Unlocking the energy storage potential of silicon: Morphological and electrochemical mechanism studies of Si-NiO/graphene//AC/MnO₂ composites for sustainable energy storage applications

Silicon (Si) is an abundant battery-active material with attractive physicochemical characteristics, but limitations in practical applications are volume expansion and poor cycle stability. To address these issues, this study developed a hybrid energy storage system combining battery-type and supercapacitor-type mechanisms. Si-NiO/graphene composites were used as battery-type anodes, while AC/MnO₂ served as supercapacitor-type cathodes assembled in a coin cell device configuration. The addition of Si effectively reduced the diameter of NiO agglomeration (to ∼2–5 μm) and increased the porosity by 75.85%, leading to an increase in the surface area from 36.434 to 288.266 m2/g. As a result, the Si-NiO/graphene composites exhibited enhanced electrochemical performance, with a decrease in charge transfer resistance (Rct) from 16.1 Ω to 10.75 Ω and a 127% increase in specific capacitance from 7.61 to 17.29 F/g. The device assembled on a coin cell device yielded an energy density of 7.176 Wh/kg and a power density of 391.837 W/kg. In addition, the voltage drop during galvanostatic cycling was reduced from 0.09 V to 0.04 V, enabling a higher maximum power output (1111 W/kg vs. 2500 W/kg). Furthermore, Si promoted a dominant capacitive charge storage contribution over a wide range of scan rates, indicating the ability to alleviate diffusion limitations and ensure stable ion intercalation pathways. These results demonstrate that Si serves as an effective structural and electrochemical enhancer for NiO/graphene electrodes, offering a promising strategy for high-performance energy storage systems.