CN116779972A

CN116779972A - Electrolyte and lithium ion battery

Info

Publication number: CN116779972A
Application number: CN202310981887.6A
Authority: CN
Inventors: 段凯嘉; 张昌明; 李枫; 谢海芳; 胡大林
Original assignee: Guangdong Highpower New Energy Technology Co Ltd
Current assignee: Guangdong Highpower New Energy Technology Co Ltd
Priority date: 2023-08-07
Filing date: 2023-08-07
Publication date: 2023-09-19

Abstract

The application relates to an electrolyte and a lithium ion battery. The electrolyte comprises: lithium salt, additive and organic solvent, wherein the additive comprises fluorovinylene carbonate, 1-methyl-1, 3-propane sultone, 2-difluoro methyl acetate and tetra-nitrile siloxane compound. According to the scheme provided by the application, four additives are added into the electrolyte in a combined way, and the four additives are synergistic with each other, so that the lithium ion battery can have good high-temperature continuous charging, high-temperature circulation, high-temperature storage performance and low-temperature discharge performance under high charging voltage.

Description

Electrolyte and lithium ion battery

Technical Field

The application relates to the technical field of batteries, in particular to electrolyte and a lithium ion battery.

Background

The lithium ion battery has the advantages of high specific energy, good quick charge and discharge capability, small self discharge and the like, and is widely applied to consumer electronic products and power batteries. As the use conditions of electronic products and power batteries become more and more complex, the requirements on lithium ion batteries are also higher and higher, and particularly the requirements on battery capacity and service life are also higher and higher.

The performance of lithium ion batteries is affected by a combination of several criteria, of which the energy density and cycle performance of the battery are two particularly critical criteria. In the application of lithium ion batteries, the commonly used charging voltage is typically 4.2V. The current lithium ion battery is difficult to bear higher charging voltage, and the high-temperature storage and high-temperature cycle performance of the battery are poor under the high charging voltage, so that the capacity of the battery is reduced, and the service life of the battery is shortened.

Disclosure of Invention

In order to solve or partially solve the problems in the related art, the application provides an electrolyte and a lithium ion battery, which can improve the comprehensive performances of high-temperature continuous charging, high-temperature circulation, high-temperature storage, low-temperature discharge performance and the like of the lithium ion battery under high charging voltage.

The first aspect of the application provides an electrolyte comprising a lithium salt, an additive and an organic solvent, wherein the additive comprises fluorovinylene carbonate, 1-methyl-1, 3-propane sultone, 2-difluoro methyl acetate and a tetra-nitrile siloxane compound.

In some embodiments, the fluorovinylene carbonate is present in the electrolyte at a mass ratio of a%, the 1-methyl-1, 3-propane sultone is present in the electrolyte at a mass ratio of b%, the 2, 2-methyl difluoroacetate is present in the electrolyte at a mass ratio of c%, and the tetranitrile siloxane compound is present in the electrolyte at a mass ratio of d%, wherein 0.4.ltoreq.b/a.ltoreq. 0.8,2 (a+b)/d.ltoreq.3, and 2.ltoreq.c/d.ltoreq.5.

In some embodiments, the fluoroethylene carbonate accounts for a% of the electrolyte, wherein a is more than or equal to 0.01 and less than or equal to 20.

In some embodiments, the mass ratio of the 1-methyl-1, 3-propane sultone in the electrolyte is b%, wherein b is more than or equal to 0.01 and less than or equal to 5.

In some embodiments, the mass ratio of the 2, 2-difluoro methyl acetate in the electrolyte is c%, wherein, c is more than or equal to 5 and less than or equal to 30.

In some embodiments, the mass ratio of the tetranitrile siloxane compound in the electrolyte is d%, wherein d is 0.01-5.

In some embodiments, the lithium salt is a fluorine-containing lithium salt selected from one or more of hexafluorophosphate, hexafluoroarsenate, lithium trifluorosulfonyl, lithium difluoro (trifluoromethylsulfonyl) imide, lithium bis (fluorosulfonyl) imide, lithium tris (trifluoromethylsulfonyl) methyllithium, lithium tetrafluoroborate, lithium hexafluoroantimonate, lithium hexafluorotantalate; and/or

The organic solvent is selected from two or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl formate, ethyl propionate, propyl propionate, methyl butyrate and tetrahydrofuran.

