Microstructure Design of Carbon-Coated Nb2O5–Si Composites as Reversible Li Storage Materials

We report on a rationally designed microsphere composite consisting of Nb2O5 and Si nanoparticles for application to secondary Li batteries as an anode material. The micron-sized spherical Nb2O5–Si composite was first prepared using a solvothermal method with appropriate surfactants. Heat treatment was performed to achieve both crystallization of Nb2O5 and pyrolysis of the carbon precursor, yielding a carbon-coated Nb2O5–Si microsphere composite. To characterize the synthesized material, X-ray diffraction, electron microscopy, and X-ray photoelectron spectroscopy were employed. The electrochemical test results demonstrated that the composite electrode delivered a high capacity of approximately 900 mAh g−1 after 100 cycles. This improved cycling stability can be attributed to the microstructure. In the microsphere composite, Si nanoparticles play a role as the main active material for Li storage; however, they suffer large volume changes during Li insertion and extraction cycling. In our composite material, crystallized orthorhombic Nb2O5 buffers the volume change and facilitates rapid Li transport through its microspheres. Additionally, the carbon coating layer acts as a secondary buffering medium and propels fast electronic/ionic transport.