Lithium-ion secondary battery and non-aqueous electrolyte

Abstract

The present invention addresses the problem of providing a lithium ion secondary battery that has excellent safety during use at high temperatures, which is considered important for in-vehicle secondary batteries for electric automobiles and the like, while additionally having excellent battery characteristics such as initial capacity and high-rate charge at low temperatures.　The present invention provides a lithium ion secondary battery which is provided with a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte solution that contains a nonaqueous solvent and an electrolyte salt, and which is characterized in that the nonaqueous electrolyte solution contains a pivalic acid ester having 12 to 13 carbon atoms in an amount of not less than 0.1% by volume but less than 5% by volume relative to the nonaqueous solvent, the pivalic acid ester having a flash point of 90°C or more and a viscosity of 2 to 2.3 cp at 25°C.

Description

[Technical Field]

[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. [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 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 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] However, because chain carbonates have extremely low flash points, the flash points of DMC, EMC, and DEC are affected, lowering the flash point of the entire electrolyte to around 25°C. This raises concerns about the safety of batteries used at high temperatures when used as a power source for in-vehicle secondary batteries, such as electric vehicles. For this reason, battery safety has become a major focus for in-vehicle secondary batteries, such as those used in 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] Patent Document 1 proposes a lithium secondary battery 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, by using a nonaqueous solvent (e.g., EC, PC) as the solvent for the electrolyte, in which a tertiary carboxylic acid ester in which the alkyl group (R4) bonded to the oxygen atom has 4 to 20 carbon atoms.

[0005] Furthermore, Patent Document 2 proposes a lithium secondary battery 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 solution that contains at least three specific lithium salts as an electrolyte and further contains a tertiary carboxylic acid ester in which the alkyl group (R4) bonded to an oxygen atom has 1 to 6 carbon atoms.

[0006] In this prior art, Patent Document 1 shows that adding tertiary carboxylic acid esters such as methyl pivalate, ethyl pivalate, butyl pivalate, hexyl pivalate, octyl pivalate, decyl pivalate, or dodecyl pivalate to the nonaqueous 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 pivalates has the problem of lowering the flash point of the entire electrolyte solution. Using pivalates that do not lower the flash point of the entire electrolyte solution results in high viscosity, slowing the migration of lithium ions and resulting in a decrease in rapid charging characteristics, especially at low temperatures. This problem was not previously recognized and was revealed through the inventors' research and development of electrolyte solutions, i.e., it is a newly discovered issue.

[0007] Furthermore, Patent Document 2 shows that the discharge capacity retention rate after high-temperature storage at 60°C during charging was improved and that an increase in impedance was suppressed (Examples), but Patent Document 2 also fails to teach any problems such as lowering the flash point of the entire electrolyte or causing a decrease in rapid charging characteristics at low temperatures. While emphasis has traditionally been 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 has not been 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 anticipated. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 4691871 [Patent Document 2] Patent No. 6575521 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[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 hexyl pivalate's flash point. 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 lists sec-octyl pivalate as an example of a pivalate ester, indicating that the ester moiety of the pivalate 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 or reducing fast charging characteristics at low temperatures, and does not specifically disclose a pivalate 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 find it difficult to conceive of the present invention, which uses a pivalate 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 pivalate 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 R4 in the tertiary carboxylic acid ester has 1 to 6 carbon atoms, and similarly to Patent Document 1, no problem to be solved by the present invention is taught. Therefore, it would not be easy for a person skilled in the art to arrive at the pivalic acid ester of the present invention and its addition amount from Patent Document 2. Incidentally, paragraph

[0034] of Patent Document 2 describes n-heptyl pivalate (R4 has 7 carbon atoms), in which R4 does not have 1 to 6 carbon atoms. However, this is written between n-butyl pivalate (R4 has 4 carbon atoms) and n-hexyl pivalate (R4 has 6 carbon atoms), and a person skilled in the art would recognize this as a typographical error for n-pentyl pivalate (R4 has 5 carbon atoms).

[0013] As described above, the present invention is an invention that has discovered a previously unrecognized problem and found that the problem can be solved by using a specific amount of a specific pivalic acid ester. Furthermore, in the present invention, it is preferable to use LiN(SO2F2)2 as the electrolyte salt in the non-aqueous electrolyte, and it is preferable to use ethylene carbonate / propylene carbonate = 49 / 51 to 10 / 90 (volume ratio) as the non-aqueous solvent, which has a high flash point. It has been found that the effects of the present invention can be further enhanced by combining these electrolyte salts and non-aqueous solvents.

