is a promising cathode candidate for Li-ion batteries because of its high discharge capacity; however, its reaction mechanism during cycling has not been sufficiently explicated. Observations of Mn and O binding energy shifts in operando hard X-ray photoelectron spectroscopy measurements enabled us to determine the charge-compensation mechanism of Li 2 MnO 3 . The O 1s peak splits at an early stage during the first charge, and the concentration of lower-valence O changes reversibly with cycling, indicating the formation of a low-valence O species that intrinsically participates in the redox reaction. The O 1s peak-splitting behavior, which indicates the number of valences of O in Li 2 MnO 3 , is supported by the computational results for an O3 to O1 structural transition. This is in agreement with the results of our previous study, wherein we confirmed this O3 to O1 transition based on in situ surface X-ray diffraction analysis, X-ray photoelectron spectroscopy, and firstprinciples formation energy calculations.
We
evaluated the structural change of the cathode material Li2MnO3 that was deposited as an epitaxial film with
an (001) orientation in an all-solid-state battery. We developed an in situ surface X-ray diffraction (XRD) technique, where
X-rays are incident at a very low grazing angle of 0.1°. An X-ray
with wavelength of 0.82518 Å penetrated an ∼2 μm-thick
amorphous Li3PO4 solid-state electrolyte and
∼1 μm-thick metal Li anode on the Li2MnO3 cathode. Experiments revealed a structural change to a high-capacity
(activated) phase that proceeded gradually and continuously with cycling.
The activated phase barely showed any capacity fading. First-principles
calculations suggested that the activated phase has O1 stacking, which
is attained by first delithiating to an intermediate phase with O3
stacking and tetrahedral Li. This intermediate phase has a low Li
migration barrier path in the [001] direction, but further delithiation
causes an energetically favorable and irreversible transition to the
O1 phase. We propose a mechanism of structural change with cycling:
charging to a high voltage at a sufficiently low Li concentration
typically induces irreversible transition to a phase detrimental to
cycling that could, but not necessarily, be accompanied by the dissolution
of Mn and/or the release of O into the electrolyte, while a gradual
irreversible transition to an activated phase happens at a similar
Li concentration under a lower voltage.
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