A secondary battery that uses lithium ions and an anhydrous electrolyte solution as components.

Abstract

Invention details;

Description

Lithium-ion secondary battery and non-aqueous electrolyte

[0001] The present invention relates to a lithium ion secondary battery that is excellent in battery safety when used at high temperatures and also has battery properties such as initial capacity and rapid charging at low temperatures, and to a nonaqueous electrolyte solution used in the lithium ion secondary battery.

[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 secondary batteries using 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] However, because chain carbonates have extremely low flash points, the flash points of DMC, EMC, and DEC lower the flash point of the entire electrolyte to around 25°C. Therefore, there are concerns about the safety of batteries used at high temperatures as a power source for on-board secondary batteries, such as electric vehicles. For this reason, battery safety has become a major concern for on-board secondary batteries, such as those used in recent electric vehicles, and non-flammable all-solid-state electrolytes have been attracting attention, but in reality, they are still inferior in terms of performance.

[0004] In Patent Document 1, a non-aqueous solvent (e.g., EC, PC) is used as a solvent for an electrolyte solution, and an alkyl group (R ４ By using a non-aqueous solvent containing a tertiary carboxylic acid ester of the formula (I) having 4 to 20 carbon atoms, a lithium secondary battery has been proposed that has a long charge-discharge cycle life, excellent battery characteristics such as electric capacity and storage characteristics in a charged state, and can suppress swelling of the battery when used at high temperatures.

[0005] In addition, Patent Document 2 discloses a lithium battery containing at least three specific lithium salts as an electrolyte, and further including an alkyl group (R ４ A lithium secondary battery has been proposed that can improve the capacity retention rate after high-temperature storage and suppress an increase in impedance after high-temperature storage by using a nonaqueous electrolyte containing a tertiary carboxylic acid ester having 1 to 6 carbon atoms.

[0006] In this prior art, Patent Document 1 shows that adding a tertiary carboxylic acid ester such as methyl pivalate, ethyl pivalate, butyl pivalate, hexyl pivalate, octyl pivalate, decyl pivalate, or dodecyl pivalate to a non-aqueous solvent of an electrolyte solution can improve the 50-cycle discharge capacity retention rate of a lithium secondary battery (Examples). However, the use of these pivalate esters has the problem of lowering the flash point of the entire electrolyte solution. Using a pivalate ester that does not lower the flash point of the entire electrolyte solution results in an increase in viscosity, which slows the migration of lithium ions and leads to a decrease in rapid charging characteristics, especially at low temperatures. This problem was not previously recognized and was revealed through the research and development of electrolyte solutions by the present inventors, i.e., it is a newly discovered issue.

[0007] Furthermore, Patent Document 2 shows that the discharge capacity retention rate after high-temperature charging and storage at 60°C was improved and an increase in impedance was suppressed (Examples), but Patent Document 2 also does not teach any problems such as lowering the flash point of the entire electrolyte or causing a decrease in rapid charging characteristics at low temperatures. In the past, while emphasis was placed on the performance of on-board secondary batteries for electric vehicles and the like when stored at high temperatures after charging, the need for battery safety and rapid charging at low temperatures was not recognized, and it appears that the rise of non-flammable all-solid-state electrolytes and the effect of rapid charging characteristics at low temperatures were not predicted.

[0008] Patent No. 4691871 Patent No. 6575521

[0009] The present invention aims to solve the above problems and to provide a lithium ion secondary battery that is excellent in battery safety during use at high temperatures, which is considered important for in-vehicle secondary batteries such as those used in recent electric vehicles, and that also has excellent battery characteristics such as initial capacity and rapid charging at low temperatures.

[0010] The present inventors have conducted extensive research to solve the above-mentioned problems, and have found that the above-mentioned problems can be solved in a lithium ion secondary battery including a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent, by using a pivalic acid ester having 12 to 13 carbon atoms and having a flash point of 90°C or higher and a viscosity of 2 to 2.3 cp at 25°C, and by adding the pivalic acid ester to the non-aqueous solvent in an amount of 0.1% by volume or more and less than 5% by volume.

