An electrolyte and its application

Abstract

This invention relates to an electrolyte and its application. The electrolyte comprises the following components: lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, a phosphate ester compound, and fluoroethylene carbonate; the product of the molality of the phosphate ester compound and the lithium hexafluorophosphate is not greater than 0.05 (mol / 1000 g). 2 This invention solves the technical problem in lithium-ion batteries that phosphate ester electrolytes cannot be matched with lithium hexafluorophosphate as a lithium salt by designing the product of the molar concentrations of some components of the electrolyte.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage technology, and in particular to an electrolyte and its application. Background Technology

[0002] Currently, the organic electrolyte materials used in the lithium battery industry are mainly composed of carbonate solvents and LiPF6 lithium salts. Their performance degrades significantly at high temperatures (above 60°C) and at low temperatures (below -10°C). Moreover, fires and explosions occur frequently, making battery safety an obstacle to battery development.

[0003] In addition, there are two other technical issues in the field of lithium batteries.

[0004] The first issue is that phosphate ester electrolytes are incompatible with lithium hexafluorophosphate as the lithium salt. This is because lithium hexafluorophosphate undergoes an ester-fluoride exchange reaction with phosphate esters, leading to decomposition and gas production, which affects battery performance. In July 2018, an experimental team comprised of Professor Cao Yuliang's group at Wuhan University, Professor Xiao Lifen's group at Wuhan University of Technology, and Professor Liu Jun's group at Pacific Northwest National Laboratory in the United States reported a non-flammable phosphate ester electrolyte in the world's top energy journal, *Nature Energy*, suitable for current lithium-ion batteries and ensuring battery safety. This report used a high concentration of lithium difluorosulfonylimide to replace lithium hexafluorophosphate and triethyl phosphate as the solvent, achieving safe battery operation and avoiding the first technical problem. However, using a single lithium difluorosulfonylimide as the sole lithium salt presents problems such as corrosion of the current collector aluminum foil and high cost, preventing the achievement from being applied industrially.

[0005] The second issue is that when using lithium hexafluorophosphate as the main salt in the electrolyte, the mass percentage of fluoroethylene carbonate cannot exceed 1%. This is because excess fluoroethylene carbonate reacts with lithium hexafluorophosphate to produce acid, which ultimately generates gases such as hydrogen and phosphorus oxyfluoride, causing battery swelling and capacity decay. In 2024, Professors Hu Jiangtao and Zhang Qianling from Shenzhen University, Professor Pan Feng from Peking University, and Xiao Biwei from the Research Institute of New Materials Technology (Guangdong) proposed a novel non-flammable electrolyte, publishing an article entitled "Ultra-high rate performance of single-crystalline NMC cathodes enabled by a TEP-based electrolyte" in the internationally renowned journal Nano Energy. This project used triethyl phosphate (TEP), fluoroethylene carbonate (FEC), and lithium tetrafluorophosphate to form a non-flammable electrolyte for research on single-crystal NMC83 cathodes, exhibiting approximately 209 mAh g⁻¹ at 0.1C and 0.5C, respectively. -1 and 192 mAh g -1The battery exhibits high discharge capacity at high rates. After 300 cycles at 1C, the capacity retention rate increased from 46.1% to 88.2%, avoiding the second technical problem. However, the above data still lags behind the performance of commercially available batteries, and issues such as high cost and current collector corrosion hinder its widespread industrial application.

[0006] Phosphate esters possess high thermal stability and flame retardancy, decomposing to generate phosphorus free radicals when batteries are at high temperatures or overheated, thus blocking the combustion reaction chain and reducing the risk of battery fire. However, the industrialization of electrolytes composed of phosphate esters and lithium hexafluorophosphate is hindered by factors such as rapid capacity decay during battery cycles, high internal resistance, concentration polarization, and low capacity utilization. Furthermore, adding only single-salt electrolytes without lithium hexafluorophosphate leads to corrosion of the aluminum current collector at the positive electrode, making the addition of lithium hexafluorophosphate a necessity. Therefore, improving the compatibility between phosphate ester electrolytes and lithium hexafluorophosphate salts has become a pressing technical problem that needs to be solved in the development of low-cost flame-retardant electrolytes. Summary of the Invention

[0007] To address the aforementioned technical problems, the present invention aims to provide an electrolyte and its application. By designing the product of the molar concentrations of some components of the electrolyte, the technical problem that phosphate ester electrolytes cannot be matched with lithium hexafluorophosphate as a lithium salt in lithium-ion batteries is solved.

