Definition of Redox Centers in Reactions of Lithium Intercalation in Li₃RuO₄ Polymorphs

Abstract

Cathodes based on layered LiMO 2 are the limiting components in the path toward Li-ion batteries with energy densities suitable for electric vehicles. Introducing an overstoichiometry of Li increases storage capacity beyond a conventional mechanism of formal transition metal redox. However, the role and fate of the oxide ligands in such intriguing additional capacity remain unclear. This reactivity was predicted in Li 3 RuO 4 , making it a valuable model system. A comprehensive analysis of the redox activity of both Ru and O under different electrochemical conditions was carried out, and the effect of Li/Ru ordering was evaluated. Li 3 RuO 4 displays highly reversible Li intercalation to Li 4 RuO 4 below 2.5 V vs Li + /Li 0 , with conventional reactivity through the formal Ru 5+ −Ru 4+ couple. In turn, it can also undergo anodic Li extraction at 3.9 V, which involves O states to a much greater extent than Ru. This reaction competes with side processes such as electrolyte decomposition and, to a much lesser extent, oxygen loss. Although the associated capacity is reversible, reintercalation unlocks a different, conventional pathway also involving the formal Ru 5+ −Ru 4+ couple despite operating above 2.5 V, leading to chemical hysteresis. This new pathway is both chemically and electrochemically reversible in subsequent cycles. This work exemplifies both the challenge of stabilizing highly depleted O states, even with 4d metals, and the ability of solids to access the same redox couple at two very different potential windows depending on the underlying structural changes. It highlights the importance of properly defining the covalency of oxides when defining charge compensation in view of the design of materials with high capacity for Li storage.

A New Approach to Stable Cationic and Anionic Redox Activity in O3‐Layered Cathode for Sodium‐Ion Batteries

“…The EXAFS data revealed no change in the RuO distances on charge and vice versa on discharge (Figure 4d); however, the distances varied in the RuRu/Ni shell, which is associated with the neighboring Ni 2+/3+ redox reaction, reflecting the local structural change. These phenomena are consistent with the results for Ru-based Li-rich compounds studied by Tarascon et al [44] and Cabana et al, [45] demonstrating that the hybridization between Ru 5p and 4d states is enhanced in a distorted octahedral coordination, increasing the pre-edge peak. Furthermore, we observed Ru 3p XPS spectra for the first cycle to prove the inactivity of Ru in Na[Ni 2/3 Ru 1/3 ]O 2 in the voltage range 2-4.1 V (Figure S4, Supporting Information).…”

Elucidation of Active Oxygen Sites upon Delithiation of Li3IrO4

“…While other oxides have conclusively been shown to display unconventional LOR activity upon delithiation, many display chemical hysteresis, whereby subsequent reduction does not return the compound to its original crystal and/or electronic structure. 16,17,[34][35][36][37] Since the final step of O 2 loss is irreversible, 19 we will only elaborate on the In other Li-rich transition metal oxides, LOR is most often associated with the onset of a sharp feature in resonant inelastic X-ray scattering (RIXS) maps, located at ~523 eV emission when the excitation was at ~531 eV. 23,24,27,28 RIXS maps were also measured for Li x IrO 4 (Figure 4), with an estimated flux (5 x 10 10 -1 x 10 11 photons/s), below the reported threshold of sample damage reported in the literature.…”

Elucidation of Active Oxygen Sites upon Delithiation of Li3IrO4

“…Li 1 IrO 4 , with the material returning to its initial state. This behavior distinguishes Li 3 IrO 4 from other lithiated oxides where LOR unlocks a different pathway involving conventional CR,17,34,35,37,47 including parent Li 3 RuO 4 36,48. This report clearly pinpoints the O states that enable the shift of the redox centers to the ligands.…”

“…While other oxides have conclusively been shown to display unconventional LOR activity upon delithiation, many display chemical hysteresis, whereby subsequent reduction does not return the compound to its original crystal and/or electronic structure. 16,17,[34][35][36][37] Since the final step of O 2 loss is irreversible, 19 we will only elaborate on the In other Li-rich transition metal oxides, LOR is most often associated with the onset of a sharp feature in resonant inelastic X-ray scattering (RIXS) maps, located at ~523 eV emission when the excitation was at ~531 eV. 23,24,27,28 RIXS maps were also measured for Li x IrO 4 (Figure 4), with an estimated flux (5 x 10 10 -1 x 10 11 photons/s), below the reported threshold of sample damage reported in the literature.…”

“…Our DFT calculations show that this effect is related to the statistical Li/Ir disorder within the transition metal layer of the oxide. Since O 2 loss does not occur when the oxidation is limited to x = 1,19 we can indisputably ascribe the behavior of Li 3 IrO 4 -Li 1 IrO 4 to LOR, beyond the bounds of CR 36. Interestingly, although evolution of O 2 accompanied by amorphization is induced when the last 1/3 of Li are removed, the O spectra indicate that ligands in a-IrO 3 remained highly oxidized despite the evolution of O 2 19.…”