Morphology Evolution of Tin-Based Oxide Hierarchical Structures Synthesized by Molten Salt Approach and Their Applications as Anode for Lithium Ion Battery

A molten salt strategy with SnO₂ nanoparticles as precursor in a mixed molten salt system (NaCl + Na₂CO₃) was used to prepare tin-based oxide microstructures with various morphologies, including porous hollow, bimodal mesoporous and solid core-porous shell structures in a large scale. The morphology of the as-synthesized products exhibits a strong correlation with the weight ratios of NaCl/Na₂CO₃ in the mixed solvent. With NaCl:Na₂CO₃ = 1:1, the liquefaction of eutectic salt and the reaction between SnO₂ nanoparticles and Na₂CO₃ would happen simultaneously, which allows formation of the porous hollow particles by Ostwald ripening and decomposition of Na₂CO₃. As the weight ratios of NaCl to Na₂CO₃ are increased to 2:1 and 4:1, the morphology of oxides is changed from porous hollow structure into bimodal mesoporous structure and solid core-porous shell structure, respectively. The morphologies of as-synthesized nanostructures are determined by the release rate of CO₂ from solvent decomposition and the aggregation rate of small nanocrystals at high temperature. Porous hollow oxides exhibit the best performance as anode for lithium ion battery. The enhanced performance can originate from the structural features, which alleviate the volume changes and mechanical stress during charging/discharging cycling.