secondary batteries

Abstract

To provide an electrolyte that suppresses an increase in reaction resistance at an interface between the electrolyte and an active material layer.SOLUTION: An electrolyte includes a lithium imide salt and an ionic liquid, and a molar ratio (ionic liquid / lithium imide salt) of the ionic liquid to the lithium imide salt is 1.5 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to an electrolyte and a secondary battery. [Background technology]

[0002] In recent years, development of secondary batteries that use electrolytes containing flame-retardant ionic liquids has been progressing. For example, Patent Document 1 discloses a non-aqueous electrolyte lithium-ion battery that includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte containing a lithium salt and an ionic liquid (Claim 1, Paragraph 0006). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-204133 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when an electrolyte containing an ionic liquid is used in a secondary battery containing a separator, cracks occur in the solid electrolyte interphase (SEI) film formed at the interface between the electrolyte and the active material layer during charge and discharge, and a new SEI film is reformed at the cracked area. As a result, liquid decomposition of the electrolyte progresses and a thick film is formed, which tends to increase the reaction resistance at the interface between the electrolyte and the active material layer and reduce the capacity retention rate. Therefore, in view of the above circumstances, the problem to be solved by the present disclosure is to provide an electrolyte that suppresses an increase in reaction resistance at the interface between the electrolyte and the active material layer, and a secondary battery including the electrolyte. [Means for solving the problem]

[0005] The means for solving the above problems include the following means. <1> comprising a lithium imide salt and an ionic liquid, An electrolyte in which the molar ratio of the ionic liquid to the lithium imide salt (ionic liquid / lithium imide salt) is 1.5 or less. <2> Both the lithium imide salt and the ionic liquid contain bis(fluorosulfonyl)amide ions (FSA) as anionic species; <1> The electrolyte according to claim 1. <3> The ionic liquid contains 1-ethyl-3-methylimidazolium (EMIM) as a cationic species. <1> or <2> The electrolyte according to claim 1. <4> a positive electrode layer; a negative electrode layer; The cathode layer and the anode layer are disposed between the cathode layer and the anode layer. <1> ~ <3> a separator layer containing the electrolyte according to any one of the above items; A secondary battery comprising: <5> the negative electrode layer contains a Si-based active material as a negative electrode active material, <4> The secondary battery according to claim 1. [Effects of the Invention]

[0006] According to the present disclosure, there are provided an electrolyte that suppresses an increase in reaction resistance at the interface between the electrolyte and an active material layer, and a secondary battery including the electrolyte.

[0007] 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. In the present specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range. In addition, in the present specification, the upper or lower limit of a numerical range may be replaced with a value shown in the examples.

[0008] Each component may contain multiple types of the corresponding substance. When referring to the amount of each component in a composition, if there are multiple substances corresponding to each component in the composition, the amount refers to the total amount of those multiple substances present in the composition, unless otherwise specified.

[0009] [Electrolytes] The electrolyte according to the present disclosure is an electrolyte containing a lithium imide salt and an ionic liquid, in which the molar ratio of the ionic liquid to the lithium imide salt (ionic liquid / lithium imide salt) is 1.5 or less. The electrolyte according to the present disclosure, having the above-described structure, can suppress an increase in reaction resistance at the interface between the electrolyte and the active material layer. The mechanism of this action is not entirely clear, but is presumed to be as follows. By setting the molar ratio of ionic liquid to lithium imide salt in the electrolyte within the above range, the viscosity of the electrolyte is increased, and an increase in resistance due to outflow of ionic liquid from the electrode can be suppressed. Furthermore, by increasing the concentration of ionic liquid, which has traditionally been thought to have low ionic conductivity, it is thought that an increase in reaction resistance at the interface between the electrolyte and the active material layer can be suppressed.

[0010] The electrolyte according to the present disclosure can be used for both positive and negative electrodes. In particular, since the electrolyte contains an ionic liquid, it is softer than an electrolyte that does not contain an ionic liquid, and is therefore useful for solid-state batteries that include an electrode (e.g., a negative electrode, more specifically, a silicon-containing negative electrode) that contains an active material that easily expands and contracts.

[0011] [Lithium imide salt] The electrolyte according to the present disclosure contains a lithium imide salt. Examples of the lithium imide salt include lithium bis(trifluoromethanesulfonyl)imide (LiTFSA), lithium bis(fluorosulfonyl)imide (LiFSA), lithium difluoro(oxalato)borate (LiDFOB), and lithium bis(oxalato)borate (LiBOB). The lithium imide salt may be used alone or in combination of two or more.

[0012] Among the above, the lithium imide salt preferably contains a bis(fluorosulfonyl)amide ion (FSA) as an anion species, and more preferably contains lithium bis(fluorosulfonyl)imide (LiFSA), from the viewpoint of further suppressing an increase in reaction resistance at the interface between the electrolyte and the active material layer.