In some embodiments, the concentration of lithium salt in the electrolyte is 0.5mol/L to 2mol/L; preferably, the concentration of the lithium salt is 0.9mol/L to 1.3mol/L.

The second aspect of the application provides a lithium ion battery, which comprises a positive electrode plate, a negative electrode plate, a diaphragm and the electrolyte in any embodiment.

In some embodiments, the lithium ion battery has a maximum charge voltage of 4.55V and a maximum storage temperature of 60 ℃.

The technical scheme provided by the application can comprise the following beneficial effects:

according to the technical scheme, four additives such as fluorovinylene carbonate, 1-methyl-1, 3-propane sultone, 2-methyl difluoroacetate and tetranitrile siloxane compound are added into the electrolyte in a combined manner to realize mutual synergistic effect, so that the lithium ion battery can have good high-temperature continuous charging, high-temperature circulation, high-temperature storage performance and low-temperature discharge performance under high charging voltage, and further has longer service life; meanwhile, the manufacturing process is simple, and the production cost is proper.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application as claimed.

Detailed Description

In order that the application may be readily understood, the application will be described in detail. Before the present application is described in detail, it is to be understood that this application is not limited to particular embodiments described. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

Where a range of values is provided, it is understood that each intervening value, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the application. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges, and are also encompassed within the application, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the application.

Unless defined otherwise, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. Although any methods and materials equivalent to those described herein can also be used in the practice or testing of the present application, the preferred methods and materials are now described.

In the related art, since the high-temperature storage and high-temperature cycle performance of the lithium ion battery are deteriorated at a high charging voltage, the battery capacity is reduced and the life is shortened.

In view of the above problems, the embodiment of the application provides an electrolyte and a lithium ion battery, which can improve the high-temperature continuous charging, high-temperature circulation, high-temperature storage and low-temperature discharge performance of the lithium ion battery.

An embodiment of the present application shows an electrolyte comprising a lithium salt, an additive and an organic solvent, wherein the additive comprises fluorovinylene carbonate, 1-methyl-1, 3-propane sultone, methyl 2, 2-difluoroacetate and a tetranitrile siloxane compound.

Wherein the fluorovinylene carbonate (FVC) has the following structural formula:

the structural formula of the 1-methyl-1, 3-propane sultone is as follows:

methyl 2, 2-difluoroacetate (MFA) has the following structural formula:

the tetranitrile siloxane compound may be, for example, 1, 2-tetranitrile-1, 2-bis (trimethylsilyl) ethane having the structural formula:

specifically, at the first cycle of the lithium ion battery, since the electrolyte and the anode material react on the solid-liquid phase interface, a layer of SEI film is formed at the anode. In addition, in the recycling of lithium ion batteries, a CEI film is formed at the interface between the electrolyte and the positive electrode.

When the lithium ion battery is charged continuously at high temperature, the potential of the positive electrode side is higher, and oxygen release is easily caused on the positive electrode side, so that electrolyte is decomposed and CEI film is broken. In the present application, various additives are added to the electrolyte. Wherein the tetranitrile siloxane compound contains a cyano group and a Si-O bond. The cyano group can be complexed with the transition metal of the positive electrode to play a role in stabilizing the structure of the positive electrode material, meanwhile, si-O bond can be combined with hydrogen and metal ions, the positive electrode active material is protected, meanwhile, the positive electrode/electrolyte interface is optimized, a stable passivation film can be formed at the positive electrode interface and the negative electrode interface, and the passivation film has good thermal stability and strong oxidation resistance under extreme environment, and can effectively stabilize a battery system, so that the high-temperature storage performance and the high-temperature cycle performance of the battery are improved. When the content of the tetranitrile siloxane compound is high, the high-temperature continuous charging performance of the battery can be effectively improved, but in the cyclic process, the tetranitrile siloxane compound which is not consumed at the positive electrode side participates in the negative electrode SEI film formation, and the negative electrode SEI film performance is deteriorated.

