Lithium ion battery and application thereof

Abstract

The application belongs to the technical field of lithium ion battery processing, and particularly relates to a lithium ion battery and application thereof. The lithium ion battery comprises: 1) an electrolyte; wherein the additive comprises FEC, DFEC, ADN and HTCN; 2) a positive electrode sheet; 3) a negative electrode sheet; wherein the active substance comprises a silicon material and graphite; and 4) a separator. The lithium ion battery satisfies 8 <= 8*(alpha*beta / 2+sigma 2 )+30*ln(sigma*u / 5+gamma 2 / 25)+10 / min(sigma*u*gamma*v,sigma u / v,100*w*alpha / beta) <= 130, wherein alpha, beta, gamma and w are mass percentage contents of FEC, DFEC, ADN and HTCN in the electrolyte in sequence, sigma g / Ah is a liquid retention coefficient of the battery, u mu m is a median particle size of the silicon material, and v is a mass percentage of the silicon material in the active substance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of lithium ion battery processing, and particularly relates to a lithium ion battery and application thereof. BACKGROUND

[0002] In order to improve the volume energy density of lithium ion batteries, major lithium ion battery manufacturers have begun to use a mixed material of graphite and silicon as a negative active material on a large scale. Graphite itself has stable performance, a mature production process and low cost, but its low gram capacity of 372 mAh / g limits the improvement of the gram capacity of the negative electrode. Silicon has a maximum theoretical gram capacity of 4200 mAh / g, which is much higher than that of graphite, and is conducive to improving the gram capacity of the negative electrode, thereby improving the energy density of the lithium ion battery. However, the silicon material itself will have a huge volume change during charging, causing the binder to fail, the distance between the negative active materials to increase, and the active material to separate from the current collector, thereby causing the lithium ion battery to have an irreversible thickness increase during the cycle process. At the same time, the huge volume expansion of the silicon negative electrode will cause the SEI on the surface of the silicon negative electrode to be in an unstable state, and the SEI will be in a repeated process of damage-repair for a long time, causing the active Li + to be continuously consumed, resulting in capacity attenuation. Therefore, improving the cycle performance and inhibiting the thickness increase during the cycle process are the biggest problems currently faced by the silicon negative electrode lithium ion battery.

[0003] By reducing the liquid retention coefficient of the lithium ion battery, the amount of electrolyte floating inside the lithium battery can be effectively reduced, the influence of the floating liquid on the adhesion of the separator can be reduced, and the lithium precipitation and thickness increase during the cycle process can be inhibited. However, after the liquid retention coefficient is reduced, the battery is prone to have a cycle diving phenomenon in the later cycle stage, so generally, low thickness increase and long cycle are incompatible.

[0004] Therefore, there is an urgent need to develop an electrolyte suitable for a high-voltage lithium cobaltate-graphite / silicon mixed negative electrode system, which can inhibit the thickness increase of the battery during the cycle process and inhibit the diving problem of the low liquid retention coefficient battery in the later stage of long cycle, thereby avoiding safety hazards. SUMMARY

[0005] The following is a summary of the subject matter described in detail in this document. This summary is not intended to limit the scope of protection of the claims.

[0006] The present application aims to at least solve the technical problems of the thickness increase and fast cycle attenuation of the silicon negative electrode system during the cycle process in the prior art, and the cycle diving of the low liquid retention coefficient battery in the later stage of long cycle. To this end, the present application proposes a lithium ion battery and application thereof. The electrolyte used in the present application is particularly suitable for a high-voltage lithium cobaltate-graphite / silicon mixed negative electrode system, can inhibit the thickness increase of the battery during the cycle process, and inhibit the diving problem of the low liquid retention coefficient battery in the later stage of long cycle, thereby avoiding safety hazards.

