Reactions of the Li₂MnO₃ Cathode in an All-Solid-State Thin-Film Battery during Cycling

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.