Electrolyte and lithium ion battery

Abstract

The application discloses an electrolyte and a lithium ion battery, which comprise an organic solvent, a lithium salt and an additive; wherein the organic solvent comprises solvent A, solvent B and solvent C, and the additive comprises additive D and additive E; compared with the prior art, the application adopts a plurality of solvents and a high lithium salt system, and the additive D and the additive E are combined, so that the electrolyte system has a high ion transference number, the solvation structure of Li + is effectively changed, the energy barrier during solvation and desolvation of Li + is significantly reduced, the solvation and desolvation process of Li + is accelerated, a thin and hard inorganic layer is formed on the inner layer of the SEI of the silicon negative electrode, the stability of the SEI is effectively improved, the polarization in the charging and discharging process is reduced, and the cycle performance, the fast-charging performance and the stability of the silicon negative electrode system lithium ion battery are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and particularly relates to an electrolyte and a lithium ion battery. BACKGROUND

[0002] A 3C digital lithium ion battery is generally composed of a positive electrode, a negative electrode, a separator, an electrolyte and other auxiliary materials. The most commonly used positive electrode active material is LiCoO2, which has a very high compaction density and a high voltage characteristic. The most common negative electrode material is artificial graphite, which has a high theoretical specific capacity (372 mAh / g), and after years of development, its actual specific capacity is close to its theoretical specific capacity, thus limiting the improvement of the energy density of the lithium ion battery.

[0003] Silicon negative electrodes are the next generation of negative electrode materials with application prospects, but they will undergo a huge volume change (about 300%) during charging and discharging, which will cause the failure of the binder, the separation of the active material and the current collector, etc. The existing negative electrode binders such as styrene-butadiene rubber SBR and polyvinylidene fluoride PVDF are easily affected by the silicon negative electrode system and lose their binding effect, thus causing the cycle performance to deteriorate. Moreover, the SEI on the surface of the silicon negative electrode will repeatedly break due to the volume change of the silicon particles, and the broken position will grow new SEI, which will cause Li + consumption and thus deteriorate the cycle performance.

[0004] Therefore, it is urgent to develop a new type of negative electrode and electrolyte combination to improve the specific capacity of the negative electrode and enhance the stability of the SEI on the surface of the silicon negative electrode, thus improving the cycle performance of the high-voltage LiCoO2-hard carbon / SiC (or SiOx) system and the fast charging performance of the silicon negative electrode. SUMMARY

[0005] The application aims to provide an electrolyte to improve the low negative electrode capacity and poor SEI film stability of the current high-silicon negative electrode system battery.

[0006] To achieve the above-mentioned purpose, the application adopts the following technical scheme:

[0007] An electrolyte comprises an organic solvent, a lithium salt and an additive.

[0008] The organic solvent comprises solvent A, solvent B and solvent C, the solvent A is shown in the following formula 1, the solvent B is shown in the following formula 2, and the solvent C is shown in the following formula 3.

[0009]

[0010] The additive comprises additive D and additive E, the additive D is shown in the following formula 4, and the additive E is shown in the following formula 5.

[0011]

[0012] R1-R 10 each independently is selected from at least one of alkyl group having 1-10 carbon atoms, fluoroalkyl group having 2-4 carbon atoms, fluoroalkenyl group having 2-4 carbon atoms, N-containing heterocycle, S-containing heterocycle.

[0013] Preferably, the mass content of the solvent A in the electrolyte is w; the mass content of the solvent B in the electrolyte is x; the mass content of the solvent C in the electrolyte is y; the mass content of the additive D in the electrolyte is 2-15%; the mass content of the additive E in the electrolyte is z; wherein w, x, y, z satisfy the following relationship:

[0014] 0.1≤w / (x+y)≤0.5;

[0015] 0.003≤z / (2x+y)≤0.08;

[0016] 0.1≤x / y≤1;

[0017] 0.007≤z / (2w+y)≤0.15.

[0018] Preferably, the w is 5-20%; the x is 5-50%; the y is 10-70%; the z is 0.4-5%.

