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

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.