An electrolyte for a lithium-rich manganese-based high-voltage lithium-ion battery and the lithium-ion battery itself.

Abstract

This invention relates to the technical field of electrolytes, and discloses an electrolyte for lithium-rich manganese-based high-voltage lithium-ion batteries and the lithium-ion battery itself. The electrolyte comprises a lithium salt and a solvent; the solvent is a fluorinated solvent or a mixture of a non-fluorinated solvent and a fluorinated solvent; the concentration of the lithium salt in the solvent is 0.8~1.5 mol / L. This invention improves the high-voltage stability of the electrolyte by using a fluorinated solvent with a higher oxidation voltage to replace the non-fluorinated cyclic and chain carbonates in traditional solvents, thus mitigating the impedance increase during cycling in lithium-rich manganese-based lithium-ion batteries and improving the discharge capacity and capacity retention after 500 cycles, thereby enhancing high-voltage cycling performance.

Description

Technical Field

[0001] This invention relates to the technical field of electrolytes, and in particular to an electrolyte for lithium-rich manganese-based high-voltage lithium-ion batteries and the lithium-ion battery itself. Background Technology

[0002] With the increasing global energy demand, the need for energy storage systems is also growing. As an energy storage device, lithium-ion batteries have attracted widespread attention due to their advantages such as high energy density, good cycle performance, and long calendar life. They are widely used in large-scale energy storage systems, electric vehicles, mobile electronic devices, power tools, and home energy storage. With the continued expansion of the electric vehicle industry, the demand for battery energy density is increasing. To improve battery energy density, one approach is to increase battery capacity, and another is to increase the battery's operating voltage. Lithium-rich manganese-based oxide cathode materials combine the advantages of high operating voltage and high capacity, making them an excellent high-energy-density cathode material. However, they also have some unresolved issues. For example, higher operating voltages can lead to electrolyte decomposition, oxygen release causing changes in material structure, transition metal dissolution, and a decrease in discharge voltage, ultimately resulting in poor battery cycle life, reduced energy density, and poor safety performance. Adding appropriate additives to the electrolyte can form a stable CEI film on the cathode surface during battery formation, preventing contact between the electrolyte and the cathode material and reducing side reactions. Conversely, using solvents with higher oxidation voltages can improve the electrolyte's stability at high voltages, thereby reducing electrolyte decomposition. Both methods can improve the performance of lithium-rich manganese-based cathodes.

[0003] Patent CN115763977A discloses a high-voltage electrolyte suitable for lithium-rich manganese-based cathode batteries. This electrolyte contains a fluorinated organic solvent, lithium salt, and functional additives. The use of an electrolyte containing alkoxy-containing polyfluorinated triphosphazene additives forms a CEI protective film on the cathode surface, improving the electrolyte's high-voltage resistance, oxidation resistance, and high-temperature resistance, thus enhancing the battery's high-voltage cycle performance. However, this design is for high-nickel ternary materials. For lithium-rich manganese-based cathodes, the structural changes, oxygen release, and excessive metal deposition caused by the material phase transition during the first charge are more severe than with ternary materials, making the formation of a stable CEI film more challenging.

[0004] Patent CN110112465A discloses an electrolyte for a lithium-rich manganese-based cathode material system and a lithium-ion battery. The electrolyte includes a solvent, a lithium electrolyte salt, and additives, including fluoroethylene carbonate, thiophene-2-methoxyborate pinacol ester, and di(2,2,2-trifluoroethyl) carbonate. This improves the interfacial compatibility between the electrolyte and the positive and negative electrodes, and enhances the battery's high-temperature and high-pressure cycle performance. Although the patent uses a lithium-rich manganese cathode material, the capacity retention rate in the given battery cycle performance examples is not significantly improved. Summary of the Invention

[0005] To address the shortcomings of conventional electrolyte solvents such as cyclic and chain carbonates, including poor electrochemical stability under high voltage, easy decomposition, and poor CEI film formation stability, this invention provides an electrolyte and lithium-ion battery for lithium-rich manganese-based high-voltage cathode batteries. Fluorinated solvents, including fluorinated cyclic / chain carbonates and fluorinated ethers, are used to partially or completely replace non-fluorinated solvents in traditional electrolytes. Boron-containing additives are added to improve the high-voltage stability of the electrolyte and enhance the film formation characteristics of both positive and negative electrodes, thereby improving the cycle performance of lithium-rich manganese-based cathode lithium batteries.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] In a first aspect, the present invention provides an electrolyte for a lithium-rich manganese-based high-voltage lithium-ion battery, the electrolyte comprising a lithium salt and a solvent; the solvent being a fluorinated solvent or a mixture of a non-fluorinated solvent and a fluorinated solvent; and the concentration of the lithium salt in the solvent being 0.8–1.5 mol / L.

