Electrolyte and lithium ion battery

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to an electrolyte and a lithium ion battery, the electrolyte comprises a compound A containing an ethylene carbonate structure and an additive B; the mass percentage content of the compound A in the electrolyte is x%, and the mass percentage content of the additive B in the electrolyte is y% and is 1lt; x / (5y) lt; 18; the LUMO values of the compound A and the additive B are relatively low, and the compound A and the additive B can form a film on a negative electrode prior to other compounds in the electrolyte, so that the lithium ion content in the SEI of the negative electrode is effectively improved on the premise of not deteriorating charging, and the SEI stability is improved; the combination of the compound A containing the ethylene carbonate structure and the additive B can effectively inhibit the thickness increase of the battery in the cycle process and improve the hot box effect of the battery, thereby effectively improving the thermal stability and charging cycle performance of the lithium ion battery.

Description

Technical Field

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

[0002] In order to improve the energy density of lithium-ion batteries, we can increase the voltage of the positive electrode active material, or increase the gram capacity of the negative electrode; introducing materials with higher gram capacity as negative electrode active materials can effectively increase the gram capacity of the negative electrode; the theoretical gram capacity of silicon is as high as 4200mAh / g, which can effectively increase the gram capacity of the negative electrode, thereby improving the energy density of lithium-ion batteries.

[0003] However, the introduction of silicon into the negative electrode will also bring a series of negative effects. This is because the solid electrolyte interface (SEI) on the surface of silicon particles has low stability and is more easily decomposed. In addition, the silicon negative electrode material will undergo a huge volume change during the charge and discharge process, and the probability of SEI rupture on the negative electrode surface will further increase, which will accelerate the consumption of active lithium ions during the cycle. Coupled with the influence of factors such as more side reactions, the battery thickness will increase rapidly during the cycle, reducing the thermal stability of the battery and attenuating the battery cycle performance. Therefore, it is urgent to develop an electrolyte suitable for high-voltage silicon negative electrode systems to inhibit the thickness growth of the battery during the cycle, thereby improving battery performance and extending the battery cycle life. Summary of the invention

[0004] In view of the above problems existing in the prior art, the object of the present invention is to provide an electrolyte and a lithium ion battery.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] The present invention provides an electrolyte, comprising a compound A and an additive B, wherein the structure of the additive B is as shown in Formula 1:

[0007] R 1 -R 2 -CN

[0008] Formula 1;

[0009] Among them, R 1 for Any one of R 2 is an alkyl, alkenyl or alkynyl group having 2 to 5 carbon atoms;

[0010] The compound A comprises an additive containing an ethylene carbonate structure and a solvent;

[0011] The mass percentage of the compound A in the electrolyte is x%, the mass percentage of the additive B in the electrolyte is y%, and 1 <x / (5y)<18。

[0012] In some embodiments, R 1 A side chain group is arranged on the aromatic heterocyclic group, and the side chain group is an alkyl group, an alkoxy group or a carbonyl group having 1 to 5 carbon atoms.

[0013] In some embodiments, the additive B is at least one of the following formulas 2 to 4:

[0014]

[0015] In some embodiments, the mass percentage of the additive B in the electrolyte is 0.3% to 4%.

[0016] In some embodiments, the additive containing an ethylene carbonate structure includes bisfluoroethylene carbonate and / or fluoroethylene carbonate.

[0017] In some embodiments, the solvent containing an ethylene carbonate structure comprises ethylene carbonate.

[0018] In some embodiments, the mass percentage of the bisfluoroethylene carbonate in the electrolyte is 0.2% to 10%; and / or the mass percentage of the fluoroethylene carbonate in the electrolyte is 1% to 20%.

[0019] In some embodiments, the solvent of the electrolyte further includes at least one of propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propionate, ethyl propionate, propyl propionate, and methyl acetate.

[0020] In some embodiments, the electrolyte includes at least one lithium salt selected from the group consisting of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium dioxalate borate, lithium difluorobisoxalate borate, lithium tetrafluoroborate, lithium bistrifluoromethanesulfonyl imide, and lithium bisfluorosulfonyl imide; the mass percentage of the lithium salt in the electrolyte is 8% to 25%.

