Protic ionic liquid and ionic melts were prepared from the combination of methanesulfonic acid (CH₃SO₃H) and 1H-1,2,4-triazole (C₂H₃N₃ ) at various molar ratios. The thermal properties, crystal structure, acid–base interactions, ionic conductivity, proton conduction behavior and electrochemical stability of the system were studied. The equimolar composition, 1,2,4-triazolium methanesulfonate (C₂H₄N₃ ⁺·CH₃SO₃ ⁻ (1)), was a proton transfer salt with a melting point of around 134 °C. Single crystal and powder XRD data, as well as TGA results, revealed that the base-rich region was a mixture of 1 and 1H-1,2,4-triazole. Infrared analysis and single crystal data suggested that the C₂H₃N₃ –CH₃SO₃H system exists in a strongly hydrogen-bonded network. Systematic investigation of the ionic conductivity showed that the ionic conductivity reached local maxima at the compositions of [C₂H₃N₃ ]/[CH₃SO₃H] = 10/90 and 80/20, respectively, while it exhibited a local minimum at the equimolar composition. The temperature dependence of the ionic conductivity was found to obey the Vogel–Fulcher–Tamman (VFT) equation. The fitting of the conductivity data to the VFT equation showed that the carrier ion concentration versus the mole fraction of 1H-1,2,4-triazole exhibited a volcano shape. In addition, the C₂H₃N₃ –CH₃SO₃H system showed adequate electrochemical stability under PEMFC conditions as measured by linear sweep voltammetry. The relatively high ionic conductivity, wide electrochemical window and good thermal stability demonstrated that the C₂H₃N₃ –CH₃SO₃H system is a suitable candidate for high temperature PEMFC electrolytes.

Incremental capacity analysis (ICA) has proven to be an effective tool for determining the state of health (SOH) of Li-ion cells under laboratory conditions. This paper deals with an outstanding challenge of applying ICA in practice: the evaluation of battery series connections. The study uses experimental aging and characterization data of lithium iron phosphate (LFP) cells down to 53% SOH. The evaluability of battery series connections using ICA is confirmed by analytical and experimental considerations for cells of the same SOH. For cells of different SOH, a method for identifying non-uniform aging states on the modules’ IC curve is presented. The findings enable the classification of battery modules with series and parallel connections based on partial terminal data.