New perspectives on the multianion approach to adapt electrode materials for lithium and post-lithium batteries.

Developing new and sustainable batteries is essential for modern society. Both cationic doping ( e.g. transition metals) and anionic doping (F - , O 2- , S 2- , PO 4 3- , etc.) can be employed to improve the electrochemical behaviour of electrode materials. Herein, the anion-doping, or multianion approach, is comprehensively reviewed and investigated. It is observed that the optimized compositions of some electrode materials can involve metastable or transient states. Going beyond the inductive effect, we propose that the simultaneous use of several kinds of anions in the framework of the same host material could create a sort of energized state in the electrode material, and this could be interpreted within the "entatic state" principle which is used in coordination chemistry, catalysis, and bioinorganic chemistry. In this new hypothesis, the coordination of a cation by several anions with different properties can modify the local structure and it could enhance the electrochemical performance; this could be particularly useful for the future post-lithium batteries. The multianion approach can also be applied to electrolytes and interfaces. Finally, new theoretical calculations on multianion-based materials (carbonophosphates, thiocarbonates and Mg 8 Mn 16 O 32- z F z ) are reported here, highlighting the need for further research in the coordination sphere of metal-ligand systems for tuning battery properties.