[0008] Traditional electrolytes typically use cyclic carbonates (such as ethylene carbonate (EC) and propylene carbonate (PC)) and chain carbonates (such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC)) as the main solvents. These solvents generally have low oxidation potentials, and even with a relatively stable CEI film forming on the positive electrode surface, they are still prone to oxidative decomposition under high voltage, thus reducing the battery's cycle life. Fluorinated solvents (such as cyclic or chain fluorinated carbonates) generally have higher oxidation potentials; therefore, using fluorinated solvents to partially or completely replace non-fluorinated solvents in traditional electrolytes can improve the high-voltage stability of the electrolyte. Furthermore, fluorinated solvents readily form fluorine-rich (such as lithium fluoride) protective CEI and SEI films on the positive and negative electrode surfaces, and they also possess non-flammable properties. The combination of excellent film-forming properties, high-voltage stability, and safety makes fluorinated solvents a solution for lithium-rich manganese high-voltage positive electrode electrolytes.

[0009] This invention investigates the use of fluorinated solvent packs to partially or completely replace non-fluorinated solvents in traditional electrolytes to improve the high-voltage stability of the electrolyte, the stability of the positive electrode film formation, reduce the decomposition of the electrolyte during cycling, slow down the increase in battery impedance, and thus improve the capacity retention rate after cycling.

[0010] When only fluorinated cyclic carbonates are used to replace non-fluorinated cyclic carbonates, the high-voltage stability of the electrolyte is improved, electrolyte decomposition during cycling is reduced, the battery impedance growth rate is decreased, and the battery capacity retention rate is improved. Further optimization involves replacing all non-fluorinated solvents with fluorinated solvents (perfluorinated), which further improves the high-voltage stability of the electrolyte and enhances cycling performance.

[0011] Preferably, the fluorinated solvent is a fluorinated carbonate solvent and / or a fluorinated ether solvent.

[0012] Preferably, the fluorocarbonate solvent is one or more of fluoroethylene carbonate, trifluoromethyl ethylene carbonate, difluoroethylene carbonate, methyltrifluoroethyl carbonate, and bis(2,2,2-trifluoroethyl) carbonate.

[0013] Preferably, the fluorinated ether solvent is one or more of 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether.

[0014] Fluorinated ethers also possess high oxidation potentials and can be added to high-voltage electrolyte formulations to improve the high-voltage stability of the electrolyte. When fluorinated ether solvents (F-EPE) with high oxidation voltage are added to the aforementioned perfluorinated solvent electrolytes, the high-voltage stability of the electrolyte is further improved synergistically with the fluorinated carbonate solvent, resulting in further enhancement of cycle performance. When 20% F-EPE is added, the capacity reaches its maximum after 500 cycles. Increasing the amount of F-EPE at this point increases battery impedance, but the capacity retention rate no longer improves, thus leading to a decrease in capacity after cycling.

[0015] Preferably, the lithium salt is one or more of lithium hexafluorophosphate, lithium bis(trifluorosulfonyl)imide, lithium bis(oxalate-borate), lithium tetrafluoroborate, lithium difluorooxalate-borate, lithium difluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium perchlorate, and lithium hexafluoroarsenate.

[0016] Preferably, the electrolyte further includes boron-containing additives; the boron-containing additives are boron-containing solvent additives and / or boron-containing lithium salt additives.

[0017] Boron-containing additives can form a robust, stable, and low-impedance CEI film rich in boron on the positive electrode surface, which can effectively protect the positive electrode material and inhibit the oxidative decomposition of the electrolyte under high voltage.

[0018] The amount of boron-containing additive added is 1 to 3% of the total mass of lithium salt and solvent.

[0019] Preferably, the boron-containing solvent additive is one or more selected from trihydro(pyridine)boron, trimethyl borate, tri(2-cyanoethyl)borate, phenylboronic acid, and 2,4,6-triphenylcycloboroxane.

