Deciphering voltage decay in lithium-rich manganese-based cathodes: the pivotal role of cation mixing-driven...

Lithium-rich manganese-based oxides (LRMOs) have garnered significant attention due to high specific capacity and operating voltage; however, their practical deployment is critically hindered by severe voltage decay. Although this phenomenon is widely attributed to five interrelated mechanisms (oxygen evolution, cation mixing, lattice strain, transition metal (TM) dissolution, and phase transformation), prior investigations remain largely qualitative. A rigorous quantitative differentiation of their individual contributions remains notably absent. Herein, the interplay between these five degradation factors and voltage decay is systematically decoupled by precisely regulating the charge and discharge cut-off voltages, revealing cation mixing as the primary driving mechanism Notably, the discharge cut-off voltage exerts a significant influence on voltage decay. Under a constant charge cut-off voltage of 4.8 V, decreasing the discharge cut-off voltage from 3.0 V to 2.0 V increases the voltage decay rate from 0.73 to 3.33 mV cycle^-1 after 200 cycles, challenging the conventional wisdom that high operating voltage alone dictates voltage decay. Quantitative analysis after 200 cycles within the 2.0-4.8 V range reveals that the structural degradation is characterized by 4.54 wt% TM dissolution, a 6.55 vol% spinel-like phase transformation, an oxygen evolution mass fraction below 3.19 wt%, and a mere 3.6% lattice strain; most notably, the I₍₀₀₃₎/I₍₁₀₄₎ ratio undergoes a severe 26.45% variation, confirming extensive cation mixing, the Ni occupancy at the 3a site is 0.022, indicating a cation mixing degree of 17.1%. These findings demonstrate that cation mixing exhibits the strongest correlation with voltage decay. This study elucidates the critical role of cation mixing in voltage decay, providing a pivotal theoretical basis for mitigating voltage decay in LRMOs.