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 Å) 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.

The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2p orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li-O(2p)-Li interactions). Na[MgMn]O exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na[MgMn]O does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg remains in Na[MgMn]O, which stabilizes oxygen.

Conventional intercalation cathodes for lithium batteries store charge in redox reactions associated with the transition metal cations, e.g., Mn(3+/4+) in LiMn2O4, and this limits the energy storage of Li-ion batteries. Compounds such as Li[Li0.2Ni0.2Mn0.6]O2 exhibit a capacity to store charge in excess of the transition metal redox reactions. The additional capacity occurs at and above 4.5 V versus Li(+)/Li. The capacity at 4.5 V is dominated by oxidation of the O(2-) anions accounting for ∼0.43 e(-)/formula unit, with an additional 0.06 e(-)/formula unit being associated with O loss from the lattice. In contrast, the capacity above 4.5 V is mainly O loss, ∼0.08 e(-)/formula. The O redox reaction involves the formation of localized hole states on O during charge, which are located on O coordinated by (Mn(4+)/Li(+)). The results have been obtained by combining operando electrochemical mass spec on (18)O labeled Li[Li0.2Ni0.2Mn0.6]O2 with XANES, soft X-ray spectroscopy, resonant inelastic X-ray spectroscopy, and Raman spectroscopy. Finally the general features of O redox are described with discussion about the role of comparatively ionic (less covalent) 3d metal-oxygen interaction on anion redox in lithium rich cathode materials.

Operando mass spectroscopy demonstrates quantitatively that lithium extraction from Li2MnO3 is charge compensated by oxygen loss (O-loss) not oxidation of oxide ions which are retained within the structural framework (Oredox). This data is confirmed by X-ray absorption and emission spectroscopy. Li NMR shows that the two-phase coreshell structure, which forms on charging, is composed of an intact Li2MnO3 core and highly disordered shell containing no Li, with a composition close to MnO2. Discharge involves Li insertion into the disordered shell. CO2 and O2 are detected on charging at 15 mAg -1 , whereas charging by GITT forms only CO2; an observation in agreement with the previously described model of oxygen evolution from high voltage cathodes producing singlet O2 that reacts with the electrolyte forming CO2. The dominance of oxygen evolution over O-redox is in accord with the model of O-loss occurring when the oxide ions are undercoordinated; O in the shell devoid of Li is coordinate by only 2 Mn. ExperimentalSynthesis: Li2MnO3 was synthesized using a sol−gel method 14 . Stoichiometric amounts of LiCH3COO•2H2O (99.0%, Sigma-Aldrich), and Mn(CH3COO)2•4H2O (99.0%, Fluka) were dissolved in distilled water containing 0.1 mol of resorcinol (99.0%, Fluka), 0.15 mol of formaldehyde (Fluka 36.5% in water), and 0.25 mmol of Li2CO3 (99.0%, Sigma-Aldrich). The mixture was heated under vigorous stirring at 70 °C until an opaque uniform gel was formed. The gel was dried at 90 °C overnight, and finally heated in air first at 500 °C for 12 h and then at 800 °C for 12 h to obtain the final product. ICP-OES:Elemental analysis of the as-prepared sample was carried out after its complete dissolution in acidic solution by ion coupled plasma optical emission spectroscopy (ICP-OES) using a PerkinElmer Optima 7300DV ICP-OES. This allowed for the exact quantification of Li and Mn.Powder X-ray Diffraction (XRD): Powder X-ray diffraction analysis was carried out using a Rigaku SmartLab Xray powder diffractometer equipped with a 3 kW Cu anode.Soft X-ray Absorption and Resonant Inelastic X-ray Spectroscopy: SXAS and RIXS spectra were recorded at beamline BL27SU of the RIKEN/JASRI Spring8 synchrotron in Japan. Ex situ cathode samples were loaded onto adhesive copper tape and measured under 10 -6 Pa high-vacuum conditions. To obtain SXAS spectra at the O Kedge and Mn L-edge, partial fluorescence yield (PFY) method was employed and the fluorescence signal was recorded using silicon drift detectors (SSD). The O K-edge RIXS spectra were recorded using an XES monochromator composed of a varied-line-spacing cylindrical grating and a CCD detector with a resolution of 0.5 eV in the energy range of interest. Electrochemical Measurements:The working electrodes were prepared with a composition of 80 wt % active material, 10 wt % Super P carbon, and 10 wt % polytetrafluoroethylene (PTFE) binder. The components were combined with a pestle and mortar and then calendared to ~60 µm between two sheets of Al foil before being punched into 10 mm  pellets. T...

Lithium-rich materials, such as Li 1.2 Ni 0.2 Mn 0.6 O 2 , exhibit capacities not limited by transition metal redox, through the reversible oxidation of oxide anions. Here we offer detailed insight into the degree of oxygen redox as a function of depth within the material as it is charged and cycled. Energy-tuned photoelectron spectroscopy is used as a powerful, yet highly sensitive technique to probe electronic states of oxygen and transition metals from the top few nanometers at the near-surface through to the bulk of the particles. Two discrete oxygen species are identified, O nÀ and O 2À , where n < 2, confirming our previous model that oxidation generates localised hole states on O upon charging. This is in contrast to the oxygen redox inactive high voltage spinel LiNi 0.5 Mn 1.5 O 4 , for which no O nÀ species is detected. The depth profile results demonstrate a concentration gradient exists for O nÀ from the surface through to the bulk, indicating a preferential surface oxidation of the layered oxide particles. This is highly consistent with the already well-established core-shell model for such materials. Ab initio calculations reaffirm the electronic structure differences observed experimentally between the surface and bulk, while modelling of delithiated structures shows good agreement between experimental and calculated binding energies for O nÀ .