This work describes the carbothermal preparation of silicon-oxy-carbide (SiOC) lamellae using two-dimensional siloxene sheets and alginic acid as precursors. X-ray photoelectron spectra, X-ray diffraction, Fourier-transform infrared spectra, high-resolution transmission electron micrographs, and Raman spectra revealed the formation of lamella-like SiOC nanostructures. Symmetric supercapacitors (SSCs) were fabricated using SiOC nanostructures as electrodes and evaluated in aqueous (1 M Li2SO4) and organic (1 M TEABF4) electrolytes. SiOC SSC fabricated with Li2SO4 electrolyte operated over a voltage window of 2.0 V, with an energy density of 14.2 Wh kg−1 and a power density of 6666 W kg−1. SiOC SSC fabricated using TEABF4 electrolyte operates over a voltage window of 3.0 V and delivered a device capacitance of about 16.71 F g−1, energy density of 20.89 Wh kg−1, with excellent cyclic stability and superior rate capability. Strikingly, the high-power density of the TEABF4-based SiOC SSC (15,000 W kg−1) reached the required power target for next-generation electric vehicles and is suitable for high-performance supercapacitor devices.