Measuring cell ageing and safety effects of battery gassing

A project led by Dr Carlos Ziebert, leader of KIT’s Batteries – Calorimetry and Safety group, examines how calorimetry and thermodynamic modelling help to improve our understanding of battery gassing and its effects on cell ageing.

For long-term use of lithium-ion batteries to reach optimal performance, perfect interactions between components such as electrodes, electrolyte, separator and current collectors are necessary. During constant charge and discharge operations, along with initial formation of the battery, material reactions occur which can affect LIB performance. The reactions between liquid electrolytes and gas production have a significant impact on battery ageing and safety – this is why Modelling Battery Gassing (BattGas) is a core project in the new BMBF funded competence cluster, Battery Usage Concepts (BattUse). With €20m of funding from the German Federal Ministry of Education and Research, BattUse aims to increase our knowledge of battery behaviour to figure out when second use for energy storage is most beneficial.

The project Modeling of Battery Gassing (BattGas) aims to

The Modeling of Battery Gassing (BattGas) project started on 1 October 2020 and is coordinated by the Institute of Applied Materials – Applied Materials Physics (IAM-AWP) at Karlsruhe Institute of Technology (KIT). RWTH Aachen University’s Institute for Power Electronics and Electrical Drives (ISEA) and Münster Electrochemical Energy Technology (MEET) are the other partners.

The aim of this project is to build electrolyte ageing models in combination with battery cell models, to anticipate behaviour in the usage phase. The IAM-AWP research concentrates on two core areas. Firstly, detailed calorimetric measurements are done on active materials and electrolytes, as well as the cells themselves, in order to gain a better understanding of changes and how pressure is generated during cell use. Secondly, the findings from these calorimetric tests are built into thermodynamic modelling of electrolytes, additives and electrode materials; CALPHAD (CALculation of PHAse Diagrams) will be used for this purpose. The model provided by ISEA will be able to calculate phase diagrams and thermodynamic functions.

This model will provide greater insight into the decomposition of battery electrolyte over the course of utilization, and the associated effects on ageing. Doing so can improve safety, prolong the lifespan of LIBs and render them more sustainable. By applying this model to new materials and cells, their readying process is expedited, thus providing an opportunity to assess their performance and life expectancy in relation to electrolyte ageing during the earlier stages.

Research conducted at IAM-AWP is divided into two realms: material and cell levels. In the material level, ultra-sensitive Tian-Calvet and differential scanning calorimeters were used for calorimetric observations of the cell components. Using CALPHAD method and Thermo-Calc software, thermodynamic data of EC and EMC electrolytes were then modelled using these measurements. Regarding the cell level, cells provided by MEET were assembled and characterized to commence a calendric ageing study.

In addition, electrochemical-calorimetric tests were conducted on cells under isothermal and adiabatic conditions. Fig. 1 above illustrates the different aspects of the project. The IAM-AWP’s work on battery gassing is particularly highlighted here, as it provides an opportunity to map the impact of gas formation on cell behaviour.

Characterization of electrochemical and thermal cells

Pouch cells with a nominal capacity of 5Ah were constructed by the MEET battery production line with no formation process and then offered to KIT. Three distinct cell types were generated, all featuring graphite on the anode side and Li (Ni0.6Mn0.2Co0.2) O2 (NMC 622) on the cathode side, but varying in terms of their electrolyte composition. The reference cell was composed of a mixture of EC and EMC solvents in a ratio 3:7 without any additives.

Battery gassing and additives

To investigate the influence of additives on gas formation, cells were supplied with either 5 wt-% vinylene carbonate (VC) or fluoroethylene carbonate (FEC). At IAM-AWP, the pouch cells underwent a formation procedure and Fig. 2 displays the cycle efficiency: it was approximately 80% for all three cell variants. Moreover, based on the lower decomposition voltage of the additives, these were decomposed instead of the electrolyte and resulted in higher discharge capacity in the first cycle compared to that of reference cells. Fig. 2 further portrays different voltage curves before exceeding a potential limit of 3.0 V for the first time. After formation, degassing and resealing were conducted to remove developed gases from inside the pouch bag; then samples were taken and analysed qualitatively in a gas chromatography-mass spectrometer (GC-MS). With specific valves within this equipment, molecules with high mass can be selectively detected; gases are separated after entering their first column only to be flushed with argon and helium once certain times elapse to avoid overlapping. These flows are measured separately by two thermal conductivity detectors (TCD), while products associated with both electrolyte and additive decay–CO, CO2

The influence of temperature on battery gassing

Ten charge and discharge cycles were carried out before thermal tests were conducted on selected cells. Heat flow sensors (HFX) were attached to the centre of their pouch cell surface for measuring heat capacity, as seen in Fig. 3. The ambient temperature of the cell was increased step-by-step to 50°C, and the mean heat capacity in this range was determined from the resulting heat flow. An evaluation routine was implemented using MATLAB software for further analysis.

Cells were cycled in a climate chamber at 25°C at different speeds (0.2C, 0.5C, and 1C) for the first isothermal measurement. A thermocouple was used to measure the change in surface temperature during cycling. A heat flux sensor was also attached to the top and bottom of the cells to measure the amount of heat generated during cycling.

Thermophysical measurements on cell components and thermodynamic modelling

In order to obtain the desired thermodynamic modelling, an extensive literature search was conducted to determine the existing thermodynamic data for EC and EMC. However, due to the low temperature of most existing data (up to 50°C), more data were collected using differential scanning calorimetry (DSC) over a wider range of temperatures. These results produced heat capacity values consistent with NIST evaluated literature values. The experimentally derived heat capacities were then incorporated in a POP file, alongside other variables used to optimise the thermodynamic modelling via the CALPHAD method in a corresponding TDB file.

How industry can utilise our methods

The development of new measuring methods for the Tian-Calvet calorimeter and the establishment of a method for determining the heat capacity of cells of any format as a function of temperature has already begun. The industrial partners can immediately receive these measurements as contract measurements after the project ends, and they can be supported in establishing them in their own businesses.

In principle, the prerequisites are very good for the scientific work to quickly lead to technologically and economically usable results which can be further developed into market-relevant measurement methods within industrial cooperation. Both the cluster BattNutzung (Battery Utilization Concepts) and the umbrella concept of the BMBF for the German battery research factory serve this purpose.