A model for predication of the gravimetric and volumetric energy densities of Li-air batteries using aqueous electrolytes is developed. The theoretical gravimetric/volumetric capacities and energy densities are calculated based on the minimum weight of the electrolyte and volume of air electrode needed for completion of the electrochemical reaction with Li metal as an anode electrode. It was determined that both theoretical gravimetric/volumetric capacities and energy densities are dependent on the porosity of the air electrode. For instance, at a porosity of 70%, the maximum theoretical cell capacities are 435 mAh/g and 509 mAh/cm(3) in basic electrolyte, and 378 mAh/g and 452 mAh/cm(3) in acidic electrolyte. The maximum theoretical cell energy densities are 1300 Wh/kg and 1520 Wh/L in basic electrolyte, and 1400 Wh/kg and 1680 Wh/L in acidic electrolyte. The significant deduction of cell capacity from specific capacity of Li metal is due to the bulky weight requirement from the electrolyte and air electrode materials. In contrast, the Li-air battery using a nonaqueous electrolyte does not consume electrolyte during the discharge process and has high cell energy density. For Li-air batteries using both aqueous and nonaqueous electrolytes, the weight increases by 8-13% and the volume decreases by 8-20% after the cell is fully discharged. (C) 2008 The Electrochemical Society.