Beyond Li-ion: electrode materials for sodium- and magnesium-ion batteries

The need for economical and sustainable energy storage drives battery research today. While Li-ion batteries are the most mature technology, scalable electrochemical energy storage applications benefit from reductions in cost and improved safety. Sodium- and magnesium-ion batteries are two technologies that may prove to be viable alternatives. Both metals are cheaper and more abundant than Li, and have better safety characteristics, while divalent magnesium has the added bonus of passing twice as much charge per atom. On the other hand, both are still emerging fields of research with challenges to overcome. For example, electrodes incorporating Na+ are often pulverized under the repeated strain of shuttling the relatively large ion, while insertion and transport of Mg2+ is often kinetically slow, which stems from larger electrostatic forces. This review provides an overview of cathode and anode materials for sodium-ion batteries, and a comprehensive summary of research on cathodes for magnesium-ion batteries. In addition, several common experimental discrepancies in the literature are addressed, noting the additional constraints placed on magnesium electrochemistry. Lastly, promising strategies for future study are highlighted.