Electrolyte and lithium-ion battery

Abstract

An electrolyte and a lithium-ion battery, which mainly solve the problem of poor compatibility of an electrolyte, which takes a phosphate as a main solvent, and a graphite negative electrode in the prior art. The electrolyte comprises a lithium salt, an additive and an organic solvent, wherein the organic solvent comprises a cyclic ester and a chain ester. The mass ratio of the chain ester in the electrolyte is greater than that of the cyclic ester; and the chain ester comprises a phosphate and a linear ester. The structural formula of the phosphate ester is formula (I), wherein R is selected from a monofluoro- or polyfluoro-substituted saturated or unsaturated aliphatic hydrocarbon, or a monofluoro- or polyfluoro-substituted aromatic hydrocarbon. The linear ester is one or more of a chain carbonate, a chain carboxylic ester and a fluorocarboxylic ester. The compatibility of a phosphate electrolyte and a graphite negative electrode is improved, thereby prolonging the cycle life of a battery.

Description

Electrolyte and lithium-ion battery Technical Field

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

[0002] Lithium-ion batteries are currently the main electrochemical energy storage method in battery applications such as large-scale energy storage, electric vehicles, and consumer electronics. The most widely used solvent system is still mainly carbonate solvents, such as binary or ternary mixed solvents based on ethylene carbonate (EC) with a large dielectric constant, among which chain carbonates (dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC)), carboxylates or ethers are co-solvents. These commonly used solvents have good solubility with lithium salts, high electrolyte conductivity, and the ability to form stable SEI films at the positive and negative electrodes. Therefore, they are considered to be the best choice for lithium-ion battery electrolyte systems, especially for electric vehicle battery electrolyte solvents. However, these solvents are highly flammable, so people have to pay attention to the safety of batteries in actual use.

[0003] In order to prevent lithium-ion batteries from being abused in various ways, which may lead to electrolyte decomposition and trigger chain reactions and then combustion, most researchers in the past have adopted the method of adding flame retardant additives. Various phosphates have often been added to lithium-ion battery electrolytes as flame retardant additives to improve the safety performance of lithium-ion batteries. It is generally believed that adding a large amount of phosphate additives (5% or more) to lithium-ion battery electrolytes, although it can improve the safety performance of the battery, will have a negative impact on battery performance. One way to solve this problem is to find more efficient safety additives, and another way is to directly use phosphate as the main solvent for lithium-ion battery electrolytes. Although phosphate as the main solvent can even adapt to 4.5V lithium-rich positive electrode materials at the positive end and has great application potential, it is not compatible with the graphite negative electrode in the current mainstream lithium-ion batteries. Summary of the Invention

[0004] The object of the present invention is to provide an electrolyte and a lithium ion battery, wherein the electrolyte can improve the compatibility of a phosphate solvent and a graphite negative electrode, thereby increasing the cycle life of the lithium ion battery.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A first aspect of the present invention provides an electrolyte comprising a lithium salt, an additive, and an organic solvent, wherein the organic solvent comprises a cyclic ester and a chain ester, and the chain ester accounts for a greater proportion by mass in the electrolyte than the cyclic ester.

[0007] The chain esters include phosphate esters and linear esters;

[0008] The structural formula of the phosphate ester is wherein R is selected from a mono- or poly-fluorinated saturated or unsaturated aliphatic hydrocarbon, or a mono- or poly-fluorinated aromatic hydrocarbon;

[0009] The linear ester is one or more of chain carbonate, chain carboxylate, and fluorocarboxylate.

[0010] The present invention improves the safety performance of the battery by adding phosphate ester, and improves the compatibility of phosphate ester with the graphite negative electrode by compounding phosphate ester, linear ester and cyclic ester, thereby increasing the cycle life of the battery.

[0011] According to some specific embodiments, R is selected from mono- or poly-fluorinated saturated aliphatic hydrocarbons having 1 to 8 carbon atoms.

[0012] Furthermore, the R is selected from mono- or poly-fluorinated saturated aliphatic hydrocarbons having 1 to 5 carbon atoms.

[0013] Furthermore, the R is selected from mono- or poly-fluorinated saturated aliphatic hydrocarbons having 1 to 3 carbon atoms.

[0014] Furthermore, the R is a mono- or poly-fluorinated ethyl group.

[0015] According to some more specific embodiments, the phosphate ester is selected from One or more of .

