Comparative Study of Lithium Transport Kinetics in Olivine Cathodes for Li-ion Batteries

Crystallite size and composition effects oil the kinetics of lithium storage in olivines are studied using potentiostatic intermittent titration tests (PITT). Here we compare undoped Li1-xFePO4 of 113 nm, 42 nm, and 34 nm average particle size, and three aliovalent solute doped compositions formulated for dopant substitution on the Li (M1) site with charge compensation by Li vacancies: Li0.90Mg0.05FePO4, Li0.80Zr0.05FePO4, and Li0.70Zr0.075FePO4. The results first show that a diffusive component can be measured with reasonable accuracy in all samples, allowing determination of the lithium chemical diffusion coefficient D as a function of potential or state-of-charge. In addition, a method is illustrated for the separation of capacity due to diffusive transport from that due to the first-order phase transition. Using the combined analyses, the effects of particle size reduction and aliovalent doping are readily understood. Both effects contribute to a reduced lithium miscibility gap and a greater contribution to stored capacity of the (faster) diffusive process. Simultaneously, the rate of phase transformation within the miscibility gap is also increased. Highly doped samples exhibit a complete lithium solid solution at room temperature, and have 1-2 orders of magnitude higher D at potentials where significant capacity is stored. This translates to improved capacity retention at high cycling rates, albeit at the expense of reduced absolute capacity due to Li diffusion channel blocking.