Dual Site-Selective Substitution Strategy Achieving Anionic Redox Activity and Solid-Solution Reaction in P2-Type Cathode Materials
Xudong Ma, Zhenqian Wu, Xiao‐Ning Cheng, Xianwei Li, Qi Liu, Xiao Yu, Yong Liu
Abstract
P2-type Na 0.67 Ni 0.33 Mn 0.67 O 2 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 Na 0.63 Ca 0.05 (Ni 0.26 Li 0.07 Mn 0.67 )O 2 cathode. The substitution of Li + for Ni 2+ 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 Ca 2+ 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 Na 0.63 Ca 0.05 (Ni 0.26 Li 0.07 Mn 0.67 )O 2 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 Na 0.63 Ca 0.05 (Ni 0.26 Li 0.07 Mn 0.67 )O 2 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).