How temperature inhomogeneities affect the cycle life of Li-ion batteries

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Looking at the ageing behaviour of Li-ion batteries, the internal cell temperature is one main influencing factor. The quantification of its influence is one of the key aspects to improve cycle life and is increasingly addressed in literature. In general, an optimum temperature of about 25 °C is given with increasing degradation both at rising and declining temperatures. In order to minimise deviations from this optimum, temperature management systems are used in applications. However, these systems cause a temperature gradient within the cell, which again influences the cell behaviour. This work focuses on the interaction of different well defined and imposed external thermal boundary conditions on cyclic degradation.
Therefore, the influence of inhomogeneous and transient temperature distributions on the cycle life is examined. As the focus of this study lies on the impact of thermal boundary conditions, full charge and discharge cycles are conducted with the same constant current while self-designed cell holders enable a precise temperature control via the cell tabs and the planar surfaces of the investigated pouch cells. Thus, homogeneous and inhomogeneous conditions can be applied, both steady state and transient, to detect the influence of temperature gradients and temperature changes during charge and discharge processes. The application of plates on the planar surfaces for a defined thermal boundary condition also ensures a proper mechanical pressure on the electrode stack to achieve comparable ageing results and to improve the reproducibility of the measurements. The degradation of the cells is quantified by means of the capacity fade as well as the rise in electrochemical impedance, which are determined in regular intermediate characterisations.
In this contribution a comprehensive analysis of the degradation progress will be presented and the influence of different thermal boundary conditions will be shown. Furthermore, the degradation is correlated to these thermal boundary conditions by introducing an equivalent ageing temperature for the rate of capacity fade. In this way, inhomogeneous temperature distributions and temperature changes are taken into account and can be correlated with results under homogeneous, steady-state conditions.
In a subsequent approach, these coherencies will be included in coupled electrical-thermal cell models for numerical investigations. In combination with a thorough forensic analysis, it is then possible to demonstrate how temperature inhomogeneities effect local and overall cell ageing.

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