The work takes advantage of a newly developed measurement system which enables to investigate the thermodynamic properties of thin films including battery layer sequences. This technique, Thin-Film Calorimetry (TFC), is based on the detection of resonance frequency shifts of bulk acoustic wave resonators. Thin films with a thickness of several micrometers of the material of interest are deposited on the resonators. By measuring the temperature dependent shift of the resonance frequency, the device is working as a precise temperature sensor. The production or consumption of latent heat by the active layer(s) results in temperature fluctuations with respect to the furnace where the sensor is placed. Those information enable to extract the temperature and time dependence of phase transformations as well as the associated enthalpies. To cover a temperature range from -20 to 900 °C high-temperature stable piezoelectric resonators made of langasite crystals (La3Ga5SiO14) are applied.
Initially, metallic layers of tin and aluminum are used to test and verify this approach. The temperatures and enthalpies of solid-liquid as well as of solid-solid phase transformation are observed in the correct manner. Further, the thermodynamic data of the battery materials Li(Ni0.8Co0.15Al0.05)O2-δ (NCA) and LiMn2O4-δ (LMO) obtained by TFC are determined and discussed. Both cathode materials are amorphous after deposition and show crystallization during heating at 460 °C (NCA) and 600 °C (LMO). The associated enthalpies are 5.3 kJ/mol (55 J/g) and 17.3 kJ/mol (96 J/g), respectively.