In the present application, fluorovinylene carbonate is added to the electrolyte at the same time. The fluorovinylene carbonate can effectively modify SEI film performance in the circulation process, and can effectively improve negative electrode SEI film performance and improve the circulation performance of a battery when the fluorovinylene carbonate is combined with a tetranitrile siloxane compound. However, fluorovinylene carbonate is consumed fast in the circulation process, and in the later period of circulation, the fluorovinylene carbonate is consumed completely, so that the phenomenon of circulation water jump is easy to occur; in addition, if the content of fluorovinylene carbonate is too high, the viscosity of the electrolyte increases, which in turn deteriorates the high-temperature storage performance.

According to the application, 1-methyl-1, 3-propane sultone is added into the electrolyte, and the 1-methyl-1, 3-propane sultone can be complementary with fluorovinylene carbonate, so that the consumption speed of fluorovinylene carbonate is slowed down, meanwhile, the electrolyte can also participate in the film formation of negative electrode SEI, and the cycle life of a battery can be effectively prolonged. When the content of additives such as fluoroethylene carbonate, 1-methyl-1, 3-propane sultone and the like in the electrolyte system is high, the viscosity of the electrolyte can be obviously improved, the conductivity of the electrolyte is reduced, and then the low-temperature discharge performance of the battery is deteriorated.

According to the application, 2-methyl difluoroacetate is added into the electrolyte, the 2, 2-methyl difluoroacetate has the characteristics of low viscosity and high dielectric constant, and can effectively improve the low-temperature discharge performance of the battery, but the 2, 2-methyl difluoroacetate has poor oxidation resistance, is easy to decompose and produce gas under high-temperature continuous charging, and the excessive content of 2, 2-methyl difluoroacetate increases in side reaction of a negative electrode in the later stage of high-temperature circulation, so that the consumption of fluorovinylene carbonate can be accelerated. The tetranitrile siloxane compound in the electrolyte can effectively improve the oxidation resistance of the electrolyte, and solve the problem that the oxidation resistance of the electrolyte is poor due to the methyl 2, 2-difluoroacetate.

In summary, by adding four additives of fluorovinylene carbonate, 1-methyl-1, 3-propane sultone, 2-methyl difluoroacetate and tetranitrile siloxane compound in combination into the electrolyte, the lithium ion battery can have good high-temperature continuous charging, high-temperature circulation, high-temperature storage performance and low-temperature discharging performance under high charging voltage at the same time, and further has longer service life.

In some embodiments, the fluorovinylene carbonate is present in the electrolyte at a mass ratio of a%, the 1-methyl-1, 3-propane sultone is present in the electrolyte at a mass ratio of b%, the 2, 2-difluoroacetic acid methyl ester is present in the electrolyte at a mass ratio of c%, and the tetranitrilosiloxane compound is present in the electrolyte at a mass ratio of d%, wherein 0.4.ltoreq.b/a.ltoreq. 0.8,2 (a+b)/d.ltoreq.3, and 2.ltoreq.c/d.ltoreq.5. That is, by setting the mass ratios of the four additives, the four additives can exert the combined effect more reliably when being combined, and the high-temperature continuous charge, the high-temperature circulation, the high-temperature storage performance and the low-temperature discharge performance of the battery can reach the optimal states.

In some embodiments, the fluoroethylene carbonate has a mass ratio of a% in the electrolyte, wherein 0.01.ltoreq.a.ltoreq.20. For example, a may be 0.01, 0.05, 0.1, 0.2, 0.5, 1.0, 1.5, 2, 4, 5, 6, 8, 10, 15, 20, etc., just to name a few. By adding a proper amount of fluorovinylene carbonate into the electrolyte, the SEI film performance of the battery in the circulation process can be effectively modified, so that the circulation performance of the battery is improved.

In some embodiments, the mass ratio of 1-methyl-1, 3-propane sultone in the electrolyte is b%, wherein b is 0.01.ltoreq.5. For example, b may be 0.01, 0.05, 0.1, 0.2, 0.5, 1.0, 1.5, 2, 2.8, 4, 5, etc., just to name a few. By adding a proper amount of 1-methyl-1, 3-propane sultone into the electrolyte, the consumption speed of fluorovinylene carbonate can be slowed down, meanwhile, the fluorovinylene carbonate can also participate in the film formation of negative electrode SEI, and the cycle life of a battery can be effectively prolonged.

