Ionic Association in CH₃-(CH₂-CF₂) -CH₃(PVDF)-Li⁺-(CF₃SO₂)₂N^- for n = 1, 4: A Computational Approach

Area of Science:

• Solid-state chemistry
• Polymer electrolytes
• Computational materials science

Background:

• Ionic conductivity in solid polymer electrolytes is crucial for energy storage applications.
• Ionic association, involving interactions between polymer chains, lithium ions (Li+), and anions, significantly influences conductivity.
• Understanding these molecular interactions is key to designing advanced electrolyte materials.

Purpose of the Study:

• To investigate the molecular interactions governing ionic association in a specific polymer-lithium salt system.
• To analyze the effect of lithium ions and anions on polymer conformation and lithium coordination.
• To elucidate the relationship between molecular structure and ionic conductivity at a fundamental level.

Main Methods:

• Density functional theory (DFT) calculations were employed to model molecular interactions.
• Analysis of polymer conformation, including gauche conformations, for varying chain lengths (n=1, 4).
• Investigation of lithium coordination numbers and their changes upon anion presence.

Main Results:

• The lowest energy conformer of the pure polymer exhibits a gauche conformation for n=2, 4, but not for n=1.
• The presence of the bis(trifluoromethanesulfonyl)imide anion (Li+–(CF3SO2)2N−) reduces the lithium coordination number with the polymer from 3 to 2 for n=2, 4 systems.
• Vibrational spectra analysis provides insights into Li+ ion and Li+–anion interactions.

Conclusions:

• Molecular interactions, specifically polymer-Li+ and anion-Li+ associations, are critical determinants of ionic conductivity in solid polymer electrolytes.
• Computational modeling effectively reveals how structural changes and coordination number variations influence the behavior of ions within the polymer matrix.
• These findings contribute to a deeper molecular-level understanding of ionic transport mechanisms in polymer electrolytes.