Revealing the Molecular Origin of Driving Forces and Thermodynamic Barriers for Li⁺ Ion Transport to Electrode–Electrolyte Interfaces

Abstract

Enhancing the power of lithium ion batteries necessitates an understanding of the molecular processes governing Li + ion transfer across the electrode−electrolyte interface. Here, employing enhanced sampling molecular dynamics simulations, we investigated the driving force and thermodynamic barrier of Li + ion adsorption onto Li 0.5 CoO 2 (104) electrode in LiClO 4 -, LiPF 6 -, and LiTFSI-based EC/EMC (3:7) electrolytes. The weaker cation− anion pairing in LiTFSI compared to LiClO 4 was found to enhance the driving force for adsorption from −0.48 eV in LiClO 4 to −1.26 eV in LiTFSI for an electrode−electrolyte interface with zero cation coverage, which was accompanied by an increased thermodynamic barrier from 0.33 eV in LiClO 4 to 0.43 eV in LiTFSI at an equilibrium surface coverage of Li + ions. The hindered diffusivity of the solvent molecules in the electric double layer (EDL) at the electrode−electrolyte interface was the main contributor to the thermodynamic barrier for ion transport. The entropic component of the thermodynamic barrier was found to be more than 1 order of magnitude smaller for ClO 4 − compared to TFSI − , which can be attributed to the presence of more ClO 4 − than TFSI − in the EDL, causing more structural changes in the EDL. The strong dependence of the entropic component of the thermodynamic barrier on the EDL structure enables its decoupling from the enthalpic components (e.g., ion-pairing that can be tuned independently) and thus can be used to control the kinetics of the interfacial transport. This work provides a fundamental understanding of the thermodynamic and kinetic parameters involved in Li + ion adsorption, which is a crucial step in the performance of Li + ion batteries.