High-voltage electrolyte and lithium metal battery containing same

Abstract

The invention discloses a high-voltage electrolyte and a lithium metal battery containing the high-voltage electrolyte, and relates to the technical field of lithium batteries. The lithium metal battery comprises a positive electrode, a negative electrode, a diaphragm and electrolyte, the electrolyte comprises a lithium salt, an organic solvent, a first additive, a second additive and a basic additive, the first additive is ferrocene hexafluorophosphate (FcPF6), and the second additive is a cyclotetraphosphazene compound; the lithium metal battery meets the following conditions: 0.5% < = a * b / (a + b) < = 1%, 1% < = a < = 1.5%, and 1% < = b < = 1.5%; the electrolyte provided by the invention can prevent a positive electrode structure from collapsing under the high voltage of the ternary high-nickel material, and forms a uniform and compact interfacial film with high ionic conductivity on a positive electrode interface and a negative electrode interface, so that the growth of lithium dendrites and the decomposition of the electrolyte under the high voltage are inhibited; the method is suitable for the requirements of long service life and high safety of a high-voltage lithium metal battery system greater than or equal to 4.5 V.

Description

Technical Field

[0001] This invention belongs to the field of lithium metal battery technology, specifically relating to a high-voltage electrolyte and a lithium metal battery containing the high-voltage electrolyte. Background Technology

[0002] Lithium metal batteries, which use lithium metal as the negative electrode material, have seen rapid development in recent years. With a theoretical specific capacity as high as 3860 mAh / g and an electrochemical potential as low as -3.04 V vs. SHE, they possess the potential to achieve even higher energy densities and are considered the "ultimate solution" in the field of power batteries. Currently, some companies have achieved technological breakthroughs, optimizing electrolyte formulations to enable lithium metal batteries to achieve a cycle life of 483 cycles and an energy density exceeding 500 Wh / kg.

[0003] Lithium metal batteries are currently mainly used in applications requiring high energy density, such as electric aviation, and have been applied in stratospheric drones, industrial drones, and eVTOL aircraft. Furthermore, they can support ultra-long-range electric vehicles with a range exceeding 1000 kilometers, addressing range anxiety. However, lithium metal batteries still face many challenges. At high voltages, the irreversible reaction between lithium metal and the electrolyte consumes active lithium and the electrolyte, leading to low coulombic efficiency and short cycle life. Simultaneously, the cathode-electrolyte interface is prone to electrolyte decomposition at high voltages. Existing electrolytes often produce insufficient interfacial protective films to prevent continuous oxidation, and the cathode material itself can also experience lattice oxygen release and structural collapse, triggering the dissolution of transition metal ions, further exacerbating electrolyte decomposition and battery performance degradation. In addition, common electrolytes suffer from poor compatibility with lithium metal, uncontrollable lithium dendrite growth, and high cost and viscosity of high-concentration electrolytes, hindering the development of high-voltage lithium metal batteries. Therefore, there is an urgent need to develop a lithium metal electrolyte system that improves high-voltage performance.

[0004] Chinese patent application CN109216769A discloses a lithium metal battery electrolyte, a lithium metal battery, and a lithium-sulfur battery. The lithium metal battery electrolyte includes a solvent and lithium salts, wherein the lithium salts include lithium salt I, lithium salt II, and lithium salt III, with the following mass percentages in the electrolyte: solvent 40%–85%, lithium salt I 10%–50%, lithium salt II 0.1%–3%, and lithium salt III 0.1%–10%. The solvent is any combination of organic heterocyclic compounds containing oxogroup elements on their rings. Lithium salt I is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium salt II is lithium nitrate (LiNO3), and lithium salt III is one or a combination of boron-containing lithium salts and phosphorus-containing lithium salts. A lithium metal battery includes a positive electrode, a lithium-containing negative electrode, a separator, and the aforementioned electrolyte. A lithium-sulfur battery includes a sulfur-containing positive electrode, a lithium-containing negative electrode, a separator, and the aforementioned electrolyte. However, after 100 cycles, the capacity retention rate of this patent is only 61%-81%, so further improvement is needed. Summary of the Invention

[0005] The technical problem to be solved by this invention is how to address the issues of existing lithium metal batteries being unable to cope with high voltage and uncontrollable lithium dendrite growth.

