Patent Publication Number: US-2021194060-A1

Title: Nonaqueous electrolyte, lithium-ion battery, battery module, battery pack, and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of PCT Patent Application No. PCT/CN2 019/108601, entitled “NONAQUEOUS ELECTROLYTE, LITHIUM-ION BATTERY, BATTERY MODULE, BATTERY PACK, AND APPARATUS” filed on Sep. 2 7, 2019, which claims priority to Chinese Patent Application No. 2 01811141736.5, filed on Sep. 2 8, 2018 and entitled “NONAQUEOUS ELECTROLYTE AND LITHIUM-ION BATTERY”, both of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     This application relates to the field of batteries, and in particular, to a nonaqueous electrolyte, a lithium-ion battery, a battery module, a battery pack, and an apparatus. 
     BACKGROUND 
     Lithium-ion batteries are widely applied to electric automobiles and consumer electronic products due to their advantages such as high energy density, high output power, long cycle life, and low environmental pollution. Current requirements for lithium-ion batteries are high voltage, high power, long cycle life, long storage life, and superb safety performance. 
     At present, nonaqueous electrolyte systems that use lithium hexafluorophosphate as a conductive lithium salt and cyclic carbonate and/or linear carbonate as a solvent are widely applied in lithium-ion batteries. However, the above nonaqueous electrolytes still have many shortcomings. For example, in a high-voltage system, cycle performance, storage performance and safety performance of the above nonaqueous electrolytes need to be improved. For another example, in a lithium cobalt oxide or high nickel ternary system, safety performance such as overcharge safety and hot box safety of lithium-ion batteries needs to be improved. 
     SUMMARY 
     In view of the problems in the Background, this application is intended to provide a nonaqueous electrolyte, a lithium-ion battery, a battery module, a battery pack and an apparatus. The nonaqueous electrolyte can not only improve electrochemical performance of a lithium-ion battery under high temperature and high voltage and improve safety performance such as overcharge safety and hot box safety of a lithium-ion battery, but also ensure that the lithium-ion battery has some kinetic performance. 
     In order to achieve the above objective, in a first aspect of this application, this application provides a nonaqueous electrolyte, which includes a nonaqueous solvent, a lithium salt, and an additive. The nonaqueous solvent includes a carbonate solvent and a high oxidation potential solvent, and the additive is a fluorinated cyclic carbonate. The carbonate solvent is a linear carbonate, or a mixture of linear carbonate and cyclic carbonate, and the high oxidation potential solvent is selected from one or more of compounds represented by formula I and formula II. In formula I, R 1  and R 2  are independently selected from unsubstituted, partially halogenated or fully halogenated alkyl groups having 1 to 5 carbon atoms, and at least one of R 1  and R 2  is a partially halogenated or fully halogenated alkyl group having 1 to 5 carbon atoms. In formula II, R 3  is selected from partially halogenated or fully halogenated alkylidene groups having 1 to 6 carbon atoms. A halogen atom is selected from one or more of F, Cl, Br, and I. The fluorinated cyclic carbonate is selected from one or more of compounds represented by formula III. In formula III, R 4  is selected from partially fluorinated or fully fluorinated alkylidene groups having 1 to 5 carbon atoms. 
     
       
         
         
             
             
         
       
     
     According to a second aspect of this application, this application provides a lithium-ion battery, including a positive electrode plate, a negative electrode plate, a separator and the nonaqueous electrolyte according to the first aspect of this application. 
     According to a third aspect of this application, this application provides a battery module, including the lithium-ion battery according to the second aspect of this application. 
     According to a fourth aspect of this application, this application provides a battery pack, including the battery module according to the third aspect of this application. 
     According to a fifth aspect of this application, this application provides an apparatus, including the lithium-ion battery according to the second aspect of this application, where the lithium-ion battery is used as a power source of the apparatus. 
     Compared with the prior art, this application includes at least the following beneficial effects: 
     (1) The nonaqueous electrolyte in this application can combine advantages of high oxidation resistance and non-flammability of high oxidation potential solvents with advantages of low viscosity and a high dielectric constant of carbonate solvents. As such, the nonaqueous electrolyte can not only improve electrochemical performance of a lithium-ion battery under high temperature and high voltage, but also ensure that the lithium-ion battery has some kinetic performance. 
     (2 ) As the nonaqueous electrolyte in this application uses a mixed solvent formed by a high oxidation potential solvent and a carbonate solvent, the nonaqueous electrolyte can overcome disadvantages of conventional carbonate solvents, such as poor oxidation resistance, easy high-pressure decomposition and gas generation, a low flash point, and easy combustion, and therefore can greatly improve safety performance such as overcharge safety and hot box safety of a lithium-ion battery. 
     (3) The nonaqueous electrolyte in this application contains an additive of fluorinated cyclic carbonate, which can first form a stable interface protective film on the negative electrode, thereby inhibiting side reactions of the high oxidation potential solvent on the negative electrode, and improving electrochemical performance such as cycle life of the lithium-ion battery. In addition, impedance of the interface protective film formed by the fluorinated cyclic carbonate on the negative electrode is also higher, and impedance of continuous reactions of the oxidation potential solvent is low, so the nonaqueous electrolyte can also improve kinetic performance of the lithium-ion battery. 
     (4) The additive of fluorinated cyclic carbonate, as a special cyclic carbonate, can not only form a film on a surface of the negative electrode, but also is characterized by a high dielectric constant. After being added, the additive can improve solubility of the nonaqueous solvent to the lithium salt and effectively alleviate a defect of a low dielectric constant of the high oxidation potential solvent, thereby effectively improving ionic conductivity in the nonaqueous electrolyte, and helping to obtain a lithium-ion battery with good kinetic performance. 
     The battery module, the battery pack, and the apparatus in this application include the lithium-ion battery, and therefore have at least the same advantages as the lithium-ion battery. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of an embodiment of a lithium-ion battery; 
         FIG. 2  is an exploded view of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of an embodiment of an electrode assembly of the lithium-ion battery in  FIG. 2 , in which a first electrode plate, a second electrode plate, and a separator are wound to form a wound electrode assembly; 
         FIG. 4  is a schematic diagram of another embodiment of an electrode assembly of the lithium-ion battery in  FIG. 2 , in which a first electrode plate, a second electrode plate, and a separator are laminated along a thickness direction to form a laminated electrode assembly; 
         FIG. 5  is a perspective view of an embodiment of a battery module; 
         FIG. 6  is a perspective view of an embodiment of a battery pack; 
         FIG. 7  is an exploded view of  FIGS. 6 ; and 
         FIG. 8  is a schematic diagram of an embodiment of an apparatus using a lithium-ion battery as a power source. 
     