[0008] To achieve this objective, the present invention adopts the following technical solution: In a first aspect, the present invention provides an electrolyte comprising the following components: lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, a phosphate ester compound, and fluoroethylene carbonate; wherein the product of the mass molar concentration of the phosphate ester compound and the lithium hexafluorophosphate is not greater than 0.05 (mol / 1000 g). 2 For example, it could be 0.01 (mol / 1000g). 2 0.02 (mol / 1000g) 2 0.025 (mol / 1000g) 2 0.03 (mol / 1000g) 2 Or 0.04 (mol / 1000 g) 2 wait.

[0009] Studies have found that at certain concentrations, lithium hexafluorophosphate undergoes an ester-fluorine exchange reaction with phosphate esters, as shown in the following reaction equation: The product, lithium ethoxyphosphate (LiPF5-OR), decomposes to produce gas, as shown in the following reaction formula: Phosphorus oxyfluoride and fluoroethane are both gases, and these gaseous barriers increase the battery's internal resistance and cause capacity decay.

[0010] In this invention, by limiting the product of the molality of phosphate ester compounds and lithium hexafluorophosphate, it was found that when the product of the molality is not greater than 0.05 (mol / 1000 g). 2 When this occurs, reaction (I) will not occur. At this time, the product of the mass molar concentrations is called the first reaction blind zone concentration product. This invention solves the incompatibility problem between phosphate ester compounds and lithium hexafluorophosphate in the electrolyte, thereby solving the current collector corrosion problem.

[0011] Preferably, the phosphate ester compound includes any one or a combination of at least two of the following: trimethyl phosphate, dimethyl methyl phosphate, triethyl phosphate, diethyl methyl phosphate, tripropyl phosphate, triphenyl phosphate, fluorophosphate, or tri(fluoroalkyl) phosphate.

[0012] Preferably, the product of the fluoroethylene carbonate and lithium hexafluorophosphate molar concentrations is not greater than 0.10 (mol / 1000 g). 2 For example, it could be 0.02 (mol / 1000g). 2 0.04 (mol / 1000 g) 2 0.05 (mol / 1000g) 2 0.06 (mol / 1000 g) 2 Or 0.08 (mol / 1000g) 2 wait.

[0013] Studies have found that adding phosphate esters to the electrolyte exacerbates concentration polarization, leading to a decrease in initial battery capacity and a shortened cycle life. Further research revealed that adding an appropriate amount of fluoroethylene carbonate can resolve both concentration polarization and capacity reduction issues; however, excessive fluoroethylene carbonate accelerates battery capacity decay. Further investigation showed that fluoroethylene carbonate reacts with lithium hexafluorophosphate to form an acid, ultimately producing gases such as hydrogen and phosphorus oxyfluoride. The reaction equation is as follows: ; ; .

[0014] The hydrogen and phosphorus pentafluoride produced by the above reaction are gases, and the phosphorus pentafluoride will further react with carbonates to produce gases such as phosphorus oxyfluoride and fluoromethane, causing the battery to swell and its capacity to decrease.

[0015] In this invention, the product of the molar concentrations of lithium hexafluorophosphate and fluoroethylene carbonate is not greater than 0.10 (mol / 1000 g). 2At this time, reaction (III) will not occur. The product of the mass molar concentrations is called the second reaction blind zone concentration product, which solves the problem that the mass percentage of fluoroethylene carbonate in the electrolyte with lithium hexafluorophosphate as the main salt cannot exceed 1%, thus solving the problem of concentration polarization.

[0016] Preferably, based on the mass percentage of the electrolyte being 100%, the mass percentage of lithium bisfluorosulfonylimide is 10%-18%, for example, it can be 12%, 14%, 15%, 16% or 17%, etc.

[0017] Preferably, based on the mass percentage of the electrolyte being 100%, the mass percentage of lithium hexafluorophosphate is 1%-3%, for example, it can be 1.5%, 1.8%, 2%, 2.3% or 2.5%, etc.

[0018] In this invention, a dual main salt combination of lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate is used, and a specific amount is defined. This can reduce the concentration of lithium hexafluorophosphate in the electrolyte, solving the problem of rapid battery degradation at high temperatures, and also improve the battery's discharge capacity, capacity retention rate, and other performance characteristics.

