The development of a negative electrode for supercapacitors is very critical for the next-generation of energy-storage devices while it remains a great challenge. Herein, a flexible and free-standing nanocomposite electrode of Bi2O3-decorated electrospun carbon nanofibers (ESCNFs) was prepared via electrospinning technology and a simple solvothermal method. The electrochemical performance of the nanocomposite electrode was investigated. An asymmetric supercapacitor (ASC) device made of the ESCNF@Bi2O3 negative electrode and CF@NiCo2O4 positive electrode was developed. It achieved excellent electrochemical properties including high energy density, high power density, and outstanding cycling stability. Moreover, the ability to power light-emitting diodes also indicates the feasibility for practical use. This work holds great promise for developing next-generation supercapacitors with high energy density.