[0021] The present invention further provides a lithium ion battery comprising the above electrolyte.

[0022] In some embodiments, the negative electrode active material of the lithium ion battery includes graphite and a silicon-containing material, the mass ratio of the graphite to the silicon-containing material is z; and z satisfies: 3 <x / (10y)+2z / y<47。

[0023] In some embodiments, the silicon-containing material includes at least one of silicon carbide and silicon oxide.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention adds a compound A containing a vinyl carbonate structure and an additive B to an electrolyte system. The LUMO (lowest unoccupied molecular orbital) value of the compound A and the additive B with some vinyl carbonate structures is relatively low, and they will form a film at the negative electrode in priority to other compounds in the electrolyte that may serve as SEI components, and can effectively increase the lithium ion content in the negative electrode SEI and improve the SEI stability without deteriorating the charging performance. The combination of the compound A containing a vinyl carbonate structure and the additive B can effectively inhibit the thickness growth of the battery during the cycle process and improve the hot box performance of the battery, thereby effectively improving the thermal stability and cycle performance of the lithium ion battery without deteriorating the charging performance. DETAILED DESCRIPTION

[0026] To make the technical solutions and advantages of the present invention more clear, the present invention and its beneficial effects will be further described in detail below in conjunction with specific implementation methods, but the implementation methods of the present invention are not limited thereto.

[0027] The present invention provides an electrolyte, comprising a compound A and an additive B, wherein the structure of the additive B is as shown in Formula 1:

[0028] R 1 -R 2 -CN

[0029] Formula 1;

[0030] Among them, R 1 for Any one of R 2 is an alkyl, alkenyl or alkynyl group having 2 to 5 carbon atoms;

[0031] The compound A comprises an additive containing an ethylene carbonate structure and a solvent;

[0032] The mass percentage of compound A in the electrolyte is x%, the mass percentage of additive B in the electrolyte is y%, and 1 <x / (5y)<18。

[0033] The present invention adds an additive B to the electrolyte under the premise that the electrolyte contains a compound A with an ethylene carbonate structure, wherein the additive B is a multifunctional compound containing an aromatic heterocyclic group; the additive B has a high HOMO (highest occupied orbital) value and a low LUMO (lowest unoccupied orbital) value, and similar to the compound A with an ethylene carbonate structure, can preferentially form a film on the negative electrode to form a highly stable SEI; in addition, the additive B contains aromatic heterocyclic groups, and the aromatic heterocyclic groups are all electron-rich structures. Under the condition that the positive electrode of high voltage lacks electrons, the electron-rich characteristics enable the additive B to be efficiently adsorbed on the surface of the positive electrode active particles, so that the additive B is convenient to polymerize and form a film on the positive electrode to form a highly stable SEI; at the same time, the cyano group (-CN) in the additive B can also complex with the metal ions dissolved in the electrolyte to inhibit the side reactions that may be caused by the dissolution of the metal ions;

[0034] The combination of compound A containing ethylene carbonate structure and additive B can form a film at the positive and negative electrodes at the same time, effectively improving the stability of SEI at the positive and negative electrodes, improving the battery cycle performance and the thermal stability of the battery, and alleviating the adverse effects of the introduction of silicon into the negative electrode. Compound A containing ethylene carbonate structure helps to increase the content of LiF in the SEI film, but the ionic conductivity and electronic conductivity of LiF are extremely low, and the mechanical strength is relatively high; the electronic conductivity of the product polymerized into a film by additive B under specific conditions is higher than that of compound A, but there is a risk of electron leakage; therefore, the combination of additive B and compound A can not only improve the strength of SEI and reduce its risk of damage, but also improve the insulation performance of SEI; when the two are used together, additive B can play a better effect; by using compound A and additive B at the same time, the growth of battery thickness during the cycle can be effectively inhibited, the cycle performance of the battery and the hot box effect of the battery can be improved, and safety hazards can be avoided.

