Electrolyte additive for all-solid-state lithium-ion batteries, solid electrolyte and all-solid-state lithium-ion battery

Abstract

To provide an electrolyte additive and a solid electrolyte that can be suitably used to produce a solid electrolyte capable of imparting excellent battery capacity and cycle stability to an all-solid lithium ion battery.SOLUTION: An electrolyte additive according to the present invention is added to an electrolyte for an all solid lithium ion battery and contains a layered lithium compound. A solid electrolyte for an all-solid lithium ion battery according to the present invention includes the electrolyte additive, a base material, and an electrolyte. The electrolyte additive according to the present invention can be suitably used to produce a solid electrolyte capable of imparting excellent battery capacity and cycle stability to an all-solid lithium ion battery.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electrolyte additive for an all-solid-state lithium ion battery, a solid electrolyte, and an all-solid-state lithium ion battery. [Background technology]

[0002] All-solid-state lithium-ion batteries (AS-LiBs) are expected to be the most promising next-generation energy storage devices. A polymer gel composite solid electrolyte (SSE) is known as a solid electrolyte for AS-LiBs. The solid electrolyte is made of polyethylene oxide (PEO) as the base material (base polymer) and metal salts (e.g., lithium bis(trifluoromethanesulfonyl)imide, LiTFSI, an imide-based lithium salt) dissolved in the polymer gel composite solid electrolyte (SSE). Because of its flexibility and excellent interfacial properties, this solid electrolyte has been widely studied as a solid electrolyte for all-solid-state lithium-ion batteries.

[0003] For example, Non-Patent Document 1 discloses a three-dimensional LLZAO-PEO / LiTFSI solid electrolyte with high ion conductivity, and it is expected that such a solid electrolyte will improve battery performance. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Chemical Engineering Journal, 2020,394:124993 Summary of the Invention [Problem to be solved by the invention]

[0005] On the other hand, in recent years, there has been a demand for all-solid-state lithium ion batteries that have a large battery capacity and that have stable battery capacity even with an increased number of cycles, and there has also been a demand for the development of solid electrolytes that can provide such all-solid-state lithium ion batteries. Such solid electrolytes will greatly advance the practical application of all-solid-state lithium ion batteries, and can therefore be said to be battery materials with extremely high utility value.

[0006] The present invention has been made in view of the above, and an object of the present invention is to provide an electrolyte additive and a solid electrolyte that can be suitably used for producing a solid electrolyte that can impart excellent battery capacity and cycle stability to an all-solid-state lithium ion battery. [Means for solving the problem]

[0007] As a result of extensive research into achieving the above object, the present inventors have found that the above object can be achieved by using a layered lithium compound as a constituent component, and have thus completed the present invention.

[0008] That is, the present invention includes, for example, the subject matter described in the following sections. Item 1 An electrolyte additive to be added to an electrolyte for an all-solid-state lithium ion battery, An electrolyte additive for all-solid-state lithium-ion batteries containing a layered lithium compound. Section 2 Item 2. The electrolyte additive according to Item 1, wherein the layered lithium compound is at least one selected from the group consisting of Li3InCl6 and Li2MnO3. Section 3 A solid electrolyte for an all-solid-state lithium ion battery, Item 1 or 2. A solid electrolyte for an all-solid-state lithium ion battery, comprising the electrolyte additive according to item 1 or 2, a base material, and an electrolyte. Section 4 Item 4. The solid electrolyte according to item 3, wherein the base material comprises polyethylene oxide. Section 5 Item 3. An all-solid-state lithium ion battery comprising the solid electrolyte according to Item 3 or 4. [Effects of the Invention]

[0009] The electrolyte additive for all-solid-state lithium ion batteries of the present invention can be suitably used as an additive for producing a solid electrolyte that can impart excellent battery capacity and excellent cycle stability to all-solid-state lithium ion batteries. [Brief explanation of the drawings]

[0010] [Figure 1] The performance (charge / discharge capacity and charge / discharge cycle durability) of the batteries including the solid electrolytes obtained in Test Example 1 and Comparative Test Example 1 is shown. [Figure 2] 1 shows the performance (charge / discharge capacity and charge / discharge cycle durability) of the battery provided with the solid electrolyte obtained in Test Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0011]

[0023] In the present specification, the terms "contain" and "comprise" include the concepts of "contain," "comprise," "consist essentially of," and "consist only of."