[0019] Preferably, based on the mass percentage of the electrolyte being 100%, the mass percentage of the phosphate ester compound is 1%-10%, for example, it can be 2%, 4%, 5%, 6%, 7% or 8%, and more preferably 5%-7%.

[0020] Preferably, based on the mass percentage of the electrolyte being 100%, the mass percentage of the fluoroethylene carbonate is 1%-12%, for example, it can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or 11%, and more preferably 5%-7%.

[0021] Preferably, the electrolyte also includes an auxiliary salt.

[0022] Preferably, the auxiliary salt comprises any one or a combination of at least two of lithium nitrate, lithium difluorophosphate, lithium bis(oxalate)borate, or lithium bis(oxalate)borate.

[0023] Preferably, the mass of the co-salt is 5%-8% of the mass of the phosphate ester compound, for example, it can be 5.5%, 6%, 6.5%, 7% or 7.5%, etc.

[0024] In this invention, adding 5%-8% lithium nitrate to a phosphate ester compound can prepare an electrolyte in which the negative electrode contains a lithium nitride film.

[0025] Preferably, the electrolyte also includes a solvent.

[0026] Preferably, the solvent comprises halogenated carbonates and / or non-halogenated carbonates.

[0027] Preferably, the non-halogenated carbonate includes any one or a combination of at least two of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, or methyl ethyl carbonate.

[0028] Preferably, the halogenated carbonate includes any one or a combination of at least two of the following: fluoroethylene carbonate, difluoroethylene carbonate, difluoropropylene carbonate, ethyl trifluorocarbonate, trifluoroethyl methyl carbonate, trifluoromethyl ethylene carbonate, 4-trifluoromethyl ethylene carbonate, chloroethylene carbonate, di(2,2,2-trifluoroethyl) carbonate, methyl trifluoropropionate, ethyl 3,3,3-trifluoroethylene carbonate, methyl 2-trifluoromethylbenzoate, ethyl 4,4,4-trifluorobutyrate, or 1,1,1,3,3,3-hexafluoroisopropyl acrylate.

[0029] Preferably, with the electrolyte having a mass percentage of 100%, the solvent has a mass percentage of 60%-70%, for example, it can be 62%, 64%, 65%, 66% or 68%, etc.

[0030] Preferably, the electrolyte also includes additives.

[0031] Preferably, the additive comprises any one or a combination of at least two of the following: vinyl sulfate, 1,3-propanesulfonyl lactone, 1,3-propenesulfonyl lactone, vinyl carbonate, propylene carbonate, lithium difluorophosphate, or tris(trimethylsilyl)phosphate.

[0032] Preferably, based on the mass percentage of the electrolyte being 100%, the total mass percentage of the additives is 4%-5%, for example, it can be 4.2%, 4.4%, 4.5%, 4.6% or 4.8%, etc.

[0033] In a second aspect, the present invention provides an application of the electrolyte as described in the first aspect in lithium-ion batteries or supercapacitors.

[0034] In this invention, the above-mentioned inventive principle can also be used for the formulation design of phosphate ester-containing flame-retardant electrolytes for sodium or potassium batteries.

[0035] Compared with the prior art, the present invention has at least the following beneficial effects: When the electrolyte described in this invention is used in a battery, the safety of the resulting battery is guaranteed. The battery obtained by this invention has a 3C discharge rate of over 89.0% at room temperature, a 1C discharge rate of over 87.5% at -20℃, a capacity retention rate of over 96.0% after 800 1C charge / 1C discharge cycles at room temperature, and a capacity retention rate of over 93.0% after 800 1C charge / 1C discharge cycles at 45℃. It has excellent overall performance, especially with significant improvements in cycle performance and safety performance. Detailed Implementation

[0036] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0037] Examples 1-11, Comparative Examples 1-5 An electrolyte is provided, and the components and amounts of the electrolyte are shown in Table 1.

[0038] Table 1 In Table 1, the amounts of each component in the electrolyte are by mass percentage. TEP+NL is a mixture of triethyl phosphate and lithium nitrate, with the mass of lithium nitrate being 5% of the mass of triethyl phosphate. The additives specifically include: 0.5% ethylene carbonate (VC), 1.0% ethylene sulfate (DTD), 0.5% tris(trimethylsilyl) phosphate (TMSP), 1% propylene carbonate (PC), 1% 1,3-propanesulfonyl lactone (PS), and 0.5% lithium difluorophosphate (2F).

