Mitigating Jahn-Teller active Mn³⁺ via thermal optimization in Li-rich layered oxides: a path toward enhanced...

Achieving higher energy density in lithium-ion batteries has drawn significant attention to Li-rich layered oxide cathodes owing to their exceptional specific capacity and high operating voltage. Here, we synthesize Co-free hierarchical microsphere-like Li_1.2Mn_0.56Ni_0.24O₂ (HMLMO) cathodes via a carbonate-assisted co-precipitation route, followed by systematic calcination at 750, 850 and 950 °C to tune Mn³⁺ and oxygen defect concentrations. XRD, Raman, and HRTEM analyses confirm the coexistence of monoclinic Li₂MnO₃ and rhombohedral LiNiO₂ phases, while HRSEM and ICP-OES verified the hierarchical morphology and elemental stoichiometry. XPS analysis revealed that calcination temperature profoundly influenced the Mn³⁺/Mn⁴⁺ ratio. HMLMO-850 exhibits the lowest Mn³⁺ content and oxygen vacancy concentration. EPR analyses further corroborate the oxygen vacancy defects. HMLMO-850 delivers a discharge capacity of 225.6 mAh g^-1 at 50 mA g^-1 with 90% retention after 50 cycles. Furthermore, it achieved discharge capacities of 171 and 133.8 mAh g^-1 at 300 and 500 mA g^-1, respectively, with stable cycling for up to 300 cycles. Post-cycling ex situ XRD, Raman, XPS and FESEM analyses validate the superior structural integrity of HMLMO-850 after prolonged cycling. Thus, the results emphasise the critical role of calcination temperature in governing the electrochemical performance.