Electrochemical (PITT, EIS) and Analytical (SIMS) Determination of Li Diffusivities at the Onset of Charging LiNi0.33Mn0.33Co0.33O2 Electrodes

LiNixMnyCozO2 (NMC) materials are entering commercial lithium-ion batteries although their charging and discharging mechanism and the underlying Li kinetics are still not entirely understood. Li diffusion directly influences charging/discharging times, maximum capacities, stress formation (electrode stability), and possible side reactions. In order to measure intrinsic properties, samples without binders and conductive agents have to be investigated. The present work reports on the determination of Li diffusivities in additive-free 250 mu m thick NMC111 (x = y = z = 0.33) bulk electrodes, which were produced by ball milling, pressing, and sintering. The Li diffusivities were determined close to the delithiation onset. Different methods were applied: electrochemical impedance spectroscopy (EIS), potentiostatic intermitted titration technique (PITT), and tracer diffusion experiments using stable Li-6 isotopes and secondary ion mass spectrometry (SIMS). The obtained diffusivities from the three different techniques are in agreement with each other and with literature data assuming a two-phase delithiation mechanism. At room temperature, the chemical Li diffusivities are approximately constant below 3.5 eV. Between 3.5 and 3.6 V, there is a strong increase of diffusivities corresponding to the onset of delithiation. The relation between tracer diffusivities and chemical diffusivities is discussed.