Examining Hysteresis in Composite xLi2MnO3·(1–x)LiMO2 Cathode Structures

This paper reports the results of an initial investigation into the phenomenon of hysteresis in the charge–discharge profile of high-capacity, lithium- and manganese-rich "layered–layered" xLi2MnO3·(1–x)LiMO2 composite cathode structures (M = Mn, Ni, Co) and "layered–layered-spinel" derivatives that are of interest for Li-ion battery applications. In this study, electrochemical measurements, combined with in situ and ex situ X-ray characterization, are used to examine and compare electrochemical and structural processes that occur during charge (lithium extraction) and discharge (lithium insertion) of preconditioned cathodes. Electrochemical measurements of the open-circuit voltage versus lithium content demonstrate a ∼1 V hysteresis in site energy for approximately 12% of the total lithium content during the early cycles, which is markedly different from the hysteresis commonly observed in other intercalation materials. X-ray absorption data indicate structural differences in the cathode at the same state of charge (i.e., the same lithium content) during lithium insertion and extraction reactions. The data support an intercalation mechanism whereby the total number of lithium ions extracted at the top of charge is not reaccommodated in the structure until low states of charge are reached. The hysteresis in this class of materials is attributed predominantly to an inherent structural reorganization after an electrochemical activation of the Li2MnO3 component that alters the crystallographic site energies.