Comparisons of Ageing and Temperature Impacts on Nonlinear and Linear Dynamic Responses of Lithium-Ion Batteries

Abstract

Lithium-ion batteries are the most promising candidate to fulfill a worldwide provision of electric mobility.For a safe operation, it is essential to accurately know and determine the state of the Lithium-ion battery. Themost commonly used dynamic determination method is the Electrochemical Impedance Spectroscopy (EIS).EIS analyses the battery in regard to linear system behaviour. However, electrochemical reactions show ahighly nonlinear relation of current and voltage according to Butler-Volmer Kinetics. Additionally, chargeand discharge processes of double-layers at surface interfaces and diffusion in spherical particles can shownonlinear behaviour [1]. Therefore, information about the nonlinearities in the cell is not fully accessed withEIS. To account for this, we provide a novel approach for characterization of Lithium-ion batteries, the socalled Nonlinear Frequency Response Analysis (NFRA). NFRA methods have priorly been used to fuel cellsto determine redox kinetics [2] and as a State-of-Charge Estimator for Lead-Acid batteries [3]. For NFRA,high current amplitudes IAC are applied to a cell and the response signal is analysed. Thereby, higherharmonic responses Yn with n≥2 are observed at multiples of the fundamental frequency f1. In this study we show experimental research on Lithium-ion batteries to distinguish between ageingcharacteristics at different environmental temperatures using dynamic analysis methods, e.g. EIS and NFRA.We conducted ageing of identically constructed pouch cells with NMC as cathode and Graphite as anodematerial at 25 °C and -10 °C with a 1 C constant current (CC)/constant voltage(CV) charging and 1 C CCdischarge profile. After every 50th cycle, dynamic measurements were performed at 25 °C in a temperaturechamber. EIS was measured with an excitation amplitude of C/15 and 1.5 C for NFRA, both at a frequencyrange between 20 mHz and 5 kHz. Whereas for EIS we analyse the linear output of the system Y1, for NFRA higher harmonics Yn as well astheir sum are investigated. In Figure 1, results of the ageing study are shown. The discrete frequency of10 Hz is chosen, as it is a characteristic time constant of the electrochemical reactions, which we willdemonstrate by showing temperature dependent EIS studies. Voltage responses are exemplarly extractedprior to ageing and each 200th cycle. By solely using EIS, it is not possible to distinguish between ageing at25 °C and -10 °C, since the change of the linear output Y1 of the system is similar. For NFRA, voltageresponse for Y2 behave qualitatively similar to Y1 for EIS. However, higher harmonic Y3 shows a strongdependency of the ageing conditions, thereby enabling the possibility to distinguish between them. Further,we will present process characterization on Lithium-ion batteries with NFRA by identifying characteristicfrequency ranges for higher harmonic voltage responses Yn and correlating them to typical electrochemicaland transport processes. Finally, we will show and discuss the impact of ageing on nonlinear and linearresponses of dynamic measurements for the overall frequency range. [1] A. M. Bond, N. W. Duffy, D. M. Elton, B. D. Fleming, Characterization of Nonlinear BackgroundComponents in Voltammetry by use of Large Amplitude Periodic Perturbations and Fourier TransformAnalysis, Analytical Chemistry 81 (2009) 8801–8808.[2] Q. Mao, U. Krewer, R. Hanke-Rauschenbach, Total Harmonic Distortion Analysis for Direct MethanolFuel Cell Anode, Electrochemistry Communications 12 (2010) 1517–1519.[3] S. Okazaki, Second-Order Harmonic in the Current Response to Sinusoidal Perturbation Voltage forLead-Acid Battery, Journal of The Electrochemical Society 132 (1985) 1516. Figure 1