[0035] Furthermore, R 1 A side chain group is arranged on the aromatic heterocyclic group, and the side chain group is an alkyl group, an alkoxy group or a carbonyl group having 1 to 5 carbon atoms.

[0036] Further, the additive B is at least one of the following formulas 2 to 4:

[0037]

[0038] In the present invention, the additive B is preferably any one of the three compounds in Formula 2 to Formula 4; wherein the structure in Formula 2 is recorded as B1, the structure in Formula 3 is recorded as B2, and the structure in Formula 4 is recorded as B3; it can be seen that R in B1 2 An alkyl group with 2 carbon atoms was selected, R 1 A side chain group is also connected to the base, and the side chain group is an alkyl group with 1 carbon atom; R in B2 2An alkyl group with 2 carbon atoms was selected; R in B3 2 An alkyl group with 2 carbon atoms was selected, R 1 A side chain group is also connected, and the side chain group is an alkoxy group with 1 carbon atom.

[0039] Furthermore, the mass percentage of additive B in the electrolyte is 0.3% to 4%.

[0040] Further, the additive containing ethylene carbonate structure includes difluoroethylene carbonate (Difluoroethylene Carbonate, DFEC) and / or fluoroethylene carbonate (Fluoroethylene carbonate, FEC);

[0041] And / or, the solvent containing an ethylene carbonate structure includes ethylene carbonate (EC).

[0042] Further, the mass percentage of bisfluoroethylene carbonate in the electrolyte is 0.2% to 10%; and / or the mass percentage of fluoroethylene carbonate in the electrolyte is 1% to 20%.

[0043] Furthermore, the solvent of the electrolyte also includes at least one of propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propionate (MA), ethyl propionate (EP), n-propyl propionate (PP), and methyl acetate (MA).

[0044] Furthermore, the electrolyte includes at least one lithium salt selected from the group consisting of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium dioxalate borate, lithium difluorooxalate borate, lithium tetrafluoroborate, lithium bistrifluoromethanesulfonyl imide, and lithium bisfluorosulfonyl imide; the mass percentage of the lithium salt in the electrolyte is 8% to 25%.

[0045] In the present invention, compound A includes DFEC, FEC and solvent EC, and the additive containing ethylene carbonate structure is referred to as additive A herein; wherein, since DFEC and additive B have lower LUMO energy, they will form a film at the negative electrode in priority to EC and FEC, but eventually DFEC and additive B will work together with EC and FEC to form a film, that is, the main components of the film formation include additive A and additive B; additive A and additive B work together on SEI to improve the stability of SEI, thereby improving the thermal stability of lithium-ion batteries; and also make the content of LiF in the negative electrode SEI higher, improving Li + Solvation-desolvation of Li + The solvation structure further effectively improves the fast charging performance.

[0046] Furthermore, a film-forming additive may be added to the electrolyte of the present invention, and the film-forming additive includes at least one of 1,3-propane sultone, vinyl sulfate, tris(trimethylsilyl)borate, and tris(trimethylsilyl)phosphate; the mass percentage of the film-forming additive in the electrolyte is 0.1% to 20%.

[0047] The present invention also provides a lithium ion battery, comprising the above electrolyte.

[0048] Furthermore, the negative electrode active material of the lithium-ion battery includes graphite and a silicon-containing material, the mass ratio of graphite to the silicon-containing material is z; and z satisfies: 3 <x / (10y)+2z / y<47。

[0049] Furthermore, the silicon-containing material includes at least one of silicon carbide and silicon oxide.

[0050] When the value of the relationship x / (10y)+2z / y meets the above range, Li + The amount of EP in the first solvation sheath is reduced to the minimum, and EP basically does not participate in the Li + The solvation of Li + The transport carrier has a low viscosity, which helps to ensure the charging performance of the battery and is basically not consumed during the cycle. The main components of the film include EC, FEC, DFEC and additive B. At this time, LiF in SEI and Li 2 CO 3 The content ratio reaches the optimal level, and the stability, insulation and ion conductivity of SEI reach the optimal level, thereby optimizing the cycle performance and fast charging performance of the lithium-ion battery.

