Understanding how materials that catalyse the oxygen evolution reaction (OER) function is essential for the development of efficient energy-storage technologies. The traditional understanding of the OER mechanism on metal oxides involves four concerted proton-electron transfer steps on metal-ion centres at their surface and product oxygen molecules derived from water. Here, using in situ 18O isotope labelling mass spectrometry, we provide direct experimental evidence that the O2 generated during the OER on some highly active oxides can come from lattice oxygen. The oxides capable of lattice-oxygen oxidation also exhibit pH-dependent OER activity on the reversible hydrogen electrode scale, indicating non-concerted proton-electron transfers in the OER mechanism. Based on our experimental data and density functional theory calculations, we discuss mechanisms that are fundamentally different from the conventional scheme and show that increasing the covalency of metal-oxygen bonds is critical to trigger lattice-oxygen oxidation and enable non-concerted proton-electron transfers during OER.

/pdf/activating-lattice-oxygen-redox-reactions-in-metal-oxides-to-1qnt83uoha.pdf

Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution

Li-ion batteries have contributed to the commercial success of portable electronics and may soon dominate the electric transportation market provided that major scientific advances including new materials and concepts are developed. Classical positive electrodes for Li-ion technology operate mainly through an insertion-deinsertion redox process involving cationic species. However, this mechanism is insufficient to account for the high capacities exhibited by the new generation of Li-rich (Li1+xNiyCozMn(1−x−y−z)O2) layered oxides that present unusual Li reactivity. In an attempt to overcome both the inherent composition and the structural complexity of this class of oxides, we have designed structurally related Li2Ru1−ySnyO3 materials that have a single redox cation and exhibit sustainable reversible capacities as high as 230 mA h g−1. Moreover, they present good cycling behaviour with no signs of voltage decay and a small irreversible capacity. We also unambiguously show, on the basis of an arsenal of characterization techniques, that the reactivity of these high-capacity materials towards Li entails cumulative cationic (Mn+→M(n+1)+) and anionic (O2−→O22−) reversible redox processes, owing to the d-sp hybridization associated with a reductive coupling mechanism. Because Li2MO3 is a large family of compounds, this study opens the door to the exploration of a vast number of high-capacity materials.

Reversible anionic redox chemistry in high-capacity layered-oxide electrodes

Ultimate Limits to Intercalation Reactions for Lithium Batteries

The 5d level positions of the trivalent lanthanides in inorganic compounds

During the charging and discharging of lithium-ion-battery cathodes through the de- and reintercalation of lithium ions, electroneutrality is maintained by transition-metal redox chemistry, which limits the charge that can be stored. However, for some transition-metal oxides this limit can be broken and oxygen loss and/or oxygen redox reactions have been proposed to explain the phenomenon. We present operando mass spectrometry of (18)O-labelled Li1.2[Ni0.13(2+)Co0.13(3+)Mn0.54(4+)]O2, which demonstrates that oxygen is extracted from the lattice on charging a Li1.2[Ni0.13(2+)Co0.13(3+)Mn0.54(4+)]O2 cathode, although we detected no O2 evolution. Combined soft X-ray absorption spectroscopy, resonant inelastic X-ray scattering spectroscopy, X-ray absorption near edge structure spectroscopy and Raman spectroscopy demonstrates that, in addition to oxygen loss, Li(+) removal is charge compensated by the formation of localized electron holes on O atoms coordinated by Mn(4+) and Li(+) ions, which serve to promote the localization, and not the formation, of true O2(2-) (peroxide, O-O ~1.45 A) species. The quantity of charge compensated by oxygen removal and by the formation of electron holes on the O atoms is estimated, and for the case described here the latter dominates.

Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen

Structural determination of some MPS3 layered phases (M = Mn, Fe, Co, Ni and Cd)

The increase in energy of the sp anionic band on going from oxides to the less electronegative sulfides, selenides, or tellurides (with a similar trend in neighboring columns) facilitates anion–cation redox interactions involving the d levels of transition metal cations and the sp levels of anionic species. The interactions can induce phase transitions or gradual change within a given structural model. When a cation is reduced by electron transfer to its d levels from the sp band, holes appear at the top of the latter. Interesting soft chemistry can be carried out based on redox processes that neutralize the holes with electrons. This approach also allows particular structural types to be stabilized. Three structural domains can be recognized amongst the transition elements: 1) on the left-hand side of the periodic table layered structures are observed involving M4+ and (chalcogen)2- ions; 2) formation of sets of metal-metal bonds is then observed, the geometry of which depends on both the initial electron population and the electron transfer to the metal; 3) on the right-hand side the metals in their highest oxidation state are no longer active, and the holes at the top of the sp band are taken up by a catenation of the anions, which can lead to full polymerization of the anionic sublattice.

Jean Rouxel

Papers

Structural determination of some MPS3 layered phases (M = Mn, Fe, Co, Ni and Cd)

Anion–Cation Redox Competition and the Formation of New Compounds in Highly Covalent Systems

Insertion de lithium la structure lamellaire NiPS3

Characterization of the new series of quasi one-dimensional compounds(MX4)nY (M =Nb, Ta; X =S, Se; Y =Br, I)

Preparation et structure cristalline de I0.33NbSe4(I4Nb12Se48)