Lithium battery

Abstract

The invention provides a lithium battery. Specifically, the lithium battery comprises an electrolyte, and the electrolyte comprises a fluoro-ether solvent and a cyclic carbonate solvent; wherein the fluoroether solvent comprises at least one compound of formula (I): # imgabs0 #, R2 and R3 are respectively and independently selected from alkylene or fluoroalkylene, the total number of carbon atoms of R2 and R3 is 4, and the total number of fluorine atoms is less than or equal to 1; r1 and R4 are respectively and independently methyl or fluoromethyl, and the total number of fluorine atoms in R1 and R4 is 1-4. The electrolyte can improve the cycle performance of the medium-nickel high-voltage lithium battery, and is beneficial 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 a lithium battery. Background Art

[0002] The ternary cathode material is a new cathode material for lithium batteries, such as nickel cobalt manganese, nickel cobalt aluminum, etc. Nickel cobalt manganese includes nickel salt, cobalt salt and manganese salt, and stands out from the cathode materials of many lithium batteries with its higher working voltage and larger energy density. Based on the proportion of nickel in nickel cobalt manganese, the nickel cobalt manganese ternary cathode material includes a low-nickel cathode material, a medium-nickel cathode material and a high-nickel cathode material. Among them, the medium-nickel cathode material not only has an energy density effect similar to that of the high-nickel cathode 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 decompose, resulting in poor cycle performance of medium-nickel high-voltage batteries. Summary of the Invention

[0003] In view of this, the purpose of the present disclosure is to provide a lithium battery.

[0004] Based on the above purpose, the present disclosure provides a lithium battery, including an electrolyte and a cathode active material; the electrolyte includes a fluorinated ether solvent and a cyclic carbonate solvent; wherein, the fluorinated ether solvent includes at least one compound of formula (I):

[0005]

[0006] Wherein,

[0007] R2 and R3 are each independently selected from alkylene or fluorinated alkylene, and the total number of carbon atoms of R2 and R3 is 4, and the total number of fluorine atoms ≤ 1;

[0008] R1 and R4 are each independently methyl or fluorinated methyl, and the total number of fluorine atoms in R1 and R4 is 1 to 4;

[0009] The cathode active material includes 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.

[0010] In some embodiments, based on the total mass of the electrolyte, the mass ratio of the fluorinated ether solvent is 4% to 35%, optionally 15% to 25%; and / or

[0011] The total mass proportion of the fluoroether solvent and the cyclic carbonate solvent is 15% to 65%, optionally 25% to 60%, and more optionally 35% to 50%.

[0012] In some embodiments, R2 and R3 are each independently selected from -CH2-, -CH2-CH2-, -CH2-CH2-CH2-, -CHF-, -CHF-CH2-, -CHF-CH2-CH2-, or -CH2-CHF-CH2-.

[0013] In some embodiments, a linear carbonate solvent is also included; optionally, the linear carbonate solvent includes at least one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; optionally, the cyclic carbonate solvent includes at least one of propylene carbonate and ethylene carbonate.

[0014] In some embodiments, the linear carbonate solvent is ethyl methyl carbonate and dimethyl carbonate, and the cyclic carbonate solvent is ethylene carbonate; wherein,

[0015] Based on the total mass of the electrolyte, the mass proportion of the ethyl methyl carbonate is 35% to 45%; the mass proportion of the dimethyl carbonate is 5% to 7%; and the mass proportion of the ethylene carbonate is 15% to 25%.

[0016] In some embodiments, the electrolyte further comprises a lithium salt; optionally, based on the total mass of the electrolyte, the mass proportion of the lithium salt is 11% to 15%, optionally 13% to 14%.

[0017] In some embodiments, the lithium salt includes at least one of LiPF6, LiClO4, LiCF3SO3, LiBOB, LiODFB and LiN(SO2CF3)2; optionally, the lithium salt is LiPF6.

[0018] In some embodiments, the electrolyte further includes at least one of lithium bis(fluorosulfonyl)imide, fluoroethylene carbonate and LiBF4; optionally, based on the total mass of the electrolyte, the mass proportion of the lithium bis(fluorosulfonyl)imide is 0.1% to 5%, more optionally 0.2% to 1%; optionally, the mass proportion of the fluoroethylene carbonate is 0.1% to 10%, more optionally 0.3% to 1%; the mass proportion of the LiBF4 is 0.1% to 2%, more optionally 0.1% to 0.3%.

