(181p) Charge Transport in Nonaqueous Polyelectrolyte Solutions for Li-Ion Batteries: Ion-Ion Correlations and the True Transference Number from Molecular Dynamics Simulations

Authors: 
Fong, K. D., University of California, Berkeley
Self, J., University of California, Berkeley
Diederichsen, K. M., University of California, Berkeley
Wood, B. M., University of California, Berkeley
McCloskey, B. D., University of California, Berkeley
Persson, K., Lawrence Berkeley Lab
Conventional liquid electrolytes for Li-ion batteries are limited by low Li+ transference numbers (t+), in which the majority of the electrolyte conductivity comes from motion of the anion, rather than the electrochemically-active Li+. Increasing t+ has the potential to enable batteries with enhanced energy and power densities by eliminating detrimental concentration gradients within the cell. Nonaqueous polyelectrolyte solutions, in which anion motion is slowed through covalent attachment to a polymer chain, have attracted recent interest as potential high t+ electrolytes, with initial efforts producing transference numbers estimated to be at least twice that of conventional electrolytes. Experimental t+ measurements, however, are typically based on self-diffusion coefficient measurements and the Nernst-Einstein equation, which assumes solution ideality. We lack insight into the extent to which these assumptions hold for polyelectrolytes as well as the true transference number of these solutions. More broadly, we lack foundational understanding of the behavior of polyelectrolytes in nonaqueous solvents, as existing polyelectrolyte research has focused extensively on aqueous systems.

The present work aims to address these knowledge gaps by using all-atom classical molecular dynamics simulations to investigate the structural and transport properties of a model polyelectrolyte solution, poly(allyl glycidyl ether-lithium sulfonate) in dimethyl sulfoxide. Through analysis of the ion-ion correlations in the system, we gain direct insight into the types of ion motion which most strongly govern trends in ionic conductivity, enabling rigorous calculation of the transference number. We find that incorporating these ion correlation effects, most importantly the correlations between anionic groups on the polymer chain, results in transference numbers significantly lower than those previously estimated in experimental works. Beyond this, we find that the overall conductivity trend cannot be fully explained through structural analysis of Li+ ion pairing but is consistent with a shift towards more structural (less vehicular) Li+ diffusion as concentration increases. These results highlight the need to reconsider the approximations typically made for transport in polyelectrolyte solutions.