In some embodiments, the mass ratio of the 2, 2-difluoro methyl acetate in the electrolyte is c%, wherein, c is more than or equal to 5 and less than or equal to 30. For example, c may be 5, 8, 10, 12, 16, 20, 24, 25, 27, 28, 30, etc. The 2, 2-difluoro methyl acetate has the characteristics of low viscosity and high dielectric constant, and the low-temperature discharge performance of the battery can be effectively improved by adding a proper amount of 2, 2-difluoro methyl acetate into the electrolyte.

In some embodiments, the mass ratio of the tetranitrile siloxane compound in the electrolyte is d%, wherein d.ltoreq.0.01.ltoreq.5. For example, d can be 0.01, 0.1, 0.3, 0.5, 1.0, 1.5, 2, 3/4, 5 and the like, a stable passivation film can be formed at the interface between the positive electrode and the negative electrode by adding a proper amount of tetranitrile siloxane compound into the electrolyte, and the passivation film has good thermal stability in extreme environment, can effectively stabilize a battery system, improves the high-temperature storage performance and the high-temperature cycle performance of the battery, and can effectively improve the oxidation resistance and the high-temperature continuous charging performance of the electrolyte.

In some embodiments, the electrolyte may have the following four additives in mass ratio: a is more than or equal to 0.01 and less than or equal to 20, b is more than or equal to 0.01 and less than or equal to 5, c is more than or equal to 5 and less than or equal to 30, d is more than or equal to 0.01 and less than or equal to 5,0.4, b/a is more than or equal to 0.8,2, (a+b)/d is more than or equal to 3, and c/d is more than or equal to 2 and less than or equal to 5. By controlling the respective mass ratio and the mutual proportion of the four additives in the electrolyte, the battery can achieve the optimal coordination effect, and the high-temperature continuous charging, the high-temperature circulation, the high-temperature storage performance and the low-temperature discharge performance of the battery can achieve the optimal state.

In some embodiments, the lithium salt is a fluorine-containing lithium salt, and may be selected from one or more of hexafluorophosphate, hexafluoroarsenate, lithium trifluorosulfonyl, lithium difluoro (trifluoromethylsulfonyl) imide, lithium bis (fluorosulfonyl) imide, lithium tris (trifluoromethylsulfonyl) methyllithium, lithium tetrafluoroborate, lithium hexafluoroantimonate, and lithium hexafluorotantalate. Of course, in other embodiments, other fluorine-containing lithium salts are also possible, and the application is not limited thereto, as this is merely illustrative. The lithium salt may be a non-fluorine-containing inorganic electrolyte salt lithium salt, for example selected from lithium perchlorate, lithium tetrachloroaluminate, li ₂ B ₁₀ Cl ₁₀ 、Li ₂ B ₁₀ F ₁₀ Etc. In other embodiments, the lithium salt may also be a lithium salt that chelates orthoborates and orthophosphates, such as: lithium dioxalate borate, lithium bis (difluoromalonate) borate, lithium (malonate oxalate) borate, lithium (difluoromalonate oxalate) borate, lithium trioxalate phosphate, lithium tris (difluoromalonate) phosphate, and the like, this application disclosesPlease do not limit here.

In some embodiments, the concentration of lithium salt in the electrolyte is 0.5mol/L to 2mol/L; preferably, the concentration of the lithium salt is 0.9mol/L to 1.3mol/L. The concentration of lithium salt that is too low may deteriorate ion transport performance of the electrolyte and affect rate performance, while the concentration of lithium salt that is too high may cause viscosity of the electrolyte to become high, thereby blocking ion transport. In the present application, by setting an appropriate lithium salt concentration, the viscosity of the electrolyte can be controlled, and a high ion transport performance can be ensured.

In some embodiments, the organic solvent is selected from two or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, methyl formate, ethyl propionate, propyl propionate, methyl butyrate, tetrahydrofuran. In the application, the selected solvent is a nonaqueous solvent, namely an organic solvent, and the conductivity of the electrolyte can be improved by selecting two or more organic solvents.

