Lithium metal negative electrode based on polymer-inorganic matter composite coating

Abstract

The invention discloses a lithium metal negative pole piece, the negative pole piece comprises a lithium foil and a composite coating coated on the lithium foil, and the composite coating comprises a polymer matrix and an inorganic filler; the polymer matrix is selected from at least one of a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) and polyethylene oxide (PEO); and the inorganic filler is selected from at least one of SiO2, Al2O3 and lithium lanthanum zirconium oxide (LLZO). The composite coating combines the advantages of the two materials, the polymer matrix (especially PVDF-HFP and PEO) provides excellent flexibility, huge volume change of lithium in the deposition / separation process can be buffered, and the structural integrity of a protective layer is kept; the inorganic filler (such as SiO2, Al2O3 and LLZO) with high mechanical strength serves as a rigid framework to be uniformly dispersed in the polymer, and growth and puncture of lithium dendrites can be effectively and physically blocked and inhibited.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery negative electrodes, and particularly relates to a lithium metal negative electrode based on a polymer-inorganic compound coating. BACKGROUND

[0002] With the surge in global demand for high-efficiency energy storage systems, especially in the fields of portable electronic devices and electric vehicles, the development of battery technologies with high energy density has become a research hotspot. Lithium metal is considered as an ideal negative electrode material for building the next generation of high-energy-density batteries due to its extremely high theoretical specific capacity (3860 mAh g -1 ) and extremely low electrochemical potential (-3.04 V vs. SHE).

[0003] However, the commercial application of lithium metal negative electrodes faces severe challenges. During the charging and discharging cycles of the battery, lithium ions are unevenly deposited on the surface of the negative electrode, forming dendritic "lithium dendrites". These sharp dendrites can pierce the battery separator, causing internal short circuits, and in severe cases, can lead to safety accidents such as thermal runaway, combustion, and even explosion. At the same time, the highly active lithium metal will continuously react with the electrolyte, forming an unstable and continuously thickening solid-state electrolyte interface (SEI) film, which not only consumes limited lithium and electrolyte, but also increases the interface impedance, resulting in short battery cycle life and low coulombic efficiency.

[0004] To solve the above problems, existing technologies have adopted various strategies, such as electrolyte modification, construction of three-dimensional negative electrode framework, and construction of artificial protective layer on the surface of lithium metal. Among them, the construction of artificial protective layer is attracting attention due to its simple operation, low cost, and good compatibility with existing battery production processes. SUMMARY

[0005] Existing artificial protective layers are mainly divided into two categories: polymers and inorganics:

[0006] Polymer protective layer: has good flexibility and can adapt to the volume change of lithium negative electrode, but its mechanical strength is generally low, it is difficult to effectively block the growth of lithium dendrites, and the ionic conductivity is not high.

[0007] Inorganic protective layer: has high mechanical strength and ionic conductivity, can effectively inhibit dendrites, but its texture is brittle and is easy to crack and fail during the cycle process.

[0008] Based on the above status, the present application aims to provide a new protection technology that can have high mechanical strength, high ionic conductivity, good flexibility, and excellent electrochemical stability, in order to realize the safe and long-term cycle of lithium metal negative electrode.

[0009] In one aspect, the present application provides a lithium metal negative electrode tab, the negative electrode tab comprising a lithium foil and a composite coating coated on the lithium foil, the composite coating comprising a polymer matrix and an inorganic filler; the polymer matrix being selected from at least one of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyethylene oxide (PEO); the inorganic filler being selected from at least one of SiO2, Al2O3, lithium lanthanum zirconium oxide (LLZO).

[0010] In some embodiments, the mass ratio of the polymer matrix and the inorganic filler is 7:(1-3), preferably 7:(1.5-2.5).

[0011] In some embodiments, the thickness of the composite coating is 1-100 microns, preferably 20-70 microns.

[0012] In preferred embodiments, the composite coating further comprises a lithium salt and a conductive additive, the lithium salt being selected from lithium bis(trifluoromethane)sulfonimide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI); the conductive additive being selected from carbon black, graphene.

[0013] In preferred embodiments, the amount of the lithium salt added to the composite coating is 5-15% by mass; and / or, the amount of the conductive additive added to the composite coating is 1-5% by mass.

[0014] In more preferred embodiments, the composite coating is selected from PVDF-HFP / SiO2 composite coating, PEO / LLZO / LiTFSI composite coating, PAN / Al2O3 / carbon black / LiFSI composite coating.

