Synthesis and Electrochemistry of Stacking Fault-Free β-NaMnO₂.

Abstract

Understanding the solid-state chemistry of corrugated layered sodium manganese oxide (β-NaMnO₂) has been hindered by stacking faults (SFs), particularly in the presence of α-type planar domains among β-type corrugated MnO₂ 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-NaMnO₂ 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.