A nonaqueous electrolyte for lithium ion batteries and a lithium ion battery containing the same

Abstract

The application belongs to the technical field of lithium ion batteries, and discloses a nonaqueous electrolyte for a lithium ion battery and a lithium ion battery using the electrolyte. The nonaqueous electrolyte for the lithium ion battery contains a lithium salt, an organic solvent and an additive, the additive contains vinylene carbonate, lithium difluorophosphate, 1,3-propane sultone and ethylene sulfate, and the additive further contains at least one silicon-containing imidazole compound. The silicon-containing imidazole compound in the electrolyte and other additives jointly act to decompose and deposit a cathode protection film on the surface of a positive electrode, effectively reduce the internal resistance change of the battery during the cycle process, inhibit the side reaction of the battery under high voltage and high temperature conditions, and improve the electrical performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a non-aqueous electrolyte for lithium-ion batteries and a lithium-ion battery using the electrolyte. Background Technology

[0002] Compared to other batteries, lithium-ion batteries have advantages such as light weight, small size, high energy density, and long cycle life. Therefore, the development of lithium-ion batteries has received widespread attention, and their application in mobile phones, digital devices, electric vehicles, electric bicycles, power tools, and energy storage has been rapid. However, in lithium-ion batteries, the oxidative decomposition of the electrolyte degrades battery performance, and the dissolution of metal ions during cycling and high-temperature storage often leads to a significant decline in battery performance. As market demand for lithium-ion batteries continues to grow, higher requirements are being placed on the overall performance of the batteries. The use of additives is one of the effective ways to improve the overall performance of lithium-ion batteries. There are many types of additives, which can be classified according to their different roles in the electrolyte: film-forming additives, overcharge protection additives, conductive additives, flame-retardant additives, and electrolyte stabilizers, etc. Regardless of the type, the proportion of each additive in the electrolyte is small, but its significant function has led to extensive research and development.

[0003] On the one hand, because lithium-ion batteries use high-potential cathode active materials and low-potential anode active materials, the potential window of the electrolyte is narrower than that of the active materials. The electrolyte, exposed to the cathode and anode electrode surfaces, is prone to decomposition. Simultaneously, lithium-ion batteries are easily exposed to high-temperature environments when used in electric vehicles or energy storage devices. Furthermore, the battery temperature can rise due to instantaneous charging and current variations. Therefore, in high-temperature environments, battery lifespan and storable energy decrease. On the other hand, the main salt of lithium-ion batteries, LiPF6, easily decomposes under high temperatures or trace amounts of water, producing HF, which damages the SEI film and corrodes electrode materials, releasing transition metal ions and further promoting electrolyte decomposition, creating a vicious cycle and degrading lithium-ion battery performance. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a non-aqueous electrolyte for lithium-ion batteries and a lithium-ion battery using the electrolyte. The electrolyte of this invention contains silimidazole compounds that, together with other additives, can decompose and deposit on the positive electrode surface to form a cathode protective film. It can also effectively reduce the internal resistance change of the battery during cycling, suppress side reactions under high voltage and high temperature conditions, and improve the battery's electrical performance.

[0005] To achieve the purpose of this invention, the non-aqueous electrolyte for lithium-ion batteries of this invention contains lithium salt, organic solvent and additives. The additives contain vinylene carbonate (VC), lithium difluorophosphate (DFP), 1,3-propane sulpholol (PS) and ethylene sulfate (DTD). The additives also contain at least one silimidazole compound, which is shown in formula (I).

[0006]

[0007] In formula (I), R1, R2, and R3 may be the same or different, and R1, R2, and R3 are independently selected from hydrogen, methyl, ethyl, propyl, tert-butyl, trifluoromethyl, trifluoroethyl, perfluoroethyl, nitrile, cyanoethyl, phenyl, fluorophenyl, fluoroalkoxyphenyl, and oxygen-containing groups.

