Charging Mechanism of Li₂MnO₃

Abstract

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...

The Role of Ni and Co in Suppressing O‐Loss in Li‐Rich Layered Cathodes

“…The results are presented in Figure ; and Figure S3 (Supporting Information). As reported previously, [ 6,11,12 ] decreasing the Li‐richness leads to a clear decrease in the proportion of capacity associated with gas release, from exclusively O‐loss for Li 2 MnO 3 to mainly reversible O‐redox for compositions close to LiTMO 2 . The quantity of gas released is dependent on the surface/volume ratio of the cathode particles which was kept close to constant across the materials used in this study.…”

Delocalized Metal–Oxygen π-Redox Is the Origin of Anomalous Nonhysteretic Capacity in Li-Ion and Na-Ion Cathode Materials

“…MnO 3 38. The Li-based materials share the Li 2 M O 3 honeycomb structure, so they can be used to estimate ∆E decomp and ∆E * mig from a combination of enumerated and known structures.11,13,14 The calculated values of ∆E decomp and ∆E * mig , shown inFigure 1d, suggest that materials that are more susceptible to cation migration than Na4 Mn 6 O 14 and Li 2 IrO 3 , and that have larger driving forces for O 2 formation, exhibit voltage hysteresis.…”

Suppression of voltage-decay in Li2MnO3 cathode via reconstruction of layered-spinel coexisting phases

“…For pristine LMO, the plateau above 4.5 V appears as the typical behavior of Li-rich cathode, which corresponds to the delithiation, charge compensation and oxygen release process. 27,28,29,19 Upon discharge, S-shaped curve is presented with discharge capacity of around 200 mAh g -1 . It is interesting that the characteristic charge plateau (above 4.5 V) is shortened in T-LMO sample.…”