CN118231763A

CN118231763A - Electrolyte, lithium ion battery and electric equipment

Info

Publication number: CN118231763A
Application number: CN202311839954.7A
Authority: CN
Inventors: 王雅琪; 巫殷福; 谭友斌; 尹小强
Original assignee: BYD Co Ltd
Current assignee: BYD Co Ltd
Priority date: 2023-12-27
Filing date: 2023-12-27
Publication date: 2024-06-21

Abstract

The embodiment of the invention discloses an electrolyte, a lithium ion battery and electric equipment, wherein the electrolyte comprises lithium salt, a solvent and an additive, the additive comprises a perfluorinated organic compound shown as a formula (I),Wherein R ₁ and R ₂ are independently selected from fluorine atoms or perfluoroalkyl groups, R ₃ is selected from perfluoroalkyl groups, M is selected from Fe or Cs, and n is 2 or 3. The perfluorinated organic compound can be adsorbed onto the surface of a battery cathode through ion characteristics, so that the intercalation kinetics of lithium ions is accelerated, and the lithium phenomenon is slowly resolved; under the condition of overcharge, the voltage is stabilized through the shuttle reaction of the positive ions between the positive electrode and the negative electrode, and the function of the overcharge protection additive is achieved; and can form fluorine-containing free radicals to capture hydrogen atoms, thereby playing a role in flame retardance. The lithium ion battery can be applied to the lithium ion battery, and the cycle life and the safety performance of the battery can be effectively improved.

Description

Electrolyte, lithium ion battery and electric equipment

Technical Field

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

Background

Lithium ion batteries are widely used in the fields of electric vehicles and energy storage because of their high energy density, long cycle life and good safety performance. In a lithium ion battery system, the electrolyte plays a vital role as an intermediate bridge for connecting an anode material system and a cathode material system, and the integral performance of the battery can be improved by adding functional molecules. At present, aiming at the problem of lithium precipitation of a battery cathode, a method is commonly adopted, namely an electrochemical reduction type film forming additive is added into electrolyte, the reduction potential of the additive is higher than that of an organic solvent in the electrolyte, the additive can be decomposed in preference to the electrolyte, a solid electrolyte interface film (SEI) is formed on the surface of the cathode, and the lithium precipitation of the cathode is slowed down by enhancing the stability of the SEI film. However, there are some limitations to such additives: (1) Can be gradually depleted during long cycles of the battery resulting in failure; (2) is not suitable for use under low temperature and high magnification conditions. In addition, the lithium ion battery electrolyte generally adopts a highly flammable organic carbonate solvent, and under extreme conditions such as overheating, overcharging and the like, the battery may be thermally out of control and cause ignition or explosion, thereby generating potential safety hazards and bringing great risks. In order to further improve the safety performance of batteries, overcharge protection additives and flame retardant additives are receiving increasing attention. Therefore, it is necessary to provide a multifunctional electrolyte additive to comprehensively solve the above problems.

Disclosure of Invention

Based on the above, the embodiment of the invention provides an electrolyte, a lithium ion battery and electric equipment, wherein the electrolyte contains a perfluorinated organic compound, and the perfluorinated organic compound can be adsorbed on the surface of a battery cathode through ion characteristics so as to accelerate the intercalation kinetics of lithium ions, thereby slowly resolving the lithium phenomenon; under the condition of overcharge, the voltage is stabilized through the shuttle reaction of the positive ions between the positive electrode and the negative electrode, and the function of the overcharge protection additive is achieved; and can form fluorine-containing free radicals to capture hydrogen atoms, thereby playing a role in flame retardance. The electrolyte is applied to the lithium ion battery, so that the cycle life and the safety performance of the battery can be effectively improved.

In a first aspect, embodiments of the present invention provide an electrolyte comprising a lithium salt, a solvent, and an additive comprising a perfluorinated organic compound of formula (I),

Wherein R ₁ and R ₂ are independently selected from fluorine atoms or perfluoroalkyl groups, R ₃ is selected from perfluoroalkyl groups, M is selected from Fe or Cs, and n is 2 or 3.

In an embodiment of the present invention, the perfluorinated organic compound includes at least one of iron perfluoro (2-methyl-3 oxahexanoate) and cesium perfluoro (2-methyl-3 oxahexanoate).

In an embodiment of the present invention, the ratio of the perfluorinated organic compound to the mass of the electrolyte excluding the lithium salt is 0.01% -5%.

In an embodiment of the present invention, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bistrifluoromethylsulfonylimide, lithium trifluoromethylsulfonate, lithium bistrifluorosulfonylimide, lithium hexafluoroarsenate, lithium difluorophosphate, lithium tetrafluoroborate, lithium difluorooxalato borate, lithium dioxaato borate, lithium perchlorate, and lithium tetrafluoroaluminate.

In an embodiment of the present invention, the concentration of the lithium salt in the electrolyte is 0.1mol/L to 4mol/L.

In an embodiment of the present invention, the solvent includes at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, propylene carbonate, dipropyl carbonate, methylpropyl carbonate, ethylene propylene carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, gamma-butyrolactone, tetrahydrofuran, and dipropylene glycol dimethyl ether.