    
    
     REFERENCE SIGNS ARE DESCRIBED AS FOLLOWS 
       1 . battery pack; 
       2 . upper box body; 
       3 . lower box body; 
       4 . battery module; 
       5 . lithium-ion battery; 
       51 . housing; 
       52 . electrode assembly; 
       521 . first electrode plate; 
       521   a . first current collector; 
       521   b . first active material layer; 
       522 . second electrode plate; 
       522   a . second current collector; 
       522   b . second active material layer; 
       523 . separator; 
       524 . first tab; 
       525 . second tab; and 
       53 . top cover assembly. 
     DESCRIPTION OF EMBODIMENTS 
     The following describes in detail a nonaqueous electrolyte, a lithium-ion battery, a battery module, a battery pack, and an apparatus according to this application. 
     A nonaqueous electrolyte according to a first aspect of this application is described first. The nonaqueous electrolyte includes a nonaqueous solvent, a lithium salt, and an additive. The nonaqueous solvent includes a carbonate solvent and a high oxidation potential solvent, and the additive is a fluorinated cyclic carbonate. 
     The carbonate solvent is a linear carbonate, or a mixture of linear carbonate and cyclic carbonate, and the high oxidation potential solvent is selected from one or more of compounds represented by formula I and formula II. In formula I, R 1  and R 2  are independently selected from unsubstituted, partially halogenated or fully halogenated alkyl groups having 1 to 5 carbon atoms, and at least one of R 1  and R 2  is a partially halogenated or fully halogenated alkyl group having 1 to 5 carbon atoms. In formula II, R 3  is selected from partially halogenated or fully halogenated alkylidene groups having 1 to 6 carbon atoms. In R 1 , R 2 ,  and R 3,  a halogen atom is selected from one or more of F, Cl, Br, and I, and further, the halogen atom is F. The alkyl and alkylidene groups may be straight-chained or branched. When the alkyl group or the alkylidene group is partially or fully halogenated, the halogen atom may be of one or more types. 
     
       
         
         
             
             
         
       
     
     The fluorinated cyclic carbonate is selected from one or more of compounds represented by formula III. In formula III, R 4  is selected from partially fluorinated or fully fluorinated alkylidene groups having 1 to 5 carbon atoms. There may be one or more fluorine atoms. 
     
       
         
         
             
             
         
       
     
