High-voltage flame-retardant electrolyte, preparation method thereof and lithium ion battery

Abstract

The application provides a high-voltage flame-retardant electrolyte, which comprises an additive, a fluorinated carbonate solvent and a lithium salt, wherein the additive has one or more compounds selected from the following formula (I) or formula (II): formula (I); wherein R1, R2 and R3 are each independently selected from alkyl, alkoxy and fluorine-containing alkyl with carbon atom number of 1-10; formula (II); wherein X1, X2, X3 and X4 are each independently selected from alkyl, alkoxy and fluorine-containing alkyl with carbon atom number of 1-10. The electrolyte has excellent lithium ion conductivity, high-voltage resistance, flame-retardant property, safety performance and electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the field of organic electrolytes for lithium-ion batteries, specifically relating to a high-voltage flame-retardant electrolyte and its preparation method, and lithium-ion batteries. Background Technology

[0002] With the development and utilization of new energy sources, lithium-ion batteries, as high-performance energy storage devices, have been widely adopted and successfully applied in portable electronic devices, electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles. The electrolyte is one of the core materials in a lithium-ion battery, acting as a bridge for charge transfer between the positive and negative electrodes and participating in almost all reaction processes occurring within the battery. Currently, there are three main technological routes for electrolyte development: liquid electrolytes, polymer electrolytes, and solid inorganic electrolytes. Due to limitations in ionic conductivity and compatibility between the positive and negative electrode interfaces, the electrolytes used in lithium-ion batteries are mostly flammable organic carbonate systems.

[0003] In recent years, safety accidents caused by the misuse of lithium-ion batteries have occurred frequently. Generally speaking, under abusive conditions such as overcharging, over-discharging, short circuits, and collisions, the chemical energy stored in the battery is rapidly converted into heat energy, damaging the solid electrolyte film on the surface of the positive and negative electrodes. This accelerates the violent chemical reactions between the components in the electrolyte and the positive and negative electrodes. The decomposition of organic solvents in the electrolyte produces hydroxyl radicals and hydrogen radicals. These radicals undergo chain reactions, generating a large amount of heat. The heat generated intensifies the reaction between the electrolyte and the lithium-intercalated negative electrode, ultimately affecting the battery's safety. Research has found that partially or completely replacing carbonates in the electrolyte with flame-retardant solvents can transform flammable electrolytes into flame-retardant or non-flammable ones, reducing the battery's heat release and self-heating rate, and improving the electrolyte's thermal stability. Currently, the most studied non-flammable solvents include phosphate esters, fluorinated solvents, and phosphazenes.

[0004] Fluorinated solvents contain CF bonds, which are more stable than CH bonds, reducing the generation of hydrogen radicals at high temperatures. Fluorinated carbonates are the most widely used in the battery field. Fluorinated carbonates often have lower HOMO and LUMO energy levels than carbonate solvents, exhibiting stronger oxidation resistance and weaker reduction resistance in electrochemistry. However, CEI films formed solely from fluorinated carbonates have a single composition, rich in low-ionic-conductivity LiF, resulting in poor performance at high current densities and failing to fully utilize the high-voltage performance of fluorinated carbonates. Patent CN109830752A reports a non-flammable high-voltage lithium-ion battery electrolyte based on fluorinated carbonates. This electrolyte exhibits excellent electrochemical performance, high positive electrode capacity retention, and good cycle stability, but it does not regulate the composition of the CEI film, thus failing to achieve high-current operation of the battery. By introducing high-voltage electrolyte additives into the fluorinated electrolyte, the composition of the CEI film is enriched, improving interfacial ionic conductivity and stability, resulting in a kinetically sound CEI film and further enhancing its electrochemical performance. CN 114695961A discloses a lithium-ion battery additive that improves the high-temperature and high-pressure cycle performance of the battery by adding a carbamate compound. Under high pressure, the carbamate preferentially decomposes to form a positive electrode protective film, reducing the contact between the electrolyte and the electrode and inhibiting the oxidative decomposition and parasitic reactions of the electrolyte. However, the decomposition products of this additive have a single function and cannot improve the lithium-ion conductivity of the positive electrode CEI film. Furthermore, the electrolyte does not have flame-retardant properties and has poor safety performance.

