Electrolyte and lithium battery

Abstract

The invention discloses an electrolyte and a lithium battery, which are used for solving the technical problems that the charging temperature is increased, the side reaction of a system is increased, the lithium precipitation risk of a negative electrode is increased and the cycle performance is deteriorated when the charging rate of the existing lithium battery is improved. The electrolyte comprises a lithium salt, a solvent and an additive, the solvent comprises fluorocarboxylic ester, and the additive comprises an additive A. In the design, by adding the additive A into the electrolyte, when oxygen atoms in the substance structure are formed, an electron can be obtained, a five-membered ring is broken, and a sulfite product is further obtained through reaction, so that the interface film-forming impedance can be effectively reduced, and the risk of temperature rise and lithium precipitation can be reduced; meanwhile, the fluorocarboxylic ester in the solvent is matched, so that the dynamic stability and the thermal stability can be considered, the fast charging performance, the high-temperature circulation performance and the storage performance of the lithium battery are considered, and in addition, the safety performance of the lithium battery can also be improved.

Description

Technical Field

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

[0002] With the development of electronic information technology, many emerging technology fields have continuously put forward new expectations and requirements for the energy density and charging time of power sources. Among them, lithium-ion batteries are particularly widely used due to their high density, long life, and green environmental protection.

[0003] In order to shorten the charging time of lithium batteries, increasing the charging rate is one of the main means. However, increasing the charging rate will increase the charging temperature, increase the side reactions of the system, increase the risk of lithium deposition at the negative electrode, and deteriorate the cycle performance.

[0004] Therefore, finding a technical solution that can solve the above technical problems has become an important topic studied by technical personnel in this field. Summary of the invention

[0005] The invention discloses an electrolyte and a lithium battery, which are used to solve the technical problems that when the charging rate of existing lithium batteries is increased, the charging temperature will increase, the side reactions of the system will increase, the risk of lithium plating at the negative electrode will increase, and the cycle performance will deteriorate.

[0006] The present invention provides an electrolyte comprising a lithium salt, a solvent and an additive, wherein the solvent comprises a fluorocarboxylate, and the additive comprises an additive A, wherein the structural formula of the additive A is as follows:

[0007]

[0008] Wherein, R is selected from any one of a hydrogen atom, a fluorine atom, an alkyl group and its substitution products, an alkenyl group and its substitution products.

[0009] Optionally, the structural formula of the additive A is as follows:

[0010]

[0011] The additive A accounts for 0.1 to 5 wt % of the electrolyte.

[0012] Optionally, the fluorocarboxylate is ethyl fluoroacetate, and the structural formula of the ethyl fluoroacetate is as follows:

[0013]

[0014] The mass percentage of the ethyl fluoroacetate in the electrolyte is 1-20wt%.

[0015] Optionally, the additive also includes one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), 1,3-propane sultone (PS), diethylene sulfate (DTD), succinonitrile (SN), adiponitrile (ADN), 1,3,6-hexanetrinitrile (HTCN), propene sultone (PST), methylene disulfonate (MMDS), and ethylene glycol bis(propionitrile) ether (EGBE), and the mass proportion of the additive in the electrolyte is 5 to 15 wt%.

[0016] Optionally, the lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiODFB), lithium difluorobis(oxalatophosphate) (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate (LiPOF2).

[0017] Optionally, the solvent also includes two or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP), and the mass proportion of the solvent in the electrolyte is 20 to 70 wt%.

[0018] Optionally, the concentration of lithium salt in the electrolyte is 0.9M to 2M.

[0019] The present invention provides a lithium battery, characterized in that it comprises a positive electrode sheet, a negative electrode sheet, a separator and the above-mentioned electrolyte.

[0020] Optionally, in the positive electrode sheet and the negative electrode sheet, the total content of S element accounts for no less than 150 ppm by mass.

