Reactions of the Li<sub>2</sub>MnO<sub>3</sub> Cathode in an All-Solid-State Thin-Film Battery during Cycling
Kazuhiro Hikima, Yoyo Hinuma, Keisuke Shimizu, Kota Suzuki, Sou Taminato, Masaaki Hirayama, Takuya Masuda, Kazuhisa Tamura, Ryoji Kanno
Abstract
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.