Electro-Optical Correlation of Charge-Transfer Kinetics in Composition-Tuned Wurtzite Oxide Anodes for Sodium-Ion Storage

Composition-tuned Zn1−xCuxO (x = 0.0, 0.1, and 0.2) nanoparticles were synthesized via a facile co-precipitation method and investigated as wurtzite oxide anodes for sodium-ion batteries (SIBs). This work focuses on establishing an electro-optical correlation between optical band-edge modulation and electrochemical charge-transfer kinetics. X-ray diffraction confirmed the formation of the hexagonal wurtzite ZnO phase without detectable secondary phases, while FESEM and particle-size distribution analysis showed that the Zn0.9Cu0.1O sample possessed a more refined and uniform particle population than pristine ZnO. Diffuse reflectance spectroscopy (DRS) revealed a non-linear optical response with composition, where the direct optical band gap decreased from 3.08 eV for pristine ZnO to 2.66 eV for Zn0.9Cu0.1O, followed by partial recovery to 2.99 eV for Zn0.8Cu0.2O. Density functional theory (DFT) calculations supported this trend by showing the appearance of additional electronic states near the Fermi level after Cu incorporation, indicating improved electronic accessibility. Electrochemical impedance spectroscopy further confirmed that Zn0.9Cu0.1O exhibited the lowest specific charge-transfer resistance of 185.15 Ω cm2 and the highest Na+ diffusion coefficient of 5.77 × 10−11 cm2 s−1, compared with pristine ZnO and Zn0.8Cu0.2O. The improved sodium storage performance of Zn0.9Cu0.1O is, therefore, attributed to the favorable combination of particle uniformity, band-edge modulation, and interfacial charge-transfer kinetics. These findings demonstrate a composition-dependent association between optical band gap modulation and charge-transfer behavior in semiconductor-type oxide anodes for sodium-ion storage.