Hollow Carbon Nanosphere/Germanium Metal Composite Li-Ion Anode

Since their introduction, lithium ion batteries have become ubiquitous for powering portable electronic devices. However, the charging rate of graphite (by far the most common anode material) is dependent on lithium diffusion into the material. Highly graphitic hollow carbon nanospheres (HCNS) have a tremendous advantage over micron-sized graphite particles as the diffusion path length is on the nanoscale, 3 orders of magnitude shorter than graphite’s, facilitating rapid charging within a few minutes, rather than many hours. In addition, HCNS anodes cycle stably in inexpensive low melting electrolytes, cycling with excellent capacity retention and high rate at temperatures as low as -50 ˚C.  Unfortunately, HCNS anodes have lower specific capacity compared to those made with graphite (~220 mAh/g vs. ~330 mAh/g).(1)  However, the high surface area, good electrical conductivity and excellent electrochemical stability of HCNS make it a promising support material for other high capacity lithium alloying metals, suggesting the possibility of significantly higher specific capacities, much better low temperature performance and charging rates than graphite-based anodes. Here we present our studies to date on the incorporation of germanium metal which is second only to Si in gravimetric capacity as a Li alloying material (1385 mAh/g) and nearly equal to it in volumetric capacity (7372 mAh/mL), into HCNS anodes.

Syntheses were performed by multiple reduction methods of germanium precursors in the presence of HCNS to produce conductive composites of HCNS. Various Ge morphologies were observed from each synthesis method. Using these composites, electrodes were prepared with different binder materials and tested in different electrolytes at various potentials in attempt to obtain a high capacity, long cycle-life anode while maintaining nearly 100 % coulombic efficiency. A stable capacity of 950 mAh/g of Ge was obtained when cycling from 1.0 to 0.055 V and long-term coulombic efficiency values were greater than 99.7 %. TEM analysis revealed the HCNS particles to be coated with Ge. Performance comparisons of multiple Ge/HCNS composites with differing cycling environments will be presented.

(1) Michael J. Wagner, J. Thomas McKinnon, J. Cox and K. Gneshin, “Hollow Carbon Naonsphere Based Secondary Cell Electrodes”, United States Patent 8,262,942, September 11, 2012.