[0051] Example 1

[0052] This embodiment provides an electrolyte and a lithium-ion battery including the electrolyte;

[0053] The steps for preparing the electrolyte are as follows:

[0054] In a glove box filled with argon, EC, PC, DEC, and EP were mixed at a mass ratio of EC: PC: DEC: EP = 15: 15: 40: 30 to obtain a mixed solution;

[0055] Based on the total weight of the electrolyte as 100%, 14% lithium hexafluorophosphate (LiPF 6 ); then add 12% FEC, 3% DFEC, and 1.5% B1, and stir evenly to obtain an electrolyte;

[0056] The structure of B1 is shown below:

[0057]

[0058] The steps for preparing lithium-ion batteries are as follows:

[0059] The positive electrode active material is lithium cobalt oxide (LiCoO 2 ), the conductive agent is conductive carbon black (Super P, SP), the binder is polyvinylidene difluoride (Polyvinylidene difluoride, PVDF); LiCoO 2 , SP and PVDF by weight ratio LiCoO 2 : SP: PVDF = 98.5: 0.5: 1, add into N-methylpyrrolidone, mix evenly to obtain positive electrode slurry for lithium ion battery;

[0060] The prepared positive electrode slurry was coated on the current collector aluminum foil, dried at 85°C, and then cold pressed, trimmed and stripped. It was dried at 85°C under vacuum for 4 hours, and then the tabs were welded to obtain a positive electrode sheet, wherein the positive electrode gram capacity was 175 mAh / g.

[0061] The negative electrode active material is graphite and silicon carbide (SiC), the conductive agent is carbon nanotubes (CNT), the thickener is sodium carboxymethyl cellulose (CMC), and the binder is polyacrylic acid (PAA); a mixture of graphite and SiC in a weight ratio of graphite: SiC = 85:15 is used as the negative electrode active material; each substance is mixed in a weight ratio of negative electrode active material: CNT: CMC: PAA = 97:0.8:1.2:1, and deionized water is added to mix evenly to prepare a negative electrode slurry;

[0062] The prepared negative electrode slurry was coated on the current collector copper foil, dried at 85°C, and then cold pressed, trimmed and stripped in sequence, dried at 85°C under vacuum conditions for 12 hours, and then welded on the tabs to obtain the negative electrode sheet;

[0063] The positive electrode sheet, the separator and the negative electrode sheet are stacked in order, the separator is placed between the positive and negative electrode sheets, and the bare battery cell is wound; the bare battery cell has a designed capacity of 6.0Ah and a voltage range of 3.0V to 4.55V; the bare battery cell is placed in an aluminum-plastic film outer package for packaging, and then placed in a vacuum oven at 85°C for baking for 48 hours; the electrolyte is injected into the dried battery, and the injection coefficient is 1.5g / Ah; the battery after injection is packaged, allowed to stand, formed, shaped and divided into different sizes, and then packaged for the second time, and the liquid retention coefficient is controlled to be 1.2g / Ah, to obtain a lithium-ion soft-pack battery.

[0064] Example 2-11

[0065] Repeat the steps of Example 1 according to the parameters specified in Examples 2 to 11 in Table 1;

[0066] Example 12

[0067] The difference between Example 12 and Example 1 is that a mixture of graphite and SiC in a weight ratio of graphite:SiC=80:20 is used as the negative electrode active material;

[0068] Comparative Examples 1-6

[0069] Repeat the steps of Example 1 according to the parameters specified in Comparative Examples 1 to 6 in Table 1;

[0070] In Comparative Examples 1-6, the missing content due to the absence of additive B, DFEC or FEC was supplemented by the solvent, and the addition ratio of each component in the solvent remained unchanged.

