p-Toluenesulfonyl isocyanate (PTSI) has been used as a novel film-forming additive in tetramethylene sulfone (TMS)-based electrolytes and the mixed electrolytes have been examined for use in rechargeable lithium-ion batteries. The ionic conductivities of the TMS/PTSI composite electrolytes with lithium salt are mostly in the order of 10−3 Scm−1 at ambient temperature and the electrochemical stability windows are in excess of 5.0V versus Li/Li+. Compared with pure TMS-based electrolyte, the mixed electrolytes show lower melting points, better wettability and enhanced battery performance. The improved battery performance is mainly attributed to the formation of an effective solid electrolyte interface layer formed by the reductive decomposition of PTSI. The battery performance is influenced strongly by the presence of sulfonyl groups in isocyanate and the addition of different lithium salts. Moreover, the mixed electrolytes exhibit good thermal stability and low combustibility. All of the results show that the sulfonyl isocyanate/sulfone mixed electrolytes are a promising choice for use in rechargeable lithium-ion batteries.