Materials in the Li2-xMPO4F family, in which the M sites are occupied by a transition metal (M = Fe, Co, Ni, etc.), have been proposed as alternative cathodes to replace traditional cathode materials, such as LiCoO2. Depending on synthesis pathways, materials in this family can take three distinct structures, one in the triclinic space group P1 , and two with orthogonal space groups, Pbcn and Pnma [3,4], all of which permit multi-dimensional lithium transport. This, in combination with the wide range of transition metals that may occupy or be doped into the M sites, make the family potentially rich with cathode structures that operate within a desired voltage range. In addition, materials within this family are interesting due to the presence of two lithium ions per formula unit, compared to the single lithium ion present per formula unit in the olivine LiMPO4. However, extracting lithium ions from the standard Li2FePO4F past LiFePO4F requires a voltage of roughly 5 V, regardless of the polymorph . By changing the redox active transition metal species, the operating voltage for lithium extraction can be tuned.
We use DFT to perform first principles calculations of Li2-xMPO4F structures with both M sites fully filled with single transition metal species, and structures with transition metal doping in the form of Li2-xMI1-yMIIyPO4F. From our calculations, we determine ground state phase stability as a function of M, as well as structural, energetic, and electrochemical properties.
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