Nonaqueous electrolyte and lithium-ion secondary battery using the same

Abstract

The present disclosure addresses the problem of providing a non-aqueous electrolyte solution and a lithium ion secondary battery using the same, both having excellent battery characteristics and being highly safe during use at high temperatures, which is an important feature for in-vehicle secondary batteries for recent electric automobiles and the like. The non-aqueous electrolyte solution according to the present disclosure comprises a non-aqueous solvent with an electrolytic salt dissolved therein, the non-aqueous solvent containing 80-100 vol% in total of at least one selected from ethylene carbonate, propylene carbonate and γ-butyrolactone, and further containing trifluoroacetic acid ester having an alcohol group with a carbon chain length of 6-8.

Description

[Technical Field]

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

[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 lithium composite oxide containing Ni as the positive electrode and a carbon material or titanium oxide as the negative electrode are preferred. The electrolyte for these 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 or inconsistent performance. When chain carbonates (e.g., DMC, EMC, DEC) are used to achieve impregnation with 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, 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] Furthermore, Patent Document 1 discloses a method for improving the impregnation of polyolefin separators. It demonstrates that the 50-cycle discharge capacity retention rate of batteries at 20°C can be improved by using an electrolyte containing a pivalate ester in which the alkyl group (R4) bonded to the oxygen atom has 4 to 20 carbon atoms in a nonaqueous solvent (e.g., EC, PC, GBL). The examples also 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 allow microporous separators to quickly penetrate the separator's porous structure, thereby shortening the manufacturing time for lithium-ion secondary batteries. However, the prior art disclosed in Patent Document 1 has a viscosity of 2.5 cP at 25°C for octyl pivalate (R4 = n-octyl group), which is higher than GBL (viscosity 1.8 cP), the cyclic compound with the lowest viscosity used as a main solvent. This raises concerns about a decline in battery performance due to increased viscosity. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent No. 4691871 [Non-patent literature]

[0007] [Non-Patent Document 1] Journal of Fluorine Chemistry,156,136-143(2013) Summary of the Invention

[0008] A nonaqueous electrolyte according to one embodiment of the present disclosure is a nonaqueous electrolyte obtained by dissolving an electrolyte salt in a nonaqueous solvent, the 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] A lithium ion secondary battery according to one embodiment of the present disclosure includes a positive electrode, a negative electrode, a polyolefin separator, and a nonaqueous electrolyte solution containing an electrolyte salt in a nonaqueous solvent, wherein the nonaqueous electrolyte solution is the nonaqueous electrolyte solution according to one embodiment of the present disclosure. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view showing the structure of a lithium ion secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0011] 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 lithium-ion secondary battery using the same.

[0012] The non-aqueous electrolyte solution of the present disclosure is a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent, and the non-aqueous solvent is a mixture of ethylene carbonate (EC), propylene carbonate (PC), and γ-butyrolactone ( GBLThe nonaqueous solvent contains 80 to 100 volume % of at least one selected from the group consisting of EC, PC, and GBL, in a total amount, and further contains 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, making them cyclic carbonates with flash points of 80°C or higher. GBL has a flash point of 100°C, making it a cyclic compound with a flash point of 80°C or higher. In the present disclosure, the nonaqueous solvent contains 80 to 100 volume % of at least one selected from EC, PC, and GBL in a total amount. "Containing 80 to 100 volume % in total" means that when only one selected from EC, PC, and GBL is contained, the selected one is contained in 80 to 100 volume %, and when two or more selected from EC, PC, and GBL are contained, the selected two or more are contained in a total amount of 80 to 100 volume %. The above volume % is a ratio to the volume of the entire nonaqueous solvent. In the present disclosure, the nonaqueous solvent preferably contains 90 to 100 volume % of at least one selected from EC, PC, and GBL. The nonaqueous solvent preferably contains two or more 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. Because EC has a melting point of 36°C and is solid at room temperature, a high EC ratio may lead to precipitation of EC at low temperatures, resulting in a deterioration in low-temperature properties. Furthermore, a high PC ratio may lead to a decrease in the ionic conductivity of the nonaqueous electrolyte. Therefore, when the mixing ratio of EC relative 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.

[0013] The nonaqueous electrolyte of the present disclosure may contain a nonaqueous solvent other than EC, PC, and GBL. Even when the nonaqueous electrolyte 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. The nonaqueous electrolyte may also 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 compounds, and it is preferable to add them in a total amount of 1% by weight to 10% by weight based on the nonaqueous electrolyte. 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 carbonates is preferably 5% by volume or less relative to the total volume of the nonaqueous solvent.

