An electrolyte and a lithium metal battery containing the electrolyte

Abstract

This invention discloses an electrolyte and a lithium metal battery containing the electrolyte. The electrolyte comprises a phosphonate ester organic solvent, a dilithium salt, and a fluorinated alkyl ether diluent. The dilithium salt includes a borate lithium salt and lithium nitrate. The concentration of the borate lithium salt in the electrolyte is 1 mol / L–5 mol / L, and the concentration of the lithium nitrate is 1 mol / L–5 mol / L. The lithium metal battery prepared using the electrolyte of this invention can cycle stably under high voltage, exhibiting high cycle life and safety.

Description

Technical Field

[0001] This invention relates to the field of batteries, and more specifically to a high-voltage electrolyte for locally high-concentration lithium metal batteries and a lithium metal battery using the electrolyte. Background Technology

[0002] Lithium metal batteries possess extremely high energy density, making them one of the next generation of high-energy-density storage devices, widely used in high-end communication terminals, electric vehicles (EVs), aerospace, large-scale energy storage stations, and other emerging industries. However, numerous problems with lithium metal anodes hinder the development of lithium metal batteries. The two main problems with lithium metal anodes are: first, the side reactions between the highly active lithium metal anode and the organic electrolyte; and second, the uncontrolled growth of lithium dendrites. The unstable solid electrolyte interphase (SEI) film on the lithium metal surface and the resulting uneven lithium deposition are the root causes of these problems. As a crucial component of lithium metal batteries, the compatibility between the liquid electrolyte and the lithium metal anode, as well as the properties of the liquid electrolyte itself, determine the practicality of lithium metal batteries. In conventional carbonate electrolytes, the dominant solid electrolyte interface has low interfacial energy and high resistance with metallic lithium, resulting in excessively low lithium stripping coulombic efficiency (CE) and severe lithium dendrite growth. Introducing lithium nitrate is a good strategy to improve the compatibility between the electrolyte and lithium metal. Lithium nitrate is an excellent film-forming additive in lithium metal batteries, which can passivate the lithium metal anode and inhibit the growth of lithium dendrites. However, the low solubility of lithium nitrate in conventional electrolyte solvents such as carbonate solvents limits its widespread application.

[0003] Currently reported methods mainly employ ether-based electrolytes or add co-solvents to assist in the dissolution of lithium nitrate, such as sulfolane (ACS Energy Lett., 2021, 6, 5, 1839-1848) and copper fluoride (Angewandte Chemie. 2018, 130, 43, 14251-14255). However, conventional ether-based electrolytes cannot be used in high-voltage battery systems due to their narrow electrochemical window. At the same time, most co-solvents are incompatible with high-voltage cathodes, which is detrimental to the performance improvement of lithium metal batteries. Therefore, it is crucial to find a new electrolyte for high-voltage battery systems. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide an electrolyte for lithium metal batteries and a lithium metal battery using the electrolyte. Lithium metal batteries prepared using the electrolyte of this invention can cycle stably under high voltage, exhibiting high cycle life and safety.

[0005] The first aspect of the present invention provides an electrolyte comprising a phosphonate ester organic solvent, a lithium disulfide, and a fluorinated alkyl ether diluent, wherein the lithium disulfide comprises a borate lithium salt and lithium nitrate; wherein the concentration of the borate lithium salt in the electrolyte is 1 mol / L-5 mol / L and the concentration of the lithium nitrate is 1 mol / L-5 mol / L.

[0006] Furthermore, the borate lithium salt is at least one of LiBF4, lithium bis(oxalate)borate (LiBOB), and lithium difluorooxalateborate (LiODFB).

[0007] Furthermore, the phosphonate organic solvent, used to dissolve borate lithium salts and lithium nitrate, is selected from at least one of 1-propylphosphonate diethyl ester, phenylphosphonate dimethyl ester, phenylphosphonate diethyl ester, methylphosphonate dimethyl ester, methylphosphonite diethyl ester, hydroxymethylphosphonate diethyl ester, hydroxymethylphosphonate dimethyl ester, and (methoxymethyl)phosphonate diethyl ester.

