Electrolyte and secondary battery

Abstract

The present application provides an electrolyte and a secondary battery, wherein the electrolyte contains a flame retardant additive having the general structural formula shown in Formula (I): #imgabs0# wherein R, R1, R2, R3, R4, and R5 are independently selected from one or more of a hydrogen atom, a halogen, an amino group, an amide group, an ester group, an alkyl group, a halogenated alkyl group, an alkoxy group, a halogenated alkoxy group, an aryl group, a halogenated aryl group, an aryloxy group, and a halogenated aryloxy group. The electrolyte containing the flame retardant additive can have good flame retardant properties while not increasing the viscosity of the electrolyte and preventing the electrode active material from peeling off from the electrode due to co-embedding, thereby ensuring good electrochemical performance of the battery.

Description

Technical Field

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

[0002] At present, the electrolyte of secondary batteries is mainly non-aqueous organic electrolyte. When the battery overheats under conditions such as overcharging and internal short circuit, the electrolyte has hidden dangers such as easy combustion, which can easily cause the battery to explode.

[0003] The mainstream strategy to solve the flammability problem of secondary battery electrolytes is to add flame retardant additives to the electrolyte. Commonly used flame retardant additives include phosphates such as trimethyl phosphate and triethyl phosphate. However, conventional phosphates have a high viscosity, and their addition amount in the electrolyte is limited, which does not provide a good flame retardant effect. In addition, conventional phosphates have a strong coordination effect with lithium ions, which will be embedded in the negative electrode along with lithium ions, resulting in the stripping of the negative electrode active material, and then causing the battery cycle performance to deteriorate. Summary of the Invention

[0004] In view of this, the present application provides an electrolyte and a secondary battery to solve the problem that the existing battery electrolyte is flammable and the addition of existing flame retardant additives leads to degradation of battery cycle performance.

[0005] Specifically, the first aspect of the present application provides an electrolyte, which includes an electrolyte salt, a non-aqueous organic solvent, and an additive, wherein the additive includes at least one flame retardant additive having a general structural formula as shown in formula (I):

[0006]

[0007] In formula (I), R, R1, R2, R3, R4, and R5 are independently selected from one or more of hydrogen, halogen, amino, amide, ester, alkyl, halogenated alkyl, alkoxy, halogenated alkoxy, aryl, halogenated aryl, aryloxy, and halogenated aryloxy.

[0008] Firstly, the phosphorus element in the flame retardant additive structure of formula (I) can release phosphorus-containing free radicals at high temperatures. These phosphorus-containing free radicals can combine with hydrogen free radicals generated by the electrolyte to block the hydrogen free radical chain reaction, thereby preventing or hindering the combustion process of the electrolyte and improving the safety performance of the electrolyte. Secondly, the structure of the flame retardant additive contains a large benzene ring, which is not easily co-intercalated with active ions (such as lithium ions) in the electrode during battery cycling, causing the electrode active material to peel off. Third, the co-existence of the benzene ring and the F (fluorine) element connected to the P element in the structure of the flame retardant additive can make the flame retardant additive have a lower viscosity. Its addition to the battery electrolyte will not cause the electrolyte viscosity to increase excessively or the conductivity to decrease. Therefore, its addition amount can be large, which can ensure that the electrolyte has an excellent flame retardant effect (e.g., it can greatly shorten the self-extinguishing time of the electrolyte) while avoiding the problem of excessive viscosity increase and poor battery cycle and rate performance caused by adding traditional phosphate flame retardants; and the presence of fluorine element can also enhance the flame retardant effect of the above-mentioned flame retardant additive. In addition, the flame retardant additive shown in formula (I) has a low viscosity and good interfacial wettability, which can solve the problem of uneven SEI film formation on the electrode surface and poor battery cycle performance caused by poor wettability to the electrode / diaphragm brought by traditional flame retardant electrolytes.

[0009] Therefore, the flame retardant additive provided in this application can impart good flame retardancy to the electrolyte without increasing electrolyte viscosity or causing co-embedding to cause the electrode active material to peel off from the electrode, thereby ensuring that the battery has good cycle performance and rate capability. As a result, the battery can achieve both high safety performance and good electrochemical performance.

