Fig. 2: Online Raman measurements in a hollow-core fibre.

a, Raman light emerging from the fibre facet (left, image scale bar is 50 µm) with a spectrally dispersed image (right). b, Raman spectra tracked during sequential sample infiltration. Red dashed lines indicate when the pump switched on; t_1−a indicate the times at which the sample fluid reaches the fibre core. Dashed white lines indicate when the pump is switched off, followed by a switch of the sample syringe. Solid horizontal lines indicate times at which the spectra shown in c were taken. c, obtained spectra of the different solvent mixtures. Highlighted Raman bands relevant to battery chemistry are ethylene carbonate breathing mode at 893 cm⁻¹ (dark red dotted line), PF₆ anion mode at 740 cm⁻¹ (green dashed line), and vinylene carbonate -HC = CH- band centred at 1628 cm⁻¹ (not present in these solutions and spectra). The shaded area indicates EC and EMC bands at 1700−1850 cm⁻¹ relevant to lithium solvation mechanisms (see Fig. 4) Inset i demonstrates the sample exchange time as monitored by the Raman intensity of the IPA (819 cm⁻¹) and EMC skeleton (∼900 cm⁻¹) modes (arrows in c). Inset ii shows rescaled EC and EMC bands at 1700−1850 cm⁻¹.