Bipolarization of cathode particles as underlying mechanism for voltage hysteresis and the first charge cycle overvoltage of intercalation batteries

Abstract

Understanding the mechanisms at the basis of the charge/discharge characteristics of cathode materials is mandatory for lithium-ion and post-lithium-ion batteries optimal development. Here, we theoretically propose bipolarization of cathode particles, as a noble governing mechanism for charge-discharge behaviors, capable to justify many phenomena including the voltage hysteresis (VH) and the first-charge cycle overvoltage, etc. The here proposed original approach is based on previous models, simple concepts and experimental evidences. After representation of the approach, two important phenomena are explained in view of it. The first charge cycle overvoltage is justified by the bipolarization mechanism. VH phenomenon is caused by bipolarization and subsequently neutralization of the particles. Ab-initio calculations are used to estimate VH values based on the approach and the results are in agreement with our experiments. Our approach sheds light on the relevant characteristics of cathode materials cycling properties and allows designing strategies for optimization of the characterizations by controlling and engineering the mechanism. As practical results, different parts of V-C curvature and their corresponding reactions could be distinguished. Accordingly, a new assessment field is opened to determine quality of the cathode powder, electrode preparation and cell assembling. This paper is a foundation to justify other observed phenomena including memory-effect, voltage overshooting, beginning/ending of (dis)charge curvatures and different obtained voltage for one cathode etc.