Non-aqueous electrolyte and lithium cobalt oxide battery

Abstract

The application belongs to the technical field of lithium ion batteries, and specifically discloses a nonaqueous electrolyte, which comprises a lithium salt, an organic solvent and an additive; the additive is lithium tetrafluoro-oxalato-phosphate (TFOP) and 1,4-dicyano-2-butene (DCB). After the lithium cobaltate battery is applied with the nonaqueous electrolyte, the high-voltage cycle performance is improved, and the gas production phenomenon is improved. In addition, the nonaqueous electrolyte can also improve the floating performance of the lithium cobaltate battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to the electrolyte of lithium cobalt oxide batteries. Background Technology

[0002] Lithium-ion batteries are widely used rechargeable batteries, possessing advantages such as high energy density, high open-circuit voltage, low self-discharge rate, long cycle life, and good safety. The performance of lithium rechargeable batteries largely depends on whether the organic electrolyte can react and form a stable and uniform passivation film on the electrode surface. However, during high-voltage cycling, stress caused by electrode deformation can lead to passivation film failure, ultimately resulting in battery capacity decay. Therefore, improving the stability of the passivation film formed by the electrolyte on the positive and negative electrode surfaces is crucial for improving battery cycle life.

[0003] On the other hand, to meet the market's high energy density requirements, the voltage needs to be further increased to 4.6 V. However, expanding the voltage window can lead to irreversible phase transitions or even structural collapse of the lithium cobalt oxide cathode material during long-term cycling, increasing interfacial side reactions, and causing problems such as cobalt metal surface dissolution and oxygen evolution, resulting in rapid deterioration of battery performance.

[0004] In addition, during cycling, especially under high-voltage conditions, lithium cobalt oxide batteries also suffer from the problem of battery swelling caused by the decomposition products or gases generated by the decomposition of organic electrolyte in contact with the electrodes, which reduces battery performance. Summary of the Invention

[0005] To address the problems existing in the prior art, the main objective of this invention is to provide a non-aqueous electrolyte for lithium cobalt oxide batteries, thereby improving the high-voltage cycle performance and reducing gas production in lithium cobalt oxide batteries. Another objective of this invention is to provide a lithium cobalt oxide battery.

[0006] To achieve the above objectives, the present invention provides the following specific technical solutions.

[0007] First, the present invention provides a non-aqueous electrolyte for use in lithium cobalt oxide batteries, the non-aqueous electrolyte comprising lithium salt, organic solvent and additives; the additives being lithium tetrafluorooxalate phosphate (TFOP) and 1,4-dicyano-2-butene (DCB).

[0008] In a further preferred embodiment, the lithium tetrafluorooxalate phosphate has a mass percentage content of 0.5-1.5% in the electrolyte; and the 1,4-dicyano-2-butene has a mass percentage content of 0.4-0.8% in the electrolyte.

[0009] In a further preferred embodiment, the lithium salt is at least one selected from LiPF6, LiAsF6, LiClO4, LiBF4, LiB(C2O4)2, LiBF2C2O4, LiN(SO2F)2, LiN(SO2CF3)2, LiPO2F2, LiPF2(C2O4)2, and LiPF4C2O4.

[0010] In a further preferred embodiment, the lithium salt content in the non-aqueous electrolyte is 8.5-16% by mass.

[0011] In a further preferred embodiment, the organic solvent is two or more selected from dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, ethyl butyrate, methyl propyl carbonate, halogenated ethylene carbonate, propylene carbonate, methyl propyl carbonate, γ-butyrolactone, γ-valerolactone, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and 2,2-difluoroethyl acetate.

[0012] Based on the same inventive concept, the present invention provides a lithium cobalt oxide battery containing the above-mentioned non-aqueous electrolyte, wherein the charging cut-off voltage of the lithium cobalt oxide battery is ≥4.5V.

