Synergistic biphasic engineering and dual-site high-entropy doping enable stable sodium storage in layered oxide cathodes

O3-type layered transition metal oxide cathodes have attracted considerable attention due to their high sodium storage capacity and straightforward synthesis process. However, their practical applications are limited by irreversible phase transitions, transition metal dissolution, and sluggish Na⁺ diffusion kinetics. Herein, a unique high-entropy oxide (HEO), Na_0.88K_0.02Ni_0.24Li_0.06Mg_0.07Fe_0.1Mn_0.41Ti_0.1Sn_0.02O₂ is constructed by combining biphasic engineering and dual-site high-entropy doping for stable sodium storage. This synergistic effect significantly improves structural stability, enhances particle integrity, suppresses transition metal dissolution, accelerates electrochemical reaction kinetics, and mitigates electrolyte decomposition during the electrochemical cycling. Therefore, the HEO cathode demonstrates exceptional electrochemical performance, delivering a remarkable rate capability of 74.19 mAh g^-1 at 10 C and outstanding cycling stability with 82.68% capacity retention after 1,000 cycles. In addition, the practical viability of HEO is confirmed by its outstanding air stability and stable operation of full cells. These findings underscore the potential of synergistic effect of biphasic engineering and dual-site high-entropy doping in developing high-performance cathode materials for sodium-ion batteries.