Nonaqueous electrolyte and clay-type lithium ion secondary battery using same

Abstract

The present disclosure addresses the problem of providing a nonaqueous electrolyte that has excellent battery safety during use at high temperatures, which is considered important for in-vehicle secondary batteries for recent electric vehicles and the like, while additionally having excellent battery characteristics, and a clay-type lithium ion secondary battery using the nonaqueous electrolyte. A nonaqueous electrolyte according to the present disclosure is obtained by dissolving an electrolytic salt in a nonaqueous solvent. The nonaqueous solvent contains at least one kind selected from ethylene carbonate, propylene carbonate, and γ-butyrolactone in an amount of 80-100 vol% in total, and further contains trifluoroacetic acid ester having a carbon chain length of 6-8 in an alcohol group.

Description

Nonaqueous electrolyte and clay-type lithium-ion secondary battery using the same

[0001] The present disclosure relates to a non-aqueous electrolyte solution that provides excellent battery safety during high-temperature use and also has good initial capacity and cycle characteristics, and to a clay-type lithium-ion secondary battery using the same.

[0002] In recent years, lithium-ion secondary batteries have been widely used not only as power sources for small electronic devices, but also as power sources for electric vehicles and power storage. A lithium-ion secondary battery is primarily composed of a positive electrode, a nonaqueous electrolyte, a separator, and a negative electrode. In particular, lithium-ion secondary batteries that use a Ni-containing lithium composite oxide as the positive electrode and a carbon material or titanium oxide as the negative electrode are preferred. The electrolyte for such lithium-ion secondary batteries is preferably a combination of a cyclic carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) with a chain carbonate such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), or diethyl carbonate (DEC).

[0003] Non-flammable all-solid-state electrolytes have been attracting attention as a power source for automotive secondary batteries, such as those used in electric vehicles. However, their performance remains poor. To improve the thermal stability of automotive secondary batteries using liquid electrolytes, one approach is to use cyclic compounds with high flash points, such as EC, PC, and gamma-butyrolactone (GBL), in the non-aqueous electrolyte. However, non-aqueous electrolytes composed solely of cyclic compounds lack the ability to impregnate polyolefin separators, resulting in battery failure and inconsistent performance. When chain carbonates (e.g., DMC, EMC, DEC) are used to achieve impregnation of polyolefin separators, the low flash point of the chain carbonates lowers the flash point of the entire electrolyte to around 25°C.

[0004] Non-Patent Document 1 proposes an electrolyte solution in which trifluoroacetic acid ester is added to an EC / EMC-based electrolyte solution. Because EMC, a chain carbonate, is used here, the impregnation of polyolefin separators is not an issue. As a result of focusing on the effect of the carbon chain length of trifluoroacetic acid ester, Literature 1 states that n-hexyl trifluoroacetate has a long carbon chain, which reduces the dissociation ability of LiPF6 and reduces the mobility of solvated lithium ions, and therefore methyl trifluoroacetate or ethyl trifluoroacetate, which have a shorter carbon chain length in the alcohol group of trifluoroacetic acid ester, are preferable.

[0005] In addition, as a method for improving the impregnation property of a polyolefin separator, Patent Document 1 discloses a method for adding an alkyl group (R ４ It has been shown that using an electrolyte containing a pivalate ester of butyl pivalate, hexyl pivalate, octyl pivalate, or decyl pivalate, with an alcohol group carbon chain length of 4 or more, can improve the 50-cycle discharge capacity retention rate of a battery at 20°C. Furthermore, the examples describe that pivalate esters with an alcohol group carbon chain length of 4 or more, such as butyl pivalate, hexyl pivalate, octyl pivalate, and decyl pivalate, have high affinity for separators and rapidly penetrate the porous structure of microporous separators, thereby shortening the manufacturing time of lithium-ion secondary batteries. However, in the prior art disclosed in Patent Document 1, the viscosity of octyl pivalate (R = n-octyl group) at 25°C is 2.5 cP, which is higher than that of GBL (viscosity 1.8 cP), which has the lowest viscosity among cyclic compounds used as the main solvent, raising concerns about a deterioration in battery performance due to increased viscosity.