[0019] Preferably, the structural formula of the solvent A is:

[0020]

[0021] The structural formula of the solvent B is:

[0022]

[0023] The structural formula of the solvent C is:

[0024]

[0025] The structural formula of the additive D is:

[0026]

[0027] The structural formula of the additive E is:

[0028]

[0029] Preferably, the mass ratio of the solvent A, solvent B and solvent C is (13-15):(38-42):

[0030] (43-47).

[0031] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium bisoxalate borate, lithium difluoroxalate borate, lithium tetrafluoroborate, lithium bistrifluoromethanesulfonimide and lithium bisfluorosulfonimide, and the content of the lithium salt is 8-25% of the total mass of the electrolyte.

[0032] Preferably, the additive further comprises a film-forming additive, and the film-forming additive is at least one of 1,3-propanesultone, ethylene sulfate and fluorinated ethylene carbonate.

[0033] Preferably, the mass of the film-forming additive is 0.1-20% of the total mass of the electrolyte.

[0034] In addition, the application further provides a lithium ion battery comprising a positive electrode sheet, a separator, a negative electrode sheet and the above-mentioned electrolyte.

[0035] Preferably, the negative electrode active material of the negative electrode sheet comprises hard carbon and silicon carbon, the gram capacity of the hard carbon is greater than or equal to 380 mAh / g, the gram capacity of the silicon carbon is greater than or equal to 1800 mAh / g, and the total gram capacity of the negative electrode sheet is greater than or equal to 600 mAh / g.

[0036] The application has the following beneficial effects: by using A, B and C as solvents and adding D and E, the application can significantly affect the directional migration of Li + , significantly reduce the energy barrier of Li + solvation and desolvation, accelerate the solvation and desolvation process of Li + , reduce the polarization in the charging and discharging process, improve the fast charging performance, form a thin and hard inorganic layer in the inner layer of the SEI on the surface of the silicon negative electrode, effectively improve the stability of the SEI and improve the cycle performance. In addition, the synergistic use of the additive D and the additive E can form a more uniform and dense SEI film, the SEI film has small impedance, and the cycle and high-temperature storage performance of the battery can be further improved. The electrolyte of the application can significantly improve the problems of low negative electrode capacity and poor SEI film stability of the current high-silicon negative electrode system battery. DETAILED DESCRIPTION

[0037] In order to make the technical solutions and advantages of the application clearer, the technical solutions of the application will be described clearly and completely below in combination with specific embodiments. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0038] In the first aspect of the present application, the present application provides an electrolyte, comprising an organic solvent, a lithium salt, an additive; wherein the organic solvent includes carbonates, carboxylic acid esters; specifically including solvent A, solvent B and solvent C, the solvent A is shown as formula 1 below, the solvent B is shown as formula 2 below, and the solvent C is shown as formula 3 below;

[0039]

[0040] The additive includes additive D and additive E, the additive D is shown as formula 4 below, and the additive E is shown as formula 5 below;

[0041]

[0042] R1-R 10 Each of R1-R4 is independently selected from at least one of substituted or unsubstituted alkyl with carbon number of 1-10, fluorinated alkyl with carbon number of 2-4, fluorinated alkenyl with carbon number of 2-4, N-containing heterocycle, and S-containing heterocycle.

[0043] The lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiDFP), lithium difluoro bisoxalate phosphate (LiODFP), lithium tetrafluoro oxalate phosphate (LiOTFP), lithium bisoxalate borate (LiBOB), lithium difluoro oxalate borate (LiODFB), lithium tetrafluoroborate (LiBF4), lithium bis-trifluoromethanesulfonimide (LiTFSI), and lithium bisfluorosulfonimide (LiFSI).

[0044] The preparation steps of the above electrolyte of the present application include: in an argon-filled glove box, mixing solvent A, B and C according to the mass ratio of (13-15):(38-42):(43-47) to form a mixed solution; then slowly adding 8-25wt% of lithium salt based on the total weight of the electrolyte into the mixed solution, and finally adding 0.4%-5wt% of additive E, 2%-15wt% of additive D based on the total weight of the electrolyte, and stirring uniformly to obtain a lithium ion battery electrolyte.

