance value of the evaluation cell for each example was determined using the method described above. The resistance value before storage, with the resistance value after storage used as the reference, is shown in Table 1 as the relative resistance value. Table 1 also shows the resistance increase rate (%) calculated from the obtained relative resistance value = (relative resistance value after storage - relative resistance value before storage) / relative resistance value before storage × 100.

[0033] [Table 1]

[0034] As shown in Table 1, it was found that the composite solid electrolytes of the examples had a more suppressed increase in resistance than the composite solid electrolytes of the comparative examples.

Claims

1. a sulfide solid electrolyte; a polymer electrolyte containing a lithium imide salt and a polymer having an alkyl side chain having 4 or more carbon atoms; Contains The polymer is an ether-based polymer in which the alkyl side chains are linked via ether bonds, has a weight average molecular weight (Mw) of 47.5 kg / mol or more in terms of polystyrene as determined by gel permeation chromatography (GPC), has 6 or more and 8 or less carbon atoms in the alkyl side chains, and is a compound represented by the following general formula (1): Composite solid electrolyte. 【Chemistry 1】 (In the above general formula (1), m represents an integer of 6 or more and 8 or less, and n represents an integer of 1 or more.)

2. 2. The composite solid electrolyte according to claim 1, wherein a molar ratio of the polymer to the lithium imide salt (polymer / lithium imide salt) is 4 or more and 7 or less.

3. A solid-state battery comprising the composite solid electrolyte according to claim 1 or 2.

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