The limitations of energy density of battery/double-layer capacitor asymmetric cells

The formula describing the energy density of asymmetric cells, which consists of a battery-type electrode (such as lithium intercalated compound) and an electrochemical capacitor-type electrode (such as activated carbon), was derived. From the formula, the optimal mass (or volume) ratio of battery electrode to capacitor electrodes and electrolyte can be obtained for achieving the maximum theoretical gravimetric (or volumetric) energy density. The voltage swing of the cell during charge and discharge cycles was also described. Relationships between the energy density, ion concentration of the electrolyte, specific capacity of battery electrode, specific capacitance of capacitor electrode, and maximum operational voltage were also given. Three specific asymmetric systems, including carbon/LiPF6 ethylene carbonate: dimethyl carbonate (EC:DMC)/LixTi(5)O(12), carbon/LiPF6 EC:DMC/WO2, and Ni(OH)(2)/KOH H2O/carbon were evaluated for their maximum theoretical energy density and swing voltage. It was found that for asymmetric cells using nonaqueous electrolyte, the maximum energy density (about 30 Wh/kg) was limited mainly by the electrolyte due to the low ion concentration; however, for asymmetric cells using aqueous electrolytes, the maximum energy density (about 40 Wh/kg) was limited mainly by the capacitor electrode. The maximum operational voltage always plays an important role in the maximum energy density. (C) 2003 The Electrochemical Society.