Phenyl tris-2-methoxydiethoxy silane (PTMS) has been studied as an additive to the PC-based electrolyte of lithium ion batteries with graphite anode. An electrolyte of 1.0M LiPF6 in propylene carbonate (PC):dimethyl carbonate (DMC) (1:1 v/v) containing phenyl tris-2-methoxydiethoxy silane can suppress the co-intercalation of PC. The main reason for this suppression is that silane can form a cross-polymerized network of SiOSi bonds on the graphite surface, thus restraining PC molecules from co-intercalating with lithium ions into graphite electrodes. The electrochemical properties of the modified electrolyte were characterized by cyclic voltammetry, charge/discharge tests, SEM, XPS and FT-IR.