Suppressing Voltage Decay in O2-Type Li-Rich Layered Cathode Materials through Microstrain Alleviation

O2-type Li-rich layered oxides (LLOs) typically display stable cyclic properties and low voltage decay owing to the reversible migration of transition metals (TMs) between face-shared TMO₆ and LiO₆. However, the intrinsic relationship between microstrain and voltage decay in O2-type LLOs remains unclear. Herein, an O2-type manganese-based layered cathode material, Li_2/3□_1/3Ni_0.25Mn_0.75O₂ (LNMO-RT), is synthesized through a Li⁺/Na⁺-ion exchange reaction at room temperature. The phase transition from a P2-type to O2-type layered structure induces a significant change in unit-cell volume, resulting in pronounced microstrain. To tackle this issue, a low-temperature thermal treatment at 300 °C is employed to prepare O2-type Li_2/3□_1/3Ni_0.25Mn_0.75O₂ (LNMO-HT). High-resolution transmission electron microscopy (HRTEM) images coupled with geometric phase analysis (GPA) demonstrate that LNMO-HT exhibits suppressed lattice distortion and reduced microstrain compared to LNMO-RT. This, in turn, proves advantageous for charge transfer and aids in mitigating voltage decay. The electrochemical performance of LNMO-HT is demonstrated to be excellent, displaying negligible voltage decay (0.8 mV per cycle) and outstanding long-term cycling stability, retaining nearly 91% of its initial capacity after 50 cycles at 0.1 C. In situ X-ray diffraction (XRD) measurements during the first cycle for LNMO-HT reveal minimal changes in lattice parameters, indicating excellent structural stability. This finding highlights the efficacy of low-temperature thermal treatment in eliminating lattice dislocation and strain, offering a novel design approach for developing O2-type LLOs with suppressed voltage fading.