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Lithium metal penetrations through the liquid-electrolyte-wetted porous separator and solid electrolytes are a major safety concern of next-generation rechargeable metal batteries. Penetrations were frequently discovered to occur through only a few isolated channels, as revealed by “black spots” on both sides of the separator or electrolyte, which manifest a highly localized ionic flux or current density. Predictions of the penetration time have been difficult due to the hidden and unclear dynamics in these penetration channels. Here, using glass capillary cells, we investigate for the first time the unexpectedly sensitive influence of channel geometry on the concentration polarization and dendrite initiation processes. The characteristic time for the complete depletion of salt concentration at the surface of the advancing electrode, i.e. Sand's time, exhibits a nonlinear dependence on the curvature of the channel walls along the axial direction. While a positively deviated Sand's time scaling exponent can be used to infer a converging penetration area through the electrolyte, a negatively deviated scaling exponent suggests that diffusion limitations can be avoided in expanding channels, such that the fast-advancing tip-growing dendrites will not be initiated. The safety design of rechargeable metal batteries will benefit from considering the true local current densities and the conduction structures. 

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.

 

Rechargeable alkali metal anodes hold the promise to significantly increase the energy density of current battery technologies. But they are plagued by dendritic growths and solid‐electrolyte interphase (SEI) layers that undermine the battery safety and cycle life. Here, a non‐porous ingot‐type sodium (Na) metal growth with self‐modulated shiny‐smooth interfaces is reported for the first time. The Na metal anode can be cycled reversibly, without forming whiskers, mosses, gas bubbles, or disconnected metal particles that are usually observed in other studies. The ideal interfacial stability confirmed in the microcapillary cells is the key to enable anode‐free Na metal full cells with a capacity retention rate of 99.93% per cycle, superior to available anode‐free Na and Li batteries using liquid electrolytes. Contradictory to the common beliefs established around alkali metal anodes, there is no repeated SEI formation on or within the sodium anode, supported by the X‐ray photoelectron spectroscopy elemental depth profile analyses, electrochemical impedance spectroscopy diagnosis, and microscopic imaging.