关键词: LiF, Sn, 真空热蒸镀, 临界电流密度, 长循环性能

Abstract:

The growing demands for electric vehicles and consumer electronics，as well as the expanding renewable energy storage market，have promoted extensive research on energy storage technologies with low cost，high energy density and safety. Lithium (Li) metal and solid-state electrolytes are considered as important components for next-generation batteries because of their great potential for improvements in energy density and safety performance. Inorganic garnet-type solid electrolytes with high Li-ion conductivity (about 10^-3 S·cm^-1) and high shear modulus (55 GPa) are considered to be ideal solid-state electrolytes，however，the issue of Li dendrite growth still obstructs their practical application. Herein，a simple and efficient strategy was developed to suppress the Li dendrite formation in the garnet solid electrolytes. A composite modification layer made of 2 nm LiF and 2 nm Sn thin layers was prepared on the surface of the Li_6.5La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) solid electrolyte by the high vacuum evaporation. The composite modification layer combined the advantages of LiF and Sn，which effectively improves the interfacial contact between the Li metal and LLZTO electrolyte，and promotes the uniform Li plating/stripping. The LiF-Sn composite modification layer was deposited on the surface of garnet electrolyte to increase the interfacial wettability between the garnet electrolyte and Li metal，which blocks the injection of electrons into the bulk phase of garnet. The LiF-Sn modification layer effectively enhanced the interfacial contact and inhibited the growth of lithium dendrites. Benefiting from the LiF-Sn interfacial modification，the cross-sectional SEM image shows the intimate contact between the LLZTO-LiF-Sn and the Li metal without any voids. In addition，the interfacial impedance of Li/garnet electrolyte interface decreased from 969 Ω·cm² to 3.5 Ω·cm². Meanwhile, the critical current density of the Li symmetric cell increased to 1.3 mA·cm^-2, and the Li symmetric cell could be cycled stably for 200 h at a current density of 0.4 mA·cm^-2. After disassembling the short-circuited Li/LLZTO/Li cell and reacting the Li metal with alcohol solution，it was found that Li dendrites had grown into the LLZTO pellet. However，the surface of the LiF-Sn-protected LLZTO remained smooth without dark spots from dendrites. The excellent electrochemical performance clearly shows that the LiF-Sn composite modification can effectively inhibit the formation of Li dendrite inside the garnet SSE, proving that this interfacial engineering provides a practical solution for addressing the key challenge of Li/LLZTO interface. At the same time，high vacuum evaporation is a matured industrial technology with large-scale application prospects and can be widely used to solve solid-state interface problems.

Key words: LiF, Sn, Vacuum thermal evaporation, Critical current density, Long cycle performance