Electrolyte for high-specific-energy lithium metal battery and lithium metal battery

Abstract

The invention discloses an electrolyte for a high-specific-energy lithium metal battery and the lithium metal battery, and belongs to the technical field of secondary batteries. The invention relates to an electrolyte for a high-specific-energy lithium metal battery. The electrolyte comprises a lithium salt, a solvent and an additive, the solvent comprises fluorinated cyclic carbonate and sulfones; the fluoro cyclic carbonate is prepared from at least one of fluoro ethylene carbonate, propylene trifluorocarbonate and difluoro ethylene carbonate; the sulfones comprise at least one of dimethyl sulfone, methyl ethyl sulfone, sulfolane, trifluoromethyl ethyl sulfone, trifluoropropyl methyl sulfone and methoxy methyl fluorosulfone. According to the electrolyte for the high-specific-energy lithium metal battery disclosed by the invention, the fluoro cyclic carbonate is matched with the sulfone solvent to form a main solvent system with high oxidation resistance, so that the high-pressure decomposition of the electrolyte and the side reaction on the surface of lithium metal are reduced, and the growth of lithium dendrites is inhibited, thereby realizing stable circulation of the high-specific-energy lithium metal battery.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to an electrolyte for a high-energy-density lithium metal battery and a lithium metal battery. Background Art

[0002] Traditional lithium-ion batteries, which use graphite as the negative electrode, have a theoretical energy density limit of approximately 300 Wh / kg, while current high-end products only reach 250–300 Wh / kg. New energy vehicles, drones, and other fields are demanding doubled range (>500 km), urgently requiring new systems with energy densities >500 Wh / kg. Lithium metal batteries, a rechargeable battery technology that uses metallic lithium (Li) directly as the negative electrode, offer a theoretical specific capacity of 3860 mAh / g (10 times that of graphite) and the lowest potential (-3.04 V). When paired with a high-nickel or sulfur cathode, the theoretical energy density can exceed 500 Wh / kg, making them a core next-generation technology. In order to meet the ultra-high energy density requirements of batteries, the electrolyte not only faces challenges such as oxidative decomposition under ultra-high voltage (>4.5V), but the high activity of lithium metal also causes two major problems: dendrite growth, during charging and discharging, lithium is unevenly deposited to form dendrites, which may pierce the diaphragm and cause a short circuit; interfacial side reactions, in which metallic lithium and the electrolyte continue to react, consuming active lithium and generating an unstable SEI film, resulting in a sharp drop in cycle life.

[0003] Patent CN 119627227A designs a locally high-concentration electrolyte using 1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether as a diluent. This optimizes the lithium ion solvation structure, forming a solvation structure dominated by contact ion pairs / aggregated ion pairs. This promotes the formation of a stable and robust SEI film, while also achieving dense and uniform deposition of lithium metal, significantly improving the cycling performance of lithium metal batteries. While the use of this novel diluent optimizes the lithium ion solvation structure and enhances the cycling stability of lithium metal batteries, the upper cycle voltage is only 4.4V, failing to address the electrolyte's poor oxidation resistance at high voltages, leading to poor battery cycle life. Patent CN118610587A replaces linear ether solvents with phosphate-based solvents to enhance the electrolyte's antioxidant capacity. Furthermore, a novel diluent, a fluorinated phosphazene flame retardant, is introduced to ensure battery safety. Furthermore, a lithium protective additive is used to mitigate the adverse effects of phosphates on the interface, achieving stable cycling of high-voltage lithium metal batteries. Although the solvent design takes into account both the high-voltage resistance and the stability of the electrolyte to lithium metal, the oxidation resistance potential of the electrolyte has not been significantly improved, and the use of phosphate-based main solvents will also reduce the conductivity of the electrolyte and affect the battery kinetic performance. Summary of the Invention

[0004] The purpose of the present invention is to provide an electrolyte with a high upper limit voltage and stable cycle suitable for high specific energy lithium metal batteries; another purpose of the present invention is to provide an electrolyte with a high upper limit voltage and stable cycle suitable for high specific energy lithium metal batteries.

