Electrolyte for lithium ion secondary battery

Abstract

The purpose of the present invention is to provide an electrolyte solution for a lithium ion secondary battery, the electrolyte solution being capable of optimizing the amount of cyclic carbonate, suppressing a decomposition reaction of a graphite-based negative electrode, and improving battery performance in a low-temperature region. This electrolyte solution for a lithium ion secondary battery is characterized by containing: 10-20 vol% of an ethylene carbonate (EC) and / or a propylene carbonate (PC) as a cyclic carbonate; 0.38 mol / L to 0.75 mol / L of at least one of lithium hexafluorophosphate (LiPF6), lithium fluoroborate (LiBF4) and lithium bis (oxalato) borate (LiBOB) as a first lithium salt; and an imine lithium salt, the total amount of the imine lithium salt and the first lithium salt being in the range of 0.5 mol / L to 1.5 mol / L, and the molar ratio represented by the cyclic carbonate / first lithium salt being in the range of 3 to 5.

Description

[0001] This application is a divisional application of a patent application with the same name having an application number of 201880083634.4 and filed on November 28, 2018. Technical Field

[0002] The present invention relates to an electrolyte for a lithium-ion secondary battery. Background Art

[0003] In recent years, power sources for mobile communications such as mobile phones or portable personal computers have increasingly been required to be miniaturized and have a high energy density (high capacity), and the practical application of electric vehicles, hybrid vehicles that use electric power for part of the power, and hybrid trains is being promoted. Moreover, from the viewpoint of environmental protection, not only the storage of nighttime electricity, but also the development of power storage sources combined with solar cells or wind power generation is being gradually promoted. Under such circumstances, lithium-ion secondary batteries have attracted much attention. However, the charge and discharge efficiency of lithium-ion secondary batteries decreases due to repeated charge and discharge, so there is a need for lithium-ion secondary batteries with little deterioration in battery performance over time.

[0004] As an electrolyte used in a lithium-ion secondary battery, Patent Document 1 discloses: i) a non-aqueous organic solvent containing propylene carbonate (PC) and ethylene carbonate (EC), and ii) a non-aqueous electrolyte containing lithium bis(fluorosulfonyl)imide (LiFSI). Moreover, the mixing ratio of the above propylene carbonate and ethylene carbonate is preferably 1:0.1 to 2 by weight. In a lithium-ion secondary battery containing this non-aqueous electrolyte, a strong SEI film is formed on the negative electrode during initial charging, and thus the output characteristics at low temperature and normal temperature, the cycle characteristics at high temperature and normal temperature, and the capacity characteristics after high-temperature storage can be improved.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2015-523701 Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] In the prior art, as non-aqueous electrolytes, various non-aqueous organic solvents have been studied so far. As polar solvents, propylene carbonate (PC) or ethylene carbonate (EC) is used. However, propylene carbonate (PC) has a problem of irreversible decomposition reaction with a graphite-based negative electrode. Therefore, a non-aqueous organic solvent based on ethylene carbonate (EC) is generally used. However, since ethylene carbonate (EC) has a high melting point, there is a problem that the viscosity of the non-aqueous organic solvent increases in a low-temperature region, resulting in a decrease in battery characteristics.

[0010] Regarding such a problem, in the above-mentioned Patent Document 1, by adjusting the mixing ratio of propylene carbonate (PC) and ethylene carbonate (EC) as organic solvents, the problems that occur when using propylene carbonate (PC) and ethylene carbonate (EC) separately can be solved, the respective advantages of these solvents can be exerted, and the synergistic effect brought about by mixing organic solvents can be exhibited.

[0011] However, there are the following problems in the electrolyte of Patent Document 1: it is difficult to completely suppress the irreversible decomposition reaction of the graphite-based negative electrode caused by propylene carbonate (PC), and it is desired to reduce the usage amount of propylene carbonate (PC) as much as possible. On the other hand, if the amount of ethylene carbonate (EC) is increased, the viscosity in the low-temperature region increases. Therefore, an object of the present invention is to provide an electrolyte for a lithium-ion secondary battery, which optimizes the amount of cyclic carbonates such as ethylene carbonate (EC) or propylene carbonate (PC), can suppress the decomposition reaction of the graphite-based negative electrode, and improve the battery performance in the low-temperature region.

