Iodine-Based Electrolyte Chemistry Enabling Reversible Ca Metal Anodes

Electrolyte chemistry is of paramount importance for tackling the challenge of irreversible Ca deposition/stripping caused by ionic-insulating solid electrolyte interphases (SEIs). Current research has been mainly concentrating on the boron center-based electrolytes despite their complex synthetic procedure and leaves aside others because of a virtually inhibited electrochemical response. Herein, we report a kind of iodine-based electrolytes comprising CaI₂ salt paired with auxiliary iodides, in which the latter elevates the I^- concentration to reconfigure electrical double-layer structures of a low-solubility CaI₂ electrolyte, thus accelerating Ca²⁺ desolvation and Ca²⁺ diffusion across SEI. Consequently, the optimized iodine electrolytes enable a high average Coulombic efficiency of 96.5% under 0.5 mAh cm^-2 and a decent Ca reversibility at a large current density of 1.5 mA cm^-2, showing competitive or even better performance than boron-based counterparts. As a proof of concept, full cells are demonstrated by coupling Ca metal anodes with an organic cathode, yielding an average output voltage of ∼2.1 V with outstanding stability for over 250 cycles. These findings expand the realm of Ca electrolyte chemistry, constituting a vital step in the development of efficient Ca systems.