Copper‐Induced Phase Transitions in NaMn _1‐x Cu _x O ₂ : Structural Insights from Operando XAS, DFT Calculations, and Electrochemical Evaluation Using Laurus Nobilis‐Derived Hard Carbon

Abstract This study investigates the effect of Cu ²⁺ doping on NaMn _{1‐ x} Cu _x O ₂ layered cathodes. It also explores their integration with Laurus nobilis ‐derived hard carbon (HC) anodes for sodium‐ion batteries (SIBs). Cu doping, particularly at x = 0.20, stabilizes the β ‐NaMnO ₂ phase, suppresses Jahn–Teller distortions, and improves the structural stability of the MnO ₂ framework. In situ X‐ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations confirm that Cu improved Na ⁺ diffusion kinetics and reduces charge‐transfer resistance, despite its electrochemical inactivity. X‐ray Difraction (XRD), Raman, and Fourier transform infrared spectroscopy (FTIR) analyses reveal phase destabilization and segregation at higher Cu concentrations, while XPS indicates shifts in the Mn/Cu oxidation states, consistent with improved electronic conductivity and multivalent redox behavior. The scanning electron microscope (SEM and transmission electron microscopy (TEM) images demonstrate Cu‐induced morphological transitions toward denser, more crystalline structures. Brunauer–Emmett–Teller (BET) measurements reveal that the L. nobilis ‐derived hard carbon (HC) anode possesses a high surface area and hierarchical porosity, which facilitated efficient Na + storage and rapid ion transport. Full‐cell tests demonstrate high reversible capacity (≈126 mAh g ⁻¹ ), excellent rate capability, and 56% capacity retention over 250 cycles. This work demonstrates that Cu doping and porous HC anodes synergistically enhance the structural and electrochemical performance of SIBs, thereby providing a sustainable strategy for advanced energy storage.