A new reversible and high-performance potassium-selenium (K-Se) battery, using confined selenium/carbonized-polyacrylonitrile (PAN) composite (c-PAN-Se) as cathode and metallic potassium as anode, is reported in this work. The PAN-derived carbon matrix could effectively confine the small Se molecules and provide a sufficient buffer for the volume changes. The reversible formation of small-molecule trigonal Se (Se1, P3121) phase could essentially inhibit the formation of polyselenides and account for outstanding electrochemical performance. The carbonate-based electrolyte further synergistically diminishes the shuttle effect by inhibiting the formation of polyselenides in the meantime. The as-prepared K-Se battery shows a reversible capacity of 1904mAhcm−3 after 100 cycles at 0.2C and rate retention of 89% from 0.1 to 2C. In addition, the charge-discharge mechanism is also investigated via the combination of in-situ and ex-situ synchrotron X-ray diffraction (XRD), and Raman spectroscopy analysis. The results reveal that the introduction of K+ ions leads to the cleavage of C-Se bonds, the rearrangement of selenium atoms, and the final formation of the main product K2Se. Moreover, the reversible formation of trigonal Se (Se1, P3121) phase was detected in the reaction with K+. These findings not only can advance our understanding of this family of batteries, but also provide insight into chemically-bonded selenium composite electrodes, which could give guidance for scientific research and the optimization of Se and S electrodes for the K-S, Na-S, Li-S, Na-Se, and Li-Se batteries.