[0012] 1. Electrolyte additives The electrolyte additive for all-solid-state lithium-ion batteries (hereinafter referred to as "electrolyte additive") of the present invention is an electrolyte additive to be added to an electrolyte for an all-solid-state lithium-ion battery, and contains a layered lithium compound. Such an electrolyte additive can be suitably used as an additive for producing a solid electrolyte that can impart excellent battery capacity and cycle stability to an all-solid-state lithium-ion battery. Excellent cycle stability means, for example, that the battery performance is not likely to deteriorate even after repeated charge and discharge, and in particular, that the capacity is not likely to deteriorate even after repeated charge and discharge.

[0013] The electrolyte additive of the present invention can contain a layered lithium compound as a main component. For example, the electrolyte additive of the present invention can contain 50% by mass or more of the layered lithium compound, preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more. The electrolyte additive of the present invention may be only the layered lithium compound.

[0014] The layered lithium compound is a lithium compound formed into a layer, and examples thereof include lithium compounds formed into a sheet, film, or flake shape.Whether the layered lithium compound is formed into a layer can be confirmed, for example, by observing the layered lithium compound with an electron microscope or the like.The layered lithium compound is preferably in the form of a nanosheet.

[0015] The lithium compound for forming the layered lithium compound can include various compounds containing lithium, such as an oxide containing lithium, a salt containing lithium, etc. The oxide containing lithium may be a so-called composite oxide containing other metals in addition to lithium, and the salt containing lithium may also contain other metals in addition to lithium.

[0016] When the layered lithium compound is an oxide containing lithium, specific examples thereof include Li2MnO3, Li2MoO3, Li2HfO3, Li2ZrO3, Li2ScO3, LiCoO2, LiFeO2, LiNiO2, LiTiO2, LiVO2, and LiScO2. When the layered lithium compound is a salt containing lithium, specific examples thereof include halogen compounds containing lithium, such as Li3GaCl6, Li3GaBr6, Li3GaI6, Li3AlCl6, Li3AlBr6, and Li3AlI6.

[0017] The layered lithium compound is preferably at least one selected from the group consisting of lithium-containing oxides and lithium-containing halides, and more preferably Li2MnO3. In this case, the electrolyte additive of the present invention is more likely to improve the battery capacity and cycle stability of the all-solid-state lithium-ion battery.

[0018] The layered lithium compound may be in the form of, for example, a powder, a lump, a granule, or a pellet.

[0019] The method for producing the layered lithium compound is not particularly limited, and the layered lithium compound can be produced by, for example, a known method. Alternatively, the layered lithium compound can be obtained from a commercial product or the like.

[0020] The layered lithium compound can be produced, for example, by solution combustion reaction (SCR). In this solution combustion reaction, specifically, an aqueous solution containing a metal-containing oxidizer and a fuel is heated to cause autoignition, thereby producing the layered lithium compound. When the layered lithium compound is an oxide containing lithium, it is particularly suitable to use the solution combustion reaction. The product obtained by the solution combustion reaction is, for example, an oxide of the metal contained in the oxidizer.

[0021] The oxidizing agent used in the solution combustion reaction is a compound containing at least Li. Examples of compounds containing Li that can be used include a wide variety of lithium salts, such as chlorides, halides, inorganic acid salts, and organic acid salts. Examples of inorganic acid salts of lithium include one or more salts selected from the group consisting of lithium nitrates, sulfates, hydrochlorides, chlorates, perchlorates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates. Among these, lithium nitrates are preferred. Examples of organic acid salts of lithium include one or more salts selected from the group consisting of lithium acetates, oxalates, formates, and succinates.

