Long-cycling nitrate electrolyte and lithium metal battery

Abstract

This invention discloses a long-cycle nitrate ester electrolyte and lithium metal, belonging to the field of lithium metal battery technology. The long-cycle nitrate ester electrolyte includes a non-aqueous solvent, a lithium salt, and a diluent; the lithium salt accounts for 6-30% of the molar percentage of the electrolyte, and the non-aqueous solvent accounts for 10-70% of the molar percentage of the electrolyte. This invention uses fluoroethyl nitrate as a solvent, and the nitrate ester functional groups it contains can generate LiN-rich compounds on the surface of the lithium metal anode. x O y The solid electrolyte interface with Li3N, along with the fluorinated groups promoting the formation of fluorine-containing SEI structures, effectively improves the uniformity of lithium deposition and extraction on the lithium metal anode surface and inhibits the formation of lithium dendrites. Compared with traditional carbonate electrolytes, this electrolyte improves the problem of lithium metal anode dendrite growth, thereby improving the long-cycle stability and safety of lithium metal batteries.

Description

Technical Field

[0001] This invention belongs to the field of lithium metal battery technology, specifically relating to a long-cycle nitrate ester electrolyte and lithium metal. Background Technology

[0002] With the rapid development of electronic products and the widespread application of electric vehicles, the development of electrochemical energy storage technologies with high energy density and high safety is urgently needed. Over the past 30 years, commercial lithium-ion battery technology using graphite as the primary anode material has matured, but the theoretical specific capacity of graphite anodes is limited (372 mAh g / g). -1 This has led to a bottleneck in the development of high-energy-density lithium-ion batteries (<300Wh / kg). -1 However, current energy density batteries cannot meet the energy storage needs of modern society, so it is urgent to develop high-energy-density and high-safety battery systems.

[0003] In many lithium-ion battery systems, lithium metal battery systems, which use lithium metal as the anode based on electrochemical conversion reactions, can achieve twice the energy density of lithium-ion batteries. This is because lithium metal anode materials have a high theoretical specific capacity (3860 mAh g⁻¹). -1 It features low electrochemical potential (-3.04 V compared to the standard hydrogen electrode) and low density (0.534 g / cm³). -3 This makes lithium metal batteries an ideal candidate for high-energy battery systems.

[0004] However, lithium metal batteries still face many obstacles in practical applications. First, the uneven nucleation and growth of lithium metal leads to the formation of uncontrolled lithium dendrites and dead lithium, which in turn poses a risk of puncturing the separator and pulverizing the electrode, causing safety issues such as short circuits and thermal control problems. On the other hand, the high reactivity of the lithium metal anode makes it prone to reacting with the electrolyte to form a solid electrolyte interphase (SEI) film. During long-term cycling, the unstable SEI film breaks down, causing the lithium metal anode to be continuously exposed and react with the electrolyte, reducing the battery's coulombic efficiency and cycle life.

[0005] For lithium metal systems, electrolyte engineering is crucial for improving battery performance. Electrolytes enhance cycle performance by increasing the electrochemical stability of the electrode interface. Therefore, designing a suitable electrolyte solution is one way to address the existing problems of lithium metal batteries. Summary of the Invention

[0006] To overcome the shortcomings of existing technical solutions, the purpose of this invention is to provide a long-cycle nitrate ester electrolyte that can generate LiN-rich electrolyte on the surface of a lithium metal anode. x O yThe solid electrolyte interface with Li3N, along with the fluorinated groups, promotes the formation of fluorine-containing SEI structures, effectively improving the uniformity of lithium deposition and extraction on the lithium metal anode surface, suppressing the formation of lithium dendrites, and improving the cycle performance of the battery.

[0007] In one aspect, this application provides a long-cycle nitrate ester electrolyte, comprising a non-aqueous solvent, a lithium salt, and a diluent.

