Reversible Li+ Storage in a LiMnTiO4 Spinel and Its Structural Transition Mechanisms

Abstract

In this work, LiMnTiO4 (a structural analogue of classic spinel LiMn2O4) with a disordered cubic spinel structure (Fd3̅m) has been synthesized by a low-temperature sol–gel route. The as-obtained LiMnTiO4 exhibits excellent cycling stability in a wide voltage range from 1.5 to 4.8 V with high discharge capacities of 290, 250, and 140 mA h g–1 at a C/40, C/19, and 1C rate, respectively. Combined long- and short-range structural characterization techniques are used to reveal the correlation between structure and electrochemical behavior. During cycling, the charge/discharge profiles of LiMnTiO4 evolve from initially two well-separated plateaus into sloping regimes. In the early stage of discharge, LiMnTiO4 undergoes phase transitions from an initial spinel phase to mixtures of predominant rock-salt (Fm3̅m) and tetragonal (I41/amd) structures along with a decrease in crystallite size from 12 nm to 3 to 4 nm. During further cycling, the spinel/rock-salt phase transition was found to be reversible with the cubic framework remaining intact. The presence of the tetragonal phase after the first discharge suggests that the Mn3+ Jahn–Teller distortion is partially involved during lithiation from Li1–yMn3+yTiO4 to Li1+xMn3–xTiO4 and the fraction of such a tetragonal phase remains at about 30–40% during subsequent cycling.