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.

Automated summary

This study investigates the crystal structure variation and redox reactions of LiMn1.5Ni0.5O4 cathode material at different voltages using operando X-ray diffraction. The results show that a two-phase reaction occurs at low voltages, with the presence of cubic and tetragonal spinel phases. The redox reaction takes place over a wide voltage range, with small capacity fading. Exothermic reactions are observed during charge and discharge stages.