A low-temperature electrolyte for lithium batteries

Abstract

The present invention discloses a low-temperature electrolyte for lithium batteries, including an organic solvent, a lithium salt and an additive, wherein the mass percentage of the organic solvent, the lithium salt and the additive is 70-90:5-20:0.2-3; the organic solvent is composed of a cyclic carbonate, a chain carbonate and a chain carboxylate, and the mass percentage is: 10-20:10-30:10-80, the cyclic carbonate is one or both of ethylene carbonate and propylene carbonate, the chain carbonate is one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate and methylpropyl carbonate, and the lithium salt is one or more of lithium hexafluorophosphate, lithium perchlorate and lithium tetrafluoroborate. Compared with the single administration of auxiliary additives, the present invention uses the above-mentioned ratio of amino-containing nicotinonitrile compound in combination with the auxiliary additive to significantly improve the low-temperature performance of the electrolyte.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery electrolytes, and in particular to a low-temperature electrolyte for lithium batteries. Background Art

[0002] In recent years, lithium batteries have been increasingly used in various fields, and the requirements for their performance and application environment have continued to increase, such as high-power discharge, ultra-low temperature discharge below -30°C, and ultra-long cycle life of 10,000 times. Among them, the demand in the low-temperature field is becoming increasingly strong. However, lithium batteries have many problems under low-temperature conditions: First, as the temperature drops, discharge becomes increasingly difficult, until discharge becomes impossible. Second, under low-temperature charging conditions, the battery's constant current charge ratio is very low, making it impossible to fully charge. In addition, lithium deposition is prone to occur at the negative electrode during low-temperature charging, and the battery after lithium deposition is prone to spontaneous combustion or explosion, causing safety accidents.

[0003] The main factors that contribute to the poor low-temperature performance of lithium batteries are as follows: First, as the temperature drops, the viscosity of the electrolyte increases, and the migration rate of lithium ions in the electrolyte decreases; second, lithium ions need to pass through the SEI film in the process of migrating from the negative electrode to the electrolyte, and its composition and thickness greatly affect the migration rate of lithium ions; third, during low-temperature charging, especially low-temperature high-rate charging, active lithium may precipitate on the surface of the negative electrode, and the active lithium reacts irreversibly with the electrolyte, further increasing the thickness of the SEI film, making lithium ion migration more difficult, and the low-temperature performance of the lithium battery worse, and even causing safety accidents.

[0004] The SEI film is a passivation film composed of inorganic and organic compounds that forms on the surface of the carbon anode during the initial charge of a lithium-ion battery's electrolyte. This film is formed by the reduction of various components of the electrolyte, which have a higher reduction potential than lithium. This SEI film prevents subsequent reduction reactions of the electrolyte at the anode during the battery's charge and discharge processes, contributing to a longer cycle life for the battery. However, the SEI film affects the rate at which lithium ions embed into the anode, significantly impacting the battery's rate capability and low-temperature performance.

[0005] To ensure the performance of lithium-ion batteries, some negative electrode film-forming additives are usually added to the electrolyte to improve the composition and performance of the SEI, such as vinylene carbonate (VC), methylene carbonate or vinyl carbonate. However, these film-forming additives have obvious disadvantages. The SEI film they form has a large impedance, which will lead to an increase in the battery's DC internal resistance and deterioration of low-temperature performance. The higher the additive content, the more obvious the deterioration. Especially under low-temperature conditions, low-temperature charging and lithium deposition are prone to occur, affecting battery safety. If the amount of such additives is reduced or not used, the cycle life of the battery will be difficult to guarantee.

[0006] When discharging ternary lithium-ion batteries at low temperatures and high currents, researchers have found that their cycle life is short and their internal resistance increases rapidly. Analysis has shown that this is due to excessively high positive electrode potential during high-current discharge, leading to continuous decomposition of the electrolyte at the positive electrode. Summary of the Invention

[0007] Based on the technical problems existing in the background technology, the present invention proposes a low-temperature electrolyte for lithium batteries.