[0006] The present invention solves the above-mentioned technical problems through the following technical means: This invention proposes a high-voltage electrolyte comprising the following components: lithium salt, organic solvent, first additive, second additive, and base additive; the first additive is ferrocene hexafluorophosphate, and the second additive is a cyclotetraphosphazene compound; the cyclotetraphosphazene compound has the following structure: R1-R8 are each independently selected from halogen atoms or alkoxy groups.

[0007] Preferably, with the total mass fraction of the electrolyte being 100%, the electrolyte satisfies the following conditions: 0.5% ≤ a×b / (a+b) ≤ 1%, and 1% ≤ a ≤ 1.5%, 1% ≤ b ≤ 1.5%; wherein a is the mass percentage of the first additive in the electrolyte, and b is the mass percentage of the second additive in the electrolyte.

[0008] Preferred, ferrocene hexafluorophosphate (FcPF6), CAS number 11077-24-0, has the following structure: .

[0009] Preferably, the electrolyte satisfies the following conditions: 0.5%≤a×b / (a+b)≤0.75%, and 1%≤a≤1.5%, 1%≤b≤1.5%.

[0010] Preferably, the cyclotetraphosphononitrile compound is selected from at least one of the following compounds:

[0011] Preferably, the base additive includes at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), tris(trimethylsilyl)phosphate (TMSP), and methanedisulfonate (MMDS).

[0012] With the total mass fraction of the electrolyte being 100%, the mass percentage of the basic additive in the electrolyte is 0.5% to 2%.

[0013] Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorobis(oxalato) phosphate, lithium bis(trifluorosulfonyl)imide, and lithium bis(fluorosulfonyl)imide. The final concentration of the lithium salt in the electrolyte is 0.5~1.5 mol / L.

[0014] Preferably, the organic solvent includes at least one of the following: carbonates having 3 to 5 carbon atoms, carboxylic acid esters having 2 to 6 carbon atoms, ethers having 4 to 10 carbon atoms, and nitriles having 2 to 4 carbon atoms; More preferably, the organic solvent is at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0015] With the total mass fraction of the electrolyte being 100%, the mass percentage of the organic solvent is 70-90%.

[0016] Preferably, the electrolyte is composed of the following components: lithium salt, organic solvent, first additive, second additive, and basic additive; the first additive is ferrocene hexafluorophosphate, the second additive is a tetrafluorocyclophosphamide compound, and based on the total mass fraction of the electrolyte being 100%, the organic solvent is 70-90% by mass, the first additive is 1-1.5% by mass, the second additive is 1-1.5% by mass, the basic additive is 0.5-2% by mass, and the remainder is lithium salt.

[0017] The present invention also proposes a lithium metal battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte; wherein the electrolyte is the aforementioned high-voltage electrolyte.

[0018] Preferably, the active material of the positive electrode is NCM811 ternary high-nickel positive electrode material (LiNi). 0.8 Co 0.1 Mn 0.1 O2).

[0019] Preferably, the active material of the negative electrode is lithium metal.

[0020] Preferably, the membrane material is a polyolefin.

[0021] The beneficial effects of this invention are as follows: The lithium metal battery provided by this invention, by ensuring that the parameters of the first additive and the second additive satisfy a specific relationship of 0.5%≤a×b / (a+b)≤1% (where 1%≤a≤1.5%, 1%≤b≤1.5%), can exhibit significant advantages under high-voltage conditions of ternary high-nickel materials: Fe in the first additive FcPF6 + During battery cycling, this electrolyte can fill the vacancies of Mn ions dissolved from the positive electrode material through kinetic interactions, thereby effectively suppressing the collapse of the positive electrode structure. Simultaneously, it can form a thin and dense CEI film at the positive electrode interface and a fluorine- and phosphorus-rich inorganic SEI film at the negative electrode interface. Through this dual interfacial stabilization, it can both inhibit the growth of lithium dendrites and block the decomposition of the electrolyte under high voltage conditions. Furthermore, the synergistic effect of the first and second additives can efficiently capture trace amounts of water and hydrofluoric acid in the electrolyte, preventing their corrosion of the positive electrode material and reducing the dissolution of transition metal ions. This multi-dimensional improvement enhances the cycle stability and rate performance of the battery, making it particularly suitable for the long lifespan and high safety requirements of ≥4.5V high-voltage lithium metal battery systems.

