Electrolyte for negative-electrode-free lithium metal battery and negative-electrode-free lithium metal battery

Abstract

The invention provides an electrolyte for a negative-electrode-free lithium metal battery and the negative-electrode-free lithium metal battery. The electrolyte comprises an organic solvent, a lithium salt and an additive, the organic solvent is a mixture of fluorinated 1, 4-dimethoxybutane and 1, 1, 2, 2-tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropyl ether, and the volume ratio of the fluorinated 1, 4-dimethoxybutane to the 1, 1, 2, 2-tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropyl ether is (2-5): 1; the lithium salt comprises at least one of lithium bis (fluorosulfonyl) imide and lithium difluoro (oxalato) borate; the additive comprises at least one of lithium iodide and lithium nitrate, and the content of the additive in the electrolyte is 0.1 wt.%-1wt.%. The electrolyte adopting the technical scheme of the invention can be beneficial to construction of a compact SEI film and effectively inhibit generation of lithium dendrites, so that the consumption of lithium is reduced, and the coulombic efficiency and the cycle performance of the battery are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of negative electrode-free lithium metal batteries, and in particular relates to an electrolyte for negative electrode-free lithium metal batteries and a negative electrode-free lithium metal battery. Background Art

[0002] Currently, state-of-the-art lithium-ion batteries are reaching their theoretical limits of energy density, prompting a search for alternative battery systems. Lithium metal has an excellent specific capacity (3860 mAh / g) and a low working potential (-3.04 V compared to a standard hydrogen electrode). Despite these advantages, recent research directions for lithium metal battery technology have focused on using thicker metal lithium foils to improve cycling stability by compensating for lithium. However, lithium depletion is an inherent problem caused by uncontrollable lithium dendrite growth, and its high cost, complex battery manufacturing process, and safety issues make it unlikely that lithium metal anodes will be adopted in actual batteries. Therefore, anode-free lithium metal batteries (AFLMBs) have attracted a lot of attention as a new option to overcome the shortcomings of lithium metal batteries. Since the anode of anode-free lithium metal batteries does not require the use of metal lithium foil and only uses lithium from the cathode material for reaction, it can maximize the energy density. However, the coulombic efficiency (CE) and cycling stability of such batteries are not satisfactory compared with those of lithium metal batteries, mainly because there is no lithium inventory to replenish the lost lithium. In addition, Li-metal batteries and anode-free Li-metal batteries also suffer from Li dendrite-related problems such as unstable solid electrolyte interface (SEI), formation of “dead Li”, electrolyte depletion, and short-circuiting due to the deposition of metallic Li on the Cu current collector during cycling.

[0003] The negative electrode interface has a significant impact on the performance of negative electrode-free lithium metal batteries, and is also directly affected by the function of the electrolyte, so the corresponding performance is highly dependent on the choice of electrolyte. This shows the importance of electrolyte to the performance of negative electrode-free lithium metal batteries. However, the performance of negative electrode-free lithium metal battery electrolytes disclosed in other patents still needs to be further improved. Summary of the invention

[0004] In view of the above technical problems, the present invention discloses an electrolyte for a negative electrode-free lithium metal battery and a negative electrode-free lithium metal battery, which can effectively inhibit the generation of lithium dendrites, make up for the shortcomings of poor antioxidant stability of ordinary ether solvents, and improve the coulombic efficiency and cycle performance of the battery. Wherein, the negative electrode-free refers to the absence of negative electrode active material.

[0005] To this end, the technical solution adopted by the present invention is:

[0006] An electrolyte for a negative electrode-free lithium metal battery, comprising an organic solvent, a lithium salt and an additive;

[0007] The organic solvent is a mixture of fluorinated 1,4-dimethoxybutane and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and the volume ratio of the fluorinated 1,4-dimethoxybutane to 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 2 to 5:1;

[0008] The lithium salt includes at least one of lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate;

[0009] The additive comprises at least one of lithium iodide and lithium nitrate, and the content of the additive in the electrolyte is 0.1wt.% to 1wt.%.

