Promotional role of nano TiO2 for pomegranate-like SnS2@C spheres toward enhanced sodium ion storage

The rational design of the anodes for sodium ion batteries (SIB) is highly significant to improve the sodium storage performance. Here we demonstrate a sulfuration-in-microsphere strategy for the first time to construct a pomegranate-like spheres for enhanced sodium ion storage anode comprised of SnS2 and TiO2 nanoparticles encapsulated within hollow carbon spheres (SnS2 NP/TiO2@C). The inner nano TiO2 acts as hard particle to cut SnS2 into nanoparticles and suppresses its aggregation during repeated cycling, thus maintaining the inner nanostructure and creating shorter Na+ diffusion pathway. The carbon shell acts as electric conductive layer as well as volume expansion buffer layer to maintain structural integrity. Benefiting from the unique architecture, the hybrid anode maintains a high reversible capacity of 624.3 mAh g−1 after 100 cycles at 0.2 A g−1 and 540.3 mAh g−1 after 100 cycles at 0.5 A g−1, while that of SnS2@C is only 232.3 mAh g−1 after 100 cycles at 0.2 A g−1. Even at a high rate of 5.0 A g−1 for 600 cycles, the SnS2 NP/TiO2@C composite still maintained a reversible capacity of 338.2 mAh g−1, corresponding to a superior cycling retention rate of 82.4%. In addition, the higher utilization rate of SnS2 in SnS2 NP/TiO2@C (74.6%) than in SnS2@C (21.7%) suggests that Na+ diffusion kinetics in SnS2 NP/TiO2@C spheres is much superior to that in SnS2@C. Furthermore, the corresponding kinetics analysis demonstrates that the pseudo-capacitive effect in SnS2 NP/TiO2@C contributes a lot to the total capacities (pseudo-capacitive contribution of 82.8% for SnS2 NP/TiO2@C, 71.2% for SnS2@C). This work provides a conception to manufacture high-performance SnS2-based anodes for sodium ion battery.