Nickel manganese hydroxides with thin-layer nanosheets and multivalences for high-performance supercapacitor

Abstract We propose a simple and effective one-step hydrothermal strategy to prepare various nickel manganese layered double hydroxides (NiMn-LDHs) by changing the concentrations of NiCl2∙6H2O and MnCl2∙4H2O under a fixed molar ratio of Ni to Mn of 3:1. The SEM and TEM results indicate that the obtained NiMn-LDHs are composed of thin-layer nanosheets with well-improved dispersity. Furthermore, the NiMn-LDHs displayed by the XPS analysis contain multivalent Ni and Mn ions, in which the relative contents of Ni2+ and Mn3+ are increasing with the decrease of reactant concentration. When the concentration reduce to 0.0125 mol L-1, the optimal Ni1Mn1-LDH exhibits both the thin-layer nanosheets structure with average thickness of about 15 nm and the highest contents of Ni2+ and Mn3+. Applied as an electrode material for supercapacitors, the Ni1Mn1-LDH can deliver an excellent electrochemical performance with a maximum specific capacity of 624 C g-1 at a current density of 0.5 A g-1 and above 80% capacitance retention over 1000 cycles. The electrochemical performance of Ni1Mn1-LDH is further investigated by assembling two-electrode asymmetric supercapacitor with carbon spheres. The specific capacity is 106 C g-1 at 0.5 A g-1 and the capacitance retention is 75% over 1000 cycles. This work offers a facile way to tune simultaneously the nanostructure and multivalence of the NiMn-LDHs, and also provides a confirmation about the important affection of them on the enhanced capacitive performance for supercapacitor electrode.