Fig. 4: Monitoring Li⁺ solvation to the different solvent carbonyl groups.

a, b Evolution of the carbonyl group (C = O) Raman bands during the formation cycle. Dashed and solid curves are spectra taken at times highlighted in b The deconvolution based on the solid purple line in a suggests that the peaks comprise a combination of solvent C = O stretch bands for un-solvated Li⁺ (dark blue, ~1750 cm⁻¹, for EMC and dark red, ~1805 cm⁻¹, for EC) and solvated Li⁺ (light blue, 1735 cm⁻¹, for Li⁺EMC and dark red, ~1787 cm⁻¹, for Li⁺EC). With the broadband (and thus lower-resolution) grating used here, differences between solvated and non-solvated Li⁺ cannot be simply resolved. c, Trend of C = O band intensity ratio between EC C = O (solvated and un-solvated 1782–1817 cm⁻¹) and EMC C = O (solvated and un-solvated bands, 1730–1765 cm⁻¹). The shaded region indicates a 10% confidence interval (estimated from typical fluctuations in the Raman signal). The complex interplay of lithium solvation dynamics with solvents (EC, EMC) and additive (VC) is segmented into different regimes (coloured horizontal lines I-V) with a different behaviour at each distinct state of charge.