An electrolyte and a lithium battery

Abstract

This invention discloses an electrolyte and a lithium battery to address the technical problems of existing lithium batteries where increasing the charging rate leads to increased charging temperature, increased system side reactions, increased risk of lithium plating at the negative electrode, and deterioration of cycle performance. The electrolyte of this invention includes a lithium salt, a solvent, and additives. The solvent includes a fluorocarboxylic acid ester, and the additive includes additive A. In the above design, by adding additive A to the electrolyte, the oxygen atom in the substance gains an electron during formation, breaking the five-membered ring and further reacting to obtain a sulfite product. This effectively reduces interfacial film-forming resistance, lowers temperature rise, and reduces the risk of lithium plating. Simultaneously, the fluorocarboxylic acid ester in the solvent balances kinetic and thermal stability, thus achieving a balance between the fast-charging performance, high-temperature cycling, and storage performance of the lithium battery. Furthermore, it also improves the safety performance of the lithium battery.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and more particularly to an electrolyte and a lithium battery. Background Technology

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

[0003] To shorten the charging time of lithium batteries, increasing the charging rate is one of the main methods. However, increasing the charging rate will lead to problems such as higher charging temperature, increased side reactions in the system, increased risk of lithium plating at the negative electrode, and deterioration of cycle performance.

[0004] Therefore, finding a technical solution that can solve the above-mentioned technical problems has become an important research topic for those skilled in the art. Summary of the Invention

[0005] This invention discloses an electrolyte and a lithium battery to solve the technical problems of existing lithium batteries, which lead to increased charging temperature, increased system side reactions, increased risk of lithium plating at the negative electrode, and deterioration of cycle performance when the charging rate is increased.

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

[0007]

[0008] R is selected from any one of hydrogen atoms, fluorine atoms, alkyl groups and their substitutes, alkenyl groups and their substitutes.

[0009] Optionally, the additive A has the following structural formula:

[0010]

[0011] The additive A has a mass percentage of 0.1 to 5 wt% in the electrolyte.

[0012] Optionally, the fluorocarboxylic acid ester is ethyl fluorocarbonate, and the structural formula of ethyl fluorocarbonate is as follows:

[0013]

[0014] The ethyl fluoroacetate has a mass ratio of 1-20 wt% in the electrolyte.

[0015] Optionally, the additive further includes one or more of the following: fluoroethylene carbonate (FEC), vinylene carbonate (VC), 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), succinic anionyl nitrile (SN), adiponitrile (ADN), 1,3,6-hexanetrionitrile (HTCN), propenesulfonate lactone (PST), methanedisulfonate methylene ester (MMDS), and ethylene glycol bis(propionitrile) ether (EGBE), wherein the additive accounts for 5 to 15 wt% of the electrolyte.

[0016] Optionally, the lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalate borate (LiODFB), lithium difluorodioxalate phosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate (LiPOF2).

[0017] Optionally, the solvent may further include two or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP), wherein the mass percentage 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 includes a positive electrode, a negative electrode, a separator, and the electrolyte described above.

[0020] Optionally, the total mass percentage of sulfur in the positive electrode and the negative electrode is not less than 150 ppm.

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

[0022] The positive electrode membrane includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder;

[0023] The negative electrode membrane includes 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 materials, 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 this invention, by adding additive A, the oxygen atom in the structure of this substance can gain an electron during formation, the five-membered ring breaks, and further reaction yields sulfite products, which can effectively reduce interfacial film-forming resistance, reduce temperature rise and lithium plating risk; at the same time, in combination with fluorocarboxylic acid esters in the solvent, it can take into account both kinetic and thermal stability, thereby taking into account the fast charging performance, high temperature cycling and storage performance of lithium batteries, and can also improve the safety performance of lithium batteries. Detailed Implementation

[0027] This invention discloses an electrolyte and a lithium battery to solve the technical problems of existing lithium batteries, which lead to increased charging temperature, increased system side reactions, increased risk of lithium plating at the negative electrode, and deterioration of cycle performance when the charging rate is increased.

[0028] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to specific embodiments. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort 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 fluorocarboxylic acid ester, and the additive comprises additive A, wherein the structural formula of additive A is as follows:

[0030]

[0031] R is selected from any one of hydrogen atoms, fluorine atoms, alkyl groups and their substitutes, alkenyl groups and their substitutes.

[0032] In the electrolyte of this invention, by adding additive A, the oxygen atom in the structure of this substance can gain an electron during formation, the five-membered ring breaks, and further reaction yields sulfite products, which can effectively reduce interfacial film-forming resistance, reduce temperature rise and lithium plating risk; at the same time, in combination with fluorocarboxylic acid esters in the solvent, it can take into account both kinetic and thermal stability, thereby taking into account the fast charging performance, high temperature cycling and storage performance of lithium batteries, and can also improve the safety performance of lithium batteries.

