JP7474985B2 - Solid electrolyte for lithium ion battery and method for producing same, lithium ion battery - Google Patents

Description

The present invention relates to a solid electrolyte for lithium ion batteries, a method for producing the same, and a lithium ion battery.

Lithium-ion batteries are used in a wide range of applications, such as electric vehicles, and are now indispensable in the industrial world. Current lithium-ion batteries, for example, have a metal anode made of a material with a low mass density (e.g., 0.59 g·cm ⁻³ ) and a high electrochemical potential (e.g., −3.04 V vs. the standard hydrogen electrode). This allows the lithium-ion battery to have an excellent theoretical specific capacity (e.g., 3860 mAh·g ⁻¹ ).

On the other hand, there is a concern in recent years that there is a potential danger of fire, explosion, etc. when lithium-ion batteries are used on a large scale. Specifically, lithium metal anodes react easily with liquid organic electrolytes at the interface, which may cause combustion and explosion due to the so-called "thermal runaway" of the battery. In addition, the safety of lithium-ion batteries may be reduced due to factors such as the growth of lithium dendrites, and various serious safety issues may be potentially present.

From this perspective, materials for use in lithium-ion batteries have been actively researched even recently, and solid-state electrolytes (SSEs) are one of the materials that have attracted attention. For example, Patent Document 1 discloses a technology for improving ionic conductivity using a sulfide solid electrolyte material.

JP 2019-003927 A

However, sulfide-based solid electrolytes such as those disclosed in Patent Document 1 and the like have the risk of generating toxic hydrogen sulfide gas, and have been problematic from the standpoint of safety, etc. On the other hand, in recent years, although the use of ionic liquids to improve battery performance has been devised, the performance requirements for lithium-ion batteries are increasing, and these have not necessarily provided satisfactory performance. From this perspective, there has been a strong demand for the development of an electrolyte with high ionic conductivity and excellent electrochemical stability.

The present invention has been made in view of the above, and aims to provide a solid electrolyte that has high ionic conductivity and excellent electrochemical stability and can provide high stability to lithium ion batteries, a method for producing the same, and a lithium ion battery that includes the solid electrolyte.

As a result of intensive research into achieving the above object, the inventors discovered that the above object can be achieved by using a layered composite oxide having a crystal structure of a layered double hydroxide of a metal containing at least aluminum, and thus completed the present invention.

That is, the present invention includes, for example, the subject matter described in the following sections.
Item 1
The present invention relates to a layered composite oxide and an ionic liquid containing a lithium salt.
The layered composite oxide is formed of an oxide of a metal including aluminum and a metal M (excluding aluminum),
The layered composite oxide has a crystal structure of a layered double hydroxide.
Item 2
Item 2. The solid electrolyte for lithium ion batteries according to Item 1, wherein the metal M other than aluminum is at least one metal selected from the group consisting of Li, Mg, Zn, and Ni.
Item 3
Item 3. The solid electrolyte for lithium ion batteries according to item 1 or 2, wherein the content of the layered composite oxide is 10 to 60 mass% based on the total mass of the layered composite oxide, the lithium salt, and the ionic liquid.
Item 4
A step 1 of heat-treating a raw material containing an aluminum source, a metal M source (excluding the aluminum source), and water in the presence of an alkaline agent;
A step 2 of calcining the product obtained by the heat treatment to obtain a calcined product;
and step 3 of mixing the fired product with a solution containing a lithium salt and an ionic liquid, followed by drying to obtain a solid electrolyte.
Item 5
Item 5. The method according to item 4, wherein the product obtained in step 1 and the fired product obtained in step 2 both have layered crystal structures.
Item 6
Item 6. The method according to item 4 or 5, wherein the metal other than aluminum M is at least one metal selected from the group consisting of Li, Mg, Zn and Ni.
Item 7
Item 4. A lithium ion battery comprising the solid electrolyte for lithium ion batteries according to any one of items 1 to 3.

The solid electrolyte of the present invention has high ionic conductivity and excellent electrochemical stability, and can provide high stability to lithium ion batteries. Therefore, the solid electrolyte of the present invention is suitable as an electrolyte for lithium ion batteries.