[0012] Technical solution for solving the problem

[0013] In order to solve the above-mentioned problems, the electrolyte for a lithium-ion secondary battery of the present invention is characterized by comprising: ethylene carbonate (EC) and / or propylene carbonate (PC) as cyclic carbonates in an amount of 10% by volume to 20% by volume; at least one selected from lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), and lithium bis(oxalato)borate (LiBOB) as a first lithium salt in an amount of 0.38 mol / L to 0.75 mol / L; and an imide lithium salt, the total of the above-mentioned imide lithium salt and the above-mentioned first lithium salt being in the range of 0.5 mol / L to 1.5 mol / L, and the molar ratio represented by the above-mentioned cyclic carbonate / above-mentioned first lithium salt being in the range of 3 to 5.

[0014] Effect of the invention

[0015] According to the present invention, since the lithium ions of the imide lithium salt are easily dissociated, it is not necessary to use a large amount of polar solvents. Therefore, the amount of cyclic carbonates such as ethylene carbonate (EC) required to dissociate the first lithium salt is optimized, the decomposition reaction of the graphite-based negative electrode is suppressed, and the viscosity of the whole organic solvent is reduced, especially the battery performance in the low-temperature region is improved. In addition, through the description of the following embodiments, problems, structures, and effects other than the above will be further clarified. Description of the drawings

[0016] Figure 1 It is a perspective view of the appearance of a flat-wound lithium-ion secondary battery.

[0017] Figure 2It is an exploded perspective view of a flat wound lithium ion secondary battery.

[0018] Figure 3 It is an exploded perspective view of an electrode winding.

[0019] Figure 4 It is a graph showing the ionic conductivity with respect to the molar ratio of ethylene carbonate (EC) / LiPF 6 of.

[0020] Figure 5 It is a graph showing the DCR comparison results between Example 1 and Comparative Example 1. Detailed implementation mode

[0021] Hereinafter, embodiments of the present invention will be described using the accompanying drawings and the like. The present invention is not limited to these descriptions, and those skilled in the art can make various changes and modifications within the scope of the technical idea disclosed in this specification. In addition, in all the drawings used to describe the present invention, components having the same function are denoted by the same reference numerals, and their repeated descriptions are sometimes omitted.

[0022] As an embodiment of a lithium ion secondary battery using the electrolyte of the present invention, based on Figures 1 to 3 the structure of a flat wound lithium ion secondary battery will be described. Figure 1 It is an external perspective view of a flat wound lithium ion secondary battery, Figure 2 and it is an exploded perspective view thereof.

[0023] The flat wound lithium ion secondary battery 100 has a battery case 1 and a battery cover 6. The battery case 1 has a side surface composed of a pair of opposed wide side surfaces 1b with relatively large areas and a pair of opposed narrow side surfaces 1c with relatively small areas, and a bottom surface 1d, and has an opening 1a above it.

[0024] Inside the battery case 1, a winding 3 is housed together with an insulating protective film 2, and the opening 1a of the battery case 1 is sealed by the battery cover 6. The battery cover 6 is a substantially rectangular flat plate, and is welded in such a way as to block the upper opening 1a of the battery case 1, and the battery case 1 is sealed. The battery cover 6 is provided with a positive electrode external terminal 14 and a negative electrode external terminal 12. The winding 3 is charged through the positive electrode external terminal 14 and the negative electrode external terminal 12, and electric power is supplied to an external load. An exhaust valve 10 is integrally provided with the battery cover 6. If the pressure inside the battery container rises, the exhaust valve 10 opens, and gas is discharged from the inside, and the pressure inside the battery container decreases. Thereby, the safety of the flat wound lithium ion secondary battery 100 can be ensured.

[0025] The wound winding 3 is wound into a flat shape, so it has a pair of bent portions facing each other with a semicircular cross-section and a flat portion continuously formed between the pair of bent portions. The wound winding 3 is inserted into the battery case 1 from one bent portion side in such a manner that the winding axis direction is along the transverse width direction of the battery case 1, and the other bent portion side is disposed on the upper opening side.

