Regulating Bottom-Up Sodium Deposition with a Triple-Gradient Scaffold for High-Capacity and Long-Life Sodium Metal Batteries

Achieving long-term stability of sodium metal anodes at high currents and capacities remains a formidable hurdle for the achievement of practical rechargeable sodium metal batteries. Herein, a triplegradient framework that integrates gradient orientation, conductivity, and sodiophilicity is proposed to regulate bottom-up sodium deposition and enable long-term operation of dendrite-free sodium metal batteries. The triple-gradient framework consists of two essential layers. The top layer is composed of highly oriented and electron-insulating oxidized polyacrylonitrile fibermat, while the bottom layer consists of randomly oriented carbon nanofibers that are electron-conductive and decorated with silver to enhance their sodiophilicity. Finite element simulations demonstrate that the triple-gradient scaffold optimizes the distribution of the electric field and Na+ ions within the skeleton, facilitating preferential "bottom-up" sodium deposition and inhibiting dendrite growth, even at high current densities and capacities. As a result, the triple-gradient framework enables stable sodium plating/stripping for over 1500 h at a current density and capacity of 5 mA cm-2 and 5 mAh cm-2 in symmetrical cells, accompanied by a remarkable cumulative capacity of 3750 mAh cm-2, outperforming that of nongradient, single-gradient, and dual-gradient counterparts. The triple-gradient design offers new opportunities for fabricating high-energy-density metal batteries with improved stability.