Electrolytic solution and lithium ion battery

Abstract

To provide an electrolytic solution and a lithium ion battery capable of overcoming a problem that a melting point of the electrolyte solution is high, an ionic conductivity is low, and a performance at low temperature is poor.SOLUTION: An electrolytic solution includes a solvent, a lithium salt, and an additive. The solvent includes a cyclic carbonate, a linear carbonate, and ethyl acetate. The cyclic carbonate includes ethylene carbonate, and the lithium salt includes lithium bis (fluorosulfonyl) imide and lithium hexafluorophosphate. A mass percentage of the cyclic carbonate accounts for 5% to 10% of the total amount of the solvent. The mass percentage of the linear carbonate accounts for 20% to 60% of the total amount of the solvent. The mass percentage of the ethyl acetate accounts for 50% to 95% of the total amount of the solvent, and the mass percentage of the lithium bis (fluorosulfonyl) imide accounts for 8% to 20% of the total amount of the electrolytic solution.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electrolyte and a lithium-ion battery. [Background technology]

[0002] Currently, a common problem with lithium-ion batteries is a significant decrease in energy density and power density under low-temperature conditions. Typical lithium-ion batteries use cyclic carbonates (e.g., ethylene carbonate (EC) and propylene carbonate (PC)) and linear carbonates (e.g., dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC)) as solvents, and typically use lithium hexafluorophosphate as the lithium salt. Because the solvents have high melting points and high viscosities, electrolytes with such combinations typically perform poorly at low temperatures.

[0003] Ethylene carbonate (EC) is a cyclic carbonate solvent commonly used in lithium-ion battery electrolytes. Its dielectric constant (dielectric constant) is 89, which is higher than that of propylene carbonate (65). The addition of ethylene carbonate promotes the dissociation of lithium hexafluorophosphate, improving the ionic conductivity of the electrolyte. Furthermore, ethylene carbonate has a higher decomposition potential on the anode side than propylene carbonate, allowing it to preferentially participate in the formation of the solid electrolyte membrane (SEI), thereby significantly improving the cycling performance of lithium-ion batteries. Therefore, ethylene carbonate has become an essential solvent component in conventional commercial electrolytes.

[0004] However, ethylene carbonate has a high freezing point (its melting point is 35°C to 38°C). Under low-temperature conditions, such as minus 40°C, ethylene carbonate is likely to precipitate, blocking the pores in the separator film or electrode sheet, thereby increasing impedance and affecting battery performance. Furthermore, ethylene carbonate precipitation also reduces the dielectric constant of the mixed solvent, reducing the dissociation of lithium hexafluorophosphate. To address this issue, linear carbonates with low melting points are commonly used. For example, dimethyl carbonate (melting point: 4.6°C), ethyl methyl carbonate (EMC) (melting point: -53°C), or diethyl carbonate (melting point: -74.3°C) can be used to partially replace EC to lower the melting point of the mixed solvent. However, the dielectric constants of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate are only 3.12, 2.96, and 2.82, respectively. The introduction of large amounts of low-dielectric-constant solvents obviously does not help the dissociation of lithium hexafluorophosphate, thereby reducing the ionic conductivity of the electrolyte and affecting the battery's performance at low temperatures.

[0005] Therefore, it is an urgent task to improve the ionic conductivity of the electrolyte and the performance of the battery at low temperatures while ensuring that the melting point of the electrolyte meets the requirements for practical application. Summary of the Invention [Problem to be solved by the invention]

[0006] To provide an electrolyte and a lithium ion battery that can overcome the problems of electrolytes having a high melting point, low ionic conductivity, and poor performance at low temperatures. [Means for solving the problem]

[0007] The technical problem that the present invention aims to solve is to overcome the problems of prior art electrolytes, such as high melting points, low ionic conductivity, and poor performance at low temperatures. The present invention provides an electrolyte and a lithium-ion battery. When used in a lithium-ion battery, the electrolyte provided by the present invention has a low melting point, high ionic conductivity, and low battery DCR, and can improve performance at low temperatures.

[0008] The present invention solves the above technical problems by the following technical solutions.