[0020] Preferably, the boron-containing lithium salt additive is one or more of lithium tetraborate, lithium difluorooxalate borate, lithium dioxalate borate, and lithium tetrafluoroborate.

[0021] Preferably, the electrolyte comprises lithium salt, fluorinated solvent, and boron-containing additive; the fluorinated solvent is 70-80% fluorinated carbonate solvent and 20-30% fluorinated ether solvent by mass fraction; the amount of boron-containing additive added is 1-3% of the total mass of lithium salt and solvent, and it is a mixture of boron-containing solvent additive and boron-containing lithium salt additive.

[0022] Further optimization shows that when perfluorinated solvents are used in combination with boron-containing additives, the two have a synergistic effect, further reducing battery impedance and further improving high-voltage cycle performance.

[0023] Secondly, the present invention also provides a lithium-ion battery comprising the above-mentioned electrolyte.

[0024] Preferably, in the lithium-ion battery, the positive electrode active material is a lithium-rich manganese-based positive electrode material.

[0025] Preferably, in the lithium-ion battery, the negative electrode active material is one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon, silicon-carbon, and silicon-oxygen.

[0026] Preferably, the diaphragm is one or more of the following: PP diaphragm, PE diaphragm, PE / PP double-layer composite membrane, PI electrospun diaphragm, PP / PE / PP triple-layer composite diaphragm, ceramic diaphragm, and PVDF coated diaphragm.

[0027] Compared with the prior art, the present invention has the following beneficial effects: by using fluorinated solvents (including fluorinated cyclic and chain carbonates and fluorinated ethers) with higher oxidation voltage to replace non-fluorinated cyclic and chain carbonates in traditional solvents, the present invention improves the high-voltage stability of the electrolyte, slows down the increase in impedance during cycling of lithium-rich manganese-based lithium-ion batteries, and improves the discharge capacity and capacity retention rate after 500 cycles, thereby enhancing the high-voltage cycling performance. Detailed Implementation

[0028] The technical solution of the present invention is illustrated below with specific embodiments, but the scope of protection of the present invention is not limited thereto:

[0029] An electrolyte for a lithium-rich manganese-based high-voltage lithium-ion battery comprises a lithium salt, a solvent, and a boron-containing additive. The solvent is a fluorinated solvent or a mixture of a non-fluorinated solvent and a fluorinated solvent. The boron-containing additive is a boron-containing solvent additive and / or a boron-containing lithium salt additive.

[0030] The concentration of lithium salt in the solvent is 0.8–1.5 mol / L, and the amount of boron-containing additive added is 1–3% of the total mass of lithium salt and solvent.

[0031] The lithium salt is one or more of lithium hexafluorophosphate, lithium bis(trifluorosulfonyl)imide, lithium bis(oxalate-borate), lithium tetrafluoroborate, lithium difluorooxalate-borate, lithium difluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium perchlorate, and lithium hexafluoroarsenate.

[0032] The fluorinated solvent is a fluorinated carbonate solvent and / or a fluorinated ether solvent. Fluorinated carbonate solvents are one or more of fluoroethylene carbonate (FEC), trifluoromethyl ethylene carbonate (TFPC), difluoroethylene carbonate (DFEC), methyltrifluoroethyl carbonate (FEMC), and bis(2,2,2-trifluoroethyl) carbonate (FDEC). Fluorinated ether solvents are one or more of 2,2,2-trifluoroethyl ether (BTFE), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE), and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE).

[0033] The non-fluorinated solvent is one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC).

[0034] The boron-containing solvent additive is one or more of the following: trihydro(pyridine)boron (PBF), trimethyl borate (TMB), tri(2-cyanoethyl)borate (TCEB), phenylboronic acid (PBA), and 2,4,6-triphenylcycloboronic acid (TPBX).

[0035] The boron-containing lithium salt additive is one or more of lithium tetraborate (LTB), lithium difluorooxalate borate (LiDFOB), lithium bis(oxalate borate) (LiBOB), and lithium tetrafluoroborate (LiBF4).

[0036] A lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte.

