Crystal structure variation of LiMn1.5Ni0.5O4 high-voltage spinel cathode during low-voltage redox reaction

Using operando X-ray diffraction, crystal structure variation of high-purity spinel cathode LiMn1.5Ni0.5O4 at voltages of 2.1–3.5 V vs. Li anode was studied. Results showed that the low-voltage Li+ extraction–insertion reaction proceeded in a two-phase reaction with a long voltage plateau around 2.8 V, where cubic and tetragonal spinel phases coexist. Their lattice volume difference was approximately 4.9%. That difference, which is small compared to that of ordinary LiMn2O4 spinel compound, implies that the Jahn-Teller distortion can slightly influence Li+ extraction-insertion reaction reversibility. Therefore, the redox reaction occurred over the wide voltage range of 2.5–4.9 V, at which simultaneous calorimetric measurements were conducted. Capacity fading during redox cycling was sufficiently small, but not negligible. A large exothermic reaction was detected at both charge and discharge stages, although only a small exothermic reaction was observed in both 2.8 V and 4.7 V plateau regions. The irreversibility can be attributed to the reaction at the transition region between high-voltage redox and low-voltage redox.