[0015] In another aspect, the present application provides a method for preparing a lithium metal negative electrode tab, comprising: mixing a polymer matrix and an inorganic filler in a solvent to obtain a composite coating slurry;

[0016] coating the composite coating slurry on the surface of a lithium foil, drying and curing to obtain a lithium metal negative electrode tab with a composite coating on the surface.

[0017] In some embodiments, the solvent is selected from N-methylpyrrolidone, acetonitrile, dimethylformamide.

[0018] In some embodiments, the mass ratio of the polymer matrix and the inorganic filler is 7:(1-3), preferably 7:(1.5-2.5).

[0019] In some embodiments, the mixing comprises magnetic stirring at room temperature for 12 hours, followed by ultrasonic treatment for 30 minutes.

[0020] In some embodiments, the drying and curing comprises vacuum drying at 60-100℃ for 6-12 hours.

[0021] In yet another aspect, the present application provides a battery comprising the lithium metal negative electrode tab.

[0022] Technical effects

[0023] The present application constructs a polymer-inorganic composite protective layer on the surface of the lithium metal negative electrode. The protective layer is formed by scientifically compounding a polymer matrix with inorganic functional fillers. The composite coating combines the advantages of both materials. The polymer matrix (especially PVDF-HFP, PEO) provides excellent flexibility, which can buffer the huge volume change of lithium during deposition / detachment and maintain the structural integrity of the protective layer; the inorganic filler (such as SiO2, Al2O3, LLZO) with high mechanical strength uniformly dispersed in the polymer as a rigid skeleton can effectively physically block and inhibit the growth and penetration of lithium dendrites.

[0024] The safety of the lithium metal negative electrode of the present application can be significantly improved. The composite coating of the present application effectively inhibits the growth of lithium dendrites, fundamentally reduces the risk of internal short circuit of the battery, and greatly improves the safety performance of the battery. By isolating the lithium metal from the electrolyte, the interface side reaction is reduced, the coulombic efficiency and capacity retention rate are significantly improved, and the cycle life of the battery is prolonged. The data of the examples of the present application show that by selecting specific polymer matrix and inorganic filler, the capacity retention rate of the obtained battery after 50 cycles can reach more than 90%, while the capacity of the unprotected negative electrode is severely attenuated after 8 cycles.

[0025] The selective addition of lithium salt (LiTFSI, LiFSI) in the composite coating can increase the lithium ion concentration and migration number in the protective layer, construct a fast ion transport channel, reduce the interface impedance, and improve the ion conductivity. The addition of conductive additives (carbon black, graphene) can improve the electric field distribution on the surface of the coating, guide the uniform deposition of lithium ions, and thus inhibit the formation of dendrites from the source. The fast ion transport channel reduces the interface impedance of the battery, resulting in smaller voltage polarization during charging and discharging, and reduces the polarization of the battery. The uniform lithium ion flow and low impedance interface enable the battery to work stably at high current, showing more excellent rate performance and improving the rate performance.

[0026] The present application uses a simple slurry coating-drying and curing process to prepare the composite protective layer, which includes pre-mixing all components (polymer, inorganic filler, etc.) in a solvent to form a uniform slurry, then directly coating on the surface of the lithium negative electrode by using mature methods such as blade coating and spraying, and finally curing into a film by mild drying conditions. The process is simple and easy to mass produce: the preparation method has short process flow, simple operation, and does not require complex equipment, which is easy to realize large-scale and automated production.

[0027] The process used by the present application is highly compatible with the existing electrode manufacturing process of lithium ion batteries (NMP system scraping coating), is convenient for introduction into the existing production line, and reduces the industrialization threshold. DETAILED DESCRIPTION

[0028] The technical content of the present application will be clearer and more convenient to understand by introducing the preferred embodiments of the present application. The present application can be embodied in many different forms of embodiments, and the protection scope of the present application is not limited to the embodiments mentioned in the text.

[0029] Example 1: Preparation of negative electrode sheet and battery using PVDF-HFP / SiO2 composite coating

[0030] Preparation of composite coating slurry: 7g of PVDF-HFP (polyvinylidene fluoride-hexafluoropropylene copolymer) and 2g of nano-SiO2 particles (average particle size about 50nm) were added to 50mL of NMP (N-methyl pyrrolidone) solvent, and magnetically stirred at room temperature for 12 hours, and then ultrasonically treated for 30 minutes to form a uniform and stable composite coating slurry.