[0037] Positive electrode: The positive electrode active material is a lithium-rich manganese-based positive electrode material. The positive electrode active material, conductive agent, and binder polyvinylidene fluoride (PVDF) are dispersed in an appropriate amount of N-methylpyrrolidone at a mass ratio of 93:2.5:2:2.5, and then thoroughly stirred according to the homogenization process. The uniformly dispersed positive electrode slurry is evenly coated onto aluminum foil, and after baking, rolling, slitting, and stamping, the positive electrode sheet is obtained.

[0038] Negative electrode: The negative electrode active material is one or more of synthetic graphite, natural graphite, soft carbon, hard carbon, silicon, silicon-carbon, and silicon oxide. One or more negative electrode active materials, a conductive agent, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) are added to a planetary mixing tank in a mass ratio of 96:1:2:1, and a uniformly dispersed negative electrode slurry is prepared according to the slurry preparation process. The negative electrode slurry is then uniformly coated onto copper foil, and after baking, rolling, slitting, and stamping, a negative electrode sheet is obtained.

[0039] Separators: one or more of the following: PP separator, PE separator, PE / PP double-layer composite membrane, PI electrospun separator, PP / PE / PP triple-layer composite separator, ceramic separator, and PVDF coated separator.

[0040] After the positive and negative electrodes are stamped, the positive electrode is placed in an oven at 110-140℃ and the negative electrode is placed in an oven at 90-100℃ for 20-30 hours. When the moisture content of the electrodes meets the requirements, the positive electrode, negative electrode, and separator are placed in a stacking machine to form a bare cell. The bare cell is then packaged into a stamped aluminum-plastic film bag. After the packaged dry cell is dried at 80-95℃ for 8-15 hours, the electrolyte of this invention is injected into the dry cell. After the cell undergoes resting, formation, high-temperature resting, degassing and sealing, and capacity testing, a lithium-ion battery is obtained.

[0041] Example 1

[0042] 1. Electrolyte

[0043] Lithium salt: Lithium hexafluorophosphate, with a concentration of 1.0 mol / L in the solvent.

[0044] Solvents: FEC (fluorocarbonate solvent) and EMC (non-fluorinated solvent) in a mass ratio of 3:7.

[0045] Control the moisture content in the glove box to be below 10 ppm and the moisture content of the solvent to be below 10 ppm. Using a pipette, accurately transfer different solvents from the glove box in a specific ratio to aluminum bottles, stir thoroughly, and then store in a 0°C incubator for 1 hour. Then, add lithium hexafluorophosphate to the mixed solvent at a concentration of 1.0 mol / L while stirring.

[0046] 2. Lithium-ion batteries (including positive electrode, negative electrode, separator, and electrolyte)

[0047] Positive electrode: The positive electrode active material is lithium-rich manganese-based positive electrode material Li. 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2. Positive electrode active material, conductive agent, and binder polyvinylidene fluoride (PVDF) are dispersed in a mass ratio of 93:2.5:2:2.5 into 20% of the total solid mass of N-methylpyrrolidone, and then thoroughly stirred according to the homogenization process. The uniformly dispersed positive electrode slurry is then evenly coated onto aluminum foil, and after baking, rolling, slitting, and stamping, positive electrode sheets are obtained.

[0048] Negative Electrode: The active material for the negative electrode is natural graphite. Natural graphite, conductive agent, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) are added to a planetary mixing tank in a mass ratio of 96:1:2:1, and a uniformly dispersed negative electrode slurry is prepared according to the slurry preparation process. The negative electrode slurry is then uniformly coated onto copper foil, and after baking, rolling, slitting, and stamping, the negative electrode sheet is obtained.

[0049] Diaphragm: PP diaphragm.

[0050] After the positive and negative electrodes are stamped, the positive electrode is placed in an oven at 130°C and the negative electrode is placed in an oven at 100°C for 25 hours. Once the moisture content of the electrodes meets the requirements, the positive electrode, negative electrode, and separator are placed in a stacking machine to form a bare cell. The bare cell is then packaged into a stamped aluminum-plastic film bag. After the packaged dry cell is dried at 90°C for 11 hours, the electrolyte of this invention is injected into the dry cell. The cell undergoes resting, formation, high-temperature resting, degassing and sealing, and capacity testing to obtain a lithium-ion battery.

[0051] Example 2

[0052] The difference from Example 1 is that the electrolyte uses TFPC (fluorocarbonate solvent) and EMC (non-fluorinated solvent) in a mass ratio of 3:7.

