The electrochemical role of the SEI in Li-ion batteries and design principles for electrode surface modification

The solid electrolyte interphase (SEI) formed on the electrode in Li-ion batteries plays an important role not only in surface durability but also in charge transfer reactions. Herein, design principles for large-current batteries are presented after simulating various surface-modified graphite electrodes. Activation energies for the solvation and desolvation processes in the charge transfer reaction of the Li⁺ ion were determined at equilibrium using a hybrid method combining first-principles (density functional theory) and solution theory (reference interaction site method), in which electrochemical behaviors are well described. It was possible to show how the surface morphology influences the charge transfer reaction through the activation energy of the solvation/desolvation process. The activation energy of the charge transfer reaction on the graphite electrode modified with a LiF cluster or a CHO group is smaller (∼0.3 eV) than that on H-terminated, LiF-layered, or OH-terminated graphite (∼0.6 eV). For the LiF cluster and the isolated CHO group, there is a large three-dimensional free space around the Li⁺ ion in the transition state during the solvation/desolvation process, resulting in lower activation energy. The H-terminated, LiF-layered, and OH-terminated graphite have flat surfaces, providing limited space for solvation, resulting in half solvation rather than full solvation. The solvation/desolvation process on the graphite modified with a LiF cluster or CHO group is very weakly coupled with the charge transfer, and thus the anodic transfer coefficient approaches unity (symmetry factor of 0).