Structural and Electrochemical Investigation of Fluoro(hydroxy)phosphate Based Cathode Materials for Secondary Batteries

Abstract

Structural and electrochemical investigation of LiFePO4OH hydroxyphosphate material. The synthesis and structure of this material is explained. The hydrothermally synthesized material crystallizes in triclinic structure with P-1 space group. The electrochemistry of this material was revisited in both aqueous and organic electrolytes. In organic commercial electrolyte, a stable discharge capacity of 140 mAh g-1 up to 60 cycles at a current density of 1 mA cm-2 was obtained making it the best-reported capacity for this material until date. Further, the material was tested in 21m LiTFSi + 7m LiOTf aqueous electrolyte in the potential window -1.0 to 1.0 V vs. Ag/AgCl reference electrode. It delivered a discharge capacity of 153 mAh g-1 with good cyclability. Notably, LiFePO4OH, which is reported as a cathode for LIBs in organic electrolyte, worked in the anodic range for aqueous LIBs. This is first time an iron based tavorite material was reported as an anode for aqueous LIBs. An electrochemical model was also developed to improve the performance of the material by employing various geometrical and kinetic parameters. It was further used to study the commercial aspects of the material. Though the redox potential of LiFePO4OH (2.4-2.6 V) is low as compared to commercial LiFePO4 (3.5 V) cathode, but it can still be a good option for stationary storage applications where high energy density is often not required. The possible future directions to explore fluoro(hydroxy)phosphate materials for electrochemical applications.