Boosting reversible anionic redox reaction with Li/Cu dual honeycomb centers

Abstract

The anionic redox reaction (ARR) is a promising charge contributor to improve the reversible capacity of layered-oxide cathodes for Na-ion batteries; however, some practical bottlenecks still need to be eliminated, including a low capacity retention, large voltage hysteresis, and low rate capability. Herein, we proposed a high-Na content honeycomb-ordered cathode, P2–Na_5/6[Li_1/6Cu_1/6Mn_2/3]O₂ (P2-NLCMO), with combined cationic/anionic redox. Neutron powder diffraction and X-ray diffraction of P2-NLCMO suggested P2-type stacking with rarely found P6₃22 symmetry. In addition, advanced spectroscopy techniques and density functional theory calculations confirmed the synergistic stabilizing relationship between the Li/Cu dual honeycomb centers, achieving fully active Cu³⁺/Cu²⁺ redox and stabilized ARR with interactively suppressed local distortion. With a meticulously regulated charge/discharge protocol, both the cycling and rate capability of P2-NLCMO were significantly improved, demonstrating reasonable capacity and eliminating voltage hysteresis. Overall, this work contributes a well-defined layered oxide cathode with combined cationic/anionic redox towards rational designing advanced Na-ion batteries.