  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 emerges as a
critical question to be investigated in systematic experiments under practical
conditions. Here, we use the ideally stable ingot-type sodium metal anode as a
model system to identify the fast-charging limits, i.e. highest safe current
density, of metal anodes. Our 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 we developed as the Young-Laplace overpotential.
Our 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 the penetration-free operation.

Materials Science;Chemical Physics