[0031] Coating of composite coating: the composite coating slurry was uniformly coated on the surface of lithium metal foil by scraping method.

[0032] Drying and curing: the coated lithium metal foil was placed in a vacuum drying oven and dried at 60°C for 12 hours under vacuum to remove the solvent, and a lithium metal negative electrode with a PVDF-HFP / SiO2 composite protective layer on the surface was obtained.

[0033] Battery assembly and testing: the above negative electrode was assembled with NCM811 positive electrode (4mg cm -2 ), separator (PP) and electrolyte (1mol / L LIPF6 EC:DEC (1:1V / V%)) into CR2032 type button cell.

[0034] Constant current charge and discharge test was carried out at 25°C in the voltage range of 2.8V-4.3V at 0.1C, the first cycle efficiency was tested, and the capacity retention rate was tested after 50 cycles, and the results are shown in Table 1.

[0035] Example 2: Preparation of negative electrode sheet and battery using PEO / LLZO / LiTFSI composite coating

[0036] Preparation of composite coating slurry: 7g of PEO (polyethylene oxide), 2g of LLZO (lithium lanthanum zirconium oxide) powder and 1g of LiTFSI (lithium bis(trifluoromethane) sulfonamide) were dissolved or dispersed in 50mL of acetonitrile (AN) solvent, and magnetically stirred at room temperature for 12 hours to obtain a uniform composite coating slurry.

[0037] Coating composite coating: The composite coating slurry was uniformly coated on the surface of the lithium metal foil by using the doctor blade method.

[0038] Drying and curing: The coated lithium metal foil was placed in a vacuum drying oven and dried at 80°C for 12 hours under vacuum to remove the solvent, obtaining a lithium metal anode with a PEO / LLZO / LiTFSI composite protective layer on the surface.

[0039] Battery assembly and testing: Same as Example 1, and the test results are shown in Table 1.

[0040] Example 3: Preparation of anode electrode sheets and batteries using PAN / Al2O3 / Carbon black / LiFSI composite coating

[0041] Preparation of composite coating slurry: 7g of PAN (polyacrylonitrile), 2.5g of Al2O3 fiber, 0.5g of carbon black and 1g of LiFSI (lithium bis(fluorosulfonyl)imide) were dispersed in 50mL of DMF (dimethylformamide) solvent. After magnetic stirring at room temperature for 12 hours and ultrasonic treatment for 30 minutes, a uniform composite coating slurry was obtained.

[0042] Coating composite coating: The composite coating slurry was uniformly coated on the surface of the lithium metal foil by using the doctor blade method.

[0043] Drying and curing: The coated lithium metal foil was placed in a vacuum drying oven and dried at 100°C for 6 hours under vacuum to remove the solvent, obtaining a lithium metal anode with a PAN / Al2O3 / Carbon black / LiFSI composite coating on the surface.

[0044] Battery assembly and testing: Same as Example 1, and the test results are shown in Table 1.

[0045] Comparative Example 1: Preparation of anode electrode sheets and batteries without coating protection

[0046] The anode electrode sheets and batteries were prepared by the same method as in Example 1, with the only difference being that the lithium metal foil was not protected by a coating and was directly used as the anode electrode sheet. The batteries prepared in this comparative example were tested by the same method as in Example 1, and the results are shown in Table 1.

[0047] Comparative Example 2: Preparation of anode electrode sheets and batteries using PVDF-HFP coating

[0048] The anode electrode sheets and batteries were prepared by the same method as in Example 1, with the only difference being that SiO2 was not added when preparing the composite coating slurry. The batteries prepared in this comparative example were tested by the same method as in Example 1, and the results are shown in Table 1.

[0049] Comparative Example 3: Preparation of anode electrode sheets and batteries using PEO / LiTFSI coating

[0050] The negative electrode sheet and battery were prepared by the same method as Example 2, the only difference being that no LLZO was added when preparing the composite coating slurry. The battery prepared in this comparative example was tested by the same method as Example 1, and the results are shown in Table 1.

[0051] Comparative Example 4: Preparation of negative electrode sheet and battery with PEO / LLZO / LiTFSI coating

[0052] The negative electrode sheet and battery were prepared by the same method as Example 2, the only difference being that 4 g of LLZO was added when preparing the composite coating slurry. The battery prepared in this comparative example was tested by the same method as Example 1, and the results are shown in Table 1.

