Lithium ion (li-ion) batteries are gradually increasing their volumetric power and energy densities due to requirements imposed by electric vehicles and portable electronics applications. In consequence, modern lithium ion battery cells and battery packs are having much more compact design where a high amount of energy is encapsulated in a small volume. This imposes a need for an accurate thermal modeling of li-ion batteries in order to avoid battery cells overheating, which leads to safety concerns (e.g. thermal runaways) and accelerates battery cell's performance degradation.

Conventional methods for two- and three-dimensional thermal modeling of lithium ion batteries, usually require a coupled electrochemical battery cell model and a lot of knowledge about battery cell internal composition which are not provided and often protected by the battery cell’s manufacturers. This paper proposes an alternative method for battery cell 2D modeling based on the extended concept of electrothermal impedance spectroscopy. Moreover, a verification of the accuracy of the obtained results will be performed.

Electrothermal impedance spectroscopy and proposed approach

Barsoukov et al. were the first ones presenting the concept of electrothermal impedance spectroscopy (ETIS) in [1]. Later the concept was further extended and improved by Schmidt et al., by introducing internal heat excitation and a more accurate frequency-based measurement method [2]. ETIS is a non-destructive, relatively easy to implement, ‘entropy-free’ and not requiring any a priori knowledge about cell internal composition method, which is used for determining battery cell’s heat capacity and heat conductivity. The ETIS method is based on applying a specific heat flow to the battery and measuring the amplitude and phase delay of resulting battery temperature response [2].  For the frequency-based method, this procedure is repeated for several heat excitation frequencies and thus the thermal impedance function is defined. So far, the ETIS method has been used for one point measurements. This work is extendig the ETIS concept to multi-point measurements and study the accuracy of the two-dimensional thermal model parameterized by the means of the multi-point ETIS measurement.

Laboratory setup and description of experiments

A laboratory setup was built based on a high-bandwidth Kepco galvanostat and several high-precision temperature sensors located at different points of the high-power LiMO₂/Li₄Ti₅O₁₂ battery cell (Fig.1). Frequency-based method with internal heat generation was applied and results for two spots, on the battery cell surface, are presented in Fig.2. The obtained results are demonstrating different ‘local’ thermal impedances of the battery cell and in consequence uneven battery cell heating (Fig.3 ad Fig.4). These spot dependent thermal impedance functions can be used later for two-dimensional thermal modeling of battery cell by means e.g. Cauer model [3].

Final version of the paper

In the final version of the paper, the detailed results from multi-point ETIS will be presented and discussed. A two-dimensional equivalent thermal circuit based battery model will be developed and parametrized by means of multi-point ETIS. Finally, the accuracy of this two-dimensional battery thermal model [3], developed based on the multi-point ETIS, will be analyzed.

References:

1. E. Barsoukov and J.H. Jang and H. Lee, J. Power Sources 109, 313 (2002).
2. J.P. Schmidt and D. Manka and D. Klotz and E Ivers-Tiffee, J. Power Sources, 196 (19): 8140 (2011).
3. A. Samba and N. Omar and H. Gualous and Y. Firouz and P.V den Bossche and J. V. Mierlo and T.I. Boubekeur, Electrochemica Acta, 117, 246 (2014).