[0045] Compared with conventional electrolyte, the present application can significantly affect the directional migration of Li+ during the charging and discharging of the silicon negative electrode system lithium ion battery by using A, B and C three compounds as solvents, and cooperating with additives D and E, significantly reducing the energy barrier of Li+ during solvation and desolvation, and accelerating Li+. + + + ​​The solventation and desolvation process of Li+ is accelerated, the polarization in the charging and discharging process is reduced, and the fast charging performance is improved. Meanwhile, the compound D with better film-forming performance than EC is used as a key film-forming additive, and a thin and hard inorganic layer is formed in the inner layer of the SEI film on the surface of the silicon negative electrode, effectively improving the stability of the SEI and the cycle performance. In addition, the synergistic use of additives D and E can form a more uniform and dense SEI film. Since the impedance of the SEI film is small, the cycle and high-temperature storage performance of the battery is further improved.

[0046] In summary, the electrolyte of the present application can significantly improve the cycle and high-temperature storage performance of the battery, and solve the problems of low negative electrode capacity and poor SEI film stability in the current high-silicon negative electrode system battery.

[0047] The reaction mechanism of the electrolyte prepared by the above method is as follows: the additive D can affect the directional migration of Li+ during the charging and discharging of the battery, effectively change the solventation structure of Li+, significantly reduce the energy barrier of Li+ during solventation and desolvation, accelerate the solventation and desolvation process of Li+, reduce the polarization in the charging and discharging process, and improve the fast charging performance. + + + + + + + + +

[0048] In one embodiment, the weight ratio of the solvents A, B and C is (13-15):(38-42):(43-47), and the specific ratio can be 13:38:43, 14:39:44, 15:40:44, 15:40:45, and can include but is not limited to the listed values. Preferably, the weight ratio of A, B and C is 15:40:45.

[0049] ​​​​​​​​​In one embodiment, the mass content of the solvent A in the electrolyte is w; the mass content of the solvent B in the electrolyte is x; the mass content of the solvent C in the electrolyte is y; the mass content of the additive D in the electrolyte is 2-15%; the mass content of the additive E in the electrolyte is z; and the following relationship is satisfied among A, B, C, and E:

[0050] 0.1≤w / (x+y)≤0.5;

[0051] 0.003≤z / (2x+y)≤0.08;

[0052] 0.1≤x / y≤1;

[0053] 0.007≤z / (2w+y)≤0.15.

[0054] In one embodiment, the content w is 5-20%; the content x is 5-50%; the content y is 10-70%; and the content z is 0.4-5%; specifically, w can be 5%, 10%, 15%, or 20%; x can be 5%, 10%, 15%, 20%, 25%, 30%; y can be 10%, 20%, 30%, 40%, 50%, 60%, or 70%; and z can be 0.4%, 0.8%, 1.2%, 1.6%, 2.0%, 4.0%, 4.4%, 4.8%, or 5.0%; the above values can include but are not limited to the listed values, and the above values are controlled within the relationship, which can make the performance of the battery better, better inhibit the expansion of the silicon electrode, and improve the strength of the SEI film. In one embodiment, the content of the lithium salt is 8-25% of the total mass of the electrolyte, specifically, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 23%, 24%, or 25%; the above values can include but are not limited to the listed values.

[0055] In one embodiment, at least one of the film-forming additives is selected from 1,3-propanesultone, vinyl sulfate, and fluoroethylene carbonate.

[0056] In one embodiment, the mass of the film-forming additive is 0.1-20% of the total mass of the electrolyte, specifically, 0.1%, 0.5%, 1%, 5%, 7%, 10%, 12%, 15%, 17%, 19%, or 20%; the above values can include but are not limited to the listed values, and preferably, the mass of the film-forming additive is 4% of the total mass of the electrolyte.

[0057] In a second aspect according to the present application, the present application further provides a lithium ion battery, comprising an electrolyte, a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the electrolyte is the electrolyte described above.

