Unravelling the electrochemical activation and the reaction mechanism of maricite-NaFePO4 using multimodal operando techniques

Abstract

The electrochemical properties of maricite NaFePO4 can be activated by ball milling with carbon. The origin of such activation is still unclear, as this material does not exhibit apparent open channels for Na+diffusion. Herein, a complementary multi-technique approach is applied to investigate the effect of ball milling on maricite NaFePO4 prepared by hydrothermal synthesis and its electrochemical mechanism. Our findings confirm the partial nano-sizing, amorphisation and oxidation during ball milling, and allow the elucidation of different mechanisms contributing to the total capacity delivered during (de)sodiation. Although only 15% of the capacity is explained by Na+ insertion/extraction of bulk crystalline NaFePO4 maricite, 75% of the total capacity is attributed to simultaneous Fe3+/Fe2+ redox activity. The remaining 25% extra-capacity does not seem to be related to Fe3+/Fe2+ activity, but rather to surface activity, associated with the new species formed during ball milling and electrochemical cycling