[0008] Furthermore, the fluorinated alkyl ether diluent is selected from at least one of tetrafluoroethyl-tetrafluoropropyl ether (HFE), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), bis(2,2,2-trifluoroethyl) ether (BTFE), and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoro ether (TFETFE), which have non-solventizing and low dielectric properties, preferably at least one of tetrafluoroethyl-tetrafluoropropyl ether (HFE) and bis(2,2,2-trifluoroethyl) ether (BTFE).

[0009] Furthermore, the volume ratio of the added diluent to the phosphonate organic solvent is 1:20-20:1, preferably 1:5-5:1, and examples, but not limited to, are 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, 4:1, etc.

[0010] Furthermore, in the electrolyte, the concentration of borate lithium salt is 1 mol / L-5 mol / L, preferably 2 mol / L-3 mol / L; and the concentration of lithium nitrate is 1 mol / L-5 mol / L, preferably 2 mol / L-3 mol / L.

[0011] The electrolyte of this invention can be prepared using conventional methods. For example, under an inert atmosphere, a certain amount of organic solvent is used to dissolve lithium borate salt and lithium nitrate separately. After complete dissolution, the two solutions are mixed and stirred evenly. Then, a diluent is added to obtain the electrolyte.

[0012] A second aspect of the present invention provides a lithium metal battery, comprising a positive electrode material, a negative electrode material, and the electrolyte described above.

[0013] Furthermore, the cathode material is selected from at least one of lithium-ion transition metal phosphates with an olivine structure, lithium-ion intercalated transition metal oxides with a layered structure, and lithium-ion transition metal mixed oxides with a spinel structure, preferably lithium-iron phosphate batteries, lithium-ternary cathode batteries, lithium-cobalt oxide batteries, or lithium-sulfur batteries.

[0014] Furthermore, the negative electrode material is selected from at least one of lithium sheets, lithium foils, or lithium / fiber composite materials.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] (1) This invention provides a high-voltage resistant, locally high-concentration double lithium salt electrolyte. The lithium salt in this electrolyte is a double lithium salt electrolyte composed of borate lithium salt and lithium nitrate. Phosphonates are used as the organic solvent, and fluorinated alkyl ethers with non-solventizable and low dielectric properties are used as the diluent. This allows the construction of an SEI membrane with a special structure. The presence of the fluorinated alkyl ether diluent results in a large amount of inorganic component LiF in the SEI membrane, while the presence of lithium nitrate results in the presence of LiN in the SEI. x O y Inorganic components, including LiF and LiN. x O y The synergistic effect can effectively enhance the mechanical strength and conductivity of the SEI film, making the transport of lithium ions in the SEI film more uniform and rapid, thus improving the cycle life and stability of lithium metal batteries. The resulting lithium metal batteries retain up to 96% of their capacity after 150 cycles at 1C.

[0017] (2) In the electrolyte of the present invention, two lithium salts simultaneously transport lithium ions. Since borate lithium salts have good high voltage resistance and film-forming effect, no other additives need to be added to the electrolyte to assist film formation, which effectively reduces costs. The local high-concentration dual lithium salt electrolyte uses phosphonate ester solvents, so there is no need to introduce other co-solvents. Lithium nitrate has high solubility in phosphonate ester solvents, which makes the electrolyte have high conductivity.

[0018] (3) In the electrolyte of the present invention, phosphonate solvents have good flame retardancy, and fluoroether diluents also have flame retardancy. All solvent components in the local high-concentration double lithium salt electrolyte have flame retardant effects, which makes the lithium metal battery have better safety performance and non-flammability in extreme cases.