[0019] In some embodiments, the compound of formula (I) is selected from at least one of the following compounds:

[0020]

[0021]

[0022] As can be seen from the above, the present disclosure provides a lithium battery, comprising an electrolyte and a positive electrode active material, wherein the electrolyte comprises a fluoroether solvent and a cyclic carbonate solvent; wherein the fluoroether solvent comprises at least one compound of formula (I): Wherein, R2 and R3 are each independently selected from an alkylene group or a fluoroalkylene group, and the total number of carbon atoms in R2 and R3 is 4, and the total number of fluorine atoms is ≤1; R1 and R4 are each independently a methyl group or a fluoromethyl group, and the total number of fluorine atoms in R1 and R4 is 1 to 4. Such an electrolyte can improve the cycle performance of a medium nickel high voltage lithium battery, and is conducive to the long-term stable operation of the lithium battery. DETAILED DESCRIPTION

[0023] In order to make the objectives, technical solutions and advantages of the present disclosure more clearly understood, the present disclosure is further described in detail below in conjunction with specific embodiments.

[0024] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the embodiments of the present disclosure should have the common meanings understood by persons having ordinary skills in the field to which the present disclosure belongs.

[0025] The "range" used in the embodiments of the present disclosure is defined in the form of a lower limit and an upper limit, and 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 be inclusive or exclusive of end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60 to 130 and 70 to 120 is listed for a particular parameter, it is understood that a range of 60 to 120 and 70 to 130 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following ranges can all be expected: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4 and 2 to 5. In the present disclosure, unless otherwise specified, the numerical range "a to b" represents an abbreviation of any real number combination between a and b, wherein a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0026] If not otherwise specified, all embodiments and optional embodiments of the present disclosure may be combined with each other to form a new technical solution.

[0027] Unless otherwise specified, all technical features and optional technical features of the present disclosure can be combined with each other to form a new technical solution.

[0028] If there is no special explanation, the "include" and "comprising" mentioned in the present disclosure represent open or closed forms. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0029] If not specifically stated, in the present disclosure, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B".

[0030] In the related art, the solvent system of common lithium-ion battery electrolyte is mainly composed of a mixed solvent of cyclic carbonates and chain carbonates. Compared with ether solvents, cyclic carbonates and chain carbonates have better antioxidant properties, but they still have the risk of decomposition under high voltage conditions. Therefore, developing more diverse solvent molecular structures and giving lithium-ion battery electrolyte solvent systems more choices is a research hotspot.

[0031] Fluorinated ether solvents have both the higher solubility of ether solvents for lithium salts and a more stable chemical structure after modification with fluorine atoms, and are considered strong candidates for high-voltage battery electrolytes. However, due to the high viscosity and high cost of fluoroether solvents, it is difficult to use them as a single solvent system in lithium-ion batteries. In addition, due to the long carbon chain in the structure of fluoroethers, their solubility in common lithium salts is affected to a certain extent.

[0032] In order to achieve the purpose of the present disclosure, an embodiment of the present disclosure provides an electrolyte, which includes a new fluoroether solvent, which advantageously broadens the electrochemical working window of the electrolyte and improves the cycle performance of medium-nickel high-voltage batteries.

[0033] Electrolyte

[0034] The present disclosure provides an electrolyte solution, which includes a fluoroether solvent and a cyclic carbonate solvent; wherein the fluoroether solvent includes at least one compound of formula (I):

[0035]

[0036] in,

[0037] R2 and R3 are each independently selected from an alkylene group or a fluoroalkylene group, and the total number of carbon atoms of R2 and R3 is 4, and the total number of fluorine atoms is ≤1; here, "alkylene" refers to a saturated, branched or straight-chain, or cyclic hydrocarbon residue of 1 to 18 carbon atoms, and has two monovalent residue centers derived from the same carbon atom or two different carbon atoms of the parent alkane by removing two hydrogen atoms. Typical alkylene residues include, but are not limited to, methylene (-CH2-), 1,2-ethyl (-CH2CH2-), 1,3-propyl (CH2CH2CH2-), 1,4-butyl (-CH2CH2CH2CH2-), etc.