[0016] According to some specific embodiments, the chain carbonate is selected from one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and methylpropyl carbonate.

[0017] According to some specific embodiments, the chain carboxylate is selected from one or more of methyl propionate, ethyl propionate, propyl propionate, methyl acetate, ethyl acetate, propyl acetate, methyl butyrate, ethyl butyrate, and propyl butyrate.

[0018] According to some specific embodiments, the fluorocarboxylic acid ester is selected from one or more of methyl fluoropropionate, ethyl fluoropropionate, and ethyl fluoroacetate.

[0019] Furthermore, the fluorocarboxylate is 2,2-difluoroethyl acetate (DFEA, CAS: 1550-44-3).

[0020] According to some specific embodiments, the cyclic ester is selected from one or more of γ-butyrolactone, ethylene carbonate, propylene carbonate, and fluoroethylene carbonate.

[0021] According to some specific embodiments, the molar ratio of the phosphate ester to the linear ester is 3:(1-12).

[0022] According to some specific embodiments, the molar ratio of the phosphate ester to the linear ester is 1:(0.5-4). The present invention further improves the cycle life of the battery by further optimizing the ratio of the phosphate ester to the linear ester.

[0023] Furthermore, the molar ratio of the phosphate ester to the linear ester is 1:(2-4), for example, 1:2, 1:2.5, 1:3, 1:3.5, 1:4.

[0024] According to some specific embodiments, the cyclic ester accounts for 5-20% of the total mass of the electrolyte. Further, the cyclic ester accounts for 5-15% of the total mass of the electrolyte. Further, the cyclic ester accounts for 8-12% of the total mass of the electrolyte.

[0025] According to some specific embodiments, the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, anhydrous lithium perchlorate, lithium bis(trifluoromethylsulfonyl)imide, lithium difluorobisoxalate phosphate, lithium difluorophosphate, lithium trifluoromethylsulfonate, lithium difluorobisoxalate phosphate, lithium dioxalatoborate, lithium monooxalatobisfluoroborate, and lithium bisfluorosulfonyl imide.

[0026] According to some specific embodiments, the concentration of the lithium salt in the electrolyte is 0.6 to 1.8 mol / L. Further, the concentration of the lithium salt in the electrolyte is 0.8 to 1.2 mol / L.

[0027] According to some more specific embodiments, the lithium salt is lithium hexafluorophosphate or lithium bis(oxalatoborate).

[0028] According to some more specific embodiments, the lithium salt is lithium hexafluorophosphate, or lithium bisfluorosulfonyl imide, or a combination of lithium hexafluorophosphate and lithium bisfluorosulfonyl imide.

[0029] According to some specific embodiments, the additive includes one or more of 3-phenyl-1,4,2-dioxazol-5-one, vinylene carbonate, 1,3-propane sultone, propenyl-1,3-sultone, vinyl sulfate, propenyl sulfate, 1,4-butane sultone, and lithium difluorophosphate.

[0030] According to some specific embodiments, the additive accounts for 0.1-2% of the total mass of the electrolyte, for example, 0.1%, 0.5%, 1%, 1.5% or 2%.

[0031] According to a first more specific embodiment, the electrolyte includes a lithium salt, an additive and an organic solvent, the organic solvent includes a cyclic ester and a chain ester, and the chain ester accounts for a larger proportion by mass in the electrolyte than the cyclic ester.

[0032] The chain ester includes a first chain ester (phosphate ester) and a second chain ester. The structural formula of the first chain ester is wherein R is selected from a mono- or poly-fluorinated saturated or unsaturated aliphatic hydrocarbon, or a mono- or poly-fluorinated aromatic hydrocarbon;

[0033] The second chain ester is a chain carbonate and / or a chain carboxylate, and the chain carboxylate is selected from one or more of methyl propionate, ethyl propionate, propyl propionate, methyl acetate, ethyl acetate, propyl acetate, methyl butyrate, ethyl butyrate, and propyl butyrate;

[0034] The molar ratio of the first chain ester (ie, phosphate ester) to the second chain ester is 1:(0.5-4).

[0035] This first, more specific embodiment improves the safety performance of the battery by adding a first chain ester (i.e., phosphate ester). By compounding the first chain ester, the second chain ester, and the cyclic ester, and by optimizing the ratio of each ester solvent component, the compatibility of the phosphate ester with the graphite negative electrode is improved, thereby increasing the cycle life of the battery.