In an embodiment of the present invention, the solvent is a mixture of ethylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

In an embodiment of the present invention, the additive further comprises a film forming additive comprising at least one of vinylene carbonate, fluoroethylene carbonate, ethylene sulfate, glycerol tri (propionitrile) ether, propylene sulfate, 1, 3-propane sultone, 1, 3-propene sultone, 1, 4-butane sultone, ethylene sulfite, and propylene sulfite.

In an embodiment of the present invention, the film forming additive accounts for 0.1% to 5% of the mass of the electrolyte excluding the lithium salt.

The electrolyte provided by the embodiment of the invention contains the perfluorinated organic compound, and the perfluorinated organic compound can be adsorbed on the surface of the battery cathode through the ion characteristic, so that the intercalation kinetics of lithium ions is accelerated, and the lithium phenomenon is slowly resolved; under the condition of overcharge, the stable voltage can be realized through the shuttle reaction of the positive ions between the positive electrode and the negative electrode, so that the function of the overcharge protection additive is realized; and can form fluorine-containing free radicals to capture hydrogen atoms, thereby playing a role in flame retardance. Therefore, the electrolyte is applied to the lithium ion battery, and the cycle life and the safety performance of the battery can be effectively improved.

In a second aspect, an embodiment of the present invention provides a lithium ion battery, including a positive electrode, a negative electrode, and a separator and an electrolyte disposed between the positive electrode and the negative electrode, where the electrolyte includes the electrolyte of the first aspect.

The lithium ion battery provided by the embodiment of the invention contains the perfluorinated organic compound, and the perfluorinated organic compound not only can relieve the problem of lithium precipitation of the negative electrode, but also has overcharge resistance and flame retardance, and is beneficial to improving the cycle stability and safety of the battery.

In a third aspect, an embodiment of the present invention further provides an electrical apparatus, where the electrical apparatus includes the lithium ion battery according to the second aspect.

The electric equipment provided by the embodiment of the invention comprises the lithium ion battery of the second aspect, and the lithium ion battery has long cycle life and good safety performance, so that the electric equipment can be stably used for a long time, and the service performance of the electric equipment is improved.

Drawings

In order to more clearly describe the technical solution of the embodiments of the present invention, the drawings required to be used in the embodiments of the present invention will be described below.

Fig. 1 is a schematic diagram of a lithium ion battery 100 according to an embodiment of the present invention.

Detailed Description

Embodiments of the present invention will be described below with reference to the accompanying drawings in the embodiments of the present invention.

The lithium ion battery is widely applied to the fields of production and life, such as consumer electronics, electric automobiles, medical electronics, unmanned aerial vehicles and the like. The lithium ion battery mainly comprises a positive electrode, a negative electrode, a diaphragm and electrolyte, wherein the electrolyte is one of key materials of the lithium ion battery, and plays a role in transmitting lithium ions between the positive electrode and the negative electrode. In order to further improve the functionality of lithium ion batteries, various kinds of electrolyte additives are added to the electrolyte, and the electrolyte additives are small in dosage but great in effect, and are hot spots of research in recent years.

Currently, the phenomenon of negative electrode lithium precipitation is the most important problem affecting the cycle life and safety performance of lithium ion batteries. The lithium separation of the negative electrode not only accelerates the capacity decay of the battery and causes more electrolyte consumption, but also possibly punctures the diaphragm to cause internal short circuit and thermal runaway of the battery, so that the problem of lithium separation is solved. Current research to buffer lithium by adding electrolyte additives has focused mainly on electrochemical reduction type film forming additives such as Vinylene Carbonate (VC) and the like. The electrochemical reduction type film forming additive is an organic solvent with a reduction potential higher than that of the electrolyte, can be decomposed before the electrolyte, forms an SEI film on the surface of the negative electrode, and slows down lithium precipitation of the negative electrode by changing the characteristics of the SEI film. However, there are some limitations to such additives: (1) The electrolyte is protected by decomposition before the electrolyte, and once the content of the electrolyte is exhausted, the battery capacity is greatly attenuated, so that the additive can fail under long circulation. (2) Under the conditions of low temperature, high-rate charge and discharge and the like, the diffusion of lithium ions in a solid phase is slow, and the lithium ions are accumulated on the surface of an electrode/electrolyte, so that lithium precipitation occurs, and therefore, the additive is not suitable for use under the conditions of low temperature and high rate. In addition, the lithium ion battery electrolyte generally adopts a highly flammable organic carbonate solvent, and under extreme conditions such as overheating, overcharging and the like, the battery may be thermally out of control and cause ignition or explosion, thereby generating potential safety hazards and bringing great risks. In order to further improve the safety performance of batteries, overcharge protection additives and flame retardant additives are receiving increasing attention. Therefore, it is necessary to provide a multifunctional electrolyte additive to comprehensively solve the above problems.

