Horizontal Sodium Growth via the Sodiophilicity-Driven Structural Design of Current Collectors for Anode-Free Sodium Metal Batteries.

Sodium metal batteries offer high energy densities but suffer from dendrite formation, which causes capacity loss and short circuits. The present study achieves horizontal sodium growth and stripping by employing a periodic pattern of pyrolytic carbon pillars with vertical facets coated with gold on a copper current collector. The sodiophilic gold layer reduces the energy barrier for sodium nucleation, thereby facilitating preferential sodium nucleation. The regularly spaced carbon pillars with gold-coated sidewalls act as nucleation sites, directing sodium to deposit laterally while maintaining a predominantly horizontal trajectory. Alloys of sodium and gold on the pillar sidewalls induce horizontal expansion of sodium fronts that merge with those from neighboring pillars. During stripping, sodium progressively retracts toward the pillars from these merged boundaries. Electrochemical sodiation and desodiation behaviors in the coexistence of carbon, gold, and copper, influencing subsequent sodium plating, were elucidated by cyclic voltammetry. Copper current collectors with 10-20 μm-high, gold-coated carbon pillars achieved stable half-cell cycling for 1000 cycles at 5 mA cm. The horizontal sodium growth and stripping enabled the high-power and safe operation of anode-free sodium metal full-cells at ultrahigh critical current densities (>3.0 mA cm).