Self-Assembled Monolayer Enables Slurry-Coating of Li Anode

Li metal has long been considered as the ultimate anodic material for high-energy-density batteries. Protection of Li metal in electrochemical cycling and in the manufacturing environment is critical for practical applications. Here, we present the passivation of the Li metal-carbon nanotube (CNT) composite with molecular self-assembly of a long-chain aliphatic phosphonic acid. The dynamics of the self-assembly process is investigated with sum-frequency generation spectroscopy (SFG). The aliphatic phosphonic acid molecules self-assemble on the Li metal surface via the lithium phosphate bonding, while the well-aligned long chains of the molecules help to prevent corrosion of lithium by oxygen and water in the air. As a result, the self-assembled monolayer (SAM) passivated Li-CNT composite displays excellent stability in dry or even humid air, and could be slurry-coated with organic solvents. The resulting slurry-coated Li anode exhibits a high Coulombic efficiency of 98.8% under a 33% depth of discharge (DOD) at a 1C rate in full battery cycling. The concept of molecular self-assembly on Li metal and the stability of the resulting SAM layer open vast possibilities of designed reagents for surface passivation of Li, which may eventually pave the way for practical application of Li metal in secondary batteries.