[0071] Table 1

[0072]

[0073]

[0074]

[0075] Test Case

[0076] The lithium ion batteries obtained in Examples 1 to 12 and Comparative Examples 1 to 6 were subjected to the following performance tests:

[0077] (1) Normal temperature cycle performance test

[0078] In an environment of 25°C, the divided lithium-ion battery is charged to 4.55V at 0.7C constant current and constant voltage, with a cut-off current of 0.05C, and then discharged to 3.0V at 0.5C constant current. After 400 cycles of charge and discharge, the capacity retention rate at the 400th week is calculated. The calculation formula is as follows:

[0079] 400th cycle capacity retention rate = (400th cycle discharge capacity / first cycle discharge capacity) × 100%;

[0080] Thickness growth rate at the 400th cycle = (thickness at full charge at the 400th cycle / thickness at full charge at the first cycle) × 100%;

[0081] (2) High temperature cycle performance test

[0082] In an environment of 45°C, the divided lithium-ion battery is charged to 4.55V at 0.7C constant current and constant voltage, with a cut-off current of 0.05C, and then discharged to 3.0V at 0.5C constant current. After 300 cycles of charge and discharge, the capacity retention rate at the 300th week is calculated. The calculation formula is as follows:

[0083] 300th cycle capacity retention rate = (300th cycle discharge capacity / first cycle discharge capacity) × 100%;

[0084] Thickness growth rate at the 300th cycle = (thickness at full charge at the 300th cycle / thickness at full charge at the first cycle) × 100%;

[0085] (3) 24h high temperature storage test at 85℃

[0086] Place the lithium-ion battery at room temperature and charge and discharge it once at 0.5C (voltage 3.0V~4.55V), and record the discharge capacity C of the lithium-ion battery before storage. 0 ; Then the lithium-ion battery was charged to 4.55V (100% SOC) with constant current and constant voltage, and the thickness of the lithium-ion battery before high-temperature storage was tested using a PPG battery thickness gauge (600g). 1 ; Store the lithium-ion battery in a constant temperature box at 85℃ for 24 hours. After storage, take out the lithium-ion battery and test the thermal thickness d of the battery after storage. 2 ; Calculate the thickness expansion rate of lithium-ion batteries after storage at 85°C for 24 hours;

[0087] After the lithium-ion battery is cooled at room temperature for 24 hours, it is discharged again at a constant current of 0.5C to 3.0V, and then charged to 4.55V at a constant current and voltage of 0.5C. The discharge capacity C of the lithium-ion battery after storage is recorded. 1 and charging capacity C 2 , calculate the capacity remaining rate and recovery rate of lithium-ion batteries after storage at 85℃ for 24h. The calculation formula is as follows:

[0088] Thickness expansion ratio after storage at 85℃ for 24h = ((d 2 -d 1 ) / d 1 )*100%;

[0089] After 24h storage at 85℃, the remaining capacity is (C 1 / C 0 )*100%;

[0090] Capacity recovery rate after storage at 85℃ for 24h = (C 2 / C 0 )*100%;

[0091] (4) Thermal shock performance test

[0092] In an environment of 25℃, discharge the lithium-ion battery at a given current of 0.2C to 3.0V and leave it for 5min; then charge it at a charging current of 0.2C to 4.55V. When the cell voltage reaches 4.55V, change to 4.55V constant voltage charging until the charging current is ≤ the cut-off current of 0.05C; after leaving it for 1h, put the lithium-ion battery cell into an oven, and raise the oven temperature to 135±2℃ at a heating rate of 5±2℃ / min, and keep it for 60min before stopping. The qualified judgment standard is that the cell does not catch fire or explode;

[0093] (5) 3C DC charging constant current ratio test

[0094] In an environment of 25℃, discharge the lithium-ion battery cell at 0.2C to 3.0V; leave it for 30min; charge it to 4.55V at 3C. When the cell voltage reaches 4.55V, change to 4.55V constant voltage charging until the charging current is ≤0.05C of the cut-off current; the constant current charging capacity during the whole charging process is recorded as C 3 The total capacity during the charging process is recorded as C 4 , calculate the constant current charging capacity during the entire DC charging process, the calculation formula is as follows:

[0095] 3C DC charging constant current ratio = (C 3 / C 4 )*100%;

[0096] The results of the above performance tests are shown in Table 3;

[0097] The values ​​of x, V, z and their relationship of Examples 1-12 and Comparative Examples 1-6 are shown in Table 2.

