Unexplored Orthorhombic LiMn_1-xTi_xO₂ Cathode Materials with a Stable Atomic Site Occupancy and Phase Transition

Mn-rich orthorhombic (o)-LiMn_1-xTi_xO₂ with a stable oxygen/cation site occupancy and cycling-dependent phase transition is explored as a novel Co- and Ni-free cathode material for Li-ion rechargeable batteries. Typical o-LiMnO₂ suffers from oxygen deficiency, cation mixing between Li and Mn, and monoclinic (m)-Li₂MnO₃ secondary phase with low conductivity. Together with these drawbacks, the gradual, irreversible phase transition from layered o-LiMnO₂ into spinel-like cubic (c)-Li_xMnO₂ (x ≈ 0.5) during repeated charge/discharge cycles degrades the cycling performance of o-LiMnO₂ despite the activation of electroactive c-Li_xMnO₂ (x ≈ 0.5). By contrast, o-LiMn_1-xTi_xO₂ consists of Ti-doped o-LiMnO₂ and c-LiTiO₂ as the primary and secondary phases, respectively. The presence of Ti-O bonds, stronger than the existing Mn-O bonds, improves the structural stability of Ti-doped o-LiMnO₂ by reducing the imperfections of the oxygen/cation lattices (including Mn octahedral sites associated with the Jahn-Teller distortion) in Ti-doped o-LiMnO₂ during the long-term synthesis under an inert atmosphere. In addition, the electrochemically inactive (>2 V vs Li⁺/Li) c-LiTiO₂ phase with high conductivity serves as a pillar that suppresses the severe structural collapse of Ti-doped o-LiMnO₂ through an abrupt phase/structural transition during cycling (2-4.5 V).