Unexpectedly High Cycling Stability Induced by a High Charge Cut-Off Voltage of Layered Sodium Oxide Cathodes

Initiating anionic redox chemistry in layered sodium oxide cathodes is a prevalent method to break the capacity limit set by traditional transition metal redox. However, realizing the "win-win" scenario of high capacity and high cycling stability is still challenging due to the high-voltage structural distortion and irreversible oxygen loss. Herein, a Mn activation mechanism is unveiled in a novel P2-Na0.80Li0.08Ni0.22Mn0.67O2 cathode. By elevating the charge cut-off voltage to 4.3 V, anionic redox is successfully triggered and partial oxygen loss enables the reduction of Mn upon discharge, thus activating more Mn3+/Mn4+ redox reactions in the following cycles and maintaining the total capacity almost unchanged. In situ X-ray diffraction reveals a complete solid-solution reaction with an ultralow volume change of 1.04% upon cycling. Consequently, the P2-Na0.80Li0.08Ni0.22Mn0.67O2 cathode simultaneously accomplishes a high discharge capacity (134.8 mAh g(-1) at 0.1 C) and an unexpectedly long cycling life (capacity retention of 91.5% and 85.2% after 500 and 1000 cycles at 10 C, respectively). Via systematic ex situ characterizations and theoretical computations, the charge compensation mechanism upon Na+ insertion/extraction is elucidated. This work broadens the horizons of current oxygen redox chemistry and provides a new path to design high-performance layered oxide cathode materials for sodium-ion batteries.