[0008] The lithium salt accounts for 6-30% of the molar percentage of the electrolyte, and the non-aqueous solvent accounts for 10-80% of the molar percentage of the electrolyte.

[0009] Preferably, the lithium salt accounts for 15.9-17.5% of the molar percentage of the electrolyte, and the non-aqueous solvent accounts for 29.8-36.5% of the molar percentage of the electrolyte.

[0010] The non-aqueous solvent includes fluoroethyl nitrate, and the fluoroethyl nitrate has the structural formula of Formula I, as shown below:

[0011]

[0012] In Formula I, R1 is a fluoroalkyl group, which is any one of alkyl groups having 1 to 3 fluorine atoms and 1 to 5 carbon atoms.

[0013] Preferably, R1 is any one of alkyl groups having 3 fluorine atoms and 2 to 4 carbon atoms; more preferably, it is any one of alkyl groups having 2 to 3 carbon atoms.

[0014] Preferably, the fluoroethyl nitrate is trifluoroethyl diethyl nitrate.

[0015] Preferably, the lithium salt is selected from at least one of lithium bisfluorosulfonylimide, lithium trifluoromethylsulfonylimide, and lithium hexafluorophosphate.

[0016] Preferably, the diluent is selected from at least one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,2,3-trifluoroethyl-1,2,2,2,3-hexafluoropropyl ether.

[0017] Secondly, this application provides a method for preparing a long-cycle nitrate ester electrolyte, comprising the following steps:

[0018] Under the protective atmosphere of a glove box, lithium salt is added to a non-aqueous solvent and stirred until homogeneous to obtain a mixed solution. Then, a diluent is added to the mixed solution and stirred until homogeneous to obtain a long-cycle nitrate ester electrolyte.

[0019] Preferably, the protective atmosphere is nitrogen or argon with a purity of 99.99% or higher.

[0020] Thirdly, this application provides a lithium metal battery, including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is the aforementioned long-cycle nitrate ester electrolyte.

[0021] Preferably, the positive electrode is a lithium nickel cobalt manganese oxide or a lithium-rich manganese-based positive electrode.

[0022] Preferably, the negative electrode is a lithium metal negative electrode.

[0023] Preferably, the lithium metal battery has an operating temperature of -20 to 80°C and a voltage of 3.0 to 4.6V.

[0024] The beneficial effects of this invention are:

[0025] This invention provides a long-cycle nitrate ester electrolyte, comprising a non-aqueous solvent, a lithium salt, and a diluent. The invention uses fluoroethyl nitrate ester as the solvent, whose nitrate ester functional groups can generate LiN-rich compounds on the surface of a lithium metal anode. x O y The solid electrolyte interface with Li3N, along with the fluorinated groups promoting the formation of fluorine-containing SE I structures, effectively improves the uniformity of lithium deposition and extraction on the lithium metal anode surface and inhibits the formation of lithium dendrites. Compared with traditional carbonate electrolytes, this electrolyte improves the problem of lithium metal anode dendrite growth, thereby improving the long-cycle stability and safety of lithium metal batteries. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] As a first aspect of the present invention, this application provides a long-cycle nitrate ester electrolyte, comprising a non-aqueous solvent, a lithium salt, and a diluent.

[0028] The lithium salt accounts for 6-30% of the molar percentage of the electrolyte, and the non-aqueous solvent accounts for 10-80% of the molar percentage of the electrolyte.

[0029] In some embodiments, the lithium salt accounts for 15.9 to 17.5% of the electrolyte in molar percentage, and the non-aqueous solvent accounts for 29.8 to 36.5% of the electrolyte in molar percentage.

[0030] The non-aqueous solvent includes fluoroethyl nitrate, and the fluoroethyl nitrate has the structural formula of Formula I, as shown below:

[0031]

[0032] In Formula I, R1 is a fluoroalkyl group, which is any one of alkyl groups having 1 to 3 fluorine atoms and 1 to 5 carbon atoms.