[0053] Example 3

[0054] The difference from Example 1 is that the electrolyte uses TFPC (fluorocarbonate solvent) and FEMC (fluorocarbonate solvent) in a mass ratio of 3:7.

[0055] Example 4

[0056] The difference from Example 1 is that the electrolyte uses TFPC (fluorocarbonate solvent) and FDEC (fluorocarbonate solvent) in a mass ratio of 3:7.

[0057] Example 5

[0058] The difference from Example 1 is that the solvent used in the electrolyte is TFPC (fluorocarbonate solvent), FDEC (fluorocarbonate solvent), and F-EPE (fluoroether solvent) in a mass ratio of 2.7:6.3:1.

[0059] Example 6

[0060] The difference from Example 1 is that the solvent used in the electrolyte is TFPC (fluorocarbonate solvent), FDEC (fluorocarbonate solvent), and F-EPE (fluoroether solvent) in a mass ratio of 2.4:5.6:2.

[0061] Example 7

[0062] The difference from Example 1 is that the solvent used in the electrolyte is TFPC (fluorocarbonate solvent), FDEC (fluorocarbonate solvent), and F-EPE (fluoroether solvent) in a mass ratio of 2.1:4.9:3.

[0063] Example 8

[0064] The difference from Example 1 is that the solvent used in the electrolyte is TFPC (fluorocarbonate solvent), FDEC (fluorocarbonate solvent), and BTFE (fluoroether solvent) in a mass ratio of 2.4:5.6:2.

[0065] Example 9

[0066] The difference from Example 1 is that the electrolyte uses EC (non-fluorinated solvent) and EMC (non-fluorinated solvent) in a mass ratio of 3:7, and 2 wt% PBF (boron-containing solvent additive) is added.

[0067] Example 10

[0068] The difference from Example 1 is that the electrolyte uses EC (non-fluorinated solvent) and EMC (non-fluorinated solvent) in a mass ratio of 3:7, and 2wt% LTB (boron-containing lithium salt additive) is added.

[0069] Example 11

[0070] The difference from Example 1 is that the electrolyte uses EC (non-fluorinated solvent) and EMC (non-fluorinated solvent) in a mass ratio of 3:7, and 1 wt% PBF (boron-containing solvent additive) and 1 wt% LTB (boron-containing lithium salt additive) are added.

[0071] Example 12

[0072] The difference from Example 1 is that the electrolyte uses TFPC (fluorocarbonate solvent), FDEC (fluorocarbonate solvent) and F-EPE (fluoroether solvent) in a mass ratio of 2.4:5.6:2, and 2 wt% PBF (boron-containing solvent additive) is added.

[0073] Example 13

[0074] The difference from Example 1 is that the electrolyte uses TFPC (fluorocarbonate solvent), FDEC (fluorocarbonate solvent) and F-EPE (fluoroether solvent) in a mass ratio of 2.4:5.6:2, and 2 wt% LTB (boron-containing lithium salt additive) is added.

[0075] Example 14

[0076] The difference from Example 1 is that the electrolyte uses TFPC (fluorocarbonate solvent), FDEC (fluorocarbonate solvent) and F-EPE (fluoroether solvent) in a mass ratio of 2.4:5.6:2, and 1 wt% PBF (boron-containing solvent additive) and 1 wt% LTB (boron-containing lithium salt additive) are added.

[0077] Comparative Example 1

[0078] The difference from Example 1 is that the electrolyte uses EC (non-fluorinated solvent) and EMC (non-fluorinated solvent) in a mass ratio of 3:7.

[0079] Comparative Example 2

[0080] The difference from Example 1 is that the electrolyte uses EC (non-fluorinated solvent) and EMC (non-fluorinated solvent) in a mass ratio of 1:9.

[0081] The electrolytes and lithium-ion batteries of the examples and comparative examples were tested and their performance differences were compared using the following methods:

[0082] (1) Room temperature cycling test of lithium-ion batteries

[0083] The examples and comparative examples were subjected to room temperature cycle tests on the batteries according to the following steps: The lithium-ion batteries, after being capacitated, were left to stand for at least 1 hour. Then, they were charged at a constant current and constant voltage of 1C, with a cutoff voltage of 4.5V and a cutoff current of 0.05C. Next, they were discharged at a constant current of 1C to 2.5V. The batteries were cycled using the above steps, and the discharge capacity and capacity retention rate after 500 cycles were recorded. Specific test data are shown in Table 1.