[0021] Optionally, the positive electrode sheet includes a positive electrode current collector and a positive electrode membrane, and the negative electrode sheet includes a negative electrode current collector and a negative electrode membrane;

[0022] The positive electrode film comprises a positive electrode active material, a positive electrode conductive agent and a positive electrode binder;

[0023] The negative electrode film comprises a negative electrode active material, a negative electrode conductive agent and a negative electrode binder;

[0024] The positive electrode active material includes lithium cobalt oxide, ternary material, and lithium iron phosphate, and the negative electrode active material includes graphite and / or silicon.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] In the electrolyte of the present invention, by adding additive A to the electrolyte, the oxygen atom in the substance structure can obtain an electron during chemical formation, the five-membered ring is disconnected, and further reaction is performed to obtain a sulfite product, which can effectively reduce the interface film formation impedance, reduce the temperature rise and lithium precipitation risks; at the same time, in combination with the fluorocarboxylic acid ester in the solvent, it can take into account both kinetics and thermal stability, thereby taking into account the fast charging performance, high temperature cycle and storage performance of the lithium battery, and can also improve the safety performance of the lithium battery. DETAILED DESCRIPTION

[0027] The invention discloses an electrolyte and a lithium battery, which are used to solve the technical problems that when the charging rate of existing lithium batteries is increased, the charging temperature will increase, the side reactions of the system will increase, the risk of lithium plating at the negative electrode will increase, and the cycle performance will deteriorate.

[0028] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below in conjunction with specific implementation methods. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0029] The present invention provides an electrolyte comprising a lithium salt, a solvent and an additive, wherein the solvent comprises a fluorocarboxylate, and the additive comprises an additive A, wherein the structural formula of the additive A is as follows:

[0030]

[0031] Wherein, R is selected from any one of a hydrogen atom, a fluorine atom, an alkyl group and its substitution products, an alkenyl group and its substitution products.

[0032] In the electrolyte of the present invention, by adding additive A to the electrolyte, the oxygen atom in the substance structure can obtain an electron during chemical formation, the five-membered ring is disconnected, and further reaction is performed to obtain a sulfite product, which can effectively reduce the interface film formation impedance, reduce the temperature rise and lithium precipitation risks; at the same time, in combination with the fluorocarboxylic acid ester in the solvent, it can take into account both kinetics and thermal stability, thereby taking into account the fast charging performance, high temperature cycle and storage performance of the lithium battery, and can also improve the safety performance of the lithium battery.

[0033] Furthermore, the fluorocarboxylic acid ester in the present invention is specifically fluoroethyl acetate (DFEA), and the structural formula of the fluoroethyl acetate (DFEA) is as follows:

[0034]

[0035] Wherein, the mass proportion of the ethyl fluoroacetate in the electrolyte is 1-20wt%.

[0036] It should be noted that, in some specific embodiments, the above-mentioned proportion may be 2wt%, 10wt%, 15wt%, etc., and the present invention is not limited to this.

[0037] Furthermore, the structural formula of the additive A in the present invention is as follows:

[0038]

[0039] Its mass proportion in the electrolyte is 0.1 to 5 wt %.

[0040] It should be noted that the above-mentioned proportion is preferably 0.1-2wt%. In some specific embodiments, the above-mentioned proportion can be 0.3wt%, 1wt%, 1.5wt%, etc., and the present invention is not limited to this.

[0041] Furthermore, the additives in this embodiment may also include one or more of fluorocarbonate, vinylene carbonate (VC), 1,3-propane sultone (PS), diethylene sulfate (DTD), succinonitrile (SN), adiponitrile (ADN), 1,3,6-hexanetrinitrile (HTCN), propene sultone (PST), methylene disulfonate (MMDS), and ethylene glycol bis(propionitrile) ether (EGBE), wherein the mass proportion of the additive in the electrolyte is 5 to 15 wt%.

[0042] Specifically, the fluorinated carbonate is fluorinated ethylene carbonate (FEC).

[0043] It should be noted that, in some specific embodiments, the above-mentioned proportion may be 6wt%, 10wt%, 12wt%, etc., and the present invention is not limited to this.

[0044] In addition, designers can select one or more of the above additives according to actual conditions, and this embodiment does not limit this.

[0045] Furthermore, the lithium salt in this embodiment includes one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiODFB), lithium difluorobis(oxalatophosphate) (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate (LiPOF2).

[0046] It should be noted that the designer may select one or more of the above lithium salts according to actual conditions, and this embodiment does not limit this.

[0047] Furthermore, the solvent in this embodiment also includes two or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP), and the mass proportion of the solvent in the electrolyte is 20 to 70 wt%.

[0048] It should be noted that, in some specific embodiments, the above-mentioned proportion may be 30wt%, 40wt%, 50wt%, etc., and the present invention is not limited to this.

[0049] In addition, designers can select two or more solvents according to actual conditions, and this embodiment does not limit this.

[0050] Furthermore, the concentration of lithium salt in the electrolyte of the present invention is 0.9M to 2M.

[0051] It should be noted that the above concentration is preferably 1.0 M to 1.3 M. In some specific embodiments, the concentration may be 1.1 M, 1.2 M, etc. This embodiment does not limit this.

