Determination of the Li ion chemical diffusion coefficient for the topotactic solid-state reactions occurring via a two-phase or single-phase solid solution pathway

We studied the electroanalytical behavior of LixNiO2 and LixCo0.2Ni0.8O2 el ectrodes using three electroanalytical tools, namely, slow scan cyclic volt ammetry (SSCV), potentiostatic intermittent titration (PITT), and galvanost atic intermittent titration (GITT). Based on previous studies of these elec trodes by in situ XRD, it appears that the Li-insertion mechanisms into the se electrodes are different in the 3.65 to 4.05 V (Li \ Li+) potential rang e. In the case of LixNiO2, Li intercalation in this range involves phase tr ansitions, while Li insertion into LixCo0.2Ni0.8O2 in this potential range proceeds via the formation of a solid solution. Using the above electroanal ytical tools, we calculated the diffusion coefficient of Li ions (D) and th e incremental capacity (C-int) versus electrode potential. It was interesti ng to observe typical differences in the behavior of C-int and D versus E o f these electrodes, which reflect the difference of Li insertion mechanisms . It is assumed that the behavior observed around 3.75-3.95 V (Li/Li+) for both electrodes reflects repulsive interaction amongst intercalation sites. However, for LixNiO2, short-range interactions are assumed to be dominant, leading to peak-shaped behavior of both C-int and D versus E, whereas in t he case of LixCo0.2Ni0.8O2 electrodes, long-range repulsive interactions do minate, leading to plain-shaped C-int and D versus E curves. In the potenti al range 3.50 to 3.64 V (Li/Li+), Li de-intercalation from both electrodes behaves similarly because it relates to changes in the homogeneous hexagona l phase (H-1). (C) 1999 Elsevier Science S.A. All rights reserved.