Electrolyte and lithium battery

Abstract

The present disclosure provides an electrolyte and a lithium battery, the electrolyte comprising a fluorinated ether solvent; wherein the fluorinated ether solvent comprises at least one compound of formula (I): wherein R2 is a C4 alkylene or a C4 fluoroalkylene, wherein the number of fluorine atoms in the C4 fluoroalkylene is 1; R1 and R3 are each independently methyl or fluoromethyl, and the total number of fluorine atoms in R1 and R3 is 1-4; such an electrolyte can improve the cycle performance of a medium-nickel high-voltage lithium battery, reduce the direct current resistance growth rate under high-temperature storage conditions, and is conducive to long-term stable operation of the lithium battery.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of batteries, and in particular to an electrolyte and a lithium battery. BACKGROUND

[0002] The ternary positive electrode material is a new type of positive electrode material of a lithium battery, such as nickel-cobalt-manganese, nickel-cobalt-aluminum, etc. The nickel-cobalt-manganese includes nickel salt, cobalt salt and manganese salt, and is distinguished from numerous positive electrode materials of lithium batteries due to its higher working voltage and greater energy density. Based on the proportion of nickel in the nickel-cobalt-manganese, the nickel-cobalt-manganese ternary positive electrode material includes low-nickel positive electrode material, medium-nickel positive electrode material and high-nickel positive electrode material. Among them, the medium-nickel positive electrode material not only has similar energy density effect to the high-nickel positive electrode material, but also has higher safety and lower cost, and is regarded as one of the mainstream solutions to solve the range anxiety of vehicle-mounted batteries. However, under high-voltage working conditions, unstable substances in the electrolyte are prone to decomposition, resulting in poor cycle performance of the medium-nickel high-voltage battery. SUMMARY

[0003] Therefore, the purpose of the present disclosure is to provide an electrolyte and a lithium battery.

[0004] To achieve the above purpose, the present disclosure provides an electrolyte in a first aspect, comprising a fluorinated ether solvent; wherein the fluorinated ether solvent comprises at least one compound of formula (I):

[0005]

[0006] wherein,

[0007] R2 is a C4 alkylene group or a C4 fluorinated alkylene group, wherein the number of fluorine atoms in the C4 fluorinated alkylene group is 1;

[0008] R1 and R3 are each independently methyl or fluoromethyl, and the total number of fluorine atoms in R1 and R3 is 1-4.

[0009] In some embodiments, the mass fraction of the fluorinated ether solvent is 10%-45%, or 15%-35%, based on the total mass of the electrolyte.

[0010] In some embodiments, R2 is a C4 chain alkylene group or a C4 fluorinated chain alkylene group, or a C4 straight chain alkylene group or a C4 fluorinated straight chain alkylene group.

[0011] In some embodiments, the electrolyte further comprises a non-fluorinated ether solvent; optionally, the non-fluorinated ether solvent comprises at least one of tetrahydrofuran, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether and carbonate, and more optionally, ethylene glycol dimethyl ether.

[0012] In some embodiments, the mass percentage of the ethylene glycol dimethyl ether is 40-75%, or 50-70%, based on the total mass of the electrolyte.

[0013] In some embodiments, the electrolyte further comprises a lithium salt; and the mass percentage of the lithium salt is 11-15%, or 13-14%, based on the total mass of the electrolyte.

[0014] In some embodiments, the lithium salt comprises at least one of LiPF6, LiBF4, LiClO4, LiCF3SO3, LiBOB, LiODFB and LiN(SO2CF3)2; and the lithium salt is LiPF6.

[0015] In some embodiments, the electrolyte further comprises at least one of lithium bisfluorosulfonylimide, vinylene carbonate and tetravinylsilane; and the mass percentage of the lithium bisfluorosulfonylimide is 0.1-5%, or 0.2-1%, based on the total mass of the electrolyte; the mass percentage of the vinylene carbonate is 0.1-10%, or 0.2-1%; and the mass percentage of the tetravinylsilane is 0.1-10%, or 0.2-1%.