[0019] (4) If phosphonate ester solvents are used directly as lithium salt solvents in high-concentration electrolytes without adding diluents, most batteries will be unable to charge or discharge due to the poor wettability of the membrane by a single phosphonate ester solvent. At the same time, the viscosity of high-concentration electrolytes is also too high. The inventors of this invention have creatively introduced a specific diluent into the high-concentration electrolyte, which not only solves the above problems, but also enables the construction of a special SEI film, which significantly improves the cycle life and stability of lithium metal batteries. Attached Figure Description

[0020] Figure 1 The electrolyte prepared in Example 1 of this invention, lithium difluorooxalate borate-lithium nitrate-diethyl methylphosphonate, was used to test the cycle stability of the lithium metal anode in a lithium / lithium coin cell battery. The concentrations of lithium difluorooxalate borate and lithium nitrate were 2 mol·L⁻¹. -1 and 2 mol·L -1 Current density 1 mA·cm -2 ;

[0021] Figure 2 Comparative Example 2 of this invention uses a commercially available LiPF6 / EC-EMC (3∶7, V / V) electrolyte applied to test the cycle stability of the lithium metal anode in a lithium / lithium coin cell, with a current density of 1 mA·cm⁻¹. -2 ;

[0022] Figure 3 The graph shows the charge-discharge cycle test curves at 1C rate of the dual lithium salt electrolyte of Example 1 of the present invention, and the electrolytes of Comparative Examples 1, 2 and 3, when applied to lithium metal batteries. Detailed Implementation

[0023] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0024] The present invention will be further described below with reference to specific embodiments.

[0025] Example 1

[0026] (1) Preparation of cathode material: The ternary cathode material LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), conductive carbon black, and binder polyvinylidene fluoride (PVDF) are added to a mixing tank at a mass ratio of 90:5:5. After adding an appropriate amount of N-methylpyrrolidone, the mixture is stirred thoroughly to obtain a positive electrode slurry with appropriate viscosity and solid content. The positive electrode slurry is then uniformly coated onto aluminum foil, dried, rolled, and slit to obtain a positive electrode sheet.

[0027] (2) The negative electrode material is lithium metal, and the diameter of the lithium sheet is 14mm;

[0028] (3) Electrolyte preparation: In a glove box under argon atmosphere (H2O < 0.5 ppm, O2 < 0.5 ppm), lithium difluorooxalate borate and lithium nitrate were used as electrolytes. A certain volume of diethyl methylphosphonite was measured to dissolve lithium difluorooxalate borate and lithium nitrate separately. Then, the two lithium salt solutions were mixed. After mixing, the concentration of lithium difluorooxalate borate was 4.0 mol·L⁻¹. -1 The lithium nitrate concentration is 4 mol·L⁻¹ -1 After adding diluent HFE, the concentration of lithium difluorooxalate borate was diluted to 2.0 mol·L⁻¹. -1 The lithium nitrate concentration is 2 mol·L⁻¹ -1 ;

[0029] (4) Battery assembly: Lithium / lithium symmetric batteries were assembled using lithium metal as the positive and negative electrodes and Celgard2400 as the separator; ternary positive electrode material NCM622 was assembled as the positive electrode, lithium sheet as the negative electrode, and Celgard2400 as the separator to assemble ternary positive electrode / lithium button batteries to test the cycle efficiency and stability of lithium metal materials.

[0030] Example 2

[0031] The battery was prepared using the same method as in Example 1, except that the following was added: A certain volume of diethyl methylphosphonite was measured to dissolve lithium difluorooxalate borate and lithium nitrate separately during electrolyte preparation. The two lithium salt solutions were then mixed, resulting in a lithium difluorooxalate borate concentration of 2.0 mol·L⁻¹. -1 The lithium nitrate concentration is 4 mol·L⁻¹ -1 After adding diluent HFE, the concentration of lithium difluorooxalate borate was diluted to 1.0 mol·L⁻¹. -1 The lithium nitrate concentration is 2 mol·L⁻¹ -1 .

