Cu Substitution Stabilizes Oxygen Redox in High Na Content P3‐Type Na0.75Li0.2Cu0.05Mn0.75O2 Cathode with Unexpected High Energy Density

Abstract Oxygen redox enhances the specific energy of sodium cathodes, but the other performance remains unsatisfactory. By introducing Cu into P2 lattice to replace Li cations, P3‐type Na 0.75 Li 0.2 Cu 0.05 Mn 0.75 O 2 with high Na concentration is achieved. This modification induces notable alteration in the lattice structure, specifically increasing the interplanar spacing of NaO 6 from 3.6 Å to 3.8 Å. The resultant P3‐type cathode delivers a remarkable capacity of 253 ± 1.3 mAh g −1 with energy density of 680 mWh g −1 , setting a benchmark for P3‐type sodium cathodes. The high capacity can be attributed to the activation of Mn 3+ / Mn 4+ redox pair following Cu substitution. Further investigations confirm that Mn 3+ / Mn 4+ , Cu2+/ Cu 3+ and O 2− /O n− redox pairs all contribute to the high performance. The absence of O vacancy and the reduction in phase transitions enhance the cyclic performance with capacity retention of 86.3% at 0.5C. Additionally, the small diffusion energy barrier (34.6 KJ mol −1 ) results in a high Na diffusion coefficient (1.332 × 10 −9 cm 2 s −1 ), thereby promoting superior rate behavior with a capacity of 200.8± 2.1 mAh g −1 at 5C. These results demonstrate the advantages of the P3‐type Na 0.75 Li 0.2 Cu 0.05 Mn 0.75 O 2 cathode over the other Na cathodes, suggesting high potential for application in high‐energy storage fields.