Insights into Ligand-centered Redox Mechanisms in Li-rich Metal Oxides during Battery Reactions

Abstract

Li-ion battery technology is central to achieve a sustainable society. However, the energy density of current Li-ion batteries is limited by the relatively low capacity of conventional cathodes, in comparison with that of anodes. Going beyond current design rules based on conventional cationic redox, tapping into the lattice oxygen redox displayed by Li-rich transition metal oxides could transcend the current barrier in capacity. This promise makes Li-rich transition metal oxides desirable as next-generation cathodes. However, the associated deficiencies, such as lattice oxygen release, structural reorganization, sluggish kinetics, voltage hysteresis and fade, largely impede their commercialization. These drawbacks are mainly derived from underpinning mechanisms that remain to be fully elucidated, whereby lattice oxygen redox reactions are activated and stabilized. Building a thorough picture of lattice oxygen redox could provide insights into the design of high-energy-density cathode materials. In order to understand the chemistry of lattice oxygen redox reaction, a series of Li-rich 4d/5d transition metals oxides are investigated in this thesis via various X-ray spectroscopic probes. Li2RuO3, a canonical oxide proposed for lattice oxygen redox, displayed a dramatic evolution of its crystal and electronic structure during the first cycle with a large hysteresis in both voltage and chemistry, which notably decreased in subsequent cycles. X-ray spectra demonstrated a different extent of involvement of Ru and O upon various (de)lithiation stages, indicating different pathways upon charge and discharge and possibly providing an explanation to the hysteresis. Li3RuO4 exhibited unambiguous lattice oxygen redox upon first oxidation to 3.9 V, which also competes with side processes such as electrolyte decomposition and, to a much lesser extent, oxygen loss. This process subsequently then unlocked a reversible conventional cationic redox with the formal Ru5+/Ru4+ couple in the following re-intercalation and subsequent cycles. This compound could also experience highly reversible (de)lithiation associated with a conventional cationic redox with the formal Ru5+/Ru4+ couple between 1.5 and 2.5 V. In spite of having the same O/M ratio as Li3RuO4, Li3IrO4 displayed highly reversible involvement of non-equivalent O sites with different number of non-bonding O 2p states in electrochemical reactions upon delithiation. In contrast, this compound also presented a typical conventional interaction reaction to Li4.7IrO4 accompanied by the formal oxidation of Ir in a reversible manner. Li7RuO6, with the highest O/M ratio of all oxides evaluated, showed highly reversible lithiation to Li8RuO6 between 1.5 and 2.6 V, obeying a conventional cationic redox via the formal Ru5+/Ru4+ couple. In contrast to all other Ru oxides evaluated, Li7RuO6 underwent reversible delithiation to 3.5 V with minimal hysteresis, but it was compensated by the formal Ru5+/Ru6+ redox reaction and involvement of O via covalency with Ru.