Suppressing the charge-ordering transition in LiMn2O4 through substitution of Li by Mg

The important electrode material LiMn2O4 undergoes a first order structural phase transition near room temperature. This transition, known to affect electrode characteristics, was long thought to result from the Jahn–Teller nature of Mn(III). However, Rodríguez-Carvajal and coworkers have recently established that with half-integral charge on Mn, what actually takes place at the transition is the crystallization of holes and electrons (charge ordering) in the crystal structure. Since the tendency to charge ordering is suppressed when charges on ions are not simple fractions, we have substituted Mg for Li to take Mn away from an average valence of 3.5. The transition is suppressed with as little as 4 atom% of Mg substitution for Li. The results suggest that the charge-ordering transition and Jahn–Teller distortions of the MnIIIO6 octahedra are distinct phenomena.