1 shows the results of X-ray diffraction measurement (XRD) of the solid electrolytes obtained in Examples 1 to 3. 1A shows the results of a stability test of a battery including the liquid electrolyte obtained in Comparative Example 1, and FIG. 1B shows the results of a stability test of a battery including the solid electrolyte obtained in Example 2. 4 shows the measurement results of the ionic conductivity of each electrolyte obtained in the examples. 1 shows the results of an electrochemical stability test for the liquid electrolyte obtained in Comparative Example 1 (a) and the solid electrolyte obtained in Example 2 (b). 1 shows the results of evaluation of the characteristics of a battery having the solid electrolyte obtained in Example 2. FIG. 1 is a schematic diagram illustrating the layer structure of a layered double hydroxide and the mechanism of lithium ion migration in a solid electrolyte.

The embodiments of the present invention will be described in detail below. In this specification, the expressions "contain" and "include" include the concepts of "contain," "include," "consist essentially of," and "consist only of."

1. Solid Electrolyte The solid electrolyte of the present invention contains a layered composite oxide and an ionic liquid containing a lithium salt, and the layered composite oxide is formed of a metal oxide containing aluminum and a metal M (excluding aluminum). In particular, the layered composite oxide is characterized by having a crystal structure of a layered double hydroxide.

The solid electrolyte of the present invention has high ionic conductivity and excellent electrochemical stability, and when incorporated into a lithium ion battery, it can provide high stability to the lithium ion battery.

(Layered Complex Oxide)
The layered complex oxide is an oxide containing at least two kinds of metals, and in particular has a layered structure. The layered complex oxide is, for example, a compound formed by oxidizing (for example, calcining) a layered double hydroxide. In general, it is known that a layered double hydroxide (LDH) has a structure having exchangeable anions between layers of a metal hydroxide (metal double hydroxide). In the solid electrolyte of the present invention, the layered complex oxide contains an oxide of a layered double hydroxide, and the crystal structure of the layered complex oxide has a structure having exchangeable anions between layers of a metal oxide, similar to the crystal structure of the layered double hydroxide. Whether the crystal structure of the layered complex oxide in the solid electrolyte of the present invention is the same as the crystal structure of the double hydroxide can be determined from the peak position of the spectrum obtained from X-ray diffraction measurement (XRD measurement). Specifically, by comparing the spectrum (XRD peak) obtained from the XRD measurement of the solid electrolyte with the XRD peak of the control double hydroxide, it is possible to determine whether the crystal structure of the layered composite oxide is the same as that of the double hydroxide. In particular, since the control double hydroxide has a peak at a specific position due to its layered structure, it is possible to easily determine whether the crystal structure of the layered composite oxide in the solid electrolyte is the same as that of the double hydroxide depending on whether the layered composite oxide in the solid electrolyte has this peak.

The layered composite oxide contained in the solid electrolyte of the present invention is a layered composite oxide of a metal containing at least aluminum, and may have a structure in which anions of a lithium ion electrolyte are fixed between the layers of the layered structure.

The layered composite oxide is a composite oxide of metals containing aluminum and metal M. Here, metal M is a metal other than aluminum.

The type of metal M is not particularly limited as long as it can form a layered composite oxide, and various metals such as alkali metals, alkaline earth metals, transition metals, etc. can be applied. Among them, metal M is preferably at least one metal selected from the group consisting of Li, Mg, Zn, and Ni. In this case, it is easy to form a layered composite oxide with aluminum, and the obtained solid electrolyte is likely to have high ionic conductivity and improved electrochemical stability. In particular, metal M preferably contains Li. Metal M may be used alone or in combination of two or more different types.

As long as the layered complex oxide contains aluminum and metal M, it may also contain other metals to the extent that the effects of the present invention are not impaired. The metals contained in the layered complex oxide may be only aluminum and metal M.

A particularly preferred layered composite oxide is a composite oxide of aluminum and lithium.

In the layered complex oxide, the content ratio of aluminum and metal M is not particularly limited. For example, the molar ratio of aluminum and metal M (aluminum:metal M) contained in the layered complex oxide can be 10:1 to 0.1:1, preferably 8:1 to 1:1, more preferably 6:1 to 1:1, and particularly preferably 4:1 to 2:1.

