Crystallinity transformation engineering of hydrous cobalt nickel phosphate cathodes for hybrid supercapacitor devices: Extrinsic/battery to intercalation type pseudocapacitors

The necessitating strategies of rationalizing nanostructures and crystallinity of electrode material are crucial to developing high-efficiency hybrid energy storage devices. To enhance the energy storage performance of bimetal phosphates, it is important to have a strategic blend of metal cations that can work together synergistically. So, this work describes the scalable preparation of binder-free cobalt nickel phosphates thin film cathodes with cations variation (Co/Ni) via the facile Successive Ionic Layer Adsorption and Reaction (SILAR) process. The distinct roles of cations in cobalt nickel phosphate electrodes resulted in a noteworthy structural transformation from crystalline to amorphous with an increment of Ni content. The change in cations composition influences the pseudocapacitive performance from extrinsic/battery to intercalation type, and cobalt nickel phosphate electrode with an ideal cations composition (Co:Ni) ratio of approximately 1:1 attain the highest specific capacitance (SCs) of 2142 F g−1 (1071 C g−1). To fulfil the demand for high-performance hybrid supercapacitor devices, aqueous (HASC) and solid-state (HSSC) supercapacitors are assembled using optimized cobalt nickel phosphate (S-CNP5) as a cathode. The HASC device represents maximum SCs of 120 F g−1 with specific energy (SE) and specific power (SP) of 42.59 Wh kg−1 and 1600 W kg−1, respectively. The flexible HSSC device shows impressive SCs of 89 F g−1 and SE of 31.54 Wh kg−1 at SP of 2057.14 W kg−1. The practical demonstration of an HSSC device to power a 12-white LED lamp suggests that the SILAR strategy offers commercializing insights for fabricating high-specific capacity hydrous cobalt nickel phosphate thin film cathodes.