Investigation on diffusion behavior of Li+ in LiFePO4 by capacity intermittent titration technique (CITT)

Capacity intermittent titration technique (CITT) was used to investigate the chemical diffusion coefficient (D˜) of lithium-ion in LiFePO4 cathode material. The values of D˜ at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 8.8 × 10−16 to 8.9 × 10−14 cm2 s−1 and from 1.2 × 10−16 to 8.5 × 10−14 cm2 s−1 in the voltage range from 3.2 to 4 V (vs. Li+/Li). The transfer coefficients of cathode (0.32–0.42) and anodic (0.26–0.3), and the standard rate constant (1.58 × 10−9 to 1.30 × 10−8 cm s−1) were measured from the Tafel plots of LiFePO4 in the equilibrium potential range from 3.06 to 3.45 V. From these kinetic parameters, the finite kinetics at interface was taken into account to revise the above values of D˜. The revised values of D˜ at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 2.44 × 10−15 to 2.21 × 10−13 cm2 s−1 and from 5.81 × 10−16 to 3.22 × 10−13 cm2 s−1 in the voltage range from 3.2 to 4 V. Results show that the approximation of infinite charge-transfer kinetics leads to a spurious value of D˜ which is lower than the revised value, and the spurious extent depends on the galvano-charge current of CITT experiment.