Counterion Transport and Transference Number in Aqueous and Nonaqueous Short-Chain Polyelectrolyte Solutions

Nonaqueous polyelectrolyte solutions have recently been proposed as potential battery electrolytes due to their unique ability to tune the mobility of the anion relative to that of the electrochemically active lithium ion. This could potentially be used to study the effect of concentration polarization during battery charge, a major limiting factor in achieving fast charge rates that is caused by high anion mobility. An important consideration in the design of polyelectrolyte solutions for battery applications is the solubility of the polymer in battery-relevant carbonate blend solvents. Little is understood from a transport perspective, however, about the importance of designing the polymer to be solvophillic or if it is sufficient to obtain solubility through the incorporation of appended ions alone (as with polystyrene sulfonate in water). Using a model polysulfone-based system without added salt, we investigate the conductivity, viscosity, and diffusion of polyelectrolyte solutions over a range of concentrations and molecular weights in dimethyl sulfoxide (DMSO) and water. In both solvents, sulfonated polysulfone is readily soluble and the charged group is known to dissociate, but the neutral backbone polymer is only soluble in DMSO. We find marked differences in the transport behavior of polymer solutions prepared from the two solvents, particularly at high concentrations. Comparing this transport behavior to that of the monomer in solution demonstrates a larger decrease in lithium motion in DMSO than in water, even though the bulk viscosity in water increases far more rapidly. This study sheds light on the important parameters for optimizing polyelectrolyte solution transport in different solvents.