[0016] In some embodiments, the compound of formula (I) is selected from at least one of the following compounds:

[0017]

[0018]

[0019] Based on the same inventive concept, the second aspect of the present disclosure further provides a lithium battery comprising the electrolyte according to any one of the preceding embodiments; wherein the positive active material of the lithium battery comprises Li a Ni x Mn y Co z O2; wherein 0.9

[0020] As can be seen from the above, the electrolyte and lithium battery provided by the present disclosure, the electrolyte comprises a fluorinated ether solvent; wherein the fluorinated ether solvent comprises at least one compound of formula (I): In the formula, R2 is a C4 alkylene group or a C4 fluoroalkylene group, wherein the number of fluorine atoms in the C4 fluoroalkylene group is 1; R1 and R3 are each independently a methyl group or a fluoromethyl group, and the total number of fluorine atoms in R1 and R3 is 1-4; and the electrolyte can improve the cycle performance of the middle-nickel high-voltage lithium battery and reduce the direct-current internal resistance growth rate under high-temperature storage conditions, thereby being beneficial to long-term stable operation of the lithium battery. DETAILED DESCRIPTION

[0021] To make the objects, technical solutions, and advantages of the present disclosure clearer, the present disclosure is further described in detail below with reference to specific embodiments.

[0022] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the embodiments of the present disclosure should be understood as the common meanings understood by those skilled in the art to which the present disclosure belongs.

[0023] The "range" used in the embodiments of the present disclosure is defined in the form of a lower limit and an upper limit, a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The range defined in this way can include or not include the end value, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if the ranges of 60-130 and 70-120 are listed for a particular parameter, it is understood that the ranges of 60-120 and 70-130 are also anticipated. In addition, if the minimum range values of 1 and 2 are listed, and if the maximum range values of 3, 4, and 5 are listed, the following ranges are all anticipated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present disclosure, unless otherwise stated, the numerical range "a-b" represents a shorthand notation for any real combination of integers between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed herein, and "0-5" is only a shorthand notation for these numerical combinations. In addition, when it is stated that a parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0024] If not specifically stated, all embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions.

[0025] If not specifically stated, all technical features and optional technical features of the present disclosure can be combined with each other to form new technical solutions.

[0026] If not specifically stated, the "comprising" and "including" mentioned in the present disclosure represent open-ended, and can also be closed-ended. For example, the "comprising" and "including" can represent that other components not listed can also be included, or only the listed components can be included.

[0027] If not specifically stated, in the present disclosure, the term "or" is inclusive. For example, the phrase "A or B" represents "A, B, or both A and B".

[0028] In the related art, the common solvent system of the lithium ion battery electrolyte mainly consists of mixed solvents of cyclic carbonates and chain carbonates. Compared with ether solvents, the cyclic carbonates and chain carbonates have better oxidation resistance, but they still have the risk of decomposition under high voltage working conditions. Therefore, developing more diverse solvent molecular structures to give the lithium ion battery electrolyte solvent system more choices is a research hotspot.

[0029] Fluoroether solvents have high solubility of lithium salts and more stable chemical structures after modification by fluorine atoms, and are considered as strong candidates for high-voltage battery electrolytes. However, due to the high viscosity and high cost of fluoroether solvents, it is difficult to apply them as a single solvent system in lithium ion batteries. In addition, due to the long carbon chain structure of fluoroethers, their solubility of common lithium salts is affected to a certain extent.

[0030] To achieve the purpose of the present disclosure, the embodiments of the present disclosure provide an electrolyte comprising a new fluoroether solvent, which is beneficial to widen the electrochemical working window of the electrolyte and improve the cycle performance of the middle-nickel high-voltage battery cell.

[0031] Electrolyte

[0032] The embodiments of the present disclosure provide an electrolyte comprising a fluoroether solvent; wherein the fluoroether solvent comprises at least one compound of formula (I):

[0033]

[0034] wherein,

[0035] R2 is a C4 alkylene or a C4 fluoroalkylene, wherein the number of fluorine atoms in the C4 fluoroalkylene is 1; here, the alkylene refers to a saturated, branched or straight-chain, or cyclic hydrocarbon residue of 1-18 carbon atoms, and has two monovalent residue centers derived from removing two hydrogen atoms from the same carbon atom or two different carbon atoms of the parent alkane, for example, 1,4-butyl (-CH2CH2CH2CH2-).

[0036] R1and R3are each independently methyl or fluoromethyl, and the total number of fluorine atoms in R1and R3is 1 to 4.

[0037] The electrolyte provided by the present disclosure can improve the cycle performance of the medium-nickel high-voltage lithium battery, and is helpful for long-term stable operation of the lithium battery. Here, the lithium battery can be a primary lithium battery or a secondary lithium battery, and the present disclosure does not limit this.

