Melt Synthesis of LiFePO4: Fundamentals, Versatility and Application

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With its good thermal stability, abundance in nature and benign environmental impact, LiFePO4 (LFP) cathode material is seen as one of the most promising candidate for the next generation of Li-ion batteries. Many synthetic routes have already been used for preparing LFP material, best known being solid state, sol-gel, hydrothermal, co-precipitation and microwave preparations. All of them are performed at moderate temperature and/or in a wide range of pressures. Melt process of LFP, advanced by Gauthier et al. in 2003[1], operates in the liquid phase above 1000⁰C and then benefits from increased reaction kinetics and from the thermodynamic stability of LFP in a mild reducing atmosphere. The melt synthesis allows the use of a wide range of simple raw materials as well as a possible purification strategy during the melt step or upon solidification, potentially enabling for usage of less pure non-expensive raw materials. In this work, thermodynamic considerations are first addressed for deep understanding of the Li-Fe-P-O systems. A model has recently been developed and is supported by experimental data. On this basis, recent experimental observations and progress on raw material selection (including direct use of iron ore concentrates) and systems will be reported. Eventually, melt-synthesis conditions and the latest results in controlling the purification of LiFePO4 from major phase impurities will also be covered. All these considerations allow the selection of the best conditions to prepare a high purity LiFePO4 by melt-process. This work is part of an Automotive Partnership of Canada supported program to develop and pilot the molten-synthesis process to make high purity C-LiFePO4with excellent electrochemical properties for using as a cathode material in Li-ion batteries for EVs and PHEVs application. [1] M. Gauthier, L. Gauthier, D. Lavoie, C. Michot, N. Ravet, US Patent 7,534,408 B2. (2003).