Investigation of Spin State of Mn3+ in 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. According to the ligand field theory, the Mn3+ ions in the octahedral ligand (MnO6) appear as high-spin (t2g3eg1, S = 2) and low-spin (t2g4eg0, S = 1) ions, respectively. The high-spin Jahn-Teller (J-T) Mn3+ ion is one of the reasons for the severe structural degradation and voltage decay of the Mn-based layered materials. 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 Mn3+ spin state, voltage decay and structural degradation of β-NaMnO₂ during cycling remain largely unexplored. This work aims to investigate the Mn3+ spin state evolution, voltage decay mechanism and structure evolution of β-NaMnO₂ using advanced X-ray Magnetic Circular Dichroism (XMCD), complemented by operando X-ray diffraction (XRD) and pair distribution function (PDF) analysis, to deconvolute the key factors contributing to Mn3+ spin state evolution. Concerning the long-standing analytical challenge in characterising the Mn3+ spin state which related the J-T distortion, XMCD would be critical to gaining insights into these materials degradation mechanism to establish their structure-property relationship for further performance enhancement.