High-voltage lithium ion battery electrolyte and application thereof

Abstract

The invention discloses a high-voltage lithium ion battery electrolyte and application thereof, and belongs to the field of lithium battery electrolytes. A traditional EC solvent is replaced with fluoro-carbonate, a ternary mixed solvent system is constructed by combining chain carbonate and fluoro-ether, the boron-containing lithium salt additive is added, a boron-fluorine synergistic interface regulation and control mechanism is formed, the cycling stability of the electrolyte at the high voltage of 4.6 V is remarkably improved, and a contrast experiment shows that the electrolyte has the advantages of being simple in preparation process and low in cost. The capacity retention ratio after 200 cycles is increased by 20% or above compared with that of a traditional EC-based electrolyte, the electrolyte is suitable for lithium-rich manganese-based and other high-voltage positive electrode materials, the problems of oxygenolysis and interface failure of an existing EC-based electrolyte are solved, and a key matching technology is provided for a high-energy-density battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery electrolytes, specifically relating to a high-voltage lithium-ion battery electrolyte and its applications. Background Technology

[0002] With the increasing global demand for clean energy and sustainable transportation, electric vehicles and large-scale energy storage technologies have developed rapidly. As the core power source, lithium-ion batteries have seen their energy density become a key indicator determining their driving range and performance. To overcome the energy density bottleneck of existing battery systems, research on cathode materials is moving towards higher voltages. Lithium-rich manganese-based materials, high-voltage lithium cobalt oxide, and spinel-structured LiNi are among the promising advancements. 0.5 Mn 1.5 High-voltage cathode materials such as O4 are used because they have a voltage higher than 4.5V (relative to Li). + The discharge platform of / Li) can provide higher output voltage and specific capacity, and is regarded as the ideal cathode choice for the next generation of high energy density lithium-ion batteries.

[0003] However, the practical application of high-voltage cathode materials faces severe challenges in terms of matching electrolyte systems. Currently, commercial lithium-ion batteries generally use carbonate electrolytes based on ethylene carbonate (EC). Due to its excellent film-forming properties, EC can form a stable and dense solid electrolyte interphase (SEI) film on the graphite anode surface, thereby ensuring the battery's cycle life. However, EC itself has a narrow electrochemical window, and its antioxidant decomposition potential is usually below 4.3V. When the battery operating voltage is increased to 4.5V and above, traditional EC-based electrolytes will undergo a violent oxidative decomposition reaction on the high-voltage cathode surface. This process not only consumes active lithium and electrolyte, and produces gases such as carbon dioxide, leading to battery swelling and performance degradation, but also forms an unstable, high-resistivity cathode electrolyte interphase (CEI) film on the cathode material surface. This unstable CEI film cannot effectively prevent the continuous decomposition of the electrolyte, nor can it suppress the erosion of the cathode material structure by high voltage (such as the dissolution of transition metal ions and the loss of lattice oxygen), ultimately leading to rapid capacity decay and drastic deterioration of cycle stability, which severely restricts the advantages of high-voltage cathode materials.

[0004] To alleviate this problem, existing technologies typically employ the addition of functional additives to traditional EC-based electrolytes. For example, introducing a certain proportion of high-voltage additives such as nitriles, sulfones, or fluorinated solvents aims to preferentially oxidize the cathode surface, forming a protective layer. These methods improve the high-voltage cycle performance of the battery to some extent. However, such solutions are mostly "repair-type" improvements, failing to fundamentally address the core contradiction of the thermodynamic instability of EC components at high voltages. The presence of EC remains a weakness in the overall antioxidant capacity of the electrolyte, and its decomposition products are unlikely to form a robust and ion-conducting ideal interfacial film. Therefore, relying solely on additives is insufficient to meet the long-term, stable cycling requirements at high voltages (especially 4.6V and above).

[0005] And the invention patent with publication number CN104659417A discloses an electrolyte for lithium-ion batteries, which includes fluorocarbonate compounds, fluoroether compounds, lithium difluoroborate oxalate, and lithium hexafluorophosphate. Since fluoroethylene carbonate is a strong film-forming electrolyte component, an excessively high proportion may result in an SEI film that is not the optimal microstructure (e.g., too thick, too high impedance), negatively impacting the initial coulombic efficiency and long-term cycle stability. Furthermore, the viscosity of fluorocarbonates and fluoroethers is typically higher than their unfluorinated counterparts; the overall viscosity of the mixture may still be higher than that of conventional electrolytes, leading to a decrease in overall ionic conductivity and poorer electrode wettability.