[0005] In summary, there is currently limited control over the CEI membrane of fluorocarbonate-based electrolytes, which restricts the high-voltage advantage of fluorocarbonate-based electrolytes. It is necessary to develop a fluorocarbonate-based electrolyte with excellent film-forming properties. Summary of the Invention

[0006] To address the above problems, this invention provides a high-voltage flame-retardant electrolyte, its preparation method, and a lithium-ion battery.

[0007] To address the aforementioned technical issues, the following solutions are proposed:

[0008] One of the objectives of this invention is to provide a high-voltage flame-retardant electrolyte, comprising a high-voltage electrolyte additive, a fluorocarbonate solvent, and a lithium salt.

[0009] In the above electrolyte, the high-voltage electrolyte additive is selected from one or more compounds having the structure shown in formula (I) or formula (II):

[0010]

[0011] Equation (Ⅰ);

[0012] R1, R2, and R3 are each independently selected from alkyl, alkoxy, and fluorinated alkane having 1 to 10 carbon atoms;

[0013]

[0014] Equation (II);

[0015] X1, X2, X3, and X4 are each independently selected from alkyl, alkoxy, and fluorinated alkane with 1 to 10 carbon atoms.

[0016] Preferably, in formula (I), R1, R2, and R3 are each independently selected from alkyl, alkoxy, and fluorinated alkane having 1 to 5 carbon atoms; in formula (2), X1, X2, X3, and X4 are each independently selected from alkyl, alkoxy, and fluorinated alkane having 1 to 5 carbon atoms.

[0017] Including but not limited to additive 1: Additive 2: Additive 3: Additive 4: Additive 5: Additive 6: .

[0018] Preferably, the additive is present in the electrolyte at a mass percentage of 0.01-2%, more preferably 0.8-1.2%.

[0019] Preferably, the fluorocarbonate solvent includes fluorocyclic carbonates and fluorochain carbonates, and more preferably, the volume ratio of fluorocyclic carbonates to fluorochain carbonates is 1~5:5~9, and even more preferably 3~4:6~7.

[0020] Preferably, the fluorocyclic carbonate is at least one of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and trifluoropropylene carbonate (TFPC), and more preferably fluoroethylene carbonate.

[0021] Preferably, the fluorinated chain carbonate is at least one of trifluoroethyl methyl carbonate (FEMC), bistrifluoroethyl carbonate (DFDEC), fluoroethyl methyl carbonate (AFEMC), methyl trifluoroacetate (TFMA), ethyl trifluoroacetate (TFEA), ethyl difluoroacetate (DFEA), and methyl difluoroacetate (DFMA), and more preferably trifluoroethyl methyl carbonate.

[0022] Preferably, the lithium salt is at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalateborate), and lithium di(fluorooxalateborate), and more preferably lithium hexafluorophosphate.

[0023] Preferably, the lithium salt concentration is 0.5~1.5 mol / L, and more preferably 0.8~1 mol / L.

[0024] A second objective of this invention is to provide a method for preparing the above-mentioned high-voltage flame-retardant electrolyte, comprising the following steps:

[0025] S1. Fluorinated cyclic carbonate and fluorinated chain carbonate are mixed evenly according to the volume ratio to obtain mixture A;

[0026] S2. Add the lithium salt to mixture A at a molar concentration and mix thoroughly to obtain mixture B;

[0027] S3. Add the additive to mixture B according to the mass ratio, mix evenly, and the mixture is ready.

[0028] A third objective of the present invention is to provide a lithium-ion battery comprising the aforementioned high-voltage flame-retardant electrolyte.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] This invention introduces a special additive into a high-voltage electrolyte. This additive is oxidized to generate Li3N, which has high lithium-ion conductivity. Li3N, oxidized with a fluorinated carbonate solvent to form LiF, forms grain boundaries, improving the lithium-ion conductivity at the LiF-rich interface. Furthermore, the cyano group (-CN) in the additive inhibits transition metal dissolution under high voltage, enhancing interface stability. The electrolyte of this invention produces a highly stable, low-impedance, and highly protective solid electrolyte membrane (CEI membrane), improving its high-voltage resistance in practical applications. Simultaneously, the flame-retardant properties of the fluorinated carbonate solvent enhance both safety and electrochemical performance. Lithium-ion batteries containing this electrolyte exhibit stable long-cycle capacity and excellent intrinsic safety. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 The flame retardant properties of the high-voltage flame retardant electrolyte prepared in Example 1 are shown in (a) and (b) respectively.