[0038] The compound of formula (I) in the electrolyte of the present disclosure is an ether with a relatively long carbon chain and containing at least one fluorine atom. After substitution of the fluorine atom, the charge density distribution of the compound itself is changed, and the more dispersed charge density distribution improves the oxidation resistance of the molecule, thereby improving the overall oxidation resistance of the electrolyte and showing higher stability under high-temperature cycle working conditions. In addition, since the main chain of the compound of formula (I) contains two oxygen atoms and the positions are relatively symmetric, the solvent with such a long main chain does not participate in the first solvent shell of lithium ions, and therefore does not participate in film formation during charging and discharging. However, after adding such a fluorinated ether solvent, the composition in the solvation structure of lithium ions is still changed, so that the cycle stability of the battery is significantly improved, and the growth rate of the direct current resistance (DCR) in the high-temperature storage test (HTS) is reduced.

[0039] In some embodiments, R2is a C4chain alkylene or a C4fluorinated chain alkylene, optionally a C4straight chain alkylene or a C4fluorinated straight chain alkylene.

[0040] In some embodiments, the compound of formula (I) is selected from at least one of the following compounds:

[0041]

[0042]

[0043] In some embodiments, the compound of formula (I) is selected from at least one of I-12, I-13, I-14, I-15, I-16, I-1, I-6, I-2, I-8, I-9, I-7, I-11, I-19, and I-5. In some embodiments, the compound of formula (I) is optionally selected from at least one of I-11, I-12, I-13, I-14, and I-16.

[0044] For the above compounds, the inventors found that although the oxidation stability of fluorinated ethers is relatively high, the difference in the number and position of fluorine substitution, the uniformity of the charge distribution of the overall structure, and the chemical stability of the structure also differ. The more uniform the charge distribution of the overall structure, the higher the corresponding oxidation stability, and the better the room temperature cycle performance of the mixed electrolyte with ether solvents. In addition, the change in the structure of fluorinated ethers can affect the solvation structure of lithium ions in the electrolyte, and also increase the stability of the electrolyte itself.

[0045] In some embodiments, the mass percentage of the fluorinated ether solvent is 10% to 45%, for example, 10%, 14%, 18%, 20%, 25%, 31%, 35%, 40%, or 45%; or optionally 15% to 35%. When the mass range of the fluorinated ether solvent is less than 10%, the solvent has insufficient oxidation stability, and it is difficult to improve the stability of the electrolyte. When the mass range of the fluorinated ether solvent is greater than 50%, the electrolyte has excessively high viscosity, and the number of room temperature cycles decreases. This may be because the direct current resistance gradually increases during the cycle, the internal polarization of the battery increases, and the room temperature cycle performance of the battery deteriorates.

[0046] In some embodiments, the electrolyte further comprises a non-fluorinated ether solvent. Optionally, the non-fluorinated ether solvent comprises at least one of tetrahydrofuran (THF), ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (G4), and a carbonate, and more optionally ethylene glycol dimethyl ether.

[0047] It should be noted that the carbonate includes cyclic carbonates and chain carbonates. For example, the cyclic carbonate solvent includes at least one of propylene carbonate (PC) and ethylene carbonate (EC). Optionally, the cyclic carbonate solvent is ethylene carbonate. The cyclic carbonate solvent has a high dielectric constant, which helps to dissolve lithium salt and ensure uniform distribution of lithium salt in the electrolyte, thereby ensuring normal operation of the battery. In addition to increasing the solubility of lithium salt, ethylene carbonate can form a stable solvation configuration with lithium ions, thereby improving the stability and conductivity of the electrolyte and enhancing the performance of the lithium battery. For example, the chain carbonate solvent includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The electrolyte including the chain carbonate solvent can reduce the overall viscosity and help to improve the conductivity, cycle life, and energy density of the lithium battery.

[0048] In some embodiments, the electrolyte comprises ethylene carbonate and methyl ethyl carbonate. Further, the mass percentage of the ethylene carbonate is 15-25% based on the total mass of the electrolyte; and the mass percentage of the methyl ethyl carbonate is 40-60% based on the total mass of the electrolyte. The combination of ethylene carbonate and methyl ethyl carbonate is conducive to ensuring the solubility of the solvent to the lithium salt while controlling the viscosity of the electrolyte, so as to obtain a high-performance lithium battery.