The embodiment of the application also provides a lithium ion battery, which comprises a positive pole piece, a negative pole piece, a diaphragm and the electrolyte in any embodiment. The positive pole piece, the diaphragm and the negative pole piece are stacked in sequence, and the diaphragm is positioned between the positive pole piece and the negative pole piece to play a role in isolation.

The positive electrode plate comprises a positive electrode current collector and a positive electrode membrane, and the positive electrode membrane comprises a positive electrode active substance, a conductive agent and a binder. In some embodiments, the positive electrode active material is selected from lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, ternary LiNi _x Co _y MnzO ₂ One or more of materials (wherein x+y+z=1, x+.y). The negative pole piece comprises a negative pole current collector and a negative pole membrane, and the negative pole membrane comprises a negative pole active substance, a conductive agent and a binder. In some embodiments, the negative electrode active material is selected from graphite and/or silicon, such as natural graphite, artificial graphite, mesophase micro carbon spheres (abbreviated as MCMB), hard carbon, soft carbon, silicon-carbon composites, li-Sn alloys, li-Sn-O alloys, sn, snO, snO ₂ Lithiated TiO of spinel structure ₂ -Li ₄ Ti ₅ O ₁₂ One or more of Li-Al alloys.

The highest charging voltage of the lithium ion battery is 4.55V, and the highest storage temperature is 60 ℃.

The lithium ion battery of the application can be used for power devices using the battery as a power source or various energy storage systems using the battery as an energy storage element. The electric device includes, but is not limited to, a mobile phone, a tablet, a computer, an electric toy, an electric tool, a battery car, an electric car, a ship, a spacecraft, etc., and is not limited herein.

In order that the application may be more readily understood, the application will be further described in detail with reference to the following examples, which are given by way of illustration only and are not limiting in scope of application. The starting materials or components used in the present application may be prepared by commercial or conventional methods unless specifically indicated.

Preparation of a cell

Example 1

Preparation of electrolyte:

ethylene carbonate EC, diethyl carbonate DEC and propylene carbonate PC were mixed in a mass ratio of 1:1:1 as an organic solvent. Adding fluorovinylene carbonate, 1-methyl-1, 3-propane sultone, 2-difluoro methyl acetate and 1, 2-tetra-nitrile-1, 2-bis (trimethylsilyl) ethane with the mass ratio shown in the example 1 in the table 1 into an organic solvent, uniformly mixing, and then adding a fluorine-containing lithium salt LiPF ₆ Obtaining LiPF ₆ An electrolyte with a concentration of 1.1 mol/L.

Preparation of (II) Positive electrode sheet

Lithium cobalt oxide (LiCoO) as a positive electrode active material ₂ ) Adding conductive agent CNT (Carbon nano tube) and binder PVDF (polyvinylidene fluoride) into N-methyl pyrrolidone solvent according to the mass ratio of 97:1.5:1.5, and fully stirring and mixing to form uniform anode slurry. And coating the positive electrode slurry on a positive electrode current collector aluminum foil, drying, and cold pressing to obtain a positive electrode plate.

Preparation of negative electrode plate

And fully stirring and mixing the negative electrode active material graphite, the conductive agent acetylene black, the adhesive styrene-butadiene rubber and the thickener sodium carboxymethyl cellulose in a proper amount of deionized water solvent according to the mass ratio of 96:1.2:1.5:1.3, so that uniform negative electrode slurry is formed. And (3) coating the slurry on a copper foil of a negative current collector, drying, and cold pressing to obtain a negative electrode plate.

Preparation of lithium ion Battery

The PE porous polymer film is used as a diaphragm.

And (3) stacking the positive electrode plate, the diaphragm and the negative electrode plate prepared by the steps in sequence, so that the diaphragm is positioned between the positive electrode plate and the negative electrode plate. And then winding the overlapped pole pieces and the diaphragm to obtain the winding core. And (3) placing the coiled core in an aluminum-plastic film bag formed by punching, injecting the electrolyte prepared by the method into the baked and dried battery core, and performing the procedures of vacuum packaging, standing, formation and the like to complete the preparation of the lithium ion battery.