[0033] Figure 2The flame retardant properties of the conventional carbonate electrolyte prepared in Comparative Example 1 are shown in (a) ignition image and (b) image 5 seconds after ignition.

[0034] Figure 3 The cycle performance of the coin cell prepared in Example 1;

[0035] Figure 4 The microstructure of the electrode of the coin cell prepared in Example 1 after cycling;

[0036] Figure 5 The microstructure of the electrode of the coin cell prepared in Comparative Example 1 after cycling;

[0037] Table 1 shows the electrochemical impedance of the coin cells prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 after 3 cycles. Detailed Implementation

[0038] To facilitate understanding of the present invention, the invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0039] Example 1

[0040] A high-voltage flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0041] (1) Mix 0.6 ml of fluoroethylene carbonate and 1.4 ml of trifluoroethyl methyl carbonate magnetically to obtain mixture A;

[0042] (2) Add 0.304 g (1 mol / L) of lithium hexafluorophosphate to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0043] (3) Add 0.02 g (1 wt%) of additive 1 to mixture B: The high-voltage flame-retardant electrolyte is obtained by mixing the components thoroughly.

[0044] Preparation of the positive electrode: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent and PVDF in a mass ratio of 8:1:1, grind them in a mortar until they are evenly dispersed, and add 100 drops of NMP as a dispersant. Then, coat the mixed slurry evenly on aluminum foil, bake it in an 80℃ oven for 10 h, and finally cut it into positive electrode sheets with a radius of 5 mm using a mold for later use.

[0045] A coin cell half-cell was fabricated using lithium cobalt oxide as the positive electrode, lithium foil as the negative electrode, and a microporous polyethylene film as the separator. Charge-discharge tests were conducted at a constant rate of 1C within the 2.75-4.5 V charge-discharge range. The initial discharge capacity reached 182.2 mAh / g, and after 500 cycles, the discharge capacity still remained at 170.8 mAh / g.

[0046] Example 2

[0047] A high-voltage flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0048] (1) Mix 0.6 ml of fluoroethylene carbonate and 1.4 ml of trifluoroethyl methyl carbonate magnetically to obtain mixture A;

[0049] (2) Add 0.304 g (1 mol / L) of lithium hexafluorophosphate to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0050] (3) Add 0.02 g (1 wt%) of additive 2 to mixture B: The high-voltage flame-retardant electrolyte is obtained by mixing the components thoroughly.

[0051] Preparation of positive and negative electrodes: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent, and PVDF in a mass ratio of 8:1:1, grind them in a mortar until uniformly dispersed, and add 100 drops of NMP as a dispersant. Then, evenly coat the mixed slurry onto aluminum foil, bake it in an 80℃ oven for 10 hours, and finally cut it into 5*5 cm positive electrode sheets for later use. Weigh 1 gram of graphite negative electrode material, conductive agent, and PVDF in a mass ratio of 8:1:1, grind them in a mortar until uniformly dispersed, and add 100 drops of NMP as a dispersant. Then, evenly coat the mixed slurry onto copper foil, bake it in an 80℃ oven for 10 hours, and finally cut it into 5*5 cm negative electrode sheets for later use.

[0052] A soft-pack dry cell was fabricated using high-voltage lithium cobalt oxide as the positive electrode, graphite as the negative electrode, and a microporous polyethylene film as the separator. The dry cell was dried in an oven at 80-85°C for 24 hours and then transferred to a glove box for later use. The aforementioned high-voltage flame-retardant electrolyte was injected into the dried cell, which was then allowed to stand for 24 hours. After a pre-charge formation, the cell was sealed and a second formation was performed to obtain the experimental battery of Example 2. Charge-discharge tests were conducted at a constant current of 200 mA within the 2.75-4.4 V charge-discharge range. The initial discharge capacity reached 219 mAh, and after 100 cycles, the discharge capacity remained at 217 mAh, representing a capacity retention of 99.09%.