[0058] The application also provides a preparation method of the lithium ion battery, which comprises the following steps: stacking or winding the negative electrode sheet, the separator and the positive electrode sheet in sequence to obtain an electric core, placing the electric core in a packaging shell, adding an electrolyte and sealing, and obtaining the lithium ion battery after formation, hot and cold pressing, and capacity distribution.

[0059] The positive electrode sheet can be prepared by the following method: mixing the positive electrode active material, the conductive agent, the binder and N-methyl pyrrolidone uniformly to obtain a positive electrode slurry. Then, the positive electrode slurry is coated on the positive electrode current collector, and the positive electrode sheet is obtained after drying and rolling. The positive electrode active material is at least one of transition metal oxides and polyanion compounds; preferably, the positive electrode active material is lithium cobaltate. The conductive agent is at least one of carbon nanotubes, acetylene black, ketjen black, carbon black, graphene and carbon nanofibers; preferably, the conductive agent is carbon black. The binder is at least one of polyvinylidene fluoride, polyacrylic acid, polyimide, sodium polyacrylate, butadiene rubber, sodium alginate and nitrile rubber; preferably, the binder is polyvinylidene fluoride. The positive electrode current collector can be carbon-coated foil material, metal foil material or composite foil material; preferably, the positive electrode current collector is aluminum foil.

[0060] The negative electrode sheet can be prepared by the following method: mixing the hard carbon and silicon carbon in the negative electrode active material at a weight ratio of 3:1, mixing the active material, the conductive agent, the thickening agent and the binder in deionized water at a weight ratio of 97:1.1:0.6:1.3 to obtain a negative electrode slurry, and coating the negative electrode slurry on the negative electrode current collector, and the negative electrode sheet is obtained after drying and rolling. The binder is at least one of polyaniline, polyurethane, polyvinyl alcohol, polyacrylic acid, sodium polyacrylate, sodium alginate and butadiene rubber; preferably, the binder is a mixture of polyaniline and polyurethane. The conductive agent is at least one of acetylene black, carbon black, ketjen black, graphene, carbon nanotubes and carbon nanofibers; preferably, the conductive agent is carbon nanotubes. The thickening agent is at least one of sodium carboxymethyl cellulose, alginic acid and starch; preferably, the thickening agent is sodium carboxymethyl cellulose. The negative electrode current collector can be carbon-coated foil material, metal foil material or composite foil material; preferably, the negative electrode current collector is copper foil.

[0061] The separator can be a single-layer or multi-layer separator made of at least one of glass fiber, polypropylene, polyethylene, non-woven fabric and polyvinylidene fluoride.

[0062] The application will be further described below by means of specific examples.

[0063] Example 1

[0064] Preparation of the electrolyte:

[0065] The preparation method of the electrolyte in this example is as follows:

[0066] In an argon-filled glove box, solvent A1, B1, C1 were mixed in a mass ratio of A1:B1:C1 = 15:40:45, then 15% lithium hexafluorophosphate based on the total weight of the electrolyte was slowly added to the mixed solution, and finally 2.5% additive E based on the total weight of the electrolyte, 10% additive D based on the total weight of the electrolyte, and 3% 1,3-propanesultone (film-forming additive) based on the total weight of the electrolyte were added, and stirred uniformly to obtain a lithium ion battery electrolyte. The structures of A1, B1, C1, D and E are as follows:

[0067] A1: B1:

[0068] D: E:

[0069] Preparation of negative electrode sheet:

[0070] Raw materials: hard carbon, silicon carbon (SiC), conductive agent carbon nanotube (CNT), thickening agent carboxymethyl cellulose sodium (CMC), binder polyaniline (PANI) and binder polyurethane (PU). The mixture of hard carbon and silicon carbon in a weight ratio of hard carbon: silicon carbon = 75:25 was used as the negative electrode active material. After mixing the substances in a weight ratio of negative electrode active material: CNT: CMC: PANI: PU = 97.6:0.5:0.7:0.7:0.5, deionized water was added and mixed uniformly to prepare a negative electrode slurry. The negative electrode slurry was coated on a current collector copper foil, dried at 85°C, then cold-pressed, and then edge-cut, sheet-cut and strip-cut. After drying at 85°C under vacuum for 12h, a lithium ion battery negative electrode sheet was obtained. The specific capacity of the used hard carbon was 380mAh / g, the specific capacity of the silicon carbon negative electrode was 1810mAh / g, and the total specific capacity of the negative electrode sheet was 737.5mAh / g.