[0006] Japanese Patent No. 4691871

[0007] Journal of Fluorine Chemistry, 156, 136-143 (2013)

[0008] A clay-type lithium-ion secondary battery according to one embodiment of the present disclosure is a clay-type lithium-ion secondary battery including a positive electrode, a negative electrode, a polyolefin separator, and a nonaqueous electrolyte solution containing an electrolyte salt in a nonaqueous solvent, the nonaqueous electrolyte solution being obtained by dissolving an electrolyte salt in a nonaqueous solvent, the nonaqueous solvent being a nonaqueous solvent containing 80 to 100% by volume in total of at least one selected from ethylene carbonate, propylene carbonate, and γ-butyrolactone, and further containing a trifluoroacetic acid ester having an alcohol group with a carbon chain length of 6 to 8.

[0009] FIG. 1 is a cross-sectional view showing the structure of a clay-type lithium-ion secondary battery.

[0010] An object of the present disclosure is to provide a nonaqueous electrolyte solution that is excellent in battery safety during high-temperature use, which is considered important for in-vehicle secondary batteries such as those used in recent electric vehicles, and that also has excellent battery characteristics, and a clay-type lithium-ion secondary battery using the same.

[0011] The nonaqueous electrolyte solution of the present disclosure is a nonaqueous electrolyte solution comprising an electrolyte salt dissolved in a nonaqueous solvent, the nonaqueous solvent comprising 80 to 100 volume % in total of at least one selected from ethylene carbonate (EC), propylene carbonate (PC), and γ-butyrolactone (DMC), and further comprising a trifluoroacetic acid ester having an alcohol group carbon chain length of 6 to 8. EC has a flash point of 143°C, and PC has a flash point of 133°C, and they are cyclic carbonates with flash points of 80°C or higher. GBL has a flash point of 100°C, and is a cyclic compound with a flash point of 80°C or higher. In the present disclosure, the nonaqueous solvent comprises 80 to 100 volume % in total of at least one selected from EC, PC, and GBL. "Containing 80 to 100 volume % in total" means that when only one type selected from EC, PC, and GBL is included, the selected type is contained in 80 to 100 volume %. When two or more types selected from EC, PC, and GBL are included, the selected two or more types are contained in a total of 80 to 100 volume %. The above volume % is a ratio to the total volume of the nonaqueous solvent. In the present disclosure, the nonaqueous solvent preferably contains 90 to 100 volume % in total of at least one type selected from EC, PC, and GBL. The nonaqueous solvent preferably contains two or more types selected from EC, PC, and GBL. The volume ratio of EC to PC in the nonaqueous solvent is preferably EC / PC = 10 / 90 to 100 / 0, more preferably 50 / 50 to 90 / 10, and even more preferably 60 / 40 to 80 / 20. EC has a melting point of 36°C and is solid at room temperature. Therefore, a high EC ratio may easily cause EC to precipitate at low temperatures, resulting in reduced low-temperature properties. Furthermore, if the ratio of PC is high, the ionic conductivity of the nonaqueous electrolyte may decrease. Therefore, when the mixing ratio of EC to PC is higher, it is preferable to add GBL, and the volume ratio of (EC+PC) / GBL is preferably 0 / 100 to 100 / 0, more preferably 20 / 80 to 70 / 30, and even more preferably 30 / 70 to 50 / 50.

[0012] The nonaqueous electrolyte solution of the present disclosure may contain a nonaqueous solvent other than EC, PC, and GBL. Even when the nonaqueous electrolyte solution of the present disclosure contains a nonaqueous solvent other than EC, PC, and GBL, the flash point is preferably 80°C or higher, more preferably 100°C or higher. Furthermore, the nonaqueous electrolyte solution may contain a cyclic compound other than EC, PC, and GBL that has a flash point of 80°C or higher. Examples of such cyclic compounds include fluoroethylene carbonate (FEC, flash point 122°C), vinylene carbonate (VC, flash point 80°C), succinic anhydride (SA, flash point 157°C), maleic anhydride (MA, flash point 102°C), 1,3-propane sultone (PS, flash point >110°C), and ethylene sulfate (ES, flash point >100°C). When PC or GBL is used, it is preferable to use one or more selected from these six, and it is preferable to add them in a total amount of 1% by weight to 10% by weight based on the nonaqueous electrolyte solution. Furthermore, the nonaqueous electrolyte solution of the present disclosure may or may not contain any of the chain carbonates DMC, EMC, DEC, and dibutyl carbonate (DBC) as the nonaqueous solvent. However, if it does contain any of these, the combined content of these is preferably 5% by volume or less relative to the total volume of the nonaqueous solvent.

