Origin of Faster Capacity Fade for Lower Electrolyte Amounts in Lithium Metal Batteries: Electrolyte “Dry‐Out”?

In lithium metal batteries, the cycle life relevantly declines with decreasing electrolyte amount. The capacity decay is kinetically reasoned as shown by rises in cell resistances, in particular for the discharge processes, as indicated by the full capacity recovery during a constant voltage step after discharge at the end of life (EOL). Interestingly, adding fresh electrolyte after EOL only partially recovers the capacity, suggesting a different and more crucial failure origin than the assumed loss of charge carriers due to the electrolyte “dry‐out”. Contrary to the cathode, the anode has higher resistances and a thicker surface layer post mortem, which is also observed in Li‖Li cells. In addition, the resistance portion of the electrolyte itself remains comparatively low during cycling, suggesting that resistance rise is dominated by the Li anode and is confirmed by exchange with fresh Li, where the capacities are recovered toward initial values, again. Based on the observations, a mechanism with a faster dry‐out of Li metal pores is proposed, which decreases the electrolyte‐accessible Li metal surface area, enhances local current densities, and facilitates high surface area and dead lithium. This continuously clogs and blocks the surface, reducing the practical accessible Li and eventually causing the rollover fade.