[0039] Application Example 1-11, Comparative Example 1-5 Each provides a lithium-ion battery. The positive electrode of the lithium-ion battery uses PVDF-S5130 binder, Super-P / KS-6 composite conductive agent (mass ratio Super-P:KS-6=2:1), 811 nickel-cobalt-manganese ternary positive electrode material, and NMP (N-methylpyrrolidone) solvent. The negative electrode uses Shanshan P15, conductive agent Super-P, solvent CMC, H2O, and binder styrene-butadiene rubber (SBR) as raw materials; Positive and negative electrode slurries were prepared using a wet slurry process. The viscosity of the positive electrode was adjusted to 12000 mPa•s, and the viscosity of the negative electrode was adjusted to 2000 mPa•s. The designed N / P ratio was 1.12, and the capacity was 1800 mAh.

[0040] The lithium-ion battery was obtained by coating, slicing, rolling, slitting, drying at 140°C for 8 h, applying adhesive tape, winding the battery cell, drying at 80°C for 48 h, injecting the electrolytes described in Examples 1-11 and Comparative Examples 1-5 into the lithium-ion battery, sealing, resting for 24 h, forming, first final sealing, aging, capacity testing, and second final sealing.

[0041] Test methods For example 1-11, compare the lithium-ion battery used in example 1-5 with the lithium-ion battery to test its cycle performance and safety performance.

[0042] (1) Discharge rate performance: 1C current is 1.8 A, 3C current is 5.4 A; charging and discharging potential range is 2.75V-4.2V. The discharge rate at room temperature at 3C is the ratio of the capacity C2 of the 3C constant current discharge to the capacity C1 of the 1C constant current discharge.

[0043] (2) Cyclic performance: The charging and discharging potential range is 2.75 V-4.2 V, the charging current is 1 C (1.8 A) to 4.2 V, the constant voltage charging at 4.2 V is until the cutoff current is ≤0.02 C (0.036 A), after standing for 5 minutes, the discharge is 1 C (1.8 A) to 2.75 V, and then standing for 5 minutes; the charging and discharging cycle is repeated in this way.

[0044] (3) Low-temperature discharge performance: The discharge capacity of 1 C (1.8 A) at room temperature (25℃) is recorded as C1. After being fully charged to 4.2 V, it is frozen at -20℃ for 4 h and then discharged to 2.75 V at 1 C (1.8 A), and the discharge capacity is recorded as C2. The discharge rate at -20℃ is C2 / C1.

[0045] (4) Safety performance: The hot box test was conducted by placing 10 fully charged batteries in an explosion-proof box at 130°C for 30 minutes. The needle penetration test was conducted in an explosion-proof box, where five batteries were fully charged and a 3 mm steel needle was used to pierce the center of the battery at a speed of 10 mm / s.

[0046] The test results are shown in Tables 2 and 3.

[0047] Table 2 Table 3 The test results show that: (1) As can be seen from Application Examples 1-11, the present invention solves the technical problems in lithium-ion batteries by designing the product of the mass molar concentrations of lithium hexafluorophosphate, phosphate ester compounds and fluoroethylene carbonate in the electrolyte. This is because the electrolyte containing phosphate esters cannot be matched with lithium hexafluorophosphate as a lithium salt, and the mass percentage of fluoroethylene carbonate in the electrolyte containing lithium hexafluorophosphate as the main salt cannot exceed 1%.

[0048] (2) By comparing the application examples 1-5, it can be seen that as the TEP concentration increases, the discharge capacity at room temperature of 3 C decreases, while the discharge capacity at low temperature of -20℃ increases.

[0049] The comparison of Application Example 6-11 shows that as the FEC concentration increases, the discharge capacity at 3C room temperature increases, and the discharge capacity at -20℃ also increases.

[0050] (3) It can be seen from Application Example 1-11 and Comparative Application Example 1-5 that when the electrolyte concentration meets the concentration product of the first and second reaction blind zones, the cycle performance data of the battery is comparable to that of the blank Comparative Application Example 5. Comparative Application Example 1-4 does not meet the requirements of the concentration product of the first and second reaction blind zones, and the cycle life is significantly reduced. Upon inspection of the battery in Comparative Application Example 1-4, it was found that the battery had many bubbles.