[0009] The present invention provides an electrolyte solution including a solvent, a lithium salt, and an additive, wherein the solvent includes a cyclic carbonate and ethyl acetate, the cyclic carbonate includes ethylene carbonate, and the lithium salt includes lithium bis(fluorosulfonyl)imide.

[0010] The mass percentage of the cyclic carbonate accounts for 5% to 20% of the total amount of the solvent, and the mass percentage of the ethyl acetate accounts for 20% to 95% of the total amount of the solvent.

[0011] The mass percentage of lithium bis(fluorosulfonyl)imide is 8% to 20% of the total amount of the electrolyte.

[0012] The present invention further provides a negative electrode, a positive electrode, a separator, and a lithium ion battery each containing the above-described electrolyte solution. [Effects of the Invention]

[0013] The advantageous effects of the present invention are as follows:

[0014] The electrolyte provided by the present invention, when employed in lithium ion batteries, has a low melting point, high ionic conductivity, and low battery DCR, improving performance at low temperatures, and maintaining high energy and power densities even at temperatures down to -20°C. DETAILED DESCRIPTION OF THE INVENTION

[0015] In the present invention, the mass percentage of ethyl acetate can be 20%, 40%, 60%, 75%, 80%, 90%, or 95% of the total amount of solvent, and preferably 50% to 95%.

[0016] In the present invention, when the mass percentage of ethyl acetate is 20% to 95% of the total amount of the solvent, the electrolyte not only has excellent lithium salt solubility when used in a lithium ion battery, but also has excellent DCR at 25° C., DCR at −20° C., and discharge rate performance at −20° C. When the mass percentage of ethyl acetate is 50% to 95% of the total amount of the solvent, the DCR at −20° C. and discharge rate performance at −20° C. are even better.

[0017] In the present invention, the mass percentage of the cyclic carbonate can be 5%, 10%, 15%, or 20% of the total amount of the solvent, and preferably 5% to 10%.

[0018] In the present invention, when the mass percentage of the cyclic carbonate is 5% to 20% of the total amount of the solvent, the electrolyte not only has excellent lithium salt solubility when used in a lithium ion battery, but also has excellent DCR at 25°C, DCR at -20°C, and discharge rate performance at -20°C. When the mass percentage of the cyclic carbonate is 5% to 10% of the total amount of the solvent, the DCR at -20°C and discharge rate at -20°C are even better.

[0019] In the present invention, the mass percentage of ethylene carbonate preferably accounts for 5% to 20%, for example, 5%, 10%, 15%, or 20%, of the total amount of the solvent.

[0020] In the present invention, the cyclic carbonate further includes propylene carbonate, and the mass percentage of propylene carbonate preferably accounts for 0% to 10%, for example 10%, of the total amount of the solvent.

[0021] In the present invention, the mass percentage of lithium fluorosulfonylimide can be 8%, 10%, 14%, 16%, 18%, or 20% of the total amount of the electrolyte, and preferably 12% to 20%.

[0022] In the present invention, when the mass percentage of lithium fluorosulfonylimide is 8% to 20% of the total amount of the electrolyte, the electrolyte not only has excellent lithium salt solubility when used in a lithium ion battery, but also has excellent DCR at 25° C., DCR at −20° C., and discharge rate performance at −20° C. When the mass percentage of lithium fluorosulfonylimide is 12% to 20% of the total amount of the electrolyte, the DCR at 25° C., DCR at −20° C., and discharge rate performance at −20° C. are even better.

[0023] In the present invention, the mass percentage of the lithium salt preferably accounts for 12% to 20%, for example, 12%, 18%, or 20%, of the total amount of the electrolyte.

[0024] In the present invention, the lithium salt further includes lithium hexafluorophosphate.

[0025] The mass percentage of lithium hexafluorophosphate preferably accounts for 2% to 10%, for example, 2%, 4%, 8%, or 10%, of the total amount of the electrolyte.

[0026] In the present invention, the solvent further comprises a linear carbonate.

[0027] The linear carbonate is preferably one or more of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0028] When the linear carbonate includes dimethyl carbonate, the mass percentage of dimethyl carbonate can account for 20% to 30% of the total amount of the solvent.

[0029] When the linear carbonate includes ethyl methyl carbonate, the mass percentage of ethyl methyl carbonate can account for 15% to 30% of the total amount of the solvent.