[0038] R1 and R4 are each independently a methyl group or a fluoromethyl group, and the total number of fluorine atoms in R1 and R4 is 1 to 4.

[0039] The electrolyte provided by the present disclosure can improve the cycle performance of the medium nickel high voltage lithium battery and help the lithium battery to operate stably for a long time. Here, the lithium battery can be a primary lithium battery or a secondary lithium battery, which is not limited by the present disclosure.

[0040] The electrolyte disclosed in the present invention includes a cyclic carbonate solvent, which has a high dielectric constant and is helpful in dissolving lithium salts, compensating for the insufficient solubility of fluoroethers in lithium salts, and making the lithium salts evenly distributed in the electrolyte, thereby ensuring the normal operation of the battery.

[0041] The compound of formula (I) in the electrolyte disclosed in the present invention is an ether with a relatively long carbon chain and containing at least one fluorine atom. After the fluorine atom is substituted, the charge density distribution of the compound itself is changed. The more dispersed charge density distribution improves the oxidation resistance of the molecule, thereby improving the overall antioxidant performance of the electrolyte and showing higher stability under high-temperature cycle conditions.

[0042] In some embodiments, R2 and R3 are each independently selected from -CH2-, -CH2-CH2-, -CH2-CH2-CH2-, -CHF-, -CHF-CH2-, -CHF-CH2-CH2-, or -CH2-CHF-CH2-. For example, R2 is -CH2-CH2-, and R3 is -CHF-CH2-; for another example, R2 is -CH2-, and R3 is -CH2-CHF-CH2-; for another example, R2 is -CHF-, and R3 is -CH2-CH2-CH2-, etc.

[0043] In some embodiments, the compound of formula (I) is selected from at least one of the following compounds:

[0044]

[0045]

[0046] In some embodiments, the compound of formula (I) is selected from at least one of I-4, I-5, I-8, I-10, I-13, I-15, I-16, and I-19. In some embodiments, optionally, the compound of formula (I) is selected from at least one of I-5, I-10, I-13, I-15, and I-16.

[0047] For the above compounds, the inventors found that although the oxidative stability of fluoroethers is relatively high, the difference in the number and position of fluorine substitutions causes the difference in the uniformity of the charge distribution of the overall structure of the compound, and the chemical stability of the structure also varies. The more uniform the charge distribution of the overall structure, the higher the corresponding oxidative stability, and the mixed electrolyte with ether solvents has better room temperature cycle performance. In addition, since the change in the fluoroether structure will affect the solvation structure of lithium ions in the electrolyte, it can also increase the stability of the electrolyte itself.

[0048] In some embodiments, the total mass proportion of the fluoroether solvent and the cyclic carbonate solvent is 15% to 65%, optionally 25% to 60%, and more optionally 35% to 50%.

[0049] In some embodiments, based on the total mass of the electrolyte, the mass proportion of the fluoroether solvent is 4% to 35%, for example, 10%, 14%, 18%, 20%, 25%, 31%; optionally, 15% to 25%. During the cycle, after the fluoroether is added, the improvement in the thermal stability of the electrolyte is accompanied by an increase in the viscosity of the electrolyte. High thermal stability helps to improve the performance of the battery cell, while high viscosity will lead to a decrease in the performance of the battery cell. The mass range of fluoroether solvents is less than 10%, and the antioxidant stability of the solvent is insufficient, making it difficult to improve the stability of the electrolyte. The mass range of fluoroether solvents is greater than 50%, which will lead to excessive viscosity of the electrolyte and a decrease in the number of cycles at room temperature. This may be because the DC internal resistance gradually increases during the cycle, the internal polarization of the battery cell increases, and the number of cycles at room temperature of the battery cell deteriorates.

[0050] In some embodiments, the cyclic carbonate solvent includes at least one of propylene carbonate (PC) and ethylene carbonate (EC). Cyclic carbonate solvents have a higher dielectric constant, which helps to dissolve lithium salts and evenly distribute them in the electrolyte, thereby ensuring the normal operation of the battery. Optionally, the cyclic carbonate solvent is ethylene carbonate. In addition to increasing the solubility of lithium salts, ethylene carbonate can also form a stable solvation configuration with lithium ions, improving the stability and conductivity of the electrolyte, thereby improving the performance of the lithium battery.

