(226bc) Mechanical Properties of Porous Polymer Separator for Lithium-Ion Batteries

Aging and degradation of Li-ion batteries over many charge and use cycles are caused by both chemical and mechanical processes. The studies of the latter are usually limited to the consideration of the processes in electrodes, particularly the expansion of the electrodes due to lithium intercalation. However, expansion of the electrodes also causes the compression of the separator between them. Recent studies showed that the compression causes the separator pore shrinkage and worsening of the ion transport through it, which has significant impact on the battery performance [1–3]. Therefore, it is necessary to know the mechanical properties of the separators relevant to the battery operation conditions. So far, the reported mechanical properties are limited to those derived from the tensile test [4,5], while tensile tests do not represent the loads in a battery.

Here we present the results of compressive tests of commercial porous polypropylene (PP) separator Celgard 3501 at different strain rates. The experiments were carried out for dry polymers and immersed in fluid (DMC – typical organic solvent for electrolyte). We focused on two characteristic mechanical properties of the separator: effective Young’s modulus determined in the region of small deformation, and yield stress. We found that both of these properties differ for the dry material and immersed in fluid. We also found strong dependence of both on the strain rate, particularly we observed the noticeable stiffening of the wet sample at high strain rates. This effect cannot be explained by viscoelastic properties of the polymer, and related to poroelastic behavior – fluid drainage from the pores.

We developed a quantitative model taking into account both non-linear viscoelastic and poroelastic properties of the separator. Our model is capable of determining the material parameters relevant to the compression of the separator during battery operation. We performed simulations using Dynaflow finite element code [6] and calculated the effective Young’s modulus corresponding to the experimental conditions. Our model shows that poroelastic effects have to be taken into account when interpreting the experimental results on fluid-immersed compression testing of the separators at the strain rates above 10⁻³ s⁻¹. The results of our work will be further used in the development of predictive model for studying the coupled mechanics and electrochemistry of lithium-ion batteries. Such model can serve as a tool for optimization the materials and geometric parameters of lithium-ion cells to find a path towards extending their performance and lifecycle.

References
[1] C. T. Love, Thermomechanical analysis and durability of commercial micro-porous polymer Li-ion battery separators, Journal of Power Sources 196 (5) (2011) 2905–2912.
[2] C. Peabody, C. B. Arnold, The role of mechanically induced separator creep in lithium-ion battery capacity fade, Journal of Power Sources 196 (19) (2011) 8147–8153.
[3] Y. Pan, Z. Zhong, Modeling the Ion Transport Restriction in Mechanically Strained Separator Membranes, Journal of the Electrochemical Society 161 (4) (2014) A583–A586.
[4] A. Sheidaei, X. Xiao, X. Huang, J. Hitt, Mechanical behavior of a battery separator in electrolyte solutions, Journal of Power Sources 196 (20) (2011) 8728–8734.
[5] I. Avdeev, M. Martinsen, A. Francis, Rate- and Temperature-Dependent Material Behavior of a Multilayer Polymer Battery Separator, Journal of Materials Engineering and Performance 23 (1) (2014) 315–325.
[6] J. H. Prevost, DYNAFLOW: A Nonlinear Transient Finite Element Analysis Program. Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ. http://www.princeton.edu/~dynaflow/ (last update 2014) (1981).