Abstract
Redox chemistry is at the origin of many energy-related electrochemical devices, among which Li-ion batteries (LIBs) have become the leading energy storage technology for portable electronics and vehicle electrification. Throughout its history, LIB technology has relied on cationic redox reactions as the sole source of energy storage capacity. However, in 2013 our group evolved this technology by demonstrating the reversible activation of the anionic network, in particular oxygen with the formation of perroxo (O2)n- groups upon Li removal. This development has enabled exacerbated increases in energy storage capacity by a factor of 2. This discovery, which has received attention from the battery research community, represents a transformational approach to creating advanced energy materials, not only for energy storage, but also for water splitting applications, as both involve peroxo species. However, as is often the case with new discoveries, it's important to understand and rationalize the basic science behind them. Specifically, what are the ion and electron transport mechanisms in these Li-driven anionic redox reactions and others ?
That's what this lecture has tried to answer by describing the origin and the various key scientific steps, achieved not without pitfalls, that led to the advent of this new paradigm. The possibility of preparing the CoO2 phase is certainly one of them, as the existence of such a compound with a Co oxidation state below 4 suggests oxygen oxidation ((O2)n- ; holes on O), recalling J. Rouxel's earlier work on hole chemistry in chalcogenides. However, we have shown how this idea, highly controversial in the battery research community, was realized 15 years later with the advent of the new generation of Li-rich lamellar oxides, notably Li2MnO3. Thanks to the synergy between varied substitution chemistry involving metals (4d and 5d), the use of conventional characterization tools (X-rays, neutrons, microscopy), but also of hitherto little-used spectroscopies (EPR, XPS, XAS, RIXS) and fundamental theoretical studies, it was possible to understand the fundamental origin of anionic redox. The latter is linked by the presence of a non-bonding oxygen band located in the vicinity of the " d' " band in the electronic structure of these materials, which extra-capacitance can easily explain. What remained to be understood, however, was the departure of O2 and the densification of the surface at high potential, the disparity between the charge potential traces 1stcharge and 1stdischarge potential, the wide hysteresis, and even the specific nature of the oxidized (O2)n species. Recent work indicating the formation of molecular O-O analogous to the way in which O2 binds to the iron of the hemoglobin molecule (metal-ligand donations (Fe O-O)), will be reported and discussed.