[0014] That is, the present invention is specified by the following items. (1) A 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 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 a flash point of 90°C or more and a viscosity of 2 to 2.3 cp at 25°C, relative to the non-aqueous solvent. (2) The lithium ion secondary battery according to (1) above, wherein the pivalic acid ester is at least one selected from the group consisting of n-heptyl pivalate, 2-ethylhexyl pivalate, and 2-octyl pivalate. (3) The lithium ion secondary battery according to (1) or (2) above, characterized in that the non-aqueous solvent contains 0.5 to 3 mol / L of LiN(SO2F)2 as an electrolyte salt, or the non-aqueous solvent contains 0.5 to 3 mol / L of LiN(SO2F)2 and LiPF6, and the weight ratio of the LiN(SO2F)2 to the LiPF6 is LiN(SO2F)2 / LiPF6=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 comprising an electrolyte salt dissolved in a non-aqueous solvent, the non-aqueous electrolyte solution containing 0.1% by volume or more and less than 5% by volume of a pivalic acid ester having 12 to 13 carbon atoms and a flash point of 90°C or more and a viscosity of 2 to 2.3 cp at 25°C, relative to the non-aqueous solvent. (6) The nonaqueous electrolyte solution according to (5) above, which comprises 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(SO2F)2 and LiPF6, the LiN(SO2F)2 and the LiPF6 are dissolved in a total amount of 0.5 to 3 mol / L in the non-aqueous solvent, and the weight ratio of the LiN(SO2F)2 to the LiPF6 is LiN(SO2F)2 / LiPF6=100 / 0 to 1 / 99. (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. [Effects of the Invention]

[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. DETAILED DESCRIPTION OF THE INVENTION

[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, 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 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 usable in nonaqueous electrolyte solutions for lithium ion secondary batteries. Examples of the nonaqueous solvent include nonaqueous solvents with a flash point of 120°C or higher, and preferred examples include cyclic carbonates with a flash point of 120°C or higher. Preferred cyclic carbonates with a flash point of 120°C or higher include EC (flash point 143°C), PC (flash point 133°C), and mixtures thereof. If the proportion of EC in the non-aqueous solvent is high, EC's melting point is 36°C, making it a solid at room temperature. This can lead to EC precipitation at low temperatures, potentially resulting in reduced low-temperature properties. Furthermore, if the proportion of PC is high, the ionic conductivity of the non-aqueous electrolyte can be reduced. Therefore, when EC and PC are used in combination, the volume ratio of EC to PC in the non-aqueous solvent is preferably 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 PC can be replaced with this.

[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 pivalic acid straight-chain ester include n-heptyl pivalate, and preferred examples of the pivalic acid branched 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 but less than 5% by volume relative to the non-aqueous solvent. Here, 0.1% by volume or more but less than 5% by volume relative to the non-aqueous solvent means that the amount of pivalate ester is 0.1% by volume or more but 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% by volume or more and less than 5% by volume, 1% by volume or more and less than 5% by volume, 0.5% by volume or more and less than 4.5% by volume (0.5 to 4.5% by volume), and 1% by volume or more and less than 4.5% by volume (1 to 4.5% by volume). In the present specification, the designation "A to B" means A to B. 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% by volume or more and less than 5% by volume with respect to the non-aqueous solvent, and preferred ranges of the total amount added include 0.5% by volume or more and less than 5% by volume, 1% by volume or more and less than 5% by volume, 0.5 to 4.5% by volume, or 1 to 4.5% by volume.

[0019] In the case of batteries 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 the following compounds to the non-aqueous solvent: pentafluorophenyl methanesulfonate (flash point 155°C), 2-propynyl methanesulfonate (flash point 124°C), a chain compound containing two S=O skeletons and one SO skeleton, and 1,3-propane sultone (flash point >110°C), which have a flash point of 110°C or higher. 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. The content of these compounds in the non-aqueous electrolyte is preferably within a range of 0.1 to 5% by mass relative to the total non-aqueous electrolyte. Furthermore, the non-aqueous solvent of 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 non-aqueous solvent and a pivalic acid ester that results in a flash point of the non-aqueous 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 non-aqueous 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 non-aqueous 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 other pivalic acid ester have either one of a flash point of 90°C or higher or a viscosity at 25°C of 2 to 2.3 cp, in order to minimize the effect 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 used 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. Examples include LiN(SO2F)2, LiPF6, LiN(SO2CF3)2, and LiBF4. LiN(SO2F)2 is preferred because it has high chemical thermal stability and can improve battery performance at high temperatures. Furthermore, since LiPF6 has the effect of supplementarily improving battery performance at low temperatures, it is preferable to add a certain amount of LiPF6. This is presumably because the solubility of the Li salt in the pivalic acid ester in the battery is increased, allowing Li ions to move more smoothly 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. 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 1 / 99, 100 / 0 to 50 / 50, 100 / 0 to 70 / 30, 95 / 5 to 50 / 50, or 90 / 10 to 70 / 30. The nonaqueous electrolyte solution of the present invention can be prepared by dissolving a pivalic acid ester and an electrolyte salt in a nonaqueous 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 must 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, and 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 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] The positive electrode active material of the positive electrode in the present invention may be, for example, LiCoO2, LiNiO2, LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, LiCo 0.15 Ni 0.8 Al 0.05 O2, 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 Al0.05 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] 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 preferably used as the 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. [Example]