[0036] According to a second more specific embodiment, the electrolyte includes a lithium salt, an additive and an organic solvent, the organic solvent includes a cyclic ester and a chain ester, the chain ester includes a phosphate ester and a fluorocarboxylate ester, and the structural formula of the phosphate ester is wherein R is selected from a mono- or poly-fluorinated saturated or unsaturated aliphatic hydrocarbon, or a mono- or poly-fluorinated aromatic hydrocarbon;

[0037] The fluorocarboxylic acid ester is selected from one or more of methyl fluoropropionate, ethyl fluoropropionate, and ethyl fluoroacetate.

[0038] This second, more specific embodiment can effectively improve the room temperature cycle performance of the battery by compounding cyclic esters, phosphates and fluorocarboxylates. At the same time, the use of fluorocarboxylates is more effective in improving the flame retardant properties of the electrolyte than carboxylates or chain carbonates that are not substituted by fluorine.

[0039] According to a third more specific embodiment, the electrolyte includes a lithium salt, an additive and an organic solvent, the lithium salt is lithium bis(fluorosulfonyl)imide, or a combination of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, the organic solvent includes a cyclic ester and a chain ester, the chain ester includes a phosphate ester and a chain carbonate and / or a chain carboxylate, and the structural formula of the phosphate ester is wherein R is selected from a mono- or poly-fluorinated saturated or unsaturated aliphatic hydrocarbon, or a mono- or poly-fluorinated aromatic hydrocarbon;

[0040] The chain carboxylic acid ester is selected from one or more of methyl propionate, ethyl propionate, propyl propionate, methyl acetate, ethyl acetate, propyl acetate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl fluoropropionate, ethyl fluoropropionate, and ethyl fluoroacetate.

[0041] This third more specific embodiment, through the compounding of cyclic esters, phosphates, and chain carbonates and / or chain carboxylates, and the optimization of lithium salts, enables the battery to have better room temperature cycle performance and rate performance while having better flame retardant performance.

[0042] A second aspect of the present invention provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the electrolyte is the electrolyte described above.

[0043] According to some specific embodiments, the active material of the negative electrode is graphite.

[0044] According to some specific embodiments, the active material of the positive electrode is lithium cobalt oxide material.

[0045] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0046] The present invention improves the compatibility of the phosphate solvent and the graphite negative electrode by optimizing the solvent system of the electrolyte, thereby increasing the cycle life of the battery. DETAILED DESCRIPTION

[0047] The present invention is further described below with reference to the following examples. However, the present invention is not limited to the following examples. The implementation conditions used in the examples may be further adjusted according to the specific requirements of the application. Unspecified implementation conditions are conventional conditions in the industry. The technical features involved in the various embodiments of the present invention may be combined with each other as long as they do not conflict with each other.

[0048] Unless otherwise specified, the raw materials used in the specific examples or comparative examples of the present invention are all commercially available products or can be prepared by conventional methods.

[0049] Compound 1 is Tris(2,2,2-trifluoroethyl) phosphate, CAS No.: 358-63-4;

[0050] Compound 2 is Tris(2,2-difluoroethyl) phosphate, CAS No.: 358-64-5;

[0051] Compound 3 is Tris(2-fluoroethyl) phosphate, CAS No.: 358-65-6;

[0052] Compound 4 is Tris(1,1,2,2-tetrafluoroethyl) phosphate, CAS number: 1268855-78-2, was prepared in-house. Trimethyl phosphate (28 g, 0.2 mol) and NaI (89.9 g, 0.6 mol) were added sequentially to 170 mL of anhydrous acetonitrile. The reaction mixture was heated to 85°C and refluxed. 1-Chloro-1,1,2,2-tetrafluoroethane (218.4 g, 1.6 mol) was slowly introduced through a dip tube for 60-72 hours. After the reaction was complete, the mixture was cooled to room temperature and diluted with diethyl ether (1600 mL). The organic phase was washed with deionized water (2 × 500 mL) and saturated sodium thiosulfate solution (2 × 500 mL). The organic layer was dried over anhydrous sodium sulfate, filtered, and concentrated. Purification by column chromatography afforded a colorless liquid weighing 38.3 g, with a purity of 99.1% and a yield of 48.1%.

