The Bonding Nature and Adhesion of Li and Polyacrylic Acid (PAA) Coating for Li Dendrite Prevention

Abstract

Vehicle electrification requires lithium ion batteries (LIB) with higher energy density to make possible the extended driving range at a lower cost. One promising approach is to adopt the lithium metal as the anode due to its ultrahigh capacity and lowest reduction potential; however, Li dendrite growth is an inevitable bottleneck that can lead to capacity loss and short circuit. It is recently shown that Li polyacrylic acid (Li-PAA) polymer as a passivation layer can exhibit high elasticity, high binding, and excellent stability to mitigate the Li dendrite growth during Li plating/stripping processes. However, the experimental limitation to the measurement of the interfacial adhesion of the interface of Li metal and PAA polymer, Li/PAA(polymer), and other polymer-based passivation layers has retarded the coating design. The uncertainty of the PAA structures (crystalline, semicrystalline, or amorphous) due to varied polymer preparation techniques and post-preparation conditions also limits direct calculation of the work of adhesion/separation using density functional theory (DFT) calculations. In this work, from atomistic understandings of the bonding nature at the Li/PAA interface, we have proposed a strategy to quantify the interfacial adhesion (work of separation) of the Li/PAA(polymer) based on simpler Li/PAA(oligomer) interfaces using DFT calculations. A total of fifteen interfaces of Li/PAA(tetramers), Li/PAA(hexamers), and Li/PAA(nanomers) were studied. It was found that the delamination of Li/PAA(oligomer) interfaces is affected by: a) the O atoms in PAA bonding with multiple surface Li atoms, b) the chemical reactions, including H incorporation into the Li substrate and surface LiOH from COOH reacting with Li surface, leading to the enhanced ionic Li-O bonding, and c) the interactions of ionized Li (incorporated into the PAA oligomers) with the surrounding metallic Li atoms on the surface. The characteristic interfacial bonding energy is then obtained based on the most likely delamination configuration and by fitting the corresponding work of separation using two approaches: as a function of the number of bonding O and as a function of the number of LiO bonds and ionized Li atoms. The areal density associated with the interfacial bonding is obtained from the practical PAA density and statics of the DFT results as well. The interfacial adhesion of Li/PAA(polymer) is quantified to be 0.97 J/m2, and is found to be comparable to that of Li/Li2O interface and much larger than Li/LiF and Li/Li2CO3 interfaces. We attribute the high interface adhesion of Li/PAA(polymer) to the high density of open O atoms of PAA, allowing more Li-O bonds across the Li/PAA interface. This study provides a strategy to estimate the adhesion of artificial polymer-based passivation layers on Li metal, which is expected to contribute to the development of polymer-based coatings for the Li dendrite growth mitigation in LIBs.