Elucidation on Abnormal Capacity Increase in SiO₂@C Core-Shell Nanospheres Anode for Lithium-Ion Battery

An abnormal capacity increase stage has been observed in nanostructured SiO₂ after the initial capacity drop stage. To investigate the Li⁺ storage kinetic mechanism for each stage, SiO₂@C core-shell nanospheres with a total diameter of ∼108 to 170 nm but an adjustable C shell thickness of ∼4 to 31 nm have been fabricated. First, the existence form and specific content of SiO₂ nanoparticles with a size of ∼6-10 nm, which are embedded in the outer C shell of SiO₂@C core-shell nanospheres, were confirmed by SEM, TEM, BET, and TGA, respectively. It was found that the initial stage for capacity drop happens at 15-43 cycles and is followed by an enhancement stage, which presents an increase of ∼120 to 180% in capacity relative to the lowest capacity value during cycling. Among them, the sample of P-1 with a diameter of 109 nm for the SiO₂ core and thickness of 31 nm for the C shell delivers the highest specific capacity of 1060 mAh/g at 100 mA/g and a capacity increase rate of ∼180% through 300 cycles. XPS analysis for the delithiation process indicates that the capacity drop and increase stage involves the partial oxidation of Li silicate, which is correlated to the formation of Li₂Si₂O₅. Our study can be used to explain the mechanism of the abnormal capacity increase phenomenon for the SiO₂ anode and provide a high-capacity anode material for LIB application.