The electrochemical performance of 3d metal oxide (MO) electrode materials for Li-ion batteries was studied in the form of Li/CoO(Co3O4) half-cells. Reversible capacity in the 750-1000 mAh/g range was achieved and sustained over numerous charge-discharge cycles both at room temperature and at 55degrees C. The studied oxides were then used as negative-electrode active materials to assemble larger plastic MO/LiCoO2 Li-ion cells, which exhibited an average output voltage of 2 V and a stable reversible specific energy of 120 Wh/kg during extended cycling at ambient and elevated temperatures. This value can be compared to 180 Wh/kg obtained for similar C/LiCoO2 Li-ion cells. Based on modeling, several scenarios involving material considerations present the optimum method for boosting the energy density of such MO/LiCoO2 Li-ion systems. (C) 2002 The Electrochemical Society.