Examples 2 to 16

The lithium ion batteries corresponding to examples 2 to 16 were respectively prepared by the same preparation method as described above according to the mass ratio of each additive in the following table 1.

Comparative examples 1 to 7

The lithium ion batteries corresponding to examples 1 to 7 were respectively prepared by the same preparation method as described above according to the mass ratio of each additive in the following table 1.

TABLE 1

According to the mass ratio of each additive in each embodiment in table 1, the corresponding preset ratio value is calculated, and the calculation result is shown in table 2.

TABLE 2

Testing battery

The lithium ion batteries fabricated in the above examples and comparative examples were subjected to the corresponding performance tests, respectively, according to the following methods, and the test data in table 3 were calculated.

(one) 45 ℃ cycle test:

the testing method comprises the following steps: in a constant temperature box at 45+/-2 ℃, a lithium ion battery is charged to 4.55V at a constant current and a constant voltage of 1C, the cut-off current is 0.05C, then the lithium ion battery is discharged to 3V according to 1C, and charge and discharge cycles are carried out for a plurality of times according to the conditions. The capacity retention of each cell after 300 and 500 cycles was calculated, respectively.

The calculation formula is as follows: capacity retention (%) = discharge capacity for cycle number (mAh)/discharge capacity for the third cycle (mAh) x 100%.

Each example and comparative example was tested using 5 batteries. The average of the capacity retention after each group of 5 cells was cycled through different cycles is reported in table 3.

(II) high-temperature storage test at 60 ℃):

the testing method comprises the following steps: after the lithium ion battery is kept stand for 2 hours at 25+/-2 ℃, the lithium ion battery is charged and discharged according to the temperature of 1C/0.5C, and the charging and discharging voltage is 3.0V-4.55V. And then after the battery is fully charged, placing the battery in a test box at 60 ℃ for 60 days, and calculating the thickness expansion rate of the stored battery.

The calculation formula is as follows: the thickness swelling ratio (%) = (the thickness of the battery after the storage on the nth day)/(the initial thickness of the battery) ×100%. The calculation results are recorded in table 3.

(III) testing low-temperature discharge performance:

the testing method comprises the following steps: discharging the separated battery to 3.0V at 0.2C under the environment condition of 25 ℃ and standing for 5min; and charged to 4.55V at 0.2C. When the cell voltage reaches 4.55V, constant voltage charging is performed according to 4.55V instead until the charging current is less than or equal to the given cutoff current of 0.05C, and the cell is left for 5min. And transferring the full-charged battery into a high-low temperature box, setting the temperature to-20 ℃, and placing the full-charged battery in the box for 120min after the temperature of the temperature box is reached. Taking out the battery, discharging at 0.2C to a final voltage of 3.0V, and standing for 5min; and then the temperature of the high-low temperature box is adjusted to 25+/-3 ℃, and the battery is placed in the box for 60 minutes after the temperature of the high-low temperature box reaches the set temperature. The battery was removed and charged to 4.55V at 0.2C. When the voltage of the battery cell reaches 4.55V, constant voltage charging is carried out by changing the voltage into 4.55V until the charging current is less than or equal to the given cutoff current of 0.05C, and the battery cell is left for 5min. The capacity retention rate of the battery after discharging at-20 ℃ at low temperature of 3.0V was calculated.

The calculation formula is as follows: -20 ℃ discharge 3.0V capacity retention (%) = (-20 ℃ discharge to 3.0V discharge capacity/25℃discharge to 3.0V discharge capacity). Times.100%.

(IV) 55 ℃ continuous charging test:

the testing method comprises the following steps: the cell was placed in a 55 ℃ test chamber, allowed to stand for 1 hour, discharged to 3.0V at 0.2C at 55 ℃, and allowed to stand for 10min. And then the battery is charged to 4.55V at a constant current of 1.0C at 55 ℃, and the charging is continued for 30 days without setting off current. And calculating the thickness expansion rate of the battery after continuous charging.