[0033] Furthermore, the fluorocarboxylic acid ester in this invention is specifically ethyl fluorocarbonate (DFEA), and the structural formula of ethyl fluorocarbonate (DFEA) is as follows:

[0034]

[0035] The ethyl fluoroacetate has a mass percentage of 1-20 wt% in the electrolyte.

[0036] It should be noted that in some specific embodiments, the above proportions can be 2wt%, 10wt%, 15wt%, etc., and the present invention does not limit them.

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

[0038]

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

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

[0041] Furthermore, the additives in this embodiment may also include one or more of the following: fluorocarbonate, vinylene carbonate (VC), 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), succinic anionyl nitrile (SN), adiponitrile (ADN), 1,3,6-hexanetrionitrile (HTCN), propenesulfonate lactone (PST), methanedisulfonate methylene ester (MMDS), and ethylene glycol bis(propionitrile) ether (EGBE), wherein the additives constitute 5 to 15 wt% of the electrolyte.

[0042] Specifically, the aforementioned fluorocarbonate is fluoroethylene carbonate (FEC).

[0043] It should be noted that in some specific embodiments, the above percentages may be 6wt%, 10wt%, 12wt%, etc., and the present invention does not limit them.

[0044] In addition, designers may choose one or more of the above-mentioned additives according to the actual situation, and this embodiment does not impose any restrictions on this.

[0045] Furthermore, the lithium salt in this embodiment includes one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalate borate (LiODFB), lithium difluorodioxalate phosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate (LiPOF2).

[0046] It should be noted that designers may choose one or more of the above-mentioned lithium salts according to actual conditions, and this embodiment does not impose any restrictions on this.

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

[0048] It should be noted that in some specific embodiments, the above percentage can be 30wt%, 40wt%, 50wt%, etc., and the present invention does not limit this.

[0049] In addition, designers may choose two or more of the above solvents according to the actual situation, and this embodiment does not impose any restrictions on 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 preferred concentration is 1.0M to 1.3M, but in some specific embodiments, the concentration can be 1.1M, 1.2M, etc. This embodiment does not impose any limitations on this.

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

[0053] It should be noted that in this invention, the positive electrode, negative electrode, and separator are wound or stacked to form a battery cell, which is then installed in a housing and the aforementioned electrolyte is injected into the housing to produce a lithium battery.

[0054] Furthermore, in the negative electrode sheet of the present invention, the total content of sulfur element by mass is not less than 150 ppm.

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

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

[0057] The positive electrode membrane includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder;

[0058] The negative electrode membrane includes 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 lithium, and lithium iron phosphate.

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

[0061] The technical solution of the present invention will be further described in detail below with reference to several preferred embodiments. These embodiments are implemented on the premise of the technical solution of the invention, and provide detailed implementation methods and specific operation processes. However, the protection scope of the present invention is not limited to the following embodiments.

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

[0063] Lithium battery manufacturing

[0064] The positive electrode sheet is prepared by mixing the positive active material lithium cobalt oxide (LiCoO2), the conductive agent CNT (carbon nanotubes), and the binder PVDF (polyvinylidene fluoride) in NMP (N-methylpyrrolidone) solvent at a weight ratio of 97:1.5:1.5. This slurry is then coated onto the positive current collector Al foil. After drying, cold pressing, slitting, sheet forming, welding, and adhesive application, a positive electrode sheet that meets the winding requirements is produced.

[0065] Fabrication of the negative electrode sheet: The negative electrode active material (graphite negative electrode), conductive agent (SP, conductive carbon black), thickener (CMC, carboxymethyl cellulose), and binder (SBR, styrene-butadiene rubber) are mixed thoroughly in an appropriate amount of deionized water solvent at a mass ratio of 96.3:1:1.2:1.5 to form a uniform negative electrode slurry. This slurry is then coated onto the negative electrode current collector (Cu foil). Following processes such as drying, cold pressing, slitting, sheet forming, welding, and adhesive application to the tabs, a negative electrode sheet meeting winding requirements is produced.

[0066] Preparation of electrolyte: The preparation steps of electrolyte are as follows: different types of solvents are mixed to obtain a mixed solvent, and then LiPF6 is slowly added to obtain a mixed solution with a LiPF6 concentration of 1.2 mol / L. After the lithium salt is completely dissolved, the corresponding additives are added to obtain the electrolyte.

