(16b) First-Principles Guided Design of Ionomers for Facile Ion Transport
Ionic polymers are of interest as single-ion conductors for use as electrolytes in electrochemical devices, including lithium ion batteries. Current ion conductivities of the best ionomers are roughly 100X too small for practical applications and have a small fraction of their Li+ counterions participating in conduction. The design space for ionomer optimization is extensive, involving at least 20 possible anions and 30 polar functional groups that may help solvate ions. We are developing an ab initio methods based approach towards a ?bottom up? design strategy to recommend anion-side chain-backbone functional group combinations that will maximize Li+ conductivity. We will detail the development of a four-state model, in which ions are allowed to reside as ion pairs, triple-ions, quadrupole ion clusters, or ?free-ions? solvated by the polymer backbone. Ab initio calculations are used to evaluate the relative energy of these states, and this analysis is used to explain experimental physical properties (glass transition temperature, dielectric constant, conducting ion concentration) of a series of ionomers. A series of solvation models, including explicit solvation with quantum-mechanically treated solvent and continuum solvation models, are evaluated for their impact on relative concentrations of ion species.