[0071] Preparation of positive electrode sheet:

[0072] Raw materials: cathode active material lithium cobaltate LiCoO2, conductive agent Super P, and binder polyvinylidene fluoride PVDF. After mixing the substances in a weight ratio of LiCoO2: Super P: PVDF = 97.5:1:1.5, adding N-methyl pyrrolidone (NMP), and mixing uniformly, a lithium ion battery positive electrode slurry was prepared. The positive electrode slurry was coated on a current collector aluminum foil, dried at 85°C, then cold-pressed, and then edge-cut, sheet-cut and strip-cut. After drying at 85°C under vacuum for 4h, the tab was welded, and a lithium ion battery positive electrode sheet was prepared.

[0073] Preparation of battery:

[0074] The positive electrode sheet, the separator, and the negative electrode sheet prepared above are stacked in order with the separator between the positive electrode sheet and the negative electrode sheet to obtain a bare battery cell; the bare battery cell is placed in an aluminum plastic film outer package, the electrolyte prepared above is injected into the dried battery, and the battery is packaged, left to stand, formed, shaped, and tested to complete the preparation of the lithium ion soft package battery.

[0075] Example 2

[0076] The method for preparing the electrolyte in this example is different from that in Example 1 only in the additive ratio in the electrolyte, as shown in Table 1, and the total content of the electrolyte is adjusted to 100% by adjusting the solvent content.

[0077] The other aspects are the same as those in Example 1, which will not be described here again.

[0078] Example 3

[0079] The method for preparing the electrolyte in this example is different from that in Example 1 only in the additive ratio in the electrolyte, as shown in Table 1, and the total content of the electrolyte is adjusted to 100% by adjusting the solvent content.

[0080] The other aspects are the same as those in Example 1, which will not be described here again.

[0081] Example 4

[0082] The method for preparing the electrolyte in this example is different from that in Example 1 only in the additive ratio in the electrolyte, as shown in Table 1, and the total content of the electrolyte is adjusted to 100% by adjusting the solvent content.

[0083] The other aspects are the same as those in Example 1, which will not be described here again.

[0084] Example 5

[0085] The method for preparing the electrolyte in this example is different from that in Example 1 only in the additive ratio in the electrolyte, as shown in Table 1, and the total content of the electrolyte is adjusted to 100% by adjusting the solvent content.

[0086] The other aspects are the same as those in Example 1, which will not be described here again.

[0087] Example 6

[0088] The method for preparing the electrolyte in this example is different from that in Example 1 only in the solvent ratio in the electrolyte, as shown in Table 1.

[0089] The other aspects are the same as those in Example 1, which will not be described here again.

[0090] Example 7

[0091] The method for preparing the electrolyte of this example is different from that of example 1 only in the solvent ratio in the electrolyte, see table 1 for details.

[0092] The others are the same as example 1, and will not be repeated here.

[0093] Example 8

[0094] The method for preparing the electrolyte of this example is different from that of example 1 only in the solvent ratio in the electrolyte, see table 1 for details.

[0095] The others are the same as example 1, and will not be repeated here.

[0096] Comparative example 1

[0097] The method for preparing the electrolyte of this example is different from that of example 1 only in that the additive in the electrolyte is only additive E, see table 1 for details, and the total content of the electrolyte is adjusted to 100% by adjusting the solvent content.

[0098] The others are the same as example 1, and will not be repeated here.