[0013] The nonaqueous electrolyte solution of the present disclosure contains a trifluoroacetic acid ester having an alcohol group with a carbon chain length of 6 to 8. The alcohol group with a carbon chain length of 6 to 8 means that the number of carbon atoms derived from the alcohol in the trifluoroacetic acid ester is 6 to 8. The content of the trifluoroacetic acid ester having an alcohol group with a carbon chain length of 6 to 8 is preferably 0.1 to 5 mass%, more preferably 0.1 to 4 mass%, and even more preferably 0.1 to 3 mass%, relative to the total mass of the nonaqueous electrolyte solution.

[0014] The trifluoroacetic acid ester in the present disclosure is not particularly limited as long as the alcohol group has a carbon chain length of 6 to 8. If the carbon number is 5 or less, impregnation (penetration) into the separator will be insufficient, resulting in reduced battery performance. Furthermore, if the carbon number is 9 or more, the viscosity will increase, resulting in reduced battery performance. Examples of trifluoroacetic acid esters in the present disclosure having an alcohol group with a carbon chain length of 6 to 8 include linear esters such as n-hexyl trifluoroacetate, n-heptyl trifluoroacetate, and n-octyl trifluoroacetate, and branched esters such as 2-ethylhexyl trifluoroacetate, 2-octyl trifluoroacetate, 3-octyl trifluoroacetate, and 4-octyl trifluoroacetate. One or more selected from these are preferred, and one or more selected from n-hexyl trifluoroacetate, n-heptyl trifluoroacetate, and 2-ethylhexyl trifluoroacetate are more preferred.

[0015] The pivalic acid ester added to the non-aqueous electrolyte solution of the present disclosure is not particularly limited as long as the alcohol group has a carbon chain length of 6 to 8. An alcohol group having a carbon chain length of 6 to 8 means that the pivalic acid ester has 6 to 8 carbon atoms derived from the alcohol. If the carbon number is 5 or less, the battery performance will be reduced due to insufficient permeability into the separator. Furthermore, a hydrocarbon group having 9 or more carbon atoms will increase the viscosity, resulting in reduced low-temperature battery performance. The content of the pivalic acid ester having an alcohol group with a carbon chain length of 6 to 8 is preferably 0.1 to 5 mass%, more preferably 0.1 to 4 mass%, and even more preferably 0.1 to 3 mass%, relative to the total mass of the non-aqueous electrolyte solution. Preferred pivalic acid esters in the present disclosure include linear esters such as n-hexyl pivalate, n-heptyl pivalate, and n-octyl pivalate, and branched esters such as 2-ethylhexyl pivalate, 2-octyl pivalate, 3-octyl pivalate, and 4-octyl pivalate. One or more selected from these are preferred, and one or more selected from n-hexyl pivalate, n-heptyl pivalate, and 2-ethylhexyl pivalate are preferred.

[0016] In the nonaqueous electrolyte solution of the present disclosure, if the amount of the trifluoroacetic acid ester and pivalic acid ester added is large, there is a risk of excessive permeation into the separator, which may result in a decrease in cycle characteristics, so it is preferable to add them as little as possible. On the other hand, if the amount is too small, there is a risk of insufficient permeation into the separator, which may result in a decrease in battery capacity. Therefore, in the case of using a trifluoroacetic acid ester and a pivalic acid ester in combination in the present disclosure, the added amount is preferably such that the total content of the trifluoroacetic acid ester and the pivalic acid ester is 0.5 to 5 mass% based on the total mass of the nonaqueous electrolyte solution.

[0017] When a trifluoroacetic acid ester and a pivalic acid ester are used in combination, the ratio of addition of each ester is not limited as long as the total viscosity of the trifluoroacetic acid ester and the pivalic acid ester does not exceed 1.8 cP at 25° C. In particular, a pivalic acid ester having an alcohol group with a carbon chain length of 6 to 8 is preferred because, although it has a higher viscosity than a trifluoroacetic acid ester, it has a higher ability to dissociate lithium ions and improves battery performance.

[0018] When a graphite negative electrode is used in a battery, it is preferable to add chain 2-propynyl methanesulfonate (PMS, flash point 124°C) to the non-aqueous solvent in order to suppress the reductive decomposition of PC or GBL on the graphite negative electrode. These compounds are preferably added in an amount ranging from 0.1 wt % to 5 mass % relative to the total amount of the non-aqueous electrolyte.

