# Iodine-Based Electrolyte Chemistry Enabling Reversible Ca Metal Anodes

## TL;DR

This paper introduces iodine-based electrolytes that improve the performance of calcium metal anodes in batteries.

## Contribution

The study introduces iodine-based electrolytes as a novel alternative to boron-based ones for calcium metal anodes.

## Key findings

- Iodine-based electrolytes achieved a high average Coulombic efficiency of 96.5%.
- The electrolytes showed decent calcium reversibility at a high current density of 1.5 mA cm–2.
- Full cells using these electrolytes delivered a stable output voltage of ∼2.1 V over 250 cycles.

## Abstract

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.