[0011] In the prior art disclosed in Patent Document 1, hexyl pivalate (R4 = n-hexyl group) has a flash point of 77°C, which is more than 50°C lower than the main solvents EC (flash point 143°C) and PC (flash point 133°C). Therefore, the more hexyl pivalate is added, the lower the flash point of the entire electrolyte solution is due to the influence of the flash point of hexyl pivalate. On the other hand, octyl pivalate (R4 = n-octyl group) has a flash point of 103°C, which is higher than the flash point of hexyl pivalate, but its viscosity is 2.5 cp, which is higher than the viscosity (2.3 cp) of PC used as the main solvent. This slows the movement of lithium ions and reduces rapid charging characteristics, especially at low temperatures. Furthermore, Patent Document 1 describes sec-octyl pivalate as one of the examples of pivalic acid esters, indicating that the ester moiety of the pivalic acid ester is branched, but does not specifically suggest whether the ester group is a 2-octyl group, a 3-octyl group, or a 4-octyl group. Patent Document 1 does not teach the problems of lowering the flash point of the entire electrolyte solution or causing a decrease in rapid charging characteristics at low temperatures, and does not specifically disclose a pivalic acid ester having 12 to 13 carbon atoms and a flash point of 90°C or higher and a viscosity of 2 to 2.3 cp at 25°C. Even a person skilled in the art would not easily have conceived of the present invention, which uses a pivalic acid ester having 12 to 13 carbon atoms and a flash point of 90°C or higher and a viscosity of 2 to 2.3 cp at 25°C, and further adds the pivalic acid ester in an amount of 0.1% by volume or more but less than 5% by volume relative to the non-aqueous solvent.

[0012] Furthermore, the prior art disclosed in Patent Document 2 differs from the pivalic acid ester of the present invention in that R ４ has 1 to 6 carbon atoms, and similarly to Patent Document 1, Patent Document 2 does not teach the problem of the present invention at all, so it is not easy for a person skilled in the art to arrive at the pivalic acid ester and its addition amount in the present invention from Patent Document 2. Incidentally, in paragraph

[0034] of Patent Document 2, it is stated that R ４ n-heptyl pivalate (R ４ n-butyl pivalate (R ４(carbon number 4) and n-hexyl pivalate (R ４ n-pentyl pivalate (R ４ A person skilled in the art would recognize that this is a typographical error in the number of carbon atoms (5).

[0013] As described above, the present invention is an invention that has discovered a problem that has not been recognized in the past and found that the problem can be solved by using a specific amount of a specific pivalic acid ester. ２ F) ２ It is preferable to use, as the non-aqueous solvent, ethylene carbonate / propylene carbonate = 49 / 51 to 10 / 90 (volume ratio) which has a high flash point, and it has been found that by combining these electrolyte salts and non-aqueous solvents, the effects of the present invention can be further enhanced.

