Unraveling the Transition from Alluaudite to Triphylite Phases during LiFePO4 Hydrothermal Synthesis

Abstract

Lithium iron phosphate (LFP) and its derivatives are an extremely promising class of cathode materials for lithium-ion batteries with an ever-expanding range of applications. The development of improved, cost-effective methods to synthesize this class of materials is a challenging task, and in this work, we explored the synthesis of LiFePO4 under hydrothermal conditions without using the traditional three-fold lithium excess. By partially replacing LiOH with NaOH, we were able to synthesize single-phase LFP demonstrating good electrochemical performance. Studying the synthesis stages, we identified an unexpected alluaudite-triphylite phase transformation. A careful examination of this intermediate phase through powder X-ray diffraction, Mössbauer spectroscopy, Fourier-transform infrared spectroscopy, and inductively coupled plasma atomic emission spectroscopy brought us to discover a new sodium iron bis(hydrogen phosphate) phosphate — Na0.7Fe3(HPO4)2(PO4) — that forms at the first stage of the synthesis. Further hydrothermal treatment facilitates Na→Li exchange, and instability of the alluaudite-type framework with high Li content results in the formation of the triphylite LiFePO4 phase.