Homogenized mechanical properties for the jellyroll of cylindrical Lithium-ion cells

doi:10.1016/j.jpowsour.2013.04.135

Journal of Power Sources

Volume 241, 1 November 2013, Pages 467-476

https://doi.org/10.1016/j.jpowsour.2013.04.135 Get rights and content

Highlights

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A method was developed to estimate the structural strength of cylindrical cells.
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The procedure was illustrated on a commercial 18650 lithium-ion cell.
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A finite element model of the cell was then created.
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The model was validated in local crush scenarios.
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The model predicts loads, deformation, and possibility of short circuit.

Abstract

A hybrid experimental/analytical approach was developed for extracting the average mechanical properties of cylindrical Li-ion cells. By using the principle of virtual work, and estimating the load transfer mechanism inside the cell, the stress–strain relation for the jellyroll was calculated for the case where the cell was crushed between two flat plates. The procedure was illustrated on an example of a commercial 18650 cell. A finite element model of the cell was then developed using the crushable foam material in LS Dyna. The model calibrated with this method closely predicts kinematic of the cell during two different load cases used for validation. These cases include local crush by a hemispherical punch and indentation by a rigid rod. The load and displacement during deformation, as well as onset of electric short circuit observed from experiments were closely predicted from simulations. It was found that the resistance of the cell comes primarily from the jellyroll. Additional analytical calculations showed that the shell casing and the end-caps provide little contribution to the overall crash resistance of the cell in the loading cases studied in this paper.

Introduction

Individual Lithium-ion battery cells consist of a jellyroll packaged inside a soft pouch or hard steel or aluminum shell casing. The jellyroll, in turn is composed of layers of electrode/separator assembly, which is rolled, or stacked inside the casing, depending on the form factor of the battery (pouch, cylindrical, and prismatic). The components of the electrode/separator assembly are the coated copper, and aluminum foils that are kept apart by a polymeric separator. The coating itself is a mixture of active powders with a binder and soaked in electrolyte. Typical thickness of coated aluminum or copper foils is about 0.1–0.2 mm. The metal foils contribute to about 10–15 μm each, and the remaining thickness comes from graphite or lithium metal oxide (or phosphate) powder. The thickness of the polymeric separator is also in the order of 10–25 μm. In a typical consumer electronic, lithium-ion cell can have about 20 layers (see Fig. 1). The number of layers for the automotive size cells could be much larger. It is evident that the battery cells are spanning at least three orders of magnitude in the length scale.

The mechanical response of individual components of the cell under certain loading conditions was studied by several authors. Shim et al published results of uniaxial tensile tests on copper and aluminum foils to model blanking of current collectors and formation of burrs that may affect performance of the battery under charging and discharging cycles and accidental loads [1]. The tensile strength of the active material is related to strength of the binder and it is generally, relatively low. Ways of increasing the tensile strength were studied by Liu et al. [2]. They performed tensile tests on coated cathode and reported a fivefold increase in the tensile strength by adding short carbon nanofibers to the matrix. Sheidaei et al studied properties of polypropylene separators in tension, and found a very strong anisotropy [3]. The anisotropic structure of this polymer is due to the manufacturing technique that produces porous sheets through crazing. Results from Venugopal et al. as well as Anand and Di Leo also confirm anisotropic behavior of the separator [4], [5]. All the above publications have shown an order of magnitude difference between the strength of separator in machine direction versus transverse direction. Breaking strain and the strength of polymeric separators was reported by Djion et al. [6].

No publication could be found in the literature that presents a comprehensive study of mechanical properties of all five components of the jellyroll. Likewise, no effort has been reported in literature to integrate the properties of the five layers into one homogenized representative volume element (RVE). While, this line of research is being conducted by the present investigating team, a different approach is proposed here to determine average constitutive properties of the RVE. In a small pouch cell, the length and width of the cell are about an order of magnitude larger than its thickness. Also, the small size of the cells allows using a standard 200 KN testing machine to test the cell under compression without a need to cut a specimen. The compression tests on these cells produce a uniaxial strain condition that would directly give the stress strain of the jellyroll, as reported by Sahraei et al. [7]. The properties found with this method were sufficient to model the behavior of batteries in several loading conditions.

