Li2S has a high theoretical capacity of 1166 mAh g-1, but it suffers from limited rate and cycling performance. Herein we reported in-situ synthesis of thermally exfoliated graphene-Li2S (in-situ TG-Li2S) nanocomposite and its application as a superior cathode material alternative to sulfur. Li2S nanoparticles with the size of ~8.5 nm homogeneously anchored in graphene nanosheets were prepared via chemical reduction of pre-sublimed sulfur by lithium triethylborohydride (LiEt3BH). The in-situ TG-Li2S nanocomposite exhibited an initial capacity of 1119 mAh g-1 Li2S (1609 mAh g-1 S) with a negligible charged potential barrier in the first cycle. The discharge capacity retained 791 mAh g-1 Li2S (1137 mAh g-1 S) after 100 cycles at 0.1C and exceeded 560 mAh g-1 Li2S (805 mAh g-1 S) at a high rate of 2C. Moreover, coupling the composite with Si thin film anode, a Li2S/Si full cell was produced, delivering a high specific capacity of ~900 mAh g-1 Li2S (1294 mAh g-1 S). The outstanding electrode performance of in-situ TG-Li2S composite was attributed to the well dispersed small Li2S nanoparticles and highly conductive graphene nanosheets, which provided merits of facile ionic and electronic transport, efficient utilization of the active material, and flexible accommodation of volume change.