Unveiling the effect of alloy-type coating for lithium metal plating and stripping on anode-free lithium metal batteries

Li dendrite formation is the root cause of capacity fading in anode-free lithium metal batteries (AFLMBs). Previous studies have shown that Au coating on Cu can seed Li nucleation by forming Li3Au for smooth Li plating and improves the cycle life of Li||Cu cells. However, unlike Li||Cu cells with excessive Li supply, in practical AFLMBs (i.e., full-cell), the active lithium is limited from the lithium intercalation cathode. The introduction of alloy-type coatings may adversely affect the cycling stability due to the pulverization effect during alloying with Li (e.g., the volume expansion rate from Au to Li3Au is 337%). Here, we evaluate the full-cell performance of AFLMBs focusing on different thicknesses of Au coating (20, 50, and 100 nm) and four coating layers (alloy-type: Au, Ag; non-alloy-type: Ni, Ti). Our results show that although the alloy-type coatings (i.e., Au and Ag) lower the lithiation overpotential, the full-cells equipped with these coatings experience faster capacity fading than those with pristine and non-alloy-type coatings. When the Au-coated Cu is paired with high-mass loading NMC622 cathode, the full-cells AFLMBs exhibit inferior performance with a low initial Coulombic efficiency (89.97% and 79.75% for 20 and 100 nm thick Au coating) and accelerated capacity fading, producing plated Li with porous structure and carbonyl-rich solid electrolyte interphase. This study underscores the necessity of employing a full-cell configuration for evaluating Li plating-stripping efficacy in AFLMBs, emphasizing a transition from laboratory research to practical applications.