阅读说明：本技术 一种在金属锂负极表面预制sei膜的方法 (Method for prefabricating SEI film on surface of metal lithium cathode ) 是由 陈飞 周翠芳 柴方荣 周建中 李明钧 佘伟华 于 2020-06-24 设计创作，主要内容包括：本发明属于锂离子电池负极技术领域,具体涉及一种在金属锂负极表面预制SEI膜的方法,包括如下步骤：(1)将配比质量的PVDF-HFP和PEO加入并溶于第一溶剂中,25～50℃下搅拌至全溶；(2)将配比质量的锂盐溶于第二溶剂中,再加入配比质量的添加剂,超声分散2～4h；(3)混合上述步骤(1)的溶液和步骤(2)的溶液,搅拌1～2h,至粘度为3000～6000cP,得到粘流性液体；(4)将上述步骤(3)的粘流性液体均匀涂覆于锂电池负极集流体的表面,干燥,得到预制SEI膜。本发明预制的高强度、弹性SEI膜,可显著减少金属锂负极的锂用量,充放电过程中能随金属锂表面波动位移而不破损,从而防止锂枝晶产生。(The invention belongs to the technical field of lithium ion battery cathodes, and particularly relates to a method for prefabricating an SEI film on the surface of a lithium metal cathode, which comprises the following steps: (1) PVDF-HFP and PEO in proportion and mass are added and dissolved in a first solvent, and stirring is carried out at 25-50 ℃ until the materials are completely dissolved; (2) dissolving lithium salt in a second solvent according to a ratio by mass, adding an additive according to a ratio by mass, and performing ultrasonic dispersion for 2-4 hours; (3) mixing the solution obtained in the step (1) and the solution obtained in the step (2), and stirring for 1-2 hours until the viscosity is 3000-6000 cP to obtain viscous liquid; (4) and (4) uniformly coating the viscous fluid liquid obtained in the step (3) on the surface of the lithium battery negative current collector, and drying to obtain the prefabricated SEI film. The prefabricated high-strength elastic SEI film can obviously reduce the lithium consumption of the metal lithium cathode, and can move along with the fluctuation of the surface of the metal lithium without being damaged in the charging and discharging process, thereby preventing the generation of lithium dendrites.)

1. A method for prefabricating an SEI film on the surface of a lithium metal negative electrode is characterized by comprising the following steps:

(1) PVDF-HFP and PEO in proportion and mass are added and dissolved in a first solvent, and stirring is carried out at 25-50 ℃ until the materials are completely dissolved;

(2) dissolving lithium salt in a second solvent according to a ratio by mass, adding an additive according to a ratio by mass, and performing ultrasonic dispersion for 2-4 hours;

(3) mixing the solution obtained in the step (1) and the solution obtained in the step (2), and stirring for 1-2 hours until the viscosity is 3000-6000 cP to obtain viscous liquid;

(4) and (4) uniformly coating the viscous fluid liquid obtained in the step (3) on the surface of the lithium battery negative current collector, and drying to obtain the prefabricated SEI film.

2. The method for prefabricating the SEI film on the surface of the lithium metal anode according to claim 1, wherein the mass of the first solvent is calculated as 100g, the mass of the second solvent is 30-60 g, the mass of PVDF-HFP is 15-30 g, the mass of the lithium salt is 0.05-0.2 mol, the mass of PEO is 0.5-10% of the mass of PVDF-HFP, and the mass of the additive is 0-10 g.

3. The method for prefabricating the SEI film on the surface of the lithium metal anode according to claim 1, wherein the first solvent is one or two of N, N-dimethylformamide and N-methylpyrrolidone.

4. The method for prefabricating the SEI film on the surface of the lithium metal anode according to claim 1, wherein the lithium salt is selected from one or more of lithium difluorophosphate, lithium bis (trifluoromethylsulfonyl) imide, lithium bis (oxalato) borate, lithium difluorooxalato borate and lithium nitrate.

5. The method for prefabricating the SEI film on the surface of the lithium metal anode according to claim 1, wherein the second solvent is one or two of acetonitrile and acetone.

6. The method for prefabricating the SEI film on the surface of the lithium metal anode according to claim 1, wherein the additive is selected from one of alumina nanopowder or nanowire, titanium oxide nanopowder or nanowire, boron oxide nanopowder or nanowire, silicon carbide nanopowder or nanowire, fumed silica and the like.

7. The method for prefabricating the SEI film on the surface of the lithium metal negative electrode according to claim 1, wherein the current collector of the lithium battery negative electrode is selected from copper foil or lithium foil.

