(Invited) Raman Microscopy: What Can the Technique Tell Us?
Authors: Flores Eibar , J. Berg Erik , Novak Petr 
Published: February 2020

The lithium-ion battery (LIB) technology is the backbone of current portable power for both, electronics and electric transport sectors. Many further incremental improvements of this technology will be based on an integral understanding of the dynamics of the cycling electrodes, from the cell level all the way down to the atomic scale.1 Given the complexity of such processes, the electrode materials should ideally be characterized operando, i.e., within their typical working environment and during cycling. Operando characterization of fundamental electrode processes is typically based on the understanding of lattice and electron structures of the material as obtained by X-ray diffraction and spectroscopies, respectively. However, they might require synchrotron-based sources and/or complex experimental setups for achieving enough resolving power, while keeping acquisition times below cycling timescales.2

Raman spectroscopy is a highly versatile, accessible, and easy-to-use technique for characterizing electrode materials via their inherent atomic vibrations. The number of Raman active vibrations, their frequencies, and intensities are determined by the lattice symmetry and electron structure. Thus, a Raman spectrum potentially reveals coordination structures, oxidation states, defects, multiple phases, and other relevant properties of the electrode material.3

We have developed an electrochemical Raman cell for cycling cathode materials and simultaneously recording their Raman spectra with high time resolution. This work is focused on the applications of operando Raman spectroscopy on the family of layered LiMO2 oxides, which are currently the most attractive cathodes for LIBs. We apply traditional and advanced data analysis techniques for obtaining meaningful information from comparing the time-resolved spectra with the electrochemical characteristics of the electrode material, having the ultimate goal of establishing clear relations between spectral features and the material’s properties and dynamics.

As a result, we have provided experimental evidence of the insulator-metal transition of LiCoO2, after establishing a relation between Raman intensity and electronic conductivity. In addition, an improved spectral quality and time resolution had revealed that the multiple redox peaks of LiNi0.8Co0.15Al0.05O2 (NCA) originate from a lithium-vacancy ordering phenomena that remained undetected by conventional diffraction techniques. Finally, we provide evidence of a complex multi-step Ni redox reactions in LiNi1-a-bCoaMnbO2 (NCMs) as revealed by the evolution of the Raman spectral profiles during cycling.3,4

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Flores, E., Novák, P. & Berg, E. J.: In situ and Operando Raman Spectroscopy of Layered Transition Metal Oxides for Li-ion Battery Cathodes. Front. Energy Res.
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Flores, E., Vonrüti, N., Novák, P., Aschauer, U. & Berg, E. J.: Elucidation of LixNi0.8Co0.15Al0.05O2 redox chemistry by operando Raman spectroscopy. Chem. Mater. (2018); doi:10.1021/acs.chemmater.8b01384