Unlocking the Inaccessible Energy Density of Sodium Vanadium Fluorophosphate Electrode Materials by Transition Metal Mixing

摘要

Sodium (Na) vanadium (V) fluorophosphate NaxV2(PO4)2F3 (NVPF) is a highly attractive intercalation electrode material due to its high operation voltage,large capacity,and long cycle life. However,several practical issues limit the full utilization of NVPF's energy density: (l) the high voltage plateau associated with extracting the "third" Na ion in the reaction N,VPF —> VPF (~4.9 V vs Na/Na+) appears above the electrochemical stability window (ESW) of most practical electrolytes (~4.5 V vs Na/Na+); and (2) a sudden drop in Na-ion diffusivity is observed near composition Na1V2(P04)2F3. Therefore,it is important to investigate the potential substitution of V by other transition metals (TMs) in NVPF derivatives,which can practically access the extraction of the third Na-ion.