Ion Transport and the True Transference Number in Nonaqueous Polyelectrolyte Solutions for Lithium Ion Batteries

Abstract

Nonaqueous polyelectrolyte solutions have been recently proposed as high Li + transference number electrolytes for lithium ion batteries. However, the atomistic phenomena governing ion diffusion and migration in polyelectrolytes are poorly understood, particularly in nonaqueous solvents. Here, the structural and transport properties of a model polyelectrolyte solution, poly(allyl glycidyl ether-lithium sulfonate) in dimethyl sulfoxide, are studied using all-atom molecular dynamics simulations. We find that the static structural analysis of Li + ion pairing is insufficient to fully explain the overall conductivity trend, necessitating a dynamic analysis of the diffusion mechanism, in which we observe a shift from largely vehicular transport to more structural diffusion as the Li + concentration increases. Furthermore, we demonstrate that despite the significantly higher diffusion coefficient of the lithium ion, the negatively charged polyion is responsible for the majority of the solution conductivity at all concentrations, corresponding to Li + transference numbers much lower than previously estimated experimentally. We quantify the ion–ion correlations unique to polyelectrolyte systems that are responsible for this surprising behavior. These results highlight the need to reconsider the approximations typically made for transport in polyelectrolyte solutions.

Transport properties of lithium-ion in green nonaqueous solutions and polymer ionic exchange membranes: an attempt to elaboration of ecological Li-ion battery

“…This finding is in quite qualitative accordance with the results presented in Fig. 3B that were calculated for the true transference data obtained from ionic conductivity using nonaqueous polyelectrolyte solutions data [13]. The true transference number (t Li+ ) calculated from ionic conductivity data is plotted along with the TN using molecular dynamics simulation with values for the charge of the anionic species (z − ) of both −1 and z polymer = − 43.…”

“…The low cation transference number generally less than 0.5 corresponds to the electrolyte in which the anion is highly mobile, whereas the electrochemically active Li + moves more sluggishly as a result of its bulky solvation shell. In consequence, the concentration gradient forms in the electrolyte, which limits the material utilization, promotes the lithium plating and generates the concentration overpotentials, all of which contribute to lowering the power density, and thus reducing the cell's lifetime [13].…”

“…These non-ideal ion correlations substantially decrease the cation transference number. The ion transport mechanisms will thus inform the design of improved polyelectrolyte systems for Li-ion batteries and generally enhance understanding of charge transport in polyelectrolyte solutions [13].…”

“…Due to its importance, Li + transport and the true transference number in nonaqueous polyelectrolyte solutions for Li-ion batteries have been recently investigated and are studied using all-atom molecular dynamic simulations [13]. An optimal composition was therefore sought: maximizing electrical conductivity and minimizing conductive salt content.…”

Conductivity–modulus–Tg relationships in solvent‐free, single lithium ion conducting network electrolytes

“…In dual ion conductors, the transference number of lithium (t Li + ) is usually less than 0.5, although the measurement and interpretation of these values are still the subjects of much discussion. [3,[44][45][46][47] According to the simulation work by Monroe and Newman, [48] Li dendrites will not form when t Li + is equal to 1 due to the absence of concentration gradients in the system and in particular at the dendrite front. [3,7] High transference numbers are pursued by fixing the anions onto the polymer matrix, forming a single-ion conductor.…”