[0026] The exposed portion 34c of the positive electrode foil of the wound winding 3 is electrically connected to the positive electrode external terminal 14 provided on the battery cover 6 via the positive electrode current collector plate (current collecting terminal) 44. In addition, the exposed portion 32c of the negative electrode foil of the wound winding 3 is electrically connected to the negative electrode external terminal 12 provided on the battery cover 6 via the negative electrode current collector plate (current collecting terminal) 24. Thus, electric power is supplied from the wound winding 3 to an external load through the positive electrode current collector plate 44 and the negative electrode current collector plate 24, and external generated electric power is supplied to the wound winding 3 through the positive electrode current collector plate 44 and the negative electrode current collector plate 24 for charging.

[0027] In order to electrically insulate the positive electrode current collector plate 44 and the negative electrode current collector plate 24, and the positive electrode external terminal 14 and the negative electrode external terminal 12 from the battery cover 6 respectively, the battery cover 6 is provided with a gasket 5 and an insulating plate 7. In addition, after injecting the electrolyte into the battery case 1 from the liquid injection port 9, the liquid injection plug 11 is joined to the battery cover 6 by laser welding to seal the liquid injection port 9, and the flat wound type lithium ion secondary battery 100 is sealed.

[0028] Here, as the forming materials of the positive electrode external terminal 14 and the positive electrode current collector plate 44, for example, aluminum alloy can be cited. As the forming materials of the negative electrode external terminal 12 and the negative electrode current collector plate 24, for example, copper alloy can be cited. In addition, as the forming materials of the insulating plate 7 and the gasket 5, for example, insulating resin materials such as polybutylene terephthalate, polyphenylene sulfide, and perfluoroalkoxy fluororesin can be cited.

[0029] The electrolyte of the electrolyte injected into the battery case 1 contains at least one selected from lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), and lithium bis(oxalato)borate (LiBOB) as the first lithium salt in an amount of 0.38 mol / L to 0.75 mol / L. In addition, the electrolyte contains an imide lithium salt, and the total of the first lithium salt and the imide lithium salt is in the range of 0.5 mol / L to 1.5 mol / L. As the imide lithium salt, lithium bis(fluorosulfonyl)imide (LiFSI) is preferably used. LiFSI can use commercially available products or materials synthesized by existing well-known methods. Or, as other imide lithium salts, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is also preferably used.

[0030] As the organic solvent in the electrolyte, a cyclic carbonate as a polar solvent is used in an amount of 10% by volume to 20% by volume. Moreover, it is preferable to mix a chain carbonate in an amount of 80% by volume to 90% by volume. If the cyclic carbonate decreases, the first lithium salt (for example, lithium hexafluorophosphate (LiPF 6 )) cannot be sufficiently dissociated, and the electrical performance deteriorates. In addition, when, for example, ethylene carbonate is used as the cyclic carbonate, if the cyclic carbonate increases, the conductivity may decrease due to an increase in the viscosity of the entire electrolyte, and particularly the battery performance at low temperatures may decrease.

[0031] Therefore, for the amount of the cyclic carbonate in which any one of lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ) and lithium bis(oxalato)borate (LiBOB) as the first lithium salt is sufficiently dissociated, the ratio of the number of moles of the cyclic carbonate to the number of moles of the first lithium salt (cyclic carbonate / first lithium salt) is set in the range of 3 to 5. The range of 3.5 to 4.5 is preferable.

[0032] As the cyclic carbonate, ethylene carbonate (EC), propylene carbonate (PC) or both are used. Fluoroethylene carbonate, vinylene carbonate, etc. as other cyclic carbonates may also be included. The cyclic carbonate may be used alone or in combination of two or more. In addition, when a substance other than ethylene carbonate (EC) and propylene carbonate (PC) is used as the cyclic carbonate, the total amount of all the cyclic carbonates does not exceed 20% by volume.

[0033] When using a chain carbonate, its type is not limited, and various known chain carbonates can be applied. Specifically, ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), etc. can be cited. Any one of the cyclic carbonates can be used alone, or two or more can be used in combination.