     At present, carbonate solvents are generally used in lithium-ion batteries as electrolytes. As this type of solvent has poor oxidation resistance, the solvent may have slight oxidation at even about 4V at room temperature (25° C.). As the voltage and temperature increase, the solvent has more and more substantial oxidization and gas generation. In addition, as this type of solvent has a low flash point (generally below 35° C.), the solvent may easily burn when exposed to an open flame, and emits a large amount of heat. Therefore, lithium-ion batteries using conventional carbonate solvents have high potential hazards in safety performance. 
     In the nonaqueous electrolyte of this application, a high oxidation potential solvent and a carbonate solvent are used to form a mixed solvent. As the high oxidation potential solvent has advantages of high oxidation resistance and non-flammability, and can overcome disadvantages of poor oxidation resistance, easy high-pressure decomposition and gas generation, a low flash point, easy combustion and the like of conventional carbonate solvents, the nonaqueous electrolyte of this application may greatly improve safety performance such as overcharge safety and hot box safety of a lithium-ion battery, and may also improve electrochemical performance such as storage life and cycle life of a high-temperature and high-voltage battery system. 
     However, although the high oxidation potential solvent has the advantages of high oxidation resistance and non-flammability, the high oxidation potential solvent has poor compatibility with a negative electrode and may have side reactions on the negative electrode. In addition, as viscosity of the high oxidation potential solvent is higher than that of the carbonate solvent, after the high oxidation potential solvent is added, overall viscosity of the nonaqueous electrolyte increases substantially, ionic conductivity decreases, and electrochemical performance such as cycle life and kinetic performance of the lithium-ion battery might be adversely affected. However, the additive of fluorinated cyclic carbonate can first form a stable interface protective film on the negative electrode, thereby inhibiting side reactions of the high oxidation potential solvent on the negative electrode, and improving electrochemical performance such as cycle life of the lithium-ion battery. In addition, impedance of the interface protective film formed by the fluorinated cyclic carbonate on the negative electrode is also higher, and impedance of continuous reactions of the oxidation potential solvent is low, so the nonaqueous electrolyte can also improve kinetic performance of the lithium-ion battery. In addition, the additive of fluorinated cyclic carbonate, as a special cyclic carbonate, can not only form a film on a surface of the negative electrode, but also is characterized by a high dielectric constant. After being added, the additive can improve solubility of the nonaqueous solvent to the lithium salt and effectively alleviate a defect of a low dielectric constant of the high oxidation potential solvent, thereby effectively improving ionic conductivity in the nonaqueous electrolyte, and helping to obtain a lithium-ion battery with good kinetic performance. 
     In the nonaqueous electrolyte of this application, if a weight percentage of the high oxidation potential solvent is low, the effect in alleviating the disadvantages of carbonate solvents such as poor oxidation resistance, easy high-pressure decomposition and gas generation, a low flash point and easy combustion is not obvious. If the weight percentage of the high oxidation potential solvent is high, overall viscosity of the nonaqueous electrolyte increases greatly, and the ionic conductivity decreases, which has greater impact on kinetic performance of the lithium-ion battery. As such, in some embodiments, based on a total weight of the nonaqueous electrolyte, the weight percentage of the high oxidation potential solvent is 10% to 60%. In this case, the advantages of high oxidation resistance and non-flammability of the high oxidation potential solvent may be better combined with the advantages of low viscosity and a high dielectric constant of carbonate solvents. Therefore, not only safety performance such as overcharge safety and hot box safety and electrochemical performance under high temperature and high voltage of the lithium-ion battery may be improved, but also the lithium-ion battery may be ensured to have some kinetic performance. Further, based on the total weight of the nonaqueous solvent, the weight percentage of the high oxidation potential solvent is 20% to 40%. 
     In the nonaqueous electrolyte of this application, when a weight percentage of the carbonate solvent is low, the effect in alleviating the disadvantages of the high oxidation potential solvent, such as high viscosity, is not obvious, and overall viscosity of the nonaqueous electrolyte is higher and ionic conductivity is lower, thereby having greater impact on kinetic performance of the lithium-ion battery. When the weight percentage of the carbonate solvent is high, the nonaqueous electrolyte has poor oxidation resistance, easy high-pressure decomposition and gas generation, a low flash point, and easy combustion, thereby having greater impact on safety performance such as overcharge safety and hot box safety of the lithium-ion battery. As such, in some embodiments, based on a total weight of the nonaqueous electrolyte, the weight percentage of the carbonate solvent is 40% to 90%. In this case, the advantages of high oxidation resistance and non-flammability of the high oxidation potential solvent may be better combined with the advantages of low viscosity and a high dielectric constant of carbonate solvents. Therefore, not only safety performance such as overcharge safety and hot box safety and electrochemical performance under high temperature and high voltage of the lithium-ion battery may be improved, but also the lithium-ion battery may be ensured to have some kinetic performance. Further, based on the total weight of the nonaqueous solvent, the weight percentage of the carbonate solvent is 60% to 80%. 
     In the nonaqueous electrolyte of this application, when content of the fluorinated cyclic carbonate is low, side reactions of the high oxidation potential solvent on the negative electrode may not be completely suppressed, affecting the effect in improving performance of the lithium-ion battery. When content of the fluorinated cyclic carbonate is high, the fluorinated cyclic carbonate reacts on the positive electrode and generates gas, thereby also affecting the effect in improving performance of the lithium-ion battery. In some embodiments, based on the total weight of the nonaqueous electrolyte, the weight percentage of the fluorinated cyclic carbonate is 0.1% to 10%. Further, based on the total weight of the nonaqueous electrolyte, the weight percentage of the fluorinated cyclic carbonate is 0.5% to 5%. 
     In the nonaqueous electrolyte of this application, in some embodiments, the high oxidation potential solvent contains at least one F atom, and presence of the F atom may better improve the oxidation resistance and flame retardancy of the high oxidation potential solvent. 
     In the nonaqueous electrolyte of this application, in some embodiments, in formula I, R 1  and R 2  are independently selected from unsubstituted, partially fluorinated or fully fluorinated alkyl groups having 1 to 5 carbon atoms, and at least one of R 1  and R 2  is a partially fluorinated or fully fluorinated alkyl group having 1 to 5 carbon atoms. Further, R 1  and R 2  are independently selected from —CH 3,  —CF 3,  —CH 2  CH 3,  —CF 2  CH 3,  —CH 2  CF 3,  —CF 2  CF 3,  —CH 2  CH 2  CH 3,  —CF 2  CH 2  CH 3,  —CH 2  CH 2  CF 3,  —CH 2  CF 2  CF 3,  —CF 2  CH 2  CF 3,  —CF 2  CF 2  CH 3,  and —CF 2  CF 2  CF 3,  and at least one of R 1  and R 2  is —CF 3,  —CF 2  CH 3,  —CH 2  CF 3,  —CF 2  CF 3,  —CF 2  CH 2  CH 3,  —CH 2  CH 2  CF 3,  —CH 2  CF 2  CF 3,  —CF 2  CH 2  CF 3,  —CF 2  CF 2  CH 3,  or —CF 2  CF 2  CF 3.    
     In the nonaqueous electrolyte of this application, in some embodiments, in formula II, R 3  is selected from partially fluorinated or fully fluorinated alkylidene groups having 1 to 6 carbon atoms. Further, R 3  is selected from —CHFCH 2  CH 2  CH 2  —, —CF 2  CH 2  CH 2  CH 2  —, —CF 2  CH 2  CH 2  CHF—, —CF 2  CH 2  CH 2  CF 2  —, —CH 2  CH 2  CHFCH 2  —, —CH 2  CHFCHFCH 2  —, —CH 2  CH 2  CH(CF 3 )CH 2  —, —CF 2  CH 2  CH 2  CH 2  CH 2  —, —CF 2  CH 2  CH 2  CH 2  CF 2  —, —CH 2  CH 2  CH 2  CHFCH 2  —, —CH 2  CHFCH 2  CHFCH 2  —, —CH 2  CHFCH 2  CHFCHF—, —CH 2  CH 2  CH 2  CH 2  CHF—, —CH 2  CH 2  CH 2  CH(CF 3 )CH 2  —, —CF 2  CH 2  CH 2  CH 2  CH 2  CH 2  —, —CH 2  CH 2  CH 2  CH 2  CHFCH 2  —, —CH 2  CHFCH 2  CH 2  CHFCH 2  —, —CF 2  CH 2  CH 2  CH 2  CH 2  CF 2  —, —CH 2  CH 2  CH(CH 3 )CH 2  CHFCH 2  —, and —CH 2  CH 2  CH(CF 3 )CH 2  CHFCH 2  —. 
     In the nonaqueous electrolyte of this application, when the substituents R 1  and R 2  have a large number of carbon atoms and a large molecular weight, the high oxidation potential solvent normally has high viscosity, and the nonaqueous electrolyte may have reduced overall ionic conductivity. This will affect the effect in improving electrochemical performance such as kinetic performance and cycle life of the lithium-ion battery. In some embodiments, R 1  and R 2  are independently selected from unsubstituted, partially halogenated or fully halogenated alkyl groups having 1 to 3 carbon atoms, and at least one of R 1  and R 2  is a partially halogenated or fully halogenated alkyl group having 1 to 3 carbon atoms. Further, R 1  and R 2  are independently selected from unsubstituted, partially fluorinated or fully fluorinated alkyl groups having 1 to 3 carbon atoms, and at least one of R 1  and R 2  is a partially fluorinated or fully fluorinated alkyl group having 1 to 3 carbon atoms. 
     In the nonaqueous electrolyte of this application, when the substituent R 3  has a large number of carbon atoms and a large molecular weight, the high oxidation potential solvent normally has high viscosity, and the nonaqueous electrolyte may have reduced overall ionic conductivity. This will affect the effect in improving electrochemical performance such as kinetic performance and cycle life of the lithium-ion battery. In some embodiments, R 3  is selected from partially halogenated or fully halogenated alkylidene groups having 1 to 4 carbon atoms. Further, R 3  is selected from partially fluorinated or fully fluorinated alkylidene groups having 1 to 4 carbon atoms. 
     In the nonaqueous electrolyte of this application, in some embodiments, the high oxidation potential solvent may be specifically selected from one or more of the following compounds: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In the nonaqueous electrolyte of this application, in some embodiments, the fluorinated cyclic carbonate is one or two of fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC). 
     In the nonaqueous electrolyte of this application, the high oxidation potential solvent is selected from one or more of the compounds represented by formula I and formula II. By performing comparison between the two, the compound represented by formula I is characterized by lower viscosity and a lower dielectric constant, while the compound represented by formula II is characterized by higher viscosity and a higher dielectric constant. Therefore, in some embodiments, the high oxidation potential solvent includes both the compound represented by formula I and the compound represented by formula II. Further, the high oxidation potential solvent may only include the compound represented by formula I. 
     In the nonaqueous electrolyte of this application, in some embodiments, a weight of the compound represented by formula I accounts for 30% to 100% of a total weight of the high oxidation potential solvent, and a weight of the compound represented by formula II accounts for 0% to 70% of the total weight of the high oxidation potential solvent. 
     In the nonaqueous electrolyte of this application, the carbonate solvent is a linear carbonate, or a mixture of cyclic carbonate and linear carbonate. The linear carbonate is characterized by low viscosity, and a defect of high viscosity of the high oxidation potential solvent may be effectively alleviated by adding the linear carbonate. The cyclic carbonate is characterized by a high dielectric constant. After the cyclic carbonate is added, solubility of the nonaqueous solvent to a lithium salt may be effectively improved, and a defect of a rather low dielectric constant of the high oxidation potential solvent is effectively improved. This may effectively improve ionic conductivity of the nonaqueous electrolyte and contribute to obtaining a lithium-ion battery with good kinetic performance. 
     In some embodiments, based on the total weight of the nonaqueous solvent, a weight percentage of the cyclic carbonate is 0% to 10%. The weight percentage of the cyclic carbonate may be 0. In this case, the carbonate solvent is only a linear carbonate, and does not include a cyclic carbonate. The cyclic carbonate is characterized by a high dielectric constant. After being added, the cyclic carbonate may increase solubility of the nonaqueous solvent to the lithium salt, further reduce overall viscosity of the nonaqueous electrolyte, and increase ionic conductivity of nonaqueous electrolyte. However, the cyclic carbonate is prone to oxidize and generate gas on the positive electrode, and heat release is higher. Therefore, when content of the cyclic carbonate is high, the effect in improving storage performance and safety performance of the lithium-ion battery may be affected. Further, when the carbonate solvent is a mixture of cyclic carbonate and linear carbonate, based on the total weight of the nonaqueous solvent, the weight percentage of the cyclic carbonate is greater than 0 and less than or equal to 10%, and the weight percentage of the cyclic carbonate is further 0.5% to 7%. 
     In some embodiments, the weight percentage of the cyclic carbonate is 0%. 
     The cyclic carbonate may be selected from one or more of compounds represented by formula IV, and the linear carbonate may be selected from one or more of compounds represented by formula V. In formula IV, R 11  is selected from unsubstituted alkyl groups having 1 to 5 carbon atoms. In formula V, R 12  and R 13  are selected from unsubstituted alkyl groups having 1 to 5 carbon atoms, and R 12  and R 13  may be the same, or may be different. In R 11 , R 12  and R 13,  the alkyl group may be straight-chained or branched. 
     
