Thin-Film Calorimetry – Novel Analytical Tool for In Situ Characterization of Lithium-Ion Batteries

Precise knowledge of the thermodynamic and calorimetric properties of the active materials used in lithium ion batteries is a key issue for the improvement of the entire system, considering especially lifetime and safety aspects. In current research the role of thin-film batteries becomes more and more important. Here, it is crucial that thin films behave, in general, differently compared to bulk electrodes. Therefore, conventional calorimetric techniques are not suitable for these requirements.

The presented measurement technique Thin-Film Calorimetry (TFC) is based on high-temperature stable piezoelectric langasite (La₃Ga₅SiO₁₄) resonators, so that the measurement range extends to about 1000 °C. Their resonance frequency f_R exhibits a strong temperature dependence. By measuring f_R, the device is working as a precise temperature sensor. Thin films with a thickness in the range from 100 nm to several micrometres of the material of interest are applied to the resonators. The production or consumption of latent heat during phase transformation of these layer(s) results in temperature fluctuations with respect to the furnace where the sensor is placed. They are reflected in frequency deviations f_Tr. As the dependence of the resonance frequency on temperature as well as mass and specific heat capacity of the resonator are known, f_Tr is used to calculate the amount of heat Q_Tr generated (exothermic) or consumed (endothermic) by the phase transformation. For the present system, the uncertainty of the measured quantity of heat ΔQ_Tr is estimated to be 0.6 mJ. Dividing Q_Tr by the film’s mass gives the enthalpy ΔH. Thereby, this mass is accessible by the Thin-Film Calorimeter itself when operating the device prior and after film deposition in the microbalance mode. Figure 1 shows the TFC setup.

Metallic layers of tin and aluminium are used to test and establish this technique. The enthalpies of solid-liquid as well as of solid-solid phase transformations are observed in the correct manner.

To establish this technique in the field of battery materials several electrode materials as well as solid electrolytes are investigated. For example, the anode material molybdenum disulphide is characterized. A phase transformation indicated by a frequency shift is observed by annealing the thin film up to a temperature of 478 °C (see Figure 2). Combining these thermodynamic data with structural investigations, this transformation is identified as crystallization with an associated enthalpy of -183.2 J/g. Another example is the cathode material lithium manganese oxide (LMO) which is investigated in the temperature range from room temperature up to 775 °C under varying atmospheres. In ambient air, three phase transformations at 330 °C, 410 °C, and 600 °C are measured while at measurements performed under 0.5% H₂/Ar four phase transformations at 390 °C, 470 °C, 730 °C, and 758 °C occur. The associated thermodynamic parameters as well as the structural changes are evaluated. By this, a thermodynamic and structural mapping of the materials can be given as shown in Figure 3.

Current research focuses on the investigation of all-solid-state thin-film electrochemical cells which are monitored at constant temperature during dis-/charging. Thereby, the heat generation with respect to the state of charge is investigated.

The newly developed measurement technique Thin-Film Calorimetry (TFC) enables to investigate thin films including battery layer sequences. Thereby, even in-situ measurements during (de-)lithiation of thin-film cells are performable. The temperature range extends far beyond the common operation range of lithium ion batteries. Even if this is not necessarily apparent at first glance, this feature enables to determine calorimetric data up to the synthesis temperatures. By this, information is gathered about the applicable thermodynamic process conditions a material can stand during battery production.