Nickel-rich layered oxide cathodes such as NMC811 (LixNi0.8Mn0.1Co0.1O2) currently have the highest practical capacities of
cathodes
used commercially, approaching 200 mAh/g. Lithium is removed from
NMC811 via a solid-solution behavior when delithiated toxLi> 0.10, maintaining the same layered (O3 structure)
throughout as observed via operando diffraction measurements. Although
it is possible to further delithiate NMC811, it is kinetically challenging,
and there are significant side reactions between the electrolyte and
cathode surface. Here, small format, NMC811-graphite pouch cells were
charged to high voltages at elevated temperatures and held for days
to access high states of delithiation. Rietveld refinements on high-resolution
diffraction data and indexing of selected area electron diffraction
patterns, both acquiredex situ, show that NMC811
undergoes a partial and reversible transition from the O3 to the O1
phase under these conditions. The O1 phase fraction depends not only
on the concentration of intercalated lithium but also on the hold
temperature and hold time, indicating that the phase transition is
kinetically controlled.1H NMR spectroscopy shows that
the proton concentration decreases with O1 phase fraction and is not,
therefore, likely to be driving the O3–O1 phase transition.