8. The method for prefabricating the SEI film on the surface of the lithium metal anode according to claim 1, wherein the drying conditions are as follows: drying at 40-60 ℃ for 1-2 h, then heating to 100-150 ℃, and vacuum drying for 6-12 h.

9. The lithium metal negative electrode prepared by the method for prefabricating the SEI film on the surface of the lithium metal negative electrode according to any one of claims 1 to 8.

10. A lithium ion battery comprising the lithium metal negative electrode according to claim 9.

Technical Field

The invention belongs to the technical field of lithium ion battery cathodes, and particularly relates to a method for preparing a high-strength and elastic SEI film on the surface of a lithium metal cathode.

Background

The lithium ion battery is one of the commonly used secondary batteries, has high energy density and excellent cycle performance, and is the mainstream choice of the current electric automobile battery. The commonly used negative electrode material is graphite, the theoretical capacity is lower and is only 375mAh g^-1This has not been able to satisfy the demand for long endurance of mobile energy sources, particularly electric vehicles, and thus, the demand for the development of high energy density batteries is becoming more urgent.

The lithium metal has the lowest reduction potential (-3.04V, for standard hydrogen electrode), and the highest specific capacity of 3860mAh g^-1Therefore, the lithium metal is used as the negative electrode, and the battery has the advantage of high energy density. In the 70's of the 20 th century, many inorganic compounds were found to exhibit reversible electrochemical lithium ion deintercalation behavior, and these findings have stimulated research into the use of metallic lithium. In 1972, Exxon led to the use of lithium metal as the negative electrode and TiS₂In the case of a positive lithium battery, it was confirmed that the lithium metal battery has good cyclability, but lithium dendrite occurs during a long cycle, i.e., needle-shaped lithium metal grows on the surface of lithium, thereby causing short circuit and ignition of the battery, and the development of a lithium metal secondary battery is thus cut into a bottleneck. In 1989, the company SONY uses petroleum coke as a negative electrode, LiCoO₂As the positive electrode, the safety problem of the battery is solved. Through the rapid development of more than 30 years, the performance of the lithium ion battery is remarkably improved, the cost is reduced to a lower level, and the requirement of people on the high-energy-density battery at the present stage cannot be met. Therefore, research on the metal lithium negative electrode is repeated, and the principle, process and the like of the metal lithium battery are deeply researched, so that the contradiction between high energy density and safety of the lithium battery is expected to be solved in principle.

Studies have shown that the failure or safety problems of metallic Li cathodes are mainly due to dendrites formed during the charge-discharge cycle. The formed lithium dendrite penetrates through the diaphragm to cause short circuit in the battery, so that safety accidents are caused; lithium dendrites also react with the electrolyte to form a new SEI film consuming the electrolyte and the lithium dendrites break down upon discharge to form "dead lithium" resulting in reduced cycling performance. Furthermore, metallic Li is accompanied by almost unlimited volume expansion during charge-discharge cycles, which also leads to extreme instability of the surface SEI film, further aggravating the formation of metallic Li dendrites. This infinite volume expansion due to dendrite formation greatly limits the practical application of metallic Li anodes.

Electrodeposited lithium metal is more prone to dendrite growth than other metals because the spontaneously formed SEI film of the lithium metal negative electrode is unstable, leading to cycle stability problems, which are mainly manifested in the following areas:

(1) instability of SEI film; the SEI film spontaneously generated by the reaction of the metallic lithium and the electrolyte has low strength and poor toughness, the SEI film is broken by the separation of the lithium from the support of the metallic lithium during the discharging process, the lithium can be preferentially deposited to form dendrites due to the high reactivity of the broken part of the SEI film, and a new SEI film can be formed on the surfaces of the dendrites of the lithium, so that the instability of the SEI film on the surface of the metallic lithium cathode is far more serious than that of the graphite cathode with the carrier due to vicious cycle in the charging and discharging process; interface gaps are also generated by lithium dissolution in the discharging process, and the electrolyte enters the interface gaps from the damaged part and reacts with the lithium metal to generate a new SEI film. The SEI film formed by the lithium metal and the electrolyte spontaneously has low strength and poor elasticity, and is an important factor causing poor cycle performance of the lithium metal negative electrode, so that a high-strength elastic SEI film needs to be constructed on the surface of the lithium metal.

(2) Instability of the "interfacial spacing"; compared with the graphite cathode, the metallic lithium cathode can generate relatively infinite volume change along with the charging and discharging process, if the single-side specific capacity of the electrode reaches 3mAh & cm for commercial use^-2It is necessary to produce a thickness variation of about 14.6 μm, and the resulting "interfacial separation" causes the energy level of the charge transition to rise or even exceed it with difficulty. The microscopic non-uniform dissolution-deposition of metallic lithium also causes variations in surface roughness, resulting in non-uniform "interfacial spacing" that exacerbates lithium dendrite growth. Therefore, a good SEI film should dynamically shift with the movement of the lithium metal surface to minimize the "interfacial distance".