Based on this, the embodiment of the invention provides an electrolyte, which comprises lithium salt, a solvent and an additive, wherein the additive comprises a perfluorinated organic compound shown as a formula (I),

The electrolyte provided by the embodiment of the invention contains the perfluorinated organic compound, the compound can be adsorbed to the surface of the battery cathode in a characteristic way, the diffusion rate and the intercalation rate of lithium ions are improved, the phenomenon of lithium precipitation of the cathode is relieved, and the electrolyte can be used as an overcharge protection additive and a flame retardant additive, so that the cycle stability and the safety of the battery can be effectively improved. On the one hand, when the anions of the perfluorinated organic compound are adsorbed on the surface of the negative electrode of the battery, the adsorbed particles are negatively charged, so that a potential difference between a compact layer and a dispersion layer can be formed, the diffusion and intercalation of lithium ions are further promoted, and the concentration of lithium ions accumulated on the surface of the negative electrode can be reduced by introducing the perfluorinated organic compound, so that the occurrence of a lithium precipitation phenomenon can be reduced. And unlike film forming additive, the perfluoro organic compound is absorbed into the inner Helmholtz layer through ionic property, affects the intercalation dynamics of lithium ions and reduces the concentration of lithium ions on the surface of the cathode to slowly analyze lithium, does not participate in the formation of SEI film, is not consumed along with the increase of the circulation times, and can continuously play a role in the long circulation process. On the other hand, the perfluorinated organic compound contains ferrous ions or trivalent cesium ions, and as the oxidation-reduction potential of the perfluorinated organic compound is in a battery voltage overcharge region, the ferrous ions or the trivalent cesium ions undergo oxidation reaction to become ferric ions and tetravalent cesium ions when the battery is overcharged, so that the oxidative decomposition of electrolyte under high voltage is prevented, the loss of active substances is reduced, and the safety performance of the battery is improved. In addition, the ferrous ions and the trivalent cesium ions have better solubility in the electrolyte, which is beneficial to better play. Therefore, ferrous ions or trivalent cesium ions can absorb redundant charges through a shuttle reaction between the anode and the cathode to stabilize the voltage, and the battery can be improved in overcharge resistance. In addition, fluorine atoms in the perfluorinated organic compound can form fluorine-containing free radicals during gasification, and the fluorine-containing free radicals can capture hydrogen atoms, so that the chain reaction of the hydrogen free radicals is inhibited, and the flame retardant effect can be achieved. In conclusion, the perfluorinated organic compound is a multifunctional additive with negative electrode lithium precipitation prevention, overcharge prevention and flame retardance, and can be applied to a lithium ion battery, compared with a single-functional additive, the perfluorinated organic compound can better improve the overall performance of the battery, and meanwhile, the dosage of other additives can be reduced, so that the capacity of the battery is improved, and the preparation cost of the battery can be reduced.

In some embodiments of the application, M is Fe and n is 2; in some embodiments, M is Cs and n is 3.

In the embodiment of the present invention, perfluoroalkyl refers to a substituted alkyl group in which all hydrogen atoms in the alkyl molecule are replaced with fluorine atoms, and may be a C ₁-C₁₈ perfluoroalkyl group, that is, the number of carbon atoms of the perfluoroalkyl group may be 1,2,3, 4, 5, 6, 7, 8, 10, 13, 15, 18, or the like. Specifically, the perfluoroalkyl group may be, but is not limited to, trifluoromethyl, pentafluoroethyl, perfluoropropyl, perfluorobutyl, perfluoropentyl, perfluorohexyl, perfluoroheptyl, or perfluorooctyl.

In the present embodiment, when R ₁ and/or R ₂ and R ₃ are perfluoroalkyl groups, they may be the same perfluoroalkyl groups or different perfluoroalkyl groups.

In an embodiment of the present invention, the perfluorinated organic compound includes at least one of iron perfluoro (2-methyl-3 oxahexanoate) (FPFA) and cesium perfluoro (2-methyl-3 oxahexanoate) (CPFA), and FPFA and CPFA are easily available, have good solubility, and can well inhibit electrode lithium precipitation.

In the embodiment of the invention, the ratio of the perfluorinated organic compound to the electrolyte except for the lithium salt is 0.01-5%, so that the lithium precipitation problem of the battery cathode can be effectively relieved, the overcharge resistance and the flame retardant property of the battery are improved, the capacity of the battery can be considered, the waste of the perfluorinated organic compound can be avoided, and the cost can be controlled. In some embodiments, the ratio of the perfluorinated organic compound to the mass of the electrolyte other than the lithium salt may be, for example, 0.01%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.3%, 2.6%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.3%, 4.5%, or 5%.

In an embodiment of the present invention, the perfluorinated organic compound may be prepared using the following method: dissolving perfluoro (2-methyl-3-oxahexanoic acid), ferrous acetate or cesium trichloride and Polyacrylonitrile (PAN) in Dimethylformamide (DMF) solution, and continuously stirring at 60 ℃ for 12 hours to obtain a uniform and stable solution; carrying out electrostatic spinning on the obtained solution under high voltage of 20kV, wherein the distance between a needle head and a roller is 12cm, so as to obtain a spinning film; calcining the obtained spinning film at 500 ℃ for 3 hours to obtain FPFA or CPFA.