[0053] Compound 5 is Tris(perfluoroethyl) phosphate, CAS number: 447411-37-2, is prepared in-house. Trimethyl phosphate (28 g, 0.2 mol) solution and NaI (89.9 g, 0.6 mol) were added sequentially to 170 mL of anhydrous acetonitrile. The reaction mixture was heated to 85°C and refluxed. 1-Chloro-1,1,2,2,2-pentafluoroethane (247.2 g, 1.6 mol) was slowly introduced through a dip tube for 72-80 hours. After completion of the reaction, the mixture was cooled to room temperature and diluted with diethyl ether (1600 mL). The organic phase was washed with deionized water (2 × 500 mL) and saturated sodium thiosulfate solution (2 × 500 mL). The organic layer was dried over anhydrous sodium sulfate, filtered, and concentrated. Purification by column chromatography afforded a colorless liquid weighing 44.8 g, with a purity of 99.2% and a yield of 49.6%.

[0054] Unless otherwise specified, “%” in the following examples and comparative examples refers to mass percentage.

[0055] Comparative Example 1

[0056] In a nitrogen-filled glove box (H2O <10ppm, O2 <10ppm), 1 equivalent of LiPF6 was weighed and dissolved in Compound 1 to prepare an electrolyte containing 1 mol / L LiPF6. Finally, 10% FEC (fluoroethylene carbonate) was added to the electrolyte to prepare the target electrolyte.

[0057] Comparative Example 2

[0058] In a nitrogen-filled glove box (H2O <10ppm, O2 <10ppm), 1 equivalent of LiPF6 was weighed and dissolved in Compound 1 to prepare an electrolyte containing 1 mol / L LiPF6. Finally, 10% FEC and 0.5% VC were added to this electrolyte to obtain the target electrolyte.

[0059] Comparative Example 3

[0060] In a nitrogen-filled glove box (H2O <10ppm, O2 <10ppm), 1 equivalent of LiPF6 was weighed and dissolved in Compound 1 to prepare an electrolyte containing 1 mol / L LiPF6. Finally, 10% FEC, 1% PDO (3-phenyl-1,4,2-dioxazol-5-one), and 0.5% VC were added to this electrolyte to obtain the target electrolyte.

[0061] Comparative Example 4

[0062] In a nitrogen-filled glove box (H2O <10 ppm, O2 <10 ppm), 1 equivalent of LiPF6 was weighed and dissolved in compound 1 to prepare an electrolyte containing 1 mol / L LiFSI. Finally, 10% FEC was added to the electrolyte to prepare the target electrolyte.

[0063] Comparative Example 5

[0064] In a nitrogen-filled glove box (H2O <10ppm, O2 <10ppm), 1 equivalent of LiPF6 was weighed and dissolved in compound 1 to prepare an electrolyte containing 1 mol / L LiFSI. Finally, 10% FEC and 0.5% VC were added to the electrolyte to obtain the target electrolyte.

[0065] Comparative Example 6

[0066] In a nitrogen-filled glove box (H2O <10ppm, O2 <10ppm), 1 equivalent of LiPF6 was weighed and dissolved in compound 1 to prepare an electrolyte containing 1 mol / L LiFSI. Finally, 10% FEC, 1% PDO, and 0.5% VC were added to this electrolyte to obtain the target electrolyte.

[0067] Comparative Example 7

[0068] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), EC and EMC were mixed at a molar ratio of 1:3 to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte containing 1 mol / L LiPF₆, thereby obtaining the target electrolyte.

[0069] Example 1

[0070] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 1 and EP were mixed in a 3:1 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0071] Example 2

[0072] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 1 and EMC were mixed at a molar ratio of 3:1 to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0073] Example 3

[0074] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 1 and DFEA were mixed in a 3:1 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the desired electrolyte solution.

