Ruddlesden-Popper type La2NiO4+δ oxide coated by Ag nanoparticles as an outstanding anion intercalation cathode for hybrid supercapacitors

Abstract

Electrode materials play the important role in electrochemical capacitors (ECs). Ruddlesden-Popper type La2NiO4+δ oxides were investigated as an anion intercalation cathode. It has a special crystal structure, which leads to an interesting oxygen transportation. Ag shows significant conductivity, fast rates of oxide ion dissociation and diffusion, a relatively low price, and high oxygen solubility. La2NiO4+δ oxide was synthesized by citrate method, and then was coated by Ag nanoparticles (La2NiO4+δ@Ag) to further improve the cycling stability and capacity. XRD, SEM, TEM and XPS were used to characterize the phase composition, microstructure and surface chemistry. The electrochemical performances in 1 M KOH electrolyte at room temperature were investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests. The capacity at 1 mV s−1 of La2NiO4+δ@Ag is 466.4 C g−1, and the average efficiency at 10 A g−1 approximates to 93% for 10,000 cycles, showing superior performance stability. Oxygen intercalation, migration and extrusion mechanisms of the La2NiO4+δ@Ag are related to oxygen transportation in the perovskite LaNiO3 layers and the rock salt LaO layers. The superior cycling stability and electrode performance can be ascribed to Ruddlesden-Popper structure of La2NiO4+δ, the role of Ag and the effective unitization of redox process. The assembled La2NiO4+δ@Ag//AC hybrid supercapacitor device shows an excellent energy density of 44.7 Wh kg−1 at a power density of 800 W kg−1.