A Raman-Based Investigation of the Fate of Li₂MnO₃in Lithium- and Manganese-Rich Cathode Materials for Lithium Ion Batteries

Abstract

The relationship between structure and electrochemical performance of lithium-and manganese-rich cathode materials with the general formula xLi 2 MnO 3 •(1-x)LiMO 2 is under intensive study world-wide in the context of its importance to the development of high energy/high capacity lithium batteries. One of the issues raised in these studies is the fate of the Li 2 MnO 3 component as a function of voltage and repeated cycling. We have performed Raman spectroscopy based measurements that shed light on the transformations of the Li 2 MnO 3 phase as a function of state-of-charge. We find that on charging the Li 2 MnO 3 phase appears to de-lithiate at a rate at least equal to that of the LiMO 2 phase, whereas, on discharge a Li 2 MnO 3 -like component reforms later in the discharge than does the LiMO 2 phase. The absence of X-ray diffraction evidence for the presence of Li 2 MnO 3 after the first charge/discharge cycle can be reconciled by the possibility that the C2/m structured Li 2 MnO 3 exists in domains thick enough to diffract; but during subsequent discharging, a perturbed C2/m phase forms in conjunction with the re-lithiated LiMO 2 phase in thin sheets (electrochemically induced topotaxy) that are not thick enough to diffract but form in sufficient volume to give Raman scattering similar to that of In contrast to conventional layered cathode materials for lithium ion batteries, such as LiNi x Co y Mn z O 2 with x + y + z ∼ = 1.0 leading to an R-3m-like structure, the lithium-and manganese-rich version of this composition (often designated as LMR-NMC) has a layered-layered structure that can be better described as xLi 2 MnO 3 · (1-x)LiMO 2 (M = Ni-Mn-Co).1 It has been clearly demonstrated that this LMR-NMC material consists of not only the R-3m phase but also the Li 2 MnO 3 phase with C2/m structure, with the two structures integrated at the domain level. The two component nature of the pristine material has been investigated by a wide range of techniques, including electrochemistry, X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), nuclear magnetic resonance spectroscopy (NMR), and Raman spectroscopy. The high lithium content in the C2/m phase gives LMR-NMC a much higher energy density making it a promising high energy cathode material for lithium ion batteries. Whereas it has been clearly demonstrated that the C2/m phase is no longer detected, e.g., by XRD, after activation at high voltage against lithium (see for example Li et al.2 ), Yu and Yanagida 3 claim evidence for a synergetic relationship between the C2/m component and the R-3m component of LMR-NMC materials wherein the two layered phases stabilize one another during charge and discharge. In prior publications our group showed that the Li 2 MnO 3 component activated in the first charge/discharge cycle is continuously active throughout the whole operating voltage window (2.0 to 4.6 V) during the charge process, but is more active below 3.6 V during the discharge process. [4][5] Ongoing research rela...

On the Localized Nature of the Structural Transformations of Li2MnO3 Following Electrochemical Cycling

“…Additionally, the STEM observation of Li 2 MnO 3 -structure close to the surface of discharged particles (e.g., Figure 3 b), where the highest degree of delithiation is expected during charge, suggests that this region of Li 2 MnO 3 reformed during discharge rather than represents an area that did not participate in the electrochemistry. The assumption of partial Li 2 MnO 3 ↔ Li x Mn 4/3 O 4 reversibility is in agreement with recent NMR reports [ 7,38 ] that 7% of Li removed from the transition metal planes returns to its original sites upon discharge; with the observation by Raman spectroscopy [ 39 ] [ 40 ] Similar to our STEM images of cycled particles, our irradiation experiments, which are conducted by rastering a highintensity electron probe over the entire particle, show pockets of damage in the oxide bulk. As noted above, the structural transformations, resulting from either electrochemical cycling or electron beam irradiation, occur in localized areas both at particle surfaces and in the oxide bulk.…”

In Situ Formed Layered-Layered Metal Oxide as Bifunctional Catalyst for Li-Air Batteries

“…The Mn-O stretching from Li 2 MnO 3 with C2/m space group can be seen at 423 cm −1 . 23,24 XRD data together with Raman spectrum supports the view that the composite oxide structure has two phases at the bulk level.…”

Composition and Impedance Heterogeneity in Oxide Electrode Cross‐Sections Detected by Raman Spectroscopy

“…Maps of the following types were obtained: (1) spot‐to‐spot maps wherein specific locations were targeted, these targeted locations being at the centers of 20 or more NCM523 crystallite agglomerates throughout the cross‐section of the laminate and (2) area maps wherein Raman spectra were recorded across a rectangular grid of points in a stepwise/serpentine manner. All baseline corrections, intensity normalizations, and spectra averaging were performed with Wire 4.4 and GRAMS AI software using well established, internally consistent procedures employed regularly in our laboratory . Because the nickel‐rich NCM materials are in general poor Raman scatterers compared to the manganese‐rich and cobalt‐rich NCM materials, obtaining spectra of adequate quality in terms of signal‐to‐noise required many accumulations per spot probed.…”

“…Whereas the latter techniques provide average information from all particles within the path of the X‐ray or electron beam, Raman spectroscopy can be used to obtain information from individual oxide agglomerates and even individual particles within the agglomerates. Because the frequencies and intensities of Raman‐active bands emanating from NCM lattices are sensitive to crystal symmetry, coordination geometry, and oxidation states, the technique has been extensively used to characterize transition metal oxides in battery electrodes …”