An asymmetric supercapacitor, exploiting nm-scale conformal coating of conducting polymer (CP) on aligned carbon nanotubes (A-CNTs) as the negative electrode and ultra-high density A-CNTs as the positive electrode, has been developed. The conformal CP coating on the A-CNTs enhances charge storage while the aligned nanotube morphology provides straight and fast ion transport pathways. The A-CNT electrode, densified using a unique mechanical method, possesses high volumetric capacitance while preserving the aligned morphology to maintain the high power, provides an ideal positive electrode for the asymmetric supercapacitor. By complementary tailoring of the asymmetric electrodes, the device exhibits a wide operation voltage of 4V with maximum energy and power densities of 82.8WhL−1 and 130.6kWL−1 in volumetric performance. In this paper, a new method was introduced which is simple but can determine directly the energy storage efficiency of a supercapacitor cell. An equivalent circuit was developed to model the performance of each electrode and investigate the asymmetric design of the cell.