Anodes and Sodium-Free Cathodes in Sodium Ion Batteries

The increase in electricity generation poses growing demands on energy storage systems, thus offering a chance for the success of the reliable and cost-effective energy storage technologies. Sodium ion batteries are emerging as such a technology, which is however not yet mature enough to enter the market. At the crux of building practical sodium ion batteries is the development of electrode materials that promise sufficient cost- and performance-competitiveness. As such, herein, all typical sodium storage materials are discussed, considering their fabrication methods and sodiation mechanisms in detail. A comprehensive cross-literature and cross-material comparison, which also includes the related thermodynamic analysis of their sodiation products, is also provided. The review focusses particularly on anodes and sodium-free cathodes, as they both play the role of the acceptor rather than the donor of sodium ions in their operation in batteries; their difference lies in the (de-)sodiation voltage. In the discussion, special attention is paid to contradictory observations and interpretations in contemporary sodium ion battery research, since debates on these controversies are likely to fuel future sodium battery research.