In the invention, the main function of the lithium salt is to provide lithium ions, so that the battery is ensured to have enough lithium ions in the charge and discharge process, and the lithium ions are transmitted between the positive electrode and the negative electrode and are inserted into and separated from the negative electrode material in the charge and discharge process. In some embodiments, the lithium salt may include at least one of lithium hexafluorophosphate, lithium bis (trifluoromethylsulfonyl) imide, lithium trifluoromethylsulfonate, lithium bis (fluorosulfonyl) imide, lithium hexafluoroarsenate, lithium difluorophosphate, lithium tetrafluoroborate, lithium difluorooxalato borate, lithium dioxaato borate, lithium perchlorate, and lithium tetrafluoroaluminate.

In the embodiment of the invention, the concentration of the lithium salt in the electrolyte is 0.1mol/L-4mol/L, so that the conductivity and the ion transmission rate of the battery can be ensured, the waste of the lithium salt can be avoided, and the cost can be controlled. In some embodiments, the concentration of lithium salt in the electrolyte may be 0.1mol/L, 0.5mol/L, 1mol/L, 1.5mol/L, 2mol/L, 2.5mol/L, 3mol/L, 3.5mol/L, or 4mol/L.

In the invention, the solvent effectively dissolves lithium salt, so that ions between the anode and the cathode can freely move, thereby realizing the flow of current. In some embodiments, the solvent may include at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, propylene carbonate, dipropyl carbonate, methylpropyl carbonate, ethylene propylene carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, gamma-butyrolactone, tetrahydrofuran, and dipropylene glycol dimethyl ether. The solvent has high chemical stability, low volatility, high dielectric constant and low viscosity, and is favorable for improving the stability and safety of the battery. In some embodiments, the solvent may be a mixture of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

In the embodiment of the invention, the additive can also comprise a film forming additive, and the film forming additive can prevent the electrode material from reacting with the electrolyte and has a stable structure by promoting the surface of the electrode material to form a stable and effective SEI film, so that the generation of lithium precipitation can be further reduced. Specifically, the film-forming additive may include at least one of vinylene carbonate, fluoroethylene carbonate, ethylene sulfate, glycerol tri (propionitrile) ether, propylene sulfate, 1, 3-propane sultone, 1, 3-propene sultone, 1, 4-butane sultone, ethylene sulfite, and propylene sulfite. In some embodiments, the film forming additive can be vinylene carbonate, and the perfluoro organic compound and the vinylene carbonate are matched for use, so that the structural stability of the electrode surface can be more effectively improved, and the effect of inhibiting lithium precipitation is better.

In the embodiment of the invention, the ratio of the film forming additive to the electrolyte except the lithium salt is 0.5% -5%, so that the structural stability of the battery electrode can be further improved, the problem of lithium precipitation can be relieved, the capacity of the battery can be considered, the waste of the film forming additive can be avoided, and the cost can be controlled. In some embodiments, the film forming additive is present in a ratio of 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.3%, 2.6%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.3%, 4.5%, 4.7% or 5% relative to the mass of the electrolyte other than the lithium salt.

The embodiment of the invention also provides a lithium ion battery, which comprises a positive electrode, a negative electrode, a diaphragm positioned between the positive electrode and the negative electrode and electrolyte, wherein the electrolyte comprises the electrolyte in any embodiment. The lithium ion battery provided by the embodiment of the invention contains the perfluorinated organic compound, and the perfluorinated organic compound not only can relieve the problem of lithium precipitation of the negative electrode, but also has overcharge resistance and flame retardance, and is beneficial to improving the cycle stability and safety of the battery.

In one embodiment, as shown in fig. 1, the lithium ion battery 100 includes a negative electrode 10, an electrolyte 20, a separator 30, a positive electrode 40, and a battery case 50, wherein the electrolyte 20 and the separator 30 are disposed between the negative electrode 10 and the positive electrode 40, and the battery case 50 is used for packaging the negative electrode 10, the electrolyte 20, the separator 30, and the positive electrode 40.

In an embodiment of the present application, the positive electrode 40 may include a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the negative electrode 10 may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, wherein the positive electrode active material layer may include a positive electrode active material, a conductive agent, and a binder, and the negative electrode active material layer may include a negative electrode active material, a conductive agent, and a binder, which are not particularly limited and may be selected according to practical application requirements.

In an embodiment of the present invention, the positive electrode current collector may be an aluminum foil; the negative electrode current collector may be copper foil.

In the embodiment of the invention, the positive electrode active material can be a phosphate positive electrode active material or a ternary positive electrode active material, and specifically, the positive electrode active material can be one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadyl phosphate, lithium vanadium fluorophosphate, lithium titanate, lithium nickel cobalt manganate and lithium nickel cobalt aluminate; the anode active material may be one or more of a graphite material, a hard carbon material, a soft carbon material, a Si-based material, and a Sn-based material.

In an embodiment of the present invention, the conductive agent may be one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotube.

In an embodiment of the present invention, the binder may be one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and chitosan derivatives.

In an embodiment of the present invention, the separator may be a polypropylene film (PP), a polyethylene film (PE), or a PP/PE composite film.