[0022] Of course, implementing any product or method of the present invention does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical terms used below have the same meaning as understood by those skilled in the art.

[0024] Unless otherwise specified, the test materials and reagents used in the following examples are commercially available or prepared by known methods.

[0025] Unless otherwise specified, all techniques or conditions described in the embodiments can be performed in accordance with the techniques or conditions described in the literature in this field or in the product manual. Unless otherwise specified, the quantitative experiments in the following embodiments are all repeated three times or more, and the results are averaged.

[0026] Example 1 A high-voltage electrolyte comprises the following components: lithium salt (lithium hexafluorophosphate), organic solvent, first additive, second additive, and base additive; the first additive is ferrocene hexafluorophosphate, and the second additive is... Based on the total mass fraction of the electrolyte being 100%, the mass percentage of organic solvent is 83.5% (the organic solvents are ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC)), the mass percentage of the first additive is 1%, the mass percentage of the second additive is 1%, the mass percentage of the base additive is 1.5% (1% DTD and 0.5% TMSP), and the remainder is lithium salt.

[0027] The preparation method of the above electrolyte is as follows: 1. In an inert atmosphere glove box with water / oxygen index <0.1ppm, ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are mixed uniformly at a mass ratio of EC:DMC:EMC=3:2:5. After mixing, fully dried lithium hexafluorophosphate (LiPF6) is added. After the lithium salt is completely dissolved, the first additive FcPF6, the second additive compound 2, and the basic additives (DTD, TMSP) are added and mixed evenly to obtain the electrolyte.

[0028] The method for preparing a lithium metal battery containing this high-voltage electrolyte is as follows: 2. Preparation of the positive electrode: Lithium-nickel-cobalt-manganese composite oxides (such as LiNi) 0.8 Co 0.1 Mn 0.1 O2), conductive agent Super P, binder PVDF and carbon nanotubes (CNT) are mixed evenly in a mass ratio of 97.5:0.8:0.7:1 to prepare a battery positive electrode slurry with a certain viscosity. Then, it is evenly coated on the current collector aluminum foil with a coating amount controlled at 360g / m². After drying at 85℃, it is cold-pressed, slit, and sliced, and then dried under vacuum at 85℃ for 4 hours to produce a lithium metal battery positive electrode sheet that meets the requirements.

[0029] 3. Preparation of the negative electrode: Select a lithium metal foil with a thickness of 10~50 µm (purity ≥99.9%) and cut it into a shape that matches the size of the positive electrode using a precision cutting device. Then, in a glove box protected by inert gas, lay the cut lithium foil directly on a copper current collector with a thickness of 8~12 µm and gently roll it to make a lithium metal negative electrode that meets the requirements.

[0030] 4. Preparation of the diaphragm: A polyolefin substrate (polyethylene or polypropylene) with a thickness of 20~50 µm is selected. The polyolefin and plasticizer are melt-blended and cast into a film. The substrate is biaxially stretched to form micropores. The plasticizer is then removed by extraction with an organic solvent and dried to obtain a diaphragm.

[0031] 5. Preparation of lithium metal batteries: The positive electrode, negative electrode, and polyolefin separator prepared by the above process are stacked to form a lithium metal battery with a thickness of 1.5 mm, a width of 30 mm, and a length of 50 mm, with a capacity of 2.4 Ah. The battery is then vacuum baked at 60°C for 2 hours and injected with the electrolyte obtained in step 1 above to complete the battery fabrication.

[0032] Examples 2-7 and Comparative Examples 1-4 The differences between Examples 2-7, Comparative Examples 1-4, and Example 1 lie in the content of the first additive, the type and content of the second additive in the electrolyte, as detailed in Table 1, and the type of the second additive, as shown in Table 2. The preparation methods for the positive electrode, negative electrode, electrolyte, separator, and lithium metal battery can refer to Example 1.

[0033] Table 1. Parameter table for Examples 1-7 and Comparative Examples 1-4

[0034] Table 2 Types of Second Additives

[0035] Performance testing: The lithium metal batteries prepared in the above embodiments and comparative examples were subjected to the following performance tests.

