Electrochemical reduction and oxidation of a graphite electrode were carried out in nonaqueous electrolytes that consisted of ethylene carbonate/diethyl carbonate (1:1) solvents where LiClO4 and NaClO4 were codissolved. Reversible lithium intercalation into graphite occurred accompanied with no remarkable side reactions in the potential range between 1.5 and 0.02 V vs Li/(1 mol dm(-3) Li+), that is, no sodium deposition and no sodium intercalation were observed. Comparing to results in a sodium-free electrolyte, i.e., a conventional electrolyte in Li-ion batteries, the initial irreversibility was noticeably suppressed by codissolving sodium ions, and kinetics of the lithium intercalation were enhanced with better capacity retention during cycling. Additionally, the redox behavior of graphite for lithium-ion batteries was improved by sodium ion coexistence, with no inferior influences on the negative and LiMn2O4 positive electrodes and electrolyte. When the sodium ion concentration increased to 0.22 mol dm(-3), the superior reversibility of lithium intercalation was observed in comparison with that in a sodium-free system; however, the higher concentration of sodium ions deteriorated the lithium intercalation properties. We found that a surface layer formed on graphite electrode got less resistive as a lithium-ion conductor by entrapping sodium during the initial electroreduction with optimal Na+ concentration. (c) 2007 The Electrochemical Society.