Measurement of Lithium Diffusion Coefficient in Li _y FeSO₄F

Lithium diffusion coefficient (D) in Li _y FeSO₄F is investigated by means of three electrochemical techniques, namely potentiostatic intermittent titration technique (PITT), galvanostatic intermittent titration technique (GITT), and galvanostatic charge/discharge at different C-rates. The analytic equations used for analyzing PITT and GITT are based on radial diffusion in spherical particles, which is more suitable to the type of electrode studied here than the original equations derived by Wen et al. [J. Electrochem. Soc., 126, 2258 (1979).] for a slab geometry. While PITT and GITT analyses with the analytic equations are only valid in the partial solid solution regions (y≤0.29 and y≥0.83 in Li _y FeSO₄F), the analysis of galvanostatic charge/discharge with a mathematical model allows for the determination of D over the entire lithium composition range. The average D values found with PITT and GITT analyses are in good agreement with each other, and of the same order as those reported for Li _y FePO₄ in the literature (ca. 10₋₁₄ cm₂/s); However, D is larger for Li _y FeSO₄F in the Li-rich composition range than in the Li-poor one, whereas it is the opposite for Li _y FePO₄. This result could explain why Li _y FeSO₄F has a better rate capability on discharge than Li _y FePO₄.