[0007] To achieve the above object, the present application adopts the following technical solutions:

[0008] The present application provides a kind of lithium ion battery, comprising:

[0009] Electrolyte;The electrolyte includes lithium salt, solvent and additive, the additive includes fluoroethylene carbonate, difluoroethylene carbonate, hexanedinitrile, 1,3,6-hexane trinitrile;

[0010] Positive pole piece;

[0011] Negative pole piece;The active substance of the negative pole piece includes silicon material and graphite;And,

[0012] Separating membrane;

[0013] The lithium ion battery meets 8≤8*(alpha*beta / 2+sigma 2 )+30*ln(sigma*u / 5+gamma 2 / 25)+10 / min(sigma*u*gamma*v,sigma u / v,100*w*alpha / beta)≤130, wherein, alpha is the mass percentage content of the fluoroethylene carbonate in electrolyte, beta is the mass percentage content of the difluoroethylene carbonate in electrolyte, gamma is the mass percentage content of the hexanedinitrile in electrolyte, w is the mass percentage content of the 1,3,6-hexane trinitrile in electrolyte, sigma g / Ah is the liquid retention coefficient of the lithium ion battery, u μm is the median particle size of the silicon material, v is the mass percentage content of the silicon material in the active substance.

[0014] The present application introduces a new fluorocarbon compound: difluoroethylene carbonate (DFEC) in electrolyte system, DFEC has high film forming effect, which can form SEI with high LiF content in formation process, improve the ratio of LiF / Li2O in SEI and the ionic conductivity of SEI, improve the charge and discharge performance, which is beneficial to solve the thickness growth and rapid cycle decay of silicon negative electrode system in the cycle process, and the problem of diving in the later stage of long cycle of low liquid retention coefficient battery. In addition, fluoroethylene carbonate (FEC) as negative electrode film forming additive can reduce impedance and improve cycle performance, hexanedinitrile (AND) and 1,3,6-hexane trinitrile (HTCN) can improve high temperature storage and cycle performance, the above additives and DFEC jointly act, synergistically improve the cycle performance of battery.

[0015] In addition, the present application also constructs the relationship between different parameters, when the above conditions are met, DFEC and Li + Most easily combined, the energy barrier is reduced to the minimum, and DFEC can preferentially combine with Li +The FEC and Li combine and undergo a single electron reduction reaction to generate LiF with a higher Young's modulus on the surface of the silicon particles, thereby increasing the ratio of LiF / Li2O in the SEI, making the SEI film more capable of withstanding the huge volume change of the silicon negative electrode during the charging and discharging process, thereby prolonging the cycle life of the lithium ion battery. When the value of the above relationship is less than 8, the FEC and Li + The FEC and Li combine and undergo a single electron reduction reaction to generate LiF with a higher Young's modulus on the surface of the silicon particles, thereby increasing the ratio of LiF / Li2O in the SEI, making the SEI film more capable of withstanding the huge volume change of the silicon negative electrode during the charging and discharging process, thereby prolonging the cycle life of the lithium ion battery. When the value of the above relationship is less than 8, the FEC and Li + The FEC and Li combine and undergo a single electron reduction reaction to generate LiF with a higher Young's modulus on the surface of the silicon particles, thereby increasing the ratio of LiF / Li2O in the SEI, making the SEI film more capable of withstanding the huge volume change of the silicon negative electrode during the charging and discharging process, thereby prolonging the cycle life of the lithium ion battery. When the value of the above relationship is less than 8, the FEC and Li

[0016] In summary, the present application establishes a relationship between different parameters, determines an electrolyte suitable for a graphite / silicon material mixed negative electrode system, inhibits the increase in the thickness of the battery during the cycle process, and inhibits the diving problem of the battery in the later stage of the long cycle, thereby avoiding safety hazards.

[0017] In some embodiments of the present application, the mass percentage content of the fluorinated ethylene carbonate in the electrolyte is 1% to 20%.

[0018] When the FEC content is too low, the film cannot be effectively formed on the negative electrode surface, the negative electrode active material cannot be protected, and the side reaction on the negative electrode surface is more likely to occur, affecting the cycle performance.

[0019] In some embodiments of the present application, the mass percentage content of the fluorinated ethylene carbonate in the electrolyte is 1% to 20%.

[0020] DFEC is a key substance for inhibiting the expansion of the silicon negative electrode, and under the premise of not deteriorating the charging, an appropriate amount of DFEC can increase the LiF content in the SEI and reduce the increase in the thickness of the battery during the cycle process.

[0021] In some embodiments of the present application, the mass percentage content of the fluorinated ethylene carbonate in the electrolyte is 1% to 20%.

[0022] When the ADN content is too low, there is no obvious improvement effect, and when the content is too high, the room temperature cycle and low temperature discharge performance will be significantly deteriorated.

[0023] In some embodiments of the present application, the 1,3,6-hexanetricarbonitrile has a mass percentage content of 0.3% to 3% in the electrolyte, for example, 0.5% to 2.5%, 1% to 2.5%, 1% to 2%.