[0006] Therefore, developing an electrolyte with high oxidation stability and good interfacial compatibility is the key to breaking through the bottleneck of practical application of high-voltage cathode materials. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a high-voltage electrolyte that significantly improves the cycling stability of the electrolyte in voltage windows above 4.5V by replacing EC with fluorinated solvents, synergistic use of ternary mixed solvents, and boron-fluorine interface regulation, thus solving the compatibility problem between high-voltage cathode materials and electrolytes.

[0008] This invention adopts a precise control approach, using low-viscosity chain carbonates as the main component, introducing a small amount of fluorinated solvents and borates as "special additives", and combining them with flexible compound lithium salts. High performance is achieved through the synergistic effect of multiple components at low dosages, while comprehensively considering process adaptability and practical performance.

[0009] The specific technical solution of this invention is as follows: This invention provides a high-voltage lithium-ion battery electrolyte, comprising: A mixed solvent, composed of chain carbonates, fluorocarbonates and fluoroethers; Lithium salt, dissolved in the mixed solvent; And lithium borate salt additives; The fluorocarbonate is used to replace the traditional ethylene carbonate to improve the electrolyte's antioxidant capacity and works synergistically with the lithium borate salt additive to form a stable boron-fluorine composite interface layer on the positive electrode surface.

[0010] By replacing traditional ECs with fluorocarbonates (such as fluoroethylene carbonate), the high electronegativity of fluorine atoms is used to increase the solvent oxidation potential (>5.0V), suppressing solvent decomposition under high voltage. At the same time, the fluorinated solvent undergoes a defluorination reaction on the positive electrode surface to generate an inorganic interface layer containing LiF, which enhances the interfacial mechanical strength and ion conductivity, thus solving the interfacial failure problem of EC-based electrolytes.

[0011] Preferably, the mass ratio of the chain carbonate, fluorocarbonate and fluoroether in the mixed solvent is 1:0.05~1:0.05~1.

[0012] By optimizing the mass ratio (1:0.05~1:0.05~1) of chain carbonates (which regulate viscosity and lithium salt solubility), fluorocarbonates (which dominate high-stability interfacial film formation), and fluoroethers (which reduce viscosity and improve safety), a synergistic balance between solvent polarity, electrochemical window, and safety is achieved, providing a suitable environment for ion transport and interfacial stability under high voltage.

[0013] Furthermore, the chain carbonate is one or more of ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dimethyl carbonate (DMC); The fluorocarbonate is one or more of fluoroethylene carbonate (FEC), fluoroethyl methyl carbonate (FEMC), and fluorodiethyl carbonate (FDEC); The fluoroether is one or more of hydrofluoroether (HFE), methyl nonafluorobutyl ether (M9FBE), and octafluoropentyl tetrafluoroethyl ether (OFPEFE).

[0014] Preferably, the concentration of the lithium salt is 0.05~5 mol / L, and it is a compound salt of at least two of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium trifluoroacetate (LiCF3COO).

[0015] Boron-containing lithium salts (such as LiDFOB) decompose under high voltage to generate interface components containing BO and BF, which together with LiF generated by fluorinated solvents form a "boron-fluorine composite interface layer". This fills the interface film defects and improves oxidation resistance, suppresses the loss of lattice oxygen in the cathode material and the continuous decomposition of the electrolyte, thereby enhancing the stability at the interface chemical level.

[0016] Preferably, the lithium borate salt additive is one or more of lithium difluorooxalate borate (LiDFOB), lithium dioxoacetic acid borate (LiBOB), and lithium tetracyanoborate (LiBCN).

[0017] Furthermore, the lithium borate salt additive accounts for 0.1% to 3% of the total mass of the electrolyte.

[0018] This invention also provides a method for preparing a high-voltage lithium-ion battery electrolyte: 1. Solvent pretreatment: In an argon glove box (oxygen content ≤0.01ppm, water content ≤0.01ppm, model such as Mbraun Labstar or VAC glove box), mix chain carbonate, fluorocarbonate and fluoroether in mass ratio, add 3A or 4A molecular sieve (1%~3% of solvent mass), and stir at room temperature for 24 hours to remove water and impurities.

[0019] 2. Lithium salt dissolution: Add lithium salt (such as a mixture of LiPF6 and LiTFSI) according to the required concentration, stir at 500~1000 rpm for 2~4 hours until completely dissolved.

[0020] 3. Additive mixing: Add lithium borate salt additives (such as LiDFOB), stir at 300~500 rpm, and stir for 1~2 hours.