[0032] Example 3

[0033] The battery was prepared using the same method as in Example 1, except that: when preparing the electrolyte, a certain volume of diethyl methylphosphonite was measured to dissolve lithium difluorooxalate borate and lithium nitrate separately, and then the two lithium salt solutions were mixed. After mixing, the concentration of lithium difluorooxalate borate was 4.0 mol·L⁻¹. -1 The lithium nitrate concentration is 2 mol·L⁻¹ -1 After adding diluent HFE, the concentration of lithium difluorooxalate borate was diluted to 2.0 mol·L⁻¹. -1 The lithium nitrate concentration is 1 mol·L⁻¹ -1 .

[0034] Example 4

[0035] The battery was prepared using the same method as in Example 1, except that: when preparing the electrolyte, a certain volume of diethyl methylphosphonite was measured to dissolve lithium difluorooxalate borate and lithium nitrate separately, and then the two lithium salt solutions were mixed. After mixing, the concentration of lithium difluorooxalate borate was 4.0 mol·L⁻¹. -1 The lithium nitrate concentration is 4 mol·L⁻¹ -1 After adding diluent HFE, the concentration of lithium difluorooxalate borate was diluted to 1.0 mol·L⁻¹. -1 The lithium nitrate concentration is 1 mol·L⁻¹ -1 .

[0036] Example 5

[0037] The battery was prepared using the same method as in Example 1, except that: when preparing the electrolyte, a certain volume of diethyl methylphosphonite was measured to dissolve lithium difluorooxalate borate and lithium nitrate separately, and then the two lithium salt solutions were mixed. After mixing, the concentration of lithium difluorooxalate borate was 6 mol·L⁻¹. -1 The lithium nitrate concentration is 4 mol·L⁻¹ -1 After adding diluent HFE, the concentration of lithium difluorooxalate borate was diluted to 3.0 mol·L⁻¹. -1 The lithium nitrate concentration is 2 mol·L⁻¹ -1 .

[0038] Example 6

[0039] The battery was prepared using the same method as in Example 1, except that the dual electrolytes were lithium tetrafluoroborate and lithium nitrate, and the concentration of lithium tetrafluoroborate was diluted to 2.0 mol·L⁻¹ after adding the diluent HFE. -1 The lithium nitrate concentration is 2 mol·L⁻¹ -1 .

[0040] Example 7

[0041] The battery was prepared using the same method as in Example 1, except that the dual electrolytes were lithium bis(oxalateborate) and lithium nitrate, and the concentration of lithium bis(oxalateborate) was diluted to 2.0 mol·L⁻¹ after adding the diluent HFE. -1 The lithium nitrate concentration is 2 mol·L⁻¹ -1 .

[0042] Example 8

[0043] The battery was prepared using the same method as in Example 1, except that the organic solvent was dimethyl phenylphosphonate.

[0044] Example 9

[0045] The battery was prepared using the same method as in Example 1, except that the organic solvent was diethyl phenylphosphonate.

[0046] Example 10

[0047] The battery was prepared using the same method as in Example 1, except that BTFE was added as the diluent.

[0048] Comparative Example 1

[0049] The battery was prepared using the same method as in Example 1, except that only lithium difluorooxalate borate was added as the electrolyte, and lithium nitrate was not added. After adding the diluent HFE, the concentration of lithium difluorooxalate borate was 2 mol·L⁻¹. -1 .

[0050] Comparative Example 2

[0051] The battery was prepared using the same method as in Example 1, except that a commercially available LiPF6 / EC-EMC (3:7, V / V) electrolyte was used.

[0052] Comparative Example 3

[0053] The battery was prepared using the same method as in Example 1, except that the concentration of lithium difluorooxalate borate was diluted to 0.5 mol·L⁻¹ after the addition of diluent HFE. -1 The lithium nitrate concentration is 0.5 mol·L⁻¹ -1 .