(Ionic liquid containing lithium salt)
The solid electrolyte of the present invention further contains an ionic liquid containing a lithium salt, in addition to the layered composite oxide.

The type of lithium salt is not particularly limited, and various lithium salts can be mentioned, for example, known lithium salts used in lithium ion batteries can be widely applied.Specific examples of the lithium salt include various lithium imide salts such as lithium bis(trifluoromethanesulfonyl)imide (LiTFIL) and lithium bis(fluorosulfonyl)imide (LiFSI), as well as lithium bisoxalate borate (LiBOB), LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiSbF ₆ and the like.These lithium salts may be contained alone or in combination of two or more in the solid electrolyte.

The lithium salt is particularly preferably lithium bis(trifluoromethanesulfonyl)imide (LiTFIL).

The type of ionic liquid is not particularly limited, and various ionic liquids can be used. For example, known ionic liquids used in lithium ion batteries can be widely used. Specific examples of ionic liquids include 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI), trimethylpropylammonium bis(trifluoromethanesulfonyl)imide (TMBATFSI), 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMI-FSI), N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13-TFSI), N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME ... Examples of the ionic liquid include N-methyl-N-propyl piperidinium tetrafluoroborate (PP13-BF4), 1-methyl-1-propyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (PY13-TFSI), and 1-methyl-1-propyl pyrrolidinium tetrafluoroborate (PY13-BF4). These ionic liquids may be contained alone or in combination with one another in the solid electrolyte.

The ionic liquid is particularly preferably 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI).

(Solid electrolyte)
As described above, the solid electrolyte contains a layered composite oxide and an ionic liquid containing a lithium salt. In one embodiment, the solid electrolyte is present between the layers of the layered composite oxide such that anions of the lithium salt and/or anions of the ionic liquid are inserted, and lithium ions may also be present between the layers. Although not necessarily limiting, it is presumed that the lithium ions can freely travel between the layers, thereby avoiding interference of the anions. As a result, the solid electrolyte of the present invention is considered to not only provide a lithium ion migration path, but also to be capable of increasing the ionic conductivity of the solid electrolyte and increasing the electrochemical stability.

In the solid electrolyte, the content of the layered complex oxide is not particularly limited. For example, the content of the layered complex oxide in the solid electrolyte can be 10 to 60 mass% based on the total mass of the layered complex oxide, lithium salt, and ionic liquid. In this case, the solid electrolyte can have a higher ionic conductivity and can be prevented from becoming liquid. The content of the layered complex oxide is preferably 20 to 50 mass%, more preferably 28 to 40 mass%, based on the total mass of the layered complex oxide, lithium salt, and ionic liquid.

In the solid electrolyte, the content ratio of the lithium salt and the ionic liquid is not particularly limited. For example, in the solid electrolyte, the content ratio of the lithium salt and the ionic liquid can be 40 to 90 mass% with respect to the total mass of the layered complex oxide, the lithium salt, and the ionic liquid. In this case, the solid electrolyte can have a higher ionic conductivity and can be prevented from becoming liquid. The content ratio of the lithium salt and the ionic liquid is preferably 50 to 80 mass% with respect to the total mass of the layered complex oxide, the lithium salt, and the ionic liquid, and more preferably 60 to 72 mass%. In addition, the concentration of the lithium salt in the ionic liquid can be, for example, 0.5 to 5 mol·L ⁻¹ .

Although the solid electrolyte of the present invention contains an ionic liquid, the electrolyte itself is solid or gel-like, not liquid. By appropriately adjusting the amounts of the layered double hydroxide, lithium salt, and ionic liquid, the electrolyte can be made solid or gel-like. Therefore, the solid electrolyte of the present invention is, more accurately, a "quasi-solid electrolyte."

The solid electrolyte of the present invention, configured as described above, has high ionic conductivity and excellent electrochemical stability, and when incorporated into a lithium ion battery, it can provide high stability to the lithium ion battery. Therefore, the solid electrolyte of the present invention is suitable as an electrolyte for lithium ion batteries.

