Le Chatelier's principle enables stable and sustainable aqueous sodium/magnesium-ion batteries

Prussian blue analogue (PBA) based aqueous batteries assembled with organic materials are an up-and-coming and promising technology for less demanding applications. By avoiding scarce, costly, and toxic transition metals (e.g. Ni/Co/Cu), the technology may become low-cost, more environmentally benign, and also safer than today's alternatives. Here we rely on a PBA using the FeII/III redox pair together with aqueous low-to-medium concentrated fluorine/perchlorate-free electrolytes and organic materials to create long-term performant cells. The performance in terms of capacity, coulombic efficiency, cell voltage, and energy density are all comparable with previously reported aqueous PBA-based batteries, while the cycling performance is substantially improved by practically implementing Le Chatelier's principle. Additionally, we investigate the redox process(es) and find no proof for any proton storage, but that both Na+ and Mg2+ likely are active, why we classify it as an aqueous Na/Mg-ion battery. We present a new Prussian blue analogue based aqueous battery that by three salts in low-to-medium concentrations in the electrolyte substantially enhances the cycling performance and give promise for low-cost and sustainable energy storage.