[0010] In formula (I), R and R1 to R5 can be the same or different groups. Adjusting them can adjust the viscosity, polarity, or flame retardancy of the flame retardant additive. Among them, amino groups, amide groups, and ester groups can increase the polarity of the flame retardant additive and help reduce the viscosity of the electrolyte. Among them, amino groups can include primary amino groups (-NH2), secondary amino groups (such as alkylamino groups), and tertiary amino groups (such as dialkylamino groups).

[0011] In the present application, when R, R1, R2, R3, R4 or R5 is a halogen, it specifically refers to fluorine (F), chlorine (Cl), bromine (Br) or iodine (I), preferably F and / or Br. Similarly, the halogen in the haloalkyl, haloalkoxy, haloaryl and haloaryloxy groups can include one or more of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), with bromine (Br) being preferred, mainly because the C-Br bond is easily broken and it is easier to release hydrogen bromide that can capture free radicals in the electrolyte. Among them, the halo can be fully halogenated or partially halogenated, for example, a trifluoromethyl group is a fully fluorinated alkyl group, and a difluoromethyl group is an alkyl group partially substituted with fluorine.

[0012] In addition, the alkyl group, haloalkyl group, alkoxy group, and haloalkoxy group may be chain-like (such as straight-chain or branched) or cyclic. For example, the alkyl group may be a chain-like alkyl group or a cyclic alkyl group, and the haloalkyl group may be a halogen-substituted chain-like alkyl group or a halogen-substituted cyclic alkyl group. In the embodiment of the present application, the number of carbon atoms of the alkyl group, haloalkyl group, alkoxy group, and haloalkoxy group involved is 1-10, and the specific number of carbon atoms may be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and further may be 1-6; the number of carbon atoms of the aryl group, haloaryl group, aryloxy group, and haloaryloxy group is 6-10, and the specific number of carbon atoms may be 6, 7, 8, 9, 10. In addition, the aryl group may be an aryl group containing or not containing ring heteroatoms (such as N, O, S, P). In addition, the structure of the aryl group, haloaryl group, aryloxy group, and haloaryloxy group is larger, which can better avoid the flame retardant additive from being co-embedded in the electrode with the active ions during the battery cycle.

[0013] Among them, the number of carbon atoms in each substituent in the above-mentioned flame retardant additive is small, not exceeding 10, which is conducive to controlling the molecular weight of the flame retardant additive to be small and increasing the content of phosphorus in the molecular structure. With the help of phosphorus, the hydrogen free radicals generated by the electrolyte can be combined to hinder the combustion of the electrolyte, thereby improving the flame retardant effect of the additive.

[0014] In the embodiment of the present application, when one or more of R, R1, R2, R3, R4, and R5 are halogen, haloalkyl, haloalkoxy, halogenated aryl, or halogenated aryloxy, it is beneficial to enhance the flame retardant properties of the above-mentioned flame retardant additive and give the electrolyte better safety performance. This is mainly because halogens will produce hydrogen halides during thermal decomposition, and hydrogen halides can capture hydrogen radicals and hydroxyl radicals generated by the decomposition of the electrolyte, thereby greatly reducing the concentration of free radicals in the electrolyte, thereby slowing down or terminating the free radical chain reaction of electrolyte combustion, and further improving the safety performance of the electrolyte. In particular, since the C-Br bond formed between the bromine atom and the carbon atom is easier to break than the chemical bond formed by other halogen atoms and C atoms, it is easier to generate HBr that can capture free radicals in the electrolyte. The haloalkyl, haloalkoxy, halogenated aryl, or halogenated aryloxy is preferably a bromoalkyl, bromoalkoxy, bromoaryl, or bromoaryloxy. That is, one or more of R1, R2, R3, R4, and R5 is a brominated alkyl group, a brominated alkoxy group, a brominated aryl group, or a brominated aryloxy group; the brominated group may be fully brominated or partially brominated.

[0015] In some embodiments of the present application, the total mass content of halogen in the flame retardant additive is greater than or equal to 10%. In this case, the electrolyte can have a better flame retardant effect and greatly improve the high-temperature stability of the electrolyte. The halogen content here also includes F connected to Formula (I).