       
         
         
             
             
         
       
     
     In some embodiments, the cyclic carbonate may be specifically selected from one or more of ethylene carbonate and propylene carbonate. 
     In some embodiments, the linear carbonate may be specifically selected from one or more of ethyl methyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, ethylene propyl carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate and dibutyl carbonate. 
     In the nonaqueous electrolyte of this application, as viscosity of the high oxidation potential solvent is typically larger than that of the carbonate solvent, ionic conductivity of the nonaqueous electrolyte may be easily affected, thereby affecting charge and discharge capacity, cycle life, and kinetic performance of the lithium-ion battery. Therefore, ionic conductivity of the nonaqueous electrolyte may be improved by using the high oxidation potential solvent in combination with a suitable type of carbonate solvent to further improve charge and discharge capacity, cycle life, and kinetic performance of the lithium-ion battery. In some embodiments, room-temperature ionic conductivity of the nonaqueous electrolyte is controlled to be greater than or equal to 5.0 mS/cm. Among carbonate solvents, linear carbonate is generally characterized by low viscosity. As such, in some embodiments, the carbonate solvent includes at least a linear carbonate. Further, the carbonate solvent includes at least one of ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. The linear carbonate has even lower viscosity, and therefore may effectively make up a defect of low ionic conductivity of the nonaquous electrolyte due to high viscosity of the high oxidation potential solvent. 
     In the nonaqueous electrolyte of this application, a specific type of the lithium salt is not specifically limited. The lithium salt may be any lithium salt used in existing batteries. For example, the lithium salt may be specifically selected from one or more of LiPF 6,  LiN(CF 3 SO 2  ) 2 ,  LiN(FSO 2  ) 2 ,  LiBF 4,  LiCF 3 SO 3,  and LiClO 4.  In order to further improve kinetic performance of the lithium-ion battery, in some embodiments, the lithium salt is LiPF 6 . 
     In the nonaqueous electrolyte of this application, specific concentration of the lithium salt is not specifically limited either, and may be adjusted according to an actual need. For example, concentration of the lithium salt may specifically be 0.7 mol/L to 2 mol/L. 
     In the nonaqueous electrolyte of this application, in some embodiments, the nonaqueous electrolyte may further include a film-forming additive, and the film-forming additive helps to form a protective film of superior performance on a surface of the negative electrode or the positive electrode, thereby further improving electrochemical performance such as kinetic performance, cycle life, and storage life of the lithium-ion battery. 
     In some embodiments, based on the total weight of the nonaqueous electrolyte, a weight percentage of the film-forming additive is 0.01% to 10%. Further, based on the total weight of the nonaqueous electrolyte, the weight percentage of the film-forming additive is 0.1% to 5%. 
     In some embodiments, the film-forming additive may be specifically selected from one or more of a cyclic carbonate compound with an unsaturated bond, a sulfate compound, a sulfite compound, a sultone compound, a disulfonate compound, a nitrile compound, an aromatic compound, an isocyanate compound, a phosphazene compound, a cyclic anhydride compound, a phosphite compound, a phosphate compound, a borate compound, and a carboxylate compound. 
     Further, the film-forming additive may be specifically selected from one or more of 1,3-propane sultone (PS), ethylene sulfate (DTD), vinylene carbonate (VC), 1,3-propene sultone (PES), adiponitrile (ADN), and succinonitrile (SN). These types of film-forming additives help to form a stable protective film on a surface of the negative electrode or the positive electrode, and effectively inhibit side reactions of the high oxidation potential solvent on the negative electrode, thereby effectively improving electrochemical performance such as kinetic performance, cycle life, and storage life of the lithium-ion battery. 
     Further, the film-forming additive includes at least one of PS and DTD. Because both PS and DTD can form a film on a surface of the positive electrode, oxidation resistance of the nonaqueous electrolyte may be further improved, and gas generation may be reduced, thereby further improving storage performance (especially high-temperature storage performance) and safety performance such as over charge safety and hot box safety of the lithium-ion battery. In particular, DTD can not only form a film on a surface of the positive electrode, but also form a stable interface protective film on the negative electrode, inhibiting side reactions of the high oxidation potential solvent on the negative electrode, thereby better improving cycle performance and kinetic performance of the lithium-ion battery. 
     Next, a lithium-ion battery according to a second aspect of this application is described. 
       FIG. 1  is a perspective view of an embodiment of a lithium-ion battery  5 .  FIG. 2  is an exploded view of  FIG. 1 .  FIG. 3  is a schematic diagram of an embodiment of an electrode assembly  52  of the lithium-ion battery  5  in  FIG. 2 , in which a first electrode plate  521 , a second electrode plate  522 , and a separator  523  are wound to form a wound electrode assembly.  FIG. 4  is a schematic diagram of another embodiment of an electrode assembly  52  of the lithium-ion battery  5  in  FIG. 2 , in which a first electrode plate  521 , a second electrode plate  522 , and a separator  523  are laminated along a thickness direction to form a laminated electrode assembly. 
     With reference to  FIG. 1  to  FIG. 4 , the lithium-ion battery  5  includes a housing  51 , the electrode assembly  52 , a top cover assembly  53 , and an electrolyte (not shown). 
     The electrode assembly  52  is accommodated in the housing  51 . The electrode assembly  52  includes the first electrode plate  521 , the second electrode plate  522 , the separator  523 , a first tab  524 , and a second tab  525 . The separator  523  separates the first electrode plate  521  from the second electrode plate  522 . 
     The first electrode plate  521  includes a first current collector  521   a  and a first active material layer  521   b  provided on a surface of the first current collector  521   a . The first active material layer  521   b  contains a first active material. The first active material layer  521   b  may be provided on one surface or two surfaces of the first current collector  521   a  depending on an actual need. The second electrode plate  522  includes a second current collector  522   a  and a second active material layer  522   b  provided on a surface of the second current collector  522   a  . The second active material layer  522   b  may be provided on one surface or two surfaces of the second current collector  522   a  depending on an actual need. The second active material layer  522   b  contains a second active material. The first active material and the second active material implement deintercalation of lithium ions. Electrical polarities of the first electrode plate  521  and the second electrode plate  522  are opposite. To be specific, one of the first electrode plate  521  and the second electrode plate  522  is a positive electrode plate, and the other of the first electrode plate  521  and the second electrode plate  522  is a negative electrode plate. The first tab  524  may be formed by cutting the first current collector  521   a , or may be formed separately and fixedly connected to the first current collector  521   a . Similarly, the second tab  525  may be formed by cutting the second current collector  522   a  , or may be formed separately and fixedly connected to the second current collector  522   a.    
     A quantity of the electrode assemblies  52  is not limited, and may be one or more. 
     The electrolyte is injected into the housing  51  and impregnates the electrode assembly  52 . Specifically, the electrolyte impregnates the first electrode plate  521 , the second electrode plate  522 , and the separator  523 . 
     It is noted that the lithium-ion battery  5  shown in  FIG. 1  is a tank type battery, but is not limited thereto. The lithium-ion battery  5  may be a pouch type battery, which means that the housing  51  is replaced by a metal plastic film and the top cover assembly  53  is eliminated. 
     In the lithium-ion battery  5 , as one of the first electrode plate  521  and the second electrode plate  522  is the positive electrode plate, a current collector of the positive electrode plate is a positive current collector, an active material layer of the positive electrode plate is a positive active material layer, and an active material of the positive electrode plate is a positive active material. As such, the positive electrode plate includes the positive current collector and the positive active material layer provided on a surface of the positive current collector. 
     In another word, the lithium-ion battery according to the second aspect of this application includes a positive electrode plate, a negative electrode plate, a separator and the nonaqueous electrolyte according to the first aspect of this application. 