[0010] In organic solvents, the structural formula of fluorinated 1,4-dimethoxybutane is shown in formula (1):

[0011]

[0012] Fluorinated 1,4-dimethoxybutane is synthesized by selectively functionalizing the extended ether backbone with –CF2– groups, which enables the molecule to solvate Li via a unique Li-F interaction. + ions, which is beneficial to the compatibility and high voltage tolerance of lithium metal negative electrodes. The selection of fluorinated ether solvents 1,4-dimethoxybutane fluoride and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in a suitable ratio can make up for the shortcomings of poor oxidation resistance of ether solvents, adjust the solvation structure, and provide high fluorine-containing SEI components, improve cycle stability and other characteristics. Lithium iodide additives can activate "dead lithium" through the shuttle effect, and can form a dense SEI film when used together with lithium nitrate, and improve the battery coulomb efficiency and cycle performance.

[0013] As a further improvement of the present invention, the molar concentration of the lithium salt in the electrolyte is 2.0 to 4.0 mol / L. With this technical solution, the battery has better coulombic efficiency and cycle performance.

[0014] As a further improvement of the present invention, the lithium salt is lithium bis(fluorosulfonyl)imide and lithium difluorooxalate borate, and the concentration ratio of the amount of the lithium bis(fluorosulfonyl)imide and lithium difluorooxalate borate in the electrolyte is 2-5: 1. Further, the concentration ratio of the amount of the lithium bis(fluorosulfonyl)imide and lithium difluorooxalate borate in the electrolyte is 4: 1.

[0015] As a further improvement of the present invention, in the electrolyte, the concentration of the lithium bis(fluorosulfonyl)imide is 2 to 2.5 mol / L, and the concentration of the lithium difluorooxalatoborate is 0.5 to 1 mol / L.

[0016] As a further improvement of the present invention, the volume ratio of the fluorinated 1,4-dimethoxybutane to 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 3 to 4:1.

[0017] As a further improvement of the present invention, the additives are lithium iodide and lithium nitrate.

[0018] As a further improvement of the present invention, the mass concentration of the lithium iodide and lithium nitrate is 0.1-0.3wt.%.

[0019] The present invention also discloses a method for preparing the electrolyte for the negative electrode-free lithium metal battery as described above, wherein an organic solvent, a lithium salt and an additive are mixed under the conditions that the H2O concentration is ≤0.1ppm and the O2 concentration is ≤0.1ppm to obtain the electrolyte.

[0020] The present invention also discloses a negative electrode-free lithium metal battery, comprising a positive electrode, a separator, an electrolyte and a negative electrode, wherein the electrolyte is the electrolyte for the negative electrode-free lithium metal battery as described above.

[0021] As a further improvement of the present invention, the positive electrode is made of lithium iron phosphate material.

[0022] As a further improvement of the present invention, the negative electrode is a copper current collector.

[0023] As a further improvement of the present invention, the separator is a glass fiber membrane, a polyethylene microporous membrane, a polypropylene microporous membrane or an ethylene-propylene copolymer microporous membrane.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] Compared with common ether solvents, the technical solution of the present invention adopts a mixture of fluorinated 1,4-dimethoxybutane and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether as fluorinated ether solvents, which improves the solvation structure, improves the oxidation stability, reduces the viscosity of the electrolyte, improves the conductivity and forms a stable SEI film. In addition, the technical solution of the present invention optimizes the ratio of each component in the electrolyte, further optimizes the type of additives, and is used in a negative electrode-free lithium metal battery to achieve the regulation of the SEI film, and improves the deposition morphology of lithium on the electrode surface by constructing a dense SEI film, while promoting the stability of the SEI film, effectively inhibiting the generation of lithium dendrites, thereby reducing the consumption of lithium and improving the battery coulomb efficiency and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a cycle comparison diagram of Examples 1-2 of the present invention and Comparative Examples 1-4.

[0027] Figure 2This is a SEM image of lithium deposition after 50 cycles of the battery assembled in Example 1 of the present invention.

[0028] Figure 3 This is a SEM image of lithium deposition after 50 cycles of the battery assembled in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0029] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further described below in conjunction with embodiments, but the present invention is not limited to the following embodiments. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0030] All examples and comparative examples were prepared according to the following methods.

[0031] Electrolyte: All electrolytes were prepared in a glove box with H2O concentration ≤ 0.1 ppm and O2 concentration ≤ 0.1 ppm. The solvents were first mixed according to the volume, lithium salts and additives of corresponding concentrations were weighed, added to the solvent and stirred to fully dissolve, and then left for 6 hours before use.

