Non-aqueous electrolyte and lithium secondary battery thereof

Abstract

The invention discloses a non-aqueous electrolyte and a lithium secondary battery thereof. The non-aqueous electrolyte comprises a lithium salt, a non-aqueous organic solvent and an additive, the additive comprises a phytate compound, and the phytate compound is selected from at least one of lithium phytate, sodium phytate, potassium phytate, calcium phytate, magnesium phytate, zinc phytate, manganese phytate, nickel phytate, iron phytate, barium phytate and ammonium phytate. According to the invention, the phytate compound is used as the additive, is an antioxidant and contains a phosphate radical group which has very strong chelation capability, so that the storage performance and the cycle performance of the lithium secondary battery under 4.5 V high voltage and high temperature conditions can be improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and particularly relates to a non-aqueous electrolyte and a lithium secondary battery thereof. Background Art

[0002] With the understanding of the battery failure mechanism by people, it is found that during formation and cycling, especially under high voltage and high temperature conditions, cobalt ions and oxygen in the lithium cobalt oxide cathode material will dissolve into the electrolyte, and cobalt ions and reactive oxygen in the electrolyte will catalyze or participate in the reaction decomposition of the electrolyte, resulting in gas generation and capacity loss of the battery. Metal ions will deposit on the surface of the negative electrode, accelerating the formation of dendrites, and finally piercing the separator, causing the battery to fail and catch fire. The design of the non-aqueous electrolyte for lithium secondary batteries must consider the compatibility between the positive and negative electrode materials of the battery and the electrolyte, so as to reduce the probability of the electrolyte reacting with the positive and negative electrodes and improve the stability of the SEI film. At present, some researchers in the industry can solve the problem of battery swelling to a certain extent by adding a certain amount of special additives, but it seriously affects the high-temperature cycling performance of lithium secondary batteries.

[0003] Therefore, there is an urgent need for a non-aqueous electrolyte and a lithium secondary battery thereof to solve the deficiencies of the existing technology. Summary of the Invention

[0004] In view of the above problems, the purpose of the present invention is to provide a non-aqueous electrolyte and a lithium secondary battery thereof. The additive in the non-aqueous electrolyte contains a phytate compound, which can effectively improve the cycling performance of the secondary battery at a high voltage of 4.5V and high temperature, and effectively reduce gas generation during storage.

[0005] To achieve the above purpose, the first aspect of the present invention provides a non-aqueous electrolyte, comprising a lithium salt, an organic solvent and an additive, wherein the additive comprises a phytate compound, and the phytate compound is selected from at least one of lithium phytate, sodium phytate, potassium phytate, calcium phytate, magnesium phytate, zinc phytate, manganese phytate, nickel phytate, iron phytate, barium phytate and ammonium phytate.

[0006] Compared with the prior art, the non-aqueous electrolyte of the present invention uses a phytate compound as an additive. The phytate compound contains six negatively charged phosphate groups and has a strong chelating ability. It can chelate the catalytically active cobalt ions dissolved from the positive cathode lithium cobaltate under the condition of a high voltage of 4.5V, reduce the decomposition effect of cobalt ions on the electrolyte solvent, and also reduce the proportion of cobalt ions reduced and deposited on the negative electrode, greatly improving the stability of the negative electrode interface. At the same time, as an antioxidant, the phytate compound can neutralize and capture the reactive oxygen free radicals generated by lithium cobaltate under the condition of a high voltage of 4.5V, reduce the chemical reaction between the reactive oxygen free radicals and the electrolyte, thereby protecting the content of effective additives in the electrolyte, enabling the non-aqueous electrolyte of the present invention to effectively improve the cycling performance of the lithium secondary battery at high temperature and 4.5V high voltage, and effectively reducing the gas generation during storage.

[0007] As a preferred technical solution, the phytate compound of the present invention is selected from at least one of lithium phytate, sodium phytate (CAS: 123408-98-0), potassium phytate (CAS: 129832-03-7), and calcium phytate (CAS: 7776-28-5). Among them, lithium phytate is obtained by an ion exchange reaction between sodium phytate and lithium carbonate.

[0008] As a preferred technical solution, the mass percentage of the phytate compound of the present invention in the non-aqueous electrolyte is 0.01% - 5%. Specifically, the mass percentage of the phytate compound of the present invention in the non-aqueous electrolyte can be, but is not limited to, 0.01%, 0.1%, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%.

[0009] As a preferred technical solution, the lithium salt of the present invention is selected from at least one of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiSbF6 (lithium hexafluoroantimonate), LiPF2O2 (lithium difluorophosphate), LiDTI (4,5-dicyano-2-trifluoromethylimidazole lithium), LiBOB (lithium bis(oxalato)borate), LiDFOB (lithium difluoro(oxalato)borate), LiFSI (lithium bis(fluorosulfonyl)imide), LiN(SO2RF)2, LiN(SO2F)(SO2RF), LiCl (lithium chloride), where RF = C n F 2n+1 , and n is an integer from 1 to 10.