     In the lithium-ion battery in this application, the positive electrode plate may include a positive active material, a conductive agent and a binder. The positive active material may be selected from a layered lithium-containing oxide, a spinel-type lithium-containing oxide, and the like. Specifically, the positive active material may be selected from one or more of a lithium cobalt oxide, a lithium nickel oxide, a lithium manganese oxide, a lithium nickel manganese oxide, a lithium nickel cobalt manganese oxide, and a lithium nickel cobalt aluminum oxide. However, this application is not limited to these materials, and other conventionally well-known materials that can be used as a positive active material of a lithium-ion battery may also be used. One type of these positive active materials may be used alone, or two or more types may be used in combination, at a combination ratio adjustable depending on an actual need. Types of the conductive agent and the binder are not specifically limited, and may be selected depending on an actual need. 
     When voltage of the lithium-ion battery is higher, the lithium-ion battery may have higher charge and discharge capacity and higher energy density, but a conventional electrolyte may be oxidized to produce gas and release heat. This may change service life and high-temperature performance of the lithium-ion battery, especially in the process of abuse. For example, at a 150° C. hot box, the lithium-ion battery may easily burn. However, the nonaqueous electrolyte of this application contains a high oxidation potential solvent, which can greatly improve oxidation resistance of the nonaqueous electrolyte and reduce heat release, thereby effectively improving high-temperature performance of the lithium-ion battery. As such, the nonaqueous electrolyte in this application can more significantly improve performance of a battery system with high positive electrode oxidation or high positive electrode oxidation potential, especially electrochemical performance of the lithium-ion battery under high temperature and high voltage. Gas generation of the lithium-ion battery under high temperature and high voltage may be even more significantly suppressed, and safety performance such as overcharge safety and hot box safety of the lithium-ion battery may also be more significantly improved. 
     In some embodiments, the positive active material is one or more of Li 1+x Ni a Co b M′ (1−a−b) O 2−c Y c , and Li 1+y Ni m Mn n M″ 2−m−n O 4−p Z p , where −0.1≤x≤0.2, 0.6≤a≤1, 0≤b&lt;1, 0≤(1−ab)&lt;1, 0≤c&lt;1, M′ is selected from one or more of Mn, Al, Mg, Zn, Ga, Ba, Fe, Cr, Sn, V, Sc, Ti, and Zr, and Y is selected from one or more of F, Cl, and Br; −0.1≤y≤0.2, 0.4≤m≤1.2, 0.8≤n≤1.6, 0≤(2−m−n)≤0.3, 0≤p≤1, M″ is selected from one or more of Al, Mg, Zn, Ga, Ba, Fe, Cr, Sn, V, Sc, Ti, and Zr, and Z is selected from one or more of F, Cl, and Br. Further, the positive active material may be specifically selected from one or more of LiNi 0.6 Co 0.2 Mn 0.2 O 2,  LiNi 0.8 Co 0.1 Mn 0.1 O 2,  LiNi 0.85 Co 0.15 Al 0.05 O 2 ,  and LiNi 0.5 Mn. 1.5 O 4.  As Ni content in the positive active material increases, the positive active material has increased charge and discharge capacity, but reduced thermal stability. As such, the positive active material may release a strong oxidizing substance under high temperature, oxidizing the nonaqueous electrolyte and deteriorating high-temperature performance of the lithium-ion battery. The nonaqueous electrolyte of this application contains an oxidation resistant high oxidation potential solvent, which can greatly improve oxidation resistance of the nonaqueous electrolyte and reduce heat release, thereby effectively improving high-temperature performance of the lithium-ion battery 
     In the lithium-ion battery in this application, in some embodiments, an end-of-charge voltage of the lithium-ion battery is U, where 4.3V≤U≤6V. That is, the nonaqueous electrolyte in this application may increase the end-of-charge voltage of the lithium-ion battery to 4.3V or more. 
     In the lithium-ion battery in this application, the negative electrode plate may include a negative active material, a conductive agent and a binder. The negative active material may be selected from a carbon-based material, a silicon-based material, a tin-based material, and the like. Specifically, the negative active material may be selected from soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide, silicon carbon composite, silicon alloy, tin, tin oxide, tin alloy, lithium titanate, a metal that can form an alloy with lithium, and the like. However, this application is not limited to these materials, and other conventionally well-known materials that can be used as a negative active material of a lithium-ion battery may also be used. One type of these negative active materials may be used alone, or two or more types may be used in combination, at a combination ratio adjustable depending on an actual need. Types of the conductive agent and the binder are not specifically limited, and may be selected depending on an actual need. 
     In the lithium-ion battery of this application, a specific type of the separator is not specifically limited, and the separator may be made of any separator material used in existing batteries, such as a polyolefin separator, a ceramic separator, or the like. Specifically, the separator may be a polyethylene film, a polypropylene film, a polyvinylidene fluoride film, or a multilayer composite film thereof, but this application is not limited thereto. 
     A battery module according to a third aspect of this application is described next. 
       FIG. 5  is a perspective view of an embodiment of a battery module  4 . 
     The battery module  4  provided in the third aspect of this application includes the lithium-ion battery  5  according to the third aspect of this application. 
     Referring to  FIG. 5 , the battery module  4  includes a plurality of lithium-ion batteries  5 . The plurality of lithium-ion batteries  5  are arranged in a longitudinal direction. The battery module  4  may be used as a power source or an energy storage apparatus. A quantity of lithium-ion batteries  5  included in the battery module  4  may be adjusted based on use and capacity of the battery module  4 . 
     A battery pack according to a fourth aspect of this application is described next. 
       FIG. 6  is a perspective view of an embodiment of a battery pack  1 .  FIG. 7  is an exploded view of  FIG. 6 . The battery pack  1  provided in the fourth aspect of this application includes the battery module  4  according to the third aspect of this application. 
     Specifically, referring to  FIG. 6  and  FIG. 7 , the battery pack  1  includes an upper box body  2 , a lower box body  3 , and the battery module  4 . The upper box body  2  and the lower box body  3  are assembled together to form a space for accommodating the battery module  4 . The battery module  4  is disposed in the space formed by the upper box body  2  and the lower box body  3  that are assembled together. An output electrode of the battery module  4  penetrates through one or both of the upper box body  2  and the lower box body  3  to output power or to charge from an outer source. A quantity and an arrangement of the battery modules  4  used in the battery pack  1  may be determined depending on an actual need. The battery pack  1  may be used as a power supply or an energy storage apparatus. 
     An apparatus according to a fifth aspect of this application is described next. 
       FIG. 8  is a schematic diagram of an embodiment of an apparatus using a lithium-ion battery as a power source. 
     The apparatus provided in the fifth aspect of this application includes the lithium-ion battery  5  according to the third aspect of this application, where the lithium-ion battery  5  may be used as a power source for the apparatus. In  FIG. 8 , the apparatus using the lithium-ion battery  5  is an electric automobile. The apparatus using the lithium-ion battery  5  is obviously not limited to this, but may be any electric vehicles other than electric automobiles (for example, an electric bus, an electric tram, an electric bicycle, an electric motorcycle, an electric scooter, an electric golf cart, and an electric truck), an electric vessel, an electric tool, an electronic device, and an energy storage system. The electric automobile may be a full electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. Certainly, depending on an actual use form, the apparatus provided in the fifth aspect of this application may include the battery module  4  according to the third aspect of this application. Certainly, the apparatus provided in the fifth aspect of this application may also include the battery pack  1  according to the fourth aspect of this application. 
     To make the objectives, technical solutions, and beneficial technical effects of this application clearer, this application is further described below in detail with reference to embodiment examples. It should be understood that the embodiment examples described in this specification are merely intended to explain this application, but not to limit this application. Formulations, proportions, and the like in the embodiment examples may be selected as appropriate to local conditions, which has no substantial effect on results. 
     For ease of explanation, reagents used in preparation of nonaqueous electrolyte are abbreviated as follows: 
     