[0005] The present invention discloses an electrolyte for a high-energy-density lithium metal battery, comprising a lithium salt, a solvent and an additive; the solvent comprises a fluorinated cyclic carbonate and a sulfone; the fluorinated cyclic carbonate comprises at least one of fluoroethylene carbonate (FEC), trifluoropropylene carbonate (TFPC) and difluoroethylene carbonate (DFEC); the sulfone comprises at least one of dimethyl sulfone (DMS), methyl ethyl sulfone (EMS), cyclopentane (TMS), trifluoromethyl ethyl sulfone (FMES), trifluoropropyl methyl sulfone (FPMS) and methoxymethyl fluorosulfone (MEMS).

[0006] Furthermore, the solvent further includes a fluoroether; the fluoroether includes at least one of 2,2,2-trifluoroethyl ether (BTFE), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE).

[0007] The present invention adopts fluorinated cyclic carbonates and sulfones as main solvents and fluorinated ethers as diluents. The addition of fluorinated ethers to the solvents can improve the intrinsic oxidation resistance of the solvent while reducing the viscosity of the electrolyte and the reaction activity of the electrolyte to lithium metal, thereby reducing the occurrence of side reactions on the surface of the lithium metal.

[0008] Furthermore, the mass fraction of the fluorinated cyclic carbonate in the solvent is 15-40%, the mass fraction of the sulfone in the solvent is 25-55%, and the mass fraction of the fluorinated ether in the solvent is 20-40%.

[0009] The sulfones are fluorosulfones; the fluorosulfones include at least one of trifluoromethyl ethyl sulfone, trifluoropropyl methyl sulfone, and methoxymethyl fluorosulfone.

[0010] Compared with non-fluorinated sulfones, fluorinated sulfones can further improve the oxidation resistance of the solvent while reducing the viscosity of the electrolyte, improving the conductivity, and forming a protective SEI film containing lithium fluoride on the surface of the negative electrode to inhibit the growth of lithium dendrites.

[0011] Furthermore, the solvent is a combination of trifluoropropylene carbonate, trifluoromethyl ethyl sulfone and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether; the mass fraction of trifluoropropylene carbonate in the solvent is 30%, the mass fraction of trifluoromethyl ethyl sulfone in the solvent is 40%, and the mass fraction of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in the solvent is 30%.

[0012] Furthermore, the solvent is a combination of propylene trifluorocarbonate, trifluoromethyl ethyl sulfone and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

[0013] Furthermore, the mass fraction of the trifluoropropylene carbonate in the solvent is 30%, the mass fraction of the trifluoromethyl ethyl sulfone in the solvent is 40%, and the mass fraction of the 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in the solvent is 30%.

[0014] Furthermore, the mass fraction of the additive in the electrolyte is 0.5-3%.

[0015] Furthermore, the additive includes lithium difluorooxalatoborate (LiDFOB) and diethylene sulfate (DTD); the mass ratio of the lithium difluorooxalatoborate to the diethylene sulfate is 1:1.

[0016] Adding lithium difluorooxalatoborate (LiDFOB) to the electrolyte can form a uniform and stable CEI film on the positive electrode surface, protecting the positive electrode and reducing high-voltage decomposition of the electrolyte; adding diethylene glycol sulfate (DTD) can form a protective SEI film on the negative electrode surface, reducing the occurrence of side reactions on the negative electrode surface.

[0017] Furthermore, the lithium salt includes lithium hexafluorophosphate; and the concentration of the lithium salt in the solvent is 0.8-2.0 mol / L.

[0018] The present invention also discloses a lithium metal battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte as described above.

[0019] Furthermore, the active material of the positive electrode sheet is one of lithium-rich manganese-based, lithium cobalt oxide and lithium nickel manganese oxide.

