Boosting the Energy Density of Flexible Solid-State Supercapacitors via Both Ternary NiV2Se4 and NiFe2Se4 Nanosheet Arrays

Because of the demand for sustainable energy storage devices, investigating high energy density and costeffective electrodes for flexible supercapacitors (SCs) is essential; however, the emergence of such high-performance electrodes to fulfill the requirements of industrial sectors remains a highly challenging task. Herein, we successfully demonstrated the preparation of ternary metal selenides of nickel-vanadium selenide (NixV(3-x)Se(4)) and nickel-iron selenide (NixV(3-x)Se(4)) series by a simple and low-cost hydrothermal method, followed by selenization for flexible asymmetric SC (ASC). The impacts of Ni2+ are studied and shown to lead to a significant enhancement in electrochemical properties, which varied with the stoichiometric ratio of Ni-V/Fe in NixV(3-x)Se(4) and NixFe3-xSe4 nanosheet arrays. The optimized NiV2Se4 and NiFe2Se4 electrodes displayed high specific capacities (,-329 and 261 mA h g-1, respectively, at 1 mA cm-2), excellent rate performances (capacity retentions of about 79.33 and 77.78%, respectively, even at 50 mA cm-2), and outstanding cycling stabilities (98.6 and 97.9% capacity retentions after 10 000 cycles, respectively). Most notably, the NiV2Se4//NiFe2Se4 ASC provides an excellent energy density of 73.5 W h kg-1 at a power density of 0.733 kW kg-1 and superior cycling performance (96.6% capacity retention after 10 000 cycles). The high-performance nanostructured flexible ASCs show promise in portable electronics and zero-emission transportation.