The method for producing the solid electrolyte of the present invention is not particularly limited, and various production methods can be used. For example, the solid electrolyte can be produced by a production method including steps 1, 2, and 3 described below.

2. Method for Producing a Solid Electrolyte for Lithium Ion Batteries The method for producing a solid electrolyte for lithium ion batteries of the present invention may include the following steps 1 to 3.
Step 1: A step of heat-treating a raw material containing an aluminum source, a metal M source (excluding the aluminum source), and water in the presence of an alkaline agent.
Step 2: A step of calcining the product obtained by the heat treatment to obtain a calcined product.
Step 3: A step of mixing the fired product with a solution containing a lithium salt and an ionic liquid, followed by drying to obtain a solid electrolyte.

By using a manufacturing method including steps 1 to 3 above, for example, the solid electrolyte of the present invention described above can be manufactured.

(Step 1)
In step 1, a raw material containing an aluminum source, a metal M source (excluding the aluminum source), and water is heat-treated in the presence of an alkaline agent. The product obtained thereby contains a double hydroxide containing aluminum and the metal M. Specifically, step 1 is a step of producing a double hydroxide by so-called hydrothermal synthesis.

The aluminum source is the starting material that provides aluminum to the metal that forms the product double hydroxide. The aluminum source may be aluminum itself or a compound that contains aluminum. The compound that contains aluminum may be a hydrate.

Examples of compounds that contain aluminum include aluminum chloride, aluminum halides, inorganic aluminum salts, and organic aluminum salts. Examples of inorganic aluminum salts include one or more selected from the group consisting of aluminum nitrates, sulfates, hydrochlorides, chlorates, perchlorates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates. The inorganic aluminum salts may contain two or more different anions. Examples of organic aluminum salts include one or more selected from the group consisting of aluminum acetates, oxalates, formates, and succinates.

The aluminum source is preferably a compound containing aluminum, more preferably an inorganic acid salt of aluminum, and particularly preferably an aluminum nitrate.

The metal M source is a starting material for bringing about metal M into the metal that forms the product double hydroxide. Here, the metal M is not particularly limited as long as it is a metal other than aluminum, and various metals such as alkali metals, alkaline earth metals, transition metals, etc. can be applied. Among them, the metal M is preferably at least one metal selected from the group consisting of Li, Mg, Zn, and Ni. In this case, it is easy to form a layered double hydroxide with aluminum, and the obtained solid electrolyte is likely to have high ionic conductivity and improved electrochemical stability. In particular, it is preferable that the metal M contains Li. The metal M may be used alone or in combination of two or more different types.

The metal M source may be metal M alone or a compound containing metal M. The compound containing metal M may be a hydrate.

Compounds containing metal M can be widely used, such as chlorides of metal M, halides of metal M, inorganic acid salts of metal M, and organic acid salts of metal M. Examples of inorganic acid salts of metal M include one or more salts selected from the group consisting of nitrates, sulfates, hydrochlorides, chlorates, perchlorates, carbonates, hydrogen carbonates, phosphates, and hydrogen phosphates of metal M. Inorganic acid salts of metal M can also contain two or more different anions. Examples of organic acid salts of metal M include one or more salts selected from the group consisting of acetates, oxalates, formates, and succinates of metal M.

The metal M source is preferably a compound containing metal M, more preferably an inorganic acid salt of metal M, and particularly preferably a nitrate of metal M.

The raw materials used in step 1 include an aluminum source, a metal M source, and water. The content ratio of aluminum and metal M contained in the raw materials is not particularly limited. For example, the molar ratio of aluminum and metal M contained in the raw materials (aluminum:metal M) can be 10:1 to 0.1:1, preferably 8:1 to 1:1, more preferably 6:1 to 1:1, and particularly preferably 4:1 to 2:1.

The concentrations of the aluminum source and metal M source contained in the raw materials are not particularly limited, and the concentration of the aluminum source can be, for example, 0.1 to 5 M (preferably, 0.5 to 1 M), and the concentration of the metal M source can be, for example, 0.01 to 1 M (preferably, 0.1 to 0.5 M).