[0013] [Ionic liquid] The electrolyte according to the present disclosure includes an ionic liquid, such as 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide [EMIM-FSA], N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide [PP13-FSA], N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide [PP13-FSA], cesium bis(fluorosulfonyl)imide [CsFSA], N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(fluorosulfonyl)imide [DEMEFSA], N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide [PP13TFSA], cesium bis(trifluoromethylsulfonyl)amide [CsTFSA], or N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide [DEMETFSA]. The ionic liquid may be used alone or in combination of two or more kinds.

[0014] Among the above, the ionic liquid preferably contains 1-ethyl-3-methylimidazolium (EMIM) as a cationic species, and more preferably 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide [EMIM-FSA]. EMIM-FSA is preferred from the viewpoint of further suppressing an increase in reaction resistance at the interface between the electrolyte and the active material layer by adjusting the viscosity of the ionic liquid, and from the viewpoint of battery performance.

[0015] As described above, in the electrolyte of the present disclosure, from the viewpoint of further suppressing an increase in reaction resistance at the interface between the electrolyte and the active material layer, it is preferable that both the lithium imide salt and the ionic liquid contain a bis(fluorosulfonyl)amide ion (FSA) as an anion species.

[0016] [Other ingredients] The electrolyte according to the present disclosure may further contain other components as needed within the range in which the effects of the present disclosure are achieved, such as fillers, binder resins, sulfide solid electrolytes, oxide solid electrolytes, halide solid electrolytes, polymers or rubbers other than the binder resins, and conductive additives (such as fibrous carbon materials).

[0017] The conductive additive is not particularly limited as long as it is a normal conductive additive that can be used in lithium ion batteries, and examples thereof include carbon black such as acetylene black and Ketjen black, and carbon materials such as vapor grown carbon fiber.

[0018] The conductive additive is not particularly limited as long as it is a conductive material. For example, it may be a metal such as aluminum, stainless steel (SUS), silver, gold, copper, or titanium. Alternatively, it may be carbon such as graphite or carbon black (acetylene black, ketjen black, furnace black, channel black, thermal lamp black, etc.). Alternatively, it may be an alloy or metal acid thereof. One type of conductive additive may be used alone, or two or more types may be used in combination. Furthermore, the conductive additive is not limited to particles, and may be a so-called filler-based conductive additive such as carbon nanofiber or carbon nanotube.

[0019] [Content] The molar ratio of the ionic liquid to the lithium imide salt (ionic liquid / lithium imide salt) is 1.5 or less, and from the viewpoint of further suppressing an increase in reaction resistance at the interface between the electrolyte and the active material layer, it is preferably 1.55 or less, and more preferably 1.56 or less. The lower limit of the molar ratio (ionic liquid / lithium imide salt) is not particularly limited, but may be, for example, the amount when the lithium imide salt is added until it reaches saturation.

[0020] The electrolyte of the present disclosure may contain an organic electrolyte (i.e., an organic solvent containing carrier ions according to the purpose and having ion conductivity for transporting the ions between electrodes) within the range in which the effects of the present application are achieved. For example, the proportion of the organic electrolyte is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, and most preferably 0% by mass, i.e., no organic electrolyte, relative to the total amount of the electrolyte.

[0021] [Secondary battery] The secondary battery of the present disclosure includes the electrolyte of the present disclosure. The method for manufacturing the secondary battery is not particularly limited, and known methods for manufacturing secondary batteries can be applied. For example, the methods described in the Examples can be mentioned. The secondary battery of the present disclosure also includes, for example, a positive electrode layer, a negative electrode layer, and a separator layer containing the electrolyte of the present disclosure disposed between the positive electrode layer and the negative electrode layer. Known materials can be used for the separator layer, the positive electrode layer, and the negative electrode layer.

[0022] In the secondary battery of the present disclosure, the negative electrode layer may contain a Si-based active material as the negative electrode active material. The Si-based active material is not particularly limited as long as it contains silicon and can function as an active material, but examples include silicon particles, silicon alloy particles (e.g., alloys of Si with one or more metals selected from the group consisting of Sn, Ti, Fe, Ni, Cu, Co, and Al), porous silicon, silicon clathrate compounds, silicon oxides, and mixtures thereof. The Si-based active material may be used alone or in combination of two or more types. While secondary batteries with a negative electrode layer containing a Si-based active material have advantages such as high capacity and a relatively low operating potential, they also expand and contract more during charging and discharging than other active materials. Therefore, cracks form in the SEI coating formed at the interface between the electrolyte and the active material layer during charging and discharging due to expansion and contraction, and a new SEI coating is formed at the cracked area. As a result, reaction resistance at the interface between the electrolyte and the active material layer is likely to increase. In contrast, in the secondary battery disclosed herein, the separator layer contains the electrolyte of the present disclosure, which suppresses the increase in reaction resistance at the interface between the electrolyte and the active material layer.

