Oxygen migration and proton diffusivity in transition-metal (Mn, Fe, Co, and Cu) doped Ruddlesden-Popper oxides

Layered Ruddlesden-Popper oxides La2NiO4/La3Ni2O7 (n = 1, 2) are considered as promising electrode candidates for electrochemical devices due to their excellent mixed ionic-electronic conducting properties. In this study, we systematically investigate both oxygen migration and proton diffusivity for transition-metal (Mn, Fe, Co and Cu) doped La2NiO4/La3Ni2O7 using first-principles calculations. The results show that the double-layered La3Ni2O7 exhibits better transport behavior than single-layered La2NiO4, which consistently corresponds to the experimental results. Transition-metal doping has a remarkable influence on the oxygen/proton transport of La2NiO4/La3Ni2O7. Furthermore, according to the in-depth electronic analysis, direct links between the barriers of oxygen/proton migration and the microelectronic properties have been established: the migration activity of oxygen ions is closely related to the degree of metal-O bonding and the charge difference gradient formed along the oxygen migration pathway, and the faster proton diffusion in the Co/Cu doped La2NiO4 and Mn/Fe/Co/Cu doped La3Ni2O7 is attributed to their weak dopant-proton association and the large capacity of the 'electron pocket' around the Fermi level. Therefore, our study presents a microscopic understanding of oxygen/proton migration in La2NiO4/La3Ni2O7-based perovskites and provides the design principle for high performance cathode materials.