[0014] That is, the present invention is characterized by the following features. (1) A lithium ion secondary battery including a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte solution containing an electrolyte salt in a nonaqueous solvent, wherein the nonaqueous electrolyte solution contains 0.1% by volume or more and less than 5% by volume of a pivalic acid ester having 12 to 13 carbon atoms and having a flash point of 90°C or more and a viscosity of 2 to 2.3 cp at 25°C, relative to the nonaqueous solvent. (2) The lithium ion secondary battery according to (1) above, wherein the pivalic acid ester is at least one selected from n-heptyl pivalate, 2-ethylhexyl pivalate, and 2-octyl pivalate. (3) A lithium ion secondary battery including LiN(SO 4 ) as an electrolyte salt. ２ F) ２ or LiN(SO ２ F) ２ and LiPF ６ The non-aqueous solvent contains 0.5 to 3 mol / L of the LiN(SO ２ F) ２ and the LiPF ６ The weight ratio of LiN(SO ２ F) ２ / LiPF ６= 100 / 0 to 1 / 99. (4) The lithium ion secondary battery according to any one of (1) to (3) above, wherein the non-aqueous solvent contains ethylene carbonate and propylene carbonate, and the volume ratio of the ethylene carbonate to the propylene carbonate is ethylene carbonate / propylene carbonate = 49 / 51 to 10 / 90. (5) A non-aqueous electrolyte solution obtained by dissolving an electrolyte salt in a non-aqueous solvent, the non-aqueous electrolyte solution containing 0.1 vol % or more and less than 5 vol % of a pivalic acid ester having 12 to 13 carbon atoms and having a flash point of 90°C or higher and a viscosity at 25°C of 2 to 2.3 cp, relative to the non-aqueous solvent. (6) The non-aqueous electrolyte solution according to (5) above, comprising at least one selected from the following (i), (ii), and (iii): (i) The pivalic acid ester is at least one selected from n-heptyl pivalate, 2-ethylhexyl pivalate, and 2-octyl pivalate. (ii) The electrolyte salt is LiN(SO ２ F) ２ and LiPF ６ and the LiN(SO ２ F) ２ and the LiPF ６ are dissolved in a non-aqueous solvent at a total concentration of 0.5 to 3 mol / L, and the LiN(SO ２ F) ２ and the LiPF ６ The weight ratio of LiN(SO ２ F) ２ / LiPF ６ (iii) The non-aqueous solvent is ethylene carbonate and propylene carbonate, and the volume ratio of the ethylene carbonate to the propylene carbonate is ethylene carbonate / propylene carbonate=49 / 51 to 10 / 90.

[0015] The lithium ion secondary battery of the present invention is excellent in battery safety when used at high temperatures, and also in battery properties such as initial capacity and rapid charging at low temperatures. Furthermore, by using the nonaqueous electrolyte solution of the present invention, a lithium ion secondary battery can be obtained that is excellent in battery safety when used at high temperatures, and also in battery properties such as initial capacity and rapid charging at low temperatures.

[0016] The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte solution containing an electrolyte salt in a nonaqueous solvent. The nonaqueous electrolyte solution contains 0.1% by volume or more and less than 5% by volume of a pivalic acid ester having 12 to 13 carbon atoms and having a flash point of 90°C or higher and a viscosity of 2 to 2.3 cp at 25°C. The nonaqueous solvent used in the present invention is not particularly limited as long as it is a nonaqueous solvent that can be used in nonaqueous electrolyte solutions for lithium ion secondary batteries. For example, nonaqueous solvents having a flash point of 120°C or higher are suitable, and cyclic carbonates having a flash point of 120°C or higher are preferred. Examples of cyclic carbonates having a flash point of 120°C or higher include EC (flash point 143°C), PC (flash point 133°C), and mixtures thereof are preferred. When the ratio of EC in the non-aqueous solvent is high, EC has a melting point of 36°C and is solid at room temperature. Therefore, EC precipitation may occur at low temperatures, resulting in a decrease in low-temperature characteristics. Furthermore, when the ratio of PC is high, the ionic conductivity of the non-aqueous electrolyte may decrease. Therefore, when EC and PC are used in combination, the volume ratio of EC to PC in the non-aqueous solvent is preferably EC / PC = 49 / 51 to 10 / 90, more preferably 40 / 60 to 20 / 80. When EC and PC are used in combination, the non-aqueous solvent may consist solely of EC and PC, or may contain non-aqueous solvents other than EC and PC. Therefore, when the non-aqueous solvent contains solvents other than EC and PC, the volume ratio of EC to PC refers to the volume ratio of EC and PC contained in the non-aqueous solvent. When EC and PC are used in combination, the total amount of EC and PC contained in the non-aqueous solvent is preferably 90 to 100% by volume. Another example of a cyclic carbonate having a flash point of 120°C or higher is fluoroethylene carbonate (FEC, flash point 122°C). Another example of a non-aqueous solvent in the present invention is vinylene carbonate (VC, flash point 80°C), which is a cyclic carbonate. When PC is used, 0.5 to 10% by volume of the cyclic carbonate can be replaced with this non-aqueous carbonate.