The embodiment of the invention also provides electric equipment, which comprises the lithium ion battery in any embodiment. Specifically, the electric equipment can be an electric automobile, an electric motorcycle, an electric bicycle, a mobile power supply, an unmanned aerial vehicle, a mobile phone, a computer, a camera, an electric tool, an intelligent home or a wearable device.

The electric equipment provided by the embodiment of the invention comprises the lithium ion battery, and the lithium ion battery has long cycle life and good safety performance, so that the electric equipment can be stably used for a long time, and the service performance of the electric equipment is improved.

The technical scheme of the invention is further described by specific examples and comparative examples.

Example 1

Taking 28.97 parts of Ethylene Carbonate (EC), 22.43 parts of dimethyl carbonate (DMC), 12 parts of diethyl carbonate (DEC) and 34 parts of methyl ethyl carbonate (EMC) at room temperature for mixing, and continuously stirring for 4 hours to uniformly mix; then adding 2.6 parts of perfluoro (2-methyl-3-oxahexanoic acid) iron ((C ₆F₁₁O₃)₂ Fe), carrying out ultrasonic treatment for 2 hours to uniformly mix the mixture, finally adding lithium hexafluorophosphate (LiPF ₆), carrying out ultrasonic treatment for 2 hours to completely dissolve the mixture, and obtaining the electrolyte containing 1mol/L LiPF ₆.

Example 2

Taking 29.64 parts of Ethylene Carbonate (EC), 22.73 parts of dimethyl carbonate (DMC), 12.16 parts of diethyl carbonate (DEC) and 35.46 parts of ethylmethyl carbonate (EMC) at room temperature, mixing, and continuously stirring for 4 hours to uniformly mix the materials; then adding 0.01 part of perfluoro (2-methyl-3-oxahexanoic acid) iron ((C ₆F₁₁O₃)₂ Fe), carrying out ultrasonic treatment for 2 hours to uniformly mix the mixture, finally adding lithium hexafluorophosphate (LiPF ₆), carrying out ultrasonic treatment for 2 hours to completely dissolve the mixture, and obtaining the electrolyte containing 1mol/L LiPF ₆.

Example 3

Mixing 26.42 parts of Ethylene Carbonate (EC), 18.25 parts of dimethyl carbonate (DMC), 8.64 parts of diethyl carbonate (DEC) and 41.69 parts of ethylmethyl carbonate (EMC) at room temperature, and continuously stirring for 4 hours to uniformly mix the materials; then adding 5 parts of perfluoro (2-methyl-3-oxahexanoic acid) iron ((C ₆F₁₁O₃)₂ Fe), carrying out ultrasonic treatment for 2 hours to uniformly mix the mixture, finally adding lithium hexafluorophosphate (LiPF ₆), carrying out ultrasonic treatment for 2 hours to completely dissolve the mixture, and obtaining the electrolyte containing 1mol/L LiPF ₆.

Example 4

Mixing 30.12 parts of Ethylene Carbonate (EC), 22.86 parts of dimethyl carbonate (DMC), 10.16 parts of diethyl carbonate (DEC) and 36.36 parts of ethylmethyl carbonate (EMC) at room temperature, and continuously stirring for 4 hours to uniformly mix the materials; then 0.5 part of cesium (C ₆F₁₁O₃)₃ Cs) perfluoro (2-methyl-3 oxahexanoate) is added, the mixture is uniformly mixed by ultrasonic treatment for 2 hours, and finally lithium hexafluorophosphate (LiPF ₆) is added, and the electrolyte containing 1mol/L LiPF ₆ is obtained by ultrasonic treatment for 2 hours.

Example 5

Mixing 31.42 parts of Ethylene Carbonate (EC), 21.42 parts of dimethyl carbonate (DMC), 13.16 parts of diethyl carbonate (DEC) and 31.6 parts of ethylmethyl carbonate (EMC) at room temperature, and continuously stirring for 4 hours to uniformly mix the materials; then adding 2.4 parts of cesium (C ₆F₁₁O₃)₃ Cs) perfluoro (2-methyl-3 oxahexanoate), carrying out ultrasonic treatment for 2 hours to uniformly mix the cesium ((C ₆F₁₁O₃)₃ Cs), finally adding lithium hexafluorophosphate (LiPF ₆), carrying out ultrasonic treatment for 2 hours to completely dissolve the lithium hexafluorophosphate, and obtaining the electrolyte containing 1mol/L LiPF ₆.

Example 6

Mixing 30.52 parts of Ethylene Carbonate (EC), 20.64 parts of dimethyl carbonate (DMC), 10.74 parts of diethyl carbonate (DEC) and 33.1 parts of ethylmethyl carbonate (EMC) at room temperature, and continuously stirring for 4 hours to uniformly mix the materials; then 5 parts of cesium perfluoro (2-methyl-3 oxahexanoate) (C ₆F₁₁O₃)₃ Cs) is added, the mixture is uniformly mixed by ultrasonic treatment for 2 hours, and finally lithium hexafluorophosphate (LiPF ₆) is added, and the electrolyte containing 1mol/L LiPF ₆ is obtained by ultrasonic treatment for 2 hours.