The type of alkaline agent used in step 1 is not particularly limited. For example, a wide variety of known alkalis can be used. Examples of alkalis include sodium hydroxide, potassium hydroxide, and ammonia. The amount of alkali used is also not particularly limited. For example, the amount of alkali used can be adjusted so that the pH of the raw material is 7 or higher, preferably 10 or higher.

In step 1, the conditions for the heat treatment are not particularly limited. As described above, hydrothermal synthesis is performed in step 1, and therefore, for example, the conditions can be the same as those for known hydrothermal synthesis. Specifically, in the heat treatment, the heating temperature can be, for example, 70 to 150°C, preferably 80 to 130°C, and more preferably 90 to 120°C. The heating time is also not particularly limited, and can be, for example, 3 to 24 hours. The reaction vessel used for the heat treatment is also not particularly limited, and for example, known pressure-resistant solutions can be widely used. The pressure inside the vessel in the hydrothermal synthesis can also be set appropriately.

The heat treatment produces a product, for example, as a precipitate, which can then be separated and purified by an appropriate method.

The product obtained in step 1 contains a double hydroxide of aluminum and metal M.

(Step 2)
Step 2 is a step for obtaining a fired product by firing the product obtained by the heat treatment in step 1. The method of firing is not particularly limited, and for example, a wide variety of known firing methods can be adopted.

The firing temperature can be, for example, 300 to 700°C, preferably 400 to 600°C, and more preferably 450 to 580°C. The firing time can be appropriately selected depending on the firing temperature, and can be, for example, 1 to 10 hours. There are no particular limitations on the heating rate during firing treatment.

The calcination process may be carried out in air or in an inert gas atmosphere. It is preferable to carry out the calcination in air. For example, a known heating device such as a commercially available heating furnace may be used for the calcination.

By the above-mentioned calcination, the double hydroxide obtained in step 1 can be converted into a composite oxide.

(Step 3)
Step 3 is a step for obtaining a solid electrolyte by mixing the fired product obtained in step 2 with a solution containing a lithium salt and an ionic liquid, followed by drying treatment.

The lithium salt and ionic liquid used in step 3 can be the same types as those contained in the solid electrolyte described above.

In step 3, a solution containing a lithium salt and an ionic liquid is mixed with the fired product obtained in step 2 to obtain a mixture, and the mixture is dried to obtain a solid electrolyte.

The solution containing lithium salt and ionic liquid can be, for example, an aqueous solution in which lithium salt and ionic liquid are dissolved. In the solution containing lithium salt and ionic liquid, the concentration of the lithium salt can be, for example, 0.1 to 50 mass%, and preferably 0.25 to 40 mass%. In the solution containing lithium salt and ionic liquid, the concentration of the ionic liquid can be, for example, 0.1 to 50 mass%, and preferably 1 to 40 mass%. In the solution containing lithium salt and ionic liquid, the concentration of the ionic liquid can be, for example, 50 to 99 mass%, and preferably 60 to 95 mass%, relative to the total mass of the lithium salt and ionic liquid.

The method for preparing the solution containing the lithium salt and the ionic liquid is not particularly limited. For example, the lithium salt and the ionic liquid may be mixed with water and then heated. In this case, the heating temperature may be, for example, 50 to 150°C. This heating may also be performed in a vacuum.

The method of mixing the solution containing the lithium salt and the ionic liquid with the fired product is not particularly limited, and any known mixing means can be widely used. The mixing ratio of the solution containing the lithium salt and the ionic liquid with the fired product is also not particularly limited, and for example, the total mass of the lithium salt and the ionic liquid is preferably 50 to 500 parts by mass, more preferably 80 to 300 parts by mass, and even more preferably 100 to 250 parts by mass per 100 parts by mass of the fired product. In this case, the obtained solid electrolyte is likely to have high ionic conductivity and excellent electrochemical stability.

After preparing a mixture by mixing the solution containing the lithium salt and the ionic liquid with the fired product, the mixture may be heat-treated as necessary. For example, the mixture may be heat-treated to a temperature in the range of 50 to 150°C.

