High-temperature thin-film calorimetry: a newly developed method applied to lithium ion battery materials

Abstract

A thin-film calorimeter has been developed to investigate the thermodynamic properties of thin films including battery layer sequences. A new approach, i.e., the application of high-temperature stable piezoelectric resonators as highly sensitive planar temperature sensor, is chosen. Thin films with a thickness of several micrometers of the material of interest are deposited on the resonators. The production or consumption of latent heat by the active layer(s) results in temperature fluctuations with respect to surroundings, in our case the furnace in which the sensor is placed. The temperature fluctuations can be easily monitored in situ via changes of the resonance frequency of the resonator. This enables us to extract the temperature and time dependence of phase transformations as well as the associated enthalpies. To cover a temperature range from −20 to 1000 °C, high-temperature stable piezoelectric langasite (La3Ga5SiO14) resonators are applied. Initially, aluminum and tin layers are used to test the calorimeter. The temperature and enthalpy of the solid–liquid phase transformation agree well with the literature data. Further, the thermodynamic data of the battery materials to be used as cathode, solid electrolyte, and anode in lithium ion batteries are investigated by the newly developed method. The cathode materials Li(Ni0.8Co0.15Al0.05)O2-δ (NCA) and LiMn2O4-δ (LMO) are amorphous after deposition and crystallize during heating. NCA shows this transformation at 455 °C with an enthalpy of −4.8 J/g. LMO undergoes three phase transformations at 330, 410 and 600 °C. They require initially an activation which is followed by an exothermic enthalpy. The associated energies (activation; enthalpy) are (+67.2; −50.2) J/g, (+29.3; −29.3) J/g, and (+20.4; −26.2) J/g, respectively. The solid electrolyte Li3.4V0.6Si0.4O4-δ (LVSO) shows no phase transformation up to its decomposition at about 220 °C. The anode material molybdenum disulfide (MoS2) exhibits a phase transformation at 480 °C with an enthalpy of −183.2 J/g.