The calculation formula is as follows: thickness swelling ratio (%) = (battery thickness after continuous charging on the nth day)/(initial battery thickness) ×100% on the nth day.

TABLE 3 Table 3

As can be seen from the data relating to tables 1 to 3, when the electrolytes of comparative examples 2 to 7 were each compared with comparative example 1, the high-temperature cycle performance of the battery was improved to some extent, but the high-temperature storage performance was deteriorated after the fluorovinylene carbonate was added to the electrolyte of comparative example 2; after the electrolyte of the comparative example 3 is added with 1-methyl-1, 3-propane sultone, the high-temperature circulation, high-temperature storage and high-temperature continuous charging performance of the battery are improved; after the electrolyte of the comparative example 4 is added with the 2, 2-difluoro methyl acetate, the low-temperature discharge performance of the battery is obviously improved, but the high-temperature circulation, high-temperature storage and high-temperature continuous charging performance are obviously deteriorated; after the electrolyte of the comparative example 5 is added with 1, 2-tetranitrile-1, 2-bis (trimethylsilyl) ethane, the high-temperature storage, high-temperature circulation and high-temperature continuous charging performance of the battery are obviously improved; the electrolyte of comparative example 6 can effectively improve the high-temperature cycle performance of a battery by adding fluorovinylene carbonate and 1, 2-tetranitrile-1, 2-bis (trimethylsilyl) ethane, which is based on the synergistic effect of fluorovinylene carbonate and nitrile groups of additives, the fluorovinylene carbonate can form a better protective film on the negative electrode, and the degradation of the negative electrode by the nitrile groups is relieved; the electrolyte of comparative example 7 was added with fluorovinylene carbonate and 1-methyl-1, 3-propane sultone, and the high temperature cycle and high temperature storage performance of the battery were improved, because 1-methyl-1, 3-propane sultone can participate in the negative electrode film formation, the consumption of fluorovinylene carbonate was alleviated, and the deterioration of the negative electrode by nitrile group was suppressed, but after both additives were added at the same time, the low temperature discharge performance of the battery was remarkably deteriorated, because the addition of both additives resulted in an increase in the viscosity of the electrolyte.

From example 1, it was found that, in example 1, when fluorovinylene carbonate, 1-methyl-1, 3-propane sultone, methyl 2, 2-difluoroacetate and 1, 2-tetranitrile-1, 2-bis (trimethylsilyl) ethane were simultaneously added to the electrolyte, the battery had the best combination of properties such as high-temperature cycle, high-temperature storage, low-temperature discharge and high-temperature continuous charge.

It was found from examples 2 to 6 that the use of 1-methyl-1, 3-propane sultone in the electrolyte in combination with fluorovinylene carbonate can improve the high-temperature cycle performance of the battery, but when 1-methyl-1, 3-propane sultone is too small, the effect of suppressing the gas production of fluorovinylene carbonate in high-temperature storage while alleviating the consumption of fluorovinylene carbonate is not achieved; when the 1-methyl-1, 3-propane sultone is too high, the high-temperature cycle performance is improved, the impedance is increased instead of being increased along with the increase of the content of the 1-methyl-1, 3-propane sultone, the low-temperature discharge performance of the battery is deteriorated, and the comprehensive performance is optimal only when b/a is more than or equal to 0.4 and less than or equal to 0.8.

It was found from examples 7 to 11 that the addition of 1-methyl-1, 3-propane sultone in combination with fluorovinylene carbonate to the electrolyte can alleviate the deterioration of the negative electrode by nitrile groups in the additive, but the protection of the negative electrode is insufficient when the content of 1-methyl-1, 3-propane sultone and fluorovinylene carbonate is low; when the content of the 1-methyl-1, 3-propane sultone and fluorovinylene carbonate is too high, the protection of the cathode is not obviously improved, but the impedance is increased, the low-temperature discharge performance of the battery is deteriorated, and the comprehensive performance is optimal only when the ratio of (a+b)/d is more than or equal to 2 and less than or equal to 3.