[0008] Furthermore, in some embodiments of the present invention, the compound represented by formula (I) includes, but is not limited to, the following compounds:

[0009]

[0010] Preferably, in some embodiments of the present invention, the compound represented by formula (I) accounts for 0.05-5% of the total mass of the electrolyte, for example 0.1-2%.

[0011] Furthermore, in some embodiments of the present invention, the organic solvent comprises ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC).

[0012] Preferably, in some embodiments of the present invention, the ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) are mixed in a weight ratio of 25-35:15-25:40-60.

[0013] Furthermore, in some embodiments of the present invention, the mass percentages of vinylene carbonate (VC), lithium difluorophosphate (DFP), 1,3-propanesulfonyl lactone (PS), and ethylene sulfate (DTD) in the electrolyte are 0.3-0.7%, 0.2-0.5%, 1-2%, and 0.7-1.3%, respectively.

[0014] Furthermore, in some embodiments of the present invention, the lithium salt is selected from one or more of LiPF6, LiBF4, LiDFOB, LiBOB, LiDFBOP, LiPO2F2, LiSbF6, LiAsF6, LiCF3SO3, Li(CF3SO2)3C, Li(CF3SO2)2N, Li(FSO2)2N, LiC4F9SO3, LiClO4, LiAlO2, and LiAlCl4.

[0015] Preferably, in some embodiments of the present invention, the concentration of the lithium salt in the electrolyte, based on lithium ions, is 0.5-2M, for example 1-1.5M.

[0016] Preferably, in some embodiments of the present invention, the lithium salt is LiPF6.

[0017] To achieve the objectives of this invention, this invention also provides a lithium-ion battery, which includes a positive electrode, a negative electrode, a separator, and the aforementioned non-aqueous electrolyte for lithium-ion batteries.

[0018] Furthermore, in some embodiments of the present invention, the positive electrode active material in the positive electrode sheet is at least one selected from lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium iron phosphate, lithium iron manganese phosphate, and ternary positive electrode materials, wherein the ternary positive electrode material is LiNi. 1-x-y-z Co x Mn y Al z O2, where 0≤x≤1, 0≤y≤1, 0≤z≤1 and 0≤x+y+z≤1.

[0019] Furthermore, in some embodiments of the present invention, the negative electrode active material in the negative electrode sheet is selected from natural graphite, coated natural graphite, artificial graphite, organic pyrolysis carbon, mesophase carbon microspheres, carbon fiber, silicon carbide material, silicon material, metal oxides, and Li-Sn, Li-Sn-O, Li-Al, Li-Ti, Li-Mg, Li-Ge, and Li-Si alloy materials, wherein the metal oxide is an oxide of the metal elements Sn, Ti, Cr, Bo, Fe, V, Mn, Cu, Mo, Ni, W, Zr, and Zn.

[0020] Compared with the prior art, the advantages of the present invention are as follows:

[0021] (1) The electrolyte of the present invention contains an additive with an imidazole structural group. Because it contains nitrogen atoms with lone pairs of electrons, it exhibits weak alkalinity in the electrolyte, which can remove HF generated by LiPF6 and inhibit the increase of electrolyte acidity.

[0022] (2) The electrolyte of the present invention contains silimidazole compounds that work together with other additives to suppress side reactions of the battery under high voltage and high temperature conditions, improve the thermal stability of the electrolyte, and significantly improve the high temperature storage and high temperature cycling performance of the electrolyte. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. Additional aspects and advantages of this invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. It should be understood that the following description is merely illustrative and not intended to limit the invention.

[0024] The indefinite articles “a” and “an” preceding an element or component of this invention do not impose any limitation on the quantity (i.e., number of times) of the element or component. Therefore, “an” or “a” should be interpreted as including one or at least one, and the singular form of an element or component also includes the plural form, unless the quantity clearly refers only to the singular form.

