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.
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