[0020] The positive electrode active material, conductive agent, and binder polyvinylidene fluoride (PVDF) are dispersed in an appropriate amount of N-methylpyrrolidone and then thoroughly stirred according to the homogenization process. The evenly dispersed positive electrode slurry is evenly coated on aluminum foil, and the positive electrode sheet is obtained through baking, roller pressing, slitting, and punching.

[0021] The negative electrode active material is metallic lithium. The lithium-copper composite foil is baked, rolled, slit, and punched to obtain the negative electrode sheet.

[0022] The diaphragm is selected from one of PP diaphragm, PE diaphragm, PE / PP double-layer composite membrane, PI electrostatic spinning diaphragm, PP / PE / PP three-layer composite membrane, ceramic diaphragm and PVDF coated diaphragm.

[0023] The present invention discloses an electrolyte for high-energy-density lithium metal batteries, which uses a fluorinated cyclic carbonate and a sulfone solvent to form a main solvent system with high oxidation resistance, reduces high-voltage decomposition of the electrolyte and side reactions on the lithium metal surface, inhibits the growth of lithium dendrites, and thus achieves stable circulation of high-energy-density lithium metal batteries. DETAILED DESCRIPTION

[0024] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to specific embodiments.

[0025] Example 1

[0026] Prepare electrolyte:

[0027] The lithium salt is lithium hexafluorophosphate, and the concentration in the solvent is 1.2 mol / L.

[0028] Solvent: The solvent contains 30% by mass of propylene trifluorocarbonate (TFPC), 55% by mass of ethyl methyl sulfone (EMS), and 15% by mass of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE).

[0029] Additives: lithium difluorooxalatoborate (LiDFOB) with a mass fraction of 1% of the total electrolyte mass and diethyl thioester (DTD) with a mass fraction of 1% of the total electrolyte mass.

[0030] Control the moisture content in the glove box to below 10 ppm, and the moisture content of the solvent to below 10 ppm. Use a pipette to accurately pipette different solvents in a specific ratio into the glove box, transfer them to an aluminum bottle, stir thoroughly, and store in a 0°C incubator for 1 hour. Then, add lithium hexafluorophosphate, 1% lithium difluorooxalatoborate (LiDFOB), and 1% diethylenetriaminetetraacetic acid (DTD) to the mixed solvent at a concentration of 1.2 mol / L, stirring continuously until the electrolyte is obtained.

[0031] Assembling lithium metal batteries (including cathode, anode, separator and electrolyte):

[0032] Positive electrode: The positive electrode active material is lithium-rich manganese-based positive electrode material Li 1.2 Mn 0.54 Ni0.13 Co 0.13 The positive electrode active material, conductive agent, and binder polyvinylidene fluoride (PVDF) were dispersed in a 20% solids-total N-methylpyrrolidone solution at a mass ratio of 93:2.5:2:2.5. The mixture was then thoroughly stirred according to the homogenization process. The evenly dispersed positive electrode slurry was evenly coated on aluminum foil, and the positive electrode sheets were produced through baking, roll pressing, slitting, and punching.

[0033] Negative electrode: The active material of the negative electrode is metallic lithium. The lithium-copper composite foil is baked, rolled, slit, and punched to obtain the negative electrode sheet.

[0034] Diaphragm: PE diaphragm.

[0035] After punching the positive and negative electrodes, the positive electrode is placed in an oven at 130°C, and the negative electrode is placed in an oven at 100°C for 25 hours. When the moisture content of the electrode sheets meets the requirements, the positive and negative electrode sheets and the separator are placed in a laminating machine to form bare cells, which are then encapsulated in stamped aluminum-plastic film bags. After the encapsulated dry cells are dried at 90°C for 11 hours, the electrolyte of the present invention is injected into the dry cells. After the cells are shelved, formed, shelved at high temperature, evacuated and sealed, and capacity divided, a lithium metal battery is obtained.