[0033] In some embodiments, the fluoroethyl nitrate is preferably trifluoroethyl diethyl nitrate.

[0034] In some embodiments, the lithium salt is selected from at least one of lithium bisfluorosulfonylimide, lithium trifluoromethylsulfonylimide, and lithium hexafluorophosphate.

[0035] In some embodiments, the diluent is selected from at least one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,2,3-trifluoroethyl-1,2,2,2,3-hexafluoropropyl ether.

[0036] As a second aspect of the present invention, this application provides a method for preparing a long-cycle nitrate ester electrolyte, comprising the following steps:

[0037] Under the protective atmosphere of a glove box, lithium salt is added to a non-aqueous solvent and stirred until homogeneous to obtain a mixed solution. Then, a diluent is added to the mixed solution and stirred until homogeneous to obtain a long-cycle nitrate ester electrolyte.

[0038] The protective atmosphere is nitrogen or argon with a purity of 99.99% or higher.

[0039] As a third aspect of the present invention, this application provides a lithium metal battery, including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is the above-mentioned long-cycle nitrate ester electrolyte.

[0040] The positive electrode is a lithium nickel cobalt manganese oxide or a lithium-rich manganese-based positive electrode.

[0041] The negative electrode is a lithium metal negative electrode.

[0042] The lithium metal battery operates at a temperature of -20 to 80°C and a voltage of 3.0 to 4.6V.

[0043] Unless otherwise specified, all raw materials used in the embodiments of this invention are purchased commercially. According to this invention, the lithium metal battery includes a lithium nickel cobalt manganese oxide or lithium-rich manganese-based positive electrode, a lithium metal negative electrode, a separator, and an electrolyte containing fluorinated nitrate esters. The positive electrode current collector, conductive agent, binder, and separator are all known to those skilled in the art and are not particularly limited.

[0044] Specifically, the positive electrode is preferably LiNi0.8Co0.1Mn0.1O2, the active material content is preferably 90% to 95%, the current collector is preferably Al foil, the negative electrode is preferably lithium metal with a thickness of 80 mm, the separator is preferably a polypropylene (PP) separator, the conductive agent is preferably a combination of single-walled carbon nanotubes and carbon black (SWCNT+SP), and the binder is preferably polyvinylidene fluoride (PVDF).

[0045] Name Abbreviation:

[0046] Diethyl trifluoroethyl nitrate: TFNC

[0047] Lithium difluorosulfonylimide: LiFS I

[0048] 1,1,2,2-Tetrafluoroethyl-2,2,3,3-Tetrafluoropropyl ether: TTE

[0049] Dimethyl methylphosphonate: DMMP

[0050] Diethyl carbonate: DEC.

[0051] The preparation reaction process of the trifluoroethyl diethyl nitrate is as follows:

[0052]

[0053] Acetonitrile (3.5 mol, 143.0 g) was used as the reaction solvent. Ethyl nitrate (1.15 mol, 105.8 g) was added to the reaction solvent and mixed thoroughly. The temperature of the solvent system was raised to 60 °C, and polyethylene glycol (3.1 g) was added to the mixture as a catalyst and stirred thoroughly. Then, 2,2,2-trifluoroethanol (1.38 mol, 138.0 g) was added and reacted for 10 h to obtain a mixture of trifluoroethyl diethyl nitrate. Finally, the obtained mixture was subjected to solid-liquid separation and purified by vacuum distillation under a vacuum of about -1.3 kPa to obtain trifluoroethyl diethyl nitrate.

[0054] Example 1

[0055] A method for preparing a long-cycle nitrate ester electrolyte, prepared according to the proportions in Table 1, includes the following steps:

[0056] In a glove box with an argon atmosphere and a moisture content of less than 1 ppm and an oxygen content of less than 0.01 ppm, 1 mol of LiFS I was added to 2 mol of TFNC and stirred for 0.5 h. Then, 3 mol of TTE was added and stirred for 1 h to obtain the target electrolyte.

