Thickness-Dependent Engineering of Sodiophilic Sputtered (In, Ge, Ti, Ni) Interlayers for Anode-Free Sodium Batteries

Interfacial engineering of current collectors is central to controlling Na nucleation in anode-free Na batteries. Nanometric sputtered metal interlayers (Ti, Ni, Ge, In; 50–500 nm) on stainless-steel collectors are correlated with Na plating/stripping behavior by resolving their crystal structure, strain/amorphousness, and surface chemistry via XRD, Raman spectroscopy, and XPS. Nonalloying Ti and Ni provide only limited nucleation improvement, whereas alloy-forming interlayers reconfigure the Na/current-collector interface. Ultrathin In (∼50 nm) forms a stable Na–In interphase that suppresses nucleation barriers and minimizes voltage hysteresis during extended cycling. Amorphous Ge acts as a thickness-dependent Na-hosting interlayer whose stability can be unlocked through tailored first-plating protocols. Overall, structure- and thickness-dependent design rules are established for sputtered interlayers to engineer sodiophilic interfaces in anode-free Na batteries and related metal-anode systems.