Construction of Multishelled Binary Metal Oxides via Coabsorption of Positive and Negative Ions as a Superior Cathode for Sodium-Ion Batteries.

Multishelled binary metal oxide, which can exert a synergetic effect of different oxides, is a promising electrochemical electrode material. However, it is challenging to synthesize this kind of binary metal oxide 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 a chelating agent to inhibit hydrolysis and precipitation, a series of multishelled binary metal oxide hollow spheres (Fe2(MoO4)3, NiMoO4, MnMoO4, CoWO4, MnWO4, etc.) were obtained via coabsorption of negative and positive metal ions. In addition, the chelation between a metal ion and citric acid is systematically validated by NMR, MS, Raman, and UV-vis. In particular, multishelled Fe2(MoO4)3 hollow spheres show excellent electrochemical performance as cathode material for sodium-ion batteries benefited from their 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 multishelled multielement metal oxides and greatly enrich the diversity of hollow multishelled structures.