Directing High-Efficiency Na Plating with Carbon–Aluminum Junction Interfaces for Anode-Free Na Metal Batteries

Anode-free sodium metal batteries are highly promising for future energy storage but suffer from much faster cycling degradation as they are sensitive to even trace levels of irreversible side reactions. This work focuses on the most practical Al foil current collectors and systematically examined the effect of nanosized carbon coating on improving the Na plating and stripping stability. We identified that the carbon–Al junction interface generated by carbon coating enabled more uniform Na deposition with lower overpotentials, delivering higher than 99.8% Faradaic efficiencies for a wide range of cycling currents between 0.5 and 3.0 mA cm–2. This performance is much better than the 96.4% efficiency observed on uncoated Al foils under the same conditions and is also confirmed under lean electrolyte and freezing electrolyte conditions and can be attributed to the stronger interfacial binding and enhanced sodiophilic properties of the carbon–aluminum junction sites. These sites not only ensure uniform Na plating but also eliminates side reactions that would otherwise cause electrolyte depletion. As a result, Na-metal free full cells assembled with a high capacity Na3V2(PO4)3 cathode delivered ∼93% capacity retention for 100 cycles, higher than the ∼42% of retention of uncoated Al foil.