The mixture obtained by mixing the fired product with a solution containing a lithium salt and an ionic liquid is dried to evaporate the solvent other than the ionic liquid, thereby obtaining a solid electrolyte. The temperature of the drying process in this case is not particularly limited, and can be performed under appropriate conditions.

In step 3, the calcined composite oxide is mixed with lithium salt and ionic liquid, and the crystal structure of the composite oxide is restored to that of a double hydroxide (the so-called memory effect; see also FIG. 6 below). In step 2, the double hydroxide is calcined to form a composite oxide, but in step 3, it is mixed with lithium salt and ionic liquid, and it is presumed that the memory effect is exerted, causing the crystallinity of the composite oxide to be restored to that of a double hydroxide.

The method for producing a solid electrolyte may also include steps other than steps 1, 2, and 3.

The lithium ion battery of the present invention includes the above-mentioned solid electrolyte of the present invention as an electrolyte. The lithium ion battery of the present invention may also include the solid electrolyte obtained by the production method including the above-mentioned steps 1 to 3 as an electrolyte.

As long as the lithium ion battery of the present invention has the solid electrolyte, other configurations can be the same as, for example, known lithium ion batteries.

For example, the negative electrode (cathode) can be produced by applying a slurry containing an active material, a conductive agent, a binder, etc. to a current collector and then forming the current collector. The components contained in the slurry can be the same as those used to produce known lithium-ion batteries.

Examples of the active material include LiFePO ₄ , LiFeO ₂ , LiMnPO ₄ , LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O _4. Examples of the conductive agent include Super P (conductive carbon black), as well as various carbon materials such as acetylene carbon black, graphite, vapor-grown carbon fiber, carbon nanotubes, and activated carbon. Examples of the binder include polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), and carboxymethyl cellulose. The solvent contained in the slurry is not particularly limited, and examples thereof include N-methyl-2-pyrrolidone.

Current collectors include metal foils such as aluminum, titanium, platinum, molybdenum, stainless steel, and copper. The shape of the current collector can be, for example, a porous body, foil, plate, or mesh made of fibers.

In addition, a wide variety of known materials can be used as the anode, such as a lithium metal anode.

The size and shape of the lithium ion battery can be determined appropriately depending on the application. There are also no particular limitations on the method for assembling the lithium ion battery, and the lithium ion battery can be obtained by a method similar to a known assembly method.

The lithium ion battery of the present invention has excellent stability because it contains the above-mentioned solid electrolyte as a component, and also has good capacity reversibility and high coulombic efficiency.

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

Example 1
1.18 g of LiNO ₃ and 22.82 g of Al(NO ₃ ) ₃ ·9H ₂ O were dissolved in 100 mL of distilled water to obtain a LiNO ₃ -Al(NO ₃ ) ₃ ·9H ₂ O solution. After dissolving 10.75 g of NaOH in 50 mL of distilled water, it was slowly added to the above LiNO ₃ -Al(NO ₃ ) ₃ ·9H ₂ O solution to adjust the pH to 10 or more, and after rapid stirring for 1 hour, it was transferred to a stainless steel autoclave lined with Teflon (registered trademark) and held at 100 ° C for 6 hours to perform hydrothermal synthesis. Then, it was naturally cooled to room temperature, and the white precipitate of the contents was separated by centrifugation, and then it was washed three times in succession with deionized water and absolute ethanol, and finally dried at 60 ° C for 24 hours to obtain a product (step 1). The obtained product was designated Li-AlLDH.

Next, the obtained Li-AlLDH was heated to 550°C at a heating rate of 2°C/min in a heating furnace and maintained at this temperature for 4 hours to obtain a fired product (step 2). On the other hand, LiTFIL (lithium bis(trifluoromethanesulfonyl)imide) as a lithium salt was dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI) as an ionic liquid, and heated at 120°C for 12 hours in a vacuum to prepare an aqueous solution of the lithium salt and the ionic liquid (Li-IL-H ₂ O solution). In this solution, the concentration of the lithium salt was 0.4 mass%, and the concentration of the ionic liquid was 1.1 mass%. The Li-IL-H ₂ O solution and the fired product were mixed so that the ratio of the mass of the fired product to the total mass of the lithium salt and the ionic liquid (Li-AlLDH:Li-IL) was 1:1.5, and the mixture was rapidly stirred at 80° C. for 12 hours to obtain a mixture. The mixture was left to stand in an oven at 60° C. for 12 hours, and then transferred to a vacuum oven and further dried at 60° C. for 12 hours. This resulted in a solid electrolyte (step 3). The obtained solid electrolyte was designated as "Li-AlLDH/Li-IL (1:1.5)."