[0053] Example 3

[0054] A high-voltage flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0055] (1) Mix 0.8 ml of trifluoroethylene carbonate and 1.2 ml of trifluoroethyl methyl carbonate magnetically to obtain mixture A;

[0056] (2) Add 0.304 g (1 mol / L) of lithium hexafluorophosphate to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0057] (3) Add 0.02g (1 wt%) of additive 3 to mixture B: And 0.04g (2wt%) of additive 2: The high-voltage flame-retardant electrolyte is obtained by mixing the components thoroughly.

[0058] Preparation of positive and negative electrodes: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent, and PVDF in a mass ratio of 8:1:1, grind them in a mortar until uniformly dispersed, and add 100 drops of NMP as a dispersant. Then, evenly coat the mixed slurry onto aluminum foil, bake it in an 80℃ oven for 10 hours, and finally cut it into 5*5 cm positive electrode sheets for later use. Weigh 1 gram of graphite negative electrode material, conductive agent, and PVDF in a mass ratio of 8:1:1, grind them in a mortar until uniformly dispersed, and add 100 drops of NMP as a dispersant. Then, evenly coat the mixed slurry onto copper foil, bake it in an 80℃ oven for 10 hours, and finally cut it into 5*5 cm negative electrode sheets for later use.

[0059] A soft-pack dry cell was fabricated using high-voltage lithium cobalt oxide as the positive electrode, graphite as the negative electrode, and a microporous polyethylene film as the separator. The dry cell was dried in an oven at 80-85°C for 24 hours and then transferred to a glove box for later use. The aforementioned high-voltage flame-retardant electrolyte was injected into the dried cell, which was then allowed to stand for 24 hours. After a pre-charge formation, the cell was sealed and a second formation was performed to obtain the experimental battery of Example 3. Charge-discharge tests were conducted at a constant current of 200 mA within the 2.75-4.4 V charge-discharge range. The initial discharge capacity reached 215 mAh, and after 100 cycles, the discharge capacity remained at 212 mAh, representing a capacity retention of 98.60%.

[0060] Example 4

[0061] A high-voltage flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0062] (1) Mix 0.6 ml of fluoroethylene carbonate, 0.7 ml of fluoroethyl methyl carbonate and 0.7 ml of bis(trifluoroethyl) carbonate magnetically until homogeneous to obtain mixture A;

[0063] (2) Add 0.304 g (1 mol / L) of lithium hexafluorophosphate to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0064] (3) Add 0.03 g (1.5 wt%) of additive 4 to mixture B: The high-voltage flame-retardant electrolyte is obtained by mixing the components thoroughly.

[0065] Preparation of positive and negative electrodes: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent, and PVDF in a mass ratio of 8:1:1, grind them in a mortar until uniformly dispersed, and add 100 drops of NMP as a dispersant. Then, evenly coat the mixed slurry onto aluminum foil, bake it in an 80℃ oven for 10 hours, and finally cut it into 5*5cm positive electrode sheets for later use. Weigh 1 gram of graphite negative electrode material, conductive agent, and PVDF in a mass ratio of 8:1:1, grind them in a mortar until uniformly dispersed, and add 100 drops of NMP as a dispersant. Then, evenly coat the mixed slurry onto copper foil, bake it in an 80℃ oven for 10 hours, and finally cut it into 5*5cm negative electrode sheets for later use.

[0066] A soft-pack dry cell was fabricated using high-voltage lithium cobalt oxide as the positive electrode, graphite as the negative electrode, and a microporous polyethylene film as the separator. The dry cell was dried in an oven at 80-85°C for 24 hours and then transferred to a glove box for later use. The aforementioned high-voltage flame-retardant electrolyte was injected into the dried cell, which was then allowed to stand for 24 hours. After a pre-charge formation, the cell was sealed and a second formation was performed to obtain the experimental battery of Example 4. Charge-discharge tests were conducted at a constant current of 200 mA within the 2.75-4.4 V charge-discharge range. The initial discharge capacity reached 214 mAh, and after 100 cycles, the discharge capacity remained at 201 mAh, representing a capacity retention of 93.92%.