[0030] The mass percentage of the linear carbonate preferably accounts for 20% to 60% of the total amount of the solvent, for example, 20%, 30%, 45%, or 60%.

[0031] In the present invention, the additive is preferably one or more of vinylene carbonate, fluoroethylene carbonate, ethylene sulfate, and 1,3-propane sultone.

[0032] When the additive contains vinylene carbonate, the mass percentage of the vinylene carbonate accounts for 0.5% to 3% of the total amount of the electrolyte.

[0033] When the additive contains fluoroethylene carbonate, the mass percentage of the fluoroethylene carbonate accounts for 1% to 5% of the total amount of the electrolyte.

[0034] In the present invention, the mass percentage of the additive preferably accounts for 2% to 5%, for example 3%, of the total amount of the electrolyte.

[0035] In a preferred embodiment, the additive is 1% vinylene carbonate and 2% fluoroethylene carbonate.

[0036] In the present invention, the negative electrode preferably includes a negative electrode current collector and a negative electrode active material applied to the negative electrode current collector.

[0037] The negative electrode active material preferably comprises a graphite material or a graphite material-silicon material composite.

[0038] The negative electrode current collector can be made of a conductive material that does not undergo chemical change, such as copper, stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum-cadmium alloy, or "copper, stainless steel material, or aluminum-cadmium alloy surface-treated with carbon, nickel, titanium, or silver." Furthermore, micro-embossing may be formed on the surface of the negative electrode current collector to enhance adhesion of the negative electrode active material. The negative electrode current collector can have various shapes, such as a film, sheet, foil, mesh, porous body, foam, or nonwoven fabric.

[0039] In the present invention, the thickness of the negative electrode current collector may be 4.5 μm to 10 μm, for example, 6 μm or 8 μm.

[0040] In the present invention, the negative electrode further comprises a conductive agent, a binder, and a thickener.

[0041] The binder is a component provided to bind the active material to the conductive agent. For example, one or more of SBR, PAA, CMC, and PVDF can be used as the binder.

[0042] The conductive agent is not particularly limited as long as the conductive agent exhibits high conductivity without causing a chemical change in the battery to which it is applied. For example, carbon nanotubes, carbon black, or graphene can be used as the conductive material.

[0043] The thickener is not particularly limited. The addition of a thickener can increase the system viscosity of each component of the negative electrode active material layer. Generally, a carboxymethyl cellulose sodium solution can be selected as the thickener.

[0044] In a preferred embodiment, the negative electrode satisfies the following conditions: the negative electrode active material is artificial graphite, the conductive agent is Super P, the thickener is sodium carboxymethyl cellulose, and the binder is styrene-butadiene rubber. The mass ratio of the negative electrode active material, conductive agent, thickener, and binder is 96:1:1:2.

[0045] In the present invention, the positive electrode preferably includes a positive electrode current collector and a positive electrode active material applied to the positive electrode current collector.

[0046] The positive electrode active material preferably comprises one or more of lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0047] The positive electrode current collector may be made of a material that exhibits high conductivity without chemical change, such as, without limitation, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel that has been surface-treated with carbon, nickel, titanium, silver, or the like. Micro-embossing may be formed on the surface of the positive electrode current collector to enhance adhesion of the positive electrode active material. The positive electrode current collector may have various shapes, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0048] In the present invention, the thickness of the positive electrode current collector may be a thickness common in the technical field, such as 16 μm or 13 μm.

[0049] In the present invention, the positive electrode further contains a binder and a conductive agent.

[0050] In a preferred embodiment, the positive electrode satisfies the following conditions: the positive electrode active material is lithium iron phosphate, the binder is polyvinylidene fluoride, and the conductive agent is Super P. The mass ratio of the positive electrode active material, binder, and conductive agent is 98:1:1.

[0051] In the present invention, a separator film is used, which is an insulating film having high ion permeability and mechanical strength. The pore size of the separator film is generally 0.01 μm to 10 μm, and its thickness is generally 5 μm to 300 μm. Examples of separator films include sheets or nonwoven fabrics made of chemically resistant and water-repellent olefin polymers such as polypropylene, glass fibers, polyethylene, etc. When a solid electrolyte solution such as a polymer is used as the electrolyte solution, the solid electrolyte solution may also be used as the separator.