[0013] The advantages of the present 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 from specific embodiments thereof. Detailed Implementation

[0014] The embodiments of the present invention are described in detail below. These embodiments are exemplary and are only used to explain the present invention, and should not be construed as limiting the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0015] In a first aspect, the present invention provides a non-aqueous electrolyte for use in lithium cobalt oxide batteries, the non-aqueous electrolyte comprising a lithium salt, an organic solvent, and additives; the additives comprising lithium tetrafluorooxalate phosphate (TFOP) and 1,4-dicyano-2-butene (DCB).

[0016] Furthermore, in the electrolyte of the present invention, the mass percentage of lithium tetrafluorooxalate phosphate in the electrolyte is 0.5-1.5%, which can be 0.5%, 1.0%, 1.5%, etc.; the mass percentage of 1,4-dicyano-2-butene in the electrolyte is 0.4-0.8%, which can be 0.4%, 0.5%, 0.6%, 0.8%, etc.

[0017] The application of the non-aqueous electrolyte containing lithium tetrafluorooxalate phosphate (TFOP) and 1,4-dicyano-2-butene (DCB) in lithium cobalt oxide batteries not only enhances their high-voltage cycling performance but also improves gas production issues, as detailed below: Compared to other common lithium salt additives, such as difluorooxalate phosphate, difluorooxalate borate, and lithium dioxalate borate, lithium tetrafluorooxalate phosphate has the lowest LUMO energy level and stronger electron affinity. After being added to the electrolyte, it will be reduced before other additives decompose and adhere to the surface of the negative electrode material to participate in the formation of a thin SEI film with low resistance, which can effectively improve the high-voltage cycle performance of lithium cobalt oxide batteries. The inventors discovered that adding lithium tetrafluorooxalate phosphate to the electrolyte causes gas production, especially when the addition amount reaches more than 1%, the gas production phenomenon is more serious. The main reason is that excessive tetrafluorooxalate phosphate ions are more likely to open the ring under high voltage, and are oxidized at the positive electrode to generate CO2, and the gas overflows to form the gas production phenomenon. The addition of 1,4-dicyano-2-butene (DCB) reduces the gas production level of lithium cobalt oxide batteries and further improves their high-voltage cycle performance. The relevant mechanism may be that DCB, through the complexation of cyano groups with metal ions, can prevent metal ions from damaging the SEI film. In addition, DCB has double bonds with a higher s component of sp2 hybrid orbitals. Compared with sp3 hybrid carbon, its electron cloud is closer to the atomic nucleus. Specifically, it has higher electronegativity and can easily gain electrons on the negative electrode surface to undergo reduction and decomposition to form an SEI film, which can improve the oxidation resistance of the electrolyte, thereby improving the high-voltage cycle performance of lithium cobalt oxide batteries and reducing the gas production level of the batteries. However, as the amount of 1,4-dicyano-2-butene added gradually increases, especially when the amount of addition reaches more than 1%, the thickness of the film layer also increases accordingly, which leads to a gradual increase in the interfacial impedance of the battery, an increase in the internal resistance of the battery, a deterioration in the high-voltage cycle performance of the lithium cobalt oxide battery, a rapid decay, and ultimately affects the overall performance of the battery. This solution combines lithium tetrafluorooxalate phosphate with 1,4-dicyano-2-butene and further limits the addition ratio of the two, which significantly suppresses the gas generation phenomenon of lithium cobalt oxide batteries while maintaining excellent high-voltage cycle performance. It effectively prevents the disadvantages of high impedance or severe gas generation when the dosage of a single additive is high, and also solves the problem that the dosage of a single additive is too low to improve battery performance. Furthermore, experimental verification showed that adding only 1,4-dicyano-2-butene to the electrolyte without adding lithium tetrafluorooxalate phosphate resulted in unsatisfactory improvements in high-voltage cycle performance and gas production in lithium cobalt oxide batteries. Simultaneously, the inventors discovered that replacing lithium tetrafluorooxalate phosphate with other similar lithium salts, or replacing 1,4-dicyano-2-butene with other similar nitriles, did not achieve the aforementioned effects.