[0098] Table 2

[0099]

[0100]

[0101] Table 3

[0102]

[0103] It can be seen from the test results in Table 3 that compared with Example 1, different structures of additive B were used in both Example 2 and Example 3, and only the addition amount of additive B was adjusted in Examples 7-9; the performance test results of the lithium-ion batteries obtained in Example 1 and Examples 7-9 were overall better than those in Example 2 and Example 3, proving that when the additive B is B1, better effects can be achieved than B2 and B3.

[0104] Compared with Example 1, the addition amount of DFEC was adjusted in Examples 4-6, and the overall performance test results of the obtained lithium-ion batteries were relatively good, proving that when the addition amount of DFEC in this solution is maintained within the range of 0.2% to 10%, it can have a certain improvement effect on the thermal stability performance and charge cycle performance of the lithium-ion battery.

[0105] Compared with Example 1, the addition amount of FEC was adjusted in Examples 10 and 11, and the overall performance test results of the obtained lithium-ion batteries were relatively good, proving that when the addition amount of FEC in this solution is maintained within the range of 1% to 10%, it can have a certain improvement effect on the thermal stability performance and charge cycle performance of the lithium-ion battery.

[0106] Compared with Example 1, the weight ratio of graphite to SiC, that is, the z value, was adjusted in Example 12, further proving that when the relationship x / (10y)+2z / y in this solution is maintained within the range of 3 < x / (10y)+2z / y < 47, it can have a certain improvement effect on the thermal stability performance and charge cycle performance of the lithium-ion battery.

[0107] Compared with Example 1, in Comparative Example 1, only DFEC was used, and additives B and FEC were not used; in Comparative Example 2, only additive B was used, and DFEC and FEC were not used; in Comparative Example 3, only FEC was used, and DFEC and additive B were not used; Comparative Example 6 was used as a blank control group, and DFEC, FEC, and additive B were not used. It can be seen from Table 2 that the performance test results of the lithium-ion batteries obtained in Comparative Examples 1-3 are generally better than those in Comparative Example 6. This is because in Comparative Examples 1-3, when any one of DFEC, FEC, or additive B is added to the electrolyte, the film formation on the negative electrode can, to a certain extent, improve the cycle performance and increase the thermal stability of the SEI. In addition, Comparative Example 2 using additive B shows better performance in the thermal shock pass rate test than Comparative Examples 1 and 3, while Comparative Examples 1 and 3 show better performance in the thickness expansion rate test than Comparative Example 2. It can be seen that the use of additive A containing a vinylene carbonate structure can improve the problem of volume change during the charge and discharge process of lithium-ion batteries, effectively inhibit the thickness growth of the battery during the cycle, and improve the cycle efficiency of the battery. However, additive A has no improvement effect on the phenomenon of battery thermal runaway. The use of additive B can effectively inhibit the heat accumulation inside the battery and improve the battery thermal box effect, but it has no improvement effect on the battery thickness growth problem. When additive B is used in combination with additive A containing a vinylene carbonate structure, as in Example 1, the improvement of the battery volume expansion rate, charge cycle, and thermal box effect can be achieved simultaneously, and the improvement degree is better than that of the first additive or the second additive used alone, such as in Comparative Examples 1-3.

[0108] In Comparative Examples 4 and 5, although the addition amounts of additive B, DFEC, and FEC and the weight ratio of graphite to SiC are controlled within the ranges given in this solution, the performance of the obtained lithium-ion batteries is also not ideal. This is because they do not satisfy the relationships 1 < x / (5y) < 18 and 3 < x / (10y) + 2z / y < 47. At this time, there may be two situations inside the battery: one is that the content of Li 2 CO 3 in the SEI is relatively high. Although it brings higher ionic conductivity and enables the lithium-ion battery to have better fast charging performance, it also makes the stability and insulation of the SEI worse. The SEI cannot maintain stability, and electrons are more likely to pass through the SEI and react with the electrolyte by side reactions. The other is that the content of LiF in the SEI is too high. Although the SEI has high stability, the ionic conductivity of LiF is low, resulting in poor fast charging performance of the lithium-ion battery. Only when the relationships 1 < x / (5y) < 18 and 3 < x / (10y) + 2z / y < 47 are satisfied, the number of EPs in the first solvation sheath around Li + is reduced to the lowest, and EPs basically do not participate in the solvation of Li + and are mainly responsible for solvating Li +The transport carrier is basically not consumed during the cycling process; therefore, the main components of the film formation include additive B, DFEC, FEC, and EC. At this time, the content ratio of LiF and Li 2 CO 3 in the SEI reaches the optimum, and the stability, insulation, and ionic conductivity of the SEI reach the best, so that the cycling performance and fast charging performance of the lithium-ion battery reach the optimum; in summary, when the relational value is not within this range, the performance of the lithium-ion battery will deteriorate.

