Hollow and yolk-shell metal oxide powders used as energy storage materials exhibit good electrochemical properties at high current density because of their shortened diffusion length and increased amount of contact area between the electrolyte and the electrode for Li + insertion/extraction. Although various types of hollow-structured oxide materials have been studied as anode and cathode electrode materials for lithium secondary batteries, [13][14][15][16][17][18][19][20][21][22][23][24][25] the hollow-structured powders cannot be readily applied as battery materials because of their low energy densities due to low tab density. The disadvantages of the hollow materials can be overcome using core@void@shellconfi gured yolk-shell-structured powder particles. The core of such yolk-shell-structured powder particles will improve the rate capability as well as the energy density of the powders by increasing the weight fraction of the electrochemically active component. [ 12 , 15 ] The structures of the metal oxides that undergo large changes in volume during Li + insertion/extraction strongly affect the electrochemical properties of the anodes produced using such materials. The voids between the yolk and the shell can also serve as a buffering space for the electroactive core material during lithium insertion and extraction; therefore, various types of yolk-shell-structured metal oxides prepared using hydrothermal, coprecipitation, and shell-by-shell deposition methods have previously been studied as anode electrode materials. [ 1-7 , 9-14 ] According to previous reports, the yolk-shell-structured powders were mainly prepared using a multistep solution method; [ 1-10 , 12-14 , 27-37 ] therefore, a scalable one-pot method should be developed for the large-scale production of yolk-shellstructured powders. We report a novel, simple one-pot method of systematically synthesizing double-shelled yolk-shell-structured powders by scalable spray pyrolysis, which is one of the gas phase reaction methods. The electrochemical properties of yolk-shell-structured SnO 2 powders prepared using the hydrothermal method have previously been well studied. [3][4][5] Therefore, in this study, SnO 2 was selected as the fi rst target material to develop the yolk-shell-structured powders with spray pyrolysis. The mechanism of the formation of the double-shelled SnO 2 yolk-shell-structured powders in spray pyrolysis was investigated.The morphologies and dot-mapping images of the SnO 2 powder particles directly prepared using spray pyrolysis are shown in Figure 1 . The SEM and TEM images show that the powder particles of all sizes exhibit uniform yolk-shell structures. Mobile spherical core powder is located inside the uniformly thick spherical shell, as shown in the SEM image in the insert of Figure 1 a. The HR-TEM images of the powder particles, as shown in Figure 1 c, reveal the double-shelled yolkshell structure of the particles. The dot mappings for the Sn and O components, as shown in Figure 1 e, are consistent with the double-shelle...

Fe2O3 yolk-shell particles with two, three, and four shells are prepared by one-pot spray pyrolysis. The discharge capacity of the Fe2O3 yolk-shell particles with two shells showing the best electrochemical properties is as high as 848 mA h g(-1) after 80 cycles at a current density of 300 mA g(-1).

A facile, one-pot method of systematically synthesizing yolk-shell materials with complex compositions is proposed. The spray pyrolysis process for obtaining yolk-shell materials is advantageous because it is highly efficient, allows high throughput, comprises a single-step reaction, and is a continuous process that yields homogeneous composition and enables facile control of the mean size.

Highly crystalline Co3O4 yolk-shell powders with a filled core and various shell numbers are prepared by a simple one-pot spray pyrolysis process. Co3O4 yolk-shell powders exhibited a high initial discharge capacity of 548 mA h g(-1) and retained their capacities very well, even