Lithium battery fluorine-containing low-temperature electrolyte and application

Abstract

The invention discloses a fluorine-containing electrolyte. The fluorine-containing electrolyte comprises a solvent and an electrolyte salt, as an example, 1, 1, 2, 2-tetrafluoroethyl-2, 2, 2-trifluoroethyl ether, ethyl acetate and fluoroethylene carbonate are adopted as a solvent part, and lithium bis (fluorosulfonyl) imide is adopted as an electrolyte. The electrolyte using 1, 1, 2, 2-tetrafluoroethyl-2, 2, 2-trifluoroethyl ether as a diluent is mainly applied to Li-S batteries and sodium ion batteries in the previous patent, the capacity of lithium salt is generally 0.5 wt%-5wt%, and the content is low. Compared with other low-temperature electrolytes, the formula adopts a local high-concentration salt concept, the solubility of the lithium salt can reach 3M * L <-1 >, and the lithium salt can be stably circulated. An SEI film rich in lithium fluoride LiF and Li2O can be generated on the surface of the negative electrode through circulation, and the desolvation energy of Li on the surface of the electrode under the low-temperature condition can be effectively reduced, so that the low-temperature performance of the electrolyte is improved.

Description

[Technical field]

[0001] The present invention belongs to the field of lithium metal batteries, and in particular, the present invention relates to an electrolyte for lithium metal batteries and a method for preparing the same. The present invention also includes the application of the above-mentioned low-temperature electrolyte and a lithium metal battery containing the low-temperature electrolyte. [Background technology]

[0002] Lithium metal batteries are increasingly favored by various manufacturers due to their high energy density, long life, lightness, environmental friendliness, and ability to achieve fast and high current charging. Among the various materials for the negative electrode, lithium metal (lithium element) has a theoretical capacity of 3,860 mAhg -1 ) ultra-high characteristics, as well as the lowest electrochemical redox potential (-3.04Vvs.SHE) and low density (0.534g cm-3), can surpass traditional graphite negative electrodes in the field of fast charging, and has become a hot research object for secondary batteries in recent years. With the further popularization of lithium metal applications in commercial batteries, it is bound to replace traditional lead-acid batteries, nickel-cadmium batteries, etc., and become the darling of the future electrification wave. And it will be used in electric vehicles, aerospace, military industry and other fields, which poses new challenges to the low temperature performance and safety of lithium metal batteries. At present, lithium metal batteries have problems such as low capacity at low temperatures, difficulty in achieving high current charging and discharging, and low coulomb efficiency. In my country's high-latitude areas, the winter temperature can reach about -30℃ to -40℃. At this temperature, many existing commercial lithium metal battery electrolytes cannot work properly, and may even cause irreversible damage to the entire battery, or have a very low capacity retention rate. This is because under low temperature conditions, there are problems such as increased electrolyte viscosity, decreased conductivity, and increased SEI interface impedance, which will lead to the growth of dendrites on the surface of the lithium negative electrode and even battery failure.

[0003] Develop a configuration scheme that can be applied to low-temperature electrolytes to improve the low-temperature performance of lithium metal batteries; the main priorities are to improve the excessive viscosity of the electrolyte at low temperatures and to improve the performance of the SEI membrane. [Summary of the invention]

[0004] The significance of the present invention lies in providing a feasible solution for low-temperature lithium metal batteries, which can still maintain a high charge and discharge capacity and a high coulombic efficiency at low temperatures; and in a low-temperature environment, a dense SEI film can be formed on the surface of the lithium metal negative electrode to inhibit the growth of dendrites on the lithium metal surface, so as to achieve improvement in the cycle performance of the low-temperature lithium metal battery.

[0005] Specifically, the present invention adopts a ternary organic solvent system, an electrolyte and an optionally added low-temperature additive.

[0006] In order to achieve stable charge and discharge at -40°C while maintaining a high charge-discharge specific capacity, the present invention adopts: a fluoro carbonate, another linear carboxylic acid ester solvent, and a diluent are used to dissolve a lithium salt together.

[0007] Among them, the fluoro carbonate can be optionally selected from fluoro carbonate, fluoro ethyl carbonate, difluoro carbonate, or any combination thereof.

