Mechanistic insights of Li⁺ diffusion within doped LiFePO₄ from Muon Spectroscopy

The Li⁺ ion diffusion characteristics of V- and Nb-doped LiFePO₄ were examined with respect to undoped LiFePO₄ using muon spectroscopy (µSR) as a local probe. As little difference in diffusion coefficient between the pure and doped samples was observed, offering D_Li values in the range 1.8-2.3 × 10^-10 cm² s^-1, this implied the improvement in electrochemical performance observed within doped LiFePO₄ was not a result of increased local Li⁺ diffusion. This unexpected observation was made possible with the µSR technique, which can measure Li⁺ self-diffusion within LiFePO₄, and therefore negated the effect of the LiFePO₄ two-phase delithiation mechanism, which has previously prevented accurate Li⁺ diffusion comparison between the doped and undoped materials. Therefore, the authors suggest that µSR is an excellent technique for analysing materials on a local scale to elucidate the effects of dopants on solid-state diffusion behaviour.