Designed Synthesis Of Hierarchical SnO₂@C Hollow Nanospheres And Their Enhanced Electrochemical Performance For Lithium Ion Battery Anode

Nowadays, with the fast development and vast demand in portable electronic devices and vehicular transport, lithium ion batteries have attracted intensive attention because of their high energy density, superior power density and enhanced cyclability. As we all know, Sn-based anode materials are regarded as a promising candidate for the next generation of lithium ion batteries(LIBs),due to its high theoretical capacity 992 mA h g-1 which is about twice than commercial graphite anode materials(372 mA h g-1).Therefore, Sn-based materials with different novel morphologies have been widely studied and reported.It is well known that hydrothermal method is one of promising approaches to synthesize different materials with varied morphology, high purity and perfect crystallization. In this paper, the double-shelled SnO2@C hollow spheres, single-shelled SnO2@C hollow spheres and double-shelled SnO2 hollow spheres were synthesized by multi-step hydrothermal methods. In the process, the core-shell SiO2 hollow spheres are the crucial template. Subsequently, the as-synthesized tin-based materials have been applied as anode materials of lithium ion batteries and the main results are as follows:(1)On the basis of our observations, we can conceive the mechanism of the transformation from solid silica spheres to hollow structures. Obviously, core-dissolution and shell-regrowth processes are concurrent but separate during the reaction to yield. In the process of the whole reaction, the NaBH4 is crucial. It is well known that the reaction between water and Na BH4 slowly generates sodium metaborate NaBO2 and H2. On the one hand, in the beginning, the solution is alkaline because of the reaction of water and NaBH4.Therefore, the silica is dissolved into solution easily, which is similar to process of silica dissolved in NaOH solution. On the other hand, monosilicate and polysilicate species are released into the solution, which eventually becomes supersaturated. At the same time, the concentration of NaBO2 also increases gradually as a result of the decomposition of NaBH4, thus causing the silicate species to precipitate and redeposit on the core surfaces.(2) The novel double-shelled SnO2@C hollow spheres possess numerous outstanding properties. Its inner active hollow spheres can effectively improve the tag energy density and its double-shelled carbon wrapping outside of each SnO2-shell can protect the integrity of structure as well as enhance electrical conductivity. This is a significant improvement in the design and synthesis of multi-shell or core-shell functional materials. More valuably, the porous hollow structure can not only facilitate liquid electrolyte fast diffusion into the double-shelled spheres but also buffer large volume changes during lithium ions insertion/extraction. Importantly, due to the prominent dispersibility and high specific surface area, the SnO2@C spheres can provide sufficient contact areas between active materials and electrolyte so as to improve its electrochemical performance. As a result, the obtained double-shelled SnO2@C nanocomposites exhibit excellent rate capability, enhanced cyclability(911 mA h g-1 after 100 circles) and high specific energy density, which are much better than single-shelled SnO2@C hollow spheres and double-shelled SnO2 hollow spheres.