New developments of energy-efficient recycling processes and systems for LIB recycling are required from the perspective of resource circulation, raw material supply and sustainability. One promising approach is based on a combined pyro- and hydrometallurgical treatment. However, tailoring respective slag compositions and processing conditions is a challenging task in terms of increasing recycling efficiency. It requires a fundamental understanding of the phase development in the slag. For this, thermodynamic data and models are needed to predict phase formation and fractions under certain compositions and solidification conditions. The influence of manganese in LIB slag systems is not fully understood. An important base system is the Li2O-MnO system, for which no thermodynamic model-compared data are available in the literature that are in a composition range relevant for LIB slags. In this work, the Li2O-MnO system is studied thermodynamically. Different compositions of Li2O-MnO in a LIB relevant slag composition field are synthesized and investigated with respect to their caloric and thermal properties with STA measurements under respective solidification conditions, such that solid-liquid transitions, as well as specific heat capacities of the mixtures, are obtained. Based on the experimental data gained in this study, model parameters of the modified quasichemical model (MQM) are optimized in the CALPHAD framework, such that the liquid phase of this system can be modelled thermodynamically for several compositions. This first survey contributes to a deeper understanding of the influence of manganese on the phase formation during solidification in lithium-containing systems.