Effect of Chemical Variations in the Structure of Poly(ethylene oxide)-Based Polymers on Lithium Transport in Concentrated Electrolytes

Polymer electrolytes constitute an attractive alternative
to current liquid electrolytes used in Li-ion batteries. Unfortunately,
the lithium-ion conductivities of the state-of-the-art polymer electrolytes
are few orders of magnitude lower than those of liquid electrolytes
at room temperature. In this work, we focus on poly­(ethylene oxide)
(PEO), which has shown the highest lithium ion conductivity in polymer
electrolytes so far. At high salt concentrations, the lithium conductivity
of a PEO electrolyte is strongly reduced because of the formation
of ionic aggregates. Using molecular dynamics simulations and rigorously
taking into account ionic correlations, we show how introducing a
secondary site with a specific chemical structure in the backbone
of PEO can greatly enhance the lithium conductivity of such concentrated
electrolytes. In addition, we demonstrate how results based on the
Nernst–Einstein equation can be highly misleading in the concentrated
regime. We identify PEO-based carbonate and sulfonyl variants that,
respectively, allow for significant ion dissociation and high cation
transference number.