Electrochemically In Situ Formed Inverse-Honeycomb Local Ordering for Low-Voltage-Hysteresis Oxygen Redox in Na/Li...

In oxygen-redox cathodes, voltage hysteresis largely arises as oxygen redox generally proceeds with electrochemical reactions followed by chemical reactions that normally drive asymmetrical charge and discharge pathways. Although different superstructures, such as ribbon and mesh units, were intentionally introduced to mitigate voltage hysteresis, those structures remain as model compounds that can hardly be implemented in real-world applications. Here, inspired by the classical Li-rich layered Li[Li_0.2Ni_0.2Mn_0.6]O₂, we design a new O3-type sodium layered oxide, NaLi_0.2Ni_0.2Mn_0.6O₂, that delivers 205.2 mA h g^{- 1} with reversible oxygen redox but shows mitigated voltage hysteresis and suppressed voltage decay. Atomic-resolution scanning transmission electron microscopy reveals that deep desodiation reconstruts an addtional inverse-honeycomb superstructure in the alkali layer, which is retained after discharge and extended cycling. Li analogues prepared via (electro)chemical ion-exchange reactions from pristine and charged-state Na precursors further reveal the critical role of this inverse-honeycomb ordering in suppressing voltage hysteresis. Thermal relaxation experiments combined with spectroscopic studies show that inverse-honeycomb ordering suppresses Ni-centered ligand-to-metal charge transfer associated with chemical relaxation, allowing the reduction process to proceed from a nonequilibrium state, thereby mitigating voltage hysteresis. Our study provides practical significance in this newly designed Na compound NaLi_0.2Ni_0.2Mn_0.6O₂ by taking advantage of in situ formation of inverse-honeycomb ordering.