[0017] The pivalic acid ester added to the non-aqueous solvent in the present invention is not particularly limited as long as it has 12 to 13 carbon atoms, a flash point of 90°C or higher, and a viscosity at 25°C of 2 to 2.3 cp. However, preferred examples of the linear pivalic acid ester include n-heptyl pivalate, and preferred examples of the branched pivalic acid ester include one or more selected from 2-ethylhexyl pivalate and 2-octyl pivalate. Particularly preferred is 2-ethylhexyl pivalate, a branched ester that can achieve the highest flash point despite its low viscosity. Furthermore, when two or more pivalic acid esters are used, mixtures of n-heptyl pivalate and 2-octyl pivalate, n-heptyl pivalate and 2-ethylhexyl pivalate, 2-octyl pivalate and 2-ethylhexyl pivalate, and n-heptyl pivalate, 2-octyl pivalate and 2-ethylhexyl pivalate are particularly preferred because they can be adjusted to have a high flash point and a low viscosity.

[0018] In the non-aqueous electrolyte solution of the present invention, the pivalate ester is added in an amount of 0.1% by volume or more and less than 5% by volume relative to the non-aqueous solvent. "0.1% by volume or more and less than 5% by volume" means that the amount of pivalate ester is 0.1% by volume or more and less than 5% by volume relative to 100% by volume of non-aqueous solvent. If the amount of pivalate ester added (content in the non-aqueous electrolyte solution) is less than 0.1% by volume relative to the non-aqueous solvent, the separator may become insufficiently permeable, resulting in a decrease in battery performance. If the amount is 5% by volume or more, the flash point of the electrolyte solution may be lowered, and excessive permeability into the separator may increase the separator resistance, resulting in a decrease in low-temperature characteristics. Preferred ranges of the amount of the pivalic acid ester added in the present invention include 0.5 vol% or more and less than 5 vol%, 1 vol% or more and less than 5 vol%, 0.5 vol% or more and 4.5 vol% or less (0.5 to 4.5 vol%), and 1 vol% or more and 4.5 vol% or less (1 to 4.5 vol%). In the present specification, the designation "A to B" means A or more and B or less. When two or more pivalic acid esters are used in the present invention, the total amount of the pivalic acid esters added in the present invention is 0.1 vol% or more and less than 5 vol% with respect to the non-aqueous solvent, and preferred ranges of the total amount added include 0.5 vol% or more and less than 5 vol%, 1 vol% or more and less than 5 vol%, 0.5 to 4.5 vol%, or 1 to 4.5 vol%.

[0019] In the case of a battery using a graphite negative electrode, in order to suppress the reductive decomposition of PC on the graphite negative electrode, it is preferable to add at least one of pentafluorophenyl methanesulfonate (flash point 155°C), a chain compound containing two S=O skeletons and one S—O skeleton, having a flash point of 110°C or higher, 2-propynyl methanesulfonate (flash point 124°C), or 1,3-propane sultone (flash point >110°C), a cyclic compound containing two S=O skeletons and one S—O skeleton, to the non-aqueous solvent. Furthermore, compounds with a flash point higher than 90°C of the pivalic acid ester of the present invention have the advantage of being able to raise the flash point of the non-aqueous electrolyte of the present invention to 120°C or higher, and the content of these compounds added to the non-aqueous electrolyte is preferably within the range of 0.1 to 5% by mass relative to the entire non-aqueous electrolyte. Furthermore, the non-aqueous solvent in the present invention does not exclude non-aqueous solvents with low flash points, such as chain carbonates such as DMC, EMC, and DEC. In the present invention, a combination of a nonaqueous solvent and a pivalic acid ester that results in a flash point of the nonaqueous solvent of 100°C or higher after the addition of a pivalic acid ester can be suitably used, and a combination that results in a flash point of 120°C or higher is more suitable. The flash point of the nonaqueous electrolyte solution in the present invention is preferably 100°C or higher, more preferably 120°C or higher. The present invention does not exclude the inclusion of a pivalic acid ester other than the pivalic acid ester of the present invention in the nonaqueous solvent. When a pivalic acid ester other than the pivalic acid ester of the present invention (hereinafter also referred to as "other pivalic acid ester") is used in combination, it is preferable that the nonaqueous solvent has either one of a flash point of 90°C or higher or a viscosity at 25°C of 2 to 2.3 cp, from the viewpoint of minimizing the influence on the effects of the present invention. Examples of such pivalic acid esters include n-octyl pivalic acid esters (number of carbon atoms: 13, flash point: 104°C, viscosity: 2.52 cp), n-nonyl pivalic acid esters (number of carbon atoms: 14, flash point: 116°C, viscosity: 3.01 cp), and 2-nonyl pivalic acid esters (number of carbon atoms: 14, flash point: 104°C, viscosity: 2.65 cp). When the pivalic acid ester of the present invention is used in combination with another pivalic acid ester, the total amount of the pivalic acid ester of the present invention and the other pivalic acid ester is preferably less than 5% by volume.