[0084] (2) DCIR test of lithium-ion battery at room temperature

[0085] The DCIR of the batteries in the examples and comparative examples was tested according to the following steps: In the above-mentioned room temperature cycling, starting from the second cycle, the DCIR of the battery at 50% SOC was measured every 50 cycles. First, the battery was capacitated by performing a 1C constant current constant voltage charge and a 1C constant current discharge in the previous cycle to determine the discharge capacity. Then, when discharged to 50% SOC, the battery was allowed to stand for one hour, and a 10-second discharge test was performed on the cell using a 2.5C pulse current. The DCIR of the battery was calculated based on the voltage change. The 50% SOC DCIR and its growth rate were recorded in the 2nd and 452nd cycles. Specific test data are shown in Table 2.

[0086] Table 1

[0087]

[0088] Table 2

[0089]

[0090] As shown in Table 1, Comparative Example 1 had the lowest discharge capacity and capacity retention rate after 500 cycles among all comparative examples and embodiments; while all embodiments had higher discharge capacity and capacity retention rates after 500 cycles than the comparative examples. This demonstrates that replacing non-fluorinated solvents in traditional electrolytes with fluorinated solvents improves battery cycle performance. Specifically, Comparative Example 2 reduced the EC content by 20% compared to Comparative Example 1. Since EC has poor high-voltage stability, reducing the EC content can reduce electrolyte decomposition under high voltage, thereby improving cycle performance.

[0091] As can be seen from Table 2, Comparative Example 1 showed the largest impedance increase at the 452nd cycle, possibly due to the higher EC content and greater electrolyte decomposition under high voltage. In Comparative Example 2, the impedance increase after cycling decreased compared to Comparative Example 1 as the EC content decreased, consistent with the improved cycling performance shown in Table 1. In Example 1, compared to Comparative Example 1, fluorinated cyclic carbonate (FEC) replaced EC, resulting in a significant increase in both the discharge capacity and capacity retention at the 500th cycle, and also an improvement over Comparative Example 2. Table 2 shows that the impedance increase after cycling in Example 1 was significantly lower than in Comparative Example 1, and also lower than in Comparative Example 2. Example 2 used another fluorinated cyclic carbonate, TFPC, to replace EC, with slightly better results than Example 1. As can be seen from Examples 1 and 2, due to the higher oxidation voltage of fluorinated carbonates, replacing non-fluorinated cyclic carbonate solvents in traditional electrolytes with fluorinated cyclic carbonates can significantly improve the cycling performance of lithium-rich manganese-based lithium batteries and reduce the increase in battery impedance during cycling.

[0092] Examples 3 and 4 used fluorinated cyclic carbonate TFPC to replace EC and fluorinated chain carbonate FEMC or FDEC to replace EMC. Compared with Examples 1 and 2, Examples 3 and 4 showed further improvements in discharge capacity and capacity retention after cycling, while the battery impedance growth during cycling was further reduced. This demonstrates that simultaneously replacing both non-fluorinated cyclic and chain carbonates in the traditional electrolyte with fluorinated carbonate further improves the high-voltage stability of the electrolyte, resulting in improved battery cycle performance.

[0093] Examples 5-7, based on Example 4, maintained the same TFPC to FDEC ratio (3:7) and added 10%, 20%, and 30% of the total solvent mass fraction of fluorinated ether solvent F-EPE to further improve the high-voltage stability of the electrolyte. Combining Tables 1 and 2, in Examples 4-7, as the F-EPE content increased, the initial discharge capacity decreased and the initial battery impedance increased. This is likely because the addition of F-EPE reduced the electrolyte conductivity, thereby increasing impedance and reducing capacity. Comparing Examples 5-7 and Example 4, adding F-EPE can further improve capacity retention and reduce impedance growth rate; however, when 20% F-EPE was added, the capacity reached its maximum at the 500th discharge cycle. Further increases in F-EPE increased battery impedance, leading to a decrease in cycle capacity.

[0094] Example 8 is based on Example 6, but uses another fluoroether solvent, BTFE, instead of F-EPE. The initial capacity, capacity retention, initial impedance, and impedance growth are essentially equivalent to those of Example 6. This demonstrates that using different fluoroether solvents can improve the high-voltage cycling performance of the electrode.

