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 cost-effective 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 (Ni_xV_3-xSe₄) and nickel-iron selenide (Ni_xFe_3-xSe₄) 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 Ni_xV_3-xSe₄ and Ni_xFe_3-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.