Development of operational strategies for lithium ion batteries to guarantee a safe and lifetime optimized performance

Kurzfassung

Important hurdles for the market launch of different types of lithium ion batteries are operation security, long calendar life, cycle durability and fast-charging capability. The main focus of this work is to identify unfavorable operation and environmental conditions which generally accelerate cell degradation such as c-rates during cycling, load profiles and temperature. All experiments are performed with different electrode materials assembled in self-build coin cells (LIR2032 body). In order to run batteries in a secure and lifetime optimized way, it is essential to determine the state of charge (SOC) and the state of health (SOH) accurately. A useful method to detect increasing cell resistances during cell operation is electrochemical impedance spectroscopy (EIS). One main reason for increasing ohmic resistances during load cycling is attributed to a growing solid electrolyte interface (SEI) located at the phase boundary between graphite anode and electrolyte. In this work we present first results of EIS measurements as a function of different operating parameters. Thereby, a useful tool to interpret EIS spectra is to use suitable equivalent circuits to simulate battery components, chemical reactions and mass transfer limitations. In addition, accelerating rate calorimetry (ARC) is used to measure the influence of exothermic reactions inside the battery and externally supplied heat in order to simulate unwanted side reactions and environmental conditions such as solar irradiation. Common methods that are used in our work are heat-wait-search and isothermal ageing to investigate the thermal cell behavior. As a supplement to the presented ARC measurements, thermogravimetric analysis (TGA) helps to allocate exothermic reactions during cell operation to different battery components.