[0075] Example 4

[0076] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 1 and EA were mixed in a 3:1 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0077] Example 5

[0078] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 1 and EP were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0079] Example 6

[0080] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 1 and EMC were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0081] Example 7

[0082] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 1 and EA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0083] Example 8

[0084] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 1 and DFEA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0085] Example 9

[0086] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 2 and EP were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0087] Example 10

[0088] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 2 and EMC were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0089] Example 11

[0090] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 2 and DFEA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0091] Example 12

[0092] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 2 and EA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0093] Example 13

[0094] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 3 and EP were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0095] Example 14

[0096] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 3 and EMC were mixed at a molar ratio of 1:3 to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0097] Example 15

[0098] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 3 and DFEA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0099] Example 16

[0100] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 3 and EA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0101] Example 17

[0102] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 4 and EP were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0103] Example 18

[0104] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 4 and EMC were mixed at a molar ratio of 1:3 to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0105] Example 19

[0106] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 4 and DFEA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0107] Example 20

[0108] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 4 and EA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0109] Example 21

[0110] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 5 and EP were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0111] Example 22

[0112] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 5 and EMC were mixed at a molar ratio of 1:3 to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0113] Example 23

[0114] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 5 and DFEA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0115] Example 24

[0116] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 5 and EA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiPF₆ was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiPF₆. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0117] Example 25

[0118] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 1 and EMC were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiTFSI was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiTFSI. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0119] Example 26

[0120] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), Compound 1 and EMC were mixed at a molar ratio of 1:3 to prepare a mixed solvent. Next, 1 equivalent of LiODFB was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiODFB. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0121] Example 27

[0122] In a nitrogen-filled glove box (H2O <10ppm, O2 <10ppm), Compound 1 and EMC were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiFSI was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiFSI. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0123] Example 28

[0124] In a nitrogen-filled glove box (H2O <10ppm, O2 <10ppm), compound 1 and EMC were mixed in a molar ratio of 1:3 to prepare a mixed solvent. Then, 0.5 equivalents of LiPF6 and 0.5 equivalents of LiFSI were weighed and dissolved in the aforementioned mixed solvent to prepare an electrolyte containing 0.5 mol / L LiPF6 and 0.5 mol / L LiFSI. Finally, 10% FEC and 0.5% VC were added to this electrolyte to prepare the target electrolyte.

[0125] Example 29

[0126] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 1 and DFEA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiTFSI was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiTFSI. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0127] Example 30

[0128] In a nitrogen-filled glove box (H₂O <10 ppm, O₂ <10 ppm), compound 1 and DFEA were mixed in a 1:3 molar ratio to prepare a mixed solvent. Next, 1 equivalent of LiODFB was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiODFB. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to obtain the target electrolyte.

[0129] Example 31

[0130] In a nitrogen-filled glove box (H2O <10ppm, O2 <10ppm), compound 1 and DFEA were mixed in a molar ratio of 1:3 to prepare a mixed solvent. Next, 1 equivalent of LiFSI was weighed and dissolved in the mixed solvent to prepare an electrolyte solution containing 1 mol / L LiFSI. Finally, 10% FEC and 0.5% VC were added to this electrolyte solution to prepare the target electrolyte.

[0131] Example 32

[0132] In a nitrogen-filled glove box (H2O <10ppm, O2 <10ppm), compound 1 and DFEA were mixed in a molar ratio of 1:3 to prepare a mixed solvent. Then, 0.5 equivalents of LiPF6 and 0.5 equivalents of LiFSI were weighed and dissolved in the aforementioned mixed solvent to prepare an electrolyte containing 0.5 mol / L LiPF6 and 0.5 mol / L LiFSI. Finally, 10% FEC and 0.5% VC were added to the electrolyte to prepare the target electrolyte.

[0133] Performance testing:

[0134] (1) The above electrolyte is assembled with the positive electrode, negative electrode, and separator into a battery, wherein the positive electrode active material is lithium cobalt oxide and the negative electrode active material is graphite. The battery assembly method refers to the existing technology. The assembled battery is subjected to basic performance test and battery performance test. The test method is as follows: 4.2V lithium cobalt oxide||graphite soft pack battery 1C rate cycle test at room temperature (25℃), with the charge and discharge cut-off voltages of 2.75V and 4.2V respectively, and the 1C rate cycle is performed until the capacity decays to 80% of the initial capacity. The number of cycles at this time is recorded as the cycle life of the battery, as shown in Table 1.