[0067] Example 5

[0068] A high-voltage flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0069] (1) Mix 0.6 ml of fluoroethylene carbonate, 0.7 ml of trifluoroethyl methyl carbonate and 0.7 ml of bis(trifluoroethyl) carbonate magnetically until homogeneous to obtain mixture A;

[0070] (2) Add 0.304 g (1 mol / L) of lithium hexafluorophosphate to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0071] (3) Add 0.03 g (1.5 wt%) of additive 5 to mixture B: And 0.03 g (1.5 wt%) of additive 6: The high-voltage flame-retardant electrolyte is obtained by mixing the components thoroughly.

[0072] Preparation of positive and negative electrodes: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent, and PVDF in a mass ratio of 8:1:1, grind them in a mortar until uniformly dispersed, and add 100 drops of NMP as a dispersant. Then, evenly coat the mixed slurry onto aluminum foil, bake it in an 80℃ oven for 10 hours, and finally cut it into 5*5cm positive electrode sheets for later use. Weigh 1 gram of graphite negative electrode material, conductive agent, and PVDF in a mass ratio of 8:1:1, grind them in a mortar until uniformly dispersed, and add 100 drops of NMP as a dispersant. Then, evenly coat the mixed slurry onto copper foil, bake it in an 80℃ oven for 10 hours, and finally cut it into 5*5cm negative electrode sheets for later use.

[0073] A soft-pack dry cell was fabricated using high-voltage lithium cobalt oxide as the positive electrode, graphite as the negative electrode, and a microporous polyethylene film as the separator. The dry cell was dried in an oven at 80-85°C for 24 hours and then transferred to a glove box for later use. The aforementioned high-voltage flame-retardant electrolyte was injected into the dried cell, which was then allowed to stand for 24 hours. After a pre-charge formation, the cell was sealed and a second formation was performed to obtain the experimental battery of Example 4. Charge-discharge tests were conducted at a constant current of 200 mA within the 2.75-4.4 V charge-discharge range. The initial discharge capacity reached 201 mAh, and after 100 cycles, the discharge capacity remained at 198 mAh, representing a capacity retention of 98.51%.

[0074] Example 6

[0075] A high-voltage flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0076] (1) Mix 0.6 ml of fluoroethylene carbonate, 0.7 ml of trifluoroethyl methyl carbonate and 0.7 ml of bis(trifluoroethyl) carbonate magnetically until homogeneous to obtain mixture A;

[0077] (2) Add 0.304 g (1 mol / L) of lithium hexafluorophosphate to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0078] (3) Add 0.03 g (1.5 wt%) of additive 6 to mixture B: The high-voltage flame-retardant electrolyte is obtained by mixing the components thoroughly.

[0079] Preparation of positive and negative electrodes: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent, and PVDF in a mass ratio of 8:1:1, grind them in a mortar until uniformly dispersed, and add 100 drops of NMP as a dispersant. Then, evenly coat the mixed slurry onto aluminum foil, bake it in an 80℃ oven for 10 hours, and finally cut it into 5*5cm positive electrode sheets for later use. Weigh 1 gram of graphite negative electrode material, conductive agent, and PVDF in a mass ratio of 8:1:1, grind them in a mortar until uniformly dispersed, and add 100 drops of NMP as a dispersant. Then, evenly coat the mixed slurry onto copper foil, bake it in an 80℃ oven for 10 hours, and finally cut it into 5*5cm negative electrode sheets for later use.

[0080] A soft-pack dry cell was fabricated using high-voltage lithium cobalt oxide as the positive electrode, graphite as the negative electrode, and a microporous polyethylene film as the separator. The dry cell was dried in an oven at 80-85°C for 24 hours and then transferred to a glove box for later use. The aforementioned high-voltage flame-retardant electrolyte was injected into the dried cell, which was then allowed to stand for 24 hours. After a pre-charge formation, the cell was sealed and a second formation was performed to obtain the experimental battery of Example 4. Charge-discharge tests were conducted at a constant current of 200 mA within the 2.75-4.4 V charge-discharge range. The initial discharge capacity reached 201 mAh, and after 100 cycles, the discharge capacity remained at 198 mAh, representing a capacity retention of 98.51%.