[0099] Comparative example 2

[0100] The method for preparing the electrolyte of this example is different from that of example 1 only in that the additive in the electrolyte is only additive D, see table 1 for details, and the total content of the electrolyte is adjusted to 100% by adjusting the solvent content.

[0101] The others are the same as example 1, and will not be repeated here.

[0102] Comparative example 3

[0103] The method for preparing the electrolyte of this example is different from that of example 1 only in the additive ratio in the electrolyte, see table 1 for details, and the total content of the electrolyte is adjusted to 100% by adjusting the solvent content.

[0104] The others are the same as example 1, and will not be repeated here.

[0105] Comparative example 4

[0106] The method for preparing the electrolyte of this example is different from that of example 1 only in the additive ratio in the electrolyte, see table 1 for details, and the total content of the electrolyte is adjusted to 100% by adjusting the solvent content.

[0107] The others are the same as example 1, and will not be repeated here.

[0108] Comparative example 5

[0109] The electrolyte in the present comparative example does not contain additives, and the total content of the electrolyte is adjusted to 100% by adjusting the solvent content, and the other conditions are the same as those in Example 1, which will not be repeated here.

[0110] Comparative Example 6

[0111] The gram capacity of the hard carbon used in the present comparative example is 370 mAh / g, and the other conditions are the same as those in Example 1, which will not be repeated here.

[0112] Comparative Example 7

[0113] The gram capacity of the silicon-carbon used in the present comparative example is 1700 mAh / g, and the other conditions are the same as those in Example 1, which will not be repeated here.

[0114] Comparative Example 8

[0115] The gram capacity of the hard carbon used in the present comparative example is 370 mAh / g, the gram capacity of the silicon-carbon is 1700 mAh / g, the gram capacity of the negative electrode is 702.5 mAh / g, and the other conditions are the same as those in Example 1, which will not be repeated here.

[0116] Comparative Example 9

[0117] The method for preparing the electrolyte in the present comparative example is different from that in Example 1 only in the solvent composition in the electrolyte, which is shown in Table 1.

[0118] The other conditions are the same as those in Example 1, which will not be repeated here.

[0119] Comparative Example 10

[0120] The method for preparing the electrolyte in the present comparative example is different from that in Example 1 only in the solvent composition in the electrolyte, which is shown in Table 1.

[0121] The other conditions are the same as those in Example 1, which will not be repeated here.

[0122] Comparative Example 11

[0123] The method for preparing the electrolyte in the present comparative example is different from that in Example 1 only in the solvent composition in the electrolyte, which is shown in Table 1.

[0124] The other conditions are the same as those in Example 1, which will not be repeated here.

[0125] Comparative Example 12

[0126] The method for preparing the electrolyte in the present comparative example is different from that in Example 1 only in the solvent composition in the electrolyte, which is shown in Table 1.

[0127] The other conditions are the same as those in Example 1, which will not be repeated here.

[0128] Comparative Example 13

[0129] The method for preparing the electrolyte of the present comparative example is different from that of Example 1 only in the amount of the lithium salt added and the ratio of the solvents, as shown in Table 1.

[0130] The other conditions are the same as those of Example 1, which are not described here.

[0131] The content of each component of the electrolyte of Examples 1-8 and Comparative Examples 1-13 is shown in Table 1.

[0132] Table 1

[0133]

[0134]

[0135]

[0136] The batteries prepared in the examples and comparative examples were respectively subjected to the following performance tests:

[0137] (1) Normal temperature cycle performance test: In a 25℃ environment, the battery after being divided into groups was charged at 0.7C constant current and constant voltage to 4.53V, with a cutoff current of 0.05C, and then discharged at 0.5C constant current to 3.0V, and the cycle was repeated. After 400 cycles of charging and discharging, the capacity retention rate at the 400th week was calculated, and the calculation formula was as follows:

[0138] Capacity retention rate at the 400th week (%) = (discharge capacity at the 400th week / discharge capacity at the first cycle) x 100%.

