Construction of multi-shelled binary metal oxides via co-absorption of metal cations and anions as superior cathode for sodium-ion battery.

Multi-shelled binary metal oxide which can exert a synergetic effect of different oxides, is promising electrochemical electrode material. However, it is challenging to synthesize this kind of binary metal oxides due to the severe hydrolysis and/ or precipitation reactions of the precursors between cations and anions of different metals. Herein, by using citric acid as chelating agent to inhibit the hydrolysis and precipitation reactions, a series of multi-shelled binary metal oxide hollow spheres (Fe2 (MoO4 )3 , NiMoO4 , MnMoO4 , CoWO4 , MnWO4 , etc.) were obtained via co-absorption of negative and positive metal ions. In addition, the chelation between metal ion and citric acid is systematically validated by NMR, MS, Raman and UV-vis. In particular, multi-shelled Fe2 (MoO4 )3 hollow spheres show excellent electrochemical performance as cathode material for sodium-ion batteries benefited from its structural superiorities. Especially, the quintuple-shelled Fe2 (MoO4 )3 hollow sphere shows the highest specific capacity (99.03 mAh g-1 ) among all Fe2 (MoO4 )3 hollow spheres, excellent stability (85.6 mAh g-1 was retained after 100 cycles at a current density of 2.2 C), and outstanding rate capability (67.4 mAh g-1 can be obtained at a current density of 10 C). This general approach can be extended to the synthesis of other multi-shelled multi-element metal oxides and greatly enrich the diversity of hollow multi-shelled structures.