[0057] Example 2

[0058] A method for preparing a long-cycle nitrate ester electrolyte, which differs from Example 1 only in that 2 mol TFNC in Example 1 is replaced with 2.3 mol TFNC.

[0059] Example 3

[0060] A method for preparing a long-cycle nitrate ester electrolyte, which differs from Example 1 only in that 2 mol TFNC in Example 1 is replaced with 1.7 mol TFNC.

[0061] Comparative Example 1

[0062] A method for preparing a long-cycle nitrate ester electrolyte, which differs from Example 1 only in that 2 mol TFNC in Example 1 is replaced with 2 mol DEC.

[0063] Comparative Example 2

[0064] A method for preparing a long-cycle nitrate ester electrolyte, which differs from Example 1 only in that 2 mol TFNC in Example 1 is replaced with 2 mol DMMP.

[0065] Table 1 Formulations of different electrolytes

[0066]

[0067]

[0068] Lithium metal cells were prepared using the electrolytes provided in Examples 1-3 and Comparative Examples 1-2. The preparation process is as follows:

[0069] (1) Preparation of positive electrode: Under low dew point conditions, LiNiNi 0.8 Co 0.1 Mn 0.1 O2, conductive agent (SP+SWCNT) and binder (PVDF) are thoroughly stirred at a mass ratio of 96.5:0.8:1.2:1.5. Then, N-methylpyrrolidone (NMP) is added as a solvent and stirred at high speed to form a positive electrode slurry. The slurry is then uniformly coated onto a 10μm aluminum foil current collector, dried, rolled, and slit to obtain the desired positive electrode sheet.

[0070] (2) Preparation of negative electrode sheet: Under low dew point conditions, the metal sheet is laser-cut to obtain lithium metal negative electrode of target size.

[0071] (3) Cell preparation: The prepared positive electrode sheet, negative electrode sheet and polyethylene separator with a thickness of 15μm are stacked in sequence to form a square cell. The cell is then installed in a soft-pack battery casing made of aluminum-plastic film. The prepared electrolyte is then injected under vacuum. After encapsulation, the cell is subjected to standing, formation, degassing, aging and capacity testing to finally obtain the desired soft-pack lithium metal cell.

[0072] The electrochemical performance of the pouch lithium metal cells prepared with the electrolytes of Examples 1-3 and Comparative Examples 1-2 was tested. The test steps and conditions are as follows:

[0073] (1) Room temperature formation test: The cells prepared in Examples 1 to 3 and Comparative Examples 1 to 2 were charged to 4.1V at room temperature (25°C) with a constant current of 0.03C; then charged to 4.3V with a constant current of 0.2C, and continued to be charged with a constant voltage, with a cutoff current of 0.05C; finally, discharged to 3.0V with a constant current of 0.03C. The ratio of the discharge capacity to the charging capacity in this process is the first-cycle efficiency.

[0074] (2) Room temperature cycling performance test: The cells prepared in Examples 1-3 and Comparative Examples 1-2 were placed in a steel fixture with a pressure of 14 Nm at room temperature (25°C), charged at a constant current of 0.1C to 4.3V, and then charged at a constant voltage with a cutoff current of 0.05C. Then, they were discharged at a constant current of 0.33C to 3.0V. The charging and discharging steps were repeated as described above until the capacity was 80% of the initial capacity. Capacity retention rate = discharge capacity after n cycles / discharge capacity of the first cycle.

