Elsevier

Journal of Power Sources

Volume 336, 30 December 2016, Pages 332-340
Journal of Power Sources

Deformation and failure mechanisms of 18650 battery cells under axial compression

https://doi.org/10.1016/j.jpowsour.2016.10.064Get rights and content

Highlights

  • Short circuit happens when an 18650 battery cell is axially compressed to 4 mm.

  • Deformation is localized in the positive terminal/endcap region.

  • Axial compression of jellyroll creates a circumferential crack in the separator in top region.

  • Deformation of shell casing and jellyroll can be simulated by FE model and explained analytically.

Abstract

An important deformation mode during ground impacts of battery packs made of cylindrical battery cells is axial compression. This type of loading subjects the cell to a complex deformation pattern and failure mechanism. The design of endcaps plays an important role in such deformations. To explore the sequence of deformation and the underlying failure mechanisms, a combined experimental/numerical study was carried out. Tests were conducted on 18650 cells, and the deformation of each component was carefully investigated and documented. There are four different stages in the force-displacement curve, corresponding with deformation of various components in the endcap assembly. A short circuit happens at a displacement of 4 mm. To clarify these observations, a detailed Finite Element model was set up, covering the geometry and the mechanical property of almost all the components of the cell. Using the simulation results, the sequence of the axial compression was revealed, which was subsequently validated by Micro CT scans as well as analytical solutions. Based on the precise analysis of the mechanical behavior, the cause of the short circuit during axial loading was clarified. Two failure mechanisms in the separator at the top section of the cell explain the possible causes of short circuit.

Introduction

Cylindrical lithium ion battery cells have been a major power source for Electric Vehicles like Tesla Model S. The vertical configuration of these cells in the floor mounted battery packs make them prone to axial deformation in case of a ground impact. Most of the work on mechanical loading of batteries have been focused on transverse direction for both pouch and cylindrical cells [1], [2], [3], [4], [5], [6], [7]. Researchers at MIT Impact and Crashworthiness Lab (ICL) have designed experimental methods to characterize through thickness mechanical propertied of pouch cells under local indentation. They have developed computational models that closely predict response of the cell in such loadings [2], [3]. Similar experimental and computational methods were applied to cylindrical cells under lateral loading, by ICL team, and this line of research was followed by Volkswagen researchers [1], [4]. Results have also been reported by ICL as well as University of Michigan, on axial loading of pouch cells in which local short amplitude buckling was observed [8], [9], [10]. An initial investigation was performed on the Tesla S ground impact accident and it was found that the road debris can produce severe indentation in the protective aluminum armor plate [11]. The computational model predicted a considerable shortening of the cylindrical cells, more than 10 mm, before a sheet separating the battery pack from passenger compartment is punctured. The cells in that study were previously represented by using homogenized material for all interior of the jellyroll.

Although, the assumption of homogenized material properties works well for transverse loading of cells, it loses much of its accuracy for axial loading, due to special assembly of anode and cathode layers, having a mismatch on top and bottom parts of cylindrical cells. Additional complexity in axial loading is the existence of cell endcap structure at positive terminal. A deformation in axial direction may cause short circuit in the cap assembly well before any failure happens in the interior of jellyroll. In axial loading, a large deformation in jellyroll will lead to local buckling patterns. Such patterns are also detected on compression of laminated and sandwich structures as studied by Karam and Gibson [12].

The objective of this paper is to investigate the modes of failure for various components of 18650 cylindrical cells under increasing axial loading. Deformation and failure in lateral loading cases are more straight forward as it only involves tension and compression of various layers in a continuous uniform way. Axial loading however involves several phenomena in a step by step sequence of deformation for various components arranged in series. Axial deformation, bending, and buckling define different stages of shell casing deformation. Also the jellyroll deformation is non uniform and localized. Mechanisms of short circuit could involve failure in the plastic gaskets, causing contact between shell casing and positive terminal, or failure in separator, leading to contact between electrodes or positive electrode and shell casing.

To identify how the deformation progresses, a detailed finite element model of an 18650 cell was developed. Special emphasis was put on high fidelity representation of endcaps, involving gaskets, safety vent, axisymmetric groove in shell casing, and several other small components. The electrodes and separators were modeled as individual components and a great deal of attention was put on representing the mismatch and interaction between the layers in the top section of jellyroll right under the positive terminal. To represent all the above details, around one million elements were used in the model. The numerical simulation provided a clear picture of the progression of the complex deformation pattern on the upper part of the cell, thus providing a much needed insight about possible causes of short circuit. The predictions were verified by CT-Scan of cells tested in identical loading configurations. It is believed that present results provide great insight for designing next generation of safer cylindrical cells.

Section snippets

Composition of the cell

An standard 18650 battery cell and its three major components are shown in Fig. 1a. Components include a safety valve at the top of the cell (Fig. 1b), jellyroll (Fig. 1c) and shell casing made of mild steel (Fig. 1d). Safety valves made by different manufactories differ in structure, but commonly several important sub-components are included, such as positive temperature coefficient (PTC) device, aluminum safety vents, steel positive terminal and gasket seal (see Fig. 1e). The jellyroll, which

Axial compression test

Axial compression tests were carried out on 18650 cells using a universal testing machine (200 kN MTS) under a loading speed of 5 mm/min (quasi-static range). The setup of the test is shown in Fig. 3a and b. Digital Image Correlation (DIC) method was used for recording the deformation of the cell and the displacement of the loading end [17]. A voltmeter connected to the data-recording system of the computer was employed to monitor the voltage of the cell. The synchronization among the

FE model

A FE model representing almost all the details of the battery cell was set up in Abaqus/explicit, using the geometric parameters in Table 1 and the material properties in Fig. 2. For the sake of computational robustness, the simple elasto-plastic material model is used for most of the metal materials. The steel of shell casing and the aluminum foil of the cathode current collector both show a certain degree of anisotropy (less than 10%) [15], [16], which is characterized by the Hill’48 model.

Analytical solution of the axial compression process

In Section 4.3, the mechanisms for the deformation of the four stages have been made so clear that it is possible to develop analytical solutions for each of them, as another validation of the simulation and analysis.

On the cause of short circuit

The numerical and analytical work presented above provides insight into the cause of the short circuit. Firstly, it happens at a displacement of 3 mm (4 mm in tests, with initial gaps), which is in Stage IV of the entire process. The main deformation at this stage is a combination of bending of the shell casing and the compression of the jelly roll. Moreover, almost all the deformation localizes at the top of the jelly roll (see Fig. 5c). Therefore, it is reasonable to conclude that the cause

Conclusions

This paper is focused on clarifying the two observations in the axial compression tests of 18650 battery cells, i.e. the different stages in the force-displacement curve and the onset of short circuit. Based on a comprehensive understanding about the mechanical properties of the battery cell components, a detailed Finite Element model was set up successfully. Simulation results showed very good correlation with test data as well as Micro CT scans. Accordingly, the sequence of the deformation

Acknowledgements

The support of the MIT Battery Modeling Consortium is gratefully acknowledged. The authors would also like to thank Dr. Milton Cornwall-Brady of MIT Koch Institute for Integrative Cancer for supporting us with the Micro CT scanning. Support of Altair Company with providing Hyperworks software for our research is greatly appreciated.

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