Enhanced Li-storage capability and cyclability of iron fluoride cathodes by non-equivalent cobalt doping

•Cubic-phase FeF3 nanocages were prepared by annealing (NH4)3FeF6 (3NH4F·FeF3) double salt.•Non-equivalent Co-doping will lower the energy gap of FeF3 and alleviate the localization degree of charge density around.•Co2+ dopants could modify the crystallization and morphology of FeF3.•Sluggish diffusion kinetics and poor conductivity of FeF3 has been improved by Co2+ dopants.•Li-storage capability and cyclability of FeF3 was enhanced by Co2+ doping. [Display omitted] Theoretical calculation reveals that non-equivalent cobalt doping of 1% (in atomic ratio, at.) could lower the energy gap of cubic-phase FeF3 from 4.27 eV to 0.27 eV, and fundamental change the charge density distribution around. Enlightened by these results, the electrochemical properties of iron fluoride cathode that limited by the intrinsic sluggish kinetics and poor electronic conductivity would be significantly improved by non-equivalent doping. Exemplified by porous FeF3 nanocages prepared by pyrolysis of fast crystallized (NH4)3FeF6 (3NH4F·FeF3) double salts, enhanced Li-storage capability and cyclability were demonstrated by non-equivalent cobalt doping due to defect-related modification of electron-/ion-conductivity and increase of active sites.