Facile Synthesis of Ge/N-Doped Carbon Spheres with Varying Nitrogen Content for Lithium Ion Battery Anodes

The simple fabrication of composites of germanium nanoparticles dispersed on nitrogen-doped carbon nanospheres (Ge/NC) of varying nitrogen content and their performance in lithium ion battery anodes are reported. A heavily nitrogen-doped carbon gel was formed by condensing m-phenylenediamine with formaldehyde (PF-gel); a less heavily N-doped gel was formed by condensing resorcinol and m-phenylenediamine with formaldehyde (RPF-gel); and an undoped gel was formed by condensing resorcinol with formaldehyde (RF-gel). Pyrolises of the gels with GeCl₄ at 750 °C produced nanocrystalline Ge composites with 7.5 atom % N-doped carbon, termed Ge/NC (PF), with 3.9% N-doped carbon, termed Ge/NC (RPF) and undoped carbon, termed Ge/C (RF). The heavily N-doped Ge/NC (PF) anode retained a reversible capacity of 684 mAhg^-1 at a specific current of 0.2 Ag^-1 after 200 cycles, versus 337 mAhg^-1 retained by anode made with Ge/NC (RPF) and 278 mAhg^-1 retained by anode made with undoped Ge/C (RF). At a specific current 2.0 Ag^-1, the capacity of the Ge/NC (PF) anode was 472 mAhg^-1, versus the 210 mAhg^-1 of the Ge/NC (RPF) anode and 83 mAhg^-1 of the Ge/C (RF) anode. The enhanced performance of the Ge/NC (PF) anode is attributed to the better electrical conductivity of Ge/NC (PF) and to the higher density of Li⁺ binding defects in its N-doped carbon.