[0053] Comparative Example 5: Preparation of negative electrode sheet and battery with PAN / Carbon black / LiFSI coating

[0054] The negative electrode sheet and battery were prepared by the same method as Example 3, the only difference being that no Al2O3 was added when preparing the composite coating slurry. The battery prepared in this comparative example was tested by the same method as Example 1, and the results are shown in Table 1.

[0055] Comparative Example 6: Preparation of negative electrode sheet and battery with PAN / Al2O3 / Carbon black / LiFSI coating

[0056] The negative electrode sheet and battery were prepared by the same method as Example 3, the only difference being that 3.8 g of Al2O3 was added when preparing the composite coating slurry. The battery prepared in this comparative example was tested by the same method as Example 1, and the results are shown in Table 1.

[0057] Table 1

[0058]

[0059]

[0060] As can be seen from Table 1, by selecting a specific polymer matrix and inorganic filler, the capacity retention rate of the battery prepared in Examples 1-3 can reach more than 90% after 50 cycles, while in the case of no protection of the negative electrode (Comparative Example 1), the capacity of the battery is severely attenuated after 8 cycles.

[0061] Comparing Example 1 with Comparative Example 2, when no SiO2 is added as an inorganic filler in the coating of the negative electrode, the first cycle efficiency of the battery prepared is reduced, and the capacity retention rate is only 82% after 50 cycles.

[0062] Compared with Comparative Examples 3 and 4, when no LLZO is added as the inorganic filler or the amount of the inorganic filler is too high in the coating of the negative electrode, the first cycle efficiency of the battery prepared in Example 2 is reduced, and the capacity retention rate is less than 80% after 50 cycles.

[0063] Compared with Comparative Examples 5 and 6, when no Al2O3 is added as the inorganic filler or the amount of the inorganic filler is too high in the coating of the negative electrode, the first cycle efficiency of the battery prepared in Example 3 is reduced, and the capacity retention rate is only about 70% after 50 cycles.

[0064] The preferred embodiments of the present application are described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations without departing from the concept of the present application. Therefore, any technical solutions obtained by logical analysis, reasoning or limited experiments based on the concept of the present application and the prior art should be within the scope of protection defined by the claims.

Claims

1. A lithium metal negative electrode plate, comprising a lithium foil and a composite coating coated on the lithium foil, wherein the composite coating comprises a polymer matrix and an inorganic filler; the polymer matrix is ​​selected from at least one of polyvinylidene fluoride-hexafluoropropylene copolymer and polyethylene oxide; and the inorganic filler is selected from at least one of SiO2, Al2O3, and lithium lanthanum zirconium oxide.

2. The lithium metal negative electrode sheet according to claim 1, wherein: The mass ratio of the polymer matrix to the inorganic filler is 7:(1-3).

3. The lithium metal negative electrode sheet according to claim 1, wherein: The thickness of the composite coating is 1 micron to 100 microns.

4. The lithium metal negative electrode sheet according to claim 1, wherein: The composite coating further comprises a lithium salt and a conductive additive, wherein the lithium salt is selected from lithium bis(trifluoromethane)sulfonyl imide and lithium bis(fluorosulfonyl)imide; and the conductive additive is selected from carbon black and graphene.

5. The lithium metal negative electrode sheet according to claim 1, wherein: The amount of the lithium salt added to the composite coating is 5% to 15% by mass; and / or The added amount of the conductive additive relative to the composite coating is 1% to 5%.

6. The lithium metal negative electrode sheet according to claim 1, wherein: The composite coating is selected from PVDF-HFP / SiO2 composite coating, PEO / LLZO / LiTFSI composite coating, and PAN / Al2O3 / carbon black / LiFSI composite coating.

7. The method for preparing the lithium metal negative electrode sheet according to any one of claims 1 to 6, comprising: Adding the polymer matrix and the inorganic filler into the solvent and mixing them to obtain a composite coating slurry; The composite coating slurry is coated on the surface of the lithium foil, dried and solidified to obtain a lithium metal negative electrode plate with the composite coating on the surface.

8. The preparation method according to claim 7, wherein The solvent is selected from N-methylpyrrolidone, acetonitrile and dimethylformamide.

9. The preparation method according to claim 7, wherein The mixing comprises magnetic stirring at room temperature for 12 hours followed by ultrasonic treatment for 30 minutes; and / or, The drying and curing comprises vacuum drying at 60° C.-100° C. for 6-12 hours.

10. A battery comprising the lithium metal negative electrode sheet according to any one of claims 1 to 6, or the lithium metal negative electrode sheet obtained by the preparation method according to any one of claims 7 to 9.

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