[0014] The non-aqueous 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 further preferably 0.1 to 3 mass%, relative to the total mass of the non-aqueous electrolyte solution.

[0015] 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 is preferred, and one or more selected from n-hexyl trifluoroacetate, n-heptyl trifluoroacetate, and 2-ethylhexyl trifluoroacetate are more preferred.

[0016] The pivalic acid ester added to the non-aqueous electrolyte solution of the present disclosure is not particularly limited as long as it is a pivalic acid ester having an alcohol group carbon chain length of 6 to 8. An alcohol group carbon chain length of 6 to 8 means that the number of carbon atoms derived from the alcohol in the pivalic acid ester is 6 to 8. If the number of carbon atoms is 5 or less, the battery performance will be reduced due to insufficient penetration 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 carbon chain length of 6 to 8 is preferably 0.1 to 5 mass%, more preferably 0.1 to 4 mass%, and 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.

[0017] 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 that the amount be as small 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 amount added 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.

[0018] 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 has a higher viscosity than a trifluoroacetic acid ester, but is preferred because it has a higher ability to dissociate lithium ions and improves battery performance.

[0019] When a graphite negative electrode is used in a battery, it is preferable to add chain-type 2-propynyl methanesulfonate (PMS, flash point 124°C) to the non-aqueous solvent to suppress the reductive decomposition of PC or GBL on the graphite negative electrode. These compounds are preferably added in an amount of 0.1 to 5% by mass based on the total amount of the non-aqueous electrolyte.

[0020] The electrolyte salt in the present disclosure is not particularly limited as long as it is an electrolyte salt that can be used in the electrolyte solution of a lithium-ion secondary battery. Examples include LiN(SO2F)2, LiN(SO2CF3)2, LiPF6, LiBF4, LiB(C2O4)2, LiBF2(C2O4), and LiPF2(C2O4). LiN(SO2F)2 is preferred because it has high chemical thermal stability and can improve battery performance at high temperatures. Furthermore, LiPF6 has the effect of supplementarily improving battery performance at low temperatures, so it is preferable to add a certain amount of LiPF6. This is presumably due to the smoother movement of Li ions near the separator. The total concentration of the electrolyte salt contained in the nonaqueous solvent is preferably 0.5 to 3 mol / L (i.e., 0.5 to 3 mol of electrolyte salt per 1 L of nonaqueous solvent), and more preferably 1 to 2 mol / L. Furthermore, when LiN(SO2F)2 is used alone or when LiN(SO2F)2 and LiPF6 are used in combination, the weight ratio of LiN(SO2F)2 to LiPF6, LiN(SO2F)2 / LiPF6, can be preferably in the range of 100 / 0 to 0 / 100, 90 / 10 to 50 / 50, or 80 / 20 to 70 / 30. The nonaqueous electrolyte solution of the present disclosure preferably contains 1 to 10 mass% of at least one selected from 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), LiBF2(C2O4), LiB(C2O4)2, and 2-propynyl methanesulfonate, based on the total mass of the nonaqueous electrolyte solution. The inclusion of these compounds further improves battery performance. The nonaqueous electrolyte solution 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 nonaqueous solvent.

[0021] The separator in the present disclosure is not particularly limited as long as it is a separator that can be used in 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.

[0022] The negative electrode in the present disclosure is not particularly limited as long as it can be used in a 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. In addition, in order to improve rapid charge and discharge, Li4Ti5O 12 Titanium oxides with spinel structure such as TiNb2O7 and Ti2Nb 10 O 29 The titanium oxide is preferably Li4Ti5O 12 Titanium oxides having a spinel structure such as the following are suitable.

[0023] The negative electrode composite is made by mixing 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).

[0024] Examples of the positive electrode active material for the positive electrode in the present disclosure include LiCoO2, LiNiO2, LiCo1 / 3Ni1 / 3Mn1 / 3O2, LiCo 0.15 Ni 0.8 Al 0.05 O2, LiNi 0.5 Co0.2 Mn 0.3 O2(NCM523), LiNi 0.8 Co 0.2 O2, LiNi 0.5 Mn 1.5 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, such as LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2 and LiCo 0.15 Ni 0.80 Al 0.05 In order to improve rapid charge and discharge, LiMn2O4 having a spinel structure and LiFePO4 having an olivine structure are preferably used.

