Unraveling the reaction mechanism of low dose Mn dopant in Ni(OH)2 supercapacitor electrode

Abstract

Mn doping is deemed as a promising strategy to improve the electrochemical performance of the α-Ni(OH)2 battery-type supercapacitor electrode. However, the internal structure evolution, the pathways and the dynamics of the proton/intercalated anion migration, as well as the functioning mechanism of Mn dopant to stabilize the layered structure during cycles remain unclear. Here, we unveil that irreversible oxidization of Mn3+ at the initial CV cycles, which will remain as Mn4+ in the NiO2 slabs after the first oxidization to effectively suppress the phase transformation from α-Ni(OH)2/γ-NiOOH to β-Ni(OH)2/β-NiOOH and further maintain the structural integrity of electrode. With a synergistic combination of theoretical calculations and various structural probes including XRD and 2H MAS solid state NMR, we decode the structure evolution and dynamics in the initial CV (cyclic voltammetry) cycles, including the absorption/desorption of hydrogen containing species, migration of intercalated anions/water molecules and the change of interlayer space. This present work elucidates a close relationship between doping chemistry and structural reliability, paving a novel way of reengineering supercapacitor electrode materials.