[0051] In some embodiments, the electrolyte further includes a linear carbonate. Exemplarily, the linear carbonate solvent includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). Since the structural main chain of the fluoroether solvent is long, its overall viscosity is high, and the electrolyte includes a linear carbonate solvent, which can reduce the overall viscosity and help improve the conductivity, cycle life and energy density of the lithium battery. In addition, DMC can improve the wettability of the overall electrolyte, which is beneficial to improving the overall cycle performance of the battery cell. At the same time, due to the addition of carboxylic acid esters, the solvation structure of lithium ions is changed, and the film-forming material during the charge and discharge process is changed, which also helps to improve the overall cycle performance of the battery cell.

[0052] In some embodiments, the electrolyte includes ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate. Further, based on the total mass of the electrolyte, the mass proportion of ethylene carbonate is 15% to 25%; the mass proportion of ethyl methyl carbonate is 35% to 45%; and the mass proportion of dimethyl carbonate is 5% to 7%. The combination of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate is conducive to ensuring the solubility of the solvent in the lithium salt while controlling the viscosity of the electrolyte, thereby obtaining a high-performance lithium battery.

[0053] In some embodiments, the electrolyte further comprises a lithium salt; optionally, based on the total mass of the electrolyte, the mass proportion of the lithium salt is 11% to 15%, optionally 13% to 14%.

[0054] In some embodiments, the lithium salt includes at least one of LiPF6, LiClO4, LiCF3SO3, LiBOB, LiODFB, and LiN(SO2CF3)2.

[0055] Optionally, the lithium salt is LiPF6. LiPF6 has good solubility in a mixture of carbonate solvents and fluoroether solvents, thereby avoiding the degradation of battery performance caused by insufficient solubility of the lithium salt.

[0056] In some embodiments, the electrolyte further includes an additive, and optionally, the additive includes at least one of lithium bis(fluorosulfonyl)imide (LiFSI), fluoroethylene carbonate (FEC), and LiBF4. Lithium fluorosulfonyl imide helps to increase the lithium ions in the electrolyte, thereby improving the overall performance of the battery; fluoroethylene carbonate (FEC) participates in film formation during the initial formation process, and the inorganic components of LiF can reduce the electronic conductivity of SEI and CEI, thereby enhancing the stability of SEI and CEI in both high temperature and room temperature cycles. The addition of LiBF4 improves the conductivity of the overall electrolyte. When used in combination with fluoroethers, it makes up for the decrease in conductivity caused by a part of the high viscosity solvent, thereby improving the cycle performance.

[0057] In some embodiments, based on the total mass of the electrolyte, the mass proportion of the lithium bis(fluorosulfonyl)imide is 0.1% to 5%, and more optionally 0.2% to 1%; optionally, the mass proportion of the fluoroethylene carbonate is 0.1% to 10%, and more optionally 0.3-1%; the mass proportion of the LiBF4 is 0.1% to 2%, and more optionally 0.1 to 0.3%.

[0058] Lithium battery

[0059] A second aspect of the present disclosure provides a lithium battery, comprising the electrolyte provided in the above disclosure.

[0060] In some embodiments, the lithium battery includes 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 battery charging and discharging process, active ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of 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 the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

[0061] Positive electrode

[0062] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0063] Exemplarily, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.

[0064] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may 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 substrate (e.g., polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.).

[0065] In some embodiments, the positive electrode active material includes lithium nickel cobalt manganese oxide, such as 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), LiNi0.5 Co 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).

[0066] Optionally, the positive electrode active material of the lithium battery includes 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. 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 film layer has poor conductivity, resulting in 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.

[0067] 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.

[0068] 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.

[0069] In some embodiments, the positive electrode plate can be prepared in the following manner: Dispersing the components for preparing the positive electrode plate, such as the positive electrode active material, conductive agent, 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.

[0070] Negative electrode

[0071] The negative electrode sheet includes a negative electrode current collector and a film layer optionally disposed on at least one surface of the negative electrode current collector.

[0072] Exemplarily, the film layer may include a negative electrode active material (eg, artificial graphite), a conductive agent, a thickener, and a binder.