[0036] Cyclic testing: The lithium metal batteries obtained in the comparative examples and embodiments were charged at a constant current of 1C to 4.5V at room temperature, then charged at a constant voltage to a current of 0.05C, and then discharged at a constant current of 1C to 2.8V. This constitutes one charge-discharge cycle. The discharge capacity of the first 5 cycles was taken as the initial discharge capacity. When the cycle reached 300 cycles, the cycle capacity retention rate was calculated as follows: Cycle capacity retention rate at 300 cycles = (Discharge capacity at 300 cycles / Initial discharge capacity) × 100% Ratio performance test: The lithium metal batteries obtained from the comparative examples and embodiments underwent five charge-discharge cycles each at 1C, 2C, and 5C. The voltage range was 2.8-4.5 V. The average 1C discharge capacity was taken as C1, the average 5C discharge capacity as C2, and the 5C capacity retention rate was... .

[0037] Room temperature storage test: The electrolytes of Comparative Examples 1-4 and Examples 1-7 were stored in a constant temperature cabinet at 25°C for 7 days, and the hydrofluoric acid (HF) content of the electrolytes before and after storage was tested. The test results are shown in Table 3.

[0038] Table 3 Performance data for Examples 1-7 and Comparative Examples 1-4

[0039] Table 1-2 shows the test results of Examples 1-7 and Comparative Examples 3-4. It is evident that when the mass percentage 'a' of the first additive and the mass percentage 'b' of the second additive in the electrolyte satisfy the relationship 0.5 ≤ a × b / (a ​​+ b) ≤ 1, and 1 ≤ a ≤ 1.5, 1 ≤ b ≤ 1.5, the resulting lithium metal battery exhibits significant advantages under the high-voltage environment of ternary high-nickel materials. Specifically, the lithium metal battery prepared in Comparative Examples 1-4 retains a capacity of ≤70.11% after 300 cycles at room temperature and a capacity retention of ≤63.19% during 5C fast charging and discharging. In contrast, the lithium metal battery prepared in Examples 1-7 retains a capacity of ≥83.18% after 300 cycles at room temperature and a capacity retention of ≥72.66% during 5C fast charging and discharging. This fully demonstrates that the novel high-voltage electrolyte provided by this invention significantly improves the cycle stability and rate performance of lithium metal batteries. Furthermore, the test results of Examples 1-7 and Comparative Examples 1-2 show that the first additive and the second additive have a synergistic effect. When either additive is missing, the resulting battery has high impedance and significantly deteriorates in cycle performance and rate performance.

[0040] HF content is an important indicator of electrolyte stability. The level of HF content not only affects battery capacity and energy density but also its safety and lifespan. Table 2 shows the changes in HF content before and after storage in Examples 1-7 and Comparative Examples 1-4. It is evident that in Comparative Examples 1-4, which do not simultaneously contain the first or second additives or whose a×b / (a+b) ratio is outside the scope of this invention, the HF content increased from approximately 18.5 ppm to about 33 ppm after 7 days of storage at room temperature. However, the electrolytes in Examples 1-7 did not show a significant upward trend in HF content after 7 days of storage, and some examples even showed a downward trend. This result indicates that appropriately adding FcPF6 and cyclotetraphosphonium nitrile compounds to the electrolyte according to the relationship 0.5≤a×b / (a+b)≤1 can remove trace amounts of water and HF, thereby preventing HF corrosion of the positive electrode material, reducing metal ion dissolution, and improving the stability of the electrode-electrolyte interface.

[0041] Example 8: A high-voltage electrolyte comprises the following components: lithium salt (lithium tetrafluoroborate), organic solvent (EC), first additive, second additive, and base additive (ethylene carbonate (VC)); the first additive is ferrocene hexafluorophosphate, the second additive is compound 1, and based on the total mass fraction of the electrolyte being 100%, the organic solvent accounts for 70% by mass, the first additive accounts for 1.5% by mass, the second additive accounts for 1.5% by mass, the base additive accounts for 2% by mass, and the remainder is lithium salt.

[0042] Example 9: A high-voltage electrolyte comprises the following components: lithium salt (lithium difluorobis(oxalato) phosphate), organic solvent (DMC), a first additive, a second additive, and a base additive (fluoroethylene carbonate (FEC)); the first additive is ferrocene hexafluorophosphate, the second additive is compound 3, and based on the total mass fraction of the electrolyte being 100%, the organic solvent accounts for 90% by mass, the first additive accounts for 1.2% by mass, the second additive accounts for 1.2% by mass, the base additive accounts for 0.5% by mass, and the remainder is lithium salt.

