High power Nb-doped Lifelong₄ Li-ion battery cathodes; pilot-scale synthesis and electrochemical properties

High power, phase-pure Nb-doped LiFePO₄ (LFP) nanoparticles are synthesised using a pilot-scale continuous hydrothermal flow synthesis process (production rate of 6 kg per day) in the range 0.01–2.00 at% Nb with respect to total transition metal content. EDS analysis suggests that Nb is homogeneously distributed throughout the structure. The addition of fructose as a reagent in the hydrothermal flow process, followed by a post synthesis heat-treatment, affords a continuous graphitic carbon coating on the particle surfaces. Electrochemical testing reveals that cycling performance improves with increasing dopant concentration, up to a maximum of 1.0 at% Nb, for which point a specific capacity of 110 mAh g⁻¹ is obtained at 10 C (6 min for the charge or discharge). This is an excellent result for a high power cathode LFP based material, particularly when considering the synthesis was performed on a large pilot-scale apparatus

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 DLi 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