Solid-state electrochemical kinetics of Li-ion intercalation into Li_1-xCoO₂: simultaneous application of electroanalytical techniques SSCV, PITT, and EIS

The electroanalytical behavior of thin Li_1-xCoO₂ electrodes is elucidated by the simultaneous application of three electroanalytical techniques: slow-scan-rate cyclic voltammetry (SSCV), potentiostatic intermittent titration technique, and electrochemical impedance spectroscopy. The data were treated within the framework of a simple model expressed by a Frumkin-type sorption isotherm. The experimental SSCV curves were well described by an equation combining such an isotherm with the Butler-Volmer equation for slow interfacial Li-ion transfer. The apparent attraction constant was -4.2, which is characteristic of a quasi-equilibrium, first-order phase transition. Impedance spectra reflected a process with the following steps: Li⁺ ion migration in solution, Li⁺ ion migration through surface films, strongly potential-dependent charge-transfer resistance, solid-state Li⁺ diffusion, and accumulation of the intercalants into the host materials. An excellent fit was found between these spectra and an equivalent circuit, including a Voigt-type analog (Li⁺ migration through multilayer surface films and charge transfer) in series with a finite-length Warburg-type element (Li⁺ solid-state diffusion), and a capacitor (Li accumulation). In this paper, we compare the solid-state diffusion time constants and the differential intercalation capacities obtained by the three electroanalytical techniques.