Unexplored Orthorhombic LiMn1–xTixO2 Cathode Materials with a Stable Atomic Site Occupancy and Phase Transition

Abstract

Mn-rich orthorhombic (o)-LiMn1–xTixO2 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)-LixMnO2 (x ≈ 0.5) during repeated charge/discharge cycles degrades the cycling performance of o-LiMnO2 despite the activation of electroactive c-LixMnO2 (x ≈ 0.5). By contrast, o-LiMn1–xTixO2 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). As a result, o-LiMn1–xTixO2 with an optimal Ti content exhibits a higher maximum discharge capacity and superior cycling performance compared to the pristine o-LiMnO2.