Electrochemical performance of a graphite electrode for lithium ion batteries are easily improved by sodium ion dissolved in an electrolyte solution as reported recently by our group. On the contrary, when potassium ions were added by dissolving 0.2moldm−3 KPF6 into a 1moldm−3 LiPF6 ethylene carbonate–diethyl carbonate (1:1, v/v) electrolyte solution prior to charge–discharge cycle, an electrochemical performance of a graphite electrode was deteriorated compared to that tested in an additive free electrolyte; larger irreversible capacity at the first cycle and worse retention of discharge capacities on successive cyclings. During the first charge in a potassium additive system, ex situ XRD observation of graphite electrodes revealed that lithium intercalation hardly proceeded in the lower potential region less than 0.1V versus Li though lithium ions were intercalated at >0.1V. Electroreduction of potassium ions on the electrode occurred instead of lithium intercalation into graphite in the lower potential region. Furthermore, the electrode surface morphology observed by electron microscopes after charge–discharge tests got less uniform in the potassium added electrolyte.