Lithium-rich and manganese-rich (LMR) layered transition metal (TM) oxide composites with general formula xLi 2 MnO 3 · (1-x)LiMO 2 (M = Ni, Co, Mn) are promising cathode candidates for high energy density lithium ion batteries. Lithium-manganese-rich TM oxides crystallize as a nanocomposite layered phase whose structure further evolves with electrochemical cycling. Raman spectroscopy is a powerful tool to monitor the crystal chemistry and correlate phase changes with electrochemical behavior. While several groups have reported Raman spectra of lithium rich TM oxides, the data show considerable variability in terms of both the vibrational features observed and their interpretation. In this study, Raman microscopy is used to investigate lithium-rich and manganese-rich TM cathodes as a function of voltage and electrochemical cycling at various temperatures. No growth of a spinel phase is observed within the cycling conditions. However, analysis of the Raman spectra does indicate the structure of LMR-NMC deviates significantly from an ideal layered phase. The results also highlight the importance of using low laser power and large sample sizes to obtain consistent data sets. Lithium-manganese-rich TM oxides form as the integrated structure of two layered phases: xLi 2 MnO 3 with C2/m space group symmetry and (1-x)LiMO 2 (M = Co, Mn, Ni, Fe, Cr) with R3m space group symmetry. While the exact structure of lithium-rich TM oxides has been the subject of much debate, a number of recent studies conclude that the material forms as the composite of these two distinct phases with nanoscale domains of Li 2 MnO 3 and LiMO 2 character. 1-6The intense interest in lithium excess cathodes stems from the potential to deliver very large reversible capacities (>200 mAh g −1 ) when charged beyond 4.5 V vs. Li 0 /Li + . 7 Part of the capacity derives from electrochemical activation of the Li 2 MnO 3 component at voltages beyond 4.4 V. Two mechanisms have been proposed for this activation. The dominant mechanism appears to be the simultaneous removal of lithium and oxygen to form a composition similar to MnO 2 . [8][9][10][11] Protons from electrolyte decomposition may also exchange for lithium. This secondary mechanism becomes more significant at higher temperature.12-14 Despite the promise of lithium rich TM oxides to deliver very high capacities, one significant limitation remains the large voltage and capacity fade observed upon extended electrochemical cycling. 15,16 A number of studies have attempted to link structural changes in lithium-rich TM oxides with electrochemical stability and cycling behavior.9,17-25 In particular, several groups have applied Raman microscopy to understand the structure and structural evolution of lithium-rich, manganese-rich TM oxides of nickel, manganese, and cobalt (hereafter LMR-NMC). 18,19,24,[26][27][28][29][30][31] Raman microscopy offers high spatial resolution (< 1μm 3 ), large field of view, and chemical specificity, making it an ideal tool to investigate both the pristine material and comp...

The dissociation of hexafluorosilicate has been reinvestigated due to recent suggestions that fluorosilicate intermediates may be present in appreciable concentrations in drinking water. 19 F NMR spectroscopy has been used to search for intermediates in the hydrolysis of hexafluorosilicate. No intermediates were observable at 10 -5 M concentrations under excess fluoride forcing conditions over the pH range of 3.5-5. A single intermediate species, assigned as SiF 5or its hydrate, was detected below pH 3.5. At moderate pH values of 4 and 5 silica oligomerization in the solutions studied made it difficult to directly determine the hexafluorosilicate equilibrium constant. Under more acidic conditions the average pK d , or negative log of the dissociation constant K d , determined by 19 F NMR measurements, was 30.6. We also investigated the behavior of hexafluorosilicate in common biological buffer reagents including phosphate/citrate, veronal/HCl buffers, and Ringer's solution. The buffer capacity of all of these systems was found to be insufficient to prevent acidic shifts in pH when hexafluorosilicate was added. The pH change is sufficient explanation for the observed inhibition of acetylcholinesterase that was previously attributed to hexafluorosilicate hydrolysis intermediates.

