Loosening the grip of polymer electrolytes: How the asymmetry of ion diffusion in conventional polyethers reveals a new design paradigm

Abstract

Battery electrolytes have been intensely studied for the last half century, but they remain a central bottleneck with respect to the stability of battery techonologies. Even the most advanced elec. vehicles still use massive arrays of small batteries within their battery packs to avoid the thermal runaway that plagues soln. electrolytes and larger (more efficient) battery designs. Polymer electrolytes would largely solve the stability problem and improve energy d., but their conductivities are presently too low to be useful. We recently discovered that ionic cond. in conventional polymers, such as polyethers, is limited by a general inverse relationship between ion soly. and diffusivity. This behavior arises from the fact that ion soly. is driven in conventional polymers through cation solvation-in most cases strong oxygen-cation interactions that promote polymer-ion complexation but restrict diffusion. We find that this behavior can be reversed in weak and strong lewis acidic polymers, such as polysilanes and polyboranes, where soly. is driven instead by anion complexation. Our study of unconventional lewis acidic polymers highlights the importance of labile solvation structures for transport and immediately suggests that we think more broadly about what is possible than the typical five and six coordinate solvation structures that have been heavily scrutinized for polyether based electrolytes. Among the barriers to designing such systems is that even the conventional space of polymer electrolytes only has fragmentary characterization and simulation. To address this we are comprehensively mapping out the design space of heteroatom (B,Si,O,S,N,P,F) d. and topol. in the polymer backbone to tease out the interplay of polymer compn., ion diffusivity, and ion soly., using mol. dynamics and automated forcefield development. Early results from charting these relationships include several new structures for next generation electrolytes; the generality of the inverse relationship between soly. and ionic diffusivity; and the crucial role played by interchain interactions in limiting ion diffusion.

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