[0075] According to the above test methods, the electrochemical performance test results of the soft-pack lithium metal cells prepared with the electrolytes of Examples 1-3 and Comparative Examples 1-2 are shown in Table 2:

[0076] Table 2

[0077]

[0078] Comparing the electrochemical performance test results of Example 1 and Comparative Examples 1-2, it can be seen that the cells of Example 1 and Comparative Example 2 have higher first-cycle efficiency and capacity retention than Comparative Example 1. This is because the first two electrolytes introduce functional groups. In SE I, the RONO2 functional group introduced in Example 1 can serve as a catalyst for the formation of Li3N and LiN. x O y The precursor, the fluorinated group can generate LiF, increasing the inorganic components in SEI and exhibiting higher Li... +The conductivity is improved, and the ion transport rate and uniformity are enhanced. In Comparative Example 2, the solvent can be alloyed with Li during the charge and discharge process to obtain the lithium-loving two-phase compound Li3P. Li3P can directionally induce uniform nucleation and dense deposition of lithium metal, reduce interfacial impedance to solve the lithium dendrite problem and extend the cycle life of the battery.

[0079] Comparing the electrochemical performance test results of Examples 1, 2, and 3, it can be seen that the molar mass of solvent TFNC added affects the performance of the battery cell. This is because different molar masses of solvent affect the electrolyte conductivity. Tests showed that the electrolyte conductivity of Example 1 was higher than that of Examples 2 and 3, exhibiting the highest Li... + This improves transmission efficiency and achieves more uniform lithium deposition, thereby enhancing the cycle performance of lithium metal cells.

[0080] Comparing the electrochemical performance test results of Example 1 and Comparative Example 2, it can be seen that in lithium metal batteries, the cycle life of Example 1 is higher than that of Comparative Example 2. This is because the SE I generated by TFNC contains more uniform and abundant inorganic components, has higher ionic conductivity, and greatly improves the ionic conductivity and stability of SE I.

[0081] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0082] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A long-cycle nitrate ester electrolyte, characterized in that, Including non-aqueous solvents, lithium salts, and diluents; The lithium salt accounts for 6-30% of the molar percentage of the electrolyte, and the non-aqueous solvent accounts for 10-70% of the molar percentage of the electrolyte.

2. The long-cycle nitrate ester electrolyte according to claim 1, characterized in that, The lithium salt accounts for 15.9-17.5% of the electrolyte in molar percentage, and the non-aqueous solvent accounts for 29.8-36.5% of the electrolyte in molar percentage.

3. The long-cycle nitrate ester electrolyte according to claim 1, characterized in that, The non-aqueous solvent includes fluoroethyl nitrate, and the fluoroethyl nitrate has the structural formula of Formula I, as shown below: In Formula I, R1 is a fluoroalkyl group, which is any one of alkyl groups having 1 to 3 fluorine atoms and 1 to 5 carbon atoms.

4. The long-cycle nitrate ester electrolyte according to claim 1, characterized in that, The lithium salt is selected from at least one of lithium bis(fluorosulfonyl)imide, lithium trifluoromethylsulfonyl)imide, and lithium hexafluorophosphate.

5. The long-cycle nitrate ester electrolyte according to claim 1, characterized in that, The diluent is selected from at least one of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and 1,2,3-trifluoroethyl-1,2,2,2,3-hexafluoropropyl ether.

6. The long-cycle nitrate ester electrolyte according to claim 1, characterized in that, The fluoroethyl nitrate is trifluoroethyl diethyl nitrate.

7. A method for preparing a long-cycle nitrate ester electrolyte as described in any one of claims 1-6, characterized in that, Includes the following steps: Under the protective atmosphere of a glove box, lithium salt is added to a non-aqueous solvent and stirred until homogeneous to obtain a mixed solution. Then, a diluent is added to the mixed solution and stirred until homogeneous to obtain a long-cycle nitrate ester electrolyte.

8. The method for preparing a long-cycle nitrate ester electrolyte according to claim 7, characterized in that, The protective atmosphere is nitrogen or argon with a purity of 99.99% or higher.

9. A lithium metal battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte is the long-cycle nitrate ester electrolyte according to any one of claims 1-6.

10. The lithium metal battery according to claim 9, characterized in that, The positive electrode is a lithium nickel cobalt manganese oxide positive electrode or a lithium-rich manganese-based positive electrode.

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