[0034] In addition, in order to improve various battery performances, the electrolyte for a lithium ion secondary battery of the present embodiment may also contain an additive as needed. As the additive, for example, sulfur-containing compounds such as dimethyl sulfone and tetramethylthiuram monosulfide; carboxylic anhydrides such as 1-methyl-2-pyrrolidone, 1-methyl-2-piperidone cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, and phenylsuccinic anhydride; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, etc. The total amount of these additives is preferably in the range of 0.1% by mass to 10% by mass in the electrolyte.

[0035] The positive external terminal 14 and the negative external terminal 12 have welding joints for welded connection with a bus bar or the like. The welding joint has a rectangular parallelepiped block shape protruding upward from the battery cover 6, and has a structure in which the lower surface faces the surface of the battery cover 6 and the upper surface is parallel to the battery cover 6 at a specified height position.

[0036] The positive connection part 14a and the negative connection part 12a have cylindrical shapes protruding from the lower surfaces of the positive external terminal 14 and the negative external terminal 12 respectively, and the front ends can be inserted into the positive electrode side through hole 46 and the negative electrode side through hole 26 of the battery cover 6. The positive connection part 14a and the negative connection part 12a penetrate the battery cover 6, protrude more toward the inside of the battery case 1 than the positive electrode current collector base 41 and the negative electrode current collector base 21 of the positive electrode current collector plate 44 and the negative electrode current collector plate 24, and the front ends are riveted to integrally fix the positive external terminal 14, the negative external terminal 12, the positive electrode current collector plate 44, the negative electrode current collector plate 24 and the battery cover 6. A gasket 5 is sandwiched between the positive external terminal 14, the negative external terminal 12 and the battery cover 6, and an insulating plate 7 is sandwiched between the positive electrode current collector plate 44, the negative electrode current collector plate 24 and the battery cover 6.

[0037] The positive electrode current collector plate 44 and the negative electrode current collector plate 24 have: a rectangular plate-shaped positive electrode current collector base 41 and a negative electrode current collector base 21 disposed opposite to the lower surface of the battery cover 6; a positive electrode side connection end portion 42 and a negative electrode side connection end portion 22 that are bent at the side ends of the positive electrode current collector base 41 and the negative electrode current collector base 21, extend toward the bottom surface side along the wide surface of the battery case 1, and are connected in a state of being opposed to and overlapping the positive electrode foil exposed portion 34c and the negative electrode foil exposed portion 32c of the wound winding 3. The positive electrode current collector base 41 and the negative electrode current collector base 21 are respectively formed with positive electrode side opening holes 43 and negative electrode side opening holes 23 for inserting the positive connection part 14a and the negative connection part 12a.

[0038] Taking the direction along the flat plane of the wound winding 3 and orthogonal to the winding axis direction of the wound winding 3 as the central axis direction, an insulating protective film 2 is wound around the wound winding 3. The insulating protective film 2 is composed of a single sheet or a plurality of film components made of a synthetic resin such as PP (polypropylene), and has a length capable of winding with the direction parallel to the flat plane of the wound winding 3 and orthogonal to the winding axis direction as the winding center.

[0039] Figure 3 It is an exploded perspective view showing a state in which a part of the electrode wound winding is unfolded.

[0040] The wound winding 3 is formed by winding the negative electrode 32 and the positive electrode 34 with the separators 33 and 35 interposed therebetween into a flat shape. For the wound winding 3, the outermost electrode is the negative electrode 32, and the separators 33 and 35 are wound outside thereof.

[0041] The separators 33 and 35 have an insulating function of preventing short circuit between the positive electrode 34 and the negative electrode 32, and also have a function of holding the non-aqueous electrolyte. Preferred examples include porous sheet materials made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. The porous sheet material made of resin can be a single-layer structure or a multi-layer structure (such as a three-layer structure of PP / PE / PP, etc.).

[0042] More preferably, a layer composed of an inorganic material (such as alumina particles, etc.) and an adhesive is provided on one side or both sides of the separators 33 and 35. Thereby, even when the lithium ion secondary battery is used under abnormal conditions (for example, when the temperature of the secondary battery rises to 160 °C or more due to overcharging or crushing, etc.), it can remain unmelted, maintain the insulating function, and ensure safety.

