Yolk-Shell Structured SiO₂@N,P Co-Doped Carbon Sphere as Highly Stable Anode Materials for Lithium-Ion Batteries

The development of lithium-ion batteries (LIBs) with high specific capacity and long lifespans has become a research hotspot in recent years due to rapidly growing energy demands in many fields such as electric vehicles, hybrid electric vehicles, and portable electronic devices. At present, graphite is the most widespread anode material for LIBs owing to low working potential and superior cyclability with high coulombic efficiency, but its unsatisfying theoretical capacity (372 mAh g^-1) has led researchers to seek for alternative anode materials with high specific capacity and rate performance. Silicon based anode material, which has extremely high theoretical capacity (4200 mAh g^-1) and low working potential (&lt;1.0 V vs. Li/Li⁺), has attracted great interests in the energy storage area. However, the huge volume expansion up to 400% during the alloying/dealloying process of Si with lithium seriously restrict its widespread application. In order to overcome the issue, SiO₂ based material, which has smaller volume expansion (100%) compared to Si, has been proposed as alternative anode material for LIBs. Moreover, SiO₂-carbon composites are highly attractive as promising anode materials owing to their improved electrical conductivity induced by carbon species. It has been found that introduction of nitrogen and phosphorus co-doping into the carbon structure can further accelerate electron transfer, which is attributed to strong dipole effect originated from difference of electronegativity of the elements (P: 2.2, C: 2.6, and N: 3.0).<br/>Herein, yolk-shell structured SiO₂@N, P co-doped carbon (SiO₂@NPC Y.S.) was fabricated by in situ thermal phosphidation and carbonization of SiO₂@PPy, followed by the partial etching of SiO₂ using dilute HF. The core-shell structured SiO₂@PPy was synthesized using hydrothermal method to conduct thermal polymerization of polypyrrole on the surface of SiO₂ sphere. The prepared SiO₂@NPC Y.S. nanospheres exhibited superior initial capacity of 1261 mAh g^-1 at the current density of 0.1 A g^-1 owing to improved electrical conductivity from heteroatom doping on carbon shell. Furthermore, the long-cycle reversible capacity can be preserved at 705 mAh g^-1 even after 300 cycles stemming from highly controlled yolk-shell structure. The unique yolk-shell structure with sufficient void space between yolk and shell has been demonstrated to be effective in alleviate the volume swelling, thereby improving the cycling performance during repeated lithium insertion/extraction process. Furthermore, the unique structure can not only prevent the aggregation of active materials but also reduce diffusion length of Li⁺ and facilitate the infiltration of electrolyte. Accordingly, this research will provide valuable insights into the future direction for the facile preparation of yolk-shell structured anode materials with highly controlled yolk size and heteroatom doped carbon shell for practical energy storage and conversion devices.