Operando X-ray Studies of Anode Materials for Lithium-ion Batteries

Abstract

Among electrical energy storage devices (EES), lithium-ion batteries (LIBs) are considered to have the highest energy and power densities. They are currently used in numerous applications including hybrid and electric vehicles, consumer electronics, grid leveling and other industrial applications. Conversion materials, as opposed to conventional intercalation/insertion compounds, show great promise because of their potentially high theoretical capacities. The goal of studying this class of materials is to develop a detailed mechanistic understanding of the reactions involved, which can lead to the development of new materials and improvement of existing ones. Given the highly complex nature of conversion reactions, analytical methods to study these mechanisms are critically important. Operando X-ray measurements, conducted at the Cornell High Energy Synchrotron Source (CHESS), provide insights into the structural and chemical changes that occur during cycling. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) are complementary techniques that yield invaluable information aiding the development of a complete mechanistic picture of battery materials. Nickel molybdate (NiMoO4) is a high capacity anode material whose conversion reaction is believed to yield reduced metal clusters and lithium oxide. Operando XAS measurements indicated that the final oxidation state of molybdenum was in a partially reduced state while the nickel appeared to be fully reduced, while operando XRD indicated that there was likely electrolyte decomposition due to the significant increase in the electrolyte background. Niobium Oxynitrides (NbOxNy) and niobium oxynitrides doped with materials such as tantalum, aluminum, titanium and silicon were also investigated. The pure niobium oxynitride crystallized in a cubic space group and exhibited a significant concentration of vacancies, which helped contribute to the long term cycle life of these materials. Upon cycling, the niobium centers are reduced to Nb3+ and subsequently reoxidized to Nb5+. All dopants resulted in amorphous mixtures that displayed enhanced capacity within the first few cycles, but compromised the overall cycle life. The potential of operando confocal XRF-XANES experiments, for the characterization of energy materials, was successfully demonstrated. These operando studies have provided a great deal of valuable mechanistic insights into conversion materials which may yield high-performance battery systems for multiple applications and guide future studies.