[0010] As a preferred technical solution, the mass percentage of the lithium salt in the non-aqueous electrolyte of the present invention is 6.5% - 15.5%. Specifically, the content of the lithium salt can be, but is not limited to, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 10%, 11%, 13.5%, 14%, 14.5%, 15%.

[0011] As a preferred technical solution, the organic solvent of the present invention is selected from at least one of chain carbonates, cyclic carbonates, carboxylic acid esters, and lactones. Among them, the chain carbonate can be at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (PMC), and ethyl propyl carbonate (PEC); the cyclic carbonate can be at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, and pentylene carbonate; the carboxylic acid ester can be at least one of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The lactone can be at least one of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0012] As a preferred technical solution, the non-aqueous electrolyte of the present invention further includes an additive, and the additive is selected from one or more of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), ethylene sulfite (ES), 1,3-propane sultone (PS), and divinyl sulfite (DTD).

[0013] As a preferred technical solution, the additive of the present invention is selected from fluoroethylene carbonate (FEC).

[0014] As a preferred technical solution, the mass percentage of the additive in the non-aqueous electrolyte of the present invention is 0.1% - 5%. Specifically, the mass percentage of the additive in the non-aqueous electrolyte can be, but is not limited to, 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%.

[0015] The second aspect of the present invention provides a lithium secondary battery, which includes a positive electrode material and a negative electrode material, further includes the aforementioned non-aqueous electrolyte, and the cut-off voltage of the lithium secondary battery is 4.5V.

[0016] As a preferred technical solution, the positive electrode material of the present invention is selected from lithium cobaltate materials. Specifically, the lithium cobaltate material is lithium cobaltate or lithium cobaltate modified by doping and coating.

[0017] As a preferred technical solution, the negative electrode material of the present invention includes at least one of graphite, artificial graphite, hard carbon, natural graphite, and mesophase microspheres. Preferably, the negative electrode material of the present invention is graphite. Detailed Embodiments

[0018] To better illustrate the purpose, technical solution and beneficial effects of the present invention, the present invention will be further described below in conjunction with specific embodiments. It should be noted that the methods described in the following embodiments are further explanatory descriptions of the present invention and should not be regarded as limitations on the present invention.

[0019] Embodiment 1 (1) Preparation of non-aqueous electrolyte: In a vacuum glove box with an argon atmosphere and a water content < 1 ppm, ethylene carbonate (EC) and propyl propionate (PP) were mixed according to a weight ratio of EC:PP = 1:4. Then lithium phytate was added, dissolved and stirred thoroughly, and then LiPF6 was added. After mixing evenly, a non-aqueous electrolyte was obtained.

[0020] (2) Preparation of the positive electrode: Lithium cobaltate LiCoO2, binder PVDF and conductive agent SuperP were mixed evenly according to a mass ratio of 95:1:4 to make a lithium secondary battery positive electrode paste with a certain viscosity. After the mixed paste was coated on both sides of the aluminum foil, it was dried and roll-pressed to obtain a positive electrode sheet.

[0021] (3) Preparation of the separator: Polyethylene (PE) with a thickness of about 15 μm was used as the separator.

[0022] (4) Preparation of the negative electrode: The negative electrode graphite material, binder PVDF and conductive agent SuperP were mixed evenly according to a mass ratio of 90:2:8 to make a lithium secondary battery negative electrode paste with a certain viscosity. After the mixed paste was coated on both sides of the copper foil, it was dried and roll-pressed to obtain a negative electrode sheet.

[0023] (5) Preparation of the lithium secondary battery: The positive electrode sheet, separator and negative electrode sheet were stacked in sequence, and then stacked as needed. After the tabs were welded, they were placed in an aluminum-plastic film for battery outer packaging. The prepared non-aqueous electrolyte was injected into the dried bare battery cell, and then vacuum packaging, standing, formation (constant current charging at 0.05C to 3.6V, and then constant current charging at 0.1C to 3.9V), shaping, capacity testing and other processes were carried out. Finally, a 2Ah soft-pack lithium secondary battery was obtained.

[0024] Embodiments 2 to 15 The lithium secondary batteries of Embodiments 2 to 15 are basically the same as those of Embodiment 1. The difference between the lithium secondary batteries of Embodiments 2 to 15 and Embodiment 1 is only the non-aqueous electrolyte. The formulations of the non-aqueous electrolytes of Embodiments 2 to 15 are shown in Table 1.

[0025] Comparative Examples 1 to 4 The lithium secondary batteries of Comparative Examples 1 to 4 are basically the same as those of Embodiment 1. The difference between the lithium secondary batteries of Comparative Examples 1 to 4 and Embodiment 1 is only the non-aqueous electrolyte. The formulations of the non-aqueous electrolytes of Comparative Examples 1 to 4 are shown in Table 1.

[0026] Table 1 Non-aqueous electrolyte formulations of Examples 1-15 and Comparative Examples 1-4

[0027] The lithium secondary batteries prepared in Examples 1-15 and Comparative Examples 1-4 were respectively subjected to high-temperature storage tests and high-temperature cycling tests according to the following methods. The specific test conditions are as follows, and the test results are shown in Table 2.