[0016] In some embodiments, R comprises halogen, haloalkyl, haloalkoxy, haloaryl, or haloaryloxy; preferably comprises haloalkyl, haloalkoxy, haloaryl, or haloaryloxy; more preferably comprises bromoalkyl, bromoalkoxy, brominated aryl, or brominated aryloxy. In some embodiments, R1, R2, R3, R4, and R5 are all hydrogen atoms.

[0017] In some embodiments, the flame retardant additive may be any one of the following substances:

[0018]

[0019] The CAS number of the flame retardant additive represented by formula (i1) is 1126-52-9, wherein the content of P element is 17.41% and the content of F element is 21.34%. The CAS number of the flame retardant additive represented by formula (i2) is 658-34-4, wherein the content of P element is 15.94%, the content of F element is 9.76%, and the content of Cl element is 18.35%. The CAS number of the flame retardant additive represented by formula (i3) is 133826-40-1, wherein the content of P element is 17.82% and the content of F element is 10.92%. In the flame retardant additive represented by formula (i4), the content of P element is 13.54% and the content of F element is 33.33%. In the flame retardant additive represented by formula (i5), the content of P element is 17.71% and the content of F element is 10.86%. In the flame retardant additive represented by formula (i6), the content of the P element is 9.84%, the content of the F element is 6.03%, and the content of the Br element is 25.4%.

[0020] Among them, the preparation method of the flame retardant additive represented by formula (i4) comprises: using methylphosphonyl fluorophenyl ester and trifluoromethyl methyl ether represented by formula (i3) as raw materials, diethyl ether as solvent, and triethylamine as reaction catalyst; using trifluoromethyl methyl ether and triethylamine in equal amounts as base material, slowly adding diethyl ether solution of methylphosphonyl fluorophenyl ester to the base material at a certain reaction temperature, stirring at a uniform speed, and continuously stirring at room temperature for 6 hours after the addition is completed, filtering the obtained reaction material, and recovering the solvent by reduced pressure distillation to obtain the substance represented by formula (i4).

[0021] The preparation method of the flame retardant additive represented by formula (i5) comprises: using methylphosphonyl fluorophenyl ester represented by formula (i3) and a primary amine (CH3NH2) as raw materials, ether as a solvent, and triethylamine as a reaction catalyst; dissolving the primary amine in ether, and adding triethylamine in an amount equal to that of the primary amine to obtain a base material, slowly dropping a methylphosphonyl fluorophenyl ester ether solution into the base material at a certain reaction temperature, stirring at a uniform speed, and continuously stirring at room temperature for 6 hours after the addition is completed, filtering the obtained reaction material, and recovering the solvent by reduced pressure distillation to obtain (amino)phosphonyl fluorophenyl ester represented by formula (i5).

[0022] The preparation method of the flame retardant additive represented by formula (i6) comprises: using methylphosphonyl fluorophenyl ester and m-bromoanisole represented by formula (i3) as raw materials, ether as solvent, and triethylamine as reaction catalyst; using m-bromoanisole and triethylamine in equal amounts as base materials, slowly adding methylphosphonyl fluorophenyl ester ether solution to the base materials at a certain reaction temperature, stirring at a uniform speed, and continuously stirring at room temperature for 6 hours after the addition is completed, filtering the obtained reaction mass, and recovering the solvent by reduced pressure distillation to obtain the substance represented by formula (i6).

[0023] In the embodiment of the present application, the mass of the above-mentioned flame retardant additive accounts for 5%-40% of the total mass of the electrolyte. The content of the above-mentioned flame retardant additive in this range can not only make the electrolyte have better flame retardant properties, but also will not affect the dissolution of electrolyte salt in the electrolyte, and thus will not affect the ionic conductivity of the electrolyte, that is, it will not affect the transmission of lithium ions in the electrolyte, so that the battery has better electrochemical performance. In particular, even if the mass of the above-mentioned flame retardant additive in the electrolyte accounts for a relatively large proportion, it will not cause an excessive increase in the viscosity of the electrolyte, and will not deteriorate the rate performance of the battery. In some embodiments, the mass of the above-mentioned flame retardant additive accounts for 10%-30% of the total mass of the electrolyte, preferably 10%-20%.

