Hollow Carbon Nanosphere/Germanium Composites for use as Li-Ion Battery Anode

Student First Name: 
Adeel
Student Last Name: 
Khan
Student Picture: 
Adeel Khan with his research team members.
Project Picture: 
Adeel Khan working on his research project.
Expected Year of Graduation: 
2015
Department/Major: 
Chemistry
Student Team Members: 
N/A
Mentor(s): 
Dr. Michael Wagner, Associate Professor of Chemistry; and Nathan Banek, Graduate Teaching Assistant
Other Team Members: 
Kevin Hays, Laura Drake, Ming Zhang
Fun Fact About Yourself: 
I helped set the Guinness world record for the largest number of Segways doing a 360° turn! Also, this project won 1st place in the undergraduate Physical and Chemical Science category at GW Research Days 2015.
Project Abstract: 

The Li-ion battery is the most widely used rechargeable battery for powering portable electronics. Because Li metal is unsafe to charge and discharge, Li-ions are a good alternative. Graphite is one material that can store Li-ions and it operates at a similar electrochemical potential to Li metal making it a good anode material without the risk of battery fires. However, graphite is currently the limiting factor to the battery in both charge rate and capacity. Diffusion of Li-ions into graphite, by intercalation, depends on the size of the graphite particles, which are microns in diameter. Our project seeks to replace graphite with graphitic hollow carbon nanospheres (HCNS). A HCNS has a diffusion path length of ~50 nm vs. ~10,000 nm of a graphite particle which allow Li-ions to intercalate at a significantly faster rates from hours down to minutes. HCNS cycles with minimal capacity fade and a long-term Coulombic efficiency of >99.9%. Additionally, HCNS can operate with inexpensive low-melting electrolytes at temperatures as low as -40°C.

Despite the advantageous properties of HCNS it has a lower gravimetric capacity than graphite (~190 mAh/g vs 372 mAh/g). HCNS is synthesized by graphitizing cellulose using nickel catalyst, but when replaced by a cobalt catalyst we observed an increase in capacity (~235 mAh/g). Utilizing HCNS as a support material for other Li-ion alloy compounds can further increase the capacity. Germanium is one such material; it has a capacity of 1385 mAh/g and high Li-ion diffusivity but undergoes a large volumetric expansion upon alloying. This makes it impractical as an anode material by itself, but stable performance was observed when we combined Ge with HCNS. Early results show very stable cycling at around 600 mAh/g that could lead to a Li-ion battery with a much larger gravimetric capacity and a rapid charge rate than what is available today.