Redox-Free Ion Insertion: Discovery of Electride Materials for Electron-Anion Exchange

In order to maintain charge neutrality, ion insertion is accompanied by reduction or oxidation of the host lattice. However, a class of materials called electrides, where vacant crystallographic lattice sites are instead occupied by a bare anionic electron, exhibits a fundamentally different mechanism. The mechanism for anion insertion into electrides—which we have named Electron-Anion Exchange (EAX)—allows for the one-to-one swap of anions and electrons as the anion hops between “vacant” electride sites. Because the participating anionic electrons are not associated with any atomic nuclei, the surrounding atoms do not experience any reduction or oxidation throughout the insertion process. The consequences of redox-free EAX are substantial. Due to the similar size and charge of anionic electrons and simple anions, the host lattice experiences virtually no electrostatic change during ion insertion and the reorganization energy becomes negligible. In order to fully exploit the effects of EAX, including little to no lattice expansion upon intercalation and exceptionally low activation energies for ion diffusion, we have designed and synthesized a novel electride material, Y₆S₄. Similar to previously known electrides such as Y₂C, Y₆S₄ has a layered structure with alternating layers of anionic electrons and host material. However, Y₆S₄ contains S pillars throughout the anionic electron layer that increase the interlayer spacing, causing the overlap with neighboring cations to be small. We predict the “pillared layered” structure of Y₆S₄ to have not only a larger capacity, but also a significantly higher ionic conductivity than its layered electride analogs. We therefore anticipate Y₆S₄ to be one of the most efficient anion intercalation materials to date—with natural applications as an electrode in anion shuttle batteries or electrochemical capacitors.