[0021] 4. Filtration and impurity removal: High-voltage electrolyte was obtained by filtration using a 0.22μm polytetrafluoroethylene (PTFE) filter membrane.

[0022] The electrolyte is applied to high-voltage cathode materials (such as lithium-rich manganese-based, LiNi). 0.5 Mn 1.5 The lithium-ion battery (O4) is assembled into button cells or pouch cells and cycled within a voltage window of 2.8~4.6V, significantly improving the cycle stability and capacity retention of the battery.

[0023] The present invention also provides a lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and the aforementioned high-voltage lithium-ion battery electrolyte.

[0024] Furthermore, the cathode is a lithium-rich manganese-based material or LiNi 0.5 Mn 1.5 O4.

[0025] This invention systematically improves the three core properties of electrolytes through a multi-dimensional design involving "fluorinated solvent substitution + mixed solvent formulation + boron-containing additive compounding": -Oxidation stability: Fluorinated solvents and boron-containing lithium salts together extend the electrochemical window to above 4.6V; -Interfacial compatibility: Progressive film formation (solvent pre-film formation + secondary reinforcement with additives) reduces interfacial resistance; - Cyclic stability: Comparative experiments show that after 200 cycles, the capacity retention rate is more than 20% higher than that of traditional EC-based electrolytes.

[0026] The beneficial effects of this invention are: This invention replaces the traditional EC solvent with fluorinated carbonate, constructs a ternary mixed solvent system by combining chain carbonate and fluorinated ether, and adds boron-containing lithium salt additives to form a boron-fluorine synergistic interface regulation mechanism, which significantly improves the cycle stability of the electrolyte at a high voltage of 4.6V. It is suitable for high-voltage cathode materials such as lithium-rich manganese-based electrolytes, solves the problems of oxidation decomposition and interface failure of existing EC-based electrolytes, and provides key supporting technologies for high-energy-density batteries. Attached Figure Description

[0027] Figure 1 The cycle performance of the electrolyte-assembled battery prepared according to the embodiments of the present invention; Figure 2 The cycle performance of the electrolyte-equipped battery prepared in the comparative example of this invention is shown. Detailed Implementation

[0028] Example 1 1. Electrolyte preparation 1.1 Mixed Solvents EMC∶DEC∶FEC∶HFE=3∶2∶1∶1 (mass ratio) (EMC was purchased from Aladdin, item number E110111; DEC was purchased from Aladdin, item number D103040; FEC was purchased from Aladdin, item number F120339; HFE was purchased from Duoduo Reagents, item number NE-000100). 1.2 Lithium Salts 5.76wt% LiPF6 + 10.89wt% LiTFSI (LiPF6 was purchased from Duoduo Chemical, catalog number NE-000012; LiTFSI was purchased from Duoduo Chemical, catalog number NE-000014). 1.3 Additives 1wt% LiDFOB (LiDFOB purchased from Aladdin, product number L303675); 1.4 Preparation process In the Mbraun Labstar glove box, 3g EMC, 2g DEC, 1g FEC and 1g HFE were mixed with 5A molecular sieve (2wt%) and stirred at room temperature for 24 hours (molecular sieve purchased from Shanghai Testing, item number 20027861). Add 0.4841g LiPF6 and 0.9149g LiTFSI, and stir at 800rpm for 3 hours; Add 0.08g LiDFOB and stir at 500rpm for 1 hour; The electrolyte was obtained by filtration through a 0.22 μm PTFE membrane.

[0029] 2. Preparation of positive electrode sheet 2.1 Slurry Formulation Lithium-rich manganese-based material: conductive carbon black: carbon nanotubes: PVDF = 85:7:3:5 (mass ratio). The lithium-rich manganese-based material was purchased from Jiangxi Hanrao Lithium-rich Technology Co., Ltd., model RM-H. 2.2 Preparation process The active material, conductive agent, binder and N-methylpyrrolidone (NMP) were mixed and stirred at 2000 rpm for 3 hours in a planetary mixer (model Thinky ARE-310) to obtain a slurry; The slurry was coated onto aluminum foil using a doctor blade coater (model Shandong Animate TBJ-B1-DJ2), with a doctor blade gap of 50μm and a coating speed of 5mm / s. The material was baked in an 80℃ forced-air oven for 12 hours and then rolled with a Hohsen HLP-200 roller press at a pressure of 8MPa to control the thickness at 120μm. The positive electrode sheet with a diameter of 10 mm was obtained by die cutting and dried in a vacuum oven at 120℃ for 24 hours, with a moisture content of <100ppm.