[0024] When the content of HTCN is too low, there is no obvious improvement effect, and when the content is too high, it will cause the normal temperature cycle and low temperature discharge performance to deteriorate significantly.

[0025] In some embodiments of the present application, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiDFP), lithium difluorophosphate bisoxalate (LiODFP), lithium tetrafluorophosphate oxalate (LiOTFP), lithium bisoxalate borate (LiBOB), lithium difluorophosphate borate oxalate (LiODFB), lithium tetrafluoroborate (LiBF4), lithium bis-trifluoromethanesulfonimide (LiTFSI), and lithium bisfluorosulfonimide (LiFSI).

[0026] In some embodiments of the present application, the lithium salt has a mass percentage content of 8% to 25.0% in the electrolyte, for example, 8% to 20.0%, 8% to 15.0%, 10% to 20.0%, 10% to 15.0%, 12% to 20.0%, 12% to 15.0%, about 14%.

[0027] In some embodiments of the present application, the solvent includes at least one of a non-fluorinated carbonate solvent, a carboxylic acid ester solvent, an ether solvent, and a C2-C5 nitrile solvent.

[0028] The non-fluorinated carbonate solvent is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (PMC), and ethyl propyl carbonate (PEC); the carboxylic acid ester solvent is selected from at least one of methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), butyl acetate, methyl propionate, ethyl propionate, propyl propionate (PP), methyl butyrate, and ethyl butyrate; the ether solvent is selected from at least one of tetrahydrofuran, 1,3-dioxolane, diethyl ether, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether; and the C2-C5 nitrile solvent is selected from at least one of acetonitrile, glutaronitrile, and malononitrile.

[0029] In some preferred embodiments of the present application, the solvent includes a non-fluorinated carbonate solvent and a carboxylic acid ester solvent.

[0030] In some preferred embodiments of the present application, the solvent comprises ethylene carbonate, propylene carbonate, diethyl carbonate, propyl propionate; more preferably, the mass ratio of the ethylene carbonate, propylene carbonate, diethyl carbonate, propyl propionate is 10-30:5-15:30-50:20-40.

[0031] In some embodiments of the present application, the positive electrode sheet comprises an aluminum foil current collector and a positive electrode film.

[0032] In some embodiments of the present application, the positive electrode film comprises a positive electrode active material lithium cobalt oxide (LiCoO2); further comprising a conductive agent and a binder; wherein the weight ratio of LiCoO2, conductive agent, binder is 98.5:0.5:1.

[0033] In some preferred embodiments of the present application, the conductive agent is selected from at least one of Super-P, acetylene black (AB), Ketjen black (KB), carbon nanotube (CNT); more preferably, the conductive agent is Super-P.

[0034] In some preferred embodiments of the present application, the binder is selected from at least one of polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), polytetrafluoroethylene (PTFE); more preferably, the binder is PVDF.

[0035] In some embodiments of the present application, the silicon material is selected from at least one of silicon, silicon oxide, silicon suboxide, silicon carbide.

[0036] In some embodiments of the present application, the weight ratio of the silicon material in the active material is 3%-20%, for example, 5%-20%, 10%-20%, 15%-20%.

[0037] If the proportion of silicon material in the active material is too low, the energy density of the lithium ion battery cannot be effectively improved; if the proportion is too high, the negative electrode side side reaction is seriously increased, resulting in serious cycle performance degradation.

[0038] In some embodiments of the present application, the median particle size of the silicon material is 2.3-7 μm, for example, 3-7 μm, 4-7 μm, 4.5-7 μm.

[0039] If the particle size of the silicon material is too large, the particle surface side reaction is less, but the capacity of the battery will gradually decrease; on the contrary, if the particle size of the silicon material is too small, the volume effect of the silicon particles in the charging and discharging process will be weakened, but the side reaction on the surface of the silicon particles will gradually intensify, resulting in deterioration of the cycle performance; therefore, the particle size needs to be in a suitable range to balance the various performances.

[0040] In some embodiments of the present application, the negative electrode sheet further comprises a copper foil current collector.

[0041] In some embodiments of the present application, the negative electrode sheet further comprises a conductive agent, a thickening agent and a binder; wherein the weight ratio of the negative electrode active material, the conductive agent, the thickening agent and the binder is 97:0.8:1.2:1.

