We adapt and apply a recently developed optimization scheme used to obtain effective potentials for aluminosilicate glasses to include the network former boron into the interaction parameter set. As input data for the optimization, we used the radial distribution functions of the liquid at high temperature generated by ab initio molecular dynamics simulations, and density, coordination, and elastic modulus of glass at room temperature from experiments. The new interaction potentials are shown to reliably reproduce the structure, coordination, and mechanical properties over a wide range of compositions for binary alkali borates. Furthermore, the transferability of these new interaction parameters allows mixing to reliably reproduce the properties of various boroaluminate and borosilicate glasses.

New interaction potentials were developed for molecular dynamics simulations to study the role of Mg and Ca in modifying the structure and properties of alkaline earth silicates and borates. Competition between the depolymerization of the silica network and the formation of new bonds between oxygen atoms and modifiers leads to the enhancement of the elastic moduli with increasing modifier content in alkaline earth silicate glasses. Compared with calcium silicate, the higher elastic moduli of magnesium silicate result from a higher connectivity of the overall glass network due to the incorporation of fourfold coordinated magnesium and a more rigid connection between oxygen atoms and modifiers. In contrast to the silicates, the effect of modifier on the elastic moduli of alkaline earth borates is dominated by the formation of fourfold coordinated boron (N4). Calcium borate with higher N4 shows a more rigid network structure and higher elastic moduli.

We apply a recently developed optimization scheme to obtain effective potentials for alkali and alkaline-earth aluminosilicate glasses that contain lithium, sodium, potassium, or calcium as modifiers. As input data for the optimization, we used the radial distribution functions of the liquid at high temperature generated by means of ab initio molecular dynamics simulations and density and elastic modulus of glass at room temperature from experiments. The new interaction potentials are able to reproduce reliably the structure and various mechanical and vibrational properties over a wide range of compositions for binary silicates. We have tested these potentials for various ternary systems and find that they are transferable and can be mixed, thus allowing us to reproduce and predict the structure and properties of multicomponent glasses.

Divalent ions (Mg, Ca, and Zn) are being considered as competitive, safe, and earth-abundant alternatives to Li-ion electrochemistry, but present challenges for stable cycling due to undesirable interfacial phenomena. We explore the formation of electroactive species in the electrolyte Ca(BH₄)₂∣THF using molecular dynamics coupled with a continuum model of bulk and interfacial speciation. Free-energy analysis and unsupervised learning indicate a majority population of neutral Ca dimers and monomers with diverse molecular conformations and an order of magnitude lower concentration of the primary electroactive charged species - the monocation, CaBH[Formula: see text] - produced via disproportionation of neutral complexes. Dense layering of THF molecules within ~1 nm of the electrode surface strongly modulates local electrolyte species populations. A dramatic increase in monocation population in this interfacial zone is induced at negative bias. We see no evidence for electrochemical activity of fully-solvated Ca²⁺. The consequences for performance are discussed in light of this molecular-scale insight.

Molecular dynamics (MD) simulations present a data-mining challenge, given that they can generate a considerable amount of data but often rely on limited or biased human interpretation to examine their information content. By not asking the right questions of MD data we may miss critical information hidden within it. We combine dimensionality reduction (UMAP) and unsupervised hierarchical clustering (HDBSCAN) to quantitatively characterize prevalent coordination environments of chemical species within MD data. By focusing on local coordination, we significantly reduce the amount of data to be analyzed by extracting all distinct molecular formulas within a given coordination sphere. We then efficiently combine UMAP and HDBSCAN with alignment or shape-matching algorithms to partition these formulas into structural isomer families indicating their relative populations. The method was employed to reveal details of cation coordination in electrolytes based on molecular liquids.

