Dual Site-Selective Substitution Strategy Achieving Anionic Redox Activity and Solid-Solution Reaction in P2-Type Cathode Materials

P2-type Na0.67Ni0.33Mn0.67O2 has been considered as the potential cathode for sodium-ion batteries. However, its practical application is plagued by Na+/vacancy ordering, harmful phase transition, and lattice oxygen loss. Herein, we develop a dual site-selective substitution strategy to fabricate a P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode. The substitution of Li+ for Ni2+ introduces lone pair oxygen via forming a Li-O-Li configuration and make O 2p close to its Fermi level due to the weakened TM-O (TM: transition metal) bond, which triggers the anionic redox for charge compensation, while the introduction of Ca2+ in a Na layer enhances the electrostatic cohesion of neighboring TM layers by forming a strengthened O-Ca-O configuration, which suppresses the glide of adjacent TM layers and reduces the excessive lattice oxygen loss. Therefore, with a dual site-selective substitution strategy, the P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode can suppress the Na+/vacancy ordering, P2-O2 phase transition, and lattice oxygen loss even at a potential of 4.35 V, achieving a reversible anionic redox and solid-solution reaction. The P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode exhibits high discharge capacity (142.7 mA h g-1 at 20 mA g-1), excellent rate capability (57.1 mA h g-1 at 2 A g-1), and cyclic stability (a capacity retention of 83.2% after 700 cycles).