[0052] The present invention provides a lithium battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and the above-mentioned electrolyte.

[0053] It should be noted that the positive electrode sheet, the negative electrode sheet, and the separator in the present invention are wound or stacked into a battery cell, which is then placed in a shell and the above-mentioned electrolyte is injected into the shell to produce a lithium battery.

[0054] Furthermore, in the negative electrode sheet of the present invention, the total content of S element accounts for no less than 150 ppm by mass.

[0055] It should be noted that additive A can be reduced at the negative electrode to generate sulfite substances, and the S element content can indicate that additive A participates in film formation.

[0056] Furthermore, the positive electrode sheet of the present invention includes a positive electrode current collector and a positive electrode membrane, and the negative electrode sheet includes a negative electrode current collector and a negative electrode membrane;

[0057] The positive electrode film comprises a positive electrode active material, a positive electrode conductive agent and a positive electrode binder;

[0058] The negative electrode film comprises a negative electrode active material, a negative electrode conductive agent and a negative electrode binder;

[0059] The positive electrode active material includes lithium cobalt oxide, ternary, and lithium iron phosphate;

[0060] The negative electrode active material includes graphite and / or silicon.

[0061] The technical solution of the present invention is further described in detail below in conjunction with several preferred embodiments. This embodiment is implemented on the premise of the technical solution of the invention, and a detailed implementation method and specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.

[0062] Unless otherwise specified, the experimental materials used in the following examples can be purchased from conventional biochemical reagent companies.

[0063] Lithium battery production

[0064] Preparation of positive electrode sheets: The positive electrode active material lithium cobalt oxide (Li CoO2), the conductive agent CNT (carbon nanotube), and the adhesive PVDF (polyvinylidene fluoride) are fully stirred and mixed in NMP (N-methylpyrrolidone) solvent at a weight ratio of 97:1.5:1.5. The slurry is coated on the positive electrode collector Al foil, and after drying, cold pressing, slitting, sheet making, welding, and ear gluing, a positive electrode sheet that meets the winding requirements is made.

[0065] Preparation of negative electrode sheet: negative electrode active material graphite negative electrode, conductive agent SP ( Conductive carbon black ), thickener CMC (carboxymethyl cellulose), and adhesive SBR (styrene-butadiene rubber) are mixed in a proper amount of deionized water solvent at a mass ratio of 96.3:1:1.2:1.5 to form a uniform negative electrode slurry. The slurry is applied to the negative electrode current collector Cu foil, and then dried, cold pressed, stripped, welded, and glued to the tabs to form a negative electrode sheet that meets the winding requirements.

[0066] Preparation of electrolyte: The preparation steps of the electrolyte are: mixing different types of solvents to obtain a mixed solvent, then slowly adding Li PF6 to obtain a mixed solution with a Li PF6 concentration of 1.2 mol / L, and after the lithium salt is completely dissolved, adding the corresponding additives to obtain the electrolyte.

[0067] Production of lithium battery: The above positive electrode sheet, separator and negative electrode sheet are wound to obtain a bare cell. The bare cell is placed in the aluminum-plastic film that has been punched to complete the top and side sealing. After high-temperature baking, liquid injection, static, formation, capacity division, testing and other processes, the lithium battery production is completed.

[0068] The types and proportions of raw materials in the electrolytes of Examples 1 to 14 and Comparative Examples 1 to 6 were changed as follows:

[0069]

[0070]

[0071]

[0072] Lithium battery performance test

[0073] 25℃ 3C / 1.0C cycle test: Charge to the upper limit voltage at 3C constant current and constant voltage at 25℃, cut-off current 0.05C, stand for 10 minutes, test the cell thickness H0, discharge to 3.0V at 1.0C, record the discharge capacity C0 as the initial, repeat 800 cycles, obtain the capacity C800 after 800 cycles, then the capacity retention rate = C800 / C0. Fully charge test cell thickness H800, thickness expansion = (H800-H0) / H0.

[0074] 45℃ 0.7C / 1.0C cycle test: charge at 3C constant current and constant voltage at 45℃ to the upper limit voltage, cutoff current 0.05C, stand for 10min, discharge at 1.0C to 3.0V, record the discharge capacity C0 as the initial, repeat 400 times, obtain the capacity C400 after 400 cycles, then the capacity retention rate = C400 / C0.

