A novel approach to prepare Si/C nanocomposites with yolk-shell structures for lithium ion batteries

A novel method was developed to successfully prepare mesoporous Si/C nanocomposites with yolk-shell structures (MSi@C). Different from the reported methods, this approach was unique, straightforward and easily scaled up. A plausible mechanism for the formation of MSi@C nanocomposites was proposed, which was in accordance with the results of transmission electron microscopy (TEM). When the mixture of mesoporous Si (M-Si) and citric acid was heated up, the volume of air adsorbed by the M-Si expanded, and the viscoelastic citric acid layers inflated just like balloons, directly leading to the formation of the yolk-shell structured MSi@C nanocomposites during the carbonization. The MSi@C nanocomposites possessed an M-Si core with diameter similar to 150 nm and a carbon shell with diameter similar to 230 nm. Such nano and mesoporous structure combined with voids between the M-Si core and carbon shell not only provides enough space for the volume expansion of M-Si during lithiation, but also accommodates the mechanical stresses/strains caused by the volume inflation and contraction. Moreover, partial graphitization of the carbon contributed to the improved electrical conductivity and rate performance of MSi@C. As a result, the prepared MSi@C exhibited an initial reversible capacity of 2599.1 mA h g(-1) and maintained 1264.7 mA h g(-1) even after 150 cycles at 100 mA g(-1), with high coulombic efficiency (CE) above 99% (based on the weight of M-Si in the electrode). Therefore, this work provided an alternative method to fabricate yolk-shell nanostructured materials with great potential as anode materials for lithium ion batteries.