[0019] The electrolyte salt in the present disclosure is not particularly limited as long as it can be used in the electrolyte solution of a clay-type lithium ion secondary battery. For example, LiN(SO ２ F) ２ , LiN(SO ２ CF ３ ) ２ , LiPF ６ , LiBF ４ , LiB(C ２ O ４ ) ２ , LiBF ２ (C ２ O ４ ) with LiPF ２ (C ２ O ４ LiN(SO ２ F) ２ is preferred because it has high chemical thermal stability and can improve battery performance at high temperatures. ６ has the effect of supplementarily improving battery performance at low temperatures, ６It is preferable to add a certain amount of LiN(SO). The reason for this is presumed to be that the movement of Li ions in the vicinity of the separator becomes smoother. The total concentration of the electrolyte salt contained in the non-aqueous solvent is preferably 0.5 to 3 mol / L (i.e., 0.5 to 3 mol of electrolyte salt per 1 L of non-aqueous solvent), more preferably 1 to 2 mol / L. In addition, LiN(SO ２ F) ２ When used alone, and LiN(SO ２ F) ２ and LiPF ６ When mixed, LiN(SO ２ F) ２ and LiPF ６ LiN(SO ２ F) ２ / LiPF ６ Preferred ranges of the ratio are 100 / 0 to 0 / 100, 90 / 10 to 50 / 50, and 80 / 20 to 70 / 30. The nonaqueous electrolyte solution of the present disclosure includes fluoroethylene carbonate (FEC, flash point 122°C), vinylene carbonate (VC, flash point 80°C), succinic anhydride (SA, flash point 157°C), maleic anhydride (MA, flash point 102°C), 1,3-propane sultone (PS, flash point >110°C), ethylene sulfate (ES, flash point >100°C), LiBF ２ (C ２ O ４ ), LiB(C ２ O ４ ) ２ and 2-propynyl methanesulfonate, and the total amount of at least one selected from these is preferably 1 to 10 mass % relative to the total mass of the non-aqueous electrolyte. By including these, battery characteristics are further improved. The non-aqueous electrolyte of the present disclosure can be produced by dissolving an electrolyte and a trifluoroacetic acid ester having an alcohol group carbon chain length of 6 to 8 and / or a pivalic acid ester having an alcohol group carbon chain length of 6 to 8 in a non-aqueous solvent.

[0020] The separator in the present disclosure is not particularly limited as long as it is a separator that can be used in clay-type lithium-ion secondary batteries. It is most preferable to use a separator made of a microporous membrane formed from a polyolefin material such as polypropylene or polyethylene, but a nonwoven fabric separator can also be used. These may have a single-layer or multilayer structure, and the separator surface may be coated with an oxide such as alumina. It is preferable to make the thickness of the separator as thin as possible to increase the volumetric energy density of the battery and improve the permeability of the electrolyte. Therefore, the thickness of the separator is preferably 20 μm or less, and more preferably 10 μm or less.

[0021] The negative electrode in the present disclosure is not particularly limited as long as it can be used in a clay-type lithium ion secondary battery, but in order to increase the volumetric energy density, graphite materials such as natural graphite and artificial graphite, and carbon materials such as hard carbon and soft carbon are preferred. ４ Ti ５ O １２ Titanium oxides with spinel structure such as TiNb2O7, Ti ２ Nb １０ O ２９ Titanium oxides of the formula (I) are preferably used, and particularly preferably Li ４ Ti ５ O １２ Titanium oxides having a spinel structure such as the following are suitable.

[0022] The negative electrode composite is prepared by kneading the negative electrode active material with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), or carboxymethyl cellulose (CMC).

[0023] The positive electrode active material of the positive electrode in the present disclosure is, for example, LiCoO ２ , LiNiO ２ , LiCo １ / ３ Ni１ / ３ Mn １ / ３ O ２ , LiCo ０．１５ Ni ０．８ Al ０．０５ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O ２ (NCM523), LiNi ０．８ Co ０．２ O ２ , LiNi ０．５ Mn １．５ O ４ In order to increase the volumetric energy density, a positive electrode active material containing a lithium composite oxide with an atomic ratio of Ni of 50% or more is preferably LiNi. ０．５ Co ０．２ Mn ０．３ O ２ , LiNi ０．６ Co ０．２ Mn ０．２ O ２ , LiNi ０．８ Co ０．１ Mn ０．１ O ２ , and LiCo ０．１５ Ni ０．８０ Al ０．０５ O ２ In order to improve rapid charge and discharge, LiMn ２ O ４ , LiFePO with an olivine structure ４ is preferably used.

