Structural evolution of a Ni/NiO_x based supercapacitor in cyclic charging-discharging: A polarized neutron and X-ray reflectometry study

Ni/Ni-oxide based supercapacitors with their excellent stability and long cycle lifetime are favorable due to their cost effectiveness and practicality. And yet, the full picture of their cyclic charging-discharging process is not well understood, and the influential factors on the cycle life of a supercapacitor are complicated. Using a combined polarized neutron and X-ray reflectometry approach, we have studied the structural evolution of a layered Ni/NiO_x supercapacitor electrode, operated in an alkaline electrolyte for cyclic charging-discharging. For the lower thousands of cycles, oxidation of Ni current-collecting backbone and dissolution of outer Ni oxide electroactive materials contribute to a total thickness consumption of 2 nm. Upon higher thousands of cycles, the two-dimensional NiO_x surface-layer evolves into a three-dimensional porous network as a result of the “hole drilling” depletion behavior of Ni oxide, i.e., dissolution of the “more porous” part and preservation of the “more compact” part in the NiO_x layer. The extra surface area of the Ni/NiO_x supercapacitor generated after cyclic charging-discharging gives rise to an increased capacitance. Evidenced by the increased derived density, the crystal defects of inner-layer NiO_x are also eliminated with cycling, probably through Ni atom rearrangement, filling of oxygen vacancies within NiO_x, or both.