Synergistic Al/Ti Co-doping and MgO Surface Engineering of O3-Type NaNi0.5Mn0.5O2 Cathodes for High-Performance Sodium-Ion Batteries

Abstract

O3-type NaNi0.5Mn0.5O2 is considered a promising cathode material for sodium-ion batteries, but its electrochemical performance is limited by rapid capacity fading associated with Jahn–Teller distortion of Mn3+ and irreversible O3–P3 phase transitions at high voltages. To address these issues, a combined strategy involving Al/Ti co-doping and MgO surface coating is proposed. The results indicate that Al/Ti co-doping modifies the local Na+ transport environment and reduces the proportion of Jahn–Teller active Mn3+, thereby improving structural stability and sodium-ion transport kinetics. In situ XRD analysis further reveals a more gradual O3–O′3–P3 structural evolution, suggesting alleviated structural strain during the high-voltage transition. Meanwhile, the ultrathin MgO coating is demonstrated to suppress electrolyte-induced side reactions and mitigate transition-metal dissolution, thereby improving interfacial stability. Warburg analysis further indicates that Al/Ti co-doping markedly enhances the apparent Na+ diffusion kinetics, while the MgO coating preserves the improved diffusion behavior despite introducing additional interfacial resistance. As a result, the optimized NMAT-1@MgO-0.5 wt % cathode delivers an initial discharge capacity of 116.7 mAh g–1 and retains 83.8% of its capacity after 100 cycles at 0.1 C, outperforming the pristine and single-modified samples. This work provides insights into the synergistic regulation of bulk structure and surface chemistry for the development of durable layered oxide cathodes for sodium-ion batteries.