Low-temperature electrolyte for lithium battery

Abstract

The low-temperature electrolyte comprises an organic solvent, a lithium salt and an additive, and the mass percentage ratio of the organic solvent to the lithium salt to the additive is (70-90): (5-20): (0.2-3); the organic solvent is composed of cyclic carbonate, chain carbonate and chain carboxylate, the mass percentage of the cyclic carbonate, the chain carbonate and the chain carboxylate is 10-20: 10-30: 10-80, the cyclic carbonate is one or two 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 dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate and methyl propyl carbonate. The lithium salt is one or more of lithium hexafluorophosphate, lithium perchlorate and lithium tetrafluoroborate. Compared with a single auxiliary additive, the low-temperature performance of the electrolyte can be remarkably improved by combining the amino-containing nicotinonitrile compound with the auxiliary additive according to the proportion.

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 been continuously improved, 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 more and more vigorous. However, lithium batteries have many problems under low-temperature conditions: first, as the temperature decreases, discharge becomes more and more difficult until it cannot be discharged; second, under low-temperature charging conditions, the battery constant current charging ratio is very low and cannot be fully charged, and lithium deposition is prone to occur at the negative electrode during low-temperature charging. The battery after lithium deposition is prone to spontaneous combustion or explosion, causing safety accidents.

[0003] The main factors for 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 membrane during the reciprocating process of migrating from the negative electrode to the electrolyte, and its composition and thickness greatly affect the migration rate of lithium ions; thirdly, 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 membrane, 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 is reduced and generated on the surface of the carbon negative electrode during the first charging process of the lithium-ion battery electrolyte because the reduction potential of the components of the electrolyte is higher than that of lithium. The generated SEI film can prevent the reduction reaction of the electrolyte at the negative electrode during the subsequent charging and discharging of the lithium-ion battery, so that the lithium-ion battery has a higher cycle life. However, the SEI film will affect the speed of lithium ion embedding into the negative electrode, and has a significant impact on the rate performance and low temperature performance of the lithium-ion battery.

[0005] In order 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 SEI, such as vinylene carbonate (VC), methylene carbonate or vinyl carbonate. However, this type of film-forming additive has obvious disadvantages. The SEI film formed by it has a large impedance, which will lead to an increase in the DC internal resistance of the battery and deterioration of low-temperature performance. The higher the content of the additive, the more obvious the deterioration. Especially under low temperature conditions, low-temperature charging and lithium precipitation are prone to occur, affecting battery safety. If the amount of such additives is reduced or not used, the cycle life of the battery is difficult to guarantee.

[0006] For ternary lithium-ion batteries, people have found that when they are discharged under low temperature and high current conditions, the battery cycle life is short and the internal resistance increases rapidly. After analysis, it is found that this is because the positive electrode potential of ternary lithium-ion batteries is too high when they are discharged under high current conditions, causing the electrolyte to continuously decompose 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 carboxylic acid ester, and the mass percentage is: 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 is a nicotinonitrile compound containing an amino group, and 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 linear carbonate is one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, and methyl propyl carbonate.

[0014] Preferably, the chain carboxylic acid ester 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 priority to the electrolyte solvent and the auxiliary additives to form a thin and continuous SEI film, the main components of which are LiN3, LiNO2, LiNO3, LiF, and Li2S with low ion passage impedance, and the above substances also have good conductivity, low SEI impedance, and the corresponding battery has good low-temperature charge and discharge performance; while 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 large impedance; and the nicotinonitrile compound forms a thin film composed of LiN3, LiNO2, LiNO3, and LiF on the positive electrode surface, preventing the electrolyte from being oxidized and decomposed on the surface of the positive electrode material under low temperature conditions;

[0018] Compared with the use of auxiliary additives alone, the use 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 in conjunction with specific embodiments.

[0020] This embodiment provides a low-temperature electrolyte for a lithium battery, 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;

[0021] The organic solvent is composed of cyclic carbonate, chain carbonate and chain carboxylic acid ester, 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 methyl propyl carbonate, and the chain carboxylic acid ester 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 additive is a nicotinonitrile compound containing an amino group, and the additive is composed of a low-temperature additive and an auxiliary additive. The nicotinonitrile compound containing an amino group 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 to form a thin and continuous SEI film. The main components of the film are LiN3, LiNO2, LiNO3, LiF, and Li2S with low ion passage impedance, and the above substances also have good conductivity, low SEI impedance, and the corresponding battery has good low-temperature charge and discharge performance; while 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 large impedance; and 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 auxiliary additive alone, the use of the nicotinonitrile compound containing an amino group 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 difluorooxalate borate. The auxiliary additive is methylene disulfonate, lithium difluorophosphate and / or lithium difluorooxalate borate, 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 added with 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 an 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 mixed evenly to prepare electrolyte 2.

[0035] Example 3

[0036] This embodiment provides a low-temperature additive C and a low-temperature electrolyte 3 added with 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 difluorooxalate 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 the electrolyte 3 was prepared after being mixed evenly.

[0040] Comparative Example 1

[0041] On the basis of Example 1, the low-temperature additive A is removed.

[0042] Comparative Example 2

[0043] On the basis of 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 electrolyte for lithium batteries prepared in Examples 1-3 and Comparative Examples 1-3 was injected into a 26650-6.0Ah battery cell (positive electrode ternary material, negative electrode graphite) to prepare a battery.

[0048] The manufactured 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 implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A low-temperature electrolyte for a lithium battery, 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 carboxylic acid ester, and the mass percentage is: 10-20:10-30:10-80; The lithium salt is one or more of lithium hexafluorophosphate, lithium perchlorate, and lithium tetrafluoroborate; The additive is a nicotinonitrile compound containing an amino group, and the additive consists of a low-temperature additive and an auxiliary additive.

2. A low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: The cyclic carbonate is one or both of ethylene carbonate and propylene carbonate.

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

4. A low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: The chain carboxylic acid ester 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.

5. A low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: The auxiliary additive is one or more of methylene disulfonate, lithium difluorophosphate, and lithium difluorooxalate borate.

6. A low-temperature electrolyte for lithium batteries according to claim 1, characterized in that: The low-temperature additive is one or more of 6-amino-4-methylnicotinonitrile, 4-amino-5-nitronicotinonitrile, and 2-amino-5-methylnicotinonitrile.

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