Fundamental consideration for electrochemical engineering of supercapattery

Supercapattery is the generic name for various electrochemical energy storage (EES) devices combining the merits of battery (high energy density) and supercapacitor (high power density and long cycling life). In this article, the principle and applications of EES devices are selectively reviewed as the background for supercapattery development. The focus is on the engineering aspects for fabrication of two types of supercapattery: (i) by coupling a battery electrode with a supercapacitor electrode, or (ii) from materials that possess both the Nernstian and capacitive charge storage capacities. Fundamental rationales are discussed in relation with the designs, such as why the device is always asymmetrical, and what materials are suitable for making supercapattery. Whilst the key is how to optimize device performance in terms of energy capacity, power capability and cycle life, cost is also discussed on resource rich materials such as nanostructured composites and redox electrolytes.