       
         
         
             
             
         
       
     
     ethylene carbonate EC, propylene carbonate PC, ethyl methyl carbonate EMC, diethyl carbonate DEC, fluoroethylene carbonate FEC, difluoroethylene carbonate DFEC, 1,3-propane sultone PS, vinylene carbonate VC 
     EXAMPLE 1 
     (1) Preparation of a Nonaqueous Electrolyte 
     A compound Al and ethyl methyl carbonate (EMC) were mixed at a weight ratio of 30:70 and used as a nonaqueous solvent, and then 1 mol/L of LiPF 6  was then dissolved as a lithium salt. After that, 0.1% FEC was added to prepare a nonaqueous electrolyte. 
     (2 ) Preparation of a Positive Electrode Plate 
     A positive active material LiNi 0.8 Co 0.1 Mn 0.1 O 2,  an acetylene black conductive agent, and a polyvinylidene fluoride (PVDF) binder were fully stirred and uniformly mixed in an N-methylpyrrolidone solvent system at a weight ratio of 94:3:3, and then the mixture was applied onto a current collector Al foil, followed by drying and cold pressing to obtain a positive electrode plate. 
     (3) Preparation of a Negative Electrode Plate 
     A negative active material artificial graphite, an acetylene black conductive agent, a styrene-butadiene rubber binder, a sodium carboxymethyl cellulose thickener were fully stirred and uniformly mixed at a weight ratio of 95:2:2:1 in a deionized water solvent system, and then the mixture was applied onto a current collector Cu foil, followed by drying and cold pressing to obtain a negative electrode plate. 
     (4) Preparation of a Separator 
     A polyethylene film was used as a separator. 
     (5) Preparation of a Lithium-Ion Battery 
     The positive electrode plate, the separator, and the negative electrode plate were laminated in order, so that the separator was interposed between the positive electrode plate and negative electrode plate to provide separation. Then the laminated product was wound to obtain an electrode assembly. The electrode assembly was placed in an outer package and dried, and the nonaqueous electrolyte was then injected. Then, after vacuum packaging, standing, chemical conversion, shaping, and other processes, a lithium-ion battery was obtained. 
     Lithium-ion batteries in embodiment examples 2 to 29 and comparative examples 1 to 4 are all prepared according to a method similar to that in Example 1, and specific differences are shown in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Compositions of the nonaqueous electrolytes prepared in Examples 1 to 29 and 
               
               
                 Comparative Examples 1 to 4 
               
            
           
           
               
               
               
               
            
               
                   
                 Composition of nonaqueous solvents and weight 
                   
                   
               
               
                   
                 percentages of .ndividual components 
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 High oxidation potential 
                 Linear 
                 Cyclic 
                 Additive 
                 Film-forming additive 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 solvent 
                 carbonate 
                 carbonate 
                 Component 
                 Percentage 
                 Component 
                 Percentage 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 A1 
                 30% 
                 EMC 
                 70% 
                 / 
                 / 
                 FEC 
                 0.1%   
                 / 
                 / 
               
               
                 Example 2 
                 A1 
                 30% 
                 EMC 
                 70% 
                 / 
                 / 
                 FEC 
                 0.5%   
                 / 
                 / 
               
               
                 Example 3 
                 A1 
                 30% 
                 EMC 
                 70% 
                 / 
                 / 
                 FEC 
                 1% 
                 / 
                 / 
               
               
                 Example 4 
                 A1 
                 30% 
                 EMC 
                 70% 
                 / 
                 / 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 5 
                 A1 
                 30% 
                 EMC 
                 70% 
                 / 
                 / 
                 FEC 
                 5% 
                 / 
                 / 
               
               
                 Example 6 
                 A1 
                 30% 
                 EMC 
                 70% 
                 / 
                 / 
                 FEC 
                 8% 
                 / 
                 / 
               
               
                 Example 7 
                 A1 
                 30% 
                 EMC 
                 70% 
                 / 
                 / 
                 FEC 
                 10% 
                 / 
                 / 
               
               
                 Example 8 
                 A1 
                 30% 
                 EMC 
                 70% 
                 / 
                 / 
                 FEC 
                 0.05%   
                 / 
                 / 
               
               
                 Example 9 
                 A1 
                 30% 
                 EMC 
                 70% 
                 / 
                 / 
                 FEC 
                 12%  
                 / 
                 / 
               
               
                 Example 10 
                 A1 
                 60% 
                 EMC 
                 35% 
                 EC 
                 5% 
                 DFEC 
                 2% 
                 / 
                 / 
               
               
                 Example 11 
                 A1 
                 50% 
                 EMC 
                 45% 
                 EC 
                 5% 
                 DFEC 
                 2% 
                 / 
                 / 
               
               
                 Example 12 
                 A1 
                 40% 
                 EMC 
                 55% 
                 EC 
                 5% 
                 DFEC 
                 2% 
                 / 
                 / 
               
               
                 Example 13 
                 A1 
                 30% 
                 EMC 
                 65% 
                 EC 
                 5% 
                 DFEC 
                 2% 
                 / 
                 / 
               
               
                 Example 14 
                 A1 
                 20% 
                 EMC 
                 75% 
                 EC 
                 5% 
                 DFEC 
                 2% 
                 / 
                 / 
               
               
                 Example 15 
                 A1 
                 10% 
                 EMC 
                 85% 
                 EC 
                 5% 
                 DFEC 
                 2% 
                 / 
                 / 
               
               
                 Example 16 
                 A1 
                 70% 
                 EMC 
                 20% 
                 EC 
                 10%  
                 DFEC 
                 2% 
                 / 
                 / 
               
               
                 Example 17 
                 A1 
                  5% 
                 EMC 
                 85% 
                 EC 
                 10%  
                 DFEC 
                 2% 
                 / 
                 / 
               
               
                 Example 18 
                 A2 
                 30% 
                 DEC 
                 65% 
                 PC 
                 5% 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 19 
                 A3 
                 30% 
                 DEC 
                 65% 
                 PC 
                 5% 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 20 
                 A4 
                 30% 
                 DEC 
                 65% 
                 PC 
                 5% 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 21 
                 AS 
                 30% 
                 DEC 
                 65% 
                 PC 
                 5% 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 22 
                 A6 
                 30% 
                 DEC 
                 65% 
                 PC 
                 5% 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 23 
                 A7 
                 30% 
                 DEC 
                 65% 
                 PC 
                 5% 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 24 
                 A1:A6 = 70:30  
                 30% 
                 DEC 
                 65% 
                 PC 
                 5% 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 25  
                 A2:A7 = 60:40 
                 30% 
                 DEC 
                 65% 
                 PC 
                 5% 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 26 
                 A1 
                 30% 
                 EMC 
                 65% 
                 EC 
                 5% 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 27 
                 A1 
                 30% 
                 EMC 
                 65% 
                 EC 
                 5% 
                 FEC 
                 2% 
                 PS 
                 2% 
               