[0036] Example 2

[0037] The difference between this embodiment and embodiment 1 is that ethyl methyl sulfone (EMS) with a total solvent mass fraction of 55% in the electrolyte is replaced with trifluoromethyl ethyl sulfone (FMES) with a total solvent mass fraction of 55%.

[0038] Example 3

[0039] The difference between this embodiment and embodiment 2 is that the mass fraction of trifluoromethyl ethyl sulfone (FMES) in the electrolyte, which has a total solvent mass fraction of 55%, is adjusted to 40%, and the mass fraction of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) is adjusted to 30%.

[0040] Example 4

[0041] The difference between this embodiment and embodiment 2 is that the mass fraction of trifluoromethyl ethyl sulfone (FMES) in the electrolyte, which has a total solvent mass fraction of 55%, is adjusted to 25%, and the mass fraction of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) is adjusted to 45%.

[0042] Example 5

[0043] The difference between this embodiment and embodiment 2 is that the additive in the electrolyte is replaced with vinylene carbonate (VC) with a mass fraction of 1% of the total electrolyte mass.

[0044] Example 6

[0045] The difference between this embodiment and embodiment 5 is that 15% of the total solvent mass fraction of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) in the electrolyte is replaced with 15% of the total solvent mass fraction of ethyl methyl carbonate (EMC).

[0046] Comparative Example 1

[0047] The difference between this comparative example and Example 1 is that the solvent in the electrolyte is replaced by ethylene carbonate (EC) with a total solvent mass fraction of 30% and ethyl methyl carbonate (EMC) with a total solvent mass fraction of 70%.

[0048] Comparative Example 2

[0049] The difference between this comparative example and Example 1 is that the electrolyte solvent is replaced with 30% by mass of propylene trifluorocarbonate (TFPC) and 70% by mass of ethyl methyl sulfone (EMS).

[0050] Comparative Example 3

[0051] The difference between this comparative example and comparative example 1 is that the ethylene carbonate (EC) with a total solvent mass fraction of 30% in the electrolyte is replaced with trifluoropropylene carbonate (TFPC) with a total solvent mass fraction of 30%.

[0052] Comparative Example 4

[0053] The difference between this comparative example and comparative example 1 is that ethyl methyl carbonate (EMC) with a total solvent mass fraction of 70% in the electrolyte is replaced with ethyl methyl sulfone (EMS) with a total solvent mass fraction of 70%.

[0054] The electrolytes and lithium metal batteries of the examples and comparative examples were tested and their performance differences were compared using the following methods:

[0055] (1) Electrolyte conductivity test

[0056] The electrolytes in the examples and comparative examples were tested for room temperature conductivity using a conductivity tester. Specific test data are shown in Table 1.

[0057] (2) Lithium metal battery room temperature formation test

[0058] The embodiment and comparative example were subjected to a room temperature formation test on the battery according to the following steps. The detailed steps are as follows: Take a fresh lithium metal battery and set it aside for more than 10 hours. Then charge it at 0.1C constant current and constant voltage at room temperature, with a cut-off voltage of 4.7V and a cut-off current of 0.05C to obtain the initial charging capacity. Then discharge it at 0.1C constant current at room temperature to 2.0V to obtain the initial discharge capacity. Then charge it at 0.1C constant current, with a cut-off voltage of 4.7V, and then discharge it at 0.1C constant current to 30% SOC. Let it stand for one hour, and then perform a 20-second discharge test on the battery cell with a 0.25C pulse current. According to the change in voltage, calculate the battery's differential capacity DCR. Specific test data are shown in Table 2.

[0059] (3) Lithium metal battery room temperature cycle test

[0060] The batteries in the Examples and Comparative Examples were subjected to room temperature cycling tests according to the following steps: A constant-capacity lithium metal battery was placed for at least 1 hour. Then, at room temperature, it was charged at a constant current of 0.2C to a cutoff voltage of 4.7V, and then discharged at a constant current of 0.2C to 2.0V. Following these steps, the battery was cycled, and the discharge capacity after the first cycle, the discharge capacity after 50 cycles, and the capacity retention were recorded. Specific test data are shown in Table 3.