[0018] In the electrolyte of this invention, the non-aqueous electrolyte contains 8.5% to 16% lithium salt by mass. There are no strict requirements for the selection of lithium salt; commonly used lithium salts in the art can be chosen, such as at least one selected from LiPF6, LiAsF6, LiClO4, LiBF4, LiB(C2O4)2, LiBF2C2O4, LiN(SO2F)2, LiN(SO2CF3)2, LiPO2F2, LiPF2(C2O4)2, and LiPF4C2O4. In practice, LiPF6 is further preferred.

[0019] In the electrolyte of this invention, the organic solvent can be any commonly used organic solvent in the art, such as two or more of the following: dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, ethyl butyrate, methyl propyl carbonate, halogenated ethylene carbonate, propylene carbonate, methyl propyl carbonate, γ-butyrolactone, γ-valerolactone, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and 2,2-difluoroethyl acetate. In practice, a commonly used organic solvent system is further preferred: a mixed solution of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate (PP) in a mass ratio of 15:20~25:15~20:40~50. The electrolyte, excluding lithium salts, additives, and other possible substances commonly used in the art, consists of organic solvents in the remainder.

[0020] In a second aspect, the present invention provides a lithium cobalt oxide battery comprising the above-mentioned non-aqueous electrolyte, wherein the charging cut-off voltage of the lithium cobalt oxide battery is ≥4.5V.

[0021] The present invention will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0022] Example 1 Non-aqueous electrolyte: 12.5 parts by mass of LiPF6, 1 part by mass of lithium tetrafluorooxalate phosphate, 0.5 parts by mass of 1,4-dicyano-2-butene, and 86 parts by mass of organic solvent. The organic solvent is a mixed solution with a mass ratio of EC / PC / DEC / PP of 15 / 20 / 15 / 50.

[0023] Comparative Example 1 Non-aqueous electrolyte: 12.5 parts by mass of LiPF6, 1 part by mass of lithium tetrafluorooxalate phosphate, and 86.5 parts by mass of organic solvent. The organic solvent is a mixed solution with a mass ratio of EC / PC / DEC / PP of 15 / 20 / 15 / 50.

[0024] Comparative Example 2 Non-aqueous electrolyte: 12.5 parts by mass of LiPF6, 0.5 parts by mass of 1,4-dicyano-2-butene, and 87 parts by mass of organic solvent. The organic solvent has a mass ratio of EC / PC / DEC / PP of 15 / 20 / 15 / 50.

[0025] Comparative Example 3 Non-aqueous electrolyte: 12.5 parts by mass of LiPF6, 1 part by mass of lithium difluorooxalate borate (LiODFB), 0.5 parts by mass of 1,4-dicyano-2-butene, and 86 parts by mass of organic solvent. The organic solvent is a mixed solution with a mass ratio of EC / PC / DEC / PP of 15 / 20 / 15 / 50.

[0026] Comparative Example 4 Non-aqueous electrolyte: 12.5 parts by mass of LiPF6, 1 part by mass of lithium bis(oxalato)borate (LiOBOB), 0.5 parts by mass of 1,4-dicyano-2-butene, and 86 parts by mass of organic solvent. The organic solvent is a mixed solution with a mass ratio of EC / PC / DEC / PP of 15 / 20 / 15 / 50.

[0027] Comparative Example 5 Non-aqueous electrolyte: 12.5 parts by mass of LiPF6, 1 part by mass of lithium tetrafluorooxalate phosphate, 0.5 parts by mass of succinic anionyl nitrile, and 86 parts by mass of organic solvent. The organic solvent is a mixed solution with a mass ratio of EC / PC / DEC / PP of 15 / 20 / 15 / 50.

[0028] Comparative Example 6 Non-aqueous electrolyte: 12.5 parts by mass of LiPF6, 1 part by mass of lithium tetrafluorooxalate phosphate, 0.5 parts by mass of hexanetrionitrile, and 86 parts by mass of organic solvent. The organic solvent is a mixed solution with a mass ratio of EC / PC / DEC / PP of 15 / 20 / 15 / 50.

