Oxygen loss, chemical expansion and delithiation in Li-rich oxides
Professor Saiful Islam and collaborators addressed the problem of oxygen loss in oxide electrodes, triggered by delithiation. This is one of the primary degradation modes in lithium-ion batteries, however, the delithiation-dependent mechanisms of oxygen loss is poorly understood.
In their paper 'Substantial oxygen loss and chemical expansion in lithium-rich layered oxides at moderate delithiation' published in Nature Materials, they investigated the oxygen non-stoichiometry in Li1.18-xNi0.21Mn0.53Co0.08O2-δ electrodes as a function of Li content by using cycling protocols with long open-circuit voltage steps at varying states of charge.
They observed substantial oxygen loss even at moderate delithiation, corresponding to 2.5, 4.0 and 7.6 mil O2 per gram of Li1.18-xNi0.21Mn0.53Co0.08O2-δ after resting at upper capacity cut-offs of 135, 200 and 265 mAhg-1 for 100h. Their observations suggested an intrinsic oxygen instability consistent with predictions of high oxygen activity at intermediate potential versus Li/Li+.
They additionally observed a large chemical expansion coefficient with respect to oxygen non-stoichiometry, which is approximately three times greater than those of classical oxygen-deficient materials (such as fluorite and perovskite oxides).
This paper challenges the conventional wisdom that deep delithiation is a necessary condition for oxygen loss in layered oxide electrodes, and highlights the importance of calendar ageing for investigating oxygen stability.
