In situ stress measurements during electrochemical cycling of lithium-rich cathodes

Layered lithium transition metal oxides (Li1+xM1-xO2, M = Ni, Mn, Co) are attractive cathode materials for lithium-ion batteries due to their high reversible capacity. However, they suffer from structural changes that lead to substantial voltage fade. In this study, we use stress as a novel way to track irreversible changes in Li1.2Mn0.55Ni0.125Co0.125O2 (LR-NMC) cathodes. A unique and unpredicted stress signature is observed during the first delithiation. Initially, a tensile stress is observed, consistent with volume contraction from lithium removal, however, the stress reverses and becomes compressive with continued charging beyond 4 V vs Li/Li+, indicating volume expansion; this phenomenon is present in the first cycle only. This irreversible stress during delithiation is likely to be at least partially due to oxygen loss and the resulting cation rearrangement. Raman spectroscopy provides evidence of the layered-to-spinel phase transition after cycling in the LR-NMC films, as well as recovery of the original spectra upon re-annealing in an oxygen environment.