Investigation of Voltage Decay Mechanism of Corrugated Zigzag O3-NaMnO2 cathode in the Sodium-ion Batteries

Rechargeable sodium-ion batteries are promising for large-scale energy storage owing to the abundance of sodium and its low cost1. O3-NaMnO2 type layered transition metal oxides have received great attention due to their high theoretical specific capacity (265 mAh g-1), cost benefits and sustainable resource supply. Nevertheless, the local Mn3+ John-Teller (J-T) distortion will trigger co-operative J-T followed by structural collapse, leading to the loss of reversible capacity upon Na+ extraction/insertion in α-NaMnO2 (monoclinic)2. Here, we propose mitigate severe co-operative J-T distortion by employ the β-NaMnO2 form (orthorhombic) as cathode which is distributing the strain more evenly across the crystal lattice3. However, the electrochemical performance remains unsatisfactory. Here, we introduce high-valence Nb into the Mn site of β-NaMnO₂ to optimize the local coordination environment and reduce lattice strain energy, thereby stabilizing the structure. Despite these efforts, the mechanisms underlying voltage decay, structural degradation, and stacking disorder in β-NaMnO₂ during cycling remain largely unexplored. This work aims to investigate the voltage decay mechanism and structure evolution of β-NaMnO₂ using advanced X-ray Absorption Spectroscopy (XAS), complemented by operando X-ray diffraction (XRD) and pair distribution function (PDF) analysis, to deconvolute the key factors contributing to degradation. Concerning the long-standing analytical challenge in characterising the voltage decay which related the local structure, operando XAS would be critical to gaining insights into these materials crystallographic and electronic structure to establish their structure-property relationship for further performance enhancement.