[0025] For 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, or graphite can be used in addition to the positive electrode active material. This is kneaded with a binder, such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVFF), styrene-butadiene copolymer (SBR), acrylonitrile-butadiene copolymer (NBR), or carboxymethyl cellulose (CMC), to form a slurry positive electrode mixture. This positive electrode material is then applied to aluminum foil as a current collector, dried, pressure-molded, and then heat-treated, for example, in a vacuum at 80°C.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] The structure of the lithium ion secondary battery of the present disclosure is not particularly limited, but examples of the shape of a secondary battery having a positive electrode, a negative electrode, and a separator include a coin-type battery, a cylindrical battery, a prismatic battery, a pouch-type battery, etc. The lithium ion secondary battery of the present disclosure can be produced by assembling the above-described positive electrode, negative electrode, and separator into the above-described structure and injecting the above-described nonaqueous electrolyte into the separator.

[0030] An example of the structure of a 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 10. The lithium-ion secondary battery 10 is composed of a positive electrode 1, a negative electrode 2, a separator 3, a non-aqueous electrolyte 4, and a case 5 that houses them. The positive electrode 1 is an electrode in which a positive electrode active material layer 1A containing a positive electrode active material is coated on the surface of a positive electrode current collector 1B. The negative electrode 2 is an electrode in which a negative electrode active material layer 2A containing a negative electrode active material is coated on the surface of a negative electrode current collector 2B.

[0031] Furthermore, the positive electrode 1 is electrically connected to a positive electrode external terminal 8 via a lead 6. The negative electrode 2 is electrically connected to a negative electrode external terminal 9 via a lead 7. The separator 3 is disposed between the positive electrode 1 and the negative electrode 2. That is, the lithium ion secondary battery 10 has a structure in which the positive electrode 1 and the negative electrode 2 are stacked with the separator 3 interposed therebetween. Furthermore, the positive electrode 1, the negative electrode 2, and the separator 3 are housed in a case 5, which is filled with a non-aqueous electrolyte 4 and is provided with a lid (not shown) to maintain airtightness.

[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. [Example]

[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) The viscometer used was a rotational viscometer. The model was a VISCOMETER DVI PrimeLV (manufactured by BROOKFIELD Corporation), 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 measurements were performed five times, and the average values ​​are shown in Table 1.

[0035] (Preparation of Electrolyte) [Example 1] 0.5 mol / L of LiPF6 and 0.5 mol / L of LiN(SO2F)2 (LiFSI) were dissolved in a non-aqueous solvent mixed at a volume ratio of EC / PC / GBL = 2 / 1 / 7. 3 mass% of n-hexyl trifluoroacetate was added to the resulting solution 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] (Production of lithium-ion secondary batteries (LIBs) and measurement of battery characteristics) NCM523 (positive electrode active material) was mixed at 93% by mass, acetylene black (conductive additive) at 3% by mass, and polyvinylidene fluoride (binder) at 4% by mass. 1-Methyl-2-pyrrolidone was added to the mixture to form a slurry. This positive electrode composite was then applied to aluminum foil. The mixture was then 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, mixed, and formed into a slurry. This negative electrode composite was then applied to copper foil. The mixture was then dried, pressure-molded, and heat-treated to prepare a negative electrode sheet. A three-layer, 20-micron microporous film consisting of polyethylene sandwiched between polypropylene sheets was used as the separator. The electrolytes of Examples 1 to 12 and Comparative Example 1 were injected into the separator to prepare coin batteries (coin-type LIBs: diameter 20 mm, thickness 3.2 mm).

[0039] This coin battery was repeatedly charged at 25°C using a charge / discharge device (ACD-MO1A, manufactured by Asuka Electronics) at a constant current and voltage of 1C, up to a maximum voltage of 4.2 V, and then discharged at a 1C rate down to a minimum voltage of 3.0 V. The cycle performance (%) was calculated by multiplying the obtained capacity (mAh / g) by 50th cycle / 1st cycle x 100. The first-cycle discharge capacity (initial capacity) was calculated relative to the first-cycle discharge capacity of an electrolyte solution containing 0.5 mol / L LiPF6 and 0.5 mol / L LiFSI dissolved in a nonaqueous solvent mixture of EC / DMC at a volume ratio of 1:2. The results are shown in Table 1.