[0073] In some embodiments, the negative electrode sheet can be prepared in the following manner: the negative electrode active material, the conductive agent, the thickener, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0074] Isolation film

[0075] In some embodiments, the secondary battery further includes a separator. The present disclosure has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

[0076] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0077] Example

[0078] Hereinafter, embodiments of the present disclosure are described. The embodiments described below are exemplary and are only used to explain the present disclosure, and should not be construed as limiting the present disclosure. If specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in the art or the product instructions are used. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be obtained commercially.

[0079] Example 1 to Example 15, Comparative Example 1

[0080] The embodiments 1 to 15 and comparative example 1 of the present disclosure are secondary lithium batteries, wherein the positive electrode active material is a medium nickel high voltage positive electrode material, and the specific preparation method thereof is as follows:

[0081] (1) LiNi 0.6 Mn 0.3 Co 0.1 Preparation of O2 positive electrode:

[0082] The positive electrode active material LiNi 0.6 Mn 0.3 Co 0.1O2, polyvinylidene fluoride as a binder, and conductive carbon black (Super P) as a conductive agent are mixed in a weight ratio of 98:1:1, N-methylpyrrolidone (NMP) is added, and the mixture is stirred under the action of a vacuum mixer until the system becomes uniform and transparent to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on an aluminum foil; the aluminum foil is dried at room temperature and then transferred to an oven for drying, and then cold pressed and cut to obtain a positive electrode (positive electrode sheet);

[0083] (2) Preparation of graphite negative electrode:

[0084] Artificial graphite is used as the negative electrode active material, Super P is used as the conductive agent, sodium carboxymethyl cellulose (CMC-Na) is used as the thickener, and styrene-butadiene rubber (SBR) is used as the binder, which are mixed in a mass ratio of 96:1:1:2, and deionized water is added to obtain the negative electrode slurry under the action of a vacuum mixer; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil; the copper foil is dried at room temperature and then transferred to an oven for drying, and then the negative electrode (pole sheet) is obtained through cold pressing and slitting.

[0085] (3) Preparation of electrolyte:

[0086] In an argon atmosphere glove box with a water content of <10 ppm, battery-grade fluoroether I-5, dimethyl carbonate (DMC), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed according to the proportions in Comparative Example 1 and Examples 1 to 15 to form a total organic solvent. LiPF6, LiFSI, LiBF4, FEC and the above-mentioned organic solvents are mixed uniformly according to the proportions in Comparative Example 1 and Examples 1 to 15 to obtain an electrolyte.

[0087] The proportions of the components in Examples 1 to 15 and Comparative Example 1 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.

[0088] (4) Preparation of isolation membrane:

[0089] Polypropylene film is used as the isolation film.

[0090] (5) Preparation of secondary batteries:

[0091] A 12 μm thick polypropylene film was used as a separator, and the positive electrode, separator, and negative electrode prepared above were stacked in sequence, so that the separator was placed between the positive and negative electrodes to play a role of isolation. Then, an aluminum-plastic film was wrapped outside, transferred to a vacuum oven for drying at 120°C, and the electrolyte prepared above was injected at 3.0 g / Ah, and then sealed, and the electrolyte was formed, and finally a soft-pack battery (i.e., a lithium-ion battery) with a capacity of 1 Ah was prepared.

[0092] Comparative Example 2 to Comparative Example 5

[0093] The difference between Comparative Examples 2 to 5 and Example 1 is the composition of the organic solvent in step (3). The components and proportions of the organic solvents in Comparative Examples 2 to 5 are shown in Table 1. In Table 2, A represents the compound represented by formula A B represents a compound of formula B

[0094] Example 16, Comparative Example 6

[0095] Example 16, Comparative Example 6 and Example 1 are different in that step (1): the positive electrode active material is replaced with LiNi 0.9 Mn 0.05 Co 0.05 The composition and proportion of the organic solvent in step (3) are shown in Table 2.

[0096] Test Method

[0097] The secondary batteries prepared in the above embodiments and comparative examples can be tested by the following method:

[0098] (1) Secondary battery high temperature cycle test

[0099] In an oven at a specified temperature (45°C), cyclic charge and discharge were performed at a current of 1C in a specified potential range (2.0V-3.8V), and the discharge capacity of each cycle was recorded. The test was terminated when the battery capacity reached 80% of the first cycle capacity. The test results are detailed in Tables 1 and 2.

