Influence of iron doping on α-NaMnO2 lattice symmetry: Insight from operando X-ray absorption, ex-situ structural analysis, and electrochemical performance using chestnut shell-derived hard carbon

The structural instability and moderate electrochemical performance of NaMnO₂ cathodes limit the use of sodium-ion batteries (SIBs). This limitation is primarily due to lattice distortions and valence variations that occur during the cycling process. To address this limitation, NaMn_1-xFe_xO₂ (0.00 ≤ x ≤ 0.50) powders were synthesized using a conventional solid-state method. Their structural and electrochemical properties were systematically investigated through a combination of structural characterization, in situ X-ray absorption spectroscopy, and computational modeling. X-ray diffraction and Rietveld refinement reveal a contraction of the β-angle from 112° to 105°, indicative of a phase transition from α to α′, with the x = 0.5 composition stabilizing as a single-phase α′ structure. Fe incorporation reduces the average Mn valence from 3.23+ to 3.18+, thereby enhancing structural stability, as corroborated by electron diffraction and density functional theory (DFT) calculations. At the same time, hard carbon (HC) derived from chestnut shells was developed as a sustainable anode material, exhibiting a disordered framework favorable for Na ⁺ storage. Electrochemical evaluation demonstrates that the x = 0.5 cathode delivers an initial half-cell capacity of 130.2 mAh/g, which declines to 77.1 mAh/g upon cycling. In contrast, the optimized electrode configuration affords improved stability. The HC anode attains a high reversible capacity of 317.3 mAh/g. Full-cell assemblies incorporating pre-sodiated HC anodes exhibit promising performance, underscoring the potential of this dual-material approach for developing high-performance, sustainable SIBs.