It is far more difficult to determine the compressive properties of the jellyroll in cylindrical cells, because there is not any simple test in which the state of stress and strain is uniform and could be measured directly. In fact, there are always regions of compressive, tensile, and shear stress in the jellyroll in even simple loading conditions. In order to overcome this difficulty, a hybrid analytical/experiment technique is proposed in this paper. This method consists of defining the average stress and strain in the deforming region as a function of the global measurable quantities such as load and displacement. Then, by comparing the analytical prediction with quantities measured from experiments, the constitutive behavior of the jellyroll in compression can be uniquely determined. For the above calibration procedure, lateral compression of a cylindrical cell between two plates was chosen. The method is quite general and is applicable to various loading situations on the cell. Two types of validation tests were conducted by performing local indentation by a cylindrical and a hemispherical punch. In both cases, very good correlation between the experimental and measured response was found. With a proper selection of the tensile cut-off value, the peak force corresponding to jellyroll failure and onset of electric short circuit is also predicted correctly.

Until recently the battery development was supported by testing alone. The present analysis provides a valuable contribution to the development of a reliable computational model of cylindrical cells, which is an extension of previously published results of the same team for pouch and cylindrical batteries [8], [9].

Section snippets

Discussion of 3D constitutive model

The most general constitutive equations to describe the jellyroll were discussed by Greve and Fehrenbach [10]. The authors distinguish between plastic flow and fracture. In order to predict failure of the jellyroll, the Coulomb–Mohr model was used by Greve and Fehrenbach. For the plastic flow, a special function with five parameters was suggested. The function predicts a nonlinear transition from the purely elastic range to the constant stress plateau followed by densification and stiffening.

The principle of virtual work

The constitutive equation of the jelly-roll will be determined from the lateral compression of the cell between rigid plates.

The quantities that could be measured in the calibration experiments are the total force P versus the cross-head displacement w (see Fig. 6). Therefore, the objective of the theoretical developments is to relate the internal stresses and strains to the measured quantities. The starting point in derivation is the principle of virtual work. $P \dot{w} = 4 \int_{V} σ_{i j} {\dot{ε}}_{i j} ⅆ V$ where $σ_{i j}$ and ${\dot{ε}}_{i j}$

Example application

The general framework developed in the previous section could be applied to any type of cylindrical cell. As an application of the above calibration procedure, the properties of jellyroll will be determined for a commercial 18650 cylindrical cell. Only one test is needed for calibration of this model. It consists of a lateral crash of a cell between two rigid plates. We have performed such tests on fully discharged cells, with end-caps removed, and the results were reported in [9]. The measured

Other contributions to the crush resistance

The shell casing is a deep drawn cylindrical structure made of aluminum or steel. It resembles a thin metal can used for various types of beverages. The crushing strength of the shell casing comes from the lateral cylindrical part and two end-caps. Only one cap is an integral part of the drawn casing. The other one is added on after inserting the jellyroll. It contains a plastic insulation ring and its strength is very small. The lateral crushing of an empty tube (with end-caps removed) was

Validation

It is very important that the validation of the proposed model be done on a different type of test than the calibration. The constitutive equation of the jelly-roll was determined in the previous section from a compression test between two flat plates. The prediction of the calibrated constitutive law has been validated with two additional types of tests, indentation with a rigid rod, and local crush with hemispherical punch. In the two chosen cases, tensile stresses appear in the cell in

Discussion and conclusion

The present paper provides a step-by-step procedure to determine the compressive properties of the jellyroll in cylindrical cells. The proposed model was calibrated from one test, and validated against two other tests. It was shown that the proposed model predicts accurately the load–displacement curve, the magnitude of the peak load, and the corresponding indentation depth causing the onset of failure. In addition to the mechanical properties, monitored during the tests were the voltage and

Acknowledgments

Support of MIT Battery Consortium, as well as Ford_MIT Alliance for this research is greatly appreciated. Authors also acknowledge support of Altair Engineering for providing Hypermesh software.

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Homogenized mechanical properties for the jellyroll of cylindrical Lithium-ion cells

Highlights

Abstract

Introduction

Section snippets

Discussion of 3D constitutive model

The principle of virtual work

Example application

Other contributions to the crush resistance

Validation

Discussion and conclusion

Acknowledgments

J. Mater. Process. Technol.

J. Power Sources

J. Power Sources

J. Power Sources

J. Power Sources

J. Power Sources

J. Power Sources

J. Power Sources

J. Mech. Phys. Solids

J. Mech. Phys. Solids