[0052] In a preferred embodiment, the separator film is a polypropylene separator film having a thickness of 12 μm.

[0053] On the one hand, in the present invention, by using a large amount of linear carbonate and carboxylic acid ester, the amount of cyclic carbonate with a high melting point and high viscosity in the electrolyte solvent can be reduced, thereby effectively solving the problem of solidification at low temperatures and a significant increase in the viscosity of the electrolyte. Furthermore, by combining a large amount of lithium bis(fluorosulfonyl)imide (LiFSI) as the main lithium salt, the high dissociation ability of LiFSI allows it to be completely dissolved and dissociated in the linear carbonate and carboxylic acid ester with a low dielectric constant, thereby providing high ionic conductivity. On the other hand, an electrolyte using LiFSI as the lithium salt has a wide liquid phase range, which prevents the electrolyte from solidifying at low temperatures. The combination of a large amount of linear carbonate, carboxylic acid ester, and a large amount of LiFSI ensures that the battery has low impedance and high capacity retention even at low temperatures. Compared with the conventional combination of a small amount of linear carbonate and lithium hexafluorophosphate, the present invention uses a specific combination of a large amount of linear carbonate, ethyl acetate, and lithium bis(fluorosulfonyl)imide, which effectively reduces the freezing point and viscosity of the electrolyte and improves the ionic conductivity and lithium ion mobility of the electrolyte, thereby helping to form a low-impedance SEI and significantly improving the rate performance of the battery at low temperatures.

[0054] All reagents and raw materials employed in the present invention are commercially available products.

[0055] Based on general knowledge in the art, the above preferred conditions can be freely combined to obtain preferred embodiments of the present invention.

[0056] The present invention will be further described below using examples, but the present invention is not limited to the scope of the described examples. In the following examples, experimental methods without specific conditions should be selected based on conventional methods and conditions or product specifications.

[0057] Example 1

[0058] Electrode preparation:

[0059] Battery-grade ethylene carbonate (EC) and ethyl acetate (EA) were mixed in a 2:8 mass ratio in an argon-atmosphere glove box with a water content of 10 ppm to form an organic solvent. The lithium salt and negative electrode additive were quantitatively added according to the electrolyte composition listed in Table 1 and mixed uniformly to obtain an electrolyte. The lithium salts were LiPF6 and LiFSI, with the mass fraction of LiPF6 being 10% and the mass fraction of LiFSI being 8%. The negative electrode additives were vinylene carbonate (VC) and fluoroethylene carbonate (FEC), with the mass fraction of VC being 1% and the mass fraction of FEC being 2%. In Table 1, the content of each component other than the solvent is expressed as a mass percentage calculated based on the total mass of the electrolyte, and the mass content of each component in the solvent is the amount present in 10 parts of the solvent.

[0060] Examples 2 to 13 and Comparative Examples 1 to 6

[0061] Electrolyte solutions were prepared according to the experimental parameters in Table 1. The electrolyte compositions of Examples 2 to 13 and Comparative Examples 1 to 6 were the same as Example 1, except that the components were added according to the ratios in Table 1.

[0062] Example of effect

[0063] 1. The method for producing a secondary lithium battery is as follows:

[0064] (1) Preparation of lithium iron phosphate cathode:

[0065] The positive electrode active material (lithium iron phosphate), polyvinylidene fluoride used as a binder, and Super P used as a conductive agent were mixed in a weight ratio of 98:1:1, and an appropriate amount of N-methylpyrrolidone (NMP) was added. The mixture was stirred under a vacuum mixer until the system became uniform and transparent, yielding a positive electrode slurry. The positive electrode slurry was uniformly applied to a 13 μm-thick aluminum foil, which was then dried at room temperature and transferred to an oven for further drying. The foil was then cold-rolled and cut to obtain a positive electrode (electrode sheet).