[0040] [Table 1] The nonaqueous electrolyte of Comparative Example 1, which did not contain the trifluoroacetic acid ester of the present disclosure, did not penetrate the separator, and the coin battery fabricated using this did not charge or discharge. On the other hand, the nonaqueous electrolytes of all Examples, which contained the trifluoroacetic acid ester of the present disclosure, penetrated the separator, and the coin batteries fabricated using these 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 even when the content of the trifluoroacetic acid ester was reduced, comparable cycle characteristics were obtained (Examples 8 and 9). Furthermore, in Example 10, the initial capacity 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. Therefore, it has been found that the electrolyte solution quickly permeates into the microporous separator during the electrolyte injection step in the mass production process of lithium ion secondary batteries, thereby enabling a reduction in the production time of lithium ion secondary batteries.

[0042] [Preparation of Electrolyte Solution] [Example 13] 0.5 mol / L of LiPF6 and 0.5 mol / L of LiN(SO2F)2 (LiFSI) were dissolved in a nonaqueous solvent containing EC / PC / GBL in a volume ratio of 2 / 1 / 7. To the resulting solution, 2% by mass of n-hexyl trifluoroacetate and 0.5% by mass of n-octyl pivalate were added. To the resulting electrolyte solution of Example 8, 2% by mass of fluoroethylene carbonate (FEC), 1% by mass of succinic anhydride (SA), and 2% by mass of LiBF2(CO4) (LiDFOB) were added. A lithium-ion secondary battery (LIB) was fabricated in the same manner as in Example 8, and its battery characteristics were measured by charge-discharge testing. The results are shown in Table 2.

[0043] [Example 14] A charge-discharge test was carried out in the same manner as in Example 13, except that the FEC in Example 13 was replaced with 2% by mass of vinylene carbonate (VC), the SA with 1% by mass of maleic anhydride (MA), and the LiDFOB with 2% by mass of LiB(C2O4)2 (LiBOB). The results are shown in Table 2.

[0044] [Example 15] A charge-discharge test was carried out 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 with 1% by mass of ethylene sulfate (ES), and the LiDFOB with 2% by mass of 2-propynyl methanesulfonate (PMS). The results are shown in Table 2.

[0045] [Table 2] 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. [Industrial Applicability]

[0046] By using the nonaqueous electrolyte solution of the present disclosure in the manufacture of lithium ion secondary batteries, it is possible to produce 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. [Explanation of symbols]

[0047] 1 positive electrode 1A Cathode active material layer 1B Positive electrode current collector 2 negative electrode 2A negative electrode active material layer 2B Negative electrode current collector 3 Separator 4 Non-aqueous electrolyte 5 cases 6 Lead 7 Lead 8 Positive external terminal 9 Negative external terminal 10 Lithium-ion secondary battery

Claims

1. A non-aqueous electrolyte solution comprising an electrolyte salt dissolved in a non-aqueous solvent, the non-aqueous solvent containing 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.

2. The non-aqueous electrolyte according to claim 1, wherein the non-aqueous solvent contains 80 to 100% by volume of at least one selected from ethylene carbonate, propylene carbonate, and γ-butyrolactone.

3. The non-aqueous electrolyte solution according to claim 1, wherein the non-aqueous electrolyte solution contains 0.1 to 5 mass % of a trifluoroacetic acid ester having an alcohol group with a carbon chain length of 6 to 8.

4. A non-aqueous electrolyte solution according to claim 1 or 2, characterized in that 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.

5. A non-aqueous electrolyte solution as described in claim 1 or 2, characterized in that it contains 0.1 to 5 mass% of trifluoroacetic acid ester having a carbon chain length of 6 to 8 in the alcohol group and 0.1 to 5 mass% of pivalic acid ester having a carbon chain length of 6 to 8.

6. The non-aqueous electrolyte solution according to claim 5, characterized in that 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. Fluoroethylene carbonate, vinylene carbonate, succinic anhydride, maleic anhydride, 1,3-propane sultone, ethylene sulfate, LiBF ２ (C ２ O ４ ), LiB(C ２ O ４ ) ２ 3. The non-aqueous electrolyte solution according to claim 1, further comprising 1 to 10 mass % in total of at least one selected from the group consisting of 2-propynyl methanesulfonate and 2-propynyl methanesulfonate.

8. A lithium ion secondary battery comprising a positive electrode, a negative electrode, a polyolefin separator, and a non-aqueous electrolyte solution containing an electrolyte salt in a non-aqueous solvent, characterized in that the non-aqueous electrolyte solution is the non-aqueous electrolyte solution described in claim 1 or 2.

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