Hierarchical N-doping germanium/carbon nanofibers as anode for high-performance lithium-ion and sodium-ion batteries

Germanium (Ge) has gained a great deal of attention as an anode material for sodium ion batteries (SIBs) and lithium ion batteries (LIBs) for its high theoretical capacity and ion diffusivity. Unfortunately, Ge particle pulverization triggered by huge volume expansion during the alloying and dealloying processes can cause rapid capacity fade. Herein we report a facile method for the preparation of ultrafine Ge nanoparticles embedded in hierarchical N-doped multichannel carbon fibers (denoted as Ge-NMCFs) by electrospinning. The hierarchical carbon matrix not only provides sufficient internal void space to accommodate the large volume expansion of Ge nanoparticles, but also provides numerous open channels for the easy access of electrolyte and Na/Li ions. As half-cell tests revealed, the composite provides discharge capacity of 303 mA h g^-1 (1st cycle) and 160 mA h g^-1 (700th cycle) for SIBs, 1146.7 mA h g^-1 (1st cycle) and 600 mA h g^-1 (500th cycle) for LIBs at a current density of 500 mA g^-1 (all the presented capacity based on the total weight of Ge/C composites). Density functional theory calculation suggests that N-doped in carbon can enhance the Na/Li ion storage and improve the electrochemical performance. This demonstration is an important step towards the development of SIBs and LIBs with much higher specific energy capacity and longer cycle stability.