The electrochemical characteristics of Li4/3Ti5/3O4 parallel to Na3V2 (PO4)(2)F-3 hybrid-ion cells are described. In Li and mixed Na/Li electrolytes these 2.4 V cells offer encouraging performance with the active materials cycling with high utilization. In all iterations the anode reaction is based on a lithium insertion mechanism. The stability of this hybrid-ion chemistry was demonstrated by representative cells completing over 70 cycles with low capacity fade. This battery chemistry may be suitable for large format applications where safety considerations are critical. Attempts to utilize a pure Na electrolyte proved unsuccessful due to the inability of the Li4/3Ti5/3O4 to undergo Na+ insertion. (C) 2007 The Electrochemical Society.