BatgasMod – Battery Gassing Modeling

Ensuring a permanently high performance of lithium-ion batteries (LIB) in long-term operation requires the perfect interaction of the components that form them, such as electrodes, electrolyte, separator and current collectors. However, during charging and discharging, but also already during initial battery formation, extensive material reactions take place that can adversely affect the behavior of the batteries. These effects must be known very precisely for the regular but also the irregular use of the batteries. Only on this basis can a product be developed and mass-produced that meets the high technical demands. Over long periods of time, the LIB should be able to withstand the operating loads without aging effects and associated performance losses, if possible. At the same time, battery safety must be guaranteed (no outgassing of toxic substances, no ignition or bursting/explosion). However, with the current state of the art, high electrical cycling, overcharging and fast charging, especially under changing temperature conditions, can severely degrade the long-term performance of the LIB.

For cell aging, the reactions of the liquid electrolyte play a significant role and are the focus of this project. The electrolyte is a mixture of organic solvents, a conducting salt and possibly additives. It exhibits complex internal chemical reactions and associated evaporation during continuous cell operation under constantly varying operating conditions. In addition, the electrolyte reacts with the electrode material already during formation (SEI formation, Solid Electrolyte Interface) and especially at higher temperatures during battery operation. This results in the formation of reaction products that can reduce the performance of the battery. Thus, the electrolyte is a major aging factor of the battery. Therefore, the goal of this project is to understand the degradation of the battery electrolyte during battery use and its effects on aging. For the material and cell studies, the liquid electrolyte (LiPF6-EC:DMC) is combined with the electrode materials NMC622 and graphite, and the additives VC (vinylene carbonate) and FEC (fluoroethylene carbonate).

For the analysis and prediction of the aging behavior, application-oriented modeling is performed at the material, cell and battery levels. For this purpose, electrolyte aging models and thermodynamic models are developed and combined. In particular, the chemical changes of the electrolytes are considered and the reaction pathways are implemented in the models. Results from experimental-analytical investigations of own work and literature are used for model construction and validation. Pouch cells are fabricated, formed and selectively aged by electrochemical cycling at different temperatures. The changes in the chemical electrolyte and SEI compositions occurring during this process are systematically investigated. For this purpose, methods such as Raman spectroscopy, gas chromatography-mass spectrometry (GC-MS), ion chromatography-mass spectrometry (IC-MS) or glow discharge spectroscopy are used. The liberated gas phases are analyzed by GC-MS. Calorimetric techniques (Accelerating Rate Calorimetry (ARC), Differential Scanning Calorimetry (DSC), Tian-Calvet Calorimetry) are used to quantify the heat effects and simultaneous gas pressure build-up occurring during the material and cell reactions. By combining modeling with physicochemical analyses, the reaction mechanisms can be efficiently elucidated.