[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] (Viscosity measurement method) The viscometer used was a rotational viscometer. 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 electrolyte into the pores of a microporous separator) An electrolyte solution was prepared by adding 3-4% by volume of pivalate ester to 100% by volume of a 1M LiN(SO2F)2EC / PC = 40 / 60 (volume ratio). A three-layer microporous membrane separator consisting of a polyethylene layer sandwiched between polypropylene layers was immersed in this electrolyte 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 as the permeability (wettability) of the nonaqueous solvent, expressed in volume percent. The results are shown in Table 1. Note that the notation "2 layers" in the table indicates that the nonaqueous 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] [Table 1]

[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 that 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 pivalate added. In other words, the use of the pivalate ester of the present invention allows for improved electrolyte permeability by reducing the amount of pivalate 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 use of a pivalate ester with a higher flash point can improve electrolyte permeability. Therefore, the addition of pivalic acid ester not only reduces the flash point of the entire electrolyte solution, but also reduces the rapid charging characteristics at low temperatures due to the increased viscosity of the electrolyte solution, which slows the movement of lithium ions. Furthermore, comparing Example 3 with Comparative Examples 2 to 4, n-octyl pivalate in Comparative Example 2 had a high flash point of 104°C, but also a high viscosity of 2.52, and when added at 3 to 4% by volume, some of it did not mix and separated into two layers. Although the viscosity of 3-octyl pivalate in Comparative Example 3 and 4-octyl pivalate in Comparative Example 4 alone 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 the permeability of the electrolyte.

[0037] As described above, by using the pivalic acid ester having a flash point of 90°C or higher and 12 to 13 carbon atoms according to the present invention, 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 electrolyte permeability 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 having a flash point of 90°C or higher and 12 to 13 carbon atoms according to the present invention, which has 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. 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 the mass production process of lithium ion secondary batteries, and it has also been found to shorten the production time of lithium ion secondary batteries. Furthermore, in the production method of clay-like lithium secondary batteries, which are composed of two electrode layers, a clay-like positive electrode and a clay-like negative electrode, rather than sheet-like positive electrode and negative electrode, and which separate 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) A non-aqueous solvent was prepared so that 2-ethylhexyl pivalate was 4% by volume relative to 100% by volume of an electrolyte solution of 1M LiN(SO2F)2 + 0.1M LiPF6EC / PC = 1 / 2 (volume ratio).

[0039] (Fabrication of lithium secondary batteries and measurement of battery characteristics) LiNi 0.8 Co 0.1 Mn 0.1The cathode active material was 80% by weight of Li4Ti5O2 (abbreviated as NCM811), 10% by weight of acetylene black (conductive additive), and 10% by weight of polyvinylidene fluoride (binder). 1-Methyl-2-pyrrolidone was added to this mixture to form a slurry, which was then applied to aluminum foil. The mixture was then dried and pressure-molded to prepare the cathode. Similarly, Li4Ti5O 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, and 1-methyl-2-pyrrolidone was added to form a slurry, which was then applied to aluminum foil. After drying, the mixture was press-molded and heated to prepare the negative electrode. The separator was a three-layer, 20-micron microporous film made of polyethylene sandwiched between polypropylene layers, and the above electrolyte was injected into it to produce a coin battery (20 mm diameter, 3.2 mm thick).

[0040] This coin battery was subjected to charge-discharge tests 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 3.0V to 1.4V. The charge-discharge conditions are shown in Table 2.

[0041] [Table 2]

[0042] [Examples 6 to 8 and Comparative Example 5] A coin battery was 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] [Table 3]

[0044] The results of Examples 5 to 8 and Comparative Example 5 showed that the electrolytes of the Examples were superior to those of 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 in Examples 5 to 8 / the value in Comparative Example 5. At high charge rates of 10C and 20C, differences occurred 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, indicating excellent rapid charging characteristics at low temperatures. Furthermore, the capacities at the first and eighth cycles were the same, indicating 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, could further improve 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 not only in safety when used at high temperatures but also in rapid charging at low temperatures.

[0045] [Table 4]

[0046] [Table 5]

[0047] [Table 6]

[0048] The lithium ion secondary battery of the present invention uses LiNi as the positive electrode active material. 0.5 Co 0.2 Mn0.3 In the case of batteries that emphasize volumetric energy density and use 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. [Industrial Applicability]

[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

1. A lithium ion secondary battery including 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 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 non-aqueous solvent.

2. 2. The lithium ion secondary battery according to claim 1, wherein the pivalic acid ester is at least one selected from the group consisting of n-heptyl pivalate, 2-ethylhexyl pivalate, and 2-octyl pivalate.

3. LiN(SO ) was used as the electrolyte salt. ２ F) ２ or LiN(SO ２ F) ２ and LiPF ６ in a non-aqueous solvent at 0.5 to 3 mol / L, and when LiN(SO 2 F) 2 is contained alone in the non-aqueous solvent, or when LiN(SO 2 F) 2 and LiPF 6 are mixed and contained in the non-aqueous solvent, ２ F) ２ and the LiPF ６ The weight ratio of LiN(SO ２ F) ２ / LiPF ６ 3. The lithium ion secondary battery according to claim 1, wherein:

4. 3. The lithium ion secondary battery according to claim 1, 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% 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 non-aqueous solvent.

6. The nonaqueous electrolyte solution according to claim 5, 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 = 100 / 0 to 1 / 99. (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.

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