Electronic structure regulation inducing robust solid electrolyte interphase for stable anode-free sodium metal batteries

Abstract

Anode-free sodium metal batteries (AFSMBs) have gained attention as next-generation storage systems with high energy density and cost-effectiveness. However, non-uniform sodium (Na) deposition and an unsteady solid electrolyte interphase (SEI) lead to dendrite-related issues and severe irreversible Na⁺ plating/stripping, greatly aggravating their cycle deterioration. In this study, we effectively modified the 3D current collector's electronic structure by introducing Zn-N_x active sites (Zn-CNF), which enhances lateral Na⁺ diffusion and the Na planar growth, enabling uniform deep Na deposition at an exceptionally high capacity of 10 ​mA ​h ​cm⁻². Furthermore, the Zn-N_x bonds enhance the adsorption capacity of PF₆⁻ and contribute to forming a stable inorganic-rich SEI layer. Consequently, Zn-CNF with the electronic structure regulation endows an ultra-low nucleation overpotential (8 ​mV) and ultra-high Coulombic efficiency of 99.94% over 1,600 cycles. Symmetric cells demonstrate stable Na⁺ plating/stripping behavior for more than 4,400 ​h at 1 ​mA ​cm⁻². Moreover, under high cathode loading (7.97 ​mg ​cm⁻²), the AFSMBs achieve a high energy density of 374 ​W h kg⁻¹ and retain a high discharge capacity of 82.49 ​mA ​h ​g⁻¹ with a capacity retention of 80.4% after 120 cycles. This work proposes a viable strategy to achieving high-energy-density AFSMBs.