Insight into Lithium Ion Transport Properties of Artificial Solid Electrolyte Interfaces By Molecular Dynamics Simulations

Abstract

Among the most promising technologies for the next generation of electrochemical energy storage devices are lithium metal anode based batteries. In practical application, however, issues such as parasitic reactions of lithium metal with liquid electrolytes or the transient inhomogeneous lithium deposition on the metal anode occur for these storage systems. To enable the use of lithium metal electrodes with liquid electrolytes, an artificial solid electrolyte interface (SEI) can be introduced, which should be affordable but also enhancing the electrochemical performance as well as calendar and cycle life of the cells. The ongoing search for feasible materials suggested the copolymer poly(3,4-ethylenedioxythiophene)-co-polyethylene glycol (PEDOT-co-PEG) as a promising candidate. The PEG homopolymer is ion-conductive whereas PEDOT is electron-conductive and offers good electrochemical stability [1]. The copolymer PEDOT-co-PEG exhibits sufficiently high ionic conductivity and may stabilize lithium electrode interfaces, thereby reducing lithium dendrite growth [1]. This work includes an analysis of the lithium ion transport properties of PEDOT-co-PEG by extensive molecular dynamics (MD) simulations, exploiting the Atomistic Polarizable Potential for Liquids, Electrolytes and Polymer (APPLE&P) method. Multiple constitutions of the copolymer as well as blend polymers of both PEDOT and PEG were studied in the presence of lithium bis(trifluoromethane)sulfonamide (LiTFSI) in different concentrations, at simulated temperatures between 300 and 500 K. Notably, the impact of selected parameters not only on the resulting polymer morphology, but also on the dissociation of the lithium salt, the mean square displacement of lithium ions, the corresponding lithium coordination, as well as the lithium transport mechanism is detailed. This work affords deeper insight and comprehensive understanding of molecular processes that govern the lithium conduction, thereby paving ways for tailored design of future materials, including the development of electrode coatings and electrolyte formulations, based on which superior electrochemical performance and lifetime for lithium metal based batteries could be achieved. References: [1] I.S. Kang, Y.-S. Lee, D.-W. Kim, J. Electrochem. Soc. 161 (2014) 53-57.