Combined density functional theory and molecular dynamics analyses on Lithium/Sodium ion interaction and diffusion in gel polymer electrolytes with poly-vinylidene fluoride scaffold, propylene carbonate/Ionic Liquid solvent, and perchlorate salt

Abstract

Choice of a proper electrolyte is very crucial to enhance the overall performance of a metal-ion battery. In this work, dispersion corrected density functional theory and classical molecular dynamics simulations are carried out to explore the metal ion (Li+ and Na+) interaction and diffusion through polyvinylidene fluoride (PVDF)-based gel polymer electrolytes. Four types of polymer/solvent/salt electrolyte systems are investigated considering PVDF scaffold for each case, and conventionally used propylene carbonate and rarely used, albeit cheaper, ionic liquid [BMIM][ClO4] as solvent, along with metal perchlorate salts LiClO4 and NaClO4. Inter-unit interactions within the electrolyte clusters, electrolyte uptake, and electrochemical stability of the electrolytes are explained by gas-phase DFT analyses, considering the polymer/solvent/salt components in 1:1:1 molecular ratio. Diffusion of the ions and ionic conductivity of the electrolytes are explored by MD simulation referring to two different size and time scales. From the DFT-based non-bonding interaction analyses, metal ions are found to exhibit strong non-covalent interactions with the adjacent O and F atoms, where the O atoms are parts of PC and [ClO4]- ions, and F atoms are from the PVDF chains. [BMIM]+, [ClO4]-, and PVDF are found to interact through weak van der Waals type hydrogen bonds. Both DFT and MD calculations suggest that, Li+ and Na+ ions are primarily coordinated with [ClO4]- ions. Dynamics studies confirm that for PVDF/IL/salt, total ionic conductivity is predominated by [BMIM]+ and [ClO4]-, exhibiting lower metal ion diffusivity as compared to PC-containing electrolytes. For both Li- and Na-ion systems, owing to the availability of higher number of charge carriers, electrolytes containing ILs show higher ionic conductivity compared to those containing PC as the solvent.