Here we report an in-operando experimental technique to detect the concentration of Mn2+ ions in electrolyte using a UV-vis probe molecule (4-(2-pyridylazo) resorcinol, PAR) and activator (1,8-bis(dimethylamino)naphthalene, proton sponge) combination. Chelation between the probe molecule and Mn2+ ion induces a color change in the electrolyte, enabling in-operando characterization of Mn2+ dissolution via spectroscopy. The probe molecule is highly selective, and can selectively differentiate trace amount of Mn2+ ions compared to concentrated Li+ ions in the electrolyte solution.
The electrolyte consisted of 1 mol/L LiPF6 in ethylene carbonate/ dimethyl carbonate (1:1 by volume). Mn2+ ions were introduced into the electrolyte by dissolving different amounts of manganese(II) acetylacetonate (Mn(acac)2). Figure 1a shows the range of color of the electrolyte with respect to the concentration of manganese ions. The electrolyte was analyzed by UV-vis spectroscopy and the absorption peak is shown to shift from 490 nm to 534 nm (Figure 1b). When the Mn2+ ion concentration is stoichiometrically lower than probe concentration, all Mn2+ ions in the solution are assumed to be chelated with the probe molecules. Thus, the absorption peak intensity of chelated molecules at the characteristic wavelength (534 nm) is proportional to concentration (Figure 1c), which follows the Beer-Lamber law. Mn2+ concentration in the electrolyte can thus be determined spectroscopically without interfering with the natural operation of Li-ion batteries.