[0022] The oxidizing agent used in the solution combustion reaction may be a compound containing at least Li, or a compound containing a metal other than Li. A wide variety of compounds containing metals other than Li can be used, including chlorides, halides, inorganic acid salts, and organic acid salts. Examples of inorganic acid salts of metals other than Li include at least one selected from the group consisting of nitrates, sulfates, hydrochlorides, chlorates, perchlorates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates. Among these, nitrates are preferred.

[0023] Examples of the metals other than Li include Mn, Mo, Hf, Zr, Sc, Co, Fe, Ni, Ti, and V, and among these, Mn is preferred.

[0024] The oxidizing agent is preferably a mixture of a nitrate of Li and a nitrate other than Li, and particularly preferably a mixture of a nitrate of Li and a nitrate of Mn. In the mixture of a nitrate of Li and a nitrate other than Li, the ratio of the nitrate of Li to the nitrate other than Li is, for example, 1:10 to 10:1, preferably 3:7 to 7:3.

[0025] In the solution combustion reaction, the fuel may be, for example, a known fuel that can be used in a known solution combustion reaction, such as a sugar, preferably sucrose.

[0026] In the solution combustion reaction, the amount of fuel used can be, for example, 10 to 300 mass % relative to the oxidant, preferably 20 to 200 mass %, more preferably 30 to 150 mass %, and even more preferably 40 to 120 mass %.

[0027] The concentration of the aqueous solution used in the solution combustion reaction is not particularly limited, and may be the same as that used in known solution combustion reactions, for example.

[0028] In the solution combustion reaction, the temperature at which self-ignition occurs is not particularly limited and can be appropriately set depending on the properties of the oxidizing agent used, etc. For example, the temperature is 100°C or higher, preferably 200°C or higher, and 1000°C or lower, preferably 800°C or lower.

[0029] Since the product obtained by the self-ignition of the solution combustion reaction is, for example, amorphous, it is preferable to further calcinate this product, which will promote crystallization and allow a sufficiently crystallized layered lithium compound to be obtained.

[0030] The firing method is not particularly limited, and can be performed, for example, under conditions of 200 to 800°C, preferably 300 to 600°C. The firing time is not particularly limited, and can be set within an appropriate range depending on the firing temperature. The firing can be performed, for example, in air or in an inert gas atmosphere.

[0031] By the above-mentioned firing, a lithium compound having a layered structure is obtained, such as the above-mentioned Li2MnO3.

[0032] When the layered lithium compound is a salt containing lithium, various production methods can be adopted in addition to the above-mentioned solution combustion reaction, and for example, known methods for producing metal halides can be widely applied. By such methods, a lithium-containing halide compound can be obtained as a layered lithium compound.

[0033] The electrolyte additive of the present invention can be used as an additive to electrolytes used for all-solid-state lithium ion batteries, and can be particularly suitably used as an additive to electrolytes for all-solid-state lithium secondary batteries.

[0034] 2. Solid electrolytes for all-solid-state lithium-ion batteries The solid electrolyte of the present invention includes the above-described electrolyte additive, a base material, and an electrolyte. The type of base material is not particularly limited. For example, a wide range of base materials for forming known solid electrolytes for all-solid-state lithium-ion batteries can be used.

[0035] The base material of the solid electrolyte is, for example, Li 10 GeP2S 12 , xLi2S-(1-x)P2S5(0.6≦x≦0.85) and Na 11 Sn2PS 12 Sulfide-based electrolytes such as Na3PSe4; Li 3x La 2 / 3-x Oxide electrolytes such as TiO3 (0≦x≦0.16); Li 1+x Al x Ti 2-x (PO4)3(0≦x≦0.5)(LATP);Li x La3M2O 12 (3≦x≦7.5, M=Ta,Nb,Zr);Na3Zr2Si2PO 12 polymer-based electrolytes; etc. Examples of polymer-based electrolytes include polyethylene oxide (PEO) and PVDF (polyvinylidene fluoride).

