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The phase-transition engineering has shown great potential in designing high-performance energy storage and catalytic materials. Herein, we report the phase transition and selenium vacancy induced by the trace boron doping into CoSe 2 , realizing an enhancement of the performance of supercapacitor. The trace amounts of boron doping (∼ 0.07 wt%) induce the phase-transition from cubic to orthorhombic structures and trigger selenium vacancies in orthorhombic cobalt diselenide (o-CoSe 2). The doped defective cobalt diselenide (o-CoSe 2−x /B) exhibits a high specific capacity of 376.3 mA h g−1 at a current density of 1 A g−1, which is much larger than that of the undoped cubic cobalt diselenide (211.3 mA h g−1) and annealed cubic cobalt diselenide (208.3 mA h g−1). A hybrid supercapacitor assembled using o-CoSe 2−x /B and activated carbon delivers a high energy density of 102.5 W h kg−1 at a power density of 850.4 W kg−1 and maintains an energy density of 34.0 W h kg−1 at 16,767.1 W kg−1. In addition, the device possesses an excellent cycling stability that remains 83.5 % of the initial capacitance after 25,000 cycles at a current density of 10 A g−1. These results demonstrate that the boron is a new efficient dopant that can dramatically improve the supercapacitor performance of CoSe 2 through the structural phase transition and defect engineering. [Display omitted] • Trace amounts of boron induces phase-transition from c-CoSe 2 to o-CoSe 2. • Se vacancies defects arise with the help of boron doping engineering. • The o-CoSe 2−x /B shows the enhanced performance as electrode for supercapacitor. • Providing an efficient dopant for design of CoSe 2 based energy storage materials. [ABSTRACT FROM AUTHOR]