Example 7

Taking 31.41 parts of Ethylene Carbonate (EC), 19.65 parts of dimethyl carbonate (DMC), 12.42 parts of diethyl carbonate (DEC) and 34.12 parts of ethylmethyl carbonate (EMC) at room temperature to be mixed, and continuously stirring for 4 hours to uniformly mix the materials; then adding 1.2 parts of iron perfluoro (2-methyl-3 oxahexanoate) and 1.2 parts of cesium perfluoro (2-methyl-3 oxahexanoate), and carrying out ultrasonic treatment for 2 hours to uniformly mix the materials; finally, lithium hexafluorophosphate (LiPF ₆) is added, and the electrolyte containing 1mol/L LiPF ₆ is obtained after ultrasonic treatment for 2 hours to completely dissolve the lithium hexafluorophosphate.

Example 8

Mixing 30.26 parts of Ethylene Carbonate (EC), 20.66 parts of dimethyl carbonate (DMC), 13.26 parts of diethyl carbonate (DEC) and 30.82 parts of ethylmethyl carbonate (EMC) at room temperature, and continuously stirring for 4 hours to uniformly mix the materials; then adding 2.6 parts of perfluoro (2-methyl-3 oxahexanoic acid) iron ((C ₆F₁₁O₃)₂ Fe) and 2.4 parts of Vinylene Carbonate (VC), carrying out ultrasonic treatment for 2 hours to uniformly mix the materials, finally adding lithium hexafluorophosphate (LiPF ₆), and carrying out ultrasonic treatment for 2 hours to completely dissolve the materials, thus obtaining the electrolyte containing 1mol/L LiPF ₆.

Comparative example 1

Taking 28.97 parts of Ethylene Carbonate (EC), 21.88 parts of dimethyl carbonate (DMC), 16.26 parts of diethyl carbonate (DEC) and 32.89 parts of ethylmethyl carbonate (EMC) at room temperature for mixing, and continuously stirring for 4 hours to uniformly mix the materials; then, lithium hexafluorophosphate (LiPF ₆) was added and the solution was completely dissolved by ultrasonic treatment for 2 hours to obtain an electrolyte containing 1mol/L LiPF ₆.

Comparative example 2

Mixing 26.97 parts of Ethylene Carbonate (EC), 20.88 parts of dimethyl carbonate (DMC), 16.26 parts of diethyl carbonate (DEC) and 32.89 parts of ethylmethyl carbonate (EMC) at room temperature, and continuously stirring for 4 hours to uniformly mix the materials; then adding 3 parts of Vinylene Carbonate (VC), and carrying out ultrasonic treatment for 2 hours to uniformly mix the materials; finally, lithium hexafluorophosphate (LiPF ₆) is added, and the electrolyte containing 1mol/L LiPF ₆ is obtained after ultrasonic treatment for 2 hours to completely dissolve the lithium hexafluorophosphate.

Comparative example 3

Mixing 26.97 parts of Ethylene Carbonate (EC), 20.88 parts of dimethyl carbonate (DMC), 16.26 parts of diethyl carbonate (DEC) and 32.89 parts of ethylmethyl carbonate (EMC) at room temperature, and continuously stirring for 4 hours to uniformly mix the materials; then adding 3 parts of fluoroethylene carbonate (FEC), and carrying out ultrasonic treatment for 2 hours to uniformly mix the materials; finally, lithium hexafluorophosphate (LiPF ₆) is added, and the electrolyte containing 1mol/L LiPF ₆ is obtained after ultrasonic treatment for 2 hours to completely dissolve the lithium hexafluorophosphate.

Comparative example 4

Taking 28.97 parts of Ethylene Carbonate (EC), 22.43 parts of dimethyl carbonate (DMC), 12 parts of diethyl carbonate (DEC) and 34 parts of methyl ethyl carbonate (EMC) at room temperature for mixing, and continuously stirring for 4 hours to uniformly mix; then adding 2.6 parts of perfluoro (2-methyl-3-oxahexanoic acid) lithium (C ₆F₁₁O₃ Li), and carrying out ultrasonic treatment for 2 hours to uniformly mix the materials; finally, lithium hexafluorophosphate (LiPF ₆) is added, and the electrolyte containing 1mol/L LiPF ₆ is obtained after ultrasonic treatment for 2 hours to completely dissolve the lithium hexafluorophosphate.

Comparative example 5

Taking 28.97 parts of Ethylene Carbonate (EC), 22.43 parts of dimethyl carbonate (DMC), 12 parts of diethyl carbonate (DEC) and 34 parts of methyl ethyl carbonate (EMC) at room temperature for mixing, and continuously stirring for 4 hours to uniformly mix; then adding 2.6 parts of ammonium perfluoro (2-methyl-3-oxahexanoate) (C ₆F₁₁O₃NH₄), and carrying out ultrasonic treatment for 2 hours to uniformly mix the mixture; finally, lithium hexafluorophosphate (LiPF ₆) is added, and the electrolyte containing 1mol/L LiPF ₆ is obtained after ultrasonic treatment for 2 hours to completely dissolve the lithium hexafluorophosphate.