[0032] Positive electrode: Weigh the positive electrode material LiFePO4, the conductive agent acetylene black and the binder PVDF in a mass ratio of 95:2.5:2.5, add an appropriate amount of N-methylpyrrolidone, mix evenly, and prepare the positive electrode slurry. Spread the positive electrode slurry on aluminum foil, roll and cut it after drying to obtain the positive electrode sheet.

[0033] Negative electrode: Use 9-micron copper foil, clean it with 1 mol / L dilute hydrochloric acid for 10 minutes, then clean it three times with deionized water and anhydrous ethanol respectively, and cut it after drying.

[0034] Battery assembly: All tests were performed on 2032 button cells, from bottom to top: positive electrode shell, positive electrode, diaphragm, electrolyte, negative electrode, gasket, shrapnel, and negative electrode shell.

[0035] Test conditions: Activate for 2 cycles at a constant current of 0.1C at 25°C to form a stable SEI film, then perform charge and discharge cycles at a constant current of 0.5C, with a charge and discharge voltage range of 2.5-4.3V.

[0036] Example 1

[0037] An electrolyte for a negative electrode-free lithium metal battery comprises an organic solvent, a lithium salt and an additive.

[0038] The organic solvents are 1,4-dimethoxybutane fluoride and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and the volume ratio used is 4:1; the lithium salts are lithium bis(fluorosulfonyl)imide and lithium difluorooxalate borate, wherein the concentration of lithium bis(fluorosulfonyl)imide in the electrolyte is 2 mol / L, and the concentration of lithium difluorooxalate borate is 0.5 mol / L; the additives are lithium iodide and lithium nitrate, and the concentrations of lithium iodide and lithium nitrate in the electrolyte are both 0.2 wt.%.

[0039] Example 2

[0040] Based on Example 1, the difference of this example is that the volume ratio of fluorinated 1,4-dimethoxybutane and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 3:1.

[0041] Example 3

[0042] Based on Example 1, the difference of this example is that the volume ratio of fluorinated 1,4-dimethoxybutane and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 2:1.

[0043] Example 4

[0044] Based on Example 1, the difference of this example is that the volume ratio of fluorinated 1,4-dimethoxybutane and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 5:1.

[0045] Example 5

[0046] Based on Example 1, the difference of this example is that the concentration of the lithium salt lithium difluorooxalate borate is 1 mol / L.

[0047] Example 6

[0048] Based on Example 1, the difference of this example is that the concentration of lithium salt lithium bis(fluorosulfonyl)imide is 2.5 mol / L.

[0049] Comparative Example 1

[0050] Based on Example 1, the difference of this comparative example is that the solvent is replaced by ethylene glycol dimethyl ether.

[0051] Comparative Example 2

[0052] Based on Example 1, the difference of this comparative example is that the solvent is replaced by ethylene glycol dimethyl ether and 1,3-dioxolane, and the volume ratio of the two is 1:1.

[0053] Comparative Example 3

[0054] Based on Example 1, the only difference of this comparative example is that no additives are added.

[0055] Comparative Example 4

[0056] Based on Example 1, the difference of this comparative example is that only lithium nitrate is used as the additive.

[0057] Comparative Example 5

[0058] Based on Example 1, the difference of this comparative example is that the concentration of lithium salt lithium bis(fluorosulfonyl)imide is 0.5 mol / L, and the concentration of lithium difluorooxalatoborate is 0.5 mol / L.

[0059] Comparative Example 6

[0060] Based on Example 1, the only difference of this comparative example is that the concentration of lithium salt lithium bis(fluorosulfonyl)imide is 4 mol / L, and the concentration of lithium difluorooxalatoborate is 2 mol / L.