[0049] In some embodiments, the mass percentage of the ethylene glycol dimethyl ether is 40-75%, or 50-70% based on the total mass of the electrolyte. The inclusion of the ethylene glycol dimethyl ether in the above range and the compound represented by formula (I) in the electrolyte is conducive to improving the normal-temperature cycle performance and high-temperature storage performance, and is particularly suitable for use with a medium-nickel positive electrode active material.

[0050] In some embodiments, the electrolyte further comprises a lithium salt; and the mass percentage of the lithium salt is 11-15%, or 13-14% based on the total mass of the electrolyte.

[0051] In some embodiments, the lithium salt comprises at least one of LiPF6, LiBF4, LiClO4, LiCF3SO3, LiBOB, LiODFB and LiN(SO2CF3)2.

[0052] Optionally, the lithium salt is LiPF6. LiPF6 has good solubility in the mixture of non-fluoro ether solvents and fluoro ether solvents, avoiding the performance degradation of the battery caused by insufficient solubility of the lithium salt.

[0053] In some embodiments, the electrolyte further comprises an additive, and the additive is selected from at least one of lithium bisfluorosulfonylimide (LiFSI), vinylene carbonate (VC) and tetravinylsilane (TVSi); optionally, the additive comprises lithium bisfluorosulfonylimide (LiFSI), vinylene carbonate (VC) and tetravinylsilane (TVSi); and more optionally, the additive comprises vinylene carbonate (VC) and tetravinylsilane (TVSi). Lithium bisfluorosulfonylimide is conducive to increasing the lithium ions in the electrolyte, thereby improving the overall performance of the battery; the combination of vinylene carbonate and tetravinylsilane (TVSi) generates a more aggregated network solid-state electrolyte interface film layer (SEI and CEI) on the surface of the positive and negative electrodes, which protects the positive and negative electrode materials during the cycle process, and is conducive to improving the cycle performance of the battery cell.

[0054] In some embodiments, the mass percentage of the lithium bisfluorosulfonylimide is 0.1% to 5%, more optionally 0.2% to 1%, based on the total mass of the electrolyte; optionally, the mass percentage of the vinylene carbonate is 0.1% to 10%, more optionally 0.2% to 1%; optionally, the mass percentage of the tetravinylsilane is 0.1% to 10%, more optionally 0.2% to 1%.

[0055] Lithium battery

[0056] The second aspect of the present disclosure provides a lithium battery comprising the electrolyte provided in the foregoing aspects of the present disclosure.

[0057] In some embodiments, the lithium battery comprises a positive electrode sheet, a negative electrode sheet, a separator between the positive electrode sheet and the negative electrode sheet, and an electrolyte. During the charging and discharging of the battery, active ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly to prevent short circuiting between the positive and negative electrodes, while allowing ions to pass through.

[0058] Positive electrode sheet

[0059] The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, the positive electrode film layer comprising a positive electrode active material.

[0060] Exemplarily, the positive electrode current collector has two opposite surfaces in the thickness direction thereof, and the positive electrode film layer is arranged on either one or both of the two opposite surfaces of the positive electrode current collector.

[0061] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be used. The composite current collector can comprise a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material (such as polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.).

[0062] In some embodiments, the positive electrode active material comprises lithium nickel cobalt manganese oxide, for example, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(also referred to as NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2(also referred to as NCM523), LiNi 0.5Co 0.25 Mn 0.25 O2 (which can also be abbreviated as NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (which can also be abbreviated as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (which can also be abbreviated as NCM811).

[0063] Optionally, the positive electrode active material of the lithium battery includes Li a Ni x Mn y Co z O2; where 0.9 < a < 1.1, 0.5 < x < 0.7, 0.1 < z ≤ 0.2 and x + y + z = 1. When 0.5 < x < 0.7, the electrochemical performance of the positive electrode active material is excellent; when x > 0.7, the stability of the positive electrode active material is poor, and there may be oxidation of the fluorinated ether solvent on the surface of the positive electrode during charge and discharge. The unstable solvent system makes it difficult to operate stably for a long time due to the diving in the later stage of the cycle at 25°C. This may be because a film layer is formed on the surface of the positive electrode after the fluorinated ether undergoes an oxidation reaction, and the poor conductivity of this film layer leads to an increase in the DC internal resistance, which can be corroborated by the increase rate of the DC internal resistance in the high-temperature storage test.

[0064] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0065] In some embodiments, the positive electrode film layer may further optionally include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0066] In some embodiments, the positive electrode plate can be prepared by the following method: Dispersing the components for preparing the positive electrode plate, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; Coating the positive electrode slurry on the positive electrode current collector, and after processes such as drying and cold pressing, the positive electrode plate can be obtained.

