A FeCo-Se@NiCo-PO<sub>4</sub> Electrode Designed by\nHierarchical Strategy for Supercapacitors and NiCo//Bi Batteries

In this work, FeCo-Se and NiCo-PO<sub>4</sub> were electrodeposited\non nickel foam (NF) successively to prepare a cathode material for\nasymmetric supercapacitors (ASCs) and NiCo//Bi batteries. FeCo-Se@NiCo-PO<sub>4</sub> combines the advantages of transition metal selenides (TMSs)\nand transition metal phosphates (TMPs). FeCo-Se electrodeposited in\nthe underlying layer can facilitate electron transfer for higher\nconductivity. NiCo-PO<sub>4</sub> in the outer layer can facilitate\nOH<sup>–</sup> ions diffusion because TMPs can be intercalated\ninto ions readily and the outer robust P–O bond of TMPs can\nstabilize the structure. Precisely because the hierarchical structure\nmaximizes the synergy between FeCo-Se and NiCo-PO<sub>4</sub>, FeCo-Se@NiCo-PO<sub>4</sub> delivers a rapid electron/ion transfer capability and superior\nelectrochemical performance. The FeCo-Se@NiCo-PO<sub>4</sub> exhibits\na high specific capacitance of 2221.5 F g<sup>–1</sup> (888.6\nC g<sup>–1</sup>) at 1 A g<sup>–1</sup>. Its aqueous\nASC shows specific capacitance of 115.8 F g<sup>–1</sup> at\n1 A g<sup>–1</sup> and all-solid-state ASC presents high reversibility.\nIts aqueous NiCo//Bi battery has superior durability of about 60%\ncapacity retention and 98% Coulombic efficiency after 2300 cycles.\nAnd its all-solid-state NiCo//Bi battery possesses a higher energy\ndensity and power density.