Nonstoichiometry and the origin of electrochemical properties of functional oxides for energy conversion and storage technologies

Abstract

It is well known and accepted that oxygen nonstoichiometry, the deviation of oxygen content from the stoichiometric composition, significantly affects electrochemical properties of functional oxides. Therefore, it is important to understand the defect formation mechanism and the influences of defect species on electrochemical properties. In this paper, layered perovskite type La2NiO4-based oxides were investigated as a model system. Oxygen nonstoichiometry was evaluated by thermogravimetry and coulometric titration and analyzed based on defect chemistry. To understand the governing factor for interstitial oxygen formation, thermochemical deformation and electronic structural variation were evaluated by in-situ X-ray diffraction measurement and soft X-ray absorption spectroscopy. While La2NiO4+δ can deform flexibly to accept interstitial oxygen, more than 90% of interstitial sites are not occupied under equilibrium state. This strongly indicates the restriction due to crystal structure is not dominant for interstitial oxygen formation. Ni L-edge and O K-edge spectra reveal that lattice oxygen works as a sink/source of electronic carrier for the interstitial oxygen formation. These analyses indicate that the interstitial oxygen formation in La2NiO4+δ is mainly limited by the electronic structural restriction. This hypothesis is supported by the fact that electron doping increases the equilibrium concentration of interstitial oxygen.