A facile synthesis of selenium sulfide (SeSx)/carbonized polyacrylonitrile (CPAN) composites is achieved by annealing the mixture of SeS2 and polyacrylonitrile (PAN) at 600 °C under vacuum. The SeSx molecules are confined by N-containing carbon (ring) structures in the carbonized PAN to mitigate the dissolution of polysulfide and polyselenide intermediates in carbonate-based electrolyte. In addition, formation of solid electrolyte interphase (SEI) on the surface of SeSx/CPAN electrode in the first cycle further prevents polysulfide and polyselenide intermediates from dissolution. The synergic restriction of SeSx by both CPAN matrix and SEI layer allows SeSx/CPAN composites to be charged and discharged in a low-cost carbonate-based electrolyte (LiPF6 in EC/DEC) with long cycling stability and high rate capability. At a current density of 600 mA g−1, it maintains a reversible capacity of 780 mAh g−1 for 1200 cycles. Moreover, it retains 50% of the capacity at 60 mA g−1 even when the current density increases to 6 A g−1. The superior electrochemical performance of SeSx/CPAN composite demonstrates that it is a promising cathode material for long cycle life and high power density lithium ion batteries. This is the first report on long cycling stability and high rate capability of selenium sulfide-based cathode material.