Engineering Zn-doped NiCo2O4 spinels for enhanced charge storage in high-performance asymmetric supercapacitors

Abstract In this work, we investigated the effect of Zn 2+ doping on the structural, morphological and electrochemical properties of NiCo 2 O 4 spinel (NCO), aiming to employ it as an active material in asymmetric supercapacitors (ASCs). The doped samples were synthesized via a microwave-assisted hydrothermal method using different Zn 2+ molar proportions (1%, 3% and 5%) and characterized by XRD, Raman, FEG-SEM, EDS, XPS and BET analyses. Zn 2+ incorporation induced structural modifications, including enlarged lattice parameters and an increase in specific surface area. The lattice expansion and associated distortion generate oxygen vacancies and wider diffusion pathways, which facilitate electrolyte ion transport and enhance the accessibility of redox-active sites. Simultaneously, the increased specific surface area provides more electrochemically active interfaces, promoting faster surface-controlled charge storage. The sample containing 5% Zn 2+ (NC5) exhibited the best electrochemical behavior, reaching a specific capacitance of 488.88 F g −1 at 1 A g −1 . In an ASC configuration with activated carbon as the anode, NC5 delivered a high energy density (75.30 Wh kg −1 ) and power density (up to 3616 W kg −1 ), presenting competitive performance compared with other doped materials reported in the literature and showing cycling stability above 4000 cycles. Overall, Zn 2+ doping enhances electrochemical performance by coupling lattice distortion-assisted ion diffusion with surface area-driven pseudocapacitive contributions, highlighting Zn-doped NCO as a promising candidate for high-performance energy storage applications. Graphical Abstract