[0067] Lithium-ion battery manufacturing: The positive electrode, separator, and negative electrode are wound together to obtain a bare cell. The bare cell is then placed in a pre-punched aluminum-plastic film to complete the top and side sealing. After high-temperature baking, electrolyte injection, settling, formation, capacity testing, and other processes, the lithium-ion battery manufacturing is completed.

[0068] The types and proportions of raw materials in the electrolytes of Examples 1-14 and Comparative Examples 1-6 are varied as follows:

[0069] Example solvent Additive 1 Additive 2 Example 1 EC:PC:DEC:PP=2:2:1:5 0.1wt% Additive A 8wt%FEC Example 2 EC:PC:DEC:PP=2:2:1:5 0.3wt% Additive A 8wt%FEC Example 3 EC:PC:DEC:PP=2:2:1:5 0.5wt% Additive A 8wt%FEC Example 4 EC:PC:DEC:PP=2:2:1:5 0.8wt% Additive A 8wt%FEC Example 5 EC:PC:DEC:PP=2:2:1:5 1.0wt% Additive A 8wt%FEC Example 6 EC:PC:DEC:PP=2:2:1:5 2.0wt% Additive A 8wt%FEC Example 7 EC:PC:DEC:PP=2:2:1:5 5.0wt% Additive A 8wt%FEC Example 8 EC:PC:DFEA:PP=2:2:1:5 0.3wt% Additive A 8wt%FEC Example 9 EC:PC:DFEA:PP=2:2:1:5 0.5wt% Additive A 8wt%FEC Example 10 EC:PC:DEC:DFEA:PP=20:20:9:1:50 0.5wt% Additive A 8wt%FEC Example 11 EC:PC:DEC:DFEA:PP=20:20:5:5:50 0.5wt% Additive A 8wt%FEC Example 12 EC:PC:DFEA:PP=20:20:15:45 0.5wt% Additive A 8wt%FEC Example 13 EC:PC:DFEA:PP=20:20:20:40 0.5wt% Additive A 8wt%FEC Example 14 EC:PC:DFEA:PP=2:2:1:5 0.8wt% Additive A 8wt%FEC Comparative Example 1 EC:PC:DEC:PP=2:2:1:5 / 8wt%FEC Comparative Example 2 EC:PC:DFEA:PP=2:2:1:5 / 8wt%FEC Comparative Example 3 EC:PC:DEC:PP=2:2:1:5 0.5wt% DTD 8wt%FEC Comparative Example 4 EC:PC:DEC:PP=2:2:1:5 2wt% PS 8wt%FEC Comparative Example 5 EC:PC:DEC:PP=2:2:1:5 / 4wt%FEC Comparative Example 6 EC:PC:DEC:PP=2:2:1:5 / 35wt%FEC

[0070] Lithium battery performance testing

[0071] 25℃ 3C / 1.0C Cyclic Test: At 25℃, charge at 3C constant current and constant voltage to the upper limit voltage, cut off current 0.05C, let stand for 10 minutes, and measure cell thickness H0. Discharge at 1.0C to 3.0V, record discharge capacity C0 as initial value, repeat for 800 cycles, and obtain capacity C800 after 800 cycles. Capacity retention rate = C800 / C0. For full charge testing, cell thickness H800 is measured, and thickness expansion = (H800 - H0) / H0.

[0072] 45℃ 0.7C / 1.0C Cyclic Test: Charge at 3C constant current and constant voltage to the upper limit voltage at 45℃, cut off current 0.05C, let stand for 10 minutes, discharge at 1.0C to 3.0V, record the discharge capacity C0 as the initial value, repeat for 400 cycles, and obtain the capacity C400 after 400 cycles. Then the capacity retention rate = C400 / C0.

[0073] 135℃ 30min thermal shock: Under 25°C conditions, discharge to 3.0V with a given current of 0.2C; rest for 5min; charge to the upper limit voltage with constant current and constant voltage of 0.2C, cut-off current 0.05C; after resting for 1h, measure the voltage and internal resistance, put the cell into an oven, monitor the cell surface temperature, the oven temperature rises to 135±2°C at a rate of 5±2°C / min and is maintained for 30min before stopping, and the standard cell is judged not to catch fire or explode.

