Hybrid polymer-liquid lithium ion electrolytes: effect of porosity on the ionic and molecular mobility

Alternative electrolyte systems such as hybrid electrolytes are much sought after to overcome safety issues related to liquid electrolytes in lithium ion batteries (LIBs). Hybrid solid-liquid electrolytes (HEs) like the heterogeneous structural battery electrolyte (SBE) consist of two discrete co-existing phases prepared by polymerization-induced phase separation: one solid polymer phase providing mechanical integrity and the other one a percolating liquid ion-conducting phase. The present work investigates the ion and the solvent mobility in a series of HEs using morphological, electrochemical impedance and NMR spectroscopic methods. All the dried HEs exhibit a porous structure with a broad pore size distribution stretching down to <10 nm diameter. Penetration of the individual components of the solution, that is the ions and the solvent, in the solid polymer phase is demonstrated. Yet, it is the pores that are the main ion conduction channels in the liquid-saturated HEs and, in general, translational mobility is strongly dependent on the volume fraction and size of the pores and, thereby, on the initial liquid electrolyte content. We also observe that the translational mobility of solvent and the ions vary differently with the pore volume fraction. This finding is explained by the presence of small mesopores where the mobility strongly depends on the specific interactions of the molecular constituent with the pore wall. These interactions are inferred to be stronger for the EC/PC solvent than for the ions. This study shows how the morphology and the chemical composition of HEs affect the ionic and molecular transport in the system.