Definitions of Pseudocapacitive Materials: A Brief Review

Pseudocapacitive materials generally offer both high capacitance and high rate capability, which has stimulated great efforts in developing the materials system and related energy storage devices. In recent years, however, with the extensive use of nanomaterials in batteries, fast redox kinetics comparable to pseudocapacitive have been achieved in many kinds of battery materials due to the much shortened ion diffusion lengths and highly exposed surface/interface as a result of nanosize effect. Consequently, the terms “pseudocapacitive materials” and “battery materials” are becoming more and more confusing. In this review, different opinions on the definition of pseudocapacitive materials and the evolution of the definitions as well as the resulting confusion will be firstly reviewed. Then, to accurately distinguish pseudocapacitive and battery materials, method with the consideration of both the electrochemical signatures (CVs and GCD) and quantitative kinetics analysis as a supplement is proposed. Finally, we end this review by discussing the possible device configurations of asymmetric supercapacitors and hybrid supercapacitors. The present review will help understanding the differences between pseudocapacitive materials and battery materials, and thus avoiding the definition confusion.