Example 2
A solid electrolyte was obtained in the same manner as in Example 1, except that the fired material and the lithium salt and ionic liquid were mixed so that the ratio of their mass to the total mass of the lithium salt and ionic liquid (Li-AlLDH:Li-IL) was 1:2.0. The obtained solid electrolyte was designated as "Li-AlLDH/Li-IL (1:2.0)."

Example 3
A solid electrolyte was obtained in the same manner as in Example 1, except that the fired material and the lithium salt and ionic liquid were mixed to obtain a mixture such that the ratio of the mass of the fired material to the total mass of the lithium salt and ionic liquid (Li-AlLDH:Li-IL) was 1:2.5. The obtained solid electrolyte was designated as "Li-AlLDH/Li-IL (1:2.5)."

(Comparative Example 1)
A mixture of the lithium salt and the ionic liquid, LiTFIL (lithium bis(trifluoromethanesulfonyl)imide), was prepared as a liquid electrolyte by dissolving the lithium salt in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI) as a lithium salt and heating the mixture at 120° C. for 12 hours in a vacuum. In this liquid electrolyte, the lithium salt:ionic liquid (mass ratio) was 1:2.65.

(Battery manufacturing)
A material consisting of LiFePO ₄ , a conductive additive (super P), and polyvinylidene fluoride (PVDF) was prepared. In this material, LiFePO ₄ :super P:PVDF=8:1:1 (mass ratio). N-methyl-2-pyrrolidone (NMP) was added as a solvent to the material, and the mixture was stirred for 12 hours to mix uniformly. The obtained slurry was coated on an Al current collector, and dried in a vacuum at 120° C. for 12 hours to produce a cathode. A block battery was assembled by a known method using this cathode, an anode (lithium metal anode), and the electrolyte obtained in the examples or comparative examples.

(Evaluation results)
1 shows the results of X-ray diffraction measurement (XRD) of the solid electrolytes obtained in Examples 1 to 3. For the X-ray diffraction measurement, a "SmartLab" manufactured by Rigaku Corporation was used, and the measurement was performed using a Cu-Kα (λ=1.540 Å) radiation source in the range of 2θ=10 to 100°.

In addition, Li-AlLDH shown in Figure 1 is the XRD spectrum of the double hydroxide of Li and Al obtained in step 1, Li-AlLDO is the XRD spectrum of the composite oxide of Li and Al obtained in step 2, and LiTFSI is the XRD spectrum of the lithium salt of bis(trifluoromethanesulfonyl)imide, which is the control. Also, (b) in Figure 1 is a partially enlarged view of (a).

The XRD peak corresponding to LiTFSI was not prominent because the lithium salt was completely dissolved in the ionic liquid. After the Li-AlLDH was calcined, the LDH (double hydroxide) was changed to LDO (composite oxide), and a decrease in crystallinity was observed. However, the crystallinity of the Li-AlLDH was restored by the addition of Li-IL, using a reconstruction method based on the so-called "memory effect." However, the lattice spacing of the restored Li-AlLDH hardly changed because the radii of NO ₃ ^- (2.64 Å) and TFSI ^- (3.26 Å) are similar.

Figure 2 shows the results of the stability test of the battery, where (a) is the result of the stability test of the battery with the liquid electrolyte obtained in Comparative Example 1, and (b) is the result of the stability test of the battery with the solid electrolyte obtained in Example 2.

The solid electrolyte obtained in Example 2 has good compatibility with Li metal, and it was found that the stability of the battery is greatly improved compared to the battery using the Li-IL liquid electrolyte in Comparative Example 1. It is presumed that this is because the solid electrolyte obtained in Example 2 had a positive effect on suppressing the formation and growth of Li dendrites, which were previously uncontrollable.