[0074] Test results of Examples 1-14 and Comparative Examples 1-6:

[0075] Test Items RT cycle 800-cycle capacity retention rate / % Thickness expansion after 800 RT cycles / % Capacity retention rate after 400 cycles at 45°C / % 135℃ for 30 minutes thermal shock Example 1 56.4% 16.5% 62.8% 0 / 5pass Example 2 72.8% 10.2% 74.5% 2 / 5 pass Example 3 80.5% 8.6% 81.3% 4 / 5 pass Example 4 82.5% 8.2% 80.6% 5 / 5 pass Example 5 82.6% 8.5% 79.7% 5 / 5 pass Example 6 82.5% 8.9% 79.6% 5 / 5 pass Example 7 80.4% 9.2% 75.7% 5 / 5 pass Example 8 79.2% 9.5% 80.5% 4 / 5 pass Example 9 83.9% 8.5% 82.6% 5 / 5 pass Example 10 80.4% 8.5% 81.4% 4 / 5 pass Example 11 82.6% 8.5% 82.1% 5 / 5 pass Example 12 83.8% 8.3% 82.5% 5 / 5 pass Example 13 80.6% 8.5% 81.7% 4 / 5 pass Example 14 83.9% 8.2% 81.5% 5 / 5 pass Comparative Example 1 55.4% 18.2% 60.7% 0 / 5pass Comparative Example 2 70.8% 15.6% 68.5% 2 / 5 pass Comparative Example 3 75.9% 12.4% 73.3% 2 / 5 pass Comparative Example 4 72.6% 11.8% 74.2% 3 / 5 pass Comparative Example 5 diving >20% (gas production) diving 2 / 5 pass Comparative Example 6 60.8% 17.8% 62.4% 0 / 5pass

[0076] The test results of Comparative Examples 1 to 6 show that the addition of fluorocarboxylic acid esters and additive A can improve the cycling performance, suggesting that the thermal stability of fluorocarboxylic acid esters and the film formation of additive A are related.

[0077] Comparative analysis of Examples 1, 5, and 6 shows that both excessively low and excessively high FEC content will affect performance. Excessively high FEC content leads to thermal shock failure, which is presumably related to severe gas production at high temperatures. Excessively low FEC content will result in a sharp drop in gas production in the later stages of long-term cycling.

[0078] The test results of Examples 1-6 and Comparative Example 1 show that the addition of additive A can improve the fast charge cycle performance, with the most significant improvement effect observed at a ratio of 0.5-0.8%. The addition of additive A also improves the thermal shock performance, which is presumably related to the appropriate fluorine substitution on the carbon chain and the low film resistance and stability of the sulfur-containing groups.

[0079] The test results of Examples 8, 9, 13 and Comparative Example 2 show that adding 0.5% of Additive A on the basis of adding fluorocarboxylic acid ester has the best effect. Adding too little has no obvious improvement effect, while adding too much has little improvement and slightly worsens the 45°C cycling performance. The combination of DFEA and Additive A takes into account both film-forming resistance and solvent stability, and improves the cycling and safety performance of the system.

[0080] The electrolyte and lithium battery provided by the present invention have been described in detail above. For those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the embodiments of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. An electrolyte, characterized in that, The mixture includes lithium salts, solvents, and additives, wherein the solvent includes fluorocarboxylic acid esters, and the additive includes additive A, wherein additive A has the following structural formula: ； Wherein, R is selected from any one of hydrogen atom, fluorine atom, alkyl group and its substituted derivatives, alkenyl group and its substituted derivatives; The structural formula of additive A is as follows: ； The mass percentage of additive A in the electrolyte conforms to the following condition: 0.5wt% < mass percentage ≤ 0.8wt%. The fluorocarboxylic acid ester is ethyl fluorocarbonate, and the structural formula of ethyl fluorocarbonate is as follows: ； The ethyl fluoroacetate has a mass percentage of 1-20 wt% in the electrolyte; The additives also include one or more of the following: fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl sulfate (DTD), succinic anionyl nitrile (SN), adiponitrile (ADN), 1,3,6-hexanetrionitrile (HTCN), methane disulfonate (MMDS), and ethylene glycol bis(propionitrile) ether (EGBE), wherein the additives constitute 5-15 wt% of the electrolyte. The solvent also includes two or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP), and the mass percentage of the solvent in the electrolyte is 20-70 wt%.

2. The electrolyte according to claim 1, characterized in that, The lithium salt includes one or more of lithium hexafluorophosphate, lithium difluorooxalate borate, lithium difluorodioxalate phosphate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium difluorophosphate.

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

4. A lithium battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte as described in any one of claims 1 to 3.

5. The lithium battery according to claim 4, characterized in that, The total sulfur content in the positive and negative electrode sheets shall be no less than 150 ppm by mass.

6. The lithium battery according to claim 4, characterized in that, The positive electrode includes a positive current collector and a positive electrode membrane, and the negative electrode includes a negative current collector and a negative electrode membrane; The positive electrode membrane includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder; The negative electrode membrane includes 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 materials, and lithium iron phosphate, and the negative electrode active material includes graphite and / or silicon.

Images