Phase and morphology change of NiCo hydroxides with controlled solvothermal synthesis for high-performance hybrid supercapacitors

Recently, hybrid supercapacitors (HSCs) garner significant attention as alternative energy storage devices that bridge the gap between lithium–ion batteries (LIBs) and electric double–layer capacitors (EDLCs). In this work, we investigate the effects of solvothermal growth with controlled mixture ratio of ethanol and water solution on morphological and electrochemical properties of NiCo hydroxides. The experimental results support our suggestion that an increase in carbonate anions accelerated by the synergistic chemical reaction causes the phase and morphology changes of NiCo hydroxides from layered double hydroxide (LDH) nanosheet to carbonate hydroxide (CH) nanowire. Moreover, it reveals that nanosheet–nanowire hierarchical NiCo hydroxides synthesized by mixture solutions exhibit excellent electrochemical performances owing to their structural merits, which provide effective ion diffusion, electron transfer, and active site accessibility. Applying these electrodes, HSCs combined with active carbon as negative electrodes are fabricated and they deliver a maximum energy density of 43.8 Wh·kg−1 at a power density of 424.3 W·kg−1, along with outstanding stability during 10,000 cycles. Our results will provide the design of a new rational structure and promising electrode materials for high–performance HSCs.