Polyanionic Insertion Materials for Sodium-Ion Batteries

Efficient energy storage is a driving factor propelling myriads of mobile electronics, electric vehicles and stationary electric grid storage. Li-ion batteries have realized these goals in a commercially viable manner with ever increasing penetration to different technology sectors across the globe. While these electronic devices are more evident and appealing to consumers, there has been a growing concern for micro-to-mega grid storage systems. Overall, the modern world demands energy in terawatt' scale. It needs a multipronged approach with alternate technologies complementing the Li-ion batteries. One such viable approach is to design and implement Na-ion batteries. With the uniform geographical distribution, abundance and materials economy of Na resources as well as a striking operational similarity to Li-ion batteries, Na-ion batteries have commercial potential, particularly for applications unrestricted by volumetric/gravimetric energy density. In pursuit of the development of Na-ion batteries, suites of oxides, sulfides, fluorides, and polyanionic materials have been reported in addition to several organic complexes. This article gives an overview of recent progress in polyanionic framework compounds, with emphasis on high-voltage candidates consisting of earth abundant elements. Guided by ternary phase diagrams, recently discovered and potential cathode candidates will be discussed gauging their performance, current status, and future perspectives.