Asymmetry of Discharge/Charge Curves of Lithium-Ion Battery Intercalation Electrodes

Nickel cobalt aluminum oxide (NCA) based lithium-ion battery electrodes exhibit a distinct asymmetry in discharge/charge behavior toward high bulk stoichiometry (low state of charge). We show that basic electrochemical relationships, that is, the Nernst equation and the Butler–Volmer equation, are able to reproduce this behavior when a two-step reaction mechanism is assumed. The two-step mechanism consists of (1) lithium-ion adsorption from the electrolyte onto the active material particle surface under electron transfer and (2) intercalation of surface-adsorbed lithium atoms into the bulk material. The asymmetry of experimental half-cell data of an NCA electrode cycled at 0.1 C rate can be quantitatively reproduced with this simple model. The model parameters show two alternative solutions, predicting either a saturated (highly covered) or a depleted surface for high bulk lithiation.