Volumenausdehnung und ihre Auswirkungen auf die Alterung einer zylindrischen Lithium-Ionen-Batterie

• Volume expansion and its effects on the ageing of a cylindrical lithium-ion battery

Willenberg, Lisa Kathrin; Sauer, Dirk Uwe (Thesis advisor); Simon, Ulrich (Thesis advisor)

Aachen : ISEA (2020, 2021)
Book, Dissertation / PhD Thesis

In: Aachener Beiträge des ISEA 147
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

The development of lithium-ion cells with higher energy densities is being driven forward by the use of alloy materials, among other things. Anodes are combined with high nickel contents in the cathode to further increase the energy density in 18650 cells. Both the dense packing of the materials and the alloy formation by silicon in the anode increase the volume expansion and thus the mechanical stress inside the cell, which accelerats cell ageing. The mechanical properties of lithium-ion cells are particularly important for applications with maximum energy density and lifetime [1], [2]. Due to the variations in volume during use of lithium-ion cells, geometric variations and the induced mechanical stresses must be taken into account when integrating the cells into modules, packs and systems [3], [4]. In this context, an unsuitable design leads to premature ageing of components at all levels. In extreme cases, a mismatch of volume requirements and device integration can lead to catastrophic failure [5]. Mechanical ageing caused by a volume change could be one of the reasons for early and unexpected cell death. Especially since the volume of graphite can change within the specified voltage limits by up to 7-12% [3], [6], [7], lithium-nickel-cobalt-aluminum-oxide by up to 5% [3], lithium-nickel-manganese-cobalt-oxide by up to 1-2% [3], [8], [9] and silicon by up to 280% [10]. The question to be answered is therefore, how serious the influence of volume expansion on cell ageing in commercially relevant cells is with regard to the application. The aim of this work is the systematic investigation of volume expansion and its effects on the ageing of lithium-ion cells. A cylindrical lithium-ion cell, the Samsung SDI INR18650 35E (Samsung 35E), is considered as an example. A strain gauge is used to monitor the volume expansion, here the diameter change. A total of 45 cells were examined under twelve different ageing conditions. The cyclic ageing conditions differ in their average state of charge and depth of discharge. Using strain gauges, a stable and reproducible method for analyzing the volume expansion of 18650 lithium-ion cells was introduced. The following main findings were elaborated in this thesis:•A complete characterization of the Samsung 35E is performed. •The characteristic curve shape of the diameter change depends on the State-of-Charge and the direction of load. The characteristic curve shape consists of three areas. A minimum at high State-of-Charge (SOC), a constant slope (plateau) at medium SOC and a flattening of the diameter change at low SOC. •The characteristic curve shape of the diameter change changes with ageing. The SOC dependent geometric changes are reversible. In addition, an irreversible increase in the diameter change over the lifetime can be observed. •The average mean diameter change and the loss of capacity correlate positively. Thus, by measuring the average mean diameter change, the State-of-Health can be determined and sudden cell death can be predicted. •The deformation of the jelly roll is caused by cyclic ageing. With the help of computed tomography images, the starting time of the deformation was determined. When the Samsung 35E ages with an average SOC of 10% and a cycle depth of 20%, the deformation of the jelly roll starts at the same time as sudden cell death. The knowledge gained should help to better design future lithium-ion cells with regard to their volume expansion and to extend their lifetime through optimized utilization.