[0043] The preparation methods of the high-voltage electrolyte in Examples 8 and 9, as well as the methods for preparing lithium metal batteries containing the high-voltage electrolyte, are the same as in Example 1, and the performance of the lithium metal batteries obtained is similar to that in Example 1.

[0044] In summary, by ensuring that the parameters of the first and second additives satisfy a specific relationship of 0.5% ≤ a × b / (a ​​+ b) ≤ 1% (where 1% ≤ a ≤ 1.5% and 1% ≤ b ≤ 1.5%), the Fe in FcPF6... + During high-voltage cycling, it can fill the Mn ion vacancies dissolved from the cathode material through kinetic action, thereby effectively suppressing the collapse of the cathode structure. At the same time, it can form a thin and dense CEI film at the cathode interface and an inorganic SEI film rich in fluorine and phosphorus at the anode interface. Through the dual interface stabilization effect, it can both inhibit the growth of lithium dendrites and block the decomposition of electrolyte under high voltage conditions. In addition, the synergistic effect of the first additive and the second additive can also efficiently capture trace amounts of water and hydrofluoric acid in the electrolyte, thereby improving the cycle stability and rate performance of the battery in multiple dimensions.

[0045] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high-voltage electrolyte, characterized in that, Includes the following ingredients: Lithium salt, organic solvent, first additive, second additive, and basic additive; the first additive is ferrocene hexafluorophosphate; the second additive is a tetraphosphazene compound; The cyclotetraphosphazene compound has the following structure: R1-R8 are each independently selected from halogen atoms or alkoxy groups.

2. The high-voltage electrolyte according to claim 1, characterized in that, With the total mass fraction of the electrolyte being 100%, the electrolyte satisfies the following conditions: 0.5% ≤ a × b / (a ​​+ b) ≤ 1%, and 1% ≤ a ≤ 1.5%, 1% ≤ b ≤ 1.5%; where a is the mass percentage of the first additive in the electrolyte; and b is the mass percentage of the second additive in the electrolyte.

3. The high-voltage electrolyte according to claim 2, characterized in that, The electrolyte meets the following conditions: 0.5%≤a×b / (a+b)≤0.75%, and 1%≤a≤1.5%, 1%≤b≤1.5%.

4. The high-voltage electrolyte according to claim 1, characterized in that, The cyclotetraphosphazene compound is selected from at least one of the following compounds: .

5. The high-voltage electrolyte according to claim 1, characterized in that, With the total mass fraction of the electrolyte being 100%, the mass percentage of the basic additive is 0.5-2%; the basic additive includes at least one of vinylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, vinyl sulfate, tris(trimethylsilyl)phosphate, and methanedisulfonate.

6. The high-voltage electrolyte according to claim 1, characterized in that, The final concentration of the lithium salt in the electrolyte is 0.5~1.5 mol / L; the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorobis(oxalato)phosphate, lithium bis(trifluorosulfonyl)imide, and lithium bis(fluorosulfonyl)imide.

7. The high-voltage electrolyte according to claim 1, characterized in that, With the total mass fraction of the electrolyte being 100%, the mass percentage of the organic solvent is 70-90%; the organic solvent includes at least one of carbonates with 3-5 carbon atoms, carboxylic acid esters with 2-6 carbon atoms, ethers with 4-10 carbon atoms, and nitriles with 2-4 carbon atoms; more preferably, the organic solvent is at least one of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate.

8. The high-voltage electrolyte according to claim 1, characterized in that, It is composed of the following components: lithium salt, organic solvent, first additive, second additive and basic additive; the first additive is ferrocene hexafluorophosphate, the second additive is a tetraphosphazene compound, and based on the total mass fraction of the electrolyte as 100%, the organic solvent mass percentage is 70~90%, the first additive mass percentage is 1~1.5%, the second additive mass percentage is 1~1.5%, the basic additive mass percentage is 0.5~2%, and the remainder is lithium salt.

9. A lithium metal battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The electrolyte is the high-voltage electrolyte according to any one of claims 1-8.

10. The battery according to claim 9, characterized in that, The active material of the positive electrode is a ternary high-nickel positive electrode material; the active material of the negative electrode is lithium metal; and the membrane material is polyolefin.

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