(65c) Alkanethiol-Passivated Germanium Nanowires As High-Performance Anode Materials for Lithium-Ion Batteries

Rechargeable lithium-ion batteries have been widely used in portable electronic devices such as cell phones, laptop computers, and digital cameras, and have been considered as an electrochemical energy storage system for powering hybrid electric vehicles (HEV) and zero-emission vehicles (ZEV). It is necessary to develop lithium-ion batteries having a higher energy density and a higher power density to meet the increasing energy demand. Germanium (Ge) has emerged as a promising anode materials due to its high theoretical capacity of 1384 mAh/g - around four times greater than commercial graphite anode (372 mAh/g). However, the practical usage of Ge as an anode material is hindered by dramatic volume changes (~300%) during the insertion/extraction process of lithium ions, causing crack and pulverization, and loss of electrode contact.

      Ge nanowires can effectively accommodate the volume changes, tolerate relaxed mechanical strain, provide channels for efficient electron transport, and alleviate pulverization. Although direct incorporation of Ge nanowires into slurry coating process shows improved electrochemical performance relative to microparticles, the anode performance decayed rapidly within a few tens of cycles. In this talk, we demonstrate that dodecanethiol monolayer passivation on Ge nanowire surface can significantly enhance their anode performance in lithium-ion batteries. The dodecanethiol-passivated Ge nanowires exhibit an excellent electrochemical performance with a reversible specific capacity of 1130 mAh/g at 0.1 C rate over 100 cycles, which is above 80% of the Ge theoretical capacity. The functionalized Ge nanowires show high-rate capability having charge and discharge capacities of ~555 mAh/g at rates as high as 11 C. An aluminum pouch type lithium cell using a LiFePO₄ cathode was also assembled to provide larger current (~30 mA) for uses on light-emitting-diodes (LEDs) and audio devices. Organic monolayer-coated Ge nanowires represent promising Ge-C anodes with very low carbon content (~2-3 wt %) for high capacity, high-rate lithium-ion batteries and are readily compatible with commercial slurry-coating process for cell fabrication.