Raman microspectroscopy is widely used for musculoskeletal tissues studies. But the fluorescence background obscures prominent Raman bands of mineral and matrix components of bone tissue. A 532-nm laser irradiation has been used efficiently to remove the fluorescence background from Raman spectra of cortical bone. Photochemical bleaching reduces over 80% of the fluorescence background after 2 h and is found to be nondestructive within 40 min. The use of electron multiplying couple charge detector (EMCCD) enables to acquire Raman spectra of bone tissues within 1-5 s range and to obtain Raman images less than in 10 min.

This paper addresses the problem of determining the number of pure chemical components in a mixture by applying the maximum likelihood estimator (MLE) of intrinsic dimension. The application here is to Raman spectroscopy data, although the method is general and can be applied to any type of data from a chemical mixture. We show that the MLE produces superior results compared to other methods on both simulated and real chemical mixtures, and is accurate even when minor components are present. Even if the signal-to-noise (SN) ratio is very low, accurate estimates can still be obtained by smoothing the data before applying the estimator, this approach is illustrated on two real datasets with high noise levels. Since the MLE is computed locally at every data point, we also show how the local estimates can be used for other applications, such as segmenting the specimen into homogeneous regions.

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Current R&D efforts in lithium-ion batteries are directed towards increasing energy density with reduced cost, improved cycle-life and safety performance. Among other important factors, developing high energy density lithium-ion cathodes is a high priority area. Recently, there has been significant progress made on a number of high voltage cathode chemistries such as LiMn1.5Ni0.5O4 1 and lithium-manganese rich NMC (LMR-NMC) 2, 3 cathodes. However, significant technical challenges still need to be addressed at both materials and electrode level for their practical use. Issues include structural and phase stability under continuous high voltage cycling, transition Metal (TM) dissolution, oxygen evolution etc. We have carried out confocal-Raman imaging and spectral analysis of LMR-NMC cathode having nominal composition of Li1.2Mn0.525Ni0.175Co0.1O2, maintained at different state of charge (SOC) and cycled 25 and 200 times between a voltage window of 4.9-2.5V. Figure 1. Raman mapping (top) and spectral analysis (bottom) of pristine LMR NMC electrode. The false color maps show carbon rich regions (red), metal oxide (cyan) and mixed colors for both carbon-LMR-NMC regions. The Raman maps and their spectral behavior change as the electrodes are charged (discharged). Both spectral position and full width at half maximum (FWHM) change with the voltage (SOC) and also under continuous electrochemical cycling. To quantify the inhomegneities of the LMR-NMC electrodes we undertake Raman analysis for cathode particle selected from different regions of electrodes charged at 3.9, 4.2, 4.5 and 4.9V, and compare to pristine electrode. Analysis was carried out at least on 5-6 cathode particles and results are summarized in terms of the ratio of their Eg to A1g band areal intensities and their respective Raman band positions. There are some interesting trends from this analysis: (i) for a given bulk electrode SOC (or voltage) the cathode particles at different electrode locations vary widely in terms of the areal ratios and band positions; (ii) The A1gband shifts consistently to red with increasing SOC or voltage. In addition, we notice a relatively larger inhomogeneity for cathode particles for the 4.2 and 4.9 V electrodes measured in terms of local spectroscopic SOC. Acknowledgement This research at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy, is sponsored by the Vehicle Technologies Program for the Office of Energy Efficiency and Renewable Energy. References: R. Santhanam, and B. Rambabu, J. Power Sources, 195, 5442–5451 (2010). M. M. Thackeray, C. Johnson, J. T. Vaughey, N. Li, and S. A. Hackney, J. Mater. Chem. 15, 2257–2267 (2005). S. K. Martha, J. Nanda, G. M. Veith, and N. J. Dudney, J. Power Sources, 199, 220–226 (2012). J. Nanda, J. Remillard, A. O’Neill, D. Bernardi, T. Ro, K. E. Nietering, J. Y. Go, and T. J. Miller, Adv. Funct. Mater. 21, 3282–3290 (2011). S. K. Martha, J. Nanda, G. M. Veith, and N. J. Dudney, J. Power Sources, 216, 179–186 (2012).