A novel energy storage system employing a KS-6 graphite cathode and niobium (V) oxide (Nb2O5) anode was developed with a 1:1 weight ratio of cathode to anode. The cell, with a voltage range of 1.5–3.5 V, showed higher capacity and better cycle performance than those of cells with other voltage ranges. At 1C and 35C rates, this cell delivered 57 and 26 mAh g−1, respectively. A graphite cathode can be increased to approximately 5.2 V, which is well above the LIB cathode material, without causing safety issues, and the operating voltage of the Nb2O5 anode was greater than that of the lithium deposition voltage. In situ X-ray diffraction results at various states of charge indicated that the mechanism of this energy storage system was intercalation and de-intercalation of PF6 − and Li+ in the KS-6 graphite cathode and in the Nb2O5 anode. This novel energy storage system was inherently safe because 1) no oxygen is released from cathode materials, 2) no lithium dendrite is used at the anode, and 3) there was no possibility of overcharge from the electrode/electrolyte reaction.