[0067] Negative electrode sheet

[0068] The negative electrode sheet includes a negative electrode current collector and a film layer optionally provided on at least one surface of the negative electrode current collector.

[0069] Exemplarily, the film layer can include a negative electrode active material (e.g. artificial graphite), a conductive agent, a thickening agent, and a binder.

[0070] In some embodiments, the negative electrode sheet can be prepared by dispersing a negative electrode active material, a conductive agent, a thickening agent, a binder, and any other components in a solvent (e.g. deionized water) to form a negative electrode slurry, coating the negative electrode slurry on a negative electrode current collector, and drying, cold-pressing, etc.

[0071] Separator film

[0072] In some embodiments, the secondary battery further includes a separator film. The type of the separator film is not particularly limited in the present disclosure, and any publicly known porous structure separator film having good chemical stability and mechanical stability can be used.

[0073] In some embodiments, the material of the separator film can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator film can be a single-layer film or a multi-layer composite film, and is not particularly limited. When the separator film is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0074] Embodiments

[0075] Hereinafter, embodiments of the present disclosure will be described. The embodiments described below are exemplary and are for the purpose of explaining the present disclosure only, and should not be construed as limiting the present disclosure. In the embodiments, specific techniques or conditions not described are performed according to the techniques or conditions described in the literature in the art or according to the product manual. The reagents or instruments not described by the manufacturer are all conventional products that can be obtained commercially.

[0076] Examples 1 to 14

[0077] Embodiments 1 to 14 of the present disclosure are secondary lithium batteries, in which the positive electrode active material is a medium-nickel high-voltage positive electrode material, and the specific preparation method is as follows:

[0078] (1) LiNi 0.6 Mn 0.3 Co 0.1 Preparation of LiNi

[0079] (1) LiNi 0.6 Mn 0.3 Co 0.1O2, polyvinylidene fluoride as a binder and conductive carbon black (Super P) as a conductive agent were mixed in a weight ratio of 98:1:1, N-methyl pyrrolidone (NMP) was added, and stirring was performed under the action of a vacuum stirrer until the system became uniform and transparent to obtain a positive electrode slurry; the positive electrode slurry was uniformly coated on an aluminum foil; the aluminum foil was dried at room temperature, then transferred to an oven for drying, and then subjected to cold pressing and slitting to obtain a positive electrode (positive electrode tab);

[0080] (2) Preparation of a graphite negative electrode:

[0081] Artificial graphite was used as a negative active material, Super P was used as a conductive agent, sodium carboxymethyl cellulose (CMC-Na) was used as a thickening agent, and styrene butadiene rubber (SBR) was used as a binder, which were mixed in a mass ratio of 96:1:1:2, deionized water was added, and a negative electrode slurry was obtained under the action of a vacuum stirrer; the negative electrode slurry was uniformly coated on a negative current collector copper foil; the copper foil was dried at room temperature, then transferred to an oven for drying, and then subjected to cold pressing and slitting to obtain a negative electrode (tab).

[0082] (3) Preparation of an electrolyte:

[0083] In an argon atmosphere glove box with a water content of <10 ppm, battery-grade fluorinated ether I-12, ethylene glycol dimethyl ether (DME), tetrahydrofuran (THF), triethylene glycol dimethyl ether (G3), ethylene carbonate (EC), and methyl ethyl carbonate (EMC) were mixed in the proportions in Examples 1-14 to form a total organic solvent. VC, TVSi, LiPF6, LiFSI were taken in the proportions in Examples 1-14 and mixed uniformly with the aforementioned organic solvent to obtain an electrolyte.

[0084] The proportions of the components in Examples 1-14 are shown in Table 1. It should be noted that the content of each component in Table 1 is the mass percentage calculated based on the total mass of the electrolyte.

[0085] (4) Preparation of a separator:

[0086] A polypropylene film was used as a separator.

[0087] (5) Preparation of a secondary battery:

[0088] A polypropylene film with a thickness of 12 μm was used as a separator, and the positive electrode, the separator, and the negative electrode prepared above were stacked in sequence with the separator between the positive electrode and the negative electrode to play a separating role. Then, the aluminum plastic film was wrapped, transferred to a vacuum oven for drying at 120°C, and then sealed after injecting 3.0 g / Ah of the electrolyte prepared above to perform electrolyte formation, and finally a soft-pack battery (i.e., a lithium ion battery) with a capacity of 1 Ah was prepared.

