Full Exploitation of Charge Compensation of O3-type Cathode Toward High Energy Sodium-Ion Batteries by High Entropy Strategy.

O3-type cathodes with sufficient Na content are considered as promising candidates for sodium-ion batteries (SIBs). However, these cathodes suffer from insufficient utilization of the active elements, restraining the delivered capacity. In this work, a high entropy strategy is applied to a typical O3 cathode NaLiNiMnO (NLNM), forming a high entropy oxide NaLiNiCuMgTiMnO (Na-HE). Results show that the active elements are fully exploited in Na-HE, with a two-electron reaction by Ni (further extended to Cu redox and even oxygen redox), vastly different from a one-electron reaction of Ni in NLNM. The full utilization of the active elements dramatically improves the output capacity of the cathode (122.6 mAh g of Na-HE versus 81 mAh g of NLNM). Moreover, the detrimental phase transition is well suppressed in Na-HE. The cathode exhibits high capacity retention of 88.7% after 100 cycles at 130 mA g, compared to only 36.4% for NLNM. These findings provide new insight for the design of new cathode materials for SIBs with high energy density and robust stability.