Thickness-Dependent Beneficial Effect of the ZnO Layer on Tailoring the Li/Li₇La₃Zr₂O₁₂ Interface

Li₇La₃Zr₂O₁₂ (LLZO)-based ceramics are well-known as the most promising solid electrolytes for all-solid-state lithium metal batteries. However, its practical application has been significantly hindered by high Li/LLZO interfacial impedance as a result of poor interfacial contact. To solve these issues, in this work, the ZnO layer was magnetron sputter-deposited on Li_6.55La_2.95Ca_0.05Zr_1.5Ta_0.5O₁₂ (LLCZTO) pellets. It was found that by introducing a 200 nm thick ZnO layer, the interfacial area specific resistance was sharply reduced to as low as 1% that of pristine LLCZTO; meanwhile, Li plating/stripping performance was improved significantly with a long life span of 320 h and a low polarization potential of 0.1 V, whereas a thicker ZnO layer of 600 nm can only improve the interface contact to a very limited extent because of the accumulated volume expansion induced by the in situ transformation of ZnO to the Li-Zn alloy, demonstrating the thickness-dependent beneficial effect of the ZnO layer on improving the Li/LLCZTO interfacial contact and therefore reducing the interfacial resistance. Accordingly, the evolution of the interfacial contact mode and the Li⁺ migration mechanism during the Li plating/stripping process without or with ZnO layers of different thicknesses were discussed in detail.