[0089] Comparative examples 1 to 4

[0090] Comparative Examples 1 to 4 and Example 1 differ in the composition of the organic solvent in step (3), the components and ratios of the organic solvent of Comparative Examples 1 to 4 being shown in Tables 1 and 2. In Table 2, A represents a compound of Formula A B represents a compound of Formula B

[0091] Example 15, Comparative example 5

[0092] Example 15, Comparative Example 5 and Example 1 differ in step (1): the positive active material is replaced by LiNi 0.9 Mn 0.05 Co 0.05 O2, the composition of the organic solvent in step (3) and the ratios, as shown in Table 2.

[0093] Test method

[0094] The secondary batteries prepared in the above examples and comparative examples can be tested by the following methods:

[0095] (1) High-temperature storage test of secondary batteries

[0096] In an oven at a specified temperature (high temperature 45°C), the battery is cycled charged at a current of 0.2C in a specified potential range (2.0V-3.8V) and cycled discharged at a current of 1C in a specified potential range (2.8V-4.35V). After three cycles of charge and discharge, the initial volume of the cell is measured and recorded as V0. The cell is stored in an oven at a specified temperature (high temperature 60°C) for a fixed time interval (60d). At each time interval, the DCR is measured. The high-temperature storage DCR growth rate is calculated from the difference between DCR 0d and DCR 60d. The test results are shown in Tables 1 and 2.

[0097] (2) Normal-temperature cycle test of secondary batteries

[0098] In an oven at a specified temperature (25°C), the battery is cycled charged and discharged at a current of 1C in a specified potential range (2.0V-3.8V). The discharge capacity of each cycle is recorded. The test is ended when the battery capacity reaches 80% of the first cycle capacity. The test results are shown in Tables 1 and 2.

[0099] Table 1

[0100]

[0101] Table 2

[0102]

[0103] Comparing Comparative Example 1 to Comparative Example 4 and Example 6, it can be seen that adding a fluorinated ether solvent to the electrolyte, whether fluorinated ether I-12, octafluoropentyl-tetrafluoroethyl ether, formula A or formula B, can improve the stability of the battery at room temperature and reduce the increase in direct current resistance under high-temperature storage conditions. Comparing Comparative Example 2 to Comparative Example 4 and Example 6, it can be seen that different fluorinated ether solvents have a large difference in the improvement effect on the cycle stability of the battery at room temperature and the increase in direct current resistance. The fluorinated ether I-12 provided in the present disclosure has a significantly better improvement effect on the cycle stability of the battery at room temperature than octafluoropentyl-tetrafluoroethyl ether, formula A and formula B. This may be because the fluorinated ether compound provided in the present disclosure, when combined with ethylene glycol dimethyl ether (DME) as an organic solvent, can provide good lithium salt solubility while maintaining good stability of the electrolyte, thereby improving the cycle performance of the battery at room temperature and reducing the increase in direct current resistance.

[0104] Comparing Comparative Example 1, Examples 1-2, Examples 5-7, Examples 9-10, it can be seen that adding fluorinated ether I-12 to the electrolyte can improve the cycle stability of the battery and reduce the increase in direct current resistance under high-temperature storage conditions. In particular, when the mass fraction of fluorinated ether I-12 is within the range of 15% to 35%, the addition of fluorinated ether I-12 significantly improves the cycle stability of the battery and reduces the increase in direct current resistance.

[0105] Comparing Example 6 to Examples 11-13, it can be found that compared to the combination of THF, G3, carbonate and fluorinated ether I-12, the combination of DME and fluorinated ether I-12 is more conducive to improving the cycle stability of the battery. This may be because G3 is not substituted with a fluorine atom and has a long chain and a large number of ether bonds, and THF has a ring structure, which is not thermodynamically stable and will decompose into film-forming substances that are not conducive to lithium ion conduction on the negative electrode during the cycle process, resulting in poor performance of the battery containing THF and G3. DME has high stability and is not easily oxidized or reduced, and has good solubility for lithium salts, which can cooperate with fluorinated ether I-12 to solve the problem of insufficient solubility of fluorinated ether I-12 for lithium salts, thereby improving the performance of the battery.

