The traditional replacements for organic salt-solutions as battery electrolytes are superionic conductors, ceramics, etc. While the use of solid electrolytes improves fire-safety and a low-barrier for an alkali metal-ion enables high ionic conductivity in these solids, poor interfacial contact with electrodes is still an inherent issue. To reduce high passivation between electrode-electrolyte layers-manifesting in poor interfacial contact, Zdilla’s group at Temple University explored solvent-in-salt concept of designing soft-solid crystals based on Pearson’s HSAB theory. In this approach, a lithium (sodium) salt is cocrystallized in a low-affinity organic solvent like N,N-dimethylformamide, adiponitrile, etc. The low-metal ion affinity of these solvents is due to the soft-basicity of carbonyl and nitrile functional groups. The solvent molecules Li+ (Na+) in a tetrahedral(octahedral) coordination while the counter-anion occupies interstitial space. Structurally, a continuous array of alkali metal ions form in the solvent matrix could enable a low-barrier ion-hopping with a soft boundary layer, enabling a better interfacial contact. Such molecular-level improvements in the electrolytes for solid-state batteries require an atomistic examination to formulate a better design principle. In my presentation, I will discuss a few approaches of computer simulations on these electrolytes which helped us in understanding the underlying complexity of these new paradigms and how the interplay of molecular interactions correlates to a set of macroscopically observed structural changes and spectroscopically observed ion-dynamics. We use classical molecular dynamics to simulate the thermal stability and structural changes in the cocrystalline electrolyte. Higher mobility of ions at the intergrain surface suggests that the grain-boundaries are the more favorable path for ion-conduction in these soft-solid electrolytes. Further, a more accurate mechanism of ion-conduction in the crystal structure (based on defect formation) is also modeled using plane-wave periodic density functional theory. Additionally, we have proposed a few design concepts based on the improvement in redox stability in this class of electrolytes using solid-state electronic structure calculations.

Soft-solid electrolytes for lithium batteries: Simulating structural and dynamical complexities

ACS Spring Meeting 2021

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