[0008] Among them, the other linear carboxylic acid ester solvent can be optionally selected from methyl formate, methyl acetate, ethyl acetate, ethyl butyrate, methyl butyrate, or any combination thereof.

[0009] Among them, through experimentation, the diluent can be optionally one of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE) or 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE).

[0010] In the present invention, the lithium salt concentration adopted is 1.0 mol·L -1 -3.0 mol·L -1 ; The electrolyte is configured according to the scheme in 1-5. Among them, the ratio of the fluoro carbonate solution: linear carboxylic acid ester solvent: diluent can be optionally 2:4:4; 3:3:4; 4:2:4; 2:5:3; 3:4:3; 4:3:3, etc. Through testing, the low-temperature performance of the battery is the best when the diluent ratio is 40%.

[0011] Since the fluoro carbonate has a relatively high melting point and will solidify into a solid at room temperature; in order to enable it to be applied to low-temperature electrolytes, a linear carboxylic acid ester solvent is introduced to lower the melting point of the electrolyte. The use of a diluent can also effectively lower the melting point of the electrolyte; taking a linear carboxylic acid ester solvent with a low melting point and low viscosity as the main component and reducing the content of the high-melting-point component is beneficial to improving the low-temperature conductivity of the electrolyte and optimizing the low-temperature performance. According to actual tests, adding 30% - 40% of the diluent (solvent fraction) can significantly reduce the viscosity of the electrolyte at low temperature and maintain better room-temperature performance. Finally, through experimentation, 40% is the optimal ratio for adding the diluent.

[0012] In one embodiment of the present invention, the electrolyte is preferably selected from lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF 4), at least one of lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), and lithium trifluoromethanesulfonate (LiCF3SO3). In one embodiment of the present invention, the electrolyte is preferably selected from at least one of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide salt (LiFSI), and lithium hexafluorophosphate (LiPF6). In one embodiment of the present invention, the electrolyte is further preferably lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0013] In one embodiment of the present invention, the concentration of the electrolyte in the electrolyte solution is 0.5 mol·L -1 ~3.0 mol·L -1 In one embodiment of the present invention, the concentration of the electrolyte in the electrolyte solution is preferably 0.5 mol·L -1 、1.0 mol·L -1 、1.5 mol·L -1 、2.0 mol·L -1 、2.5 mol·L -1 or 3.0 mol·L -1 。In one embodiment of the present invention, the concentration of the electrolyte in the electrolyte solution is further preferably 1.0 mol·L -1 ~3.0 mol·L -1 。

[0014] In one embodiment of the preparation method of the present invention, it includes: under a protective atmosphere, taking a linear carboxylic acid ester solvent and a fluorinated carbonate solvent and mixing them, then adding a diluent, stirring for 15 - 25 min, adding a formulated amount of electrolyte salt, stirring for 10 - 20 min until completely dissolved, and then standing for 18 h.

[0015] Wherein the protective atmosphere is an argon atmosphere with a moisture content lower than 10 ppm.

[0016] The preparation method of the above lithium metal battery includes the following steps:

[0017] Preparation of the positive electrode: adding a positive electrode active material (NCM 811 or NCM 111), a conductive agent (acetylene black or Super P), and a binder (polyvinylidene fluoride PVDV dissolved in an N-methylpyrrolidone NMP solution) to a solvent, mixing evenly to obtain a positive electrode slurry; evenly coating the positive electrode slurry on an aluminum foil, with the aluminum foil serving as the current collector of the positive electrode, and placing it in a vacuum oven to heat and dry for more than 12 h to obtain the positive electrode;

[0018] The negative electrode of the lithium metal battery uses a lithium sheet with a thickness of 0.15 mm.

[0019] The separator is made of a PP separator. Under the condition that argon is used as a protective atmosphere, the positive electrode, the separator and the negative electrode are stacked to form a battery cell assembly, which is then installed in a housing; and an electrolyte is injected into the battery housing.

