Unveiling the structure, magnetic, and electrochemical redox mechanism of tavorite LiFePO₄OH hydroxyphosphate lithium insertion cathode

Abstract

Affordable, non-critical metal-enriched stable polyanionic cathode chemistries are appealing for scalable energy storage applications. Tavorite LiFePO₄OH forms one such environmentally benign cathode for Li-ion batteries. Optimizing their electrochemical kinetics and cycling performance relies on elucidating the interplay between structural transformation and electrochemical redox activity during battery operation. In this study revisiting LiFePO₄OH hydroxyphosphate, we elaborate the crystal/magnetic structure and the mechanistic origin of structural transitions encountered during electrochemical cycling. Low-temperature magnetometry, Mössbauer/Raman spectroscopy and neutron diffraction revealed a long-range G-type antiferromagnetic ordering with propagation vector k = (1/2, 0, 1/2). Upon cathode optimization, it delivered the highest discharge capacity ∼133 mAh g⁻¹ with excellent cycling stability involving an Fe³⁺/Fe²⁺ redox potential centered ∼2.5 V (vs. Li/Li⁺). LiFePO₄OH exhibits a reversible biphasic transformation involving the evolution of metastable and partially amorphous Li₂FePO₄OH with local distortions around Fe atoms upon the first discharge. A permanent structural reconstruction driven by local Li rearrangements during the first cycle was confirmed by in situ and ex situ diffraction and Mössbauer spectroscopy. Tavorite-type LiFePO₄OH enriches the critical-metal free Fe-based cathode database for Li-ion batteries.