[0100] (2) Secondary battery cycle test at room temperature

[0101] In an oven at a specified temperature (25°C), cyclic charge and discharge were performed at a current of 1C in a specified potential range (2.0V-3.8V), and the discharge capacity of each cycle was recorded. The test was terminated when the battery capacity reached 80% of the first cycle capacity. The test results are detailed in Tables 1 and 2.

[0102] Table 1

[0103]

[0104]

[0105] Table 2

[0106]

[0107] Comparing Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 5, and Example 4, it can be seen that adding fluoroether I-5 to the electrolyte can significantly improve the stability of the normal temperature cycle and the high temperature cycle stability of the battery. Comparing Comparative Example 2, Comparative Example 3, Comparative Example 5 and Example 4, it can be seen that different fluoroether compounds have a large difference in the improvement effect on the cycle stability of the battery. The fluoroether I-5 provided by the embodiment of the present disclosure has a significantly better improvement effect on the cycle stability of the battery than octafluoropentyl-tetrafluoroethyl ether, Formula A and Formula B, especially octafluoropentyl-tetrafluoroethyl ether, which may be due to the large fluorine content of octafluoropentyl-tetrafluoroethyl ether but the short chain length, resulting in its thermal stability being not as good as fluoroether I-5.

[0108] Comparing Examples 1 to 5, it can be seen that adding fluoroether I-5 to the electrolyte can improve the cycle stability of the battery cell. In particular, when the mass proportion of fluoroether I-5 is within the range of 15% to 25%, the addition of fluoroether I-5 significantly improves the cycle stability of the battery cell.

[0109] By comparing Comparative Example 4, Example 4 and Implementation 10, it can be found that EC and EMC (or EC, EMC and DMC) with fluoroether I-5 are more conducive to improving the cycle stability of the battery than tetraethylene glycol dimethyl ether (G4) with fluoroether I-5. EC, EMC and DMC have better combined effects than EC and EMC. G4 is not substituted by fluorine atoms and has a long chain and a large number of ether bonds. It is thermodynamically unstable and will decompose into film-forming substances that are not conducive to lithium ion conduction on the negative electrode during the cycle, resulting in poor performance of the battery including G4. It should be noted that, except for fluoroether I-5, the compounds of formula (I) all show similar results and are not listed in detail.

[0110] By comparing Example 16 and Comparative Example 6, it can be seen that compared with the significant improvement in the cycle stability of the battery cell prepared by adding fluoroether I-5 to medium-nickel positive electrode active materials, the cycle stability of the battery cell prepared by fluoroether I-5 is not significantly improved. This may be because the material of the high-nickel positive electrode is inconsistent with the stable electrochemical window of the electrolyte including fluoroether in the present application. During the charge and discharge process, the fluoroether undergoes an oxidation reaction on the positive electrode interface and generates a film-forming substance on the positive electrode surface. The ionic conductivity of this layer of material itself is poor, and the battery cell experiences a dive in the later stage of the cycle.

[0111] Except for the fluoroether I-5, the compounds of formula (I) exhibited similar results when combined with high-nickel positive electrode materials, which will not be described in detail.

[0112] It should be noted that the positive electrode active materials of Examples 1 to 15 are only representative of the medium nickel positive electrode active materials, and are not limiting. The medium nickel positive electrode active materials disclosed herein refer to Lia 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.

[0113] Comparing Examples 4, 6 - 9, it can be seen that adding one or more of LiFSI, LiBF4, and FEC to the electrolyte can improve the cycling performance of the battery cell. Among them, FEC forms a relatively dense SEI and CEI on the surfaces of the positive and negative electrodes, which can protect the positive and negative electrode materials during cycling and is beneficial to improving the cycling performance of the battery cell.

[0114] Example 17 to Example 20

[0115] The preparation methods of Examples 17 - 20 are similar to that of Example 3, except that the structures of the fluoroethers are different. The detailed compositions and test results of the electrolytes are shown in Table 3.

[0116] Table 3

[0117]

[0118] From the test results in Tables 1 - 3, it can be seen that adding the compound of formula (I) provided in the embodiments of the present disclosure to the electrolyte can improve the cycling stability of the battery cell. Among them, I - 5, I - 8, and I - 10 have a more significant improvement in the cycling stability of the battery cell. Adding DMC, LiFSI, LiBF4, and FEC to the electrolyte can further improve the cycling stability of the battery cell.

