Nanoscale sputtered buffer layers for <i>in situ</i> fabrication of thin sodium metal anodes

Abstract Sodium (Na) metal is a highly attractive anode material for next-generation batteries due to its natural abundance and low cost, but its practical use is limited by the poor reversibility of the Na plating/stripping process and instability during cycling. Herein, nanoscale metallic films (Ti, Ni, Ge and In) with different thicknesses are grown by magnetron sputtering to coat commercial stainless steel 316L current collectors, and the substrate-dependent Na plating/stripping characteristics are thoroughly explored. The results reveal that sodiophilic interphases can be achieved by in situ formation of M–Na (M = Ge, In) alloys. A controlled protocol is developed to electrochemically form ultrathin Ge–Na or In–Na alloy buffer layers in the first cycle prior to Na plating, serving as pillars for a stable subsequent Na metal deposition and boosting the buildup of a highly efficient thin Na metal anode. Among the tested materials, 50 nm thick In coatings exhibit the most stable long-term plating/stripping process. These findings demonstrate a simple and effective interfacial engineering strategy to enhance the performance of Na metal anodes, providing a pathway toward safer and long-lasting sodium metal batteries.