[0029] Batteries were assembled using the non-aqueous electrolytes of Example 1 and Comparative Examples 1-6, respectively, as follows: The coating density is determined based on the capacity design of the battery and the capacity of the positive and negative electrode materials. The positive electrode active material is a 4.50V high-voltage lithium cobalt oxide material purchased from Tianjin Bamo; the negative electrode active material is artificial graphite purchased from Shenzhen BTR; and the separator is a PE-coated ceramic separator with a thickness of 20μm purchased from Xingyuan Materials. The positive electrode preparation steps are as follows: lithium cobalt oxide, conductive carbon black and binder polyvinylidene fluoride are mixed in a mass ratio of 96.8:2.0:1.2 and dispersed in N-methyl-2-pyrrolidone to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated on both sides of an aluminum foil, and after drying, rolling and vacuum drying, aluminum leads are welded on using an ultrasonic welding machine to obtain a positive electrode sheet with a thickness between 100 and 150 μm. The negative electrode preparation steps are as follows: graphite, conductive carbon black, styrene-butadiene rubber and carboxymethyl cellulose are mixed in a mass ratio of 95:1.5:1.5:3 and dispersed in deionized water to obtain a negative electrode slurry. The negative electrode slurry is coated on both sides of a copper foil, dried, rolled and vacuum dried, and nickel leads are welded on using an ultrasonic welding machine to obtain a negative electrode sheet with a thickness between 100 and 150 μm. The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence and wound to obtain a bare battery cell. The bare battery cell is then assembled into a battery with the non-aqueous electrolyte of Examples 1 and Comparative Examples 1-6.

[0030] The high-voltage cycle performance and high-temperature gas generation of the battery were tested using the following methods.

[0031] High-voltage cycle performance test: At 45℃, the lithium cobalt oxide battery was charged to 4.50V at a constant current of 1C, and then charged to the cutoff current of 0.05C at a constant voltage of 4.50V. The battery was then discharged at a constant current of 1C, and the discharge capacity was recorded as C0. The charge-discharge cycle was repeated 500 times to obtain the discharge capacity C500 after the 500th cycle. The capacity retention rate was calculated as C500 / C0×100%.

[0032] High-temperature gas generation performance test: At 25℃, charge at a constant current of 1C to 4.50V, maintain a constant voltage of 4.50V, and charge to the cutoff current of 0.05C. Record the battery thickness D0. Then place the battery in a 60℃ explosion-proof oven and store it for 14 days. Test the battery thickness D1 in the oven after that. Then remove the battery, cool it to room temperature, and calculate the thickness expansion rate = (D1-D0) / D0×100%.

[0033] The results of high-temperature cycling performance and gas production performance are shown in Table 1 below.

[0034] Table 1 The composition of the non-aqueous electrolyte was further adjusted, and the high-voltage cycle performance and high-temperature gas generation of the battery were tested according to the above method. The results are shown in Table 2.

[0035] Table 2 According to the test results in Tables 1 and 2, adding TFOP and DCB to the electrolyte simultaneously significantly suppresses gas generation during storage in lithium cobalt oxide batteries while maintaining good high-voltage cycling performance; adding TFOP or DCB alone does not achieve the same effect.

[0036] Adding TFOP and DCB simultaneously to the electrolyte, compared to using TFOB and DCB alone, can suppress storage gas generation and improve the high-voltage cycle performance of lithium cobalt oxide batteries. Using TFOB and DCB alone cannot simultaneously achieve both high-voltage cycle performance and storage performance. The synergistic effect of DCB with LiODFB and LiBOB is not significant. The synergistic effect of TFOB with succinic anhydride (SN) and hexanetrionitrile (HTCN) is also not significant.