[0020] The electrolyte salt in the present invention is not particularly limited as long as it can be used in the electrolyte solution of a lithium ion secondary battery. For example, LiN(SO ２ F) ２ , LiPF ６ , LiN(SO ２ CF ３ ) ２ , LiBF 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, ６ It is preferable to add a certain amount of LiN(SO ). The reason for this is presumably due to the fact that the solubility of the Li salt in the pivalic acid ester in the battery is increased, which allows Li ions to move more smoothly near the separator. 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 include 100 / 0 to 1 / 99, 100 / 0 to 50 / 50, 100 / 0 to 70 / 30, 95 / 5 to 50 / 50, and 90 / 10 to 70 / 30. The non-aqueous electrolyte solution of the present invention can be prepared by dissolving a pivalic acid ester and an electrolyte salt in a non-aqueous solvent.

[0021] The separator in the present invention is not particularly limited as long as it is usable 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. The porous sheet or nonwoven fabric may have a single-layer or multilayer structure, and the separator surface may be coated with an oxide such as alumina. The thickness of the separator needs to be as thin as possible to increase the volumetric energy density of the battery. Therefore, the thickness of the separator is preferably 20 μm or less, more preferably 10 μm or less.

[0022] The negative electrode in the present invention is not particularly limited as long as it is a negative electrode that 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 preferably used. 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.

[0023] 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).

[0024] The positive electrode active material of the positive electrode in the present invention is, for example, LiCoO ２ , LiNiO ２ , LiCo １／３ Ni １／３ Mn １／３ O２ , LiCo ０．１５ Ni ０．８ Al ０．０５ O ２ , 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, LiMn2O4 having a spinel structure and LiFePO4 having an olivine structure are preferably used.

[0025] 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.

[0026] As the combination of the positive electrode composite and the negative electrode composite used in the present invention, the above-mentioned combinations for increasing the volumetric energy density or the above-mentioned combinations for improving rapid charge and discharge can be suitably mentioned, and a battery can be produced using these combinations.

[0027] The current collector used in the present invention 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 invention, 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 secondary battery of the present invention is not particularly limited, but examples of the shape of the 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 secondary battery of the present invention can be produced by assembling the above-mentioned positive electrode, negative electrode, and separator into the above-mentioned structure and injecting the above-mentioned nonaqueous electrolyte into the separator.

[0030] The present invention will be specifically described below with reference to examples of the present invention, but the technical scope of the present invention is not limited to these examples.

[0031] [Examples 1 to 3 and Comparative Examples 1 to 4] (Method for Measuring Flash Point) Using a Pensky-Martens type flash point tester, PMA500 (manufactured by Anton Paar), the flash points of the pivalic acid esters alone of Examples 1 to 3 and Comparative Examples 1 to 5 shown in Table 1 were measured. The results are shown in Table 1.

[0032] (Method of measuring viscosity) A rotational viscometer was used. The model was a VISCOMETER DV-I PrimeLV (manufactured by BROOKFIELD). The viscosity of the pivalic acid ester alone was measured five times at 25°C, and the average value was taken as the viscosity. The results are shown in Table 1.