[0024] In the embodiment of the present application, the additive further includes a film-forming additive. Among them, the film-forming additive helps to form a uniform and dense solid electrolyte film (ie, SEI film) on the surface of the electrode, thereby improving the electrochemical performance of the secondary battery using the electrolyte. Specifically, the film-forming additive may include one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethylethylene carbonate (TFEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), styrene carbonate (PhVC), 1,3-propane sultone (1,3-PS), vinyl sulfate (DTD), etc., but is not limited thereto.

[0025] In the embodiment of the present application, the mass of the film-forming additive accounts for 1%-5% of the total mass of the electrolyte. An appropriate amount of the film-forming additive can improve the interfacial stability between the battery electrode and the electrolyte without excessively increasing the viscosity of the electrolyte.

[0026] In the present application, depending on different secondary battery systems, the electrolyte salt in the electrolyte can be lithium salt, sodium salt, potassium salt, magnesium salt, zinc salt, aluminum salt, etc. Taking lithium secondary batteries as an example, the electrolyte salt in the electrolyte is a lithium salt, which can specifically include lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (abbreviated as LiFSI, molecular formula LiN(SO2F)2), lithium bis(trifluoromethylsulfonyl)imide (abbreviated as LiTFSI, molecular formula LiN(SO2CF3)2), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluorosilicate (Li2SiF6), lithium hexafluoroantimonate (LiSbF6), lithium perchlorate (LiClO4), lithium bis(oxalatoborate) (abbreviated as LiBOB), lithium difluorooxalatoborate (abbreviated as LiDFOB), lithium bis(perfluoroethylsulfonyl)imide (LiN(C2F5SO2)2), lithium trifluoromethylsulfonate (LiCF3SO3), lithium perfluorobutylsulfonate (LiC4F9SO3), etc. One or more. In some embodiments, the lithium salt includes lithium hexafluorophosphate (LiPF6) and LiFSI.

[0027] In an embodiment of the present application, the molar concentration of the electrolyte salt in the electrolyte is 0.1 mol / L-1.5 mol / L. An appropriate amount of electrolyte salt can ensure that the electrolyte has appropriate active ion conductivity while the viscosity of the electrolyte is not too high, which is beneficial to the performance of the battery. Specifically, the molar concentration of the electrolyte salt in the electrolyte can be 0.2, 0.5, 0.8, 1, 1.2 or 1.5 mol / L; in some embodiments, the molar concentration of the electrolyte salt in the electrolyte is 1.0 mol / L-1.5 mol / L.

[0028] In an embodiment of the present application, the non-aqueous organic solvent includes one or more of a cyclic ester, a linear ester, and an ether solvent. The non-aqueous organic solvent can be mixed in any proportion. In some embodiments, the non-aqueous organic solvent includes a cyclic ester and a linear ester. Among them, the cyclic ester includes a cyclic carbonate, and the linear ester includes a linear carbonate substance and / or a carboxylate substance. This helps to ensure good solubility and low viscosity of the electrolyte salt in the electrolyte. Specifically, the cyclic carbonate can include at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, halogenated ethylene carbonate, etc. The linear carbonate substance can include at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate, etc. The carboxylate substance includes one or more of methyl propionate, ethyl propionate, propyl propionate, butyl propionate, pentyl propionate, halogenated ethyl acetate, etc.

[0029] The present application also provides a secondary battery, wherein the lithium-ion battery has an electrolyte as described in the first aspect of the present application built therein.

[0030] Specifically, the secondary battery comprises a battery casing, a battery cell contained within the casing, and an electrolyte. The battery cell comprises a positive electrode sheet, a negative electrode sheet, and a separator positioned between the positive and negative electrode sheets. The battery manufacturing method comprises the following steps: stacking the positive electrode sheet, separator, and negative electrode sheet in sequence to form a battery cell; placing the battery cell in the battery casing, injecting the electrolyte, and then sealing the battery casing to produce the secondary battery.

[0031] Among them, the negative electrode sheet, positive electrode sheet, and separator are all conventional options in the battery field. For example, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, wherein the positive electrode active material layer includes a positive electrode active material, a binder, and an optional conductive agent. The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on the negative electrode current collector, wherein the negative electrode material layer may include a negative electrode active material, a negative electrode binder, and an optional conductive agent.

