Effects of Self-Assembled Monolayers on Thermal and Lithium-ion Transport at SEI/PEO-based Polymer Electrolyte Interface: A Molecular Dynamics Study

Abstract

The formation of a stable solid electrolyte interphase (SEI) and the use of solid electrolyte in place of traditional organic liquid electrolyte are two promising strategies to mitigate dendrite-related issues associated with lithium metal anodes. However, poor interfacial compatibility between the SEI and solid electrolyte results in high thermal resistance and interfacial impedance, compromising battery safety and performance. This work investigates thermal and Li⁺ transport across the interface between LiF (serving as the SEI) and polyethylene oxide (PEO)-based solid electrolyte. Polyacrylic acid (PAA), polyacrylonitrile (PAN), and polyethylene (PE) are introduced as self-assembled monolayers (SAMs) to modulate interfacial properties. Non-equilibrium MD simulations show that all SAMs enhance interfacial thermal conductance, with PAA exhibiting the most substantial improvement—from 116.60 to 495.65 MW/(m²·K) (a 325.09% increase)—followed by PAN (196.48%) and PE (124.09%). The enhancement is attributed to the fact that SAMs facilitate vibrational coupling, and their polar functional groups (e.g., the −COOH group in PAA) strengthen non-bonded interactions. Conversely, free energy and interfacial impedance analysis reveal that SAMs hinder Li⁺ transport across the interface. The −COOH group in PAA competes with ether oxygen atoms of PEO for coordination with Li⁺, imposing additional constraints on ion mobility. PAN introduces steric hindrance, whereas PE, with minimal Li⁺ affinity and flexible chains, causes the least inhibition to ionic transport. These results highlight a trade-off between thermal and ionic transport optimization. The mechanistic insights provided in this work will be helpful for the comprehensive consideration of the interface SAM engineering, contributing to the practical application of lithium metal anodes for next-generation lithium-ion battery.