It was found from examples 12 to 16 that when the amount of methyl 2, 2-difluoroacetate added to the electrolyte was large, the low-temperature discharge performance of the battery was significantly improved, which was based on the characteristics of low viscosity and high dielectric constant of methyl 2, 2-difluoroacetate. However, the 2, 2-methyl difluoroacetate has poor oxidation resistance, is easy to decompose and produce gas under high-temperature continuous charging, and the excessive 2, 2-methyl difluoroacetate can increase side reactions at the negative electrode in the later period of high-temperature cycle, and can accelerate the consumption of fluorovinylene carbonate; when the film forming additive of the anode and the cathode is sufficient, the aim of obviously improving the low-temperature discharge performance cannot be achieved by excessively low content of the 2, 2-difluoro methyl acetate, and the comprehensive performance is optimal only when the c/d is more than or equal to 2 and less than or equal to 5.

In summary, when the mass ratio of the four additives in the electrolyte of the lithium ion battery is equal to or more than 0.4 and equal to or less than 0.8,2 (a+b)/d is equal to or less than 3, and c/d is equal to or less than 2 and is equal to or less than 5, the comprehensive performance of the battery is optimal.

The specific implementation of the foregoing embodiments has been described in detail in the embodiments related to the method, and will not be described in detail herein.

The technical features of the above-described embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above-described embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.

It should be noted that the above-described embodiments are only for explaining the present application and do not constitute any limitation of the present application. The application has been described with reference to exemplary embodiments, but it is understood that the words which have been used are words of description and illustration, rather than words of limitation. Modifications may be made to the application as defined in the appended claims, and the application may be modified without departing from the scope and spirit of the application. Although the application is described herein with reference to particular means, materials and embodiments, the application is not intended to be limited to the particulars disclosed herein, as the application extends to all other means and applications having the same function.

Claims

1. The electrolyte is characterized by comprising lithium salt, an additive and an organic solvent, wherein the additive comprises fluorovinylene carbonate, 1-methyl-1, 3-propane sultone, 2-difluoro methyl acetate and tetranitrile siloxane compound.

2. The electrolyte according to claim 1, wherein the mass ratio of the fluorovinylene carbonate in the electrolyte is a%, the mass ratio of the 1-methyl-1, 3-propane sultone in the electrolyte is b%, the mass ratio of the 2, 2-difluoroacetic acid methyl ester in the electrolyte is c%, the mass ratio of the tetranitrile siloxane compound in the electrolyte is d%, wherein 0.4.ltoreq.b/a.ltoreq. 0.8,2.ltoreq.a+b)/d.ltoreq.3, and 2.ltoreq.c/d.ltoreq.5.

3. The electrolyte according to claim 1 or 2, wherein the mass ratio of the fluorovinylene carbonate in the electrolyte is a%, wherein a is 0.01-20.

4. The electrolyte according to claim 1 or 2, wherein the mass ratio of the 1-methyl-1, 3-propane sultone in the electrolyte is b%, wherein b is 0.01.ltoreq.b.ltoreq.5.

5. The electrolyte according to claim 1 or 2, wherein the mass ratio of the methyl 2, 2-difluoroacetate in the electrolyte is c%, wherein c is 5.ltoreq.30.

6. The electrolyte according to claim 1 or 2, wherein the mass ratio of the tetranitrile siloxane compound in the electrolyte is d%, wherein d is 0.01.ltoreq.5.

7. The electrolyte according to claim 1, wherein the lithium salt is a fluorine-containing lithium salt selected from one or more of hexafluorophosphate, hexafluoroarsenate, lithium trifluorosulfonyl, lithium difluoro (trifluoromethylsulfonyl) imide, lithium bis (fluorosulfonyl) imide, lithium tris (trifluoromethylsulfonyl) methyl, lithium tetrafluoroborate, lithium hexafluoroantimonate, lithium hexafluorotantalate; and/or

8. The electrolyte according to claim 1 or 7, wherein the concentration of lithium salt in the electrolyte is 0.5mol/L to 2mol/L; preferably, the concentration of the lithium salt is 0.9mol/L to 1.3mol/L.

9. A lithium ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte of any one of claims 1 to 9.

10. The lithium ion battery of claim 9, wherein the lithium ion battery has a maximum charge voltage of 4.55V and a maximum storage temperature of 60 ℃.