Unusual Activation of Cation Disordering by Li/Fe Rearrangement in Triplite LiFeSO₄F

It is reported that cation disordering in triplite LiFeSO₄F can be activated by Li/Fe rearrangement that results from irreversible and nondestructive structural changes during the 1st charge/discharge cycle, especially during the charge. This rearrangement decreases the number of edge-shared FeO₄F₂ connection environments, compared to the pristine material. With this activation, triplite LiFeSO₄F exhibits several unexpected electrochemical features in subsequent cycles; a decrease in open-circuit voltage indicating the change in thermodynamic property, negligible volumetric change, enhanced Li diffusion, and facile phase transformation pathway. As a consequence, the cation-disordered triplite LiFeSO₄F achieves superior rate capability up to ≈66 mA h g⁻¹ at 40 C rate (1.5 min discharge) and has excellent capacity retention for 500 cycles at 5 C charge/5 C discharge rate and for 1200 cycles at 2 C charge/2 C discharge rate. Therefore, triplite LiFeSO₄F can be one of the most promising electrode materials for Li ion batteries.