Enhanced Raman Scattering from NCM523 Cathodes Coated with Electrochemically Deposited Gold

Abstract

Materials with the general composition LiMO 2 , where M is a mix of nickel, cobalt, and manganese, have been studied extensively as cathodes for lithium-based batteries. Some compositions, like LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523), are already being employed in commercial lithium-ion batteries. The study reported here focuses on Raman spectroscopic investigations of NCM523 in the laminate form typically used to fabricate lithium battery cathodes. One of the problems encountered in such studies is that the scattering from LiMO 2 -type phases gets progressively weaker as the nickel content increases. NCM523 exhibits this behavior due to the fact that half of the transition metal content is nickel. In this study we show that the intensity of the Raman scattering from the NCM523 phonons can be significantly increased by electroplating sub-micron gold clusters on NCM523 laminates. The gold tends to plate preferentially on the NCM523 particles in randomly sized clusters. These clusters stimulate the NCM523 Raman scattering to varying extents approaching 100 times the scattering intensity from uncoated laminates. Intensity enhancement variations created by heterogeneities associated with the multi-phase nature of the NCM523 laminate, the random character of the sub-micron gold deposits, and the induced preferential orientation of NCM523 grains during laminate fabrication are addressed.

In situ and Operando Raman Spectroscopy of Layered Transition Metal Oxides for Li-ion Battery Cathodes

“…Looking ahead, surface enhanced Raman spectroscopy (SERS) receives increasing attention in the field of Li-ion batteries and could provide unique insights into the local LiMO 2 structure, particularly at the interface to the electrolyte. Tornheim et al demonstrated that the Raman intensity of NCM523 can be plasmon-enhanced by two orders of magnitude when the particles of active material are electroplated with sub-micron gold clusters (Tornheim et al, 2017); hence, the spectra recording times can be potentially reduced from fractions of hour to a few seconds. However, the highly reactive gold clusters are likely not stable in contact with the electrolyte at high anodic potentials.…”

“…Furthermore, we stress that even comparisons of spectra from oxides of similar composition, particularly when measured at different setups under different experimental conditions, are a challenge and requires very mindful analysis of several effects introducing spectral variations. For instance, the oxide crystal size (Gouadec and Colomban, 2007), orientation (Inaba et al, 1997;Tornheim et al, 2017), purity (Gouadec and Colomban, 2007) and even internal stresses (Wang et al, 2005) within the oxide can significantly modulate the spectra. Experimental conditions, such as temperature, incident light polarization and wavelength, as well as sample exposure times also necessarily have to be taken into consideration, which is immediately seen in the comparison of measurements from different literature sources below.…”

“…However, most measurements are performed ex situ due to the limitations outlined above. In situ and operando Raman measurements are on the contrary desired because the Raman spectrum is sensitive to particle size (Wang et al, 2002) and preferential crystallite orientations (Tornheim et al, 2017), which can vary from sample to sample and render an ex situ comparison difficult or impossible. In addition, spectral features from different particles at a given state of charge are inherently disperse due to lateral (Lei et al, 2005) and axial (Gilbert et al, 2018) (de)lithiation inhomogeneities.…”

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

“…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. Total accumulation times per spot were ≥200 s.…”

“…An A1g + Eg two‐mode model can be used to explain the scattering, which arises from c ‐axis metal‐oxygen (M‐O) vibrations (A1g‐like modes) and a/b plane M‐O vibrations (Eg‐like modes) . The evolution of the band envelope as a function of NCM composition (in the context of the two‐mode model) has been considered by Tornheim et al Also, for a given NCM composition the 400–750 cm −1 scattering envelope changes shape as the NCM material is successively delithiated and relithiated. In Figures S1 and S2 in the Supporting Information, we provide examples of how this band envelope reshapes as a function of lithium content and physical form (powder vs laminate) of the NCM523 oxide.…”