[0008] The present invention provides a low-temperature electrolyte for a lithium battery, comprising an organic solvent, a lithium salt, and an additive, wherein the mass percentage of the organic solvent, the lithium salt, and the additive is 70-90:5-20:0.2-3;

[0009] The organic solvent is composed of cyclic carbonate, chain carbonate, and chain carboxylate, with the mass percentage being 10-20:10-30:10-80;

[0010] The lithium salt is one or more of lithium hexafluorophosphate, lithium perchlorate, and lithium tetrafluoroborate;

[0011] The additive consists of a low-temperature additive and an auxiliary additive.

[0012] Preferably, the cyclic carbonate is one or both of ethylene carbonate and propylene carbonate.

[0013] Preferably, the chain carbonate is one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, and methyl propyl carbonate.

[0014] Preferably, the chain carboxylate is one or more of methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, and propyl butyrate.

[0015] Preferably, the auxiliary additive is one or more of methylene disulfonate, lithium difluorophosphate, and lithium difluorooxalatoborate.

[0016] Preferably, the low-temperature additive is one or more of 6-amino-4-methylnicotinonitrile, 4-amino-5-nitronicotinonitrile, and 2-amino-5-methylnicotinonitrile.

[0017] The amino-containing nicotinonitrile compound provided by the present invention can participate in the formation of a passivation film on the surface of the negative electrode material in preference to the electrolyte solvent and auxiliary additives, forming a thin and continuous SEI film. The main components of the film are LiN3, LiNO2, LiNO3, LiF, and Li2S, which have low ion passage resistance. In addition, the above-mentioned substances also have good electrical conductivity, low SEI resistance, and the corresponding battery has good low-temperature charge and discharge performance. The main components of the SEI film formed by traditional electrolytes are alkyl ester lithium (ROCO2Li)n and alkyl lithium CH3(CH2)nLi, which have high resistance. In addition, the nicotinonitrile compound forms a thin film composed of LiN3, LiNO2, LiNO3, and LiF on the positive electrode surface, preventing the oxidative decomposition of the electrolyte on the surface of the positive electrode material under low temperature conditions.

[0018] Compared with the use of auxiliary additives alone, the combination of the amino-containing nicotinonitrile compound and the auxiliary additives in the above ratio can significantly improve the low-temperature performance of the electrolyte. DETAILED DESCRIPTION

[0019] The present invention will be further explained below with reference to specific embodiments.

[0020] This embodiment provides a low-temperature electrolyte for a lithium battery, comprising an organic solvent, a lithium salt, and an additive, wherein the mass percentage of the organic solvent, the lithium salt, and the additive is 70-90:5-20:0.2-3;

[0021] The organic solvent is composed of cyclic carbonate, chain carbonate, and chain carboxylate, with the mass percentage being 10-20:10-30:10-80, wherein the cyclic carbonate is one or both of ethylene carbonate and propylene carbonate, the chain carbonate is one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, and methyl propyl carbonate, and the chain carboxylate is one or more of methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, and propyl butyrate;

[0022] The lithium salt is one or more of lithium hexafluorophosphate, lithium perchlorate, and lithium tetrafluoroborate;

[0023] The additives are composed of a low-temperature additive and an auxiliary additive. The amino-containing nicotinonitrile compound provided by the present invention can participate in the formation of a passivation film on the surface of the negative electrode material in priority to the electrolyte solvent and the auxiliary additive, forming a thin and continuous SEI film. The main components of the film are LiN3, LiNO2, LiNO3, LiF, and Li2S, which have low ion passage resistance. In addition, the above-mentioned substances also have good conductivity and low SEI resistance. The corresponding battery has good low-temperature charge and discharge performance. The main components of the SEI film formed by the traditional electrolyte are alkyl ester lithium (ROCO2Li)n and alkyl lithium CH3(CH2)nLi, which have high resistance. In addition, the nicotinonitrile compound forms a thin film composed of LiN3, LiNO2, LiNO3, and LiF on the positive electrode surface, which prevents the oxidative decomposition of the electrolyte on the surface of the positive electrode material under low temperature conditions. Compared with the use of the auxiliary additive alone, the combination of the amino-containing nicotinonitrile compound and the auxiliary additive in the above ratio can significantly improve the low-temperature performance of the electrolyte.

[0024] The auxiliary additive is one or more of methylene disulfonate, lithium difluorophosphate, and lithium difluorooxalatoborate. The auxiliary additive is methylene disulfonate, lithium difluorophosphate and / or lithium difluorooxalatoborate, which will form Li2S, Li3B, and Li3P with lower impedance in the SEI film, and also improve the composition of the SEI film to a certain extent.

