Highly stable lithium anode enabled by self-assembled monolayer of dihexadecanoalkyl phosphate

Li has been considered as the ultimate anode material for high energy density secondary Li batteries. However, its practical application has been limited due to its low Coulombic efficiency (CE) and the formation of lithium dendrites. Recently, we have developed a microspherical Li-carbon nanotube (Li-CNT) composite material passivated with octadecylphosphonic acid (OPA) self-assembled monolayer (SAM) exhibiting suppressed lithium dendrite formation and improved environmental/electrochemical stability. In this work, we demonstrated the significantly enhanced passivation effects of a SAM using dihexadecanoalkyl phosphate (DHP), a molecule that is comprised of double hydrophobic alkyl chains and forms a denser SAM on surfaces with large curvature. As a result, the DHP SAM delivers superior environmental and electrochemical stability to the OPA passivated Li-CNT material. In specific, the DHP passivated Li-CNT composite (DHP-Li-CNT) delivers a high CE of 99.25% under a 33.3% depth of discharge (DOD) at 1 C, when it is paired with a LiFePO₄ cathode. The evolution of the SAM during cycling and the effects of DOD and current density on the CE of the DHP-Li-CNT anode have also been investigated. The improved SAM passivation constitutes an important step in achieving the goal of practically applicable Li anodes.