Jahn-Teller effect in LiMn₂O₄: influence on charge ordering, magnetoresistance and battery performance

The phase transition near room temperature in LiMn₂O₄ was studied using thermal expansion measurements, and directly compared with the electrochemical performance of the material. Studies based on thermal expansion indicate the onset of a first-order phase transition at T_c ∼ 220 K for the nearly half-doped material, with [Mn³⁺]/[Mn⁴⁺] ≈ 1. The T_c shifts to a higher temperature, ∼290 K, and signatures for Verwey-type charge ordering at 290 K can be observed when the fraction of Jahn-Teller Mn³⁺ in LiMn₂O₄ is increased, i.e., [Mn³⁺]/[Mn⁴⁺] > 1. These studies show that the first-order phase transition near room temperature in LiMn₂O₄ is associated with charge ordering, which ultimately is a consequence of the Jahn-Teller effect. In addition, the Jahn-Teller effect is proven to be an important cause of magnetoresistance and electrochemical capacity fading in LiMn₂O₄. Electrochemical measurements show that both materials, either with a T_c ∼ 220 K or T_c ∼ 290 K, exhibit capacity fading to almost the same extent. Electrochemical capacity retention is observed only in nanosized LiMn₂O₄, for which the phase transition anomalies are completely absent.