Single atom activated multi-stage active sites for thoroughgoing sodium utilization

Atomically dispersed metals offer advantages in guiding sodium deposition, yet the effects of single atoms on their surrounding structures and the precise tuning of coordination-governed single-atom activity remain underexplored. Herein, carbon nanofiber films with tin single atoms anchored via a dynamic coordination mode (shifting from coordination with three nitrogen atoms and one oxygen atom to coordination with one nitrogen atom and three oxygen atoms) are developed to address these challenges. The tin atoms not only enhance the sodium-ion adsorption activity of their directly coordinated nitrogen and oxygen atoms but also activate remote carbon atoms. The activation capability is strongly dependent on coordination environment, with tin atoms coordinated to more nitrogen atoms exhibiting higher activity. As a result, the optimized tin-carbon host enables uniform sodium deposition and complete stripping, allowing symmetric cells to cycle stably for 1200 h at 100 mA cm^-2 and 100 mAh cm^-2 with 100% depth of discharge. Anode-free full cells pairing the tin-carbon host with a sodium vanadate phosphate cathode achieve 94% capacity retention after 700 cycles at 10 C (6 min).