Inhomogeneous ageing in lithium-ion batteries caused by temperature gradients

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Looking at the ageing behaviour of lithium-ion batteries, the internal cell temperature is one main influencing factor. Therefore, the quantification of its impact is a key aspect of improving cycle life and is increasingly discussed in literature. In general, an optimum temperature of about 25 °C is assumed with increasing degradation both at rising and declining temperatures. In order to minimise deviations from this optimum, temperature management systems are used in automotive applications. However, these systems cause a temperature gradient within the cells, which again influences the cell behaviour and ageing. This work focuses on the local degradation mechanisms and their distribution induced by precisely defined and imposed external thermal boundary conditions during cycling.
Therefore, the influence of inhomogeneous and transient temperature distributions 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. At the same time, self-designed cell holders enable 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 reproducable ageing results and to improve the reproducibility of the measurements. The overall 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.
Subsequently, the cells are opened in an argon-filled glovebox, and the electrodes are separately characterised by various analysing techniques, including the measurement of the electrode thickness, scanning electron microscopy (SEM), X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-OES). The observed changes are then correlated with the thermal boundary conditions. These measurements reveal a strong inhomogeneity in ageing mechanisms, while the overall ageing behaviour is not significantly altered by temperature gradients but corresponds to the average temperature. The local ageing phenomena correlate with the local temperature. Thus, this work shows how temperature gradients lead to inhomogeneous temperature-dependent degradation. Temporal temperature changes, on the other hand, induce significantly different degradation effects that cannot be correlated with the average temperature.

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