[0033] (Method for measuring the permeability of an electrolyte into the pores of a microporous separator) ２ F) ２ An electrolyte solution was prepared by adding 3 to 4% by volume of pivalate ester to 100% by volume of a non-aqueous solvent with an EC / PC ratio of 40 / 60 (volume ratio). A three-layer microporous membrane separator, consisting of a polyethylene layer sandwiched between polypropylene layers, was immersed in this electrolyte solution for 15 seconds, then removed. The amount of addition at which the separator's light transmittance was visually observed to have completely changed from opaque to transparent was expressed in volume % as the permeability (wettability) of the non-aqueous solvent. The results are shown in Table 1. Note that the notation "two layers" in the table indicates that the non-aqueous solvent and the added pivalate ester did not completely mix, and all or part of the amount separated into two layers and did not permeate.

[0034]

[0035] The results in Table 1 demonstrate that an electrolyte solution using the pivalate ester of the present invention exhibits excellent permeability. Comparing the results of Examples 1 and 2 with those of Comparative Example 1, the viscosity of n-hexyl pivalate alone in Comparative Example 1 was lower than the viscosities of 2-ethylhexyl pivalate alone in Example 1 and n-heptyl pivalate alone in Example 2. However, the permeability as an electrolyte was 3.5 vol % in Example 1 and 3 vol % in Example 2, while it was 4 vol % in Comparative Example 1. Thus, Comparative Example 1, which had a lower viscosity alone, required a larger amount of addition. In other words, the use of the pivalate ester of the present invention allows for improved permeability of the electrolyte by reducing the amount of pivalate ester added, which has a lower flash point than nonaqueous solvents such as EC and PC. Furthermore, because the flash points of 2-ethylhexyl pivalate in Example 1 and n-heptyl pivalate in Example 2 are higher than that of n-hexyl pivalate in Comparative Example 1, the permeability of the electrolyte can be improved by using a pivalate ester with a higher flash point. Therefore, it is possible to solve the problem that the addition of pivalic acid ester lowers the flash point of the entire electrolyte solution, and also to solve the problem that the viscosity of the electrolyte solution increases, slowing the movement of lithium ions, resulting in a decrease in rapid charging characteristics at low temperatures. Furthermore, when Example 3 is compared with Comparative Examples 2 to 4, n-octyl pivalate in Comparative Example 2 has a high flash point of 104°C, but also a high viscosity of 2.52, and when added at 3 to 4 volume %, some of it did not mix and separated into two layers. Although the viscosity of 3-octyl pivalate alone in Comparative Example 3 and 4-octyl pivalate alone in Comparative Example 4 was lower than the viscosity of 2-octyl pivalate alone in Example 3, the permeability as an electrolyte was 3.5 vol% in Example 3 and 4 vol% in Comparative Examples 3 and 4, and a larger amount was required for Comparative Examples 3 and 4, which have a lower viscosity alone. Furthermore, the flash points of 3-octyl pivalate in Comparative Example 3 and 4-octyl pivalate in Comparative Example 4 are lower than the flash point of 2-octyl pivalate in Example 3. From these facts, it can be seen from Example 3 and Comparative Examples 2 to 4 that the present invention can achieve the effects described for the results of Examples 1 and 2 and Comparative Example 1.

[0036] Example 4 An electrolyte solution of Example 4 was prepared in the same manner as in Examples 1 to 3, except that 1 vol % of n-heptyl pivalate and 2 vol % of n-octyl pivalate were added as pivalates. The permeability of the electrolyte solution of Example 4 was evaluated in the same manner as in Examples 1 to 3. A microporous membrane separator was immersed in the electrolyte solution of Example 4 for 15 seconds, then removed and observed. The light transmittance of the separator completely changed from opaque to transparent. As shown in Comparative Example 2, n-octyl pivalate has a high flash point, but when used alone, it separates into two layers. However, by adding a small amount of n-heptyl pivalate, an electrolyte solution with excellent permeability could be obtained. Thus, even a small amount of the pivalate ester of the present invention has the effect of improving the permeability of the electrolyte. Therefore, the use of the pivalate ester of the present invention makes it possible to use pivalate esters that have a high flash point but high viscosity, which can cause problems with electrolyte permeability.