Therefore, solving the problems of safety and interface stability of the lithium metal negative electrode is a key to promote the industrialization of the lithium metal negative electrode. Aiming at the problems of the metal lithium cathode, the invention prepares the high-strength SEI film on the surface of the metal lithium, and the prepared SEI film can move along with the fluctuation of the surface of the metal lithium without being damaged in the charging and discharging process, so that the interface is stabilized, the generation of lithium dendrites is prevented, and the safe and stable work of the metal lithium cathode is realized.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide a method for prefabricating an SEI film with high strength and excellent ionic conductivity on the surface of metallic lithium, and the prefabricated SEI film can move along with the fluctuation of the surface of the metallic lithium without being damaged, stabilizes an interface, prevents lithium dendrite from generating and realizes the safe and stable work of a metallic lithium cathode.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

a method for prefabricating an SEI film on the surface of a lithium metal negative electrode comprises the following steps:

(1) PVDF-HFP (vinylidene fluoride-co-hexafluoropropylene) and PEO (polyethylene oxide) in proportion and mass are added and dissolved in a first solvent, and stirring is carried out at the temperature of 25-50 ℃ until the materials are completely dissolved;

(2) dissolving lithium salt in a second solvent according to a ratio by mass, adding an additive according to a ratio by mass, and performing ultrasonic dispersion for 2-4 hours;

(3) mixing the solution obtained in the step (1) and the solution obtained in the step (2), and stirring for 1-2 hours until the viscosity is 3000-6000 cP to obtain viscous liquid;

(4) and (4) uniformly coating the viscous fluid liquid obtained in the step (3) on the surface of the lithium battery negative current collector, and drying to obtain the prefabricated SEI film.

Preferably, the mass of the first solvent is 100g, the mass of the second solvent is 30-60 g, the mass of PVDF-HFP is 15-30 g, the amount of the lithium salt is 0.05-0.2 mol, the mass of PEO is 0.5-10% of the mass of PVDF-HFP, and the mass of the additive is 0-10 g.

Preferably, the first solvent is one or two selected from N, N-dimethylformamide and N-methylpyrrolidone.

Preferably, the lithium salt is selected from lithium difluorophosphate (LiPO)₂F₂) Lithium bis (trifluoromethylsulfonyl) imide (LiTFSI), lithium bis (oxalato) borate (LiBOB), lithium difluoro (oxalato) borate (LiODFB), lithium nitrate (LiNO)₃) One or more of them.

Preferably, the second solvent is selected from one or two of acetonitrile and acetone.

Preferably, the additive is one of alumina nanopowder or nanowire, titanium oxide nanopowder or nanowire, boron oxide nanopowder or nanowire, silicon carbide nanopowder or nanowire, fumed silica and the like.

Preferably, the negative electrode current collector of the lithium battery is selected from copper foil or lithium foil.

Preferably, the drying conditions are: drying at 40-60 ℃ for 1-2 h, then heating to 100-150 ℃, and vacuum drying for 6-12 h.

Based on one general inventive concept, another object of the present invention is to protect a lithium metal anode manufactured by the above method of prefabricating an SEI film on a surface of the lithium metal anode, and a lithium ion battery including the lithium metal anode.

Compared with the prior art, the invention has the following advantages and positive effects:

the prefabricated SEI film can obviously reduce the lithium consumption of the metal lithium cathode and reduce the cost by more than 40%; the prefabricated SEI film has high strength and certain elasticity, and can move along with the fluctuation of the surface of the lithium metal without being damaged in the charging and discharging process, so that the generation of lithium dendrites is prevented; the prefabricated SEI film can replace the diaphragm currently used, so that the cost of the battery is further reduced; the PEO is blended with the PVDF-HFP, so that the strength of the PVDF-HFP is obviously improved, and meanwhile, the ionic conductivity of the system can be improved. The two solvents have synergistic effect, so that the internal pore size distribution of the prefabricated SEI film is more uniform; the preparation process is simple and can be used for batch production.

Drawings

FIG. 1 Electron microscopy topographic map (1000 times) of pre-formed SEI film of example 1;

FIG. 2 results of the battery cycle life test of example 1;

fig. 3 results of the cycling efficiency test of the button cell of example 1.

Detailed Description

The following detailed description of specific embodiments of the invention refers to the accompanying drawings.