Molecular modeling of electrolyte and polysulfide ions for lithium-sulfur batteries

Abstract

The operation of a lithium-sulfur (Li-S) battery involves the transport of Li+ ions and soluble sulfides mostly in the form of solvated ions. Key challenges in the development of Li-S battery technology are the diffusion of Li+ in micropores filled with sulfur and eliminating the “shuttling” of polysulfides. Ion dimensions in solvated and desolvated forms are key parameters determining the diffusion coefficient and the rate of transport of such ions, while constrictivity effects due to the effect of pore size compared to ion size control both transport and filling of the pores. We present molecular simulations to determine the solvation parameters of electrolyte ions and sulfides S22−, S42−, S62−, and S82− in two different electrolyte systems: LiTFSI in DOL/DME and LiPF6 in EC/DMC. The calculated parameters include the coordination number and the geometrically optimized model and dimensions, using the van der Waals surface approach, of the solvated and desolvated ions. The desolvation energy of the electrolyte ions is also calculated. Such data is useful for the modeling and design of the pore sizes of cathode host materials to be able to accommodate the different sulfides while minimizing their “shuttling” between cathode and anode.

A Perspective on Li/S Battery Design: Modeling and Development Approaches

“…The technique is adaptable based on the complexity of the model, ranging from structureless molecules that require only Newtonian mechanics to complex molecules with many degrees of freedom, which require Euler equations, Hamilton's quaternions, and Lagrange methods [85]. MD simulations have primarily been used to elucidate the physical structures of active materials, material properties, and their dependence on electrolytes and the state of the cell [86][87][88][89][90].…”

“…More recently, Babar et al used MD to evaluate ion dimensions of the electrolyte and Li-PS in solvated and desolvated states (see Figure 2). These values were then used to determine the diffusion coefficient, rate of transport, and desolvation energy of each species [89]. MD simulations have also been used to evaluate various nanoscale sulfur electrode structures; Li et al applied the technique to assess the performance and properties of several sulfur-doped carbon nanoparticle designs.…”

“…Finally, current MD models that account for electrode structure assume that the sulfur electrode has constant and uniform pore sizes [89,90], which are typically measured by gas adsorption tests [94] or X-ray nano-computed tomography (CT) [95]. However, real sulfur electrodes have a heterogeneous pore size distribution.…”

Design of Porous Carbons for Supercapacitor Applications for Different Organic Solvent-Electrolytes

“…Equivalent circuit models [2,29] are generally used to simulate device behavior [2], to elucidate different effects and degradation with cycling [30], and for power management and control for device and system operation [29]. At the other end, molecular dynamics (MD) simulations are useful in predicting the properties of electrolyte solutions, such as viscosity, diffusion coefficient and conductivity and the size and coordination number of solvated ions [31,32]. Density functional theory (DFT) simulations elucidate interactions [33] and can predict binding energies [34] between ions and pore walls, for example.…”

“…Early in the validation of our model against experimental data, it was realized that using the van der Waals surface wrapping the molecular model of the species to determine the minimum and maximum dimensions, L min,VdW and L max,VdW , respectively, and volume of the molecule or ion [28] yielded the best fit between predictions and experimental data. Solvated ion sizes are generally within the upper limit of micropore range [32]; hence, ions can easily migrate through the separator, which has much larger pores in the macropore range of the order of several microns [47] or micron if the separator was fabricated via electrospinning [48,49]. For this reason, the model assumes a fully permeable separator of negligible thickness, so that the ion transport is governed by the porous architecture of the anode and cathode [38].…”