[0037] As described above, by using the pivalic acid ester of the present invention having a flash point of 90°C or higher and 12 to 13 carbon atoms, a battery having a flash point excellent in safety during high-temperature use can be obtained, and the amount of pivalic acid ester added that allows the electrolyte to completely penetrate the separator is smaller than in the comparative example, and the permeability of the electrolyte can be improved even with a very small amount of pivalic acid ester added, which has been found to have a high effect of improving the penetration ability into the separator.In addition, a specific range has been found in which the pivalic acid ester of the present invention having a flash point of 90°C or higher and a viscosity of 2 to 2.3 cp at 25°C can be added in a small amount, thereby achieving a high flash point and a low viscosity. It has also been found that the electrolyte solution containing the pivalic acid ester of the present invention, which has low viscosity and excellent permeability into the separator, quickly penetrates the microporous separator when a laminate or wound body consisting of a positive electrode sheet, a separator, and a negative electrode sheet is placed in a battery container and then filled with the electrolyte during mass production of lithium ion secondary batteries, thereby shortening the manufacturing time of the lithium ion secondary battery. Furthermore, in a method for manufacturing a clay-like lithium secondary battery that is composed of two electrode layers, a clay-like positive electrode and a clay-like negative electrode, rather than a sheet-like positive electrode and a sheet-like negative electrode, and that separates the two layers with a separator, using an electrolyte with a high flash point is effective from a safety perspective.

[0038] [Example 5] (Preparation of electrolyte solution) 1M LiN(SO ２ F) ２ +0.1M LiPF ６ A non-aqueous solvent was prepared so that 2-ethylhexyl pivalate was 4% by volume relative to 100% by volume of an electrolytic solution of EC / PC=1 / 2 (volume ratio).

[0039] (Fabrication of Lithium Secondary Battery and Measurement of Battery Characteristics) LiNi ０．８ Co ０．１ Mn ０．１ O ２(positive electrode active material, abbreviated as NCM811) in a ratio of 80% by weight, acetylene black (conductive additive) in a ratio of 10% by weight, and polyvinylidene fluoride (binder) in a ratio of 10% by weight. 1-Methyl-2-pyrrolidone was added to this to form a slurry, which was then applied to aluminum foil. The mixture was then dried and pressure-molded to prepare a positive electrode. Similarly, Li ４ Ti ５ O １２ A mixture of 90% by weight of LTO (negative electrode active material) and 10% by weight of polyvinylidene fluoride (binder) was added to this mixture, and 1-methyl-2-pyrrolidone was added to form a slurry, which was then applied to aluminum foil. The mixture was then dried, pressure-molded, and heat-treated to prepare the negative electrode. A three-layer, 20-micron microporous film consisting of polyethylene sandwiched between polypropylene sheets was used as the separator, and the above-mentioned electrolyte was injected into the separator to prepare a coin battery (20 mm diameter, 3.2 mm thickness).

[0040] This coin battery was subjected to a charge / discharge test at 0° C., 25° C., and 60° C. using a charge / discharge device ACD-MO1A (manufactured by Asuka Electronics) at 25° C. with the operating voltage set to 3.0 V to 1.4 V. The charge / discharge conditions are shown in Table 2.

[0041]

[0042] Examples 6 to 8 and Comparative Example 5 Coin batteries were fabricated in the same manner as in Example 5, except that the pivalic acid ester used was changed as shown in Table 3, and the battery characteristics were measured.

[0043]

[0044] The results of Examples 5 to 8 and Comparative Example 5 showed that the electrolytes of the Examples achieved superior results to the Comparative Examples in terms of initial capacity and 20C rapid charging at low temperatures (0°C). The results are shown in Tables 4 to 6. Table 4 shows the test results at 0°C, Table 5 shows the test results at 25°C, and Table 6 shows the test results at 60°C. The numbers in the tables represent capacity ratios. The battery capacity value at each test temperature and each cycle number in Comparative Example 5 was set as the standard (1.00), and the capacity ratio was calculated as the capacity ratio = each value of Examples 5 to 8 / the value of Comparative Example 5. At high charge rates of 10C and 20C, differences arose between the results of the Examples and Comparative Examples, resulting in larger capacity ratios. The capacity ratio was particularly large at the sixth cycle (20C) at 0°C, demonstrating excellent rapid charging characteristics at low temperatures. Furthermore, the same capacity at the first and eighth cycles indicates no battery capacity degradation. It was also found that a mixture of 2-ethylhexyl pivalate, which has a relatively high flash point, and n-heptyl pivalate, which has a relatively low viscosity, further improved the effects of the present invention. Furthermore, it was found that the effects of the present invention can be obtained even with n-octyl pivalate, which is a pivalate ester other than the pivalate ester of the present invention, as long as a mixture is prepared by replacing a portion of the n-octyl pivalate with n-heptyl pivalate, which is the pivalate ester of the present invention. Thus, it was found that a lithium ion secondary battery using the nonaqueous electrolyte solution of the present invention is excellent in safety when used at high temperatures and also in rapid charging at low temperatures.