[0032] Positive electrode active materials are materials that can reversibly extract and embed active ions. For lithium secondary batteries, the positive electrode active materials may include, but are not limited to, one or more of lithium oxides (such as lithium cobalt oxide, lithium nickel oxide, etc.), lithium binary oxides (such as lithium nickel manganese oxide, lithium nickel cobalt oxide, etc.), lithium ternary oxides (such as lithium nickel cobalt manganese oxide ternary materials, lithium nickel cobalt aluminum oxide ternary materials, etc.), and lithium-containing phosphates (such as lithium iron phosphate, lithium manganese iron phosphate). Negative electrode active materials include, but are not limited to, one or more of artificial graphite, natural graphite, mesocarbon microbeads (MCMB), silicon-carbon composites, silicon oxides, silicon alloys, and lithium titanate.

[0033] The secondary battery provided in the embodiment of the present application has good flame retardant properties due to the addition of the flame retardant additive represented by the above formula (1) to its electrolyte. In addition, the electrolyte has suitable viscosity, good wettability and electrical conductivity. Therefore, the secondary battery can have both excellent safety performance and good electrochemical properties such as cycle performance and rate performance. DETAILED DESCRIPTION

[0034] The technical solutions of the embodiments of the present application are further described below with reference to a number of embodiments.

[0035] Example 1

[0036] A battery electrolyte is prepared as follows: in a nitrogen-filled glove box, 40 mL of ethylene carbonate (EC) and 60 mL of ethyl methyl carbonate (EMC) are mixed to obtain a mixed organic solvent; 7.6 g of lithium hexafluorophosphate (LiPF6) and 9.35 g of lithium bis(fluorosulfonyl)imide (LiFSI) are added to the mixed organic solvent as electrolyte lithium salts; a certain amount of phenyl difluorophosphate represented by formula (i1), a flame retardant additive, is added to the mixed organic solvent; and the mixture is stirred uniformly to obtain an electrolyte. In the electrolyte, the mass of phenyl difluorophosphate accounts for 20% of the total mass of the electrolyte; the concentration of lithium hexafluorophosphate (LiPF6) is 0.5 mol / L; and the concentration of lithium bis(fluorosulfonyl)imide (LiFSI) is 0.5 mol / L.

[0037] A lithium secondary battery, specifically a graphite / LiCoO2 battery, wherein the preparation of the secondary battery specifically includes:

[0038] (1) Preparation of positive electrode sheet: The positive electrode active material - lithium cobalt oxide LiCoO2, the conductive agent (specifically Super-p carbon black), and the binder (specifically polyvinylidene fluoride) are mixed in N-methylpyrrolidone (NMP) at a weight ratio of 96:2:2, and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector aluminum foil, and after baking and rolling, a positive electrode sheet is obtained.

[0039] (2) Preparation of negative electrode sheet: The negative electrode active material (specifically artificial graphite) and the binder (specifically styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC-Na, mass ratio is 2:3) are mixed in deionized water at a mass ratio of 95:5, and stirred evenly to obtain a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector - copper foil, and after drying and rolling, a negative electrode sheet is obtained.

[0040] (3) Assembling the battery: In a nitrogen-filled glove box, the positive electrode sheet, polypropylene separator, and negative electrode sheet are stacked in order and wound into a bare cell. The bare cell is placed in a battery casing and welded. Subsequently, the electrolyte is injected into the battery casing, and the battery casing is sealed to produce a lithium-ion battery.

[0041] Example 2

[0042] The difference between the electrolyte of Example 2 and Example 1 is that the flame retardant additive contained in the electrolyte is phenyl monofluorophosphate represented by formula (i2), and its mass accounts for 15% of the total mass of the electrolyte.

[0043] According to the method described in Example 1, the electrolyte of Example 2 was prepared into a lithium ion battery.

[0044] Example 3

[0045] The difference between the electrolyte of Example 3 and Example 1 is that the flame retardant additive contained in the electrolyte is methylphosphonyl fluorophenyl ester represented by formula (i3), and its mass accounts for 20% of the total mass of the electrolyte.

[0046] According to the method described in Example 1, the electrolyte of Example 3 was prepared into a lithium ion battery.

[0047] Example 4

[0048] The electrolyte of Example 4 is different from that of Example 1 in that the flame retardant additive contained in the electrolyte is (trifluoromethyl)phenylphosphonyl fluoride represented by formula (i4).