[0020] The low-temperature electrolyte for lithium-ion batteries of the present invention has a high conductivity and a low electrode interface film impedance at low temperatures, and can significantly improve the low-temperature charge and discharge performance of lithium-ion batteries. Among them, linear carboxylic ester solvents with low melting points and low viscosities are used as the main components in the solvent composition, the content of high-melting-point components is reduced, the low-temperature conductivity is increased, and the low-temperature performance is optimized; fluorinated carbonate solvents are selected to promote the formation of an interfacial film rich in LiF on the surface of the negative electrode, improving the performance of the electrolyte at low temperatures; a diluent is added to reduce the viscosity of the electrolyte at low temperatures and promote the formation of a better-performing SEI film on the surface of the lithium metal negative electrode, improving the stable operation ability of the battery at low temperatures, reducing the interfacial impedance, and increasing the Coulomb efficiency. The lithium metal battery prepared by the present invention uses the above electrolyte, effectively improving the performance of the lithium metal battery in a low-temperature environment, having a high low-temperature capacity retention rate, and maintaining the performance at room temperature before cooling after the temperature returns to room temperature, opening up a new idea for the research of low-temperature lithium metal batteries.

Specific Embodiments

[0021] The present invention will be described in detail in conjunction with the following specific embodiments.

[0022] Example 1:

[0023] In a glove box filled with nitrogen with a humidity less than 0.1 ppm, FEC and EA were measured in a volume ratio of 4:2, and then a TFTFE diluent equal in volume to FEC was added to form a ternary organic solvent of FEC:EA:TFTFE = 4:2:4; then LiTFSI was added to make its concentration 1 mol / L; thus, the low-temperature electrolyte of the present invention was obtained.

[0024] Example 2:

[0025] In a glove box filled with nitrogen with a humidity less than 0.1 ppm, FEC and EA were measured in a volume ratio of 1:1, and then a TFTFE diluent equal in volume to 4 / 3 of the volume of FEC was added to form a ternary organic solvent of FEC:EA:TFTFE = 3:3:4; then LiTFSI was added to make its concentration 1 mol / L; thus, the low-temperature electrolyte of the present invention was obtained.

[0026] Example 3:

[0027] In a glove box filled with nitrogen with a humidity less than 0.1 ppm, measure FEC and EA in a volume ratio of 4:2, and then add an amount of TTE diluent equal to that of FEC to form a ternary organic solvent of FEC:EA:TTE = 4:2:4; then add LiTFSI to make its concentration 1 mol / L; thus obtaining the low-temperature electrolyte of the present invention.

[0028] Example 4:

[0029] In a glove box filled with nitrogen with a humidity less than 0.1 ppm, measure FEC and EA in a volume ratio of 1:1, and then add an amount of TTE diluent equal to 4 / 3 of the volume of FEC to form a ternary organic solvent of FEC:EA:TTE = 3:3:4; then add LiTFSI to make its concentration 1 mol / L; thus obtaining the low-temperature electrolyte of the present invention.

[0030] Example 5:

[0031] In a glove box filled with nitrogen with a humidity less than 0.1 ppm, measure FEC and EA in a volume ratio of 4:2, and then add an amount of TFTFE diluent equal to that of FEC to form a ternary organic solvent of FEC:EA:TFTFE = 4:2:4; then add LiTFSI to make its concentration 3 mol / L; thus obtaining the low-temperature electrolyte of the present invention.

[0032] Example 6:

[0033] In a glove box filled with nitrogen with a humidity less than 0.1 ppm, measure FEC and EA in a volume ratio of 4:2, and then add an amount of TTE diluent equal to that of FEC to form a ternary organic solvent of FEC:EA:TTE = 4:2:4; then add LiTFSI to make its concentration 3 mol / L; thus obtaining the low-temperature electrolyte of the present invention.

[0034] Experimental operation

[0035] According to the manufacturing standard of lithium metal batteries, manufacture lithium metal battery button cells for testing; the positive electrode uses a ternary material (NCM811 ternary material, that is, LiNi 0.8 Co 0.1 Mn 0.1 O2), and the negative electrode uses a lithium metal negative electrode; assemble the lithium metal battery in an argon protection atmosphere with a humidity less than 0.1 ppm, and then let it stand for 18 h. Activate at 2.8V - 4.3V, 0.1C, 30℃. Then charge at a current of 0.2C to 4.3V at 30℃, let it stand for 6 h and then discharge at 1C, with a cut-off voltage of 2.8V, and measure its discharge specific capacity; after charging again and standing at low temperature for 6 h, discharge at 1C, with a cut-off voltage of 2.8V, and measure its discharge specific capacity at low temperature.