[0036] From the viewpoint of making it easier for the electrolyte additive to exert its effects, the base material is preferably a polymer, and in particular, it is preferable to include a polymer such as polyethylene oxide or polyvinylidene fluoride, and it is particularly preferable to include polyethylene oxide.

[0037] The electrolyte additive is less likely to form aggregates in the base material and is more easily dispersed uniformly, even in polymers such as polyethylene oxide. This makes it easier for the electrolyte additive to exert its effects. When the base material is polyethylene oxide, the electrolyte additive inhibits the crystallization of the polyethylene oxide, which is thought to make it easier for the electrolyte additive to exert its effects.

[0038] The type of electrolyte contained in the solid electrolyte is not particularly limited, and examples thereof include known electrolytes used in solid electrolytes for all-solid-state lithium-ion batteries. Examples of the electrolyte include LiPF6, LiClO4, LiTFSI, NaClO4, and NaBF4. Other examples of the solid electrolyte include hybrid electrolytes obtained by mixing a known inorganic electrolyte with the electrolyte. In addition to the electrolyte additive, base material, and other electrolytes, the solid electrolyte may further include various additives used in solid electrolytes for all-solid-state lithium-ion batteries.

[0039] The content of the electrolyte additive of the present invention contained in the solid electrolyte is not particularly limited. For example, the electrolyte additive can be 0.1 to 60 mass% based on the total mass of the base material, the additives added as needed, and the electrolyte additive. In this case, when the solid electrolyte is applied to an all-solid-state lithium-ion battery, the electrochemical stability of the all-solid-state lithium-ion secondary battery can be further improved, and the charge / discharge capacity and charge / discharge cycle durability are also likely to be improved. The electrolyte additive is preferably 0.2 to 30 mass%, more preferably 0.5 to 20 mass%, and particularly preferably 1 to 10 mass%, based on the total mass of the base material, electrolyte, and electrolyte additive.

[0040] The content of the electrolyte contained in the solid electrolyte is not particularly limited, and may be, for example, the same as the amount of electrolyte added in known solid electrolytes. For example, the content of the electrolyte relative to the base material may be 1% by mass or more, preferably 3% by mass or more, more preferably 5% by mass or more, and particularly preferably 10% by mass or more, and the content of the electrolyte relative to the base material may be 50% by mass or less, preferably 40% by mass or less, more preferably 30% by mass or less, and particularly preferably 20% by mass or less.

[0041] When the solid electrolyte contains other additives, the content thereof can be, for example, 5 mass % or less, preferably 1 mass % or less, more preferably 0.1 mass % or less, and particularly preferably 0.05 mass % or less, relative to the total mass of the solid electrolyte.

[0042] The method for producing the solid electrolyte is not particularly limited, and the solid electrolyte of the present invention can be produced, for example, by a method similar to a known method for producing a solid electrolyte.

[0043] For example, a base material, the electrolyte additive of the present invention, and an electrolyte are mixed in predetermined amounts to prepare an electrolyte raw material, and the electrolyte raw material is then formed into a membrane, thereby producing a solid electrolyte. The electrolyte raw material may be prepared by either a dry method or a wet method. In the dry method, the electrolyte raw material is obtained, for example, in a powder form, and in the wet method, the electrolyte raw material is obtained, for example, in a paste form. When the electrolyte raw material is obtained in a paste form, it can be subjected to an appropriate drying process.

[0044] The method for forming a membrane from the electrolyte raw material is not particularly limited, and the membrane can be formed by a known method such as a pressing method.

[0045] The thickness of the solid electrolyte is not particularly limited, and may be the same as that of the solid electrolyte of a known all-solid-state lithium ion secondary battery, for example, 30 to 150 μm.

