For a safe operation, it is essential to accurately know and determine the state of the Lithium-ion battery. The

most 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 a

highly nonlinear relation of current and voltage according to Butler-Volmer Kinetics. Additionally, charge

and discharge processes of double-layers at surface interfaces and diffusion in spherical particles can show

nonlinear behaviour [1]. Therefore, information about the nonlinearities in the cell is not fully accessed with

EIS. To account for this, we provide a novel approach for characterization of Lithium-ion batteries, the so

called Nonlinear Frequency Response Analysis (NFRA). NFRA methods have priorly been used to fuel cells

to determine redox kinetics [2] and as a State-of-Charge Estimator for Lead-Acid batteries [3]. For NFRA,

high current amplitudes I

_{AC}are applied to a cell and the response signal is analysed. Thereby, higher

harmonic responses Y

_{n}with n≥2 are observed at multiples of the fundamental frequency f

_{1}.

In this study we show experimental research on Lithium-ion batteries to distinguish between ageing

characteristics 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 anode

material at 25 °C and -10 °C with a 1 C constant current (CC)/constant voltage(CV) charging and 1 C CC

discharge profile. After every 50^{th} cycle, dynamic measurements were performed at 25 °C in a temperature

chamber. EIS was measured with an excitation amplitude of C/15 and 1.5 C for NFRA, both at a frequency

range between 20 mHz and 5 kHz.

Whereas for EIS we analyse the linear output of the system Y_{1}, for NFRA higher harmonics Yn as well as

their sum are investigated. In Figure 1, results of the ageing study are shown. The discrete frequency of

10 Hz is chosen, as it is a characteristic time constant of the electrochemical reactions, which we will

demonstrate by showing temperature dependent EIS studies. Voltage responses are exemplarly extracted

prior to ageing and each 200th cycle. By solely using EIS, it is not possible to distinguish between ageing at

25 °C and -10 °C, since the change of the linear output Y_{1} of the system is similar. For NFRA, voltage

response for Y_{2} behave qualitatively similar to Y_{1} for EIS. However, higher harmonic Y_{3} shows a strong

dependency 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 characteristic

frequency ranges for higher harmonic voltage responses Yn and correlating them to typical electrochemical

and transport processes. Finally, we will show and discuss the impact of ageing on nonlinear and linear

responses of dynamic measurements for the overall frequency range.

[1] A. M. Bond, N. W. Duffy, D. M. Elton, B. D. Fleming, Characterization of Nonlinear Background

Components in Voltammetry by use of Large Amplitude Periodic Perturbations and Fourier Transform

Analysis, Analytical Chemistry 81 (2009) 8801–8808.

[2] Q. Mao, U. Krewer, R. Hanke-Rauschenbach, Total Harmonic Distortion Analysis for Direct Methanol

Fuel Cell Anode, Electrochemistry Communications 12 (2010) 1517–1519.

[3] S. Okazaki, Second-Order Harmonic in the Current Response to Sinusoidal Perturbation Voltage for

Lead-Acid Battery, Journal of The Electrochemical Society 132 (1985) 1516.