[0024] As the positive electrode mixture, known or commercially available conductive additives such as carbon black such as acetylene black or ketjen black, carbon nanotubes, carbon fiber, activated carbon, graphite, etc. can be used in addition to the positive electrode active material, and the positive electrode mixture is kneaded with a binder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVFF), a styrene-butadiene copolymer (SBR), an acrylonitrile-butadiene copolymer (NBR), or carboxymethyl cellulose (CMC) to form a slurry positive electrode mixture, which is then applied to an aluminum foil as a current collector, dried, pressure-molded, and then heat-treated, for example, in a vacuum at 80°C.

[0025] Suitable combinations of positive electrode composites and negative electrode composites used in the present disclosure include the above combinations for increasing the volumetric energy density or the above combinations for improving rapid charge and discharge, and a battery can be produced using these combinations.

[0026] The current collector used in the present disclosure is not particularly limited, but aluminum foil or copper foil is generally used, and a porous current collector can also be used to further improve the permeability of the electrolyte.

[0027] In the present disclosure, the solvent used for the binder is not particularly limited, and various solvents can be selected depending on the active material or binder used. Specifically, when PVDF is used as the binder, it is preferable to use N-methyl-2-pyrrolidone as the solvent, while when a rubber-based binder such as styrene butadiene rubber (SBR), polytetrafluoroethylene, polyvinyl alcohol, or carboxymethyl cellulose (CMC) is used, water is a suitable solvent.

[0028] The clay-type lithium-ion secondary battery of the present disclosure includes a positive electrode and a negative electrode having clay-like properties. The clay-type lithium-ion secondary battery of the present disclosure can be manufactured by sandwiching the clay-like electrode layers of the positive electrode and negative electrode between separators and injecting the above-mentioned nonaqueous electrolyte into the separator. In the clay-type lithium-ion secondary battery of the present disclosure, the use of the above-mentioned nonaqueous electrolyte is preferable from the standpoints of performance and safety.

[0029] An example of the structure of a clay-type lithium ion secondary battery will be described with reference to Fig. 1 . Fig. 1 is a cross-sectional view showing the structure of a lithium ion secondary battery 14. As shown in Fig. 1 , in the lithium ion secondary battery 14, the negative electrode active material layer 11b may be a layer of a negative electrode material that is a mixture of a negative electrode active material 11c, a negative electrode conductive additive 11d, and an electrolyte solution 11f. In addition, in the lithium ion secondary battery 14, the positive electrode active material layer 12b may be a layer of a positive electrode material that is a mixture of a positive electrode active material 12c, a positive electrode conductive additive 12d, and an electrolyte solution 12f.

[0030] The negative electrode material may have a clay-like property, obtained by mixing an electrolyte into a mixture composed of a negative electrode active material 11c and a negative electrode conductive additive 11d. The positive electrode material may have a clay-like property, obtained by mixing an electrolyte into a mixture composed of a positive electrode active material 12c and a positive electrode conductive additive 12d. The negative electrode 11 may be an electrode in which the negative electrode material is coated on the negative electrode current collector 11a. The positive electrode 12 may be an electrode in which the positive electrode material is coated on the positive electrode current collector 12a.

[0031] The lithium ion secondary battery 14 may further include a separator 13. The negative electrode 11, the positive electrode 12, and the separator 13 may be positioned such that the negative electrode active material layer 11b and the positive electrode active material layer 12b are in contact with the separator 13. That is, the lithium ion secondary battery 14 may have a structure in which the negative electrode 11 and the positive electrode 12 are stacked with the separator 13 interposed therebetween. The separator 13 may function as an insulating member that insulates the negative electrode 11 and the positive electrode 12. The separator 13 may be, for example, a sheet-like nonwoven fabric or a porous material. This allows the electrolyte to permeate the separator 13.

[0032] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure.

[0033] The present disclosure will be specifically described below by giving examples of the present disclosure, but the technical scope of the present disclosure is not limited to these examples.

[0034] (Viscosity Measurement Method) A rotational viscometer was used as the viscometer. The model was a VISCOMETER DVI PrimeLV (manufactured by BROOKFIELD), and the viscosity of the trifluoroacetic acid esters used in Examples 1 and 2, and the viscosity of the mixtures of trifluoroacetic acid esters and pivalic acid esters used in Examples 3 to 12 were measured at 25°C. The viscosity was measured five times, and the average value is shown in Table 1.

