Fast Charging Limits of Ideally Stable Metal Anodes in Liquid Electrolytes

Abstract Next‐generation high‐energy‐density batteries require ideally stable metal anodes, for which smooth metal deposits during battery recharging are considered a sign of interfacial stability that can ensure high efficiency and long cycle life. With the recent successes, whether the absolute morphological stability guarantees absolute electrochemical stability and safety has emerged as a critical question to be investigated in systematic experiments under practical conditions. Here, the ideally stable ingot‐type sodium metal anode is used as a model system to identify the fast‐charging limits, that is, highest safe current density, of metal anodes. The results show that metal penetration can still occur at relatively low current densities, but the overpotentials at the penetration depend on the pore sizes of the separators and surprisingly follow a simple mathematical model developed as the Young–Laplace overpotential. This study suggests that the success of stable metal batteries with even the ideally smooth metal anode requires the holistic design of the electrolyte, separator, and metal anodes to ensure penetration‐free operation.