Quantifying the dependence of battery rate performance on electrode thickness

Simultaneous optimization of capacity and rate performance in battery electrodes would be much simplified by access to a simple equation relating rate performance to electrode thickness. Although a number of equations have been proposed, data on the effect of electrode thickness on rate performance are not extensive enough to identify the most appropriate model for thickness dependence. Here, using LiNi0.815Co0.15Al0.035O2 as a model system, we use chronoamperometry as a procedure to rapidly generate capacity-rate curves for >50 different electrode thicknesses. Using a semiempirical fitting equation, we extract the characteristic time (τ) associated with charge/discharge for each thickness (LE). We find the resultant τ-LE curve to be inconsistent with minimal models based on liquidor solid-phase diffusion alone but to be in excellent agreement with a relatively simple rate model which includes liquid- and solidphase-diffusion effects as well as electrical and electrochemical limitations. Thickness-dependent impedance measurements show that the magnitudes of the electrochemical and solid-state diffusion contributions are perfectly in line with the outputs of the rate model.