Quantifying the local laser induced heating of carbon-coated Li-ion battery materials during Raman spectroscopy

Abstract

Local heating effects induced by laser illumination during Raman spectroscopy characterisation have been investigated in this article. Attempts at quantifying the amount of laser heating have been pursued, including measurement on thermocouples and low melting point metals, and analysis and interpretation of peak shifts and anti-Stokes/Stokes intensity ratios. Focus has been paid to carbon-coated lithium-ion battery materials, LFP and LTO, which normally contain 1-3 wt percent carbon in few nanometres (1–5 nm) thick coatings on the battery material particles. Under brief, very low intensity acquisition conditions, the laser light has limited penetration, and the highly absorbing carbon can only be detected. Increases in intensity to reach deeper into the substrate causes heating with damaging consequences. In air, the carbon is ignited at relatively low intensities, while in inert atmosphere, the carbon burns at higher laser intensity, by extracting oxygen from the underlying oxide substrate. Strategies to reduce the heating are discussed, however, these are difficult to implement in an operando situation, as the strategies interfere with cell assembly and function. Typical electrolytes, such as LiPF6 decompose at relatively low temperature (∼100 °C), therefore, much care must be taken when attempting Raman spectroscopy operando. While the evidence is from Raman spectroscopy, the results are relevant for many high-end experiments that make use of higher power and intensity lasers, without including likely heating effects in their analyses of results.