Thin-film calorimetry: In-situ characterization of materials for lithium-ion batteries

Thin-film calorimetry allows for qualitative and quantitative in-situ analysis of thermodynamic properties of thin films and thin-film systems from room temperature up to 1000 degrees C. It is based on highly sensitive piezoelectric langasite resonators which serve simultaneously as planar temperature sensors and substrates for the films of interest. Generation or consumption of heat during phase transformations of the films cause deviations from the regular course of the resonance frequency. Thermodynamic data such as phase transformation temperatures and enthalpies are extracted from these deviations. Thin-film calorimetry on Sn and Al thin films is performed to prove the concept. The results demonstrate high reproducibility of the measurement approach and are in agreement with literature data obtained by established calorimetric techniques. The calibration of the system is done in different atmospheres by application of defined heat pulses via heating structures. The latter replace the films of interest and simulate phase transformations to provide detailed analysis of the heat transfer mechanisms occurring in the measurement system. Based on this analysis, a data evaluation concept is developed. Application-relevant studies are performed on thin films of the lithium-ion battery materials NMC(A), NCA, LMO, and MoS2. Their phase transformation temperatures and enthalpies are evaluated in oxidizing and reducing atmospheres. Furthermore, their thermodynamic stability ranges are presented. Finally, measurements on all-solidstate thin-film batteries during electrochemical cycling are performed. They demonstrate the suitability of the system for in-situ investigations.