               
                 Example 28 
                 A1 
                 30% 
                 EMC 
                 65% 
                 EC 
                 5% 
                 FEC 
                 2% 
                 DTD 
                 2% 
               
               
                 Example 29 
                 A1 
                 30% 
                 EMC 
                 65% 
                 EC 
                 5% 
                 FEC 
                 2% 
                 PS + DTD 
                 1% + 1% 
               
               
                 Comparative 
                 / 
                 / 
                 EMC 
                 70% 
                 EC 
                 30%  
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 1 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 A1 
                 100%  
                 / 
                 / 
                 / 
                 / 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 Methyl ethyl 
                 30% 
                 EMC 
                 65% 
                 EC 
                 5% 
                 FEC 
                 2% 
                 / 
                 / 
               
               
                 Example 3 
                 sulfone 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 A1 
                 30% 
                 EMC 
                 65% 
                 EC 
                 5% 
                 PS 
                 2% 
                 / 
                 / 
               
               
                 Example 4 
               
               
                   
               
            
           
         
       
     
     Performance tests for the lithium-ion batteries are described next. 
     (1) High-Temperature Storage Gas Generation Test 
     Five lithium-ion batteries prepared in the Examples and five lithium-ion batteries prepared in the Comparative Examples were taken and placed at room temperature, charged to 4.3V at a constant current of 0.5C (that is, a current value at which the battery completely discharges its theoretical capacity in  2  h), and then charged at a constant voltage of 4.3V until that the current was lower than 0.05C, such that the lithium-ion batteries were in a 4.3V fully charged state. A volume of a fully charged battery before storage was denoted as D 0 . The fully charged battery was placed in an oven at 85° C., taken out after 10 days, and tested for a volume after storage, where the volume after storage was denoted as D 1 . 
     A volume swelling ratio of the lithium-ion battery was ε=(D1−D0)/D0 ×100%. 
     (2 ) Thermal Shock Safety Performance (Hot Box) Test 
     Five lithium-ion batteries prepared in the Examples and five lithium-ion batteries prepared in the Comparative Examples were taken and placed at room temperature, charged to 4.3V at a constant current of 0.5C, and then charged at a constant voltage of 4.3V until the current was lower than 0.05C. The lithium-ion batteries were then placed in a thermostat, and the thermostat was heated to 150° C. at a heating rate of 5° C./min. Time h 1  required for the thermostat to heat up from room temperature to 150° C. was recorded. The lithium-ion battery was then baked in the 150° C. thermostat until the lithium-ion battery caught smoke and a fire. Time h2 from when the thermostat was heated up from room temperature until when the lithium-ion battery caught smoke and a fire was recorded. 
     The thermal shock safety performance of the lithium-ion battery was characterized by time (h2 -h1) for which the lithium-ion battery withstood baking at 150° C. 
     (3) Cycle Performance Test 
     Five lithium-ion batteries prepared in the Examples and five lithium-ion batteries prepared in Comparative Examples were taken, and were repeatedly charged and discharged through the following steps, and discharge capacity retention rates of the lithium-ion batteries were calculated. 
     First, in a room temperature environment, a first cycle of charge and discharge was performed, where the lithium-ion batteries were charged at a constant current of 0.5C to an upper limit voltage of 4.3V, and then charged at a constant voltage of 4.3V until the current was lower than 0.05C, and then discharged at a constant discharge current of 0.5C until a final voltage was 3V. A discharge capacity of the first cycle was recorded. 500 charge and discharge cycles were performed according to the above operations and a discharge capacity of the 500 th  cycle was recorded. 
     Cycle capacity retention rate of the lithium-ion battery =(Discharge capacity at the 500 th  cycle/Discharge capacity at the first cycle)×100%. 
     (4) Kinetic Performance Test 
     Five lithium-ion batteries prepared in the Examples and five lithium-ion batteries prepared in the Comparative Examples were taken, and then tested in a room temperature environment. First, the lithium-ion batteries were charged at a constant current of 0.5C to an upper limit voltage of 4.3V, and then charged at a constant voltage of 4.3V until the current was lower than 0.05C, and then discharged at a constant current with different rates (0.5C,  2  C) until a final voltage was 3V. Discharge capacities at different rates were recorded. 
     The kinetic performance of the lithium-ion battery was characterized by a ratio of a discharge capacity at a rate of  2  C of the lithium-ion battery to a discharge capacity at a rate of 0.5C of the lithium-ion battery. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Performance test results of Examples 1 to 29 and the  
               
               
                 Comparative Examples 1 to 4 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Volume 
                 Hot box safety 
                 Capacity 
                   
               
               
                   
                 swelling  
                 performance  
                 retention rate 
                   
               
               
                   
                 ratio ε 
                 (h2-h1) (min) 
                 after cycling 
                 2C/0.5C 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 15% 
                 51 
                 76% 
                 59% 
               
               
                 Example 2 
                 16% 
                 49 
                 77% 
                 60% 
               
               
                 Example 3 
                 17% 
                 48 
                 79% 
                 62% 
               
               
                 Example 4 
                 19% 
                 49 
                 83% 
                 64% 
               
               
                 Example 5 
                 20% 
                 47 
                 86% 
                 67% 
               
               
                 Example 6 
                 22% 
                 43 
                 84% 
                 65% 
               
               
                 Example 7 
                 26% 
                 38 
                 80% 
                 62% 
               
               
                 Example 8 
                 14% 
                 48 
                 75% 
                 58% 
               
               
                 Example 9 
                 32% 
                 28 
                 71% 
                 43% 
               
               
                 Example 10 
                 17% 
                 57 
                 70% 
                 58% 
               
               
                 Example 11 
                 19% 
                 53 
                 75% 
                 61% 
               
               
                 Example 12 
                 20% 
                 51 
                 80% 
                 64% 
               
               
                 Example 13 
                 21% 
                 48 
                 84% 
                 67% 
               
               
                 Example 14 
                 22% 
                 37 
                 88% 
                 69% 
               
               
                 Example 15 
                 23% 
                 28 
                 89% 
                 76% 
               
               
                 Example 16 
                 16% 
                 61 
                 60% 
                 49% 
               
               
                 Example 17 
                 25% 
                 14 
                 90% 
                 79% 
               
               
                 Example 18 
                 20% 
                 48 
                 84% 
                 68% 
               
               
                 Example 19 
                 21% 
                 47 
                 85% 
                 67% 
               
               
                 Example 20 
                 19% 
                 46 
                 84% 
                 69% 
               
               
                 Example 21 
                 20% 
                 47 
                 83% 
                 68% 
               
               
                 Example 22 
                 18% 
                 50 
                 81% 
                 62% 
               
               
                 Example 23 
                 17% 
                 52 
                 80% 
                 60% 
               
               
                 Example 24 
                 19% 
                 48 
                 82% 
                 65% 
               
               
                 Example 25 
                 19% 
                 49 
                 81% 
                 64% 
               
               
                 Example 26 
                 18% 
                 52 
                 84% 
                 67% 
               
               
                 Example 27 
                 12% 
                 51 
                 84% 
                 61% 
               
               
                 Example 28 
                 13% 
                 56 
                 89% 
                 69% 
               
               
                 Example 29 
                 10% 
                 58 
                 88% 
                 64% 
               
               
                 Comparative 
                 19% 
                 50 
                 83% 
                 64% 
               
               
                 Example 1 
                   
                   
                   
                   
               
               
                 Comparative 
                 12% 
                 54 
                 50% 
                 38% 
               