[0049] According to the method described in Example 1, the electrolyte of Example 4 was prepared into a lithium ion battery.

[0050] Example 5

[0051] The electrolyte of Example 5 is different from that of Example 1 in that the flame retardant additive contained in the electrolyte is (amino)phosphonyl fluoride phenyl ester represented by formula (i5).

[0052] According to the method described in Example 1, the electrolyte of Example 5 was prepared into a lithium ion battery.

[0053] Example 6

[0054] The electrolyte of Example 6 is different from that of Example 1 in that the flame retardant additive contained in the electrolyte is (3-bromophenyl)phosphonyl fluorophenyl ester represented by formula (i6).

[0055] According to the method described in Example 1, the electrolyte of Example 6 was prepared into a lithium ion battery.

[0056] Example 7

[0057] The electrolyte of Example 7 is different from that of Example 1 in that the mass of phenyl difluorophosphate represented by formula (i1) contained in the electrolyte accounts for 5% of the total mass of the electrolyte.

[0058] Example 8

[0059] The electrolyte of Example 8 is different from that of Example 1 in that the mass of phenyl difluorophosphate represented by formula (i1) contained in the electrolyte accounts for 30% of the total mass of the electrolyte.

[0060] Example 9

[0061] The electrolyte of Example 9 is different from that of Example 1 in that the mass of phenyl difluorophosphate represented by formula (i1) contained in the electrolyte accounts for 10% of the total mass of the electrolyte.

[0062] In order to highlight the beneficial effects of the embodiments of the present application, the following comparative examples 1-5 are provided.

[0063] Comparative Example 1

[0064] An electrolyte and a battery were prepared in the same manner as in Example 1, except that the electrolyte of Example 1 did not contain a flame retardant additive.

[0065] Comparative Example 2

[0066] The electrolyte and battery were prepared in the same manner as in Example 1, except that the flame retardant additive contained in the electrolyte of Example 2 was triphenyl phosphate. Its mass proportion in the electrolyte is 20%.

[0067] Comparative Example 3

[0068] The electrolyte and battery were prepared in the same manner as in Example 1, except that the flame retardant additive contained in the electrolyte of Example 3 was trimethyl phosphate. Its mass proportion in the electrolyte is 20%.

[0069] Comparative Example 4

[0070] An electrolyte and a battery were prepared in the same manner as in Example 1, except that the flame retardant additive contained in the electrolyte of Comparative Example 4 was triphenyl phosphate, which accounted for 5% by mass in the electrolyte.

[0071] Comparative Example 5

[0072] An electrolyte and a battery were prepared in the same manner as in Example 1, except that the flame retardant additive contained in the electrolyte of Comparative Example 5 was trimethyl phosphate, which accounted for 5% by mass in the electrolyte.

[0073] The electrolytes and lithium secondary batteries of the above embodiments and comparative examples were subjected to the following performance tests, and the results are summarized in Table 1 below.

[0074] 1) Electrolyte self-extinguishing performance test:

[0075] The electrolyte of each Example or Comparative Example was dripped onto glass fiber filter paper, weighed, and ignited. The time from removal of the ignition device until the flame went out was recorded and divided by the mass of the electrolyte on the glass fiber filter paper to obtain the self-extinguishing time of the electrolyte, measured in seconds per gram. Five self-extinguishing tests were performed for each electrolyte sample, and the average value was taken.

[0076] 2) Cycle performance test of lithium secondary batteries:

[0077] After packaging the batteries from each example or comparative example, they were left overnight before being tested for capacity retention. This test was conducted in a 25°C incubator, with the lithium secondary batteries subjected to charge-discharge cycles at a current of 0.2C (0.3mA) over a voltage range of 3.0-4.25V. The capacity retention after 100 cycles was calculated as the ratio of the discharge capacity after 100 cycles to the discharge capacity at the beginning of the cycle. Twenty battery samples from the same group were tested, and the results were averaged.

