Recent advances of transition metal oxides and chalcogenides in pseudo-capacitors and hybrid capacitors: A review of structures, synthetic strategies, and mechanism studies

Supercapacitors (SCs) possesses high energy density, high power density and long cyclic performance. This re-view details the application of transition metal oxides and chalcogenides (TMOs and TMChs) in pseudo-capacitors and hybrid supercapacitors (PCs and HSCs). TMOs and TMChs with layer and spinel crystal struc-ture are discussed. The layer materials exhibit excellent capacity and rate performance but inferior cycling performance; On the contrary, spinel materials have a more stable structure and perform better cyclic perfor-mance. TMOs suffer sluggish electrochemical kinetics and poor conductivity; TMChs can make up the defects of their TMOs counterparts. Next, strategies to improve the electrochemical performance of PCs and HSCs based on TMOs and TMChs are detailed, mainly from two aspects: designing the composition, optimizing the structures/ morphologies of materials, and improving the conductivity of materials in materials' aspects; increasing the mass-loading of electrodes, fabricating free-standing electrode and assembling flexible SCs in technical aspects. Furthermore, in/ex situ techniques and theoretical calculation are discussed to explorer the fundamental mechanism of TMOs and TMChs. Lastly, the future research orientations of SCs are also timely concluded. HSCs based on TMOs and TMChs are the hotpot in the future.

Automated summary

This review provides a comprehensive overview of the application of transition metal oxides and chalcogenides in pseudo-capacitors and hybrid supercapacitors. The structural characteristics, performance, and improvement strategies are discussed. HSCs based on TMOs and TMChs are identified as a future research hotspot.