               
                 Example 2 
                   
                   
                   
                   
               
               
                 Comparative 
                 20% 
                 47 
                 84% 
                 68% 
               
               
                 Example 3 
                   
                   
                   
                   
               
               
                 Comparative 
                  4% 
                 55 
                 73% 
                 55% 
               
               
                 Example 4 
               
               
                   
               
            
           
         
       
     
     It may be seen from the test results of Comparative Examples 1 to 2 and Examples 1 to 17 that, when a mixed solvent of the high oxidation potential solvent and the carbonate solvent was used as the nonaqueous solvent, and the nonaqueous solvent was used in combination of the additive of fluorinated cyclic carbonate, high-temperature storage performance and hot box safety performance of the lithium-ion batteries could be significantly improved, and the lithium-ion batteries also had good cycle performance and kinetic performance. 
     This is because the high oxidation potential solvent had the advantages of oxidation resistance and non-flammability. After the high oxidation potential solvent was mixed with a carbonate solvent, the high oxidation potential solvent could overcome disadvantages of conventional carbonate solvents, such as poor oxidation resistance, easy high-pressure decomposition and gas generation, a low flash point,and easy combustion, thereby greatly improving the high-temperature storage performance and hot box safety performance of the lithium-ion batteries. However, although the high oxidation potential solvent had the advantages of oxidation resistance and non-flammability, the high oxidation potential solvent had poor compatibility with the negative electrode and may have side reactions on the negative electrode. In addition, viscosity of the high oxidation potential solvent was higher than that of the carbonate solvent. After the high oxidation potential solvent was added, overall viscosity of the nonaqueous electrolyte increased substantially, ionic conduction slowed down, and cycle performance and kinetic performance of the lithium-ion batteries might be deteriorated. However, the additive of fluorinated cyclic carbonate could first form a stable interface protective film on the negative electrode, thereby inhibiting side reactions of the high oxidation potential solvent on the negative electrode, and improving cycle performance the lithium-ion batteries. In addition, impedance of the interface protective film formed by the fluorinated cyclic carbonate on the negative electrode was also higher, and impedance of continuous reactions of the oxidation potential solvent was low, so kinetic performance of the lithium-ion batteries might also be improved. In addition, the additive of fluorinated cyclic carbonate was also characterized by a high dielectric constant. After being added, the additive could improve solubility of the nonaqueous solvent to the lithium salt and effectively alleviate a defect of a low dielectric constant of the high oxidation potential solvent, thereby effectively improving ionic conductivity in the nonaqueous electrolyte, and helping to obtain a lithium-ion battery with good kinetic performance. 
     In Comparative Example 1, the nonaqueous solvent was only a conventional carbonate solvent, and did not include the high oxidation potential solvent. Such conventional carbonate solvent had disadvantages such as poor oxidation resistance, easy high-pressure decomposition and gas generation, a low flash point, and easy combustion, so that high-temperature storage performance and hot box safety performance of the lithium-ion battery was poor. In Comparative Example 2 , the nonaqueous solvent was only a high oxidation potential solvent, and did include the carbonate solvent. Such high oxidation potential solvent had higher viscosity, resulting in higher overall viscosity and lower ionic conductivity of the nonaqueous electrolyte, so that cycle performance and kinetic performance of the lithium-ion battery was significantly deteriorated. 
     It may be seen from the test results of Examples 1 to 9 that, when content of the fluorinated cyclic carbonate was low, side reactions of the high oxidation potential solvent on the negative electrode might not be completely suppressed, affecting the effect in improving performance of the lithium-ion batteries. When content of the fluorinated cyclic carbonate was high, the fluorinated cyclic carbonate reacted on the positive electrode and generated gas, thereby also affecting the effect in improving performance of the lithium-ion batteries. As such, in some embodiments, based on the total weight of the nonaqueous solvent, a weight percentage of the fluorinated cyclic carbonate was 0.1% to 10%. 
     It may be seen from the test results of Examples 10 to 17 that, when a weight percentage of the high oxidation potential solvent was low, the effect in alleviating the disadvantages of carbonate solvents such as poor oxidation resistance, easy high-pressure decomposition and gas generation, a low flash point and easy combustion was not obvious. When the weight percentage of the high oxidation potential solvent was high, overall viscosity of the nonaqueous electrolyte increased greatly, and the ionic conductivity decreased, thereby having greater impact on kinetic performance of the lithium-ion batteries. As such, in some embodiments, based on the total weight of the nonaqueous solvent, the weight percentage of the high oxidation potential solvent was 10% to 60%. 
     It may be seen from the test results of Comparative Example 3 and Examples 18 to 25 that the high oxidation potential solvents that had not been fluorinated had poorer oxidation resistance and did not exhibit flame retardancy, so that the effect in improving high-temperature storage performance and hot box safety performance of the lithium-ion batteries was not desirable. 
     It may be also seen from the test results of Examples 18 to 25 that the high oxidation potential solvents of different structures also had some impact on performance of the lithium-ion batteries. The high oxidation potential solvent of a cyclic structure was characterized by higher viscosity and a higher dielectric constant, and the high oxidation potential solvent of a linear structure was characterized by lower viscosity and a lower dielectric constant. Therefore, in some embodiments, the high oxidation potential solvent included both a high oxidation potential solvent of a linear structure and a high oxidation potential solvent of a cyclic structure. Further, the high oxidation potential solvent only included a high oxidation potential solvent of a linear structure. 
     It may be seen from the test results of Comparative Example 4 and Examples 26 to 29 that, when a film-forming additive, such as PS and DTD was further added into the nonaqueous electrolyte containing the high oxidation potential solvent, the carbonate solvent and the fluorinated cyclic carbonate, comprehensive performance of the lithium-ion batteries, in particular the high-temperature storage performance and hot box safety performance of the lithium-ion batteries, were further improved. A possible reason is that both PS and DTD can form a film on a surface of the positive electrode, so that oxidation resistance of the nonaqueous electrolyte may be further improved, and gas generation may be reduced, thereby further improving high-temperature storage performance and hot box safety performance of the lithium-ion battery. In particular, DTD can not only form a film on a surface of the positive electrode, but also form a stable interface protective film on the negative electrode, inhibiting side reactions of the high oxidation potential solvent on the negative electrode, thereby better improving cycle performance and kinetic performance of the lithium-ion battery. 
     Although adding PS directly into the nonaqueous electrolyte that did not contain the fluorinated cyclic carbonate might also improve performance of the lithium-ion batteries to some extent, the effect in comprehensive improvement was poorer than that when the fluorinated cyclic carbonate was added. A possible reason is that the interface protective film formed on the positive electrode when only PS was used could not last for long, so that the interface protective film was continuously oxidized and decomposed during use of the batteries, thereby substantially increasing interface impedance of the positive electrode. In addition, even though PS might also form an interface protective film on the negative electrode, film forming quality was poor, and a thin and dense interface protective film with good kinetics of lithium ion transfer might not likely be formed on the negative electrode when only PS was used, so that the effect in improving comprehensive performance of the lithium-ion batteries, in particular in improving cycle performance and kinetic performance of the lithium-ion batteries by PS was poor. 
     According to the disclosure and teaching of this specification, a person skilled in the art of this application may further make appropriate changes or modifications to the foregoing implementations. Therefore, this application is not limited to the foregoing disclosure and the described specific implementations, and some changes or modifications to this application shall also fall within the protection scope of the claims of this application. In addition, although some specific terms are used in this specification, these terms are used only for ease of description, and do not constitute any limitation on this application.