Control of homogeneous lithium deposition governs prospects of advanced cell development and practical applications of high-energy-density lithium metal batteries. Polymer electrolytes are thus explored and employed to mitigate the growth of high-surface-area lithium species while enhancing the reversibility of the lithium reservoir upon cell cycling. Herein, an in-depth understanding of the distribution of membrane properties and lithium deposition behavior affected by the selection of polymer segment species is derived. It is demonstrated that severely localized lithium deposits featuring needle-like morphologies may be readily observed when electrostatic fields (or partial charges) and the amount of Li + coordinators of the primary and secondary polymer segment species appear rather dissimilar, leading to a sudden cell failure at early stages of cell operation. In comparison, employment of optimized copolymer electrolytes enables superior cell performance at 1C even with thicker cathodes (6.3 mg cm −2 ). Additionally, the improvement of cell-cycling stability due to enhancement of similarity of dipole moments and partial charge distributions among copolymer segments are also demonstrated for different polymer systems, contributing to avoidance of undesired lithium protrusions, also reflecting a viable concept for the design of future copolymer electrolytes.
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.
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