Combined Bulk-Interfacial Engineering of Ultrahigh-Nickel LiNi 0.9 Co 0.05 Mn 0.05 O ₂ to Regulate Electrochemical Kinetics and Phase Reversibility under Harsh Conditions

Severe thermodynamic lattice oxygen evolution and accelerated interfacial parasitic reactions significantly deteriorate the electrochemical reversibility of ultrahigh-Ni cathodes (LiNi 0.9 Co 0.05 Mn 0.05 O 2, NCM90), particularly under harsh operational conditions (4.5 V, 55 °C, and 5 C). Herein, a combined dual-modification strategy integrating Mg 2+ /Al 3+ bulk gradient doping and spinel MgAl 2 O 4 (MA) interfacial coating is proposed to concurrently modulate the structural thermodynamics and interfacial kinetics of NCM90. Electrochemical phase transition tracking (d Q /d V ) and kinetic analyses (galvanostatic intermittent titration technique and electrochemical impedance spectroscopy) reveal that the robust Al–O bonds and Mg 2+ “pillaring” effect thermodynamically mitigate lattice oxygen loss and alleviate the irreversible H2–H3 phase transition. Simultaneously, the MA coating acts as a robust physical barrier, suppressing the aggressive oxidative decomposition of the electrolyte and minimizing the accumulation of the charge-transfer resistance ( R ct ). Benefiting from this integrated electrochemical tuning, the dual-modified MA-NCM90 exhibits robust kinetics, retaining 46.8% of its capacity after 300 cycles at 4.5 V and 55 °C (a 17.7% improvement over pristine NCM90, delivering an extra 35.2 mAh g −1 ), alongside a good fast-charging capability at 5 C. This study elucidates the fundamental mechanism of bulk/interface cooperative engineering in stabilizing the solid–liquid interface and internal phase transitions of Ni-rich cathodes under harsh conditions.