[0035] (Preparation of Electrolyte Solution) [Example 1] A non-aqueous solvent was mixed in a volume ratio of EC / PC / GBL=2 / 1 / 7, and 0.5 mol / L of LiPF6 and LiN(SO ２ F) ２ 0.5 mol / L of (LiFSI) was dissolved in the solution. 3 mass% of n-hexyl trifluoroacetate was added to the solution obtained in this manner to prepare the electrolyte solution of Example 1. The amount of n-hexyl trifluoroacetate added (3 mass%) represents the ratio to the mass of the entire electrolyte solution prepared. Furthermore, the mol / L of the electrolyte represents the respective ratio to the volume of the entire electrolyte solution prepared.

[0036] [Examples 2 to 12] The electrolyte solutions of Examples 2 to 12 were prepared in the same manner as in Example 1, except that the respective contents of trifluoroacetic acid ester and pivalic acid ester were as shown in Table 1. The amounts (mass %) of trifluoroacetic acid ester and pivalic acid ester added in Table 1 represent the proportions relative to the total mass of the prepared electrolyte solutions.

[0037] Comparative Example 1 In Comparative Example 1, an electrolyte solution was prepared in the same manner as in Example 1, except that the trifluoroacetic acid ester and the pivalic acid ester were not added.

[0038] (Fabrication of Lithium-Ion Secondary Battery (LIB) and Measurement of Battery Characteristics) NCM523 (positive electrode active material) 93% by mass, acetylene black (conductive additive) 3% by mass, polyvinylidene fluoride (binder) 4% by mass were mixed, and 1-methyl-2-pyrrolidone was added to this to form a slurry to prepare a positive electrode composite, which was then applied to aluminum foil. Thereafter, the mixture was dried and pressure-molded to prepare a positive electrode. Similarly, 98% by mass of artificial graphite (negative electrode active material), 1% by mass of a styrene-butadiene copolymer binder, and 1% by mass of carboxymethyl cellulose were added to water and mixed, and a negative electrode composite was prepared as a slurry, which was then applied to copper foil. Thereafter, the mixture was dried, pressure-molded, and heat-treated to prepare a negative electrode sheet. The separator was a three-layer 20-micron microporous film made of polyethylene sandwiched between polypropylene layers, and the electrolyte solutions of Examples 1 to 12 and Comparative Example 1 were injected into each of them to prepare coin batteries (coin-shaped LIBs: diameter 20 mm, thickness 3.2 mm).

[0039] This coin battery was repeatedly charged and discharged at 25°C at a constant current and voltage of 1C rate to an upper limit voltage of 4.2 V using a charge / discharge device ACD-MO1A (manufactured by Asuka Electronics), and then discharged at a 1C rate to a lower limit voltage of 3.0 V. The cycle characteristics (%) were quantified by dividing the obtained capacity (mAh / g) by 50th cycle / 1st cycle x 100. The discharge capacity (initial capacity) at the 1st cycle was calculated by dividing the non-aqueous solvent (EC / DMC) by 1:2 (volume ratio) and LiPF ６ The results were calculated as a relative ratio compared to the first cycle discharge capacity of an electrolyte solution containing 0.5 mol / L of LiF and 0.5 mol / L of LiFSI. The results are shown in Table 1.

[0040] The nonaqueous electrolyte of Comparative Example 1, which does not contain the trifluoroacetic acid ester of the present disclosure, did not penetrate the separator, and coin batteries fabricated using this electrolyte did not charge or discharge. On the other hand, the nonaqueous electrolytes of all Examples containing the trifluoroacetic acid ester of the present disclosure penetrated the separator, and coin batteries fabricated using these electrolytes not only were able to charge and discharge, but also exhibited excellent cycle characteristics. Furthermore, by using the trifluoroacetic acid ester of the present disclosure in combination with the pivalic acid ester, even better cycle characteristics were obtained (Examples 3 to 7 and 11), and comparable cycle characteristics were obtained even when the trifluoroacetic acid ester content was reduced (Examples 8 and 9). Furthermore, the initial capacity of Example 10 was improved.

[0041] As described above, the electrolyte solution containing a trifluoroacetic acid ester and the electrolyte solution containing a trifluoroacetic acid ester and a pivalic acid ester according to the present disclosure have low viscosity and excellent permeability into the separator, which allows the electrolyte solution to quickly permeate into the microporous separator during the electrolyte injection step in the mass production of lithium ion secondary batteries, thereby shortening the manufacturing time of the lithium ion secondary battery. It has also been found that these electrolyte solutions are effective in terms of permeability and safety in clay-type lithium ion secondary batteries in which clay-like electrode layers of positive and negative electrodes are sandwiched between separators.

