Ionothermal synthesis of sodium-based fluorophosphate cathode materials

Owing to cost and abundance considerations, Na-based electrode materials are regaining interest, especially those that can be prepared at low temperatures. Here, we report the low temperature synthesis of highly divided Na-based fluorophosphates (Na2 MP O4 F, M = Fe, Mn, or mixtures) in ionic liquid media. We show that this ionothermal approach enables the synthesis of these phases at temperatures as low as 270°C, while temperatures as high as 600°C are needed to obtain similar quality phases by solid-state reactions. Moreover, owing to their highly divided character, Na2 FeP O4 F powders made via such a process show better electrochemical performances vs either Li or Na than their ceramic counterparts. In contrast, regardless of how they were made, the Na2 MnP O 4 F powders, which crystallize in a three-dimensional (3D) tunnel structure rather than in the two-dimensional (2D)-layered structure of Na 2 FeP O4 F, were poorly electroactive. Substituting 0.25 Fe for Mn in Na2 Fe1-x Mnx P O4 F is sufficient to trigger a 2D-3D structural transition and leads to a rapid decay of the materials electrochemical performances. A tentative explanation, based on structural considerations to account for such behavior, is given in this paper. © 2009 The Electrochemical Society.