[0028] High-temperature storage performance test The lithium secondary batteries of Examples 1-15 and Comparative Examples 1-4 were charged and discharged at 0.5C / 0.5C once at 25°C, with an upper limit voltage of 4.5V. Then, the batteries were charged to full charge at 0.5C under normal temperature conditions, with an upper limit voltage of 4.5V, and the battery thickness was recorded as V0. After storing the lithium secondary batteries in an 85°C environment for 6 hours, the battery thickness was measured as V1, and the thickness expansion rate = (V1 - V0) / V0 × 100%.

[0029] Cycling performance test The lithium secondary batteries of Examples 1-15 and Comparative Examples 1-4 were charged and discharged at 2.0C / 1.0C once at 45°C (the battery discharge capacity was C0), with an upper limit voltage of 4.5V. Then, they were charged and discharged at 2.0C / 1.0C for 300 cycles at 45°C (the battery discharge capacity was C1), and the capacity retention rate = (C1 / C0) × 100%.

[0030] Table 2 Performance test results of lithium secondary batteries of Examples and Comparative Examples

[0031] As can be seen from Table 2, the high-temperature cycling performance, high-temperature storage thickness expansion rate performance of the lithium secondary batteries of Examples 1-15 are superior to those of Comparative Examples 1-4. This is because the non-aqueous electrolyte of the present invention uses a phytate compound as an additive. The phytate compound contains six negatively charged phosphate groups and has strong chelating ability. It can chelate the catalytically active cobalt ions dissolved from the positive electrode lithium cobaltate under the condition of 4.5V high voltage, reduce the decomposition effect of cobalt ions on the electrolyte solvent, and also reduce the proportion of cobalt ions reduced and deposited on the negative electrode, greatly improving the stability of the negative electrode interface. At the same time, the phytate compound, as an antioxidant, can neutralize and capture the active oxygen free radicals generated by lithium cobaltate under the condition of 4.5V high voltage, reduce the chemical reaction between the active oxygen free radicals and the electrolyte, thereby protecting the content of effective additives in the electrolyte, enabling the non-aqueous electrolyte of the present invention to improve the cycling performance of the lithium secondary battery at high temperature and 4.5V high voltage, and effectively reducing the gas generation during high-temperature storage of the battery.

[0032] Furthermore, it can be seen from the test results of Examples 10 to 14 that when lithium phytate is used in combination with FEC, the performance is the best, probably because the preferential reduction of FEC reduces the consumption of phytate ions, thereby improving the overall battery performance.

[0033] It can be seen from the test results of Example 1 and Comparative Examples 3 to 4 that when the mass percentage of lithium phytate is lower than the scope of the present invention, due to insufficient phosphate chelating ability, the performance is poor; when the mass percentage of lithium phytate is higher than the scope of the present invention, due to too much phosphate content, the migration of lithium ions is hindered, so the performance is poor.

[0034] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A non-aqueous electrolyte, comprising a lithium salt, an organic solvent and an additive, characterized in that, The additive includes a phytate compound, and the phytate compound is selected from at least one of lithium phytate, sodium phytate, potassium phytate, calcium phytate, magnesium phytate, zinc phytate, manganese phytate, nickel phytate, iron phytate, barium phytate and ammonium phytate.

2. The non-aqueous electrolyte according to claim 1, characterized in that, The phytate compound is selected from at least one of lithium phytate, sodium phytate, potassium phytate and calcium phytate.

3. A non-aqueous electrolyte according to claim 1, characterized in that, The mass percentage of the phytate compound in the non-aqueous electrolyte is 0.01% to 5%.

4. A non-aqueous electrolyte according to claim 1, characterized in that, The lithium salt is selected from at least one of LiPF6, LiBF4, LiClO4, LiAsF6, LiSbF6, LiPF2O2, LiDTI, LiBOB, LiDFOB, LiFSI, LiN(SO2RF)2, LiN(SO2F)(SO2RF), and LiCl, where RF = C n F 2n+1 , and n is an integer from 1 to 10.

5. A non-aqueous electrolyte according to claim 1, characterized in that, The organic solvent is selected from at least one of chain carbonates, cyclic carbonates, carboxylic acid esters and lactones.

6. A non-aqueous electrolyte according to claim 1, characterized in that, It further includes an auxiliary agent, and the auxiliary agent is selected from at least one of vinylene carbonate, ethylene vinyl carbonate, fluoroethylene carbonate, ethylene sulfite, 1,3-propane sultone and ethylene sulfate.

7. A non-aqueous electrolyte according to claim 6, characterized in that, The auxiliary agent is selected from fluoroethylene carbonate.

8. A non-aqueous electrolyte according to any one of claims 6 or 7, characterized in that, The mass percentage of the auxiliary agent in the non-aqueous electrolyte is 0.1% to 5%.

9. A lithium secondary battery, comprising a positive electrode material and a negative electrode material, characterized in that, It further includes the non-aqueous electrolyte according to any one of claims 1 to 8, and the cut-off voltage of the lithium secondary battery is 4.5V.

10. A lithium secondary battery according to claim 9, characterized in that, The positive electrode material includes a lithium cobaltate material, and the negative electrode material includes at least one of graphite, artificial graphite, hard carbon, natural graphite and mesophase microspheres.

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