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 (Na2MPO4F, 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 degrees C, while temperatures as high as 600 degrees C are needed to obtain similar quality phases by solid-state reactions. Moreover, owing to their highly divided character, Na2FePO4F 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 Na2MnPO4F powders, which crystallize in a three-dimensional (3D) tunnel structure rather than in the two-dimensional (2D)-layered structure of Na2FePO4F, were poorly electroactive. Substituting 0.25 Fe for Mn in Na2Fe1-xMnxPO4F 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. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3236480] All rights reserved.