Lithium manganese oxide is a promising cathode material for the next generation of lithium ion batteries due to its high capacity, low cost, and low toxicity. However, LMO electrode suffer from rapid capacity loss due to degradation of the active material by manganese dissolution into the electrolyte. Significant scientific effort has recently been devoted to understanding the effect of environmental and operating parameters on Mn²⁺ dissolution (e.g. storage temperature, electrolyte acidity and overcharge/ overdischarge conditions). Yet, no experimental data exits for Mn²⁺ dissolution during lithiation and delithiation, due to lack of experimental techniques to monitor the Mn²⁺ ion concentration in real-time in the electrolyte while the battery is in operation.

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.