Operando Raman Spectroscopy of Phase Changes in Nanocrystalline Metal Oxide Lithium-Ion Battery Electrodes
Authors: Grant Alex , O'Dwyer Colm 
Published: November 2022

As the demands for clean electricity, transport and highly functional portable electronics grow faster than ever, energy storage remains the main roadblock to progression. Electrochemical devices such as lithium-ion batteries (LIBs), have been the source of most success in overcoming this roadblock. The primary issue with these devices is lifetime decay due primarily to the degradation of the electrodes within the battery. Nanocrystalline materials are often chosen,1 which enhance battery performance by reducing ion diffusion lengths and thus improving rate capabilities. However, structural changes prove difficult to monitor due to their dimensions. The electrochemical processes which cause these changes are also difficult to probe because of their metastability and lifetimes, which can be of nanosecond to sub nanosecond time domains.2 Consequently, the development of methods to capture these processes proves challenging, requiring state-of-the-art techniques. Our work demonstrates that the processes which lead to electrode degradation can in fact be captured by using novel cell designs that provide the ability to adapt analytical techniques to probe battery systems in real time.
Raman spectroscopy is particularly useful for multi-layered materials such as the porous metal oxides often used in battery electrodes.3 Information can be revealed on crystal structure, electronic structure, lattice vibrations and flake thickness of layered materials, and can be used to probe the strain, stability, charge transfer, stoichiometry, and stacking order.4 The correlation between the capacity of intercalation in an electrode to the degree of disorder in the material can also be determined.5 Such analysis can be performed during cycling, once the cell is modified to enable light penetration. A pathway is usually provided in the form of an optical window, allowing measurements to be obtained non-destructively in real-time.6 The window material must be transparent to the frequency of incident light chosen for measurements.7 These constitute spectroelectrochemical cells, providing the ability to perform electrochemical and optical measurements simultaneously.
Here, we demonstrate a non-destructive approach to monitoring battery degradation in operando. Access for Raman spectroscopy during cycling is provided by a novel cell design. Sapphire is employed as window material, transparent to the 532 nm laser beam used for our Raman measurements. We describe the process applied to a lithium-ion battery based on a metal oxide inverse opals8,9 where the interconnected order porous structure is known to facilitate stable and long term cycling in the absence of binders and conductive additives11-17. However, the same methodology can be extended to any electrode materials with Raman active phase changes at the electrode-electrolyte interface.10 This work provides information on structural and phase changes to the electrode which are compared to microscopy and electrochemical data.

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