[0109] In the present invention, by introducing additive A and additive B containing ethylene carbonate structure into the electrolyte system, additive A and additive B containing ethylene carbonate structure form a film on the negative electrode, effectively improving the cycling performance and thermal stability of the SEI. The cycling performance and thermal shock resistance of the battery are improved through the electrolyte, the growth of the battery thickness during the cycling process is inhibited, and potential safety hazards are avoided; at the same time, by controlling the values of the relational expressions 1 < x / (5y) < 18 and 3 < x / (10y) + 2z / y < 47, the SEI film can reach the best state, so that the performance of the lithium-ion battery reaches the optimum level.

[0110] The above are only partial or preferred embodiments of the present invention, and the protection scope of the present invention cannot be limited thereby. Any equivalent structural transformation made using the description of the present invention under the overall concept of the present invention, or any direct / indirect application in other related technical fields, is included in the protection scope of the present invention.

Claims

1. An electrolyte, characterized in that: It includes compound A and additive B, and the structure of the additive B is shown in Formula 1: R1-R2-CN Formula 1; Among them, R1 is Any one of; R2 is an alkyl, alkenyl or alkynyl group with a carbon number ranging from 2 to 5; The compound A comprises an additive containing an ethylene carbonate structure and a solvent; The mass percentage of the compound A in the electrolyte is x%, the mass percentage of the additive B in the electrolyte is y%, and 1<x / (5y)<18.

2. The electrolyte according to claim 1, characterized in that The aromatic heterocyclic group in R1 has a side chain group, and the side chain group is an alkyl group, an alkoxy group or a carbonyl group having 1 to 5 carbon atoms.

3. The electrolyte according to claim 1, characterized in that The additive B is at least one of the following formulas 2 to 4:

4. The electrolyte according to claim 2 or 3, characterized in that The mass percentage of the additive B in the electrolyte is 0.3% to 4%.

5. The electrolyte according to claim 1, characterized in that The additive containing ethylene carbonate structure includes difluoroethylene carbonate and / or fluoroethylene carbonate; And / or, the solvent containing ethylene carbonate structure includes ethylene carbonate.

6. The electrolyte according to claim 5, characterized in that The mass percentage of the bisfluoroethylene carbonate in the electrolyte is 0.2% to 10%; and / or the mass percentage of the fluoroethylene carbonate in the electrolyte is 1% to 20%.

7. The electrolyte according to claim 6, characterized in that The solvent of the electrolyte includes at least one of propylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propionate, ethyl propionate, propyl propionate, and methyl acetate.

8. The electrolyte according to claim 1, characterized in that The electrolyte includes at least one lithium salt selected from the group consisting of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium dioxalate borate, lithium difluorooxalate borate, lithium tetrafluoroborate, lithium bistrifluoromethanesulfonyl imide, and lithium bisfluorosulfonyl imide; the mass percentage of the lithium salt in the electrolyte is 8% to 25%.

9. A lithium ion battery, characterized in that: The electrolyte comprising the electrolyte according to any one of claims 1 to 8.

10. The lithium ion battery according to claim 9, characterized in that: The negative electrode active material of the lithium ion battery includes graphite and a silicon-containing material, the mass ratio of the graphite to the silicon-containing material is z; and z satisfies: 3<x / (10y)+2z / y<47.

Images