[0066] (2) Preparation of graphite anode:

[0067] Artificial graphite was used as the negative electrode active material, Super P as the conductive agent, sodium carboxymethyl cellulose (CMC-Na) as the thickener, and styrene-butadiene rubber (SBR) as the binder. All of the above were mixed in a mass ratio of 96:1:1:2, and then deionized water was added and the mixture was stirred under a vacuum mixer to obtain negative electrode slurry. The negative electrode slurry was uniformly applied to an 8 μm-thick negative electrode current collector copper foil, which was then dried at room temperature and transferred to an oven for drying. The copper foil was then cold-rolled and cut to obtain a negative electrode (electrode sheet).

[0068] (3) Preparation of electrolyte:

[0069] See Example 1 for the preparation of the electrolyte solution.

[0070] (4) Preparation of separator film:

[0071] A polypropylene film was used as the separator film.

[0072] (5) Fabrication of lithium-ion secondary battery:

[0073] A 12 μm thick polypropylene film (PP) was used as a separator film. The positive electrode, separator film, and negative electrode prepared above were stacked in this order, with the separator film between the positive and negative electrodes, separating them. Next, the entire assembly was wrapped in aluminum plastic film, transferred to a vacuum oven, and dried at 120°C. The electrolyte prepared above was injected at 3.0 g / Ah, and the electrolyte was chemically converted. The assembly was then sealed. Finally, a soft-pack battery (lithium-ion battery) with a capacity of 1 Ah was obtained.

[0074] 2.Performance test method:

[0075] (1) DC resistance (DCR) test for lithium-ion secondary batteries

[0076] At the specified temperature, the battery was discharged to 50% SOC (state of charge reflecting the remaining capacity of the battery) at a current of 1C, the current was increased to 4C and maintained for 30 seconds, and the difference between the updated stable voltage and the original platform voltage was detected. The ratio of this difference to the current value at 4C was the DC resistance of the battery. The result of the DCR test performed after the battery was first fully charged was the initial DCR of the battery.

[0077] (2) Low-temperature discharge capacity retention test for lithium-ion secondary batteries

[0078] After fully charging the lithium-ion secondary battery, the battery was placed in a thermostatic box at -20°C. After cooling sufficiently, the battery was discharged at a rate of 1C to the cut-off voltage, and the percentage of the battery capacity relative to the initial discharge capacity was compared.

[0079] For the lithium iron phosphate / graphite battery, the charge and discharge cut-off voltages were 2.5V to 3.65V.

[0080] Electrolyte composition of Examples 1 to 13 and Comparative Examples 1 to 6 [Table 1]

[0081] Performance test results for Examples 1 to 13 and Comparative Examples 1 to 6 [Table 2]

[0082] Table 2 shows the performance test results for Examples 1 to 13 and Comparative Examples 1 to 6. As can be seen from Examples 1 and 2 and Comparative Examples 1 and 2, when the amount of EC is within the range of 0.5 / 10 to 2 / 10 (i.e., when the mass percentage of the cyclic carbonate accounts for 5% to 20% of the total amount of the solvent), the battery performance is optimal at low temperatures. When the amount of EC is higher than 2 / 10 (see Comparative Example 1), the DCR is much higher than that of Example 1, and the low-temperature discharge rate is significantly reduced. This indicates that too much cyclic carbonate increases the melting point of the electrolyte, increases the viscosity of the electrolyte at low temperatures, reduces the conductivity of the electrolyte, and deteriorates battery performance at low temperatures. When the amount of EC is less than 0.5 / 10 (see Comparative Example 2, i.e., when the mass percentage of the cyclic carbonate in the total amount of solvent is less than 5%), the electrolyte does not contain a solvent with a sufficiently high dielectric constant, and the lithium salt does not dissolve completely, resulting in an extremely high DCR at room temperature and extremely poor performance at low temperatures. In the present invention, EC may be replaced with an appropriate amount of other cyclic carbonates (e.g., PC), and EA may be replaced with an appropriate amount of other linear carbonates (e.g., EMC, DMC). As can be seen from Examples 6 to 9, when the amount of cyclic carbonate was 0.5 / 10 to 2 / 10 (i.e., when the mass percentage of cyclic carbonate was 5% to 20% of the total solvent), as the amount of cyclic carbonate decreased and the amount of EA increased, the battery's DCR at normal temperature slightly decreased, but the DCR at low temperatures significantly decreased, and the low-temperature discharge rate increased. This indicates that decreasing the amount of cyclic carbonate and increasing the amount of EA reduces the electrolyte viscosity and increases the electrolyte conductivity, thereby improving battery performance at low temperatures. When the amount of EA was 0 (see Comparative Example 3), the electrolyte viscosity was high, so the performance of Comparative Example 3 at low temperatures was worse than that of an electrolyte containing EA. As can be seen from Examples 3, 4, and 5 and Comparative Examples 4 and 5, when the amount of LiFSI was 8% to 20%, the battery had optimal performance at low temperatures.When the amount of LiFSI is lower than 8% (see Comparative Example 4), the DCR and low-temperature performance of the battery are significantly deteriorated, and when the amount of LiFSI is higher than 20% (see Comparative Example 5), the concentration of the lithium salt is too high, which increases the viscosity of the electrolyte and reduces the dissociation degree of the lithium salt, thereby reducing the performance of the battery at low temperatures.