[0119] In addition, the electrolyte provided in the embodiments of the present disclosure is particularly suitable for medium - nickel high - voltage cathode active materials, facilitating the preparation of lithium batteries with high energy density, good safety, and low cost.

[0120] Those of ordinary skill in the art should understand that: the discussion of any above - mentioned embodiment is only exemplary, and is not intended to imply that the scope of the present disclosure (including the claims) is limited to these examples; under the concept of the present disclosure, the technical features between the above - mentioned embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of the embodiments of the present disclosure as described above, which are not provided in detail for the sake of brevity.

[0121] The embodiments of the present disclosure are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of the appended claims. Therefore, any omission, modification, equivalent substitution, improvement, etc., made within the spirit and principle of the embodiments of the present disclosure shall be included within the protection scope of the present disclosure.

Claims

1. A lithium battery, characterized in that: The invention comprises an electrolyte and a positive electrode active material; wherein the electrolyte comprises a fluoroether solvent and a cyclic carbonate solvent; wherein the fluoroether solvent comprises at least one compound of formula (I): in, R2 and R3 are each independently selected from an alkylene group or a fluoroalkylene group, and the total number of carbon atoms of R2 and R3 is 4, and the total number of fluorine atoms is ≤1; R1 and R4 are each independently a methyl group or a fluoromethyl group, and the total number of fluorine atoms in R1 and R4 is 1 to 4; The positive electrode active material includes 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.

2. The lithium battery according to claim 1, characterized in that: Based on the total mass of the electrolyte, the mass proportion of the fluoroether solvent is 4% to 35%, and optionally 15% to 25%.

3. The lithium battery according to claim 2, characterized in that: The total mass proportion of the fluoroether solvent and the cyclic carbonate solvent is 15% to 65%, optionally 25% to 60%, and more optionally 35% to 50%.

4. The lithium battery according to claim 1, characterized in that R2 and R3 are each independently selected from -CH2-, -CH2-CH2-, -CH2-CH2-CH2-, -CHF-, -CHF-CH2-, -CHF-CH2-CH2- or -CH2-CHF-CH2-.

5. The lithium battery according to any one of claims 1 to 4, characterized in that: It also includes a chain carbonate solvent; optionally, the chain carbonate solvent includes at least one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; optionally, the cyclic carbonate solvent includes at least one of propylene carbonate and ethylene carbonate.

6. The lithium battery according to claim 5, characterized in that: The chain carbonate solvent includes ethyl methyl carbonate and dimethyl carbonate, and the cyclic carbonate solvent includes ethylene carbonate; wherein, Based on the total mass of the electrolyte, the mass proportion of the ethyl methyl carbonate is 35% to 45%; the mass proportion of the dimethyl carbonate is 5% to 7%; and the mass proportion of the ethylene carbonate is 15% to 25%.

7. The lithium battery according to any one of claims 1 to 6, characterized in that: The electrolyte further comprises a lithium salt; optionally, based on the total mass of the electrolyte, the mass proportion of the lithium salt is 11% to 15%, optionally 13% to 14%.

8. The lithium battery according to claim 7, characterized in that: The lithium salt includes at least one of LiPF6, LiClO4, LiCF3SO3, LiBOB, LiODFB and LiN(SO2CF3)2; optionally, the lithium salt is LiPF6.

9. The lithium battery according to any one of claims 1 to 8, characterized in that: The electrolyte also includes at least one of lithium bis(fluorosulfonyl)imide, fluoroethylene carbonate and LiBF4; optionally, based on the total mass of the electrolyte, the mass proportion of the lithium bis(fluorosulfonyl)imide is 0.1% to 5%, more optionally 0.2% to 1%; optionally, the mass proportion of the fluoroethylene carbonate is 0.1% to 10%, more optionally 0.3% to 1%; the mass proportion of the LiBF4 is 0.1% to 2%, more optionally 0.1% to 0.3%.

10. The lithium battery according to any one of claims 1 to 9, characterized in that: The compound of formula (I) is selected from at least one of the following compounds:

Images