[0051] Furthermore, the electrolytes in Application Examples 1-11, which contained phosphate ester compounds, passed all safety tests, while the electrolytes in Comparative Application Example 5, which did not contain phosphate ester compounds, failed the safety test.

[0052] In summary, this invention solves the technical problem that phosphate ester electrolytes cannot be matched with lithium hexafluorophosphate as lithium salts in lithium-ion batteries by designing the product of the mass molar concentrations of some electrolyte components.

[0053] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. An electrolyte, characterized in that, The electrolyte comprises the following components: lithium difluorosulfonylimide, lithium hexafluorophosphate, phosphate esters, and fluoroethylene carbonate; The product of the phosphate ester compound and the molality of lithium hexafluorophosphate is not greater than 0.05 (mol / 1000 g). 2 .

2. The electrolyte according to claim 1, characterized in that, The phosphate ester compounds include any one or a combination of at least two of the following: trimethyl phosphate, dimethyl methyl phosphate, triethyl phosphate, diethyl methyl phosphate, tripropyl phosphate, triphenyl phosphate, fluorophosphate, or tri(fluoroalkyl) phosphate.

3. The electrolyte according to claim 1 or 2, characterized in that, The product of the fluoroethylene carbonate and lithium hexafluorophosphate molar concentrations is not greater than 0.10 (mol / 1000 g). 2 .

4. The electrolyte according to any one of claims 1-3, characterized in that, Based on the electrolyte having a mass percentage of 100%, the lithium difluorosulfonamide has a mass percentage content of 10%-18%. Preferably, based on the electrolyte having a mass percentage content of 100%, the lithium hexafluorophosphate has a mass percentage content of 1%-3%. Preferably, based on the electrolyte having a mass percentage content of 100%, the phosphate ester compound has a mass percentage content of 1%-10%, more preferably 5%-7%; Preferably, based on the electrolyte having a mass percentage content of 100%, the mass percentage content of the fluoroethylene carbonate is 1%-12%, more preferably 5%-7%.

5. The electrolyte according to any one of claims 1-4, characterized in that, The electrolyte also includes auxiliary salts; Preferably, the auxiliary salt comprises any one or a combination of at least two of lithium nitrate, lithium difluorophosphate, lithium bis(oxalate)borate, or lithium bis(oxalate)borate.

6. The electrolyte according to claim 5, characterized in that, The mass of the co-salt is 5%-8% of the mass of the phosphate ester compound.

7. The electrolyte according to any one of claims 1-6, characterized in that, The electrolyte also includes a solvent; Preferably, the solvent comprises halogenated carbonates and / or non-halogenated carbonates; Preferably, the non-halogenated carbonate includes any one or a combination of at least two of ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, or methyl ethyl carbonate. Preferably, the halogenated carbonate includes any one or a combination of at least two of the following: fluoroethylene carbonate, difluoroethylene carbonate, difluoropropylene carbonate, ethyl trifluorocarbonate, trifluoroethyl methyl carbonate, trifluoromethyl ethylene carbonate, 4-trifluoromethyl ethylene carbonate, chloroethylene carbonate, di(2,2,2-trifluoroethyl) carbonate, methyl trifluoropropionate, ethyl 3,3,3-trifluoroethylene carbonate, methyl 2-trifluoromethylbenzoate, ethyl 4,4,4-trifluorobutyrate, or 1,1,1,3,3,3-hexafluoroisopropyl acrylate.

8. The electrolyte according to claim 7, characterized in that, With the electrolyte having a mass percentage of 100%, the solvent has a mass percentage of 60%-70%.

9. The electrolyte according to any one of claims 1-8, characterized in that, The electrolyte also includes additives. Preferably, the additive includes any one or a combination of at least two of the following: vinyl sulfate, 1,3-propanesulfonyl lactone, 1,3-propenesulfonyl lactone, vinyl carbonate, propylene carbonate, lithium difluorophosphate, or tris(trimethylsilyl)phosphate. Preferably, the total mass percentage of the additives is 4%-5%, based on the electrolyte being 100% by mass.

10. The application of the electrolyte as described in any one of claims 1-9 in a lithium-ion battery or supercapacitor.

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