[0037] Moreover, when the mass percentage of lithium tetrafluorooxalate phosphate in the electrolyte is 0.5~1.5% and the mass percentage of 1,4-dicyano-2-butene in the electrolyte is 0.4~0.8%, the synergistic effect of the two is more prominent, the high-voltage cycle performance of the battery is better, and the high-temperature gas generation is suppressed.

[0038] Furthermore, the present invention has made a further inventive discovery that the addition of lithium tetrafluorooxalate phosphate and 1,4-dicyano-2-butene to the electrolyte also gives the battery cell better float charging performance.

[0039] Specifically, the test method for float charging performance is as follows: The assembled battery was charged at 25°C with a constant current of 1C to 4.5V, then charged at a constant voltage of 4.5V to a cutoff current of 0.05C. It was then discharged at a constant current of 1C, and the discharge capacity was recorded as C0. At 25°C, the battery was charged at a constant current of 1C to 4.6V, then charged at a constant voltage of 4.6V to a cutoff current of 0C, with a cutoff time set to 240 hours. After 10 days of float charging, the battery was discharged at 1C to 3V. It was then charged at a constant current of 1C to 4.5V, then charged at a constant voltage of 4.5V to a cutoff current of 0.05C, and then discharged at a constant current of 1C to 3V. This charge-discharge cycle was repeated three times. The discharge capacity C1 of the first cycle and C3 of the third cycle were recorded. The capacity retention rate was calculated as C1 / C0 × 100%, and the capacity recovery rate as C3 / C0 × 100%. The average value of three tests for each battery was taken. The test results are shown in Table 3 below.

[0040] Table 3 As shown in Table 3, the simultaneous addition of TFOP and DCB to the electrolyte results in exceptionally superior float-charge performance for lithium cobalt oxide batteries compared to adding only TFOP or DCB. Furthermore, replacing TFOP with a similar salt or DCB with a similar nitrile compound does not improve the float-charge performance of lithium cobalt oxide batteries. Moreover, the float-charge performance is even better when the mass percentage of lithium tetrafluorooxalate phosphate in the electrolyte is 0.5–1.5% and the mass percentage of 1,4-dicyano-2-butene in the electrolyte is 0.4–0.8%.

[0041] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A non-aqueous electrolyte, said non-aqueous electrolyte comprising a lithium salt, an organic solvent, and additives; characterized in that, The additives include lithium tetrafluorooxalate phosphate (TFOP) and 1,4-dicyano-2-butene (DCB).

2. The non-aqueous electrolyte as described in claim 1, characterized in that, The lithium tetrafluorooxalate phosphate has a mass percentage content of 0.5-1.5% in the electrolyte; the 1,4-dicyano-2-butene has a mass percentage content of 0.4-0.8% in the electrolyte.

3. The non-aqueous electrolyte as described in claim 1 or 2, characterized in that, The lithium salt is at least one of LiPF6, LiAsF6, LiClO4, LiBF4, LiB(C2O4)2, LiBF2C2O4, LiN(SO2F)2, LiN(SO2CF3)2, LiPO2F2, LiPF2(C2O4)2, and LiPF4C2O4.

4. The non-aqueous electrolyte as described in claim 3, characterized in that, The lithium salt is LiPF6.

5. The non-aqueous electrolyte as described in claim 3, characterized in that, The lithium salt content in the non-aqueous electrolyte is 8.5-16% by mass.

6. The non-aqueous electrolyte as described in claim 1 or 2, characterized in that, The organic solvent is two or more selected from dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, ethyl butyrate, methyl propyl carbonate, halogenated ethylene carbonate, propylene carbonate, methyl propyl carbonate, γ-butyrolactone, γ-valerolactone, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and 2,2-difluoroethyl acetate.

7. The non-aqueous electrolyte as described in claim 6, characterized in that, The organic solvent is a mixed solution of ethylene carbonate, propylene carbonate, diethyl carbonate, and propyl propionate in a mass ratio of 15:20~25:15~20:40~50.

8. A lithium cobalt oxide battery, characterized in that, Includes the non-aqueous electrolyte as described in any one of claims 1-7.

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