[0046] 3. All-solid-state lithium-ion batteries The all-solid-state lithium-ion battery of the present invention comprises the above-described solid electrolyte of the present invention. That is, the all-solid-state lithium-ion battery of the present invention comprises a solid electrolyte containing the electrolyte additive as a constituent element. The all-solid-state lithium-ion battery of the present invention is not particularly limited in other configuration as long as the solid electrolyte contains the electrolyte additive of the present invention, and may have, for example, a configuration similar to that of a known battery. For example, the all-solid-state lithium-ion battery may comprise a cathode, an anode, and a separator. The size and shape of the battery can be determined appropriately depending on the application of the battery.

[0047] The cathode can have a structure in which an active material is supported on a metal foil, for example. Examples of the metal foil include aluminum, titanium, platinum, molybdenum, stainless steel, and copper. The metal foil can be in the form of a porous body, foil, plate, or mesh made of fiber. A wide range of known active materials can be used as the active material for the cathode, including, for example, LiFePO4, LiCoO2, and LiNi. x Mn y Co z O2 (0.3≦x≦0.95, 0.025≦y≦0.4, 0.025≦y≦0.4), LiNi 1-y-z Co y Al z O2(0.05≦y≦0.15, 0 <z≦0.05)、LiMn2O4、LiMPO4(M=Co、Ni)、Li2FePO4F、V2O5、Li X V3O8(1.5≦x≦5.5), Li 1-X VOPO4(0.5≦x≦0.92), Li4Ti5O 12 , LiFeMO4 (M=Mn, Si), S, Se, SeS2, Na3V2(PO4)3, Na2MnP2O7, NaFePO4, Na3MnZr(PO4)3, etc.

[0048] The cathode may also contain a conductive additive, a binder, etc. The conductive additive may be a known material used in all-solid-state lithium-ion batteries, such as conductive carbon black, acetylene black, graphite, vapor-grown carbon fiber, or carbon nanotubes. The binder may be a known material used in all-solid-state lithium-ion batteries, such as polyvinylidene fluoride (PVDF).

[0049] The anode may have a structure in which an active material is supported on a metal foil. Examples of the metal foil include aluminum, titanium, platinum, molybdenum, stainless steel, and copper. The metal foil may be in the form of a porous body, foil, plate, or fibrous mesh. Examples of the active material for the anode include metals such as Li, Na, K, Mg, Al, and Zn; graphite and other carbon materials; Si(C), Si(O), or Sn-based alloys or metal oxides; and Li4Ti5O. 12 ; etc. can be mentioned.

[0050] As the separator, a known separator applied to all-solid-state lithium ion batteries can be used, and examples thereof include materials such as polyolefin resins such as polyethylene and polypropylene; polyimide; polyvinyl alcohol; fluororesins such as terminally aminated polyethylene oxide polytetrafluoroethylene; acrylic resins; nylon; aromatic aramid; inorganic glass; and ceramics, and materials in the form of porous membranes, nonwoven fabrics, woven fabrics, etc. can be used.

[0051] The method for assembling the secondary battery is not particularly limited, and the secondary battery can be obtained by the same method as that for assembling a known secondary battery. [Example]

[0052] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0053] Example 1 Li2MnO3 was synthesized by solution combustion reaction (SCR). First, an aqueous solution containing lithium nitrate (LiNO3) and manganese nitrate (Mn(NO3)2) as oxidants and sucrose as fuel was prepared. The lithium nitrate and manganese nitrate molar ratio (LiNO3:Mn(NO3)2) in this solution was 2:1, and the mass ratio of the oxidant to the fuel was 1:1. The aqueous solution was heated to 100°C, slowly evaporating the water to obtain a syrup-like reaction product. Further heating led to the formation of bubbles at 200°C, and the reaction product self-ignited upon further heating. This self-ignition yielded an amorphous product. The amorphous product was then crushed in a mortar with a pestle, and the crushed product was calcined in an aluminum crucible at 400°C in air for 1 hour. This yielded fully crystallized Li2MnO3. The formation of Li2MnO3 was confirmed by X-ray diffraction spectroscopy.