[0139] (2) High temperature cycle performance test: In a 45℃ environment, the battery after being divided into groups was charged at 0.7C constant current and constant voltage to 4.53V, with a cutoff current of 0.05C, and then discharged at 0.5C constant current to 3.0V, and the cycle was repeated. After 300 cycles of charging and discharging, the capacity retention rate at the 300th week was calculated, and the calculation formula was as follows:

[0140] Capacity retention rate at the 300th week (%) = (discharge capacity at the 300th week / discharge capacity at the first cycle) x 100%.

[0141] (3) 85℃ 24h high temperature storage test: the battery is placed at room temperature for 1 time of charging and discharging at 0.5C (4.53V-3.0V), the discharge capacity C0 of the battery before storage is recorded, then the battery is charged to 4.53V full state (100% SOC) at constant current and constant voltage, the thickness d1 of the battery before high temperature storage is tested using PPG battery thickness tester (600g), the battery is placed in a 85℃ constant temperature oven for 24h, after storage is completed, the battery is taken out and the battery thickness after storage d2 is tested, the thickness expansion rate of the battery after 85℃ storage for 24h is calculated; after the battery is cooled at room temperature for 24h, the battery is discharged to 3.0V at 0.5C again, then charged to 4.53V at 0.5C constant current and constant voltage, the discharge capacity C1 and the charge capacity C2 of the battery after storage are recorded, the capacity retention rate and the recovery rate of the battery after 85℃ storage for 24h are calculated, the calculation formula is as follows:

[0142] Thickness expansion rate after 85℃ storage for 24h = (d2-d1) / d1*100%;

[0143] Capacity retention rate after 85℃ storage for 24h = C1 / C0*100%;

[0144] Capacity recovery rate after 85℃ storage for 24h = C2 / C0*100%.

[0145] (4) Thermal shock performance: discharge at a given current of 0.2C to 3.0V under the condition of 25℃ environment; stand for 5min; charge at a charge current of 0.2C to 4.53V, when the cell voltage reaches 4.53V, change to 4.53V constant voltage charging until the charge current ≤ cutoff current 0.05C; after standing for 1h, place the cell in an oven, increase the oven temperature to 135±2℃ at a speed of 5±2℃ / min, and keep for 60min, then stop, the judgment standard is that the cell does not catch fire and does not explode.

[0146] The battery performance test results of examples 1-8 and comparative examples 1-13 are shown in table 2.

[0147] Table 2

[0148]

[0149]

[0150] Based on the test results in the table above, it can be seen that the use of compounds A1, B1, and C1 as solvents in the examples, combined with additives D and E, resulted in better performance than the comparative examples under the same test conditions. The test results of Example 1 and Comparative Examples 9-10 indicate that the use of solvents without EC compounds in this application yields better results than those using EC compounds. The test results of Example 1 and Comparative Examples 11-12 show that A1, B1, and C1 have a synergistic effect; the absence of any one of them prevents the achievement of the desired effect. The results between the examples demonstrate that different proportions of additives affect the high-temperature cycle performance, expansion rate, and capacity of the battery, with the examples within the scope of this invention showing better test results.

[0151] The test results from Examples 1-8 and Comparative Example 5 show that the combined use of additives D and E significantly affects the performance of lithium-ion batteries; additive D can affect the performance of Li-ion batteries during charge and discharge. + The targeted migration of Li can effectively change the direction of Li. + The solvation structure significantly reduces Li + The energy barriers during solvation and desolvation accelerate Li + The solvation and desolvation processes reduce polarization during charging and discharging, improving fast charging performance; additive D also has good film-forming properties, enabling the formation of a thin and hard inorganic layer within the SEI layer on the silicon anode surface, accelerating Li... + By embedding the SEI and hard carbon interlayers and forming an alloy phase with the SiC anode, lithium plating caused by high-rate charging is reduced, effectively improving the stability of the SEI and enhancing cycle performance; simultaneously, it interacts with long-chain conductive Li... + The active ingredient E is combined with other additives. Additive E has multiple branches, each with an O atom and a highly electronegative cyano-CN group, while Li... + Electron-deficient with empty 2s orbitals, additives E and PC preferentially complex with it to form solvation structures. Simultaneously, during desolvation, additive E utilizes the O and -CN on its long chain to accelerate Li... + The transfer of [something] accelerates the desolvation process, which helps improve fast charging performance.

