Theoretical Study on the Electronic Structure of Polymer-in-Salt/Ionic Liquid-Based Biodegradable Gel Polymer Electrolyte

The geometry, electronic structure, and chemical reactivity of polycaprolactone (PCL), solid polymer electrolyte (SPE; PCL/LiTFSI), and polymer-in-salt/ionic liquid-based biodegradable gel polymer electrolyte, i.e., ionogel (IG; PCL/LiTFSI/[EMIMDCA]) have been discussed on the basis of quantum chemical density functional theory (DFT) calculations using B3LYP method and LANL2DZ basis set. The interaction of Li⁺-ion with PCL and ionic liquid (IL) has been calculated in terms of electronic structures of clusters of the mixtures of PCL and IL including a Li⁺-ion. The energy values of − 6.76, − 7.02, and − 5.29 eV and − 1.11, − 3.60, and − 3.35 eV were related to HOMO-LUMO orbitals of PCL, SPE, and IG, respectively. The energy gap ( \({E}_{{\text{g}}}\) $E_{\text{g}}$ ) of the molecular systems are found 5.65 (PCL), 3.41 (SPE), and 1.95 eV (IG), respectively. The migration of the LiTFSI (Li⁺-ion) cation is what causes the lowered band gap ( \({E}_{{\text{g}}}\) $E_{\text{g}}$ ) of SPE. Finally, the narrowed \({E}_{{\text{g}}}\) $E_{\text{g}}$ of IG demonstrates that IL improves the migration of the LiTFSI cation (Li⁺-ion) across the PCL oxygen atom. The HOMO-LUMO energy levels are used to calculate the global and local chemical descriptors and electrochemical stability windows (ESWs). Mulliken population analysis (MPA) and the surface of the molecular electrostatic potential (MEP) are used to study partial charge analysis.