[0054] (Test Example 1) A mixture of polyethylene oxide (PEO) and lithium bis(trifluoromethanesulfonyl)imide (LiFTSI) in a mass ratio of 8:1 (PEO:LiFTSI) was prepared in advance. To this mixture, Li2MnO3 synthesized in Example 1 as an electrolyte additive was added so that the Li2MnO3 content was 7.5 mass%. The mixture was mixed in a mortar until it became a block. The resulting block was then kept in a vacuum oven at 80°C for 12 hours, and then pressurized in a hydraulic press at 40 MPa for 1 hour to obtain a film-like solid electrolyte. This was then cut into circular pieces with a diameter of 16 mm.

[0055] Using this solid electrolyte, a 2025-inch coin cell (all-solid-state lithium-ion battery) was assembled under an argon atmosphere. The cathode of the coin cell was prepared by weighing commercially available LiFePO4 as the positive electrode active material, Super P as a conductive additive, and PVDF as a binder in an 8:1:1 mass ratio, adding an appropriate amount of N-methyl-2-pyrrolidone (NMP), and kneading the mixture at room temperature for 12 hours. The resulting slurry was applied to an aluminum current collector and dried in a vacuum oven at 120°C for 12 hours. The anode of the coin cell was a 0.1 mm-thick lithium metal. Both the cathode and anode were cut into 12 mm-diameter circular pieces. The constant-current charge / discharge characteristics of the assembled coin cell were evaluated using a battery test system (LAND CT2001A model, LANHE Electronics Ltd.). The measurement conditions were 60°C, a voltage range of 2.5 to 4.0 V, and a current density of 0.2 C.

[0056] (Test Example 2) A coin cell was assembled in the same manner as in Test Example 1, except that LiFePO4 was changed to LiNi0.8Co0.1Mn0.1O2 in the cathode of the coin cell, and the constant current charge / discharge characteristics were evaluated.

[0057] (Comparative Test Example 1) A coin cell was assembled in the same manner as in Test Example 1 except that no electrolyte additive was used, and the constant current charge / discharge characteristics were evaluated.

[0058] Fig. 1 shows the performance (charge / discharge capacity and charge / discharge cycle durability) of the battery including the solid electrolyte obtained in Test Example 1 and Comparative Test Example 1. Fig. 2 shows the performance (charge / discharge capacity and charge / discharge cycle durability) of the battery including the solid electrolyte obtained in Test Example 2.

[0059] The results in Figures 1 and 2 show that the coin cells (Test Examples 1 and 2) with a solid electrolyte containing an electrolyte additive (Li2MnO3) had higher battery capacity than the coin cell (Comparative Test Example 1) with a solid electrolyte not containing an electrolyte additive, and furthermore, no rapid decrease in capacity was observed even with an increase in the number of cycles, and the coulombic efficiency was also stable.

[0060] [Table 1]

[0061] Table 1 shows various performance characteristics of coin cells with solid electrolytes containing the electrolyte additive (Li2MnO3). For comparison, it also shows various performance characteristics of coin cells formed using conventional electrolyte additives (the electrolyte additives described in Ref. 1 and Ref. 2), which are existing materials. Table 1 shows that the electrolyte additive (Li2MnO3) has higher lithium ion conductivity than existing materials, and its conductivity is comparable to that of existing materials.

Claims

1. An electrolyte additive to be added to an electrolyte for an all-solid-state lithium ion battery, Contains a layered lithium compound, The electrolyte additive for an all-solid-state lithium ion battery, wherein the layered lithium compound is Li 2 MnO 3 .

2. A solid electrolyte for an all-solid-state lithium ion battery, A solid electrolyte for an all-solid-state lithium ion battery, comprising the electrolyte additive according to claim 1, a base material, and an electrolyte.

3. The solid electrolyte of claim 2 wherein the base material comprises polyethylene oxide.

4. An all-solid-state lithium ion battery comprising the solid electrolyte according to claim 3.

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