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

Abstract

We studied the electroanalytical behavior of LixNiO2 and LixCo0.2Ni0.8O2 electrodes using three electroanalytical tools, namely, slow scan cyclic voltammetry (SSCV), potentiostatic intermittent titration (PITT), and galvanostatic intermittent titration (GITT). Based on previous studies of these electrodes by in situ XRD, it appears that the Li-insertion mechanisms into these electrodes are different in the 3.65 to 4.05 V (Li ∣ Li+) potential range. In the case of LixNiO2, Li intercalation in this range involves phase transitions, while Li insertion into LixCo0.2Ni0.8O2 in this potential range proceeds via the formation of a solid solution. Using the above electroanalytical tools, we calculated the diffusion coefficient of Li ions (D) and the incremental capacity (Cint) versus electrode potential. It was interesting to observe typical differences in the behavior of Cint and D versus E of 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 Cint and D versus E, whereas in the case of LixCo0.2Ni0.8O2 electrodes, long-range repulsive interactions dominate, leading to plain-shaped Cint and D versus E curves. In the potential 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 hexagonal phase (H1).