Synthesis and Electrochemistry of Stacking Fault‐Free β ‐NaMnO ₂

Understanding the solid‐state chemistry of corrugated layered sodium manganese oxide ( β ‐NaMnO 2 ) has been hindered by stacking faults (SFs), particularly in the presence of α ‐type planar domains among β ‐type corrugated MnO 2 layers. Thus, their prospects as cathode materials for Na‐ion batteries have never been assessed comprehensively. The partial substitution of Cu for the Mn in β ‐NaMnO 2 yields SF‐free β ‐phase, as confirmed via atomic scale scanning transmission electron microscopy. The SF‐free material enables the identification of unique phase transitions during electrochemical Na extraction/insertion, showing drastic gliding of the corrugated layers. The proposed gliding mechanism is validated both experimentally, using ex situ synchrotron X‐ray diffraction, and theoretically, through density functional theory calculations. Minor structural changes observed in SF‐containing materials indicated that gliding is extremely sensitive to SFs. Given that the SF‐free β ‐material exhibits excellent cycle stability in a Na cell, the corrugated layer is concluded to have high resilience and reversibility against repeated large anisotropic structural changes involving slab gliding, providing a new insight into the design of long‐life rechargeable batteries.