Structural change induced by electrochemical sodium extraction from layered O′3-NaMnO2

Sodium-ion batteries can be designed as a low-cost alternative to lithium-ion batteries, where various layered transition metal oxides are frontrunner positive electrode materials. Owing to the inexpensive and abundant Mn resources and a large reversible capacity approaching 200 mA h g−1, α type (O′3 type) NaMnO2 is considered as a competent and economical candidate for sodium-ion batteries. However, O′3 NaMnO2 suffers from rapid capacity fading during charge–discharge cycling, for which the reasons remain elusive. The current work probes the underlying mechanisms behind this capacity degradation based on the correlation between the crystal structure and electrochemical properties. O′3 type NaMnO2, having a monoclinic O3-type structure, undergoes (de)intercalation of sodium ions through numerous potential plateaus and jumps corresponding to a number of intermediate phases. In situ and ex situ X-ray diffraction analyses reveal that the structure changes with different degrees of (de)sodiation and that eight different crystalline phases (co)exist. Furthermore, we have optimized the appropriate voltage window to achieve excellent cycling stability.