[0135] Table 1 Cycle life of the above 4.2V lithium cobalt oxide||graphite battery at room temperature 1.0C

[0136] As can be seen from Table 1, Comparative Examples 1 to 6 only include phosphate solvents and cyclic carbonate solvents, and do not contain linear esters (one or more of chain carbonates, chain carboxylates, and fluorocarboxylates). From the perspective of the room temperature cycle life of battery 1C, even when using negative electrode film-forming additives VC and / or PDO, or different lithium salts, the cycle life is only 100 weeks or less, which is much shorter than the electrolyte using conventional carbonate solvents (Comparative Example 7). It can be seen that in the absence of linear esters, the electrolyte of Comparative Examples 1 to 6 is not compatible with the graphite negative electrode in the battery cell. Comparative Example 7 is a conventional carbonate electrolyte, containing EC and EMC solvents and LiPF6 lithium salt. Without the addition of any other additives, the room temperature cycle life of the battery is about 800 weeks, which is in line with the initial design of the soft-pack battery cell.

[0137] The applicant further optimized the solvent system and improved the battery performance by adjusting the solvent components and content. Based on Comparative Example 2, Examples 1 to 4 added a certain amount of linear ester and controlled the molar ratio of phosphate ester to linear ester to 3:1. From the perspective of cycle life, they all improved, reaching or slightly exceeding the level of Comparative Example 7. In Examples 5 to 8, the applicant further adjusted the molar ratio of phosphate ester to linear ester to 1:3, and the cycle life of the battery was significantly improved, reaching more than 1800 weeks. In Examples 9 to 23, the applicant adjusted the phosphate ester solvent, and the cycle life of the battery all reached more than 1700 weeks, with a maximum of 2200 weeks. In Examples 27 to 32, the applicant adjusted the lithium salt, and the cycle life of the battery reached more than 1200 weeks, with a maximum of more than 2300 weeks. In addition, by comparing the effects of different linear esters on cycle life, it was found that DFEA was better than EA, and EA was better than EMC and EP. By comparing the effects of compounds 1 to 5 on cycle life, it was found that compound 3 had the best effect.

[0138] In summary, electrolytes using phosphate esters alone or in combination with cyclic carbonates have poor compatibility with graphite anodes. When using phosphate esters as the primary solvent, using linear esters as the secondary solvent can effectively improve the electrolyte's compatibility with graphite. However, when the molar ratio of phosphate ester to linear ester is too high (e.g., 3:1), the battery cycle life decreases.

[0139] (2) Self-extinguishing time

[0140] The present invention uses the self-extinguishing time of the electrolyte to assess its safety. Generally, a self-extinguishing time of less than 5 seconds indicates good safety, while a longer self-extinguishing time indicates a less safe electrolyte. The self-extinguishing time is tested as follows: 5 mL of electrolyte is ignited over a propane flame for 10 seconds. The time from the start of combustion to extinguishment is recorded, as shown in Table 2.

[0141] Table 2 Self-extinguishing time of electrolyte

[0142] "○" and "√" in the above table both indicate that the substance is contained; in phosphate ester and linear ester, "√" indicates that the molar ratio of phosphate ester to linear ester is 3:1, and "○" indicates that the molar ratio of phosphate ester to linear ester is 1:3.

[0143] EP: ethyl propionate;

[0144] EMC: ethyl methyl carbonate;

[0145] DFEA: 2,2-difluoroethyl acetate;

[0146] EA: ethyl acetate;

[0147] EC: ethylene carbonate;

[0148] FEC: fluoroethylene carbonate;

[0149] VC: ethylene carbonate;

[0150] LiPF6: lithium hexafluorophosphate;

[0151] LiTFSI: lithium bis(trifluoromethanesulfonyl)imide;

[0152] LiODFB: lithium dioxalatoborate;

[0153] LiFSI: lithium bis(fluorosulfonyl)imide;

[0154] PDO: 3-phenyl-1,4,2-dioxazol-5-one.

[0155] Safety performance is equally important for batteries. As shown in Table 2 above, when the linear ester is selected from EP, EMC, or EA and the lithium salt is solely lithium hexafluorophosphate, the flame retardancy of the electrolyte is only slightly better than that of Comparative Example 7, with the self-extinguishing time generally exceeding 30 seconds. Although increasing the proportion of phosphate ester in the electrolyte can improve the flame retardancy to a certain extent, the room-temperature cycling performance of the battery decreases with increasing phosphate ester content. Therefore, it is difficult for the battery to achieve both excellent flame retardancy and cycling performance.

[0156] Through in-depth research, the inventors found that when DFEA is used as the linear ester based on the combination of phosphate ester, linear ester and cyclic ester, the battery has higher room temperature cycle performance, while the self-extinguishing time of the electrolyte can be reduced to less than 15s, and the flame retardant performance is better.

