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

Enhancing the power of lithium-ion batteries necessitates understanding 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 the thermodynamic barrier of Li+ ion adsorption onto Li0.5CoO2 (104) in LiClO4-, LiPF6-, and LiTFSI-based EC/EMC (3:7) electrolytes. The weaker cation-anion pairing in LiTFSI compared to LiClO4 was found to enhance the driving force for adsorption from -0.48 eV in LiClO4 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 LiClO4 to 0.43 eV in LiTFSI at 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 one order of magnitude smaller for ClO4 compared to the TFSI-, which can be attributed to the presence of more ClO4 than TFSI- in EDL, causing more structural changes in 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.