The electrolytes provided in examples 1 to 8 and comparative examples 1 to 5 were assembled into lithium ion batteries and performance tested as follows, respectively:

Preparing a positive plate: and (3) preparing positive electrode slurry by adding a certain amount of N-methyl pyrrolidone (NMP) into a lithium iron phosphate positive electrode material (LFP), a polyvinylidene fluoride (PVDF) binder and an acetylene black (SuperP) conductive agent according to the mass ratio of 8:1:1, defoaming the slurry, sieving, uniformly coating the slurry on the surface of an aluminum foil, and drying, rolling and cutting to obtain the positive electrode plate.

Preparing a negative electrode sheet: and adding deionized water into a graphite anode material, a Styrene Butadiene Rubber (SBR)/carboxymethyl cellulose (CMC) composite binder and an acetylene black (SuperP) conductive agent according to the mass ratio of 8:1:1 to prepare anode slurry, defoaming the slurry, sieving, uniformly coating the slurry on the surface of an aluminum foil, and drying, rolling and cutting to obtain the anode sheet.

Preparation of a battery: and stacking the negative electrode plate, the diaphragm and the positive electrode plate sequentially into a battery core, packaging the battery core into an aluminum plastic shell, baking the battery core to remove moisture, respectively injecting the electrolyte of the examples 1-8 and the electrolyte of the comparative examples 1-5 into the square battery core, and aging, forming, aging and capacity-dividing to obtain the lithium ion batteries of the examples 1-8 and the comparative examples 1-5.

(1) Cycle performance test at different temperatures

The method for testing the cycle performance at normal temperature (25 ℃): 1) And (3) capacity calibration: charging 0.5C constant current and constant voltage to 3.65V at 25+ -3deg.C, stopping 0.05C, and standing for 30min; discharging the current of 0.5C to 2.5V, and standing for 30min; cycling for 2 times; recording the 2 nd discharge capacity as the nominal discharge capacity of the battery; 2) Placing for 2 hours, and ensuring that the ambient temperature (25 ℃) and the battery temperature are kept consistent; 3) Constant-current and constant-voltage charging of 1.0C to 3.65V and cutoff of 0.05C; 4) Standing for 30min; 5) Constant current discharge at 1.0C to 2.5V;

6) Standing for 30min; 7) And cycling the steps 2) to 6) 4000 times, and calculating the capacity retention rate.

The method for testing the cycle performance at low temperature (0 ℃) comprises the following steps: 1) And (3) capacity calibration: as above; 2) Placing for 2 hours to ensure that the ambient temperature (0 ℃) and the battery temperature are kept consistent; 3) Constant-current and constant-voltage charging of 1.0C to 3.65V and cutoff of 0.05C; 4) Standing for 30min; 5) Constant current discharge at 1.0C to 2.5V; 6) Standing for 30min; 7) And cycling the steps 2) to 6) 4000 times, and calculating the capacity retention rate.

The batteries of examples 1 to 8 and comparative examples 1 to 5 were tested for capacity retention after 4000 cycles at normal temperature (25 ℃) and low temperature (0 ℃) respectively according to the above-described method, and the test results are shown in Table 1.

Table 1 comparison of capacity retention after 4000 cycles for the batteries of examples 1-8 and comparative examples 1-5

Sequence number	Capacity retention at room temperature (25 ℃ C.)	Capacity retention at low temperature (0 ℃ C.)
			Example 1	90.3	82.2
Example 2	76.3	63.2
			Example 3	85.4	78.6
Example 4	80.2	69.4
			Example 5	88.6	81.7
Example 6	84.4	77.4
			Example 7	87.6	82.2
Example 8	91.6	84.3
			Comparative example 1	74.2	60.4
Comparative example 2	80.2	66.4
			Comparative example 3	78.6	63.2
Comparative example 4	86.3	79.4
			Comparative example 5	87.5	80.9

As can be seen from Table 1, the capacity retention rate after 4000 cycles of examples 1 to 8 is higher and the cycle performance decay rate is slower, compared with comparative example 1, no matter under the conditions of normal temperature (25 ℃) or low temperature (0 ℃), which indicates that the lithium ion battery provided by the embodiment of the invention has good cycle stability at different temperatures. It can be seen from examples 1, 5 and comparative examples 2 to 5 that the effects of the present invention on improving the cycle performance of the battery provided by the examples (C ₆F₁₁O₃)₂ Fe and (C ₆F₁₁O₃)₃ Cs) are better than those of the additives VC, FEC, C ₆F₁₁O₃ Li and C ₆F₁₁O₃NH₄. As can be seen from examples 1 and 5, (C ₆F₁₁O₃)₂ Fe has better effects on improving the cycle performance of the battery than (C ₆F₁₁O₃)₃ Cs.. As can be seen from examples 1 to 3, 2 to 3wt% of (C ₆F₁₁O₃)₂ Fe is added to the electrolyte to help to improve the cycle performance of the battery better. As can be seen from examples 1 and 8, (C ₆F₁₁O₃)₂ Fe is used in combination with the film-forming additive VC to further improve the cycle performance of the battery.