[0061] Table 1 Test results of conductivity at room temperature

[0062] Group Conductivity (mS / cm) Example 1 3.52 Example 2 6.25 Example 3 4.83 Example 4 3.21 Example 5 6.23 Example 6 8.97 Comparative Example 1 9.02 Comparative Example 2 3.73 Comparative Example 3 8.71 Comparative Example 4 3.98

[0063] Table 2 DCR test results of lithium metal batteries

[0064] Group First charge capacity (mAh / g) First discharge capacity (mAh / g) First coulombic efficiency (%) Divided capacity DCR (mohm) Example 1 306.1 275.8 90.10 52.4 Example 2 304.9 280.1 91.87 41.2 Example 3 303.2 279.8 92.28 35.7 Example 4 301.5 276.9 91.84 39.5 Example 5 306.2 278.9 91.08 44.1 Example 6 309.1 273.4 88.45 57.2 Comparative Example 1 365.3 206.7 56.58 120.4 Comparative Example 2 312.4 272.2 87.13 61.5 Comparative Example 3 336.9 254.2 75.45 82.1 Comparative Example 4 340.1 252.6 74.27 95.5

[0065] Table 3 Lithium metal battery room temperature cycle test results

[0066] Group Discharge capacity of the first cycle (mAh / g) Discharge capacity at the 50th cycle (mAh / g) Capacity retention rate (%) Example 1 274.9 193.7 70.5 Example 2 279.5 223.4 79.9 Example 3 279.4 233.1 83.4 Example 4 276.5 229.34 83.0 Example 5 278.0 214.1 77.0 Example 6 271.8 187.8 69.1 Comparative Example 1 190.1 —— —— Comparative Example 2 270.5 168.5 62.3 Comparative Example 3 243.9 —— —— Comparative Example 4 241.4 —— ——

[0067] In Comparative Example 1, the traditional carbonate EC-EMC is used as the electrolyte solvent. Although the positive and negative electrode film-forming additives LiDFOB and DTD are added, the traditional carbonate solvent has poor oxidation resistance, is easily decomposed under high voltage, and has high reaction activity with lithium metal, resulting in abnormal first charge capacity, low first efficiency, high impedance, and low first cycle capacity. The continuous decomposition of the electrolyte causes the battery to be unable to complete 50 charge and discharge cycles. In Comparative Example 2, EC is replaced with a fluorocarbonate TFPC with high oxidation resistance, and EMC is replaced with a sulfone solvent EMS with a high oxidation potential, which significantly improves the oxidation resistance of the electrolyte. Compared with Comparative Example 1, the first efficiency is greatly improved and the impedance is greatly reduced. However, traditional sulfone solvents have shortcomings such as high viscosity, high reduction potential, and high reaction activity with lithium metal, resulting in low conductivity and low cycle capacity retention. In Comparative Example 3, only the EC in Comparative Example 1 was replaced with TFPC, and in Comparative Example 4, only the EMC in Comparative Example 1 was replaced with EMS. Although Comparative Examples 3 and 4 showed certain improvements in the first efficiency, impedance, and first cycle capacity compared with Comparative Example 1, the replacement of a single solvent had limited effect on the improvement of the electrolyte's oxidation resistance. The electrolyte decomposition during the cycle was still relatively serious, and the battery could not complete 50 charge and discharge cycles.