[0054] Comparative Example 4

[0055] The battery was prepared using the same method as in Example 1, except that no diluent was added.

[0056] Test case

[0057] Cyclic performance testing: The assembled ternary cathode / lithium metal coin cell battery was subjected to charge-discharge cycles at 1C / 1C charge-discharge rate on a Land charge-discharge tester at 25±2℃, with a voltage range of 3-4.5V. After 150 cycles, the capacity retention rate was recorded. The test results of the above examples and comparative examples are shown in Table 1. The cycle stability of the lithium metal anode tested in Example 1 and Comparative Example 2 are shown in Table 1. Figure 1 and Figure 2 The electrolytes of Example 1, Comparative Examples 1, 2, and 3 were applied to the above-mentioned lithium metal batteries, and the charge-discharge cycle test curves at 1C rate are shown in the figure. Figure 3 .

[0058] Table 1. Comparison of electrochemical performance between Examples 1-10 and Comparative Examples 1-4

[0059]

[0060]

[0061] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. An electrolyte, characterized by, The electrolyte is composed of phosphonate organic solvent, dilithium salt and fluorinated alkyl ether diluent, wherein the dilithium salt includes borate lithium salt and lithium nitrate; the concentration of borate lithium salt in the electrolyte is 1 mol / L-5 mol / L; the concentration of lithium nitrate is 1 mol / L-5 mol / L.

2. The electrolyte according to claim 1, characterized in that, The borate lithium salt is at least one of LiBF4, lithium bis(oxalate)borate, and lithium difluorooxalateborate.

3. The electrolyte according to claim 1, characterized in that, The phosphonate organic solvent is selected from at least one of 1-propylphosphonate diethyl ester, phenylphosphonate dimethyl ester, phenylphosphonate diethyl ester, methylphosphonate dimethyl ester, methylphosphonite diethyl ester, hydroxymethylphosphonate diethyl ester, hydroxymethylphosphonate dimethyl ester, and (methoxymethyl)phosphonate diethyl ester.

4. The electrolyte according to claim 1, characterized in that, The fluorinated alkyl ether diluent is selected from at least one of tetrafluoroethyl-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl) ether, and 1,1,2,2,-tetrafluoroethyl-2,2,2-trifluoroethyl ether.

5. The electrolyte according to claim 4, characterized in that, The fluorinated alkyl ether diluent is at least one of tetrafluoroethyl-tetrafluoropropyl ether and bis(2,2,2-trifluoroethyl) ether.

6. The electrolyte according to claim 1, characterized in that, The volume ratio of the added diluent to the phosphonate organic solvent is 1:20-20:

1.

7. The electrolyte according to claim 1, characterized in that, The volume ratio of the added diluent to the phosphonate organic solvent is 1:5-5:

1.

8. The electrolyte according to claim 1, characterized in that, The electrolyte contains lithium borate salts at a concentration of 2 mol / L to 3 mol / L; and / or lithium nitrate at a concentration of 2 mol / L to 3 mol / L.

9. A lithium metal battery, comprising a positive electrode material, a negative electrode material, and an electrolyte according to any one of claims 1-8.

10. The lithium metal battery according to claim 9, characterized in that, The cathode material is selected from at least one of the following: lithium-ion transition metal phosphates with an olivine structure, lithium-ion intercalated transition metal oxides with a layered structure, and lithium-ion transition metal mixed oxides with a spinel structure.

11. The lithium metal battery according to claim 10, characterized in that, The lithium metal battery is a lithium-iron phosphate battery, a lithium-ternary cathode battery, a lithium-cobalt oxide battery, or a lithium-sulfur battery.

12. The lithium metal battery according to claim 10, characterized in that, The negative electrode material is selected from at least one of lithium sheets, lithium foils, or lithium / fiber composite materials.

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