[0042] In some preferred embodiments of the present application, the conductive agent is selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes and carbon nanofibers; more preferably, the conductive agent is carbon nanotubes.

[0043] In some preferred embodiments of the present application, the thickening agent can be sodium carboxymethyl cellulose (CMC-Na).

[0044] In some preferred embodiments of the present application, the binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) or carboxymethyl chitosan (CMCS); more preferably, the binder is PAA.

[0045] In some embodiments of the present application, the injection coefficient of the lithium ion battery is 1-2 g / Ah.

[0046] In some embodiments of the present application, the liquid retention coefficient of the lithium ion battery is 1.0-1.5 g / Ah.

[0047] The range of the liquid retention coefficient in the present application is low, which can effectively inhibit the lithium precipitation phenomenon of the battery cell during the cycle process and improve the cycle performance.

[0048] The present application also provides an electronic device comprising the lithium ion battery described above.

[0049] The lithium ion battery according to the embodiments of the present application at least has the following beneficial effects:

[0050] The present application introduces a key additive DFEC, which can be preferentially formed into a film at the negative electrode by using its low reduction potential characteristics, and the SEI induced by the DFEC has a higher LiF content than the FEC, which helps to improve the stability of the SEI, reduce the rupture of the SEI film during the charge and discharge process, and improve the cycle performance of the lithium ion battery.

[0051] The relationship between the four additives DFEC, FEC, ADN, HTCN and the silicon material and the cell parameters is constructed, the ratio of two key components LiF and Li2O in SEI is adjusted to balance the film forming performance and SEI function, and the cycle diving of the cell in the low liquid retention state is inhibited to avoid the safety risk of the electronic device in the later cycle.

[0052] Terms and definitions:

[0053] Unless otherwise specified, the "liquid injection coefficient" in the present application refers to the ratio of the injection amount of electrolyte to the design discharge capacity of the battery, and the unit is g / Ah.

[0054] Unless otherwise specified, the "liquid retention coefficient" in the present application refers to the ratio of the amount of residual electrolyte in the cell after sealing to the design discharge capacity of the battery, and the unit is g / Ah.

[0055] Unless otherwise specified, "about" in the present application means that the allowable error is within ±5%. DETAILED DESCRIPTION

[0056] The concept and technical effects of the present application will be described below in conjunction with the embodiments to fully understand the purpose, features and effects of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all embodiments, and other embodiments obtained by those skilled in the art based on the embodiments of the present application without creative labor are within the scope of protection of the present application.

[0057] Embodiments

[0058] In the present application, unless specified, all equipment and raw materials can be purchased from the market or commonly used in the industry. The methods in the following embodiments are conventional methods in the art, unless otherwise specified.

[0059] EC: ethylene carbonate;

[0060] PC: propylene carbonate;

[0061] DEC: diethyl carbonate;

[0062] PP: propyl propionate;

[0063] LiPF6: lithium hexafluorophosphate;

[0064] FEC: fluoroethylene carbonate;

[0065] DFEC: difluoroethylene carbonate;

[0066] ADN: adiponitrile, dicyan;

[0067] HTCN: 1,3,6-hexanetricarbonitrile, a ternary nitrile;

[0068] The main active material LiCoO2 of the positive electrode sheet was purchased from Xiamen Tungsten New Energy;

[0069] The separator was purchased from Shenzhen Xingyuan;

[0070] The active material graphite and silicon of the negative electrode sheet were purchased from Shenzhen Betrayer.

[0071] Example 1

[0072] The present embodiment provides a lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the separator being arranged between the positive electrode sheet and the negative electrode sheet. The lithium ion battery is obtained by the following method:

[0073] 1.1 Preparation of electrolyte

[0074] The components in the electrolyte are shown in Table 1. In an argon-filled glove box, EC, PC, DEC and PP were mixed in a mass ratio of EC:PC:DEC:PP = 20:10:40:30, then 14wt% of lithium hexafluorophosphate (LiPF6) based on the total weight of the electrolyte was slowly added to the mixed solution, and finally 12wt% of FEC, 3wt% of additive DFEC, 1% of ADN, 1.5% of HTCN based on the total weight of the electrolyte were added. After stirring uniformly, the lithium ion battery electrolyte was obtained.