[0152] Based on the test results of Examples 1-8 and Comparative Examples 6-8, the effects produced by different specific capacities of the negative electrode active material in combination with the electrolyte of the present invention are also different. The specific capacities of negative electrode active materials outside the scope of the present invention do not produce as good test results as those of the present invention. Therefore, selecting a negative electrode active material with a suitable specific capacity can maximize the effect of the electrolyte of the present invention.

[0153] In summary, the electrolyte system of the present invention can form a more stable SEI on the surface of the mixed negative electrode and effectively reduce the breakage of the SEI during battery cycling, thereby extending the cycle life of the lithium-ion battery.

[0154] Those skilled in the art can make various modifications and alterations to the above embodiments on the basis of the disclosure and teachings of this specification. Therefore, the present application should not be limited to the above embodiments, and any and all modifications, equivalents or substitutes should be considered within the scope of the present application. In addition, although some specific terms are used in this specification, these terms are only for the convenience of explanation and should not be considered as limiting the present application.

Claims

1. A lithium-ion battery, characterized by, The lithium ion battery comprises a positive electrode sheet, a diaphragm, a negative electrode sheet and an electrolyte, the negative electrode active material of the negative electrode sheet comprises hard carbon and silicon carbon, the gram capacity of the hard carbon is greater than or equal to 380 mAh / g, the gram capacity of the silicon carbon is greater than or equal to 1800 mAh / g, the total gram capacity of the negative electrode sheet is greater than or equal to 600 mAh / g, and the electrolyte comprises an organic solvent, a lithium salt and an additive. The organic solvent comprises solvent A, solvent B and solvent C, the solvent A is shown in formula 1, the solvent B is shown in formula 2, and the solvent C is shown in formula 3. Formula 1, Formula 2, Formula 3 The additive comprises additive D and additive E, the additive D is shown in formula 4, and the additive E is shown in formula 5. Formula 4, Formula 5 R1~R 10 each independently is selected from at least one of an alkyl group having 1 to 10 carbon atoms, a fluoroalkyl group having 2 to 4 carbon atoms, a fluoroalkenyl group having 2 to 4 carbon atoms, an N-containing heterocycle, and an S-containing heterocycle; The mass content of the solvent A in the electrolyte is w, the mass content of the solvent B in the electrolyte is x, the mass content of the solvent C in the electrolyte is y, the mass content of the additive D in the electrolyte is 2-15%, the mass content of the additive E in the electrolyte is z, and the following relationships are met between w, x, y and z: 0.1≤w / (x+y)≤0.5; 0.003≤z / (2x+y)≤0.08; 0.1≤x / y≤1; 0.007≤z / (2w+y)≤0.15; The w is 5-20%, the x is 5-50%, the y is 10-70%, and the z is 0.4-5%, and the mass ratio of the solvent A, the solvent B and the solvent C is (13-15):(38-42):(43-47).

2. The lithium ion battery according to claim 1, wherein the structural formula of the solvent A is: The structural formula of the solvent B is: ； The structural formula of the solvent C is: ； The lithium salt is at least one selected from lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium bisoxalate borate, lithium difluorooxalate borate, lithium tetrafluoroborate, lithium bis-trifluoromethanesulfonimide and lithium bisfluorosulfonimide, and the content of the lithium salt accounts for 8-25% of the total mass of the electrolyte. ； The structural formula of the additive D is: ​ The structural formula of the additive E is: ​ 3. The lithium-ion battery of claim 1, wherein, The additive further comprises a film-forming additive, and the film-forming additive is at least one selected from 1,3-propanesultone, ethylene sulfate and fluoroethylene carbonate.

4. The lithium-ion battery of claim 1, wherein, The mass of the film-forming additive accounts for 0.1-20% of the total mass of the electrolyte.

5. The lithium-ion battery of claim 4, wherein, ​

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