Ionic Liquid Redox Catholyte for High Energy Efficiency, Low‐Cost Energy Storage

Abstract

An approach to energy storage using ionic liquids as joint ion‐conducting medium and redox active catholyte material is described. The earth‐abundant ferric ion is incorporated as an oxidizing agent in the form of the low‐melting NaFeCl4 in a 1:1 mixture with ethylmethylimidazolium tetrachloraluminate, an ambient temperature ionic liquid. Different possible anode types are considered, and the most obvious one involving liquid sodium (with special wetting of a sodium ion‐conducting ceramic separator) is tested. The high voltage >3.2 V predicted for this cell is verified, and its cyclability is confirmed. Operating at 180 °C, an unexpectedly high energy efficiency >96%, is recorded. This establishes this type of cell as an attractive candidate for energy storage. For optimum energy storage, high energy efficiency is mandated for thermal management, as well as economic reasons. The theoretical capacity of the cell is 288 Wh kg−1 (418 Wh L−1) of which 73% is realized. The cell is shown to be fail‐safe against internal shorts. As there are many degrees of freedom for developing this type of cell, it is suggested as a promising area of future research effort in the energy storage area.