Molecular Dynamics Modeling of the Conductivity of Lithiated Nafion Containing Nonaqueous Solvents

Abstract

We use molecular dynamics to predict the ionic conductivities of lithiated Nafion perfluorinated ionomeric membranes swelled in dimethyl sulfoxide (DMSO) and acetonitrile (ACN). The experimental conductivity of lithiated Nafion swollen with DMSO is two orders of magnitude higher than with ACN. Conversely, the mobility of Li + ions in a solution of LiPF 6 in ACN is approximately six times higher than in DMSO. In this work, we demonstrate that the ionic conductivity of Nafion is substantially governed by the concentration of free Li + ions, i.e. by the degree of dissociation of the Li + and SO 3 − pairs, and that the inherent mobility of Nafion is a cation-exchange membrane that has been extensively studied and used in aqueous electrochemical systems. In this work, we seek to understand the applicability of Nafion to flow batteries that use nonaqueous solvents and lithium salts. A flow battery is an electrochemical device that stores and releases energy by oxidizing and reducing redox molecules that remain dissolved in electrolytes. The active molecules are stored in external vessels and pumped to reactors to charge or discharge the battery. This arrangement provides a separation of energy and power that is absent from conventional enclosed batteries like lead acid. The all-vanadium redox flow battery, for example, is an aqueous system that relies on the V 2+ /V 3+ and VO 2+ /VO 2 + couples at the negative and positive electrodes and H + as the primary charge carrier. Nonaqueous electrolytes are being considered for redox flow batteries to enable electrochemical couples with potential differences substantially exceeding the aqueous stability limit of 1.23 V, and, consequently, increase energy density. Two key considerations in reactor design are power density and Coulombic efficiency. The membrane must possess high ionic conductivity, low electronic conductivity, and low permeability of active species (the various vanadium ions in the aqueous example mentioned above) to simultaneously achieve high area-specific power density and high Coulombic efficiency. In this work, Li + is the primary charge carrier, and the unspecified active species are assumed to be absent from the membrane. Future work could examine the behavior of active molecules in lithiated membranes containing different solvents, with a specific focus on the charge on the active molecule.Many experimental 1-9 and numerical 10-21 works discussing ion diffusion and conductivity in Nafion swollen with water have been published during the past two decades. The dependence of conductivity on water content and temperature is well understood. Furthermore, a number of theoretical 22,23 and experimental works consider Nafion swollen with methanol or water/methanol mixtures in the context of direct methanol fuel cells. 24,25 It has been demonstrated that the diffusion coefficients of water, methanol, and hydronium decrease with increasing methanol concentration. There have been far fewer studies on Nafion containing solvents other than water and methanol. A nota...

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

“…34−37 Others have performed polyelectrolyte simulations in nonexplicit continuum solvents, 38−40 but this approach often fails to adequately capture trends in chain conformation and ion dissociation. 16,41 Of the MD simulations in explicit nonaqueous solvents, 17,42 we are unaware of any which characterize the battery-relevant transport properties (such as t + ) of the solution.…”

Direct Measurement of Crossover and Interfacial Resistance of Ion-Exchange Membranes in All-Vanadium Redox Flow Batteries

“…A recent publication by the authors is dedicated to assessing the influence of membrane equivalent weight and reinforcement on the conductivity and selectivity of polymeric membranes [16]. This work, along with several others, suggests that the ionic conductivity of IEMs generally increases with increased dielectric constant of the membrane, solubility of charge balancing ion, and solvent concentration within membrane phase [17][18][19][20]. The ionic conductivity commonly decreases with increased EW and implementation of reinforcement.…”

“…As formulated in Equation (17), for a similar membrane structure and internal morphology, the variations of characteristic time with the membrane thickness is second-order. Such a second-order trend has been shown schematically in Figure 7 as a function of membrane thickness.…”

“…= (19) In Equation (19), is the elementary charge ( = 1.6 × 10 −19 C), is the Boltzmann constant, and is the temperature. [17] Therefore, plugging Equation (19) in Equation 18, we derive:…”

“…The Nernst-Einstein equation describes the mobility (u ) as a function of diffusivity for the ionic species [6]. u = qD kT (19) In Equation (19), q is the elementary charge (q = 1.6 × 10 −19 C), k is the Boltzmann constant, and T is the temperature [17]. Therefore, plugging Equation (19) in Equation (18), we derive:…”