This paper proposes a testing method that allows the monitoring of the development of volume expansion of lithium-ion batteries. The overall goal is to demonstrate the impact of the volume expansion on battery ageing. The following findings are achieved: First, the characteristic curve shape of the diameter change depended on the state-of-charge and the load direction of the battery. The characteristic curve shape consisted of three areas. Second, the characteristic curve shape of the diameter change changed over ageing. Whereas the state-of-charge dependent geometric alterations were of a reversible nature. An irreversible effect over the lifetime of the cell was observed. Third, an s-shaped course of the diameter change indicated two different ageing effects that led to the diameter change variation. Both reversible and irreversible expansion increased with ageing. Fourth, a direct correlation between the diameter change and the capacity loss of this particular lithium-ion battery was observed. Fifth, computer tomography (CT) measurements showed deformation of the jelly roll and post-mortem analysis showed the formation of a covering layer and the increase in the thickness of the anode. Sixth, reproducibility and temperature stability of the strain gauges were shown. Overall, this paper provides the basis for a stable and reproducible method for volume expansion analysis applied and established by the investigation of a state-of-the-art lithium-ion battery cell. This enables the study of volume expansion and its impact on capacity and cell death.

In this paper, the origin of the jelly roll deformation in 18650 lithium-ion batteries is examined in more detail by combining volume expansion measurements, accelerated lifetime testing, and CT imaging. Based on the presented research, a theory is developed to determine the cause of the jelly roll deformation at low states of charge (0%–20% SOC). The diameter of the cell is increasing during ageing, which reflects the increase of the internal pressure. Continuously growing cover layers of chemical degradation explain the latter. It is concluded that for releasing internal pressure, the jelly roll starts to deform. This deformation accelerates the ageing processes by electrically isolating active material from the current collector and the formation of fresh SEI. In addition, the CT images are used to determine the time of the deformation. Conclusively, it is shown that the jelly roll deformation is driven by cyclic ageing. Due to the correlation of the mean diameter change and the capacity curve, the deformation of the jelly roll and thus the sudden cell death can be predicted by measuring the mean diameter change of the battery.

This work conducts a post-mortem analysis of a cycled commercial lithium-ion pouch cell under an induced inhomogeneous pressure by using a stainless-steel sphere as a force transmitter to induce an inhomogeneous pressure distribution on a cycled lithium-ion battery. After the cycling, a macroscopic and microscopic optical analysis of the active and passive materials was executed. Also, scanning electron microscopy was used to analyze active material particles. The sphere shape results in a heterogenic pressure distribution on the lithium-ion battery and induces a ring of locally high electrochemical activity, which leads to lithium plating. Furthermore, a surface layer found on the anode, which is a possible cause of electrolyte degradation at the particle–electrolyte interface. Significant deformation and destruction of particles by the local pressure was observed on the cathode. The analysis results validate previous simulations and theories regarding lithium plating on edge effects. These results show that pressure has a strong influence on electrolyte-soaked active materials.