Carbon confined GeO 2 hollow spheres for stable rechargeable Na ion batteries.

Germanium (Ge) based nanomaterials are regarded as promising high-capacity anode materials for Na ion batteries, but suffer fast capacity fading problems caused by the alloying/de-alloying reactions of Na-Ge. Herein, we report a new method for preparing highly dispersed GeO 2 by using molecular-level ionic liquids (ILs) as carbon sources. In the obtained GeO 2 @C composite material, GeO 2 exhibits hollow spherical morphology and is uniformly distributed in the carbon matrix. The as-prepared GeO 2 @C exhibits improved Na ion storage performances including high reversible capacity (577 mA h g -1 at 0.1C), rate property (270 mA h g -1 at 3C), and high capacity retention (82.3% after 500 cycles). The improved electrochemical performance could be attributed to the unique nanostructure of GeO 2 @C, the synergistic effect between GeO 2 hollow spheres and the carbon matrix ensures the anode material effectively alleviates the volume expansion and the particle agglomeration problems.