[0106] Comparing Example 15, Comparative Example 5 and Example 6, it can be seen that the fluorinated ether I-12 can also improve the cycle stability of the prepared battery cell of the high-nickel positive electrode active material and reduce the direct current resistance growth rate. At the same time, the positive electrode active material also has a greater impact on the cycle stability of the battery cell. Compared with the high-nickel positive electrode active material (Example 15 and Comparative Example 5), the battery cell performance of the fluorinated ether I-12 combined with the medium-nickel positive electrode active material (Ni6 of Example 6) is better. This may be because the high-nickel positive electrode material has poor stability, and there may be oxidation of the fluorinated ether on the surface of the positive electrode during the charging and discharging process. The unstable solvent system causes the number of cycles at room temperature to drop in the later stage of the cycle, and the fluorinated ether generates a film-forming material on the surface of the positive electrode after oxidation. The ion conductivity of this material itself is poor, resulting in a higher growth rate of HTS-DCR 60d. In addition to the fluorinated ether I-12, the compound of formula (I) and the high-nickel positive electrode material are combined, and similar results are shown, which will not be described here.

[0107] It should be noted that the positive electrode active materials of Comparative Example 1, Examples 1-14 are only representatives of medium-nickel positive electrode active materials, but not limitations. The medium-nickel positive electrode active material of the present disclosure refers to Li a Ni x Mn y Co z O2; wherein 0.9 < a < 1.1, 0.5 < x < 0.7, 0.1 < z < 0.2 and x + y + z = 1.

[0108] Comparing Example 3, Example 4, Example 6, Example 8 and Example 14, it can be seen that the addition of LiFSI or the combination of LiFSI, VC and TVSi in the electrolyte can improve the cycle performance of the battery cell and reduce the growth rate of the direct current resistance. In particular, the combination of VC and TVSi can generate a relatively dense SEI and CEI on the surface of the positive and negative electrodes, which can protect the positive and negative electrode materials during the cycle process, and is beneficial to improve the cycle performance of the battery cell.

[0109] Examples 16 to 23

[0110] Examples 16-23 are similar to the preparation methods of Examples 1-10, except that the fluorinated ether I-12 is replaced by fluorinated ether I-16. The detailed composition of the electrolyte and the test results are shown in Table 3.

[0111] Table 3

[0112]

[0113] Examples 24 to 31

[0114] Examples 24 to 31 were prepared in a similar manner to Examples 1 to 10, except that fluoroether I-12 was replaced by fluoroether I-15. The detailed composition of electrolyte and test results are shown in Table 4.

[0115] Table 4

[0116]

[0117] Examples 32 to 39

[0118] Examples 32 to 39 were prepared in a similar manner to Examples 1 to 10, except that fluoroether I-12 was replaced by fluoroether I-8. The detailed composition of electrolyte and test results are shown in Table 5.

[0119] Table 5

[0120] Category DME Fluoroether I-8 LiPF6 LiFSI VC TVSi HTS-DCR 60d growth rate Cycle number at 25°C Example 32 43.25 43.25 13 0.5 — — 0.14 1292 Example 33 60.20 25.80 13 0.5 0.5 — 0.12 1838 Example 34 60.41 25.89 13 0.5 — 0.2 0.11 1873 Example 35 69.20 17.30 13 0.5 — — 0.10 1724 Example 36 60.55 25.95 13 0.5 — — 0.09 1799 Example 37 51.90 34.60 13 0.5 — — 0.09 1929 Example 38 60.06 25.74 13 0.5 0.5 0.2 0.09 2125 Example 39 76.50 10.00 13 0.5 — — 0.13 1420

[0121] Examples 40 to 47

[0122] Examples 40 to 47 were prepared in a similar manner to Examples 1 to 10, except that fluoroether I-12 was replaced by fluoroether I-19. The detailed composition of electrolyte and test results are shown in Table 6.

[0123] Table 6

[0124]

[0125] Examples 48 to 55

[0126] Examples 48 to 55 were prepared in a similar manner to Examples 1 to 10, except that fluoroether I-12 was replaced by fluoroether I-11. The detailed composition of electrolyte and test results are shown in Table 7.

[0127] Table 7

[0128]

[0129]

[0130] Examples 56 to 63

[0131] Examples 56 to 63 were prepared in a similar manner to Examples 1 to 10, except that fluoroether I-12 was replaced by fluoroether I-7. The detailed composition of electrolyte and test results are shown in Table 8.

[0132] Table 8

[0133]

[0134] Examples 64 to 71

[0135] Examples 64-71 were prepared in a manner similar to Examples 1-10, except that fluoroether I-12 was replaced by fluoroether I-1. The detailed composition of electrolyte and test results are shown in Table 9.