[0043] The portion of the negative electrode 32 coated with the negative electrode mixture layer 32b is larger than the portion of the positive electrode 34 coated with the positive electrode mixture layer 34b in the width direction. Thus, the portion coated with the positive electrode mixture layer 34b is configured to be sandwiched by the portion coated with the negative electrode mixture layer 32b. The exposed portions 34c of the positive electrode foil and 32c of the negative electrode foil are bundled in the planar portion and connected by welding or the like. In addition, the separators 33 and 35 are wider than the portion coated with the negative electrode mixture layer 32b in the width direction, but since the metal foil surface wound around the end is exposed at the positions of the exposed portions 34c of the positive electrode foil and 32c of the negative electrode foil, it will not become an obstacle during bundling and welding.

[0044] The negative electrode mixture layer 32b coated on the negative electrode 32 is made by dispersing a negative electrode active material and an adhesive as a binder into an appropriate solvent (such as water or N-methyl-2-pyrrolidone) and kneading, and coating to form a slurry-like material. After the solvent is removed by drying the negative electrode 32 coated with the slurry, it is made into an appropriate thickness using a pressing machine.

[0045] Examples of the negative electrode active material include carbon materials such as natural graphite, artificial graphite, hardly graphitizable carbon (hard carbon), and easily graphitizable carbon (soft carbon). Regarding graphite, by covering the surface of graphite with amorphous carbon, unnecessary reactions with the electrolyte can be prevented.

[0046] In addition, materials in which carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black as conductive aids are mixed in a graphite material, and materials in which these conductive aids are mixed with the graphite material and then covered and compounded with amorphous carbon can also be applied. Methods of mixing hardly graphitizable carbon (hard carbon), easily graphitizable carbon (soft carbon), and metal oxides (iron oxide, copper oxide, etc.) in graphite can also be used.

[0047] The positive electrode 34 has positive electrode mixture layers 34b on both sides of a positive electrode current collector foil as the positive electrode current collector. There is no particular limitation on the positive electrode active material contained in the positive electrode mixture layer 34b, and any one of known materials that can be used as the positive electrode material of a lithium ion secondary battery or a mixture of multiple kinds thereof can be used alone. As a preferred example, spinel-based (e.g., LiMn 2 O 4 etc.), layered (e.g., LiCoO 2 , LiNiO 2 ), olivine-based (e.g., LiFePO 4 etc.) can be cited. In addition, a layered lithium nickel cobalt manganese composite oxide containing Li, Ni, Co, and Mn as constituent elements (e.g., LiNi 0.33 Co 0.33 Mn 0.33 O 2 ), until the lithium ion insertion / extraction amount reaches 2 / 3, with charge and discharge, the lattice volume hardly changes, so the durability is also excellent and can be cited as a more preferred example.

[0048] In addition, as the shaft core, for example, a component composed of a resin sheet having a winding bending rigidity higher than that of the negative electrode 32, the positive electrode 34, and the separators 33 and 35 can be used.

[0049] Examples

[0050] The present invention will be described in more detail based on examples and comparative examples below, but the present invention is not limited to these examples.

[0051] (Measurement of ionic conductivity)

[0052] As the organic solvent, a material obtained by mixing ethylene carbonate (EC) as a cyclic carbonate, dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) as chain carbonates at a ratio of 50 vol%:50 vol% is used. Then, the ratio of the cyclic carbonate to the chain carbonate is set to 10 vol% / 90 vol% to 20 vol% / 80 vol%.

[0053] In such a solvent, 0.55 mol / L of LiPF 6 as the first lithium salt and 0.55 mol / L of LiFSI as the imide lithium salt are mixed, and the volume% of the cyclic carbonate EC is changed to adjust the molar ratio with LiPF 6 . The molar mass of LiPF 6 is 151.905 g / mol, and the molar mass of EC is 88.06 g / mol. In addition, the density of EC is 1.321 g / mL, the density of DMC is 1.069 g / mL, and the density of EMC is 0.975 g / mL.

