N,O Co-Doped Carbon Spheres Enable Stable Anode-Less Sodium Metal Batteries

Abstract

Anode-less sodium metal batteries (SMBs) suffer from the formation of Na dendrites and inactive Na on an anode substrate though showing advantages of high energy densities and low costs. Herein, N,O co-doped carbon spheres (NOCS), which are synthesized via a scalable polymerization and pyrolysis method, are employed as a thin and stable sodiophillic nucleation layer on the Cu foil. Combined with electrochemical measurements, Na deposition morphology observations and density functional theory calculations, it is revealed that the introduced N and O heteroatoms can greatly enhance the adsorption of Na⁺ on the carbon substrate and reduce the nucleation overpotential, thus forming sufficient seeding sites and guiding homogeneous Na deposition. Consequently, the NOCS coated Cu electrode achieves the outstanding reversibility of Na plating/stripping process over 1000 cycles at 2 mA cm⁻² with 2 mAh cm⁻² in asymmetric cells, as well as over 1000 h at 0.5 mA cm⁻² with 1 mAh cm⁻² in symmetric cells. Moreover, this modified Cu foil enables the anode-less full-cell with a high-loading Na₃V₂(PO₄)₃ cathode to deliver a high initial capacity of 103 mAh g⁻¹ with a capacity retention of 79% after 350 cycles at 200 mA g⁻¹, demonstrating the pave to the practical anode-less SMBs.