The Energy Density Perspective: From Electrochemical Capacitor to Battery/Capacitor Asymmetric Cells

摘要

A formula describing the dependence of energy density on specific capacitance, operational voltage, and salt concentration is given based on the optimized weight (or volume) ratio of the electrode material and the electrolyte. It shows that for electrochemical double-layer capacitors using non-aqueous electrolytes, the maximum energy density of the capacitor will be limited mainly by the low salt concentration of the electrolyte. The optimum mass density of the electrode can be obtained based on the value of the theoretical energy density for capacitors with different electrolytes. It also shows that for electrochemical pseudocapacitors using metal oxide electrodes, the salt concentration in the electrolyte remains constant during charge and discharge cycling. The maximum energy density of the capacitor will be limited mainly by the specific capacitance and operational voltage. 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), is also 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. Two specific asymmetric systems, including carbon/LiPF_6 EC:DMC/Li_xTi_5O_(12) and Ni(OH)_2/KOH H_2O/carbon were evaluated for their maximum theoretical energy density and swing voltage.