(2) Overcharge protection test

The overcharge protection test method comprises the following steps: 1) Constant-current and constant-voltage charging to 3.65V at 0.5C; 2) Standing for 30min; 3) Discharging to 2.5V at constant current of 0.5C; 4) Standing for 30min; 5) Cycling process steps 1) -4) 3 times; 6) And (5) charging to 5V at constant current and constant voltage at 0.5C, and recording the charging time.

The time taken for the batteries of examples 1 to 8 and comparative examples 1 to 5 to overcharged to 5V, respectively, was measured according to the above-described method, and the test results are shown in Table 2.

TABLE 2 time to charge to cutoff 5V for the batteries of examples 1-8 and comparative examples 1-5

Sequence number	Time to charge to 5V cutoff voltage (h)
		Example 1	15.2
Example 2	2.4
		Example 3	30.6
Example 4	4.6
		Example 5	14.6
Example 6	28.7
		Example 7	14.9
Example 8	15.2
		Comparative example 1	2.1
Comparative example 2	2.4
		Comparative example 3	2.6
Comparative example 4	2.1
		Comparative example 5	2.1

As can be seen from table 2, the batteries of examples 1 to 8 require a longer time to charge to a cutoff voltage of 5V than comparative example 1, indicating that the lithium ion batteries provided in the examples of the present application have the performance of preventing overcharge. The 5V cut-off voltage is far higher than the normal working voltage of the lithium iron phosphate/graphite system battery by 3.65V, and when the battery is overcharged, the electrolyte containing perfluoro (2-methyl-3 oxahexanoic acid) iron and perfluoro (2-methyl-3 oxahexanoic acid) cesium has the redox couple, so that the cut-off voltage can be reached by 5V only after a longer time, the voltage rise can be well restrained by a longer time, the overcharge of the battery is prevented, and the overcharge prevention effect is realized. It can be seen from examples 1, 5 and comparative examples 2 to 5 that the effects of the present application provided in the examples (C ₆F₁₁O₃)₂ Fe and (C ₆F₁₁O₃)₃ Cs) on improving the overcharge prevention performance of the battery are significantly better than those provided in the additives VC, FEC, C ₆F₁₁O₃ Li and C ₆F₁₁O₃NH₄ as well as from examples 1 and 5 that the effects of (C ₆F₁₁O₃)₂ Fe on improving the overcharge prevention performance of the battery are better than those provided in the examples (C ₆F₁₁O₃)₃ Cs. from examples 1 to 3, an appropriate increase in the amount of added C ₆F₁₁O₃)₂ Fe helps to better improve the overcharge prevention performance of the battery.

The foregoing examples illustrate only a few embodiments of the application and are described in detail herein without thereby limiting the scope of the application. It should be noted that it will be apparent to those skilled in the art that several variations and modifications can be made without departing from the spirit of the application, which are all within the scope of the application. Accordingly, the scope of protection of the present application is to be determined by the appended claims.

Claims

1. An electrolyte, characterized in that the electrolyte comprises lithium salt, solvent and additive, the additive comprises perfluoro organic compound shown in formula (I),

2. The electrolyte of claim 1 wherein the perfluorinated organic compound comprises at least one of iron perfluoro (2-methyl-3 oxahexanoate) and cesium perfluoro (2-methyl-3 oxahexanoate).

3. The electrolyte according to claim 1, wherein the ratio of the perfluorinated organic compound to the mass of the electrolyte excluding the lithium salt is 0.01% to 5%.

4. The electrolyte of claim 1 wherein the lithium salt comprises at least one of lithium hexafluorophosphate, lithium bis (trifluoromethylsulfonyl) imide, lithium trifluoromethylsulfonate, lithium bis (fluorosulfonyl) imide, lithium hexafluoroarsenate, lithium difluorophosphate, lithium tetrafluoroborate, lithium difluorooxalato borate, lithium dioxaato borate, lithium perchlorate, and lithium tetrafluoroaluminate.

5. The electrolyte of claim 1 wherein the concentration of the lithium salt in the electrolyte is from 0.1mol/L to 4mol/L.

6. The electrolyte of claim 1 wherein the solvent comprises at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, propylene carbonate, dipropyl carbonate, methylpropyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, ethyl butyrate, gamma-butyrolactone, tetrahydrofuran, and dipropylene glycol dimethyl ether.

7. The electrolyte of claim 6 wherein the solvent is a mixture of ethylene carbonate, dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate.

8. The electrolyte of claim 1 wherein the additive further comprises a film forming additive comprising at least one of vinylene carbonate, fluoroethylene carbonate, ethylene sulfate, glycerol tri (propionitrile) ether, propylene sulfate, 1, 3-propane sultone, 1, 3-propene sultone, 1, 4-butane sultone, ethylene sulfite, and propylene sulfite.

9. The electrolyte of claim 8 wherein the film forming additive is present in an amount of 0.1% to 5% by mass of the electrolyte excluding the lithium salt.

10. A lithium ion battery comprising a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode, and an electrolyte comprising the electrolyte of any one of claims 1-9.

11. A powered device comprising the lithium-ion battery of claim 10.