[0095] Examples 9-11, based on Comparative Example 1, added 2% by mass of boron-containing solvent additive PBF or boron-containing lithium salt additive LTB, or both at 1% by mass, to form a protective CEI film on the positive electrode surface to suppress electrolyte decomposition at high voltage. Combining Tables 1 and 2, Examples 9-11 show that when only PBF or LTB is added, the battery impedance growth rate after cycling is lower than without additives, while the capacity retention rate is higher. When both additives are added simultaneously, the impedance growth rate further decreases, and the capacity retention rate further increases. This demonstrates that boron-containing solvent additives and boron-containing lithium salt additives, alone or simultaneously, can improve the high-voltage cycling performance of the battery, with better results when used together.

[0096] Examples 12-14 are based on Example 6, with the addition of 2% by mass of boron-containing solvent additive PBF or boron-containing lithium salt additive LTB, or both at 1% by mass. This leverages the CEI film-forming properties of the boron-containing additive and the high-voltage stability of the fluorinated solvent, thereby significantly improving the high-voltage cycling performance of the battery. Comparing Examples 12-14 and 6 with Tables 1 and 2, when only PBF or LTB is added, the battery impedance growth rate after cycling is lower than without additives, and the capacity retention rate is higher. When both additives are added simultaneously, the impedance growth rate further decreases, and the capacity retention rate further increases, indicating a synergistic effect when both additives are added simultaneously. Comparing Example 14 and Comparative Example 1, in Example 14, the battery impedance growth rate decreased significantly from 103% to 40%, while the capacity retention rate increased significantly from 60% to 81%. Furthermore, comparing Examples 14 and 11, with the addition of the same boron-containing additive, compared to non-fluorinated solvents (Example 11), adding the same boron-containing additive in fluorinated solvents (Example 14) better suppresses capacity loss. It is evident that when boron-containing additives and fluorinated solvents are used in combination, their excellent CEI film-forming properties and high-voltage stability can be synergistically utilized to inhibit electrolyte oxidative decomposition, thereby significantly improving the high-voltage cycle performance of the battery.

[0097] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made using the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. An electrolyte for use in lithium-rich manganese-based high-voltage lithium-ion batteries, characterized in that, The electrolyte comprises lithium salt, solvent, and boron-containing additive; the solvent is a fluorinated solvent; by mass fraction, the fluorinated solvent is 70-80% fluorinated carbonate solvent and 20-30% fluorinated ether solvent; the amount of boron-containing additive added is 1-3% of the total mass of lithium salt and solvent, and it is a mixture of boron-containing solvent additive and boron-containing lithium salt additive; the concentration of lithium salt in the solvent is 0.8-1.5 mol / L.

2. The electrolyte for a lithium-rich manganese-based high-voltage lithium-ion battery according to claim 1, characterized in that, The fluorocarbonate solvent is one or more of fluoroethylene carbonate, trifluoromethyl ethylene carbonate, difluoroethylene carbonate, methyltrifluoroethyl carbonate, and bis(2,2,2-trifluoroethyl) carbonate.

3. The electrolyte for a lithium-rich manganese-based high-voltage lithium-ion battery according to claim 1, characterized in that, The fluorinated ether solvent is one or more of 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether.

4. The electrolyte for a lithium-rich manganese-based high-voltage lithium-ion battery according to claim 1, characterized in that, The amount of boron-containing additive added is 2% of the total mass of lithium salt and solvent.

5. The electrolyte for a lithium-rich manganese-based high-voltage lithium-ion battery according to claim 1, characterized in that, The boron-containing solvent additive is one or more of trihydro(pyridine)boron, trimethyl borate, tri(2-cyanoethyl)boronic acid, phenylboronic acid, and 2,4,6-triphenylcycloboroxane.

6. The electrolyte for a lithium-rich manganese-based high-voltage lithium-ion battery according to claim 1, characterized in that, The boron-containing lithium salt additive is one or more of lithium tetraborate, lithium difluorooxalate borate, lithium bis(oxalate borate), and lithium tetrafluoroborate.

7. A lithium-ion battery comprising the electrolyte as described in any one of claims 1-6.

8. The lithium-ion battery according to claim 7, characterized in that, In the lithium-ion battery, the positive electrode active material is a lithium-rich manganese-based positive electrode material.

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