[0075] Thermal shock at 135℃ for 30min: At 25℃, discharge to 3.0V with a given current of 0.2C; leave for 5min; charge to the upper limit voltage with a constant current and constant voltage of 0.2C, with a cut-off current of 0.05C; after leaving for 1h, measure the voltage and internal resistance, put the battery cell into the oven, monitor the surface temperature of the battery cell, and increase the oven temperature to 135±2℃ at a rate of 5±2℃ / min, and keep it for 30min before stopping. It is judged that the standard battery cell does not catch fire or explode.

[0076] Test results of Examples 1 to 14 and Comparative Examples 1 to 6:

[0077]

[0078]

[0079] The test results of Comparative Examples 1 to 6 show that the addition of fluorocarboxylic acid ester and additive A can improve the cycle performance, and it is speculated that the thermal stability of fluorocarboxylic acid ester and the film formation of additive A are related;

[0080] Comparative analysis of Examples 1, 5, and 6 shows that too low or too high FEC content will affect performance. FEC with too high a content will fail due to thermal shock, which is presumably related to severe gas production at high temperatures. FEC with too low a content will produce gas at a low temperature in the later stage of a long cycle.

[0081] The test results of Examples 1 to 6 and Comparative Example 1 show that the fast charge cycle performance can be improved by adding additive A, among which the improvement effect of 0.5-0.8% is the most obvious. At the same time, the addition of additive A also improves the thermal shock performance, which is speculated to be related to the appropriate fluorine substitution on the carbon chain and the low film impedance and stability of the sulfur-containing group;

[0082] The test results of Examples 8, 9, and 13 and Comparative Example 2 show that, on the basis of adding fluorocarboxylic acid ester, adding 0.5% Additive A has the best effect. Adding too little has no obvious improvement, adding too much has no obvious improvement, and the 45°C cycle is slightly deteriorated. The combination of DFEA and Additive A takes into account both film formation impedance and solvent stability, and improves the system circulation and safety performance.

[0083] The above is a detailed introduction to an electrolyte and a lithium battery provided by the present invention. For a person skilled in the art, according to the concept of the embodiments of the present invention, there may be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. An electrolyte, characterized in that: It includes a lithium salt, a solvent and an additive, wherein the solvent includes a fluorocarboxylate, and the additive includes an additive A, wherein the structural formula of the additive A is as follows: Wherein, R is selected from any one of a hydrogen atom, a fluorine atom, an alkyl group and its substitution products, an alkenyl group and its substitution products.

2. The electrolyte according to claim 1, characterized in that The structural formula of the additive A is as follows: The additive A accounts for 0.1 to 5 wt % of the electrolyte.

3. The electrolyte according to claim 1, characterized in that The fluorocarboxylate is ethyl fluoroacetate, and the structural formula of the ethyl fluoroacetate is as follows: The mass percentage of the ethyl fluoroacetate in the electrolyte is 1-20wt%.

4. The electrolyte according to claim 1, characterized in that The additives also include one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), 1,3-propane sultone (PS), diethylene sulfate (DTD), succinonitrile (SN), adiponitrile (ADN), 1,3,6-hexanetrinitrile (HTCN), propene sultone (PST), methylene disulfonate (MMDS), and ethylene glycol bis(propionitrile) ether (EGBE), and the mass proportion of the additives in the electrolyte is 5 to 15 wt%.

5. The electrolyte according to claim 1, characterized in that The lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiODFB), lithium difluorobis(oxalatophosphate) (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate (LiPOF2).

6. The electrolyte according to claim 1, characterized in that The solvent also includes two or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP), and the mass proportion of the solvent in the electrolyte is 20 to 70 wt%.

7. The electrolyte according to claim 1, characterized in that The concentration of lithium salt in the electrolyte is 0.9M to 2M.

8. A lithium battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte as claimed in any one of claims 1 to 7.

9. The lithium battery according to claim 8, characterized in that: In the positive electrode sheet and the negative electrode sheet, the total content of S element accounts for no less than 150 ppm by mass.

10. The lithium battery according to claim 8, characterized in that: The positive electrode sheet includes a positive electrode current collector and a positive electrode membrane, and the negative electrode sheet includes a negative electrode current collector and a negative electrode membrane; The positive electrode film comprises a positive electrode active material, a positive electrode conductive agent and a positive electrode binder; The negative electrode film comprises a negative electrode active material, a negative electrode conductive agent and a negative electrode binder; The positive electrode active material includes lithium cobalt oxide, ternary material, and lithium iron phosphate, and the negative electrode active material includes graphite and / or silicon.

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