HIGHLIGHTS• This article reviewed the recent progress on material challenges, charge storage mechanism, and electrochemical performance evaluation of supercapatteries.• Supercapatteries bridge the gap between supercapacitors (low energy density) and batteries (low power density). Fig. 1 a Ragone plot of various electrochemical energy conversion and storage devices [43]. b Schematic illustration of charge storage mechanism of EDL capacitor in porous carbon electrode. c Representation of EDLC structures: Helmholtz model, Gouy-Chapman model and Gouy-Chapman-Stern model. Schematic representation of the charge storage mechanisms in pseudocapacitor; d Intercalation (bulk redox) and e surface redox Nano-Micro Lett.(2020) 12:85Page 5 of 46 85 Table 1 Classification of various energy storage devices according to their charge storage mechanisms NFCS non-Faradaic capacitive storage = EDLC storage, CFS capacitive Faradaic storage = pseudocapacitive storage, NCFS non-capacitive Faradaic storage = battery-type storage Device Supercapattery Battery Supercapacitor Hybrid supercapacitor EDLC Pseudocapacitors

We report the synthesis of few-layered MoSe2 nanosheets using a facile hydrothermal method and their electrochemical charge storage behavior. A systematic study of the structure and morphology of the as-synthesized MoSe2 nanosheets was performed. The downward peak shift in the Raman spectrum and the high-resolution transmission electron microscopy images confirmed the formation of few-layered nanosheets. The electrochemical energy-storage behavior of MoSe2 nanosheets was also investigated for supercapacitor applications in a symmetric cell configuration. The MoSe2 nanosheet electrode exhibited a maximum specific capacitance of 198.9 F g(-1) and the symmetric device showed 49.7 F g(-1) at a scan rate of 2 mV s(-1). A capacitance retention of approximately 75% was observed even after 10 000 cycles at a high charge-discharge current density of 5 A g(-1). The two-dimensional MoSe2 nanosheets exhibited a high specific capacitance and good cyclic stability, which makes it a promising electrode material for supercapacitor applications.

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