[0075] 1.2 Preparation of positive electrode sheet

[0076] The cathode film sheet comprises 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 = 98.5:0.5:1, adding N-methyl pyrrolidone (NMP), and mixing uniformly, the lithium ion battery positive electrode slurry was prepared; the positive electrode slurry was coated on the current collector aluminum foil, dried at 85°C, then cold-pressed, and then trimmed and slitted, and dried at 85°C under vacuum conditions for 4h, and the tab was welded, to prepare the lithium ion battery positive electrode sheet.

[0077] 1.3 Preparation of negative electrode sheet

[0078] The anode film comprises anolyte graphite and silicon suboxide, conductive agent carbon nanotubes (CNTs), thickener sodium carboxymethyl cellulose (CMC), and binder polyacrylic acid (PAA), wherein the median particle size of the silicon suboxide material is u = 4.8 μm. Graphite and silicon suboxide are mixed at a weight ratio of 85:15 as the negative electrode active material. The various materials are mixed at a weight ratio of negative electrode active material:CNT:CMC:PAA = 97:0.8:1.2:1, and deionized water is added and mixed evenly to prepare a negative electrode slurry. The negative electrode slurry is coated onto a current collector copper foil, dried at 85°C, cold-pressed, then trimmed and slit, dried under vacuum at 85°C for 12 hours, and then tabs are welded to obtain the lithium-ion battery negative electrode sheet.

[0079] 1.4 Preparation of Lithium-ion Pouch Cells

[0080] The prepared positive electrode sheet (mainly LiCoO2), separator, and negative electrode sheet (active materials graphite and silicon) are stacked in sequence, with the separator positioned between the positive and negative electrodes, and wound to obtain a bare battery cell. The cell is designed to have a capacity of 5.0 Ah and a voltage range of 3.0-4.55V. The bare battery cell is placed in an aluminum-plastic film outer packaging for sealing, and then baked in an 85℃ vacuum oven for 48 hours. Electrolyte is injected into the dried battery at an injection rate of 1.5 g / Ah. After sealing, settling, formation, shaping, and capacity testing, a second sealing is performed, with a electrolyte retention rate of 1.1 g / Ah, completing the preparation of the lithium-ion soft-pack battery.

[0081] Examples 2-9

[0082] Examples 2-9 each provide a lithium-ion battery, which is carried out with reference to Example 1, except that the composition ratio of each component of the electrolyte is different, as shown in Table 1.

[0083] Example 10

[0084] This embodiment provides a lithium-ion battery, which is carried out with reference to Embodiment 1, except that the liquid retention coefficient σ during the second sealing in "1.4 Preparation of lithium-ion soft pack battery" is 1.3 g / Ah.

[0085] Example 11

[0086] This embodiment provides a lithium-ion battery, which is carried out with reference to Embodiment 1, except that the median particle size u of the silicon suboxide material in "1.3 Preparation of negative electrode sheet" is 6.5 μm.

[0087] Example 12

[0088] The present example provides a lithium ion battery, which is prepared according to the method in Example 1, with the only difference being that in the preparation of the negative electrode sheet, the silicon negative electrode material accounts for 20% of the total amount of negative electrode active material, i.e., the graphite and silicon monoxide are mixed in a weight ratio of 80:20 as the negative electrode active material.

[0089] Comparative Examples 1-4

[0090] Comparative Examples 1-4 each provide a lithium ion battery, which is prepared according to the method in Example 1, with the only difference being that the composition and ratio of each component of the electrolyte are different, as shown in Table 1.

[0091] Comparative Example 5

[0092] The present comparative example provides a lithium ion battery, which is prepared according to the method in Example 1, with the only difference being that in the preparation of the lithium ion soft package battery, the liquid retention coefficient σ is 1.9 g / Ah at the time of the second sealing.

[0093] Comparative Example 6

[0094] The present comparative example provides a lithium ion battery, which is prepared according to the method in Example 1, with the only difference being that in the preparation of the negative electrode sheet, the median particle size u of the silicon monoxide material is 8 μm.

[0095] Comparative Example 7

[0096] The present comparative example provides a lithium ion battery, which is prepared according to the method in Example 1, with the only difference being that in the preparation of the negative electrode sheet, the silicon negative electrode material accounts for 30% of the total amount of negative electrode active material, i.e., the graphite and silicon monoxide are mixed in a weight ratio of 70:30 as the negative electrode active material.

