Polyanionic Solid-Solution Cathodes for Rechargeable Batteries

As an efficient rechargeable power source, lithium-ion batteries have dominated the consumer electronics and electric vehicle markets. Their analogues, sodium-ion batteries, are emerging as promising low-cost candidates for grid electricity storage. In the pursuit of better cathode materials for Li/Na-ion batteries, major design strategies have involved cation engineering, as represented by studies on mixed transition metals and/or Li/Na-rich layered oxides. Instead of focusing on the cation, here, we report an effective anion engineering strategy that allowed us to identify a surprisingly wide range of polyanionic complete solid solutions of alluaudite-type NaxFey(PO4)3–z(SO4)z (0 ≤ z ≤ 3). These materials exhibited tunable voltage and superior capacities, depending on the mixing ratio of the two polyanions. The existence of a continuous, wide compositional domain in a mixed-polyanion system will significantly enrich materials design strategies and contribute to the development of new cathode materials for Li/Na-ion batteries.