[0030] 3. Battery assembly 3.1 Negative electrode: 14mm diameter lithium sheet; 3.2 Separator: 12μm double-sided ceramic PP diaphragm (model Celgard 2325); 3.3 Electrolyte injection volume: 80 μL; 3.4 Assembly equipment: CR2032 button cell assembly machine (model MTI Corporation MSK-110).

[0031] 4. Loop Testing 4.1 Test equipment: LAND battery test system (model CT2001A). 4.2 Test conditions: 25℃, 0.3C charge / discharge to 4.6V / 2.8V, stand for 24 hours and then cycle 200 times; 4.3 Performance indicators: initial capacity 222mAh / g, capacity retention rate 91.5% after 200 cycles.

[0032] Example 2 1. Mixed solvent: EMC∶DEC∶FEC∶HFE=3∶2∶1∶1 (mass ratio), actual addition amount is 3g EMC, 2g DEC, 2g FEC, 1g HFE; 2. Lithium salt: 5.76wt% LiPF6 + 10.89wt% LiTFSI, with actual additions of 0.4841g LiPF6 and 0.9149g LiTFSI; 3. Additive: 0.5wt% LiDFOB, actual addition amount is 0.04g; The remaining steps are the same as in Example 1.

[0033] Cyclic performance test results: initial capacity 220mAh / g, capacity retention rate 88.2% after 200 cycles.

[0034] Example 3 1. Mixed solvent: EMC∶DEC∶FEC∶HFE=3∶2∶1∶1 (mass ratio), actual addition amount is 3g EMC, 2g DEC, 2g FEC, 1g HFE; 2. Lithium salt: 5.76wt% LiPF6 + 10.89wt% LiTFSI, with actual additions of 0.4841g LiPF6 and 0.9149g LiTFSI; 3. Additives: 3wt% LiDFOB, actual addition amount is 0.25g; The remaining steps are the same as in Example 1.

[0035] Cyclic performance test results: initial capacity 218mAh / g, capacity retention rate 89.7% after 200 cycles.

[0036] Example 4 1. Mixed solvent: EMC∶DEC∶FEC∶HFE=3∶2∶1∶1 (mass ratio), actual addition amount is 3g EMC, 2g DEC, 2g FEC, 1g HFE; 2. Lithium salt: 5.76wt% LiPF6 + 10.89wt% LiTFSI, with actual additions of 0.4841g LiPF6 and 0.9149g LiTFSI; 3. Additives: 1wt% LiBOB (LiBOB purchased from Aladdin, product number L120347), actual addition amount is 0.08g; The remaining steps are the same as in Example 1.

[0037] Cyclic performance test results: initial capacity 217mAh / g, capacity retention rate 89.1% after 200 cycles.

[0038] Example 5 1. Mixed solvent: EMC∶FEC∶HFE=1∶1∶1 (mass ratio), actual addition amount is 2g EMC, 2g FEC, 2g HFE; 2. Lithium salt: 5.05wt% LiPF6 + 9.54wt% LiTFSI, with actual additions of 0.3546g LiPF6 and 0.6701g LiTFSI; 3. Additives: 1wt% LiDFOB, actual addition amount is 0.07g; The remaining steps are the same as in Example 1.

[0039] Cyclic performance test results: initial capacity 216mAh / g, capacity retention rate 91.1% after 200 cycles.

[0040] Example 6 1. Mixed solvent: EMC∶FEC∶HFE=1∶0.05∶1 (mass ratio), actual addition amount is 2g EMC, 0.1g FEC, 2g HFE; 2. Lithium salt: 5.28wt% LiPF6 + 9.97wt% LiTFSI, with actual additions of 0.2553g LiPF6 and 0.4826g LiTFSI; 3. Additives: 1wt% LiDFOB; The remaining steps are the same as in Example 1.

[0041] Cyclic performance test results: Initial capacity 218 mAh / g, capacity retention after 200 cycles 90.4%. Comparative Example 1 (Fluorocarbonate) Solvent composition: EMC∶DEC∶EC∶HFE=3∶2∶1∶1, actual addition amount is 3g EMC, 2g DEC, 1g EC, 1g HFE; Lithium salt composition: 6.50wt% LiPF6 + 10.83wt% LiTFSI, with actual addition amounts of 0.4808g LiPF6 and 0.9087g LiTFSI; The preparation of additives, positive electrode sheets, and battery assembly are the same as in Example 1.

[0042] Cycling performance: Initial capacity 215mAh / g, capacity retention 78.8% after 200 cycles.