[0036] Table 1:

[0037]

[0038] According to Table 1, the lithium metal battery using the low-temperature electrolyte of the present invention has good discharge capacity and rate performance at low temperatures.

[0039] Table 2:

[0040]

[0042] According to Table 2, it can be seen that for the lithium metal battery using the low-temperature electrolyte of the present invention, the problem of low Coulomb efficiency at low temperatures is significantly improved, which can indirectly illustrate that using this electrolyte can optimize the interface performance and make the lithium metal battery operate more stably at low temperatures.

Description of the Drawings

[0043] Figure 1 It is a graph of capacity and Coulomb efficiency at -40 °C using Example 1.

[0044] Figure 2 It is a graph of capacity and Coulomb efficiency at -40 °C using Example 2.

Claims

1. A novel electrolyte formulation ratio focuses on formulating a low-temperature electrolyte (-40 °C) for lithium metal batteries. It consists of an organic solvent, an organic diluent, and an electrolyte salt. The concentration of the electrolyte salt can be 1 mol·L -1 -3.0 mol·L -1 , and the content of the diluent accounts for 30%-50% of the total volume of the overall electrolyte; the balance is the remaining organic solvent, where the content of the linear carboxylic acid ester solvent is 10%-40%, and the content of the fluorinated carbonate solvent is 30%-40%. The electrolyte salt used is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or lithium bis(fluorosulfonyl)imide salt (LiFSI), lithium hexafluorophosphate (LiPF6), any one or a combination thereof.

2. The electrolyte for a low-temperature lithium metal battery according to claim 1, characterized in that: The linear carboxylic acid ester solvent can be any one or more of methyl formate, methyl acetate, ethyl acetate, ethyl butyrate, and methyl butyrate.

3. The electrolyte for a low-temperature lithium metal battery according to claim 1, characterized in that: the fluorinated carbonate solvent can be one or several of fluorinated carbonate, fluoroethyl carbonate, and difluorinated carbonate.

4. The electrolyte for a low-temperature lithium metal battery according to claim 1, characterized in that: The diluent can be selected from one or more of 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), ethyl perfluorobutyl ether, ethyl nonafluorobutyl ether, 2,2,2-trifluoroethyl ether, 2,2,2-trifluoroethyl methyl ether, 2,2,2-trifluoroethyl trifluoromethanesulfonate, and 1,1,1-trifluoro-2-(trifluoromethoxy)ethane.

5. According to claims 2 to 4, one or more additives can be optionally added to the electrolyte of the low-temperature lithium metal battery. The options for the additives include: any one or several of vinylene carbonate fluoride, ethylene sulfate, methylene methanedisulfonate, tris(trimethylsilyl) borate, diethyl sulfite, and dimethyl sulfite.

6. According to claim 5, the dosage of the optionally added electrolyte additive can be 0.5%-3.5% by mass percentage in the electrolyte.

7. A low-temperature lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that: The electrolyte is the low-temperature electrolyte described in any one of claims 1 to 6.

8. Application of the low-temperature electrolyte according to any one of claims 1 to 6 as an electrolyte for a lithium-ion battery.

9. The preparation method of the low-temperature electrolyte according to any one of claims 1 to 8, the method comprising the following steps: (1) Mix the first organic solvent and the second organic solvent according to the ratio, and then add the third organic solvent to form a ternary organic solvent; (2) Add the optionally added additive and stir with a magnetic stirrer until dissolved; (3) Add the electrolyte and stir thoroughly with a magnetic stirrer until dissolved; (4) Let it stand still sufficiently to obtain the low-temperature electrolyte of the lithium metal battery.

10. The low-temperature lithium-ion battery according to the claim, characterized in that: The positive electrode active materials used in the positive electrode are any one or more of LiCoO2, LiFePO4, LiMn2O4, LiMn 1-y M y PO4, LiMn 1-y M y O4 and LiNi x Co y Mn z M 1-x-y-z O2; wherein, each M is independently any one of Fe, Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, Ti, and 0≤y≤1, 0≤x≤1, 0≤z≤1, and x + y + z ≤ 1.

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