[0078] 3) Rate performance test of lithium secondary batteries:

[0079] At a temperature of 25°C, each lithium secondary battery was charged and discharged at a charge rate of 0.2C and a discharge rate of 0.2C, with a voltage range of 3.0-4.5V, and the discharge capacity at the discharge rate of 0.2C was recorded. At a temperature of 25°C, each lithium secondary battery was charged and discharged at a charge rate of 0.2C and a discharge rate of 2C, with a voltage range of 3.0-4.5V, and the discharge capacity at the discharge rate of 2C was recorded. The ratio of 2C / 0.2C discharge capacity = discharge capacity at a discharge rate of 2C / discharge capacity at a discharge rate of 0.2C is used to measure the rate performance of the battery.

[0080] Table 1 Test results of electrolytes and batteries of various embodiments and comparative examples

[0081]

[0082]

[0083] As can be seen from Table 1, compared to Comparative Example 1, in which no flame retardant was added to the electrolyte, the self-extinguishing time of the electrolytes of Examples 1-9 of the present application was significantly shortened, reflecting their better flame retardant performance. Moreover, compared to Comparative Example 3, in which the same amount of trimethyl phosphate flame retardant was added to the electrolyte, the cycle performance of the batteries of Examples 1 and 3-6 of the present application was particularly excellent. This is mainly because the flame retardant additives of the present application examples greatly avoided the phenomenon of co-embedding with lithium ions into the negative electrode during the battery cycle. Compared with Comparative Example 2, in which the same amount of triphenyl phosphate flame retardant was added to the electrolyte, the electrolytes of Examples 1 and 3-6 of the present application had a shorter self-extinguishing time, better flame retardant performance, and the viscosity of the electrolyte was not too high. The battery's rate performance was particularly excellent, and the cycle performance was also relatively good. When the amount of flame retardants triphenyl phosphate and trimethyl phosphate added to the electrolyte is small (Comparative Examples 4-5), although the cycle performance and rate performance of the battery are acceptable, they will not improve the flame retardant properties of the electrolyte well, and the self-extinguishing time of the electrolyte is too long.

[0084] The above is an exemplary embodiment of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made thereto without departing from the principles of the present application. These improvements and modifications are also considered to be within the scope of protection of the present application.

Claims

1. An electrolyte, characterized in that: The electrolyte includes an electrolyte salt, a non-aqueous organic solvent, and an additive, wherein the additive includes at least one flame retardant additive having a general structural formula as shown in formula (I): （I） In formula (I), R, R1, R2, R3, R4, and R5 are independently selected from one or more of hydrogen, halogen, amino, amide, ester, alkyl, halogenated alkyl, alkoxy, halogenated alkoxy, aryl, halogenated aryl, aryloxy, and halogenated aryloxy.

2. The electrolyte according to claim 1, wherein The number of carbon atoms in the alkyl group, halogenated alkyl group, alkoxy group and halogenated alkoxy group is 1-10; the number of carbon atoms in the aryl group, halogenated aryl group, aryloxy group and halogenated aryloxy group is 6-10.

3. The electrolyte according to claim 1, wherein The halogen in the halogenated alkyl, halogenated alkoxy, halogenated aryl and halogenated aryloxy includes one or more of fluorine, chlorine, bromine and iodine; and the halogenated group is fully halogenated or partially halogenated.

4. The electrolyte according to claim 3, wherein The halogen in the halogenated alkyl group, halogenated alkoxy group, halogenated aryl group and halogenated aryloxy group includes bromine.

5. The electrolyte according to claim 1, wherein The R is halogen, halogenated alkyl, halogenated alkoxy, halogenated aryl or halogenated aryloxy.

6. The electrolyte according to claim 1, wherein R1, R2, R3, R4, and R5 are all hydrogen atoms.

7. The electrolyte according to claim 1, wherein The total mass content of halogen in the flame retardant additive is greater than or equal to 10%.

8. The electrolyte according to any one of claims 1 to 7, wherein The flame retardant additive accounts for 5%-40% by mass in the electrolyte.

9. The electrolyte according to claim 1, wherein The additives also include film-forming additives.

10. The electrolyte according to claim 9, wherein The mass of the film-forming additive accounts for 1%-5% of the total mass of the electrolyte.

11. The electrolyte according to claim 1, wherein The molar concentration of the electrolyte salt in the electrolyte solution is 0.1 mol / L-1.5 mol / L.

12. A secondary battery comprising the electrolyte according to any one of claims 1 to 11.

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