[0025] Furthermore, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., described below refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms are not necessarily directed at the same embodiment or example. Moreover, the technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0026] Example 1

[0027] The non-aqueous electrolyte was prepared as follows: In a glove box, ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) were mixed in a weight ratio of 3:2:5, and then lithium hexafluorophosphate was added to dissolve them, preparing an electrolyte with a lithium hexafluorophosphate concentration of 1.3M. Subsequently, 0.5% vinylene carbonate (VC), 0.35% lithium difluorophosphate, 1.5% 1,3-propanesulfonyl lactone (PS), 1% ethylene sulfate (DTD), and 1% compound (1) were added to the electrolyte at mass percentages.

[0028] The prepared lithium-ion battery was injected with a non-aqueous electrolyte into a fully dried 4.4V NCM (nickel:cobalt:manganese = 6:2:2) / graphite soft-pack battery. After being placed at 45°C, formed in a high-temperature fixture, and sealed twice, the battery performance was tested to obtain the battery used in Example 1.

[0029] The electrolyte preparation methods and battery preparation methods of Examples 2-13 and Comparative Examples 1-7 are the same as those of Example 1, except for the formulation of the lithium-ion battery electrolyte. The electrolyte formulations of Examples 2-13 and Comparative Examples 1-7 are shown in Table 1.

[0030] Table 1 Electrolyte formulations for each embodiment and comparative example

[0031]

[0032]

[0033] Lithium-ion battery performance testing

[0034] 1. Room temperature cycling performance

[0035] At room temperature (25℃), the above lithium-ion battery was charged to 4.4V under 1C constant current and constant voltage conditions, and then discharged to 3.0V under 1C constant current conditions. After 500 charge-discharge cycles, the capacity retention rate after the 500th cycle was calculated:

[0036]

[0037] 2. High-temperature cycling performance

[0038] Under high temperature (45℃) conditions, the above-mentioned lithium-ion battery was charged to 4.4V under 1C constant current and constant voltage conditions, and then discharged to 3.0V under 1C constant current conditions. After 500 charge-discharge cycles, the capacity retention rate after the 500th cycle was calculated:

[0039]

[0040] 3. High-temperature storage performance

[0041] Under normal temperature (25℃) conditions, a lithium-ion battery is subjected to one 1C / 1C charge and discharge cycle (discharge capacity denoted as DC0), and then charged to 4.4V under 1C constant current and constant voltage conditions. The lithium-ion battery is then stored in a 60℃ high-temperature chamber for one month. After removal, it is discharged at 1C under normal temperature conditions (discharge capacity denoted as DC1); then subjected to 1C / 1C charge and discharge cycles at normal temperature conditions (discharge capacity denoted as DC2). The capacity retention rate and capacity recovery rate of the lithium-ion battery are calculated using the following formulas:

[0042]

[0043]

[0044] The battery performance results of the above specific embodiments and comparative examples are shown in Table 2:

[0045] Table 2. Performance test results of lithium-ion batteries in comparative and example cases.

[0046]

[0047]

[0048] The data in the table above shows that batteries using silicone imidazole compounds exhibit good performance in both room temperature and high temperature cycling, especially at high temperatures. The addition of silicone imidazole compounds improves the high-temperature performance of the batteries. This is because the silicone imidazole compounds provided by this invention are nitrogen-containing compounds. Due to the presence of lone electrons on nitrogen, they have an elimination and complexation effect on active hydrogen substances and Lewis acidic substances such as PF5 and POF3 in the battery system, reducing their destructive effects on the battery system and thus improving the battery's high-temperature and cycle stability. Examples 1-3 and 8-10 show that when the amount of silicone imidazole compound added is less than 1.5%, the high-temperature cycle performance of the battery tends to improve with increasing addition. However, when the addition amount is ≥1.5%, the high-temperature cycle performance decreases. Therefore, it can be seen that the optimal addition amount for this type of compound is 0.5%-1.5% (e.g., 1%). The results from Comparative Examples 1-7 and Examples 1-3 show that when the compound shown in Formula (I) is used in combination with the conventional additive, the electrolyte's cycle performance and high-temperature performance are better than when the compound shown in Formula (I) is added alone. Meanwhile, Comparative Examples 5-7 show that when the compound shown in Formula (I) is used with only one or two conventional additives, the effect is not outstanding.