Figure 3 shows the measurement results of the ionic conductivity of each electrolyte. The measurements were performed using a VersaSTAT4 potentiostat galvanostat electrochemical workstation (USA) and a SUS symmetric battery at a measurement temperature of 30°C and a frequency of 1,000,000-10 Hz.

3, it was found that the solid electrolytes obtained in each Example had high ionic conductivity. In particular, the solid electrolyte obtained in Example 3 had an ionic conductivity of 1.07×10 ⁻³ Scm ⁻¹ at 30° C. (4.45×10 ⁻⁴ Scm ⁻¹ in Example 1 and 8.54×10 ⁻⁴ Scm ⁻¹ in Example 2).

4 shows the results of electrochemical stability tests of the liquid electrolyte obtained in Comparative Example 1 (FIG. 4(a)) and the solid electrolyte obtained in Example 2 (FIG. 4(b)). The measurements were performed using a VersaSTAT4 potentiostat galvanostat electrochemical workstation manufactured by the United States, a SUS/electrolyte/battery battery, and under conditions of a measurement temperature of 40° C., a voltage of −1 to 5 V, and a scan rate of 1 mVs ⁻¹ .

In Fig. 4(a) (Comparative Example 1), the peaks in the range of -0.5 to 0.5 V indicate that metal ions are being inserted and removed from the lithium surface, and the peak occurring near 1/0.8 V indicates that both the ionic liquid [EMIM] ⁺ and the lithium salt [TFSI] ^- are decomposed at the interface. On the other hand, when Li-AlLDH is used as a host for Li-IL as in Example 2 (Fig. 4(b)), no new peaks appear in a wide potential range, indicating that the Li-IL has excellent stability against Li metal in a wide potential range.

Figure 5 shows the results of the characteristic evaluation of the battery having the solid electrolyte obtained in Example 2. The measurement was performed using an SD8 battery tester at a measurement temperature of 40°C and a voltage of 2.5 to 4 V.

At 0.1, 0.3, 0.5, 1, and 0.1 C, the discharge capacities were found to be 151.8, 146.6, 138.9, 107.1, and 143.9 mAh·g ⁻¹ in the first charge-discharge cycle, respectively, with good capacity reversibility and high coulombic efficiency.

Figure 6 shows a schematic diagram of the layered structure of the layered complex oxide and the mechanism of lithium ion migration. From the results of the above examples, it is presumed that the anions of the lithium ion electrolyte are fixed between the layers of the layered structure of the layered complex oxide, allowing the lithium ions to move freely back and forth between the layers of the layered complex oxide, thereby avoiding interference with the anions. Furthermore, this structure not only provides a path for the lithium ions to migrate, but is also thought to increase the ionic conductivity of the solid electrolyte and the stability of the solid electrolyte.

As mentioned above, after Li-AlLDH is sintered, LDH changes to LDO (complex oxide), but since the solid electrolyte contains Li-IL, it is presumed that the crystallinity is restored by the reconstitution method based on the memory effect, as shown in Figure 6.

Claims (4)

The present invention relates to a layered composite oxide and an ionic liquid containing a lithium salt.
The layered composite oxide is formed of a composite oxide of aluminum and lithium ,
The layered composite oxide has a crystal structure of a layered double hydroxide. 2. The solid electrolyte for lithium ion batteries according to claim 1 , wherein the content of the composite oxide is 10 to 60 mass% based on the total mass of the layered composite oxide, the lithium salt and the ionic liquid. A step 1 of heat-treating a raw material including an aluminum source, a lithium source , and water in the presence of an alkaline agent;
A step 2 of calcining the product obtained by the heat treatment to obtain a calcined product;
and step 3 of mixing the fired product with a solution containing a lithium salt and an ionic liquid and drying the mixture to obtain a solid electrolyte .
the product obtained in step 1 and the fired product obtained in step 2 each have a layered crystal structure, and the solid electrolyte obtained in step 3 is a layered composite oxide having a layered double hydroxide crystal structure . A lithium ion battery comprising the solid electrolyte for lithium ion batteries according to claim 1 or 2 .

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