Puzzles and confusions in supercapacitor and battery: Theory and solutions

Supercapacitors and batteries have been intensively investigated but there is much confusion and misleading between them in publications. In particular, some battery behaviours are incorrectly as supercapacitors and some reported “remarkable” performances like the high specific capacitance of 2188 F g−1 for Ni(OH)2 nanospheres are not true. It is likely that the energy storage mechanism and the similar device configuration/characterization techniques cause confusions. It is important to clarify and remove confusion about both theoretical and experimental aspects of these two types of energy devices. This review starts with briefing fundamentals of battery and supercapacitor specifically emphasizing the essential difference on energy storage mechanism between pseudocapacitors and batteries, in which the former undergoes rapid surface-controlled electrochemical reactions without diffusion control and phase transformation, while the latter stores energy in the crystal lattices or porous materials through much slower electrochemical reactions with limits from the phase transformation, chemical binding changes or/and reactant diffusions. Correct characterization and analysis methods such as cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy with a suggested working flowchart to distinguish them are presented while offering thoughtful discussions and explanations about the confusions. Perspectives are also offered for realizing high-energy density ECs without trading off its high power density.