Dynamics and ion mobility in polymer single ion conductors for lithium ion batteries

Janna Maranas, Penn State

Rechargeable lithium ion batteries are widely used in portable electronic devices. There are several advantages to replacing the organic liquid electrolyte with one based on a polymeric material, most notably the ability to use lithium metal as the anode material.  Such “solid polymer electrolytes” or SPEs suffer from low conductivity with significant contribution of the anion.  In this talk, the latter issue is addressed by considering a single ion conductor, in which the anion is incorporated in the polymer backbone.  The single ion conductor contains a PEO spacer, punctuated by an anion-bearing isophalate group.   In “traditional” SPEs ion motion is coupled to segmental mobility of the polymer, and a significant fraction of the lithium ions are solvated by the polymer backbone [single ions], with limited pairing and aggregation.  The behavior of the single ion conductors is more complex: single ions, ion pairs, and temperature-dependent aggregation are observed.  Polymer mobility is influenced by the amounts of single ions and aggregates, both of which form temporary cross links with the polymer chains.  The polymer relaxes in two stages: one related to single ions and corresponding to the spacer midpoint, and one related to ionic aggregates.  The slow regions are spatially correlated, leading to “dynamic phase separation”.  In some cases, ionic aggregation occurs within the slow regions.     Manipulating the single ion conductor to create more single ions does not increase ion mobility, in contrast to the expectation that single ions are the main contributors to conductivity.  Instead most highly mobile ions hop between pair states, or along the edges of ion clusters.  Within string-like aggregates, mechanisms that move charge without similar movement of ion position are observed.