Understanding the Intrinsic Electrochemistry of Ni-Rich Layered Cathodes

摘要

The demand for energy is continually increasing overtime and the key to meeting future demand in a sustainable way is with energy storage. Li-ion batteries employing layered transition metal oxide cathodes are one of the most technologically important energy storage technologies. However, current Li-ion batteries are unable to access their full theoretical capacity and suffer from performance limiting degradation over time partially originating from the cathode and partially from the interface with the electrolyte. Understanding the fundamental limitations of layered transition metal oxide cathodes requires a complete understanding of the surface and bulk of the materials in their most delithiated state.;In this thesis, we employ LiNi0.8Co0.15Al 0.05O2 (NCA) as a model system for Ni-rich layered oxide cathodes. Unlike its parent compound, LiCoO2, NCA is capable of high states of delithiation with minimal structural transitions. Furthermore, commercially available NCA has little to no transition metals in the Li layer. X-ray spectroscopies are an ideal tool for studying cathodes at high states of delithiation due their elemental selectivity, range of probing depths, and sensitivity to both chemical and electronic state information. The oxidation state of the transition metals at the surface can be probed via X-ray photoelectron spectroscopy (XPS) while both bulk and surface oxidation states as well as changes in metal oxygen bonding can be probed using X-ray absorption spectroscopy (XAS).;Using X-ray spectroscopy in tandem with electrochemical, transport and microscopy measurements of the same materials, the impedance growth with increasing delithiation was correlated with the formation of a disordered NiO phase on the surface of NCA which was precipitated by the release of oxygen. Furthermore, the surface degradation was strongly impacted by the type of Li salt used in the electrolyte, with the standard commercial salt LiPF6 suffering from exothermic decomposition at high voltages and temperatures. Substituting LiPF6with LiBF4 suppressed NCA surface degradation and the dissolution of the transition metals into the electrolyte which is responsible for the impedance growth. Even in the most extreme conditions (4.75V vs Li +/Li0 at 60 °C for > 100 hrs) the degradation (i.e. metal reduction) was restricted to the first 10-30 nm and no evidence of oxygen loss was observed in the bulk.;However, the transition metal ions were found to cease oxidizing above 4.25 V vs Li+/Li0 despite it being possible to extract 20% more lithium. Using a newly developed high efficiency resonant inelastic x-ray scattering (RIXS) spectrometer to probe the O K-edge of NCA electrodes at various conditions, it was concluded that oxygen participates in the charge compensation at the highest states of delithiation instead of the transition metals. These results are intrinsic to the physical and electronic structure of NCA and appear general to the other layered transition metal oxides currently under consideration for use as cathodes in Li-ion batteries.