Lithium salts with low coordinating anions such as bis(trifluoromethanesulfonyl)imide (TFSI) have been the state‐of‐the‐art for polyethylene oxide (PEO)‐based “dry” polymer electrolytes for 3 decades. Plasticizing PEO with TFSI‐based ionic liquids (ILs) to form ternary solid polymer electrolytes (TSPEs) increases conductivity and Li+ diffusivity. However, the Li+ transport mechanism is unaffected compared to their “dry” counterparts and is essentially coupled to the dynamics of the polymer host matrix, which limits Li+ transport improvement. Thus, a paradigm shift is hereby suggested: the utilization of more coordinating anions such as trifluoromethanesulfonyl‐N‐cyanoamide (TFSAM), able to compete with PEO for Li+ solvation, to accelerate the Li+ transport and reach a higher Li+ transference number. The Li–TFSAM interaction in binary and ternary TFSAM‐based electrolytes is probed by experimental methods and discussed in the context of recent computational results. In PEO‐based TSPEs, TFSAM drastically accelerates the Li+ transport (increases Li+ transference number by a factor 6 and the Li+ conductivity by 2–3) and computer simulations reveal that lithium dynamics are effectively re‐coupled from polymer to anion dynamics. Last, this concept of coordinating anions in TSPEs is successfully applied in LFP||Li metal cells leading to enhanced capacity retention (86% after 300 cycles) and an improved rate performance at 2C.
In this work, we report the results from molecular dynamics simulations of lithium salt-ionic liquid electrolytes (ILEs) based either on the symmetric bis[(trifluoromethyl)sulfonyl]imide (TFSI-) anion or its asymmetric analog 2,2,2-(trifluoromethyl)sulfonyl-N-cyanoamide...
We
present the results from an extensive atomistic molecular dynamics
simulation study of poly(ethylene oxide) (PEO) doped with various
amounts of lithium-bis(trifluoromethane)sulfonimide (LiTFSI) salt
under the influence of external electric field strengths up to 1 V/nm.
The motivation stems from recent experimental reports on the nonlinear
response of mobilities to the application of an electric field in
such electrolyte systems and arising speculations on field-induced
alignment of the polymer chains, creating channel-like structures
that facilitate ion passage. Hence, we systematically examine the
impact of electric field on the lithium coordination environment,
polymer structure, as well as ionic transport properties and further
present a procedure to quantify the susceptibility of both structural
and dynamical observables to the external field. Our investigation
reveals indeed a coiled-to-stretched transformation of the PEO strands
along with a concurrent nonlinear behavior of the dynamic properties.
However, from studying the temporal response of the unperturbed electrolyte
system to field application, we are able to exclude a structurally
conditioned enhancement of ion transport and surprisingly observe
a slowing down. A microscopic understanding is achieved.
With regard to methodologically enhancing the electrolyte functionality, and hence the ionic conductivity as a crucial indicator, a profound understanding of the correlated ion motion is essential. For binary ionic liquid systems it has been shown that the motional coupling between ions is determined to a large extent by the conservation of momentum, concurring with simple theoretical relations for transference numbers that serve as a guideline for optimizing the electrolyte [1,2]. Inspired by this work, we perform Molecular Dynamics (MD) simulations to unravel the complex, and rather counterintuitive, interplay of dynamical electrostatic correlations in a ternary ionic liquid - lithium salt mixture, which contribute to the overall conductivity. Considering the environmental sustainability of the electrolyte mixture in the battery context as main scope of application, we choose three anions with reduced toxic fluorine content for this study (bis(trifluormethanesulfonyl)imide (TFSI), trifluormethanesulfonyl-N-cyanoamide (TFSAM) and dicyanamide (DCA)) and study the influence of anion structure on transport behavior. From systematic analysis of the distance- and time-dependent correlational quantities we try to identify key parameters that quantify the microscopic transport properties and hence might serve as a constructive optimization concept.
[1] H. Kashyap et al., J. Phys. Chem. B, 2011, 115 (45), pp 13212–13221
[2] D. Diddens, V. Lesch and A. Heuer, Correlated Motion in Ionic Liquids. To be submitted., 2019
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