[0023] The secondary battery of the present disclosure may be a lithium-ion secondary battery. Applications of the secondary battery include power sources for vehicles, electronic devices, and electrical storage. [Example]

[0024] Examples will be described below, but the present invention is not limited to these examples in any way.

[0025] Example 1 The ionic liquid (EMIM-FSA) was mixed with lithium imide salt (LiFSA) at a molar ratio (ionic liquid / lithium imide salt) of 1.5:1, and the mixture was stirred at room temperature (23±1°C) to obtain the electrolyte of Example 1.

[0026] <Comparative Example 1> The ionic liquid (EMIM-FSA) was mixed with lithium imide salt (LiFSA) at a molar ratio (ionic liquid / lithium imide salt) of 6:1, and the mixture was stirred at room temperature (23±1° C.) to obtain the electrolyte of Example 1.

[0027] <Comparative Example 2: High-concentration organic electrolyte (4M LiFSI / DME + 120 vol% TTE)> LiFSA was mixed with DME (1,2-dimethoxyethane) to a molar concentration of 4.0 M, and the mixture was stirred at room temperature (23±1°C). Then, TTE (1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether) was added to the solution at a volume ratio of 120% by volume, and the mixture was stirred at room temperature (23±1°C) to obtain the electrolyte of Comparative Example 1.

[0028] Comparative Example 3: Dilute organic electrolyte (1.2M LiPF / FEC-EC-DMC-EMC) As a solvent, FEC (fluoroethylene carbonate), EC (ethylene carbonate), DMC (dimethyl carbonate), and EMC (ethyl methyl carbonate) were mixed at a volume ratio (FEC:EC:DMC:EMC) of 1:2:3:4. LiPF6 was then added to the solution to make the concentration 1.2 M, and the mixture was stirred at room temperature (23±1°C) to obtain an electrolyte of Comparative Example 2.

[0029] <Measurement and evaluation of reaction resistance 1: Porous Si> A coin cell (subject cell) was fabricated by laminating SUS foil, porous Si, an aramid separator containing the electrolyte of each example, Li metal as the counter electrode, and SUS foil in that order, with an aluminum laminate exterior. The secondary battery of each example was placed in a thermostatic chamber at 25° C. After charging until the battery voltage reached 2.2 V, the AC impedance of the battery was measured, and the battery was discharged so that the SOC (State Of Charge) reached 50%. The battery was charged at a constant current of 1 / 3C rate until the battery voltage reached 2.7V, and then at a constant voltage, and was terminated when the charging current reached the equivalent of 0.01C. The AC impedance measurements were performed at an AC amplitude of 10 mV and in the frequency range of 1 MHz to 10 mHz. A curve was obtained by circular fitting the waveform of the circular arc portion appearing in the Nyquist diagram obtained from the AC impedance measurements. The difference between the x-axis intercepts on the high-frequency side and the low-frequency side of the obtained curve was taken as the initial resistance. The results are shown in Table 1.

[0030] <Measurement and evaluation of reaction resistance 2: In the case of crystalline Si> Except for changing porous Si to crystalline Si, coin cells for each example were fabricated using the same method as in "Measurement and evaluation of reaction resistance 1." Impedance measurements were then performed. Then, the reaction resistance value originating from the interface between porous Si and the electrolyte in the Nyquist diagram obtained by the same method as in "Measurement and evaluation of reaction resistance 1" was calculated by circle fitting, and the increase rate of the reaction resistance value was calculated as (reaction resistance value when porous Si is used) / (reaction resistance value when crystalline Si is used) × 100 (%). The results are shown in Table 1.

[0031] [Table 1]

[0032] As shown in Table 1, the electrolytes of the examples have a low reaction resistance value compared to the electrolytes of the comparative examples, even in porous Si with a large specific surface area, and it was found that they suppress an increase in reaction resistance at the interface between the electrolyte and the active material layer.

Claims

1. A positive electrode layer, a negative electrode layer; a separator layer containing an electrolyte, the separator layer being disposed between the positive electrode layer and the negative electrode layer; Equipped with the electrolyte contains only a lithium imide salt and an ionic liquid; a molar ratio of the ionic liquid to the lithium imide salt (ionic liquid / lithium imide salt) is 1.5 or less; The negative electrode layer contains a Si-based active material as a negative electrode active material.

2. Both the lithium imide salt and the ionic liquid contain bis(fluoro) as the anionic species.

10. The secondary battery of claim 1, comprising a sulfonyl)amide ion (FSA).

3. 2. The secondary battery according to claim 1, wherein the ionic liquid contains 1-ethyl-3-methylimidazolium (EMIM) as a cationic species.

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