[0068] In Example 1, on the basis of Comparative Example 2, fluoroether TTE was added as a diluent to reduce the viscosity of the electrolyte, but TTE does not participate in the solvation of lithium ions, and the overall conductivity decreases slightly. Fluorinated ether also has the characteristic of being stable to lithium metal, and has high oxidation resistance, which can reduce the side reactions on the surface of lithium metal, reduce the decomposition of the electrolyte, and improve the capacity retention rate. Example 2 replaces the non-fluorosulfone EMS in Example 1 with fluorosulfone FMES, which further improves the oxidation resistance of the solvent while reducing the viscosity of the electrolyte, improves the conductivity, and can also form a protective SEI film containing lithium fluoride on the surface of the negative electrode, inhibiting the growth of lithium dendrites, and significantly improving the capacity retention rate. Example 3 increases the mass fraction of TTE on the basis of Example 2, improves the compatibility of the electrolyte with lithium metal, and further improves the cycle performance. Example 4 further increases the TTE content, the conductivity decreases more, the impedance is higher, and the discharge capacity is lower than that of Example 3. In Example 5, the additive in Example 2 was replaced with the conventional additive VC. Although VC can form a SEI film at the negative electrode and has a certain protective effect, the film impedance is large and it cannot form a protective CEI film at the positive electrode. Therefore, the first efficiency and cycle capacity retention rate are lower than those in Example 2. In Example 6, the fluoroether diluent in Example 5 was replaced with the conventional carbonate solvent EMC. Although EMC can also play a diluting role, it has high reactivity with lithium metal and poor oxidation resistance. Therefore, compared with Example 5, the electrolyte side reactions increased, the first efficiency decreased, the impedance increased, and the cycle performance deteriorated.

[0069] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. An electrolyte for a high specific energy lithium metal battery, characterized in that: The invention comprises a lithium salt, a solvent and an additive; the solvent comprises a fluorinated cyclic carbonate and a sulfone; the fluorinated cyclic carbonate comprises at least one of fluoroethylene carbonate, trifluoropropylene carbonate and difluoroethylene carbonate; the sulfone comprises at least one of dimethyl sulfone, methyl ethyl sulfone, cyclopentane, trifluoromethyl ethyl sulfone, trifluoropropyl methyl sulfone and methoxymethyl fluorosulfone.

2. The electrolyte for a high-energy lithium metal battery according to claim 1, characterized in that: The solvent further comprises a fluoroether; the fluoroether comprises at least one of 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether.

3. The electrolyte for a high-energy lithium metal battery according to claim 2, characterized in that: The mass fraction of the fluorinated cyclic carbonate in the solvent is 15-40%, the mass fraction of the sulfone in the solvent is 25-55%, and the mass fraction of the fluorinated ether in the solvent is 20-40%.

4. The electrolyte for a high-energy lithium metal battery according to claim 1, wherein The sulfones are fluorosulfones; the fluorosulfones include at least one of trifluoromethyl ethyl sulfone, trifluoropropyl methyl sulfone, and methoxymethyl fluorosulfone.

5. The electrolyte for a high-energy lithium metal battery according to claim 1, wherein: The solvent is a combination of propylene trifluorocarbonate, trifluoromethyl ethyl sulfone and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether; the mass fraction of propylene trifluorocarbonate in the solvent is 30%, the mass fraction of trifluoromethyl ethyl sulfone in the solvent is 40%, and the mass fraction of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether in the solvent is 30%.

6. The electrolyte for a high-energy lithium metal battery according to claim 1, wherein The mass fraction of the additive in the electrolyte is 0.5-3%.

7. The electrolyte for a high-energy lithium metal battery according to claim 1, characterized in that: The additives include lithium difluorooxalatoborate and vinyl sulfate; the mass ratio of the lithium difluorooxalatoborate to the vinyl sulfate is 1:

1.

8. The electrolyte for a high-energy lithium metal battery according to claim 1, wherein The lithium salt includes lithium hexafluorophosphate; the concentration of the lithium salt in the solvent is 0.8-2.0 mol / L.

9. A lithium metal battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte as claimed in any one of claims 1 to 8.

10. A lithium metal battery according to claim 9, characterized in that: The active material of the positive electrode sheet is one of lithium-rich manganese-based, lithium cobalt oxide and lithium nickel manganese oxide.