[0043] Comparative Example 2 (Fluorine-free ether) Solvent composition: EMC∶DEC∶FEC=3∶2∶1, actual addition amount is 3g EMC, 2g DEC, 1g FEC; Lithium salt composition: 5.99wt% LiPF6 + 11.32wt% LiTFSI, with actual addition amounts of 0.4345g LiPF6 and 0.8213g LiTFSI; The preparation of additives, positive electrode sheets, and battery assembly are the same as in Example 1.

[0044] Cycling performance: Initial capacity 220mAh / g, capacity retention 88.3% after 200 cycles.

[0045] Comparative Example 3 (without lithium borate) Solvent composition: EMC∶DEC∶FEC∶HFE=3∶2∶1∶1, actual addition amount is 3g EMC, 2g DEC, 1g FEC, 1g HFE; Lithium salt composition: 5.76wt% LiPF6 + 10.89wt% LiTFSI, with actual addition amounts of 0.4841g LiPF6 and 0.9149g LiTFSI; Additives omitted: LiDFOB is not added; the preparation of the positive electrode and the assembly of the battery are the same as in Example 1.

[0046] Cycling performance: Initial capacity 218 mAh / g, capacity retention 76.6% after 200 cycles.

[0047] Comparative Example 4 (without linear carbonate) Solvent composition: FEC:HFE = 3:1, actual addition amount is 3g FEC, 1g HFE; Lithium salt composition: 4.4878wt% LiPF6 + 8.4819wt% LiTFSI, with actual addition amounts of 0.2063g LiPF6 and 0.3898g LiTFSI; The preparation of additives, positive electrode sheets, and battery assembly are the same as in Example 1.

[0048] Cycling performance: Initial capacity 215mAh / g, capacity retention 86.1% after 200 cycles.

[0049] Comparative Example 5 (conventional EC-based electrolyte, fluorine-free solvent, lithium borate-free) Solvent composition: EMC∶DEC∶EC=3∶2∶1, actual added amount is 3g EMC, 2g DEC, 1g EC; Lithium salt composition: 5.95wt% LiPF6 + 11.25wt% LiTFSI, with actual addition amounts of 0.4312g LiPF6 and 0.8150g LiTFSI; Additives omitted: LiDFOB is not added; the preparation of the positive electrode and the assembly of the battery are the same as in Example 1.

[0050] Cycling performance: Initial capacity 210 mAh / g, capacity retention 65.1% after 200 cycles.

[0051] For specific changes in cycle performance, see Figure 1 and Figure 2 .

Claims

1. A high-voltage lithium-ion battery electrolyte, characterized in that, include: A mixed solvent, composed of chain carbonates, fluorocarbonates and fluoroethers; Lithium salt, dissolved in the mixed solvent; And lithium borate salt additives; The fluorocarbonate is used to replace the traditional ethylene carbonate to improve the electrolyte's antioxidant capacity and works synergistically with the lithium borate salt additive to form a stable boron-fluorine composite interface layer on the positive electrode surface.

2. The high-voltage lithium-ion battery electrolyte according to claim 1, characterized in that, In the mixed solvent, the mass ratio of chain carbonate, fluorocarbonate and fluoroether is 1:0.05~1:0.05~1.

3. The high-voltage lithium-ion battery electrolyte according to claim 1 or 2, characterized in that, The chain carbonate is one or more of ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate; The fluorocarbonate is one or more of fluoroethylene carbonate, fluoromethyl ethyl carbonate, and fluorodiethyl carbonate; The fluoroether is one or more of hydrofluoroether, methyl nonafluorobutyl ether, and octafluoropentyl-tetrafluoroethyl ether.

4. The high-voltage lithium-ion battery electrolyte according to claim 1, characterized in that, The concentration of the lithium salt is 0.05~5 mol / L, and it is a compound salt of at least two of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium trifluoroacetate.

5. The high-voltage lithium-ion battery electrolyte according to claim 1, characterized in that, The lithium borate salt additive is one or more of lithium difluorooxalate borate, lithium dioxoacetic acid borate, and lithium tetracyanoborate.

6. The high-voltage lithium-ion battery electrolyte according to claim 1 or 5, characterized in that, The lithium borate salt additive accounts for 0.1% to 3% of the total mass of the electrolyte.

7. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and a high-voltage lithium-ion battery electrolyte as described in any one of claims 1 to 6.

8. The lithium-ion battery according to claim 7, characterized in that, The cathode is a lithium-rich manganese-based material or LiNi. 0.5 Mn 1.5 O4.

Images