[0042] [Preparation of Electrolyte Solution] [Example 13] A non-aqueous solvent was mixed in a ratio of EC / PC / GBL=2 / 1 / 7 (volume ratio), and 0.5 mol / L of LiPF6 and 0.5 mol / L of LiN(SO ２ F) ２ 0.5 mol / L of LiFSI was dissolved in the solution obtained in this way. 2% by mass of n-hexyl trifluoroacetate and 0.5% by mass of n-octyl pivalate were mixed and added to the resulting solution, and the electrolyte solution of Example 8 was prepared by adding 2% by mass of fluoroethylene carbonate (FEC), 1% by mass of succinic anhydride (SA), and LiBF ２ (C ２ O ４ ) (LiDFOB) 2 mass % was added. A lithium ion secondary battery (LIB) was fabricated in the same manner as in Example 8, and a charge-discharge test was carried out to measure the battery characteristics. The results are shown in Table 2.

[0043] [Example 14] The FEC of Example 13 was replaced with 2% by mass of vinylene carbonate (VC), SA with 1% by mass of maleic anhydride (MA), and LiDFOB with LiB(C ２ O ４ ) ２ A charge-discharge test was carried out in the same manner as in Example 13, except that the content of (LiBOB) was changed to 2% by mass. The results are shown in Table 2.

[0044] [Example 15] A charge-discharge test was performed in the same manner as in Example 13, except that the FEC in Example 13 was replaced with 2% by mass of 1,3-propane sultone (PS), the SA was replaced with 1% by mass of ethylene sulfate (ES), and the LiDFOB was replaced with 2% by mass of 2-propynyl methanesulfonate (PMS). The results are shown in Table 2.

[0045] The results in Table 2 show that even better results can be obtained by adding FEC, VC, SA, MA, LiDFOB, LiBOB, PS, ES, or PMS to the non-aqueous electrolyte solution. Thus, it has been discovered that the lithium ion secondary battery using the non-aqueous electrolyte solution of the present disclosure is excellent in safety when used at high temperatures, as well as in initial capacity and cycle characteristics.

[0046] [Example 16] LiPF was added to a non-aqueous solvent mixed with EC / PC / GBL=2 / 2 / 6 (volume ratio). ６ To the solution thus obtained, 0.5 mass % of n-heptyl trifluoroacetate was added, and the resulting electrolyte solution of Example 16 was added with 2.8 mass % of vinylene carbonate (VC), 0.8 mass % of maleic anhydride (MA), LiB(C ２ O ４ ) ２ 1.4 mass% of (LiBOB) was added. The amount of n-heptyl trifluoroacetate added (0.5 mass%) represents the ratio to the mass of the entire electrolyte solution prepared. The mol / L of the electrolyte represents the respective ratio to the volume of the entire electrolyte solution prepared.

[0047] [Examples 17 to 20, Comparative Examples 2 to 3] The electrolyte solutions of Examples 17 to 20 and Comparative Examples 2 to 3 were prepared in the same manner as in Example 16, except that the respective contents of trifluoroacetic acid ester and pivalic acid ester were adjusted to be as shown in Table 3. The amounts (mass %) of trifluoroacetic acid ester and pivalic acid ester added in Table 13 represent the proportions relative to the total mass of the prepared electrolyte solutions.

[0048] LFP (positive electrode active material), carbon black (conductive additive), and the electrolyte solutions of Examples 16 to 20 and Comparative Examples 2 to 3 were mixed in a mass ratio of 74:1:25, and the mixture was made viscous and then applied to aluminum foil to prepare a positive electrode. The applied weight of the positive electrode was 0.078 g / cm. ２ Artificial graphite (negative electrode active material), carbon black (conductive additive), and the electrolyte solutions of Examples 16 to 20 and Comparative Examples 2 to 3 were mixed in a mass ratio of 63:2:35, and the mixture was made viscous and then applied to copper foil to prepare negative electrodes. The coating weight of the negative electrode was 0.054 g / cm. ２ A clay battery was fabricated using the positive electrode, the negative electrode, and a separator.