[0081] Example 7

[0082] A high-voltage flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0083] (1) Mix 0.6 ml of fluoroethylene carbonate and 1.4 ml of trifluoroethyl methyl carbonate magnetically to obtain mixture A;

[0084] (2) Add 0.574 g (1 mol / L) lithium bis(trifluoromethanesulfonyl)imide to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0085] (3) Add 0.02 g (1 wt%) of additive 1 to mixture B: The high-voltage flame-retardant electrolyte is obtained by mixing the components thoroughly.

[0086] Preparation of the positive electrode: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent and PVDF in a mass ratio of 8:1:1, grind them in a mortar until they are evenly dispersed, and add 100 drops of NMP as a dispersant. Then, coat the mixed slurry evenly on aluminum foil, bake it in an 80℃ oven for 10 h, and finally cut it into positive electrode sheets with a radius of 5 mm using a mold for later use.

[0087] A coin cell half-cell was fabricated using lithium cobalt oxide as the positive electrode, lithium foil as the negative electrode, and a microporous polyethylene film as the separator. Charge-discharge tests were conducted at a constant rate of 1C within the 2.75-4.5 V charge-discharge range. The initial discharge capacity reached 181.3 mAh / g, and after 500 cycles, the discharge capacity still reached 167.1 mAh / g.

[0088] Example 8

[0089] A high-voltage flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0090] (1) Stir 0.6 ml of fluoroethylene carbonate and 1.4 ml of ethyl trifluoroacetate magnetically until homogeneous to obtain mixture A;

[0091] (2) Add 0.304 g (1 mol / L) of lithium hexafluorophosphate to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0092] (3) Add 0.02 g (1 wt%) of additive 1 to mixture B: The high-voltage flame-retardant electrolyte is obtained by mixing the components thoroughly.

[0093] Preparation of the positive electrode: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent and PVDF in a mass ratio of 8:1:1, grind them in a mortar until they are evenly dispersed, and add 100 drops of NMP as a dispersant. Then, coat the mixed slurry evenly on aluminum foil, bake it in an 80℃ oven for 10 h, and finally cut it into positive electrode sheets with a radius of 5 mm using a mold for later use.

[0094] A coin cell half-cell was fabricated using lithium cobalt oxide as the positive electrode, lithium foil as the negative electrode, and a microporous polyethylene film as the separator. Charge-discharge tests were conducted at a constant rate of 1C within the 2.75-4.5 V charge-discharge range. The initial discharge capacity reached 179.8 mAh / g, and after 500 cycles, the discharge capacity remained at 165.3 mAh / g.

[0095] Example 9

[0096] A high-voltage flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0097] (1) Mix 0.6 ml of propylene trifluorocarbonate and 1.4 ml of trifluoroethyl methyl carbonate magnetically to obtain mixture A;

[0098] (2) Add 0.304 g (1 mol / L) of lithium hexafluorophosphate to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0099] (3) Add 0.02 g (1 wt%) of additive 1 to mixture B: The high-voltage flame-retardant electrolyte is obtained by mixing the components thoroughly.

[0100] Preparation of the positive electrode: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent and PVDF in a mass ratio of 8:1:1, grind them in a mortar until they are evenly dispersed, and add 100 drops of NMP as a dispersant. Then, coat the mixed slurry evenly on aluminum foil, bake it in an 80℃ oven for 10 h, and finally cut it into positive electrode sheets with a radius of 5 mm using a mold for later use.

[0101] A coin cell half-cell was fabricated using lithium cobalt oxide as the positive electrode, lithium foil as the negative electrode, and a microporous polyethylene film as the separator. Charge-discharge tests were conducted at a constant rate of 1C within the 2.75-4.5 V charge-discharge range. The initial discharge capacity reached 183.4 mAh / g, and after 500 cycles, the discharge capacity still reached 162.2 mAh / g.

[0102] Comparative Example 1

[0103] A conventional carbonate electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0104] (1) Stir 0.6 ml of ethylene carbonate, 0.7 ml of methyl ethyl carbonate and 0.7 ml of diethyl carbonate magnetically to obtain mixture A;

[0105] (2) Add 0.304 g (1 mol / L) of lithium hexafluorophosphate to the mixture A and stir magnetically until homogeneous to obtain the above conventional carbonate electrolyte.