[0049] Those skilled in the art will readily understand that the above description is merely an embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A non-aqueous electrolyte for lithium-ion batteries, characterized in that, The non-aqueous electrolyte for lithium-ion batteries contains lithium salt, organic solvent, and additives. The additives include vinylene carbonate, lithium difluorophosphate, 1,3-propane sulphol, and ethylene sulfate. The mass percentages of vinylene carbonate, lithium difluorophosphate, 1,3-propane sulphol, and ethylene sulfate in the electrolyte are 0.3-0.7%, 0.2-0.5%, 1-2%, and 0.7-1.3%, respectively. The additives also contain at least one silimidazole compound, and the compound represented by formula (I) accounts for 0.1-2% of the total mass of the electrolyte. The silimidazole compound is shown in formula (I). In formula (I), R1, R2, and R3 may be the same or different, and R1, R2, and R3 are independently selected from hydrogen, methyl, ethyl, propyl, tert-butyl, trifluoromethyl, trifluoroethyl, perfluoroethyl, nitrile, cyanoethyl, phenyl, fluorophenyl, fluoroalkoxyphenyl, and oxygen-containing groups.

2. The non-aqueous electrolyte for lithium-ion batteries according to claim 1, characterized in that, The compounds represented by formula (I) include, but are not limited to, the following compounds: 。 3. The non-aqueous electrolyte for lithium-ion batteries according to claim 1, characterized in that, The organic solvent contains ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC).

4. The non-aqueous electrolyte for lithium-ion batteries according to claim 1, characterized in that, The ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) are mixed in a weight ratio of 25-35:15-25:40-60.

5. The non-aqueous electrolyte for lithium-ion batteries according to claim 1, characterized in that, The lithium salt is selected from one or more of LiPF6, LiBF4, LiDFOB, LiBOB, LiDFBOP, LiPO2F2, LiSbF6, LiAsF6, LiCF3SO3, Li(CF3SO2)3C, Li(CF3SO2)2N, Li(FSO2)2N, LiC4F9SO3, LiClO4, LiAlO2, and LiAlCl4.

6. The non-aqueous electrolyte for lithium-ion batteries according to claim 1, characterized in that, The concentration of the lithium salt in the electrolyte is 0.5-2M, calculated based on lithium ions.

7. The non-aqueous electrolyte for lithium-ion batteries according to claim 1, characterized in that, The lithium salt concentration in the electrolyte is 1-1.5M, calculated based on lithium ions.

8. The non-aqueous electrolyte for lithium-ion batteries according to claim 1, characterized in that, The lithium salt is LiPF6.

9. A lithium-ion battery, characterized in that, The lithium-ion battery includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte for lithium-ion batteries as described in any one of claims 1-8.

10. The lithium-ion battery according to claim 9, characterized in that, The positive electrode active material in the positive electrode sheet is at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium iron phosphate, lithium iron manganese phosphate, and ternary positive electrode material. The ternary positive electrode material is LiNi1-xy-zCoxMnyAlzO2, wherein 0≤x≤1, 0≤y≤1, 0≤z≤1, and 0≤x+y+z≤1.

11. The lithium-ion battery according to claim 9, characterized in that, The negative electrode active material in the negative electrode sheet is selected from natural graphite, coated natural graphite, artificial graphite, organic pyrolysis carbon, mesophase carbon microspheres, carbon fiber, silicon carbide materials, silicon materials, metal oxides, and Li-Sn, Li-Sn-O, Li-Al, Li-Ti, Li-Mg, Li-Ge, and Li-Si alloy materials. The metal oxides are oxides of the metal elements Sn, Ti, Cr, Bo, Fe, V, Mn, Cu, Mo, Ni, W, Zr, and Zn.

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