[0054] The ionic conductivity (S / m) of the obtained electrolyte solution was measured at 0 °C. Figure 4 It shows the change of the ionic conductivity at 0 °C with respect to the molar ratio of ethylene carbonate (EC) and LiPF 6

[0055] As Figure 4 shown, the ionic conductivity at 0 °C is the highest when the molar ratio of EC / LiPF 6 is around 4. If the molar ratio is smaller or larger than this, there is a tendency to decrease. Specifically, when the molar ratio is in the range of 3 - 5, the change in the ionic conductivity at 0 °C stays within 1.5% based on the maximum value, which is appropriate. More preferably, the change in the ionic conductivity at 0 °C is within 0.5% based on the maximum value, and the molar ratio is in the range of 3.5 - 4.5.

[0056] (DCR Measurement)

[0057] First, LiNi 0.33 Co 0.33 Mn 0.33 powder as the positive electrode active material, acetylene black and graphite as the conductive aids, and polyvinylidene fluoride (PVdF) as the binder were mixed at a weight ratio of 90∶4∶3∶3, and the viscosity was adjusted using N - methyl - 2 - pyrrolidone (NMP) to form a slurry. The positive electrode slurry was coated on both sides of an aluminum foil with a thickness of 15 μm, dried and pressed to fabricate the positive electrode.

[0058] Next, natural graphite with amorphous carbon coating as the negative electrode active material, styrene - butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the dispersant were mixed at a weight ratio of 98∶1∶1, and the viscosity was adjusted using ion - exchanged water to form a slurry. The negative electrode slurry was coated on both sides of a copper foil with a thickness of 10 μm, dried and pressed to fabricate the negative electrode.

[0059] Then, a separator was sandwiched between the above - prepared positive electrode and negative electrode, and an electrode winding was fabricated according to the Figures 1 to 3 shown structure. The current collector plate of the battery cover was welded to the uncoated part of the electrode winding, the electrode winding was covered with an insulating protective film, sealed in the battery case, and the battery cover and the battery case were welded.

[0060] Next, as the non - aqueous electrolyte, in Example 1, in a solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) at a volume ratio of 16∶42∶42, LiPF as the first lithium salt 6 ​Dissolve at a concentration of 0.6 mol / L. Dissolve LiFSI, which is an imide lithium salt, at a concentration of 0.6 mol / L. Additionally, as Comparative Example 1, in a solvent obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of 28:36:36, dissolve LiPF as the first lithium salt. 6 Dissolve at a concentration of 0.6 mol / L. Dissolve LiFSI, which is an imide lithium salt, at a concentration of 0.6 mol / L.

[0061] After injecting the separately prepared electrolytes from the injection port, seal them with an injection plug to fabricate a lithium-ion secondary battery.

[0062] After initializing by charging and discharging the fabricated lithium-ion secondary battery, measure the direct current resistance (DCR) during battery capacity and state of charge (SOC).

[0063] Regarding the battery capacity, perform constant voltage - constant current charging (CC - CV charging) for a total of 2.5 hours at a charging current of 1 CA until the battery voltage reaches 4.2 V. Then, after a 30-minute pause, perform constant current discharging (CC discharging) at a discharging current of 0.02 CA until the battery voltage reaches 2.9 V to obtain the initial capacity.

[0064] Regarding the relationship between SOC - open circuit voltage (OCV), discharge the battery capacity starting from 4.2 V in 5% increments of the battery capacity, and use the voltage after a 2-hour pause as the OCV to obtain the relationship with SOC.

[0065] Regarding the DCR at SOC 50% in the low SOC region, according to the SOC - OCV relationship, perform CC - CV charging starting from SOC 0% at a charging current of 1 C until SOC 50%, control the temperature of the thermostat to -30°C and maintain it for 5 hours, then perform CC discharging, discharge at 5 CA, 10 CA, and 15 CA for 10 seconds, plot the relationship between the voltage difference that drops from the OCV within 10 seconds and the current value, and calculate the DCR based on the slope.

[0066] Figure 5 Show the results of comparing the DCRs of Example 1 and Comparative Example 1. Figure 5 It can be seen that, compared with Comparative Example 1, the -30°C DCR of Example 1 corresponding to the present invention is approximately reduced by 3%. As expected, the low-temperature output can be improved.

[0067] By forming the structure of the lithium-ion secondary battery as described above, the resistance of the lithium-ion secondary battery, especially the resistance in the low-temperature region, can be reduced, and the battery performance can be improved.

