(121g) The Effect of Host Miscibility and Polarity Contrast in Polymer Blend Electrolytes

Battery electrolytes are ionically conductive but electronically insulating materials. These materials are sandwiched between their battery’s cathode and anode, and their properties allow them to prevent the accidental electrical discharge of a charged battery when not connected to a load while still conducting ions when in operation. Highly conductive electrolytes are typically realized through dissolving a salt in a small-molecule organic host. The best performing hosts are miscible blends of low viscosity and highly polar components, which effectively impart high mobility to ions and dissociate ionic aggregates, respectively. One may naïvely assume that the ionic conductivity of this ternary electrolyte will be the volume-fraction-weighted average of each host’s intrinsic conductivity. However, it has been shown that the ionic conductivity of these electrolytes positively deviate from this linear mixing rule, allowing them to achieve a higher ionic conductivity than either of the unblended electrolytes could on its own.

Motivated by these results, we sought to investigate through molecular simulation whether such a deviation can occur in polymer blend electrolytes whose hosts have some intrinsic contrast in polarity and mobility and whether this deviation is influenced by host miscibility. However, unlike small molecule electrolytes, the phase stability of polymer blends is highly sensitive to interactions between each component, including those brought on by contrast in the polarity of each polymer. Further, it is challenging to ascertain a priori from an atomistic model the extent to which two polymer components are compatible in a blend.

Thus, we chose to examine this question using a coarse-grained molecular dynamics framework. Our coarse-grained model represents all polymers as a string of coarse-grained, bead-like repeat units in each of which is embedded a freely rotating point dipole. In addition to easily tuning host polarity contrast by independently varying the strength of the dipole moment in each of the polymers, the miscibility of each polymer can easily be changed by varying the strength of hard-core interactions. This model thus allows us to vary polarity contrast independently from the compatibility of the host polymers.

Using this model, we found that increasing host compatibility can indeed lead to improvements of the ionic conductivity relative to a simple linear mixing rule, depending on polarity contrast. We investigated the underlying electrolyte properties, such as ionic aggregation and polymer dynamics, and correlated them with ionic transport in both miscible and immiscible blends.