Effect of Al-doping on structural and cyclic stability of NiCoFe layered double hydroxides as electrode material in supercapacitors

Supercapacitors are an effective energy storage device can address the issue of storing renewable energy sources. Layered double hydroxides (LDHs) have been identified as a promising electrode material for supercapacitors due to their diverse constituent elements and adjustable structure. However, LDHs tend to experience mechanical deformation during charging and discharging, which results in poor cycle performance. In this study, an Al substitution strategy for Fe was employed to adjust the interlaced structure assembled by one-dimensional nanorods and two-dimensional nanosheets in NiCoFe-LDH, which led to an enhanced stability and electrochemical performance for supercapacitors. Excessive Al doping prompted nanorods to evolve into nanosheets, resulting in loss of the interlaced structure. When the ratio of Fe and Al is 0.75:0.25, the resulted Ni2Co1Fe0.75Al0.25-LDH maintained a much more perfect interlaced structure while exhibiting the best specific capacitance reaching 2367.39 F·g−1 at 1 A·g−1 as well as a 89.79% retention rate at 20 A·g−1 in a three-electrode unit; when paired with activated carbon (AC), it exhibited a specific capacitance of 394.14 F·g−1 at 1 A·g−1 with an energy density up to 140.14 Wh·kg−1 at a high power density level of 811.46 W·kg−1 in Ni2Co1Fe0.75Al0.25-LDH//AC unit. Moreover, after undergoing rigorous testing involving 5000 cycles at 5 A·g−1, Ni2Co1Fe0.75Al0.25-LDH//AC demonstrated a 55.31% retention rate, significantly higher than that observed for Ni2Co1Fe1-LDH//AC (17.58%). The Al doping strengthened the stability of host layer structure and further improved the interlaced structure of LDH, resulted in a significant improvement in specific capacitance, energy density and cycle performance. This improved material will exhibit better application prospects in supercapacitors.