[0136] Table 9

[0137]

[0138] Examples 72 to 79

[0139] Examples 72-79 were prepared in a manner similar to Examples 1-10, except that fluoroether I-12 was replaced by fluoroether I-2. The detailed composition of electrolyte and test results are shown in Table 10.

[0140] Table 10

[0141]

[0142] Examples 80 to 87

[0143] Examples 80-87 were prepared in a manner similar to Examples 1-10, except that fluoroether I-12 was replaced by fluoroether I-5. The detailed composition of electrolyte and test results are shown in Table 11.

[0144] Table 11

[0145]

[0146] As can be seen from the test results in Tables 1-11, the addition of the compound of formula (I) provided in the embodiments of the present disclosure to the electrolyte can improve the cycle stability of the battery cell and reduce the increase rate of direct current resistance under high-temperature storage conditions. The addition of LiFSI, VC and TVSi to the electrolyte can further improve the cycle stability of the battery cell.

[0147] In addition, the electrolyte provided in the embodiments of the present disclosure is particularly suitable for middle-nickel high-voltage positive electrode active materials, and is convenient for preparing lithium batteries with high energy density, good safety and low cost.

[0148] Those skilled in the art should understand: the discussion of the above any embodiment is only exemplary, and is not intended to imply that the scope of the disclosure (including claims) is limited to these examples; the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other changes of different aspects of the embodiments of the disclosure as described above, which are not provided in details for the sake of brevity.

[0149] Embodiments of the present disclosure are intended to cover all such alternatives, modifications, and variations as fall within the broad scope of the appended claims. Accordingly, any one of the above-mentioned alternatives, modifications, equivalents, improvements, etc. made in the spirit and principle of the embodiments of the present disclosure should be included in the scope of protection of the present disclosure.

Claims

1. An electrolyte, characterized by, The fluorinated ether solvent comprises at least one compound of formula (I): I wherein, R2 is C4 alkylene or C4 fluoroalkylene, wherein the number of fluorine atoms in the C4 fluoroalkylene is 1; R1 and R3 are each independently methyl or fluoromethyl, and the total number of fluorine atoms in R1 and R3 is 1-4; The mass percentage of the fluorinated ether solvent is 10%-45% based on the total mass of the electrolyte; and the mass percentage of the ethylene glycol dimethyl ether is 40%-75% based on the total mass of the electrolyte. The electrolyte is applied to a lithium battery, a positive active material of the lithium battery comprising Li a Ni x Mn y Co z O2; wherein 0.9 2. The electrolyte according to claim 1, characterized in that, The mass percentage of the fluorinated ether solvent is 15%-35% based on the total mass of the electrolyte.

3. The electrolyte according to claim 1 or 2, characterized in that, R2 is C4 chain alkylene or C4 fluoro chain alkylene.

4. The electrolyte according to claim 3, characterized in that R2 is C4 straight chain alkylene or C4 fluoro straight chain alkylene.

5. The electrolyte of claim 1, wherein The mass percentage of the ethylene glycol dimethyl ether is 50%-70% based on the total mass of the electrolyte.

6. The electrolyte of claim 1, wherein The electrolyte further comprises a lithium salt, wherein the lithium salt comprises at least one of LiPF6, LiBF4, LiClO4, LiCF3SO3, LiBOB, LiODFB and LiN(SO2CF3)2; The mass percentage of the lithium salt is 11%-15% based on the total mass of the electrolyte.

7. The electrolyte of claim 1, wherein The electrolyte further comprises at least one of lithium bisfluorosulfonylimide, vinylene carbonate and tetra-vinylsilane.

8. The electrolyte according to claim 7, characterized in that The mass percentage of the lithium bisfluorosulfonylimide is 0.1%-5% based on the total mass of the electrolyte; and / or The mass percentage of the vinylene carbonate is 0.1%-10%; and / or The mass percentage of the tetra-vinylsilane is 0.1%-10%.

9. The electrolyte according to claim 7, characterized in that The mass percentage of the lithium bisfluorosulfonylimide is 0.2%-1% based on the total mass of the electrolyte; and / or The mass percentage of the vinylene carbonate is 0.2%-1%; and / or The mass percentage of the tetra-vinylsilane is 0.2%-1%.

10. The electrolyte of claim 1, wherein, The compound of formula (I) is selected from at least one of the following compounds: 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 。 11. A lithium battery comprising the electrolyte according to any one of claims 1-10.

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