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<p>The model and analysis methods developed in this work are generally applicable to any polymer electrolyte/cation-anion combination, but we focus on the currently most prominent polymer electrolyte material system: poly(ethylene) oxide/Li-
bis(trifluoromethane) sulfonamide (PEO + LiTFSI). The obtained results are surprising and challenge the
conventional understanding of ionic transport in polymer electrolytes: the investigation of a technologically
relevant salt concentration range (1 - 4 M) revealed the central role of the anion in coordinating and hindering Li ion movement. Our results provide insights into correlated ion dynamics, at the same time enabling
rational design of better PEO-based electrolytes. In particular, we report the following novel observations. 1.
Strong binding of the Li cation with the polymer competes with significant correlation of the cation with the
salt anion. 2. The appearance of cation-anion clusters, especially at high concentration. 3. The asymmetry in
the composition (and therefore charge) of such clusters; specifically, we find the tendency for clusters to have a higher number of anions than cations.</p>
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