Fast Charging Limits of Ideally Stable Metal Anodes in Liquid Electrolytes

TL;DR

This study investigates the maximum safe charging rates for ideally stable sodium metal anodes in liquid electrolytes, revealing that penetration can occur at low currents and depends on separator pore size, emphasizing holistic cell design.

Contribution

It introduces a mathematical model for overpotential related to metal penetration and highlights the importance of combined electrolyte, separator, and anode design for stability.

Findings

Metal penetration occurs at low current densities.

Overpotential depends on separator pore size.

A simple Young-Laplace model describes penetration overpotential.

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 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.