[0106] Preparation of the positive electrode: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent and PVDF in a mass ratio of 8:1:1, grind them in a mortar until they are evenly dispersed, and add 100 drops of NMP as a dispersant. Then, coat the mixed slurry evenly on aluminum foil, bake it in an 80℃ oven for 10 h, and finally cut it into positive electrode sheets with a radius of 5 mm using a mold for later use.

[0107] A coin cell half-cell was fabricated using lithium cobalt oxide as the positive electrode, lithium foil as the negative electrode, and a microporous polyethylene film as the separator. Charge-discharge tests were conducted at a constant rate of 1C within the 2.75-4.5 V charge-discharge range. The initial discharge capacity reached 181.5 mAh / g, and after 500 cycles, the discharge capacity was 60.3 mAh / g.

[0108] Comparative Example 2

[0109] A flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0110] (1) Mix 0.6 ml of fluoroethylene carbonate and 1.4 ml of trifluoroethyl methyl carbonate magnetically to obtain mixture A;

[0111] (2) Add 0.574 g (1 mol / L) lithium bis(trifluoromethanesulfonyl)imide to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0112] Preparation of the positive electrode: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent and PVDF in a mass ratio of 8:1:1, grind them in a mortar until they are evenly dispersed, and add 100 drops of NMP as a dispersant. Then, coat the mixed slurry evenly on aluminum foil, bake it in an 80℃ oven for 10 h, and finally cut it into positive electrode sheets with a radius of 5 mm using a mold for later use.

[0113] A coin cell half-cell was fabricated using lithium cobalt oxide as the positive electrode, lithium foil as the negative electrode, and a microporous polyethylene film as the separator. Charge-discharge tests were conducted at a constant rate of 1C within the 2.75-4.5 V charge-discharge range. The initial discharge capacity reached 180.3 mAh / g, and after 500 cycles, the discharge capacity was 140.1 mAh / g.

[0114] Comparative Example 3

[0115] A flame-retardant electrolyte for lithium-ion batteries, the detailed preparation steps of which are as follows:

[0116] (1) Mix 0.6 ml of fluoroethylene carbonate and 1.4 ml of trifluoroethyl methyl carbonate magnetically to obtain mixture A;

[0117] (2) Add 0.574 g (1 mol / L) lithium bis(trifluoromethanesulfonyl)imide to mixture A, and stir magnetically until homogeneous to obtain mixture B;

[0118] (3) Add 0.02 g (1 wt%) of 1,3,6-hexanetrionitrile additive to mixture B: The high-voltage flame-retardant electrolyte is obtained by mixing the components thoroughly.

[0119] Preparation of the positive electrode: Weigh 1 gram of lithium cobalt oxide positive electrode material, conductive agent and PVDF in a mass ratio of 8:1:1, grind them in a mortar until they are evenly dispersed, and add 100 drops of NMP as a dispersant. Then, coat the mixed slurry evenly on aluminum foil, bake it in an 80℃ oven for 10 h, and finally cut it into positive electrode sheets with a radius of 5 mm using a mold for later use.

[0120] A coin cell half-cell was fabricated using lithium cobalt oxide as the positive electrode, lithium foil as the negative electrode, and a microporous polyethylene film as the separator. Charge-discharge tests were conducted at a constant rate of 1C within the 2.75-4.5 V charge-discharge range. The initial discharge capacity reached 183.5 mAh / g, and after 500 cycles, the discharge capacity was 152.8 mAh / g.

