Molecular Dynamics Simulations of Ion Transport in Poly-Ethylene Oxide

Abstract

Solid polymers are becoming a promising alternative option as an electrolyte material in lithium ion batteries [1,2,3]. Solid polymer electrolytes have certain advantages over traditional electrolytes – non-flammability, good electrochemical and mechanical stability, and ability to hinder the growth of dendrites at electrode surfaces [4,5]. Poly-ethylene oxide (PEO) is one such polymer which has been considered a viable option due to its ability to solvate lithium ions and its relatively high ion conductivities. However, the bulk Li-ion conductivities even in PEO at room temperature are at least an order of magnitude less than the traditional liquid electrolytes [2,6]. In this study, we explore the influence of ion concentration and pressure on the Li-ion transport mechanism and in turn on its conductivity in PEO by performing molecular dynamics (MD) simulations of a lithium salt (lithium bis(trifluoromethane) sulfonamide (LiTFSI)) in polymer matrix of poly-ethylene oxide (PEO). We explored the influence of pressure to examine if it has any role in increasing the performance of PEO as an electrolyte. We computed parameters like diffusivity, conductivity and transference numbers that are crucial in determining the performance of PEO as a potential electrolyte for lithium ion batteries. We observe that diffusivity and conductivity decrease with increase in pressure while transference number remain almost independent. The simulation framework can be further used to explore polymers other than PEO as an electrolyte material or to test different block copolymers based on PEO. [1] F. M. Gray. Solid polymer electrolytes: fundamentals and technological applications (pp. 83-93). New York: VCH (1991). [2] D. T. Hallinan Jr., & N. P. Balsara. Polymer electrolytes. Annual review of materials research, 43, 503-525 (2013). [3] S. Mogurampelly, O. Borodin, & V. Ganesan. Computer simulations of ion transport in polymer electrolyte membranes. Annual review of chemical and biomolecular engineering, 7, 349-371 (2016). [4] F. M. Gray. Polymer electrolytes. Royal Society of Chemistry (1997). [5] K. J. Harry, D. T. Hallinan, D. Y. Parkinson, A. A. MacDowell, & N. P. Balsara. Detection of subsurface structures underneath dendrites formed on cycled lithium metal electrodes. Nature materials, 13(1), 69.4 (2014). [6] A. A. Teran, M. H. Tang, S. A. Mullin, & N. P. Balsara. Effect of molecular weight on conductivity of polymer electrolytes. Solid State Ionics, 203(1), 18-21 (2011). Figure 1