[0083] According to Examples 3 and 10-13, when the solvent system remains the same and the lithium salt concentration is within the range of 12%-20%, increasing the amount of LiFSI in the lithium salt significantly reduces the DCR of the corresponding battery at normal and low temperatures, and the low-temperature discharge rate continues to increase. This is because LiFSI dissociates more easily than LiPF6, has higher solubility in the solvent, and is more conductive. Therefore, the higher the amount of LiFSI used, the better the battery performance at low temperatures.

[0084] As can be seen from Comparative Example 6, when the amount of cyclic carbonate is 3 / 10 (i.e., when the mass percentage of cyclic carbonate accounts for 30% of the total amount of solvent), even if the amount of LiFSI is increased to 18%, the DCR and low temperature performance of the corresponding battery are much worse than those of the embodiment with a lower amount of cyclic carbonate, indicating that in order for the electrolyte to ensure good conductivity and low temperature performance, the amount of cyclic carbonate needs to be reduced to 2 / 10 or less (i.e., the mass percentage of cyclic carbonate in the total amount of solvent is 20% or less). [Industrial Applicability]

[0085] The electrolyte provided by the present invention, when employed in a lithium ion battery, has a low melting point, high ionic conductivity, low battery DCR, and can improve performance at low temperatures.

Claims

1. An electrolyte solution comprising a solvent, a lithium salt, and an additive, wherein the solvent comprises a cyclic carbonate and ethyl acetate, the cyclic carbonate comprises ethylene carbonate, and the lithium salt comprises lithium bis(fluorosulfonyl)imide; the weight percentage of the cyclic carbonate is 5% to 20% of the total amount of the solvent, and the weight percentage of the ethyl acetate is 20% to 95% of the total amount of the solvent; The mass percentage of the lithium bis(fluorosulfonyl)imide is 8% to 20% of the total amount of the electrolyte.

2. 2. The electrolyte solution of claim 1, wherein the weight percentage of the ethyl acetate comprises 50% to 95% of the total amount of the solvent.

3. the lithium salt further comprises lithium hexafluorophosphate; The electrolyte solution according to claim 1 , wherein the solvent further comprises a linear carbonate.

4. 2. The electrolyte solution according to claim 1, wherein the cyclic carbonate further comprises propylene carbonate, and the mass percentage of the propylene carbonate accounts for 0% to 10% of the total amount of the solvent.

5. 4. The electrolyte solution of claim 3, wherein the weight percentage of the lithium hexafluorophosphate comprises 2% to 10% of the total amount of the electrolyte solution.

6. 4. The electrolyte solution of claim 3, wherein the mass percentage of the linear carbonate accounts for 20% to 60% of the total amount of the solvent.

7. 2. The electrolyte of claim 1, wherein the weight percentage of the additive comprises 2% to 5% of the total amount of the electrolyte.

8. 2. The electrolyte of claim 1, wherein the additive is one or more of vinylene carbonate, fluoroethylene carbonate, ethylene sulfate, and 1,3-propane sultone.

9. 4. The electrolyte of claim 3, wherein the linear carbonate is one or more of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

10. A lithium ion battery comprising a negative electrode, a positive electrode, a separator film, and the electrolyte solution of claim 1.

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