Hollow Carbon Nanosphere/Germanium Nanoparticle Composite Li-Ion Anode

Abstract

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 relatively slow due to long lithium diffusion pathlengths. 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 point 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. 372 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 materials, 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, 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 at room temperature by solution phase reduction of germanium precursors in the presence of the HCNS to produce conductive composites of Ge nanoparticles supported on HCNS. When cycling from 1.5 to 0.02 V vs. Li, a stable capacity of ~600 mAh/g electrode ( ~1200 mAh/g Ge) was obtained with average capacity fade of 0.03% per cycle. Long-term coulombic efficiency values were 99.9 and 99.6% for 20 and 40% Ge/HCNS, respectively. Owing to the high volumetric capacity of Ge and porous structure of the HCNS, areal capacities of ~2.5 mAh/cm2 could be obtained by slurry casted techniques. Performance comparisons of multiple Ge/HCNS compositions 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. Figure 1