[0157] Alternatively, by optimizing the lithium salt and introducing LiFSI into the lithium salt based on the combination of phosphate esters, linear esters, and cyclic esters, even when combined with LiPF6, the flame retardancy of the electrolyte can be improved, reducing the self-extinguishing time of the electrolyte to less than 10 seconds, and the flame retardancy of the electrolyte is even better. In addition, the combination of LiFSI and LiPF6 improves the cycle performance of the battery.

[0158] The above detailed description of the present invention is intended to enable persons familiar with the art to understand the contents of the present invention and implement them. It does not limit the scope of protection of the present invention. Any equivalent changes or modifications made based on the spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. An electrolyte, comprising a lithium salt, an additive, and an organic solvent, characterized in that: The organic solvent includes cyclic esters and chain esters, and the mass percentage of the chain esters in the electrolyte is greater than that of the cyclic esters; The chain esters include phosphate esters and linear esters; The structural formula of the phosphate ester is wherein R is selected from monofluoro- or polyfluoro-substituted saturated or unsaturated aliphatic hydrocarbons, or monofluoro- or polyfluoro-substituted aromatic hydrocarbons; The linear esters are one or more of chain carbonates, chain carboxylic acid esters, and fluorinated carboxylic acid esters.

2. The electrolyte according to claim 1, wherein: R is selected from monofluoro- or polyfluoro-substituted saturated aliphatic hydrocarbons having 1 to 8 carbon atoms.

3. The electrolyte according to claim 1, wherein: The phosphate ester is selected from one or more of them.

4. The electrolyte according to claim 1, wherein: The chain carboxylic acid esters are selected from one or more of methyl propionate, ethyl propionate, propyl propionate, methyl acetate, ethyl acetate, propyl acetate, methyl butyrate, ethyl butyrate, and propyl butyrate; the chain carbonates are selected from one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and methyl propyl carbonate; the fluorinated carboxylic acid esters are selected from one or more of methyl fluoropropionate, ethyl fluoropropionate, and ethyl fluoroacetate.

5. The electrolyte according to claim 4, characterized in that: The fluorinated carboxylic acid ester is 2,2-difluoroethyl acetate.

6. The electrolyte according to claim 1, wherein: The cyclic esters are selected from one or more of γ-butyrolactone, ethylene carbonate, propylene carbonate, and fluorinated ethylene carbonate.

7. The electrolyte according to claim 1, characterized in that: The molar ratio of the phosphate ester to the linear ester is 3:(1 - 12).

8. The electrolyte according to claim 1, wherein: The molar ratio of the phosphate ester to the linear ester is 1:(0.5 - 4).

9. The electrolyte according to claim 8, characterized in that: The molar ratio of the phosphate ester to the linear ester is 1:(2 - 4).

10. The electrolyte according to claim 1, characterized in that: The cyclic esters account for 5 - 20% of the total mass of the electrolyte.

11. The electrolyte according to claim 1, characterized in that: The lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, anhydrous lithium perchlorate, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorobis(oxalato)phosphate, lithium difluorophosphate, lithium trifluoromethanesulfonate, lithium difluorobis(oxalato)phosphate, lithium bis(oxalato)borate, lithium monooxalobis(fluoroborate), and lithium bis(fluorosulfonyl)imide; and / or, The concentration of the lithium salt in the electrolyte is 0.6 - 1.8 mol / L.

12. The electrolyte according to claim 11, characterized in that: The lithium salt is lithium hexafluorophosphate, or lithium bis(fluorosulfonyl)imide, or a combination of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

13. The electrolyte according to claim 1, wherein: The additive includes one or more of 3-phenyl-1,4,2-dioxazol-5-one, vinylene carbonate, 1,3-propane sultone, allyl-1,3-sultone, ethylene sulfate, propylene sulfate, 1,4-butane sultone, and lithium difluorophosphate; and / or, The additive accounts for 0.1 - 2% of the total mass of the electrolyte.

14. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator and an electrolyte, characterized in that: The electrolyte is the electrolyte according to any one of claims 1 to 13.

15. The lithium ion battery according to claim 14, characterized in that: The active material of the negative electrode is graphite, and the active material of the positive electrode is a lithium cobalt oxide material.

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