Exploring the Ion Solvation Environments in Solid-State Polymer Electrolytes through Free-Energy Sampling

Abstract

The success of polyethylene oxide (PEO) in solid-state polymer electrolytes for lithium-ion batteries is well established. Recently, in order to understand this success and to explore possible alternatives, we studied polyacetal electrolytes to deepen the understanding of the effect of the local chemical structure on ion transport. Advanced molecular dynamics techniques using newly developed, tailored interaction potentials have helped elucidate the various coordination environments of ions in these systems. In particular, the competition between cation−anion pairing and coordination by the polymer has been explored using freeenergy sampling (metadynamics). At equivalent reduced temperatures, with respect to the polymer-specific glass-transition temperature, two-dimensional free-energy plots reveal the existence of multiple coordination environments for the lithium (Li) ions in these systems and their relative stabilities. Furthermore, we observe that the Li-ion movement in PEO follows a serial, stepwise pathway when moving from one coordination state to another, whereas this happens in a more continuous and concerted fashion in a polyacetal such as poly(1,3-dioxalane) [P(EO-MO)]. The implication is that interconversion between coordination states of the Li ions may be easier in P(EO-MO). However, the overarching observation from our free-energy analysis is that Li-ion coordination is dominated by the polymer (in either case) and contact-ion pairs are rare. We rationalize the observed higher increase in glasstransition temperature (T g ) with salt loading in polyacetals as due to intermolecular Li-ion coordination involving multiple polymer chains, rather than just one chain for PEO-based electrolytes. This interchain coupling in the polyacetals, resulting in the higher T g , works against any gains due to variations in Li-ion coordination that might enhance transport processes over PEO. Further research is required to overcome the interdependence between local coordination and macroscopic properties to compete with PEO electrolytes at the same absolute working temperature. 36 through the free volume of the polymers assisted by the 37 segmental motion, with reasonable conductivity possible above 38 the glass-transition temperature. 6 Therefore, effective dissolu-39 tion of the cations and a low glass-transition temperature are 40 key to good ionic properties in these systems. 13 Unfortunately, 41 slow ionic conductivities and low transference numbers in

Understanding the Impact of Multi-Chain Ion Coordination in Poly(ether-Acetal) Electrolytes

“…Notably, the change in glass transition temperature in both polymers is very similar as we increase the concentration from r = 0.08 to r = 0.16. This is quite different from the observed greater increase in T g for P(EO-MO) vs. PEO between r = 0 and r = 0.08, 27,28 which we explore in more detail below. The collective variable used to describe the various solvation environments in these systems was the coordination number (CN) of the lithium ion with various types of oxygen atoms.…”

“…Molecular dynamics simulations were performed using the large-scale atomic/molecular massively parallel simulator (LAMMPS) code. 30 The interaction potential that was used is of the GAFF interaction potential functional form, 31 with slightly modified parameters that were derived in our previous work 27 (reproduced in Tables S1−S4 of the Supporting Information). The partial charges were calculated for isolated molecules using the TeraChem software 32−34 with a B3LYP hybrid density functional and the 6−311++ gss basis set using the restrained electrostatic potential (RESP) model.…”

“…The collective variable used to describe the various solvation environments in these systems was the coordination number (CN) of the lithium ion with various types of oxygen atoms. 27,39 For a better understanding of intra-versus interchain coordination, the collective variables used are the CN of the lithium ion with oxygen atoms from a specific chain of the polymer with respect to oxygen atoms from other chains. For the concentration study, the collective variables used, as before, 27 were the CN of the Li ion with oxygen atoms from the polymer and CN of the Li ion with oxygen atoms from TFSI anion.…”

“…Li Coordination in PEO. We can characterize coordination "states" with the two-dimensional coordinate [n, m], according to the number of coordinating polymer O atoms, n, or anion O atoms, m. We had previously 27 shown that within the fully polymer-coordinated states, with m = 0, [6,0] followed by [5,0] were the two most stable states, based on their sampled free energies (left panel of Figure 1). In this work, we consider further distinguishing within these polymer-coordinating states according to (p, q), where p + q = n, with p (q) denoting the number of intra-chain (inter-chain) coordinating polymer O atoms.…”

“…4,8,13−17 Molecular dynamics (MD) has been a vital tool in exploring and understanding the solvation structure and its effect on transport properties in solid polymer electrolyte (SPE) systems. 18−26 In our previous work, we had used both a regular classical molecular dynamics protocol and advanced molecular dynamics calculations (free-energy sampling using metadynamics) 27 to compare PEO with the most efficacious of the designed polyacetals 28,29 insights into the various coordination environments that exist, their relative stabilites, and how they might affect macroscopic transport properties. We showed that PEO has a stepwise pathway, with bond breaking and forming happening one after the other to move from one Li-ion coordination environment to another, while P(EO-MO) has a much more direct pathway, with the bond breaking and forming happening in a more concerted fashion.…”