[0097] Comparative Examples 8-12

[0098] Comparative Examples 8-12 each provide a lithium ion battery, which is prepared according to the method in Example 1, with the only difference being that the composition and ratio of each component of the electrolyte are different, as shown in Table 1.

[0099] Comparative Example 13

[0100] Comparative Example 13 provides a lithium ion battery, which is prepared according to the method in Example 1, with the only difference being that the composition and ratio of each component of the electrolyte are different, as shown in Table 1; in addition, in Comparative Example 13, in the preparation of the lithium ion soft package battery, the liquid retention coefficient σ is 1.3 g / Ah at the time of the second sealing, in the preparation of the negative electrode sheet, the median particle size u of the silicon monoxide material is 7 μm, and in the preparation of the negative electrode sheet, the silicon negative electrode material accounts for 20% of the total amount of negative electrode active material, and the rest is the same as in Example 1.

[0101] Table 1 Composition of each component of electrolyte of Examples 1-12 and Comparative Examples 1-13

[0102]

[0103]

[0104]

[0105] Test Example

[0106] The lithium ion batteries prepared from Examples 1-12 and Comparative Examples 1-13 were subjected to relevant performance tests.

[0107] (1) Normal temperature cycle performance test:

[0108] In a 25℃ environment, the battery after being divided into groups was charged to 4.55V at 0.7C constant current and constant voltage, with a cutoff current of 0.05C, and then discharged to 3.0V at 0.5C constant current, and the cycle was repeated. The capacity retention rate at the 400th week was calculated after 400 cycles of charging and discharging, and the calculation formula was as follows:

[0109] The capacity retention rate at the 400th week of cycle (%) = (the discharge capacity at the 400th week of cycle / the discharge capacity at the first cycle) x 100%,

[0110] The thickness growth rate at the 400th week of cycle = (the thickness at the 400th week of full state / the thickness at the first cycle of full state) x 100%.

[0111] (2) High temperature cycle performance test:

[0112] In a 45℃ environment, the battery after being divided into groups was charged to 4.55V at 0.7C constant current and constant voltage, with a cutoff current of 0.05C, and then discharged to 3.0V at 0.5C constant current, and the cycle was repeated. The capacity retention rate at the 300th week was calculated after 300 cycles of charging and discharging, and the calculation formula was as follows:

[0113] The capacity retention rate at the 300th week of cycle (%) = (the discharge capacity at the 300th week of cycle / the discharge capacity at the first cycle) x 100%,

[0114] The thickness growth rate at the 300th week of cycle = (the thickness at the 300th week of full state / the thickness at the first cycle of full state) x 100%.

[0115] (3) 85℃ 24h high temperature storage test:

[0116] The battery is placed at room temperature to charge and discharge 1 time (3.0V-4.55V) at 0.5C, the discharge capacity C0 of the battery before storage is recorded, then the battery is charged to 4.55V full state of charge (100% SOC) at constant current and constant voltage, the thickness d1 of the battery before high temperature storage is tested using a PPG battery thickness gauge (600g), the battery is placed in an 85°C constant temperature oven for storage 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°C storage for 24h is calculated; after the battery is cooled at room temperature for 24h, the battery is discharged again at 0.5C to 3.0V at constant current, then charged to 4.55V 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°C storage for 24h are calculated, the calculation formula is as follows:

[0117] Thickness expansion rate after 85°C storage for 24h = (d2-d1) / d1*100%;

[0118] Capacity retention rate after 85°C storage for 24h = C1 / C0*100%;

[0119] Capacity recovery rate after 85°C storage for 24h = C2 / C0*100%.

[0120] (4) Thermal shock performance:

[0121] Discharge at a given current of 0.2C to 3.0V at 25°C environmental conditions; stand for 5min; charge at a charge current of 0.2C to 4.55V, when the cell voltage reaches 4.55V, change to 4.55V 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°C at a speed of 5±2°C / min and keep for 60min before stopping, the judgment standard is that the cell does not catch fire or explode.

[0122] The results of the above performance tests are shown in Table 2.

[0123] Table 2 Performance test results of lithium ion battery and electrolyte

[0124]

[0125]

[0126] According to the data of the above-mentioned examples and comparative examples, it can be seen that the thickness growth and cycle attenuation of the silicon negative electrode system are faster, and the battery has the problem of diving in the later stage of long cycle due to low liquid retention coefficient. The application introduces DFEC into the electrolyte system. DFEC has a high film-forming effect. It can form a SEI with high LiF content in the negative electrode during formation, improve the ratio of LiF / Li2O in the SEI and the ionic conductivity of the SEI, and improve the charge and discharge performance. At the same time, the electrolyte can inhibit the thickness growth of the battery during the cycle process and inhibit the diving problem of the battery in the later stage of long cycle, avoiding safety hazards.

