In this work, small sulfur molecules (S2-4) confined in microporous graphitic carbon (MGC) demonstrates excellent performance in Li-S battery with carbonate-based electrolyte. The MGC is synthesized via the simultaneous activation and graphitization of peanut shell char promoted by K2FeO4, which possesses ultra-micropore (pore width <0.7 nm) volume as high as 0.65 cm3 g−1 and the predominant pore width less than 0.4 nm. After sulfur infusion, the S/MGC composite with 50.5 wt% sulfur loading exhibits superior long-term cycling stability and rate performance in S/MGC | Li full cell. The cell delivers high capacity of 1146 mAh g−1 at 0.1 C and 570 mAh g−1 even at 4 C. Moreover, the capacities of 826 and 571 mAh g−1 remain after 1000 cycles at 1 C and 2 C, respectively. In addition, the cell exhibits good storage property. The well confined S2-4 inside the interconnected ultra-micropores with graphitic carbon walls endows the S/MGC composite with good electronic conductivity and stability, while un-occupied pores within the S/MGC facilitate fast Li+ transport and kinetics of electrochemical reactions. This work offers a green and sustainable route to promote the application of Li-S batteries.