[0045]

[0046]

[0047]

[0048] The lithium ion secondary battery of the present invention has a positive electrode active material containing LiNi. ０．５ Co ０．２ Mn ０．３ O ２ In the case of a battery that emphasizes volumetric energy density and uses a mixture of natural and artificial graphite as the negative electrode active material, the rapid charging characteristics showed a similar tendency, although the difference was smaller than when LTO was used as the negative electrode active material.

[0049] By using the nonaqueous electrolyte of the present invention 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 also in battery characteristics such as rapid charging at low temperatures. The contribution of this invention to the industry is immeasurable.

Claims

DEPCT671. A secondary lithium-ion battery consisting of a positive electrode, a negative electrode, a separator, and an anhydrous electrolyte solution containing electrolyte salts in an anhydrous solvent. The anhydrous electrolyte solution contains pivalate esters in an amount of 0.1% by volume or more and less than 5% by volume relative to the anhydrous solvent. The pivalate esters have 12 to 13 carbon atoms and a flash point of 10°C.

1. Secondary batteries using lithium ions according to claim 1, where the pivalate ester is at least one substance chosen from a group consisting of n-heptyl pivalate, 2-ethylhexyl pivalate and 2-octyl pivalate.

3. Secondary batteries using lithium ions according to claim 1 or 2, which include an electrolyte salt of which is LiN(SO2F)2 from 0.5 to 3 mol / L in an anhydrous solvent or LiN(SO2F)2 and LiPF6 from 0.5 to 3 mol / L in an aqueous solvent where the weight ratio of LiN(SO2F)2 / LiPF6, which is the ratio of LiN(SO2F)2 to LiPF6, is between 100 / 0 and 1 / 994. Secondary batteries using lithium ions according to one of the claims 1 to 3, where the aqueous solvent consists of ethylene carbonate and propylene carbonate and ethylene carbonate / propylene carbonate, and the volume ratio of ethylene carbonate to propylene carbonate is between 49 / 51 and 10 / 905. Electrolyte solutions An anhydrous electrolyte consisting of an electrolyte salt dissolved in an anhydrous solvent; an anhydrous electrolyte solution containing pivalate esters in an amount equal to or greater than 0.1% by volume and less than 5% by volume relative to the anhydrous solvent; pivalate esters with 12 to 13 carbon atoms, having a flash point of 90°C or higher, and a viscosity of 2 to 2.3 centipoise at 25°C.An anhydrous electrolyte solution under Relief 5 shall consist of at least one substance selected from the groups (i), (ii), and (iii) in the following order: (i) pivalate esters are at least one substance selected from the group n-heptyl pivalate, 2-ethylhexyl pivalate, and 2-octyl pivalate; (ii) electrolyte salts are LiN(SO2F)2 and LiPF6, LiN(SO2F)2 and LiPF6 dissolved in total amounts from 0.5 to 3 mol / L in anhydrous solvent, and LiN(SO2F)2 / LiPF6, which is the weight ratio of LiN(SO2F)2 to LiPF6, ranging from 100 / 0 to 1 / 99; and (iii) the anhydrous solvent is ethylene carbonate and propylene carbonate, ethylene carbonate / propylene carbonate, which is the volume ratio of ethylene carbonate to propylene carbonate, ranging from 49 / 51 to 10 / 90.