[0049] This clay-type lithium-ion secondary battery was charged at 25°C at a constant current and voltage of 0.3C rate up to an upper limit voltage of 3.6V, and then discharged at a 0.3C rate down to a lower limit voltage of 3.0V, thereby carrying out charge and discharge. The discharge capacity at this time was measured as the 25°C capacity (mAh / g). Next, the temperature was lowered to 0°C, and the battery was charged at a 0.3C rate up to an upper limit voltage of 3.6V. The charge capacity at this time was divided by the 25°C capacity to calculate the 0°C charge rate (%). The results are shown in Table 3.

[0050] When the nonaqueous electrolytes of Comparative Examples 2 and 3, which contained only pivalic acid ester and no trifluoroacetic acid ester, were used, the 0°C charge rate was low. When the nonaqueous electrolyte of Example 16, which contained trifluoroacetic acid ester, was used, the 25°C capacity and 0°C charge rate were improved. Furthermore, Examples 17 to 20, which used a combination of trifluoroacetic acid ester and pivalic acid ester, showed excellent 25°C capacity and 0°C charge rate.

[0051] By using the nonaqueous electrolyte solution of the present disclosure in the manufacture of clay-type lithium-ion secondary batteries, it has become possible to produce clay-type lithium-ion secondary batteries that are excellent in battery safety when used at high temperatures, and that also have excellent initial characteristics and cycle characteristics. The contribution of this disclosure to industry is immeasurable.

[0052] REFERENCE SIGNS LIST 11 negative electrode 11a negative electrode current collector 11b negative electrode active material layer 11c negative electrode active material 11d negative electrode conductive additive 11f electrolyte 12 positive electrode 12a positive electrode current collector 12b positive electrode active material layer 12c positive electrode active material 12d positive electrode conductive additive 12f electrolyte 13 separator 14 lithium ion secondary battery

Claims

**Claim 1**: A clay-type lithium-ion secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution containing an electrolyte salt in a non-aqueous solvent, wherein the non-aqueous electrolyte solution is a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent, the non-aqueous solvent is a non-aqueous solvent containing at least one selected from ethylene carbonate, propylene carbonate, and γ-butyrolactone, and further contains a trifluoroacetic acid ester having an alcohol group carbon chain length of 6 to 8 characterized by a clay-type lithium-ion secondary battery. **Claim 2**: The clay-type lithium-ion secondary battery according to claim 1, characterized in that the non-aqueous solvent contains 80 to 100% by volume in total of at least one selected from ethylene carbonate, propylene carbonate, and γ-butyrolactone. **Claim 3**: The clay-type lithium-ion secondary battery according to claim 1 or 2, characterized in that the non-aqueous electrolyte solution contains 0.1 to 5% by mass of a trifluoroacetic acid ester having an alcohol group carbon chain length of 6 to 8. **Claim 4**: The clay-type lithium-ion secondary battery according to claim 1 or 2, characterized in that in the non-aqueous electrolyte solution, the trifluoroacetic acid ester having an alcohol group carbon chain length of 6 to 8 is at least one selected from n-hexyl trifluoroacetate, 2-ethylhexyl trifluoroacetate, n-heptyl trifluoroacetate, n-octyl trifluoroacetate, 2-octyl trifluoroacetate, 3-octyl trifluoroacetate, and 4-octyl trifluoroacetate. **Claim 5**: The clay-type lithium-ion secondary battery according to claim 1 or 2, characterized in that the non-aqueous electrolyte solution contains 0.1 to 5% by mass of a trifluoroacetic acid ester having an alcohol group carbon chain length of 6 to 8 and 0.1 to 5% by mass of a pivalic acid ester having a carbon chain length of 6 to 8. **Claim 6**: The clay-type lithium-ion secondary battery according to claim 5, characterized in that in the non-aqueous electrolyte solution, the pivalic acid ester having a carbon chain length of 6 to 8 is at least one selected from n-hexyl pivalate, 2-ethylhexyl pivalate, n-heptyl pivalate, n-octyl pivalate, 2-octyl pivalate, 3-octyl pivalate, and 4-octyl pivalate.

7. The non-aqueous electrolyte contains at least one selected from fluoroethylene carbonate, vinylene carbonate, succinic anhydride, maleic anhydride, 1,3-propanesultone, ethylene sulfate, LiBF ２ (C ２ O ４ ), LiB(C ２ O ４ )2 and 2-propynyl methanesulfonate, and contains 1 to 10% by mass in total. The clay-type lithium ion secondary battery according to claim 1 or 2, characterized in that.