Lithium metal fluorosulfate polymorphs as positive electrodes for Li-ion batteries: synthetic strategies and effect of cation ordering

Transition-metal fluorosulfates are currently being extensively explored for their use as cathodes in Li-ion batteries. Several new polymorphs of LiMSO4F (M Fe, Mn, Zn) crystallizing in the tavorite, triplite and sillimanite structures have captured much recent interest, but synthetic access is limited and the underlying phase stability and ion transport in these materials are poorly understood. Here we report that solvothermal routes to LiMSO4F (M Fe, Mn, Zn) offer significant advantage over both exotic ionothermal methods and solid state synthesis by enabling greater control of the chemistry. We show new limits for the onset of triplite crystallization, and report new phases in the Li[Fe,Zn]SO4F system that enable a fuller understanding of the complex chemistry and thermodynamics underlying these fascinating materials. The transformation of LiFeSO4F from the tavorite to the triplite polymorph is triggered in the absence of any substituents, proving that tavorite is an intermediate in the reaction pathway. As a result of structural changes between tavorite and triplite, their Li+ transport paths are quite different. Combined X-ray/neutron diffraction studies of the triplites suggest that distinct inter-site zig-zag paths must be involved, owing to complete cation disorder that impacts the electrochemical behavior.