[0127] In addition, binary nitriles and ternary nitriles also have a great influence on the cycle process of the silicon system. They help to inhibit the dissolution of positive metal ions at high voltage. By controlling the content of nitrile solvents in the electrolyte, the cycle performance of the silicon negative electrode lithium ion battery can be effectively improved.

[0128] The lithium ion battery of the embodiment of the application satisfies the following relationship: 8≤8*(α*β / 2+σ 2 )+30*ln(σ*u / 5+γ 2 / 25)+10 / min(σ*u*γ*v,σu / v,100*w*α / β)≤130, wherein α, β, γ, and w are the mass percentage contents of FEC, DFEC, ADN, and HTCN in the electrolyte, respectively, σ is the liquid retention coefficient of the battery, g / Ah, u is the median particle size of the silicon material, μm, v is the mass percentage of the silicon material in the active material. The lithium ion battery of the application can meet the demand for long cycle and solve the safety risk caused by electrolyte dryness in the later stage of cycle.

[0129] The above content has described the embodiments of the application in detail, but the application is not limited to the above-mentioned embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the purpose of the application. In addition, the embodiments of the application and the features in the embodiments can be combined with each other without conflict.

Claims

1. A lithium-ion battery, characterized by, The lithium ion battery comprises: an electrolyte; the electrolyte comprises a lithium salt, a solvent and an additive, the additive comprises fluoroethylene carbonate, difluoroethylene carbonate, adiponitrile, 1,3,6-hexanetricarbonitrile; a positive electrode tab; a negative electrode tab; the active material of the negative electrode tab comprises a silicon material and graphite; and a separator film; The lithium ion battery satisfies 8≤8*(a*β / 2+σ 2 )+30*ln(σ*u / 5+γ 2 / 25)+10 / min(σ*u*γ*v,σu / v,100*w*α / β)≤130, wherein a is a mass percentage content of the fluoroethylene carbonate in the electrolyte, β is a mass percentage content of the difluoroethylene carbonate in the electrolyte, γ is a mass percentage content of the adiponitrile in the electrolyte, w is a mass percentage content of the 1,3,6-hexanetricarbonitrile in the electrolyte, σ g / Ah is a liquid retention coefficient of the lithium ion battery, u μm is a median particle size of the silicon material, and v is a mass percentage content of the silicon material in the active material.

2. The lithium-ion battery of claim 1, wherein, the mass percentage content of the fluoroethylene carbonate in the electrolyte is 1% to 20%; and / or, the mass percentage content of the difluoroethylene carbonate in the electrolyte is 0.4% to 10%; and / or, the mass percentage content of the adiponitrile in the electrolyte is 0.3% to 1.5%; and / or, the mass percentage content of the 1,3,6-hexanetricarbonitrile in the electrolyte is 0.3% to 3%.

3. The lithium-ion battery of claim 1, wherein, The mass percentage content of the lithium salt in the electrolyte is 8% to 25%.

4. The lithium-ion battery of claim 1, wherein, The solvent comprises at least one of a non-fluorinated carbonate solvent, a carboxylate solvent, an ether solvent and a C2-C5 nitrile solvent.

5. The lithium-ion battery of claim 1, wherein, The positive electrode tab comprises an aluminum foil current collector and a positive electrode film; wherein the positive electrode film comprises a positive electrode active material lithium cobaltate.

6. The lithium-ion battery of claim 1, wherein, The silicon material is selected from at least one of silicon, silicon oxide, silicon monoxide and silicon carbide.

7. The lithium-ion battery of claim 1, wherein, The weight proportion of the silicon material in the active material is 3% to 20%.

8. The lithium-ion battery of claim 1, wherein, The median particle size of the silicon material is 2.3 to 7 μm.

9. The lithium-ion battery of claim 1, wherein, The liquid retention coefficient of the lithium ion battery is 1.0 to 1.5 g / Ah.

10. An electronic device, comprising: The lithium ion battery comprises any one of claims 1 to 9. The lithium ion battery comprises any one of claims 1 to 9.

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