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