Performant solid polymer electrolytes for battery applications usually have a low glass transition temperature and good ion solvation. Recently, to understand the success of PEO for solid-sate battery applications and explore alternatives, we have studied a series of polyacetals along with PEO, both from an experimental and a computational standpoint. We observed that even though the mechanism of transport may be more optimal in polyacetals, the lower glass transition temperature of the PEO-salt electrolyte system still makes it the best option, in this class of polymers, for battery applications. In this work, we explored the free-energy landscape of PEO and P(EO-MO) at various compositions and temperatures using metadynamics simulations to gain deeper insights into the various factors that affect the glass transition temperatures in these systems. In particular, we study the competition between intra- and inter-chain coordination of the cation in these systems that we had hypothesized in our previous work was responsible for the differences in the glass transition temperature. We observe that in PEO, the single-chain binding motif is thermodynamically more stable than the multi-chain binding motif, unlike P(EO-MO), where the opposite is true. We also show that multi-chain coordination, and the associated higher glass transition temperature, in P(EO-MO) is due to a larger strain energy for single-chain coordination that originates in the introduced OCO linkages (relative to PEO's consistent OCCO linkages). Furthermore, the type of pathways to move from one transition state to another in the various systems do not change at higher concentrations though the relative probability of cation-anion coordinated states increases. Calculations at different temperatures to understand the entropic effect on the stability of these coordination environments reveal that as we increase the temperature, single-chain coordination becomes relatively more stable due to the entropic cost of multi-chain coordination, reducing the number of accessible states for the polymer. The various insights into the factors that affect glass transition temperature in these systems suggest design principles for polymer electrolyte systems with lower glass transition temperatures that need further research to compete with PEO at the same absolute battery working temperatures.

The success of poly(ethylene 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 free-energy 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 glass-transition temperature (Tg) 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 higherTg, 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.

Polymer electrolytes mitigate safety concerns surrounding flammable liquid electrolytes in lithium-ion batteries. Poly(ethylene oxide) (PEO) electrolytes demonstrate viable conductivity values (∼1 × 10-3 S/cm) at elevated temperatures (>70 °C) but a relatively low Li+ current fraction (≤0.2) because strong Li+ coordination inhibits cation mobility. We have developed a series of polyacetal electrolytes by systematically varying methylene oxide (MO) and ethylene oxide (EO) units in the polymer backbone. These materials maintain high oxygen-to-carbon ratios like PEO but offer improved ion transport, revealing trends of decreasing conductivity and increasing current fraction with respect to polymer composition. In particular, the increasing current fraction measured via the Bruce-Vincent method suggests that MO units improve Li+ mobility relative to anion mobility. We calculate an overall efficacy (product of conductivity and current fraction) for each polymer/salt composition and identify two polymers - P(EO-MO) and P(EO-2MO) - that outperform PEO at high and low salt concentrations, respectively.

Polyacetal electrolytes have been demonstrated as promising alternatives to liquid electrolytes and poly(ethylene oxide) (PEO) for rechargeable lithium-ion batteries; however, the relationship between polymer structure and ion motion is difficult to characterize. Here, we study structure-property trends in ion diffusion with respect to polymer composition for a systematic series of five polyacetals with varying ratios of ethylene oxide (EO) to methylene oxide (MO) units, denoted as P(xEO-yMO), and PEO. We first use 7Li and 19F pulsed-field-gradient NMR spectroscopy to measure cation and anion self-diffusion, respectively, in polymer/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt mixtures. At 90 °C, we observe modest changes in Li+ diffusivity across all polymer compositions, while anion (TFSI-) self-diffusion coefficients decrease significantly with increasing MO content. At a given reduced temperature (T - Tg), all polyacetal electrolytes exhibit faster Li+ self-diffusion than PEO. Intriguingly, P(EO-MO) and P(EO-2MO) also show slower TFSI- anion self-diffusion than PEO at a given reduced temperature. Molecular dynamics simulations reveal that shorter distances between acetal oxygen atoms (O-CH2-O) compared to ether oxygens (O-CH2-CH2-O) promote more diverse, often asymmetric, Li+ coordination environments. Raman spectra reveal that anion-rich ion clusters in P(EO-MO) and P(EO-2MO) lead to decreased anion diffusivity, which along with increased cation diffusivity, support the viability of polyacetals as high-performance polymer electrolytes.