Dendrite-free alkali metal electrodeposition from contact-ion-pair state induced by mixing alkaline earth cation

Alkali metals are expected to be used for rechargeable metal anode batteries owing to their low electrode potentials and large capacities. However, they face the well-known fatal problem of "dendritic growth" while charging. Here, we present a detailed investigation on electrolytes where alkaline earth salts are introduced to inhibit dendrite growth in alkali metal electrodeposition. Specifically, focusing on CaTFSA2 as an exemplary additive, we reveal that dendrite-free morphology upon alkali metal electrodeposition can be attained by modifying the solvation structures in dual-cation electrolytes. Addition of Ca2+ promotes alkali cation (Li+ or Na+) to form the contact ion pairs (CIPs) with the counter anions, which replaces the solvent-separated ion pairs that commonly exist in single-cation electrolytes. The strong binding of the CIPs slows the desolvation kinetics of alkali cations and, consequently, realizes a severely constrained alkali metal electrodeposition in a reaction-limited process that is required for the dendrite-free morphology.