[0068] (Ionic conductivity measurement and evaluation of the influence on the electrode)

[0069] The material for use of organic solvents is a mixture of ethylene carbonate (EC), which is a cyclic carbonate, and ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), which are chain carbonates, at a ratio of 50 vol%:50 vol%. Moreover, the concentration of the cyclic carbonate in the electrolyte is set to the values shown in Table 1 respectively.

[0070] Add LiPF, which is a first lithium salt, to such a solvent in the amounts shown in Table 1 6 and LiFSI, which is an imide lithium salt, to prepare the electrolytes of Examples 2 to 4 and Comparative Examples 2 to 7. Measure the ionic conductivity (S / m) of the obtained electrolytes at 0 °C. The results are shown in Table 1.

[0071] In addition, instead of the electrolytes used in the above-mentioned Example 1 and Comparative Example 1, inject the electrolytes of Examples 2 to 4 and Comparative Examples 2 to 7 and seal them to fabricate lithium ion secondary batteries. Charge and discharge each lithium ion secondary battery, leave it for 1 day and then disassemble it to take out the electrodes, and conduct visual observation. Through visual inspection, confirm the discoloration of the aluminum foil as the current collector, and confirm the corrosion state through dimensional changes. Evaluate the case where corrosion is observed as having an "adverse effect", and evaluate the case where no corrosion is observed as having "no adverse effect". The results are shown in Table 1.

[0072] [Table 1]

[0073]

[0074] As shown in Table 1, when the amount of ethylene carbonate (EC) is in the range of 10 vol% to 20 vol%, including LiPF 6 at 0.38 mol / L to 0.75 mol / L and imide lithium salt (LiFSI), and the total of the imide lithium salt and the first lithium salt is in the range of 0.5 mol / L to 1.5 mol / L, and the molar ratio represented by EC / LiPF 6 is in the range of 3 to 5 for the electrolytes of Examples 2 to 4, the ionic conductivity is excellent, and no adverse effect on the electrode is observed.

[0075] Symbol Explanation

[0076] 1: Battery case; 1a: Opening; 1b: Wide side surface; 1c: Narrow side surface; 1d: Bottom surface; 2: Insulating protective film; 3: Winding group; 5: Gasket; 6: Battery cover; 7: Insulating plate; 9: Liquid injection port; 10: Exhaust valve; 11: Liquid injection plug; 12: Negative external terminal; 12a: Negative connection part; 14: Positive external terminal; 14a: Positive connection part; 21: Negative current collector plate base; 22: Negative side connection end; 23: Negative side opening hole; 24: Negative current collector plate; 26: Negative side through hole; 32: Negative electrode; 32b: Negative electrode mixture layer; 32c: Negative electrode foil exposed part; 33: Separator; 34: Positive electrode; 34b: Positive electrode mixture layer; 34c: Positive electrode foil exposed part; 35: Separator; 41: Positive current collector plate base; 42: Positive side connection end; 43: Positive side opening hole; 44: Positive current collector plate; 46: Positive side through hole; 100: Lithium ion secondary battery.

Claims

1. An electrolyte for a lithium-ion secondary battery, characterized in that, it contains: Ethylene carbonate (EC) as a cyclic carbonate: 10 vol% to 20 vol%; a mixed solvent formed by mixing ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) as linear carbonates at a ratio of 50 vol%:50 vol%; at least one selected from lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), and lithium bis(oxalato)borate (LiBOB) as the first lithium salt: 0.38 mol / L to 0.75 mol / L; and an imide lithium salt, The total of the lithium imide salt and the first lithium salt is in the range of 0.5 mol / L to 1.5 mol / L, and the molar ratio represented by the cyclic carbonate / the first lithium salt is in the range of 4.0 to 4.

5.

2. The electrolyte for a lithium-ion secondary battery according to claim 1, characterized in that, the lithium imide salt is lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

3. The electrolyte for a lithium-ion secondary battery according to claim 1, characterized in that, the chain carbonate is in the range of 80% to 90% by volume.

4. A lithium-ion secondary battery, characterized in that, it contains the electrolyte for a lithium-ion secondary battery according to any one of claims 1 to 3.

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