Unveiling the structure, magnetic, and electrochemical redox mechanism of tavorite LiFePO$_4$OH hydroxyphosphate lithium insertion cathode

Affordable, non-critical metal-enriched stable polyanionic cathode chemistries are appealing for scalable energy storage applications. Tavorite LiFePO$_4$OH 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 LiFePO$_4$OH 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 Fe$^{3+}$/Fe$^{2+}$ redox potential centered ∼2.5 V (vs. Li/Li$^+$). LiFePO$_4$OH exhibits a reversible biphasic transformation involving the evolution of metastable and partially amorphous Li$_2$FePO$_4$OH with local distortions around Fe atoms upon the first discharge. ... mehrA 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 LiFePO$_4$OH enriches the critical-metal free Fe-based cathode database for Li-ion batteries.