[0061] Loop Effect:

[0062] The first charge and discharge capacities and coulombic efficiencies of Examples 1-6 and Comparative Examples 1-6 at 0.5C are shown in Table 1 below:

[0063] Table 1 Comparison of first charge and discharge capacity and coulombic efficiency at 0.5C between the embodiment and the comparative example

[0064] First charge specific capacity (mAh / g) First discharge specific capacity (mAh / g) First coulombic efficiency (%) Comparative Example 1 162.59 147.84 90.93 Comparative Example 2 165.07 146.96 89.03 Comparative Example 3 169.27 145.91 86.2 Comparative Example 4 159.29 147.2 92.41 Comparative Example 5 163.94 146.43 89.32 Comparative Example 6 165.28 146.74 88.78 Example 1 168.96 148.87 88.11 Example 2 164.19 147.56 89.87 Example 3 166.97 147.35 88.25 Example 4 165.55 146.36 88.41 Example 5 165.65 146.72 88.57 Example 6 163.30 145.86 89.32

[0065] The capacity retention rates of Examples 1-6 and Comparative Examples 1-6 after 100 cycles at 0.5C are shown in Table 2 below:

[0066] Table 2 Comparison of capacity and capacity retention rate of the embodiment and the comparative example at 0.5C for 100 cycles

[0067]

[0068] (Capacity retention rate = C2 / C1, C1 is the first discharge capacity of the battery, C2 is the discharge capacity after 100 cycles)

[0069] The cycle comparison diagrams of Examples 1-4 and Comparative Examples 1-4 of the present invention are as follows: Figure 1 As shown, by comparing the data in Table 1 and Table 2, it can be seen that the embodiment of the present invention has a better cycle life than the comparative example.

[0070] The SEM images of lithium deposition after 50 cycles of the batteries assembled in Example 1 and Comparative Example 1 are as follows: Figure 2 and Figure 3 As shown, it can be seen that after 50 cycles of the battery, the SEI film formed in Example 1 is more compact, and the SEI film formed in Comparative Example 1 is relatively loose, and Example 1 has better interface stability.

[0071] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

Claims

1. An electrolyte for a negative electrode-free lithium metal battery, characterized in that: The invention comprises an organic solvent, a lithium salt and an additive; the organic solvent is a mixture of 1,4-dimethoxybutane fluoride and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and the volume ratio of the 1,4-dimethoxybutane fluoride and the 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 2 to 5:1; the lithium salt comprises at least one of lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate; The additive comprises at least one of lithium iodide and lithium nitrate, and the content of the additive in the electrolyte is 0.1wt.% to 1wt.%.

2. The electrolyte for a negative electrode-free lithium metal battery according to claim 1, characterized in that: The molar concentration of the lithium salt in the electrolyte is 2.0 to 4.0 mol / L.

3. The electrolyte for a negative electrode-free lithium metal battery according to claim 2, characterized in that: The lithium salt is lithium bis(fluorosulfonyl)imide and lithium difluorooxalate borate, and the concentration ratio of the lithium bis(fluorosulfonyl)imide and lithium difluorooxalate borate in the electrolyte is 2 to 5:

1.

4. The electrolyte for a negative electrode-free lithium metal battery according to claim 3, characterized in that: In the electrolyte, the concentration of the lithium bis(fluorosulfonyl)imide is 2-2.5 mol / L, and the concentration of the lithium difluorooxalatoborate is 0.5-1 mol / L.

5. The electrolyte for a negative electrode-free lithium metal battery according to claim 1, characterized in that: The volume ratio of the fluorinated 1,4-dimethoxybutane to 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is 3-4:

1.

6. The electrolyte for a negative electrode-free lithium metal battery according to claim 1, characterized in that: The additives are lithium iodide and lithium nitrate.

7. The electrolyte for a negative electrode-free lithium metal battery according to claim 5, characterized in that: The mass concentration of the lithium iodide and lithium nitrate is 0.1-0.3 wt.%.

8. The method for preparing an electrolyte for a negative electrode-free lithium metal battery according to any one of claims 1 to 7, characterized in that: Under the conditions that H2O concentration is ≤0.1ppm and O2 concentration is ≤0.1ppm, an organic solvent, a lithium salt and an additive are mixed to obtain an electrolyte.

9. A negative electrode-free lithium metal battery, comprising a positive electrode, a separator, an electrolyte and a negative electrode, characterized in that: The electrolyte is an electrolyte for a negative electrode-free lithium metal battery as claimed in any one of claims 1 to 7.

10. The negative electrode-free lithium metal battery according to claim 9, characterized in that: The positive electrode is made of lithium iron phosphate material, the negative electrode is a copper current collector, and the separator is a glass fiber membrane, a polyethylene microporous membrane, a polypropylene microporous membrane or an ethylene-propylene copolymer microporous membrane.

Images