Thermochemical Investigations on Functional Energy Storage Materials
Authors: Markus Torsten 
Published: February 2020

The profound change in the long term requirements of innovative technical solutions concerning the change in energy conversion technologies is one of the largest infrastructural and technical challenges in Germany´s history. The accelerated development of new materials is the key to fundamental innovations in the energy sector and is therefore a key element of the Energy Change.

In order to be able to understand the behavior of the interplay of different materials and the corresponding material interactions, it is important to understand the thermochemical functionality of the whole assembly. Due to the high amount of components present in these systems, experiments are as important as thermodynamic modeling. Computational Methods however require thermodynamic data as input parameters. The need for thermo chemical data is required not only in the research and development phase but also throughout the product life cycle. To overcome some of the problems encountered in the development of new and advanced materials it is necessary not only to have an understanding of the fundamental science associated but also the physical and chemical interactions of the various components within the device.

The contribution at hand will focus on the method of advanced materials design combining experimental techniques with computer based thermochemical modeling using thermodynamic databases which have been accomplished using the CALPHAD type approach. The principle procedure for this approach will be discussed and specifically described on our systematic investigations on battery materials.

The knowledge of basic thermodynamic data concerning battery relevant systems is very weak. This knowledge would be a necessary pre condition for advanced materials design based on thermodynamic modeling. This promotes the design of new and advanced materials for achieving higher performances of the devices. The capacity, voltage, rates, and energy densities of these systems are determined by the cathode material. This material is reactive with the solution components and undergoes exothermic reactions at elevated temperatures. Thus, it is mandatory to understand these processes and the relevant thermodynamic interactions.

Li-Ion batteries are well recognized as a possible and efficient way to store electric energy. However, these batteries have to meet several design criteria, such as the number of charge/discharge cycles, energy density and safety. The latter is one of the crucial requirements that has so far prevented the wide spread use of these batteries, since for instance the heat generation due to charging and discharging processes can induce thermal runaway and lead to explosions. Therefore, for safety issues the knowledge and predictability of materials behavior under operating conditions is important.

The re-development and setup of the Coulometric Titration (CT) method allows us to electrochemically measure thermodynamic properties and phase equilibrium diagrams of the respective electrode materials as well as their kinetic behavior under various conditions. For this method only electrical quantities are needed and the determination of thermodynamic properties in dependence of the composition is possible. It allows in situ investigations of the electrode material eliminating the side reactions with the electrolyte materials. One of the most common variant for this method is the Galvano static Intermittent Titration Technique (GITT) with which the measurement of diffusion rates of the intercalation agent is possible.

In addition to the determination of thermodynamic properties using the CT method, also the preparation of selective intercalated electrode material is possible on which for example calorimetric measurements for the determination of heat capacities are performed in our laboratory.

The CALPHAD method, which is highly accepted, is a semi empirical method that requires thermodynamic data as an input. Using the data, which are determined with the CT technology as well as with calorimetric measurements in addition with a sorrow literature research a consistent CALPHAD type database is set up for functional materials under operating conditions. In this context a new model to describe the temperature dependence of the heat capacity is needed since the Neumann Kopp rule for example cannot be used to describe de-intercalated electrode materials properly.

The CALPHAD database is further used for the calculation of the thermal energy produced under operating conditions of batteries.

The results are compared with experimental investigations, which are carried out by the use of a Netzsch Multi Mode Calorimeter equipped with a High Temperature Accelerating Rate Calorimeter (ARC) module with which coin cell tests are performed to measure the heat production rate in dependence of the electrical conditions of the cell.