[0025] Example 1

[0026] This embodiment provides a low-temperature additive A and a low-temperature electrolyte 1 containing the low-temperature additive A.

[0027] The low temperature additive A is shown in the following formula:

[0028]

[0029] 14.0 g of lithium hexafluorophosphate, 0.5 g of methylene disulfonate, and 0.5 g of low-temperature additive A (6-amino-4-methylnicotinonitrile) were dissolved in 85 g of an organic solvent (15 g of ethylene carbonate, 17 g of ethyl methyl carbonate, and 53 g of propyl propionate) and mixed well to prepare electrolyte 1.

[0030] Example 2

[0031] This embodiment provides a low-temperature additive B and a low-temperature electrolyte 2 added with the low-temperature additive B;

[0032] The low temperature additive B is shown in the following formula:

[0033]

[0034] 18 g of lithium hexafluorophosphate, 0.5 g of lithium difluorooxalatoborate, and 1.5 g of low-temperature additive B (4-amino-5-nitronicotinonitrile) were dissolved in 80 g of an organic solvent (15 g of ethylene carbonate, 15 g of ethyl methyl carbonate, and 50 g of ethyl propionate), and the mixture was uniformly mixed to prepare electrolyte 2.

[0035] Example 3

[0036] This embodiment provides a low-temperature additive C and a low-temperature electrolyte 3 containing the low-temperature additive C.

[0037] The low-temperature additive C is shown in the following formula:

[0038]

[0039] 20.0 g of lithium hexafluorophosphate, 0.5 g of lithium difluorooxalato iodate, and 2.0 g of low-temperature additive C (2-amino-5-methylnicotinonitrile) were dissolved in 77.5 g of an organic solvent (17.5 g of ethylene carbonate, 20 g of dimethyl carbonate, and 40 g of propyl propionate) and mixed well to prepare electrolyte 3.

[0040] Comparative Example 1

[0041] Based on Example 1, the low-temperature additive A was removed.

[0042] Comparative Example 2

[0043] Based on Example 2, the low-temperature additive B is removed.

[0044] Comparative Example 3

[0045] Based on Example 3, the low-temperature additive C is removed.

[0046] Test example low temperature performance test is as follows:

[0047] The electrolytes for lithium batteries prepared in Examples 1-3 and Comparative Examples 1-3 were injected into 26650-6.0Ah battery cells (positive electrode ternary material, negative electrode graphite) to prepare batteries.

[0048] The prepared batteries were charged at a rate of 0.2C at -20°C and at a rate of 0.5C at -40°C, and the constant current charging ratio data during charging were recorded.

[0049] The prepared battery was subjected to a 1C / 1C charge-discharge cycle test at -20°C for 300 cycles, and the capacity retention rate was recorded. The results are shown in Table 1 below:

[0050]

[0051] It can be seen from the above table that the battery prepared using the electrolyte of the present invention has excellent low-temperature performance.

[0052] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A low-temperature electrolyte for lithium batteries, comprising an organic solvent, a lithium salt and an additive, characterized in that: The mass percentage of the organic solvent, lithium salt and additive is 70-90:5-20:0.2-3; The organic solvent is composed of cyclic carbonate, chain carbonate, and chain carboxylate, with the mass percentage being 10-20:10-30:10-80; The lithium salt is one or more of lithium perchlorate and lithium tetrafluoroborate; The additives consist of low-temperature additives and auxiliary additives; The cyclic carbonate is one or both of ethylene carbonate and propylene carbonate; The chain carbonate is one or more of ethyl methyl carbonate and methyl propyl carbonate; The chain carboxylate is one or more of methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, and propyl butyrate; The auxiliary additive is one or more of methylene disulfonate, lithium difluorophosphate, and lithium difluorooxalatoborate; The low-temperature additive is one or more of 6-amino-4-methylnicotinonitrile, 4-amino-5-nitronicotinonitrile, and 2-amino-5-methylnicotinonitrile.

2. A low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: The lithium salt also includes lithium hexafluorophosphate.

3. The low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: The chain carbonate further comprises one or more of dimethyl carbonate, diethyl carbonate and dipropyl carbonate.

4. The low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: The chain carboxylic acid esters also include methyl propionate.

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