[0121] Figure 1 The image shows the flame retardant properties of the high-voltage flame-retardant electrolyte prepared in Example 1. Image (a) is the ignition image, and image (b) is the image 5 seconds after ignition, showing that the electrolyte cannot burn when exposed to an open flame. Figure 2 The image shows the flame retardant properties of the conventional carbonate electrolyte prepared in Comparative Example 1. (a) is an ignition image, and (b) is an image 5 seconds after ignition. The image shows that the electrolyte can burn when exposed to fire, indicating that the high-voltage flame-retardant electrolyte prepared in Example 1 has excellent flame retardant properties. Figure 3 The cycling performance of the coin cell prepared in Example 1 shows that the capacity retention rate is over 88% after 500 cycles, indicating that the present invention has excellent high-voltage electrochemical stability. Figure 4 The image shows the microstructure of the electrode of the coin cell prepared in Example 1 after 500 cycles. The lithium cobalt oxide particles are intact. Figure 5 The microstructure of the electrode of the coin cell prepared in Comparative Example 1 after 500 cycles is shown. The lithium cobalt oxide particles fractured, indicating that the CEI film derived from the high-voltage flame-retardant electrolyte prepared in Example 1 provides good protection for the lithium cobalt oxide particles. Table 1 shows the impedance of the coin cells prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 after 3 cycles. As shown in Table 1, the film resistance R of Example 1... f The film resistance R of Comparative Example 2 is 12.0Ω. f The resistance R is 40.5 Ω, indicating that the addition of additives effectively improved the lithium-ion conductivity of the solid electrolyte membrane. (Comparative Example 3: Membrane resistance R) f The Ω value is 39.9, indicating that commercially available nitrile additives cannot effectively improve the lithium-ion conductivity of the CEI film. It is evident that, compared to commercially available nitrile additives, this additive can effectively improve the lithium-ion conductivity of the CEI film. It is speculated that this may be because the additive of this invention is oxidized to generate Li3N with high lithium-ion conductivity, which, together with LiF generated by the oxidation of fluorinated carbonate solvent, can form grain boundaries, thereby improving the lithium-ion conductivity of the LiF-rich interface.

[0122] Table 1

[0123] Rf Rct Example 1 12.0 Ω 83.2 Ω Comparative Example 1 27.6 Ω 123.9 Ω Comparative Example 2 40.5 Ω 110.3 Ω Comparative Example 3 39.9 Ω 108.4 Ω

[0124] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A high-voltage flame-retardant electrolyte, characterized in that, The additives include an additive, a fluorocarbonate solvent, and a lithium salt, wherein the additive has one or more compounds selected from those with the structure shown in formula (I) or formula (II): Equation (Ⅰ); R1, R2, and R3 are each independently selected from alkyl, alkoxy, and fluorinated alkane having 1 to 10 carbon atoms; Equation (II); X1, X2, X3, and X4 are each independently selected from alkyl, alkoxy, and fluorinated alkane having 1 to 10 carbon atoms; the fluorinated carbonate solvent includes fluorinated cyclic carbonates and fluorinated chain carbonates; the volume ratio of fluorinated cyclic carbonates to fluorinated chain carbonates is 1~5:5~9.

2. The high-voltage flame-retardant electrolyte as described in claim 1, characterized in that, The additive has a mass percentage of 0.01~2% in the electrolyte.

3. The high-voltage flame-retardant electrolyte as described in claim 1, characterized in that, The fluorocyclic carbonate is at least one of fluoroethylene carbonate, difluoroethylene carbonate and trifluoroethylene carbonate. The fluorinated chain carbonate is at least one of trifluoroethyl methyl carbonate, bistrifluoroethyl carbonate, fluoroethyl methyl carbonate, methyl trifluoroacetate, ethyl trifluoroacetate, ethyl difluoroacetate, and methyl difluoroacetate.

4. The high-voltage flame-retardant electrolyte as described in claim 1, characterized in that, The lithium salt is at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonyl)imide, lithium bis(oxalate)borate, and lithium di(fluorooxalate)borate.

5. The high-voltage flame-retardant electrolyte according to claim 1, characterized in that, The lithium salt concentration is 0.5~1.5 mol / L.

6. The high-voltage flame-retardant electrolyte as described in claim 1, characterized in that, In the formula (Ⅰ), R1, R2, and R3 are each independently selected from alkyl, alkoxy, and fluorinated alkane having 1 to 5 carbon atoms; In formula (II), X1, X2, X3, and X4 are each independently selected from alkyl, alkoxy, and fluorinated alkane having 1 to 5 carbon atoms.

7. The method for preparing the high-voltage flame-retardant electrolyte according to any one of claims 1-6, characterized in that, Includes the following steps: S1. Mix the fluorinated cyclic carbonate and the fluorinated chain carbonate in a volume ratio to obtain a mixture A; S2. Add the lithium salt to mixture A at a molar concentration and mix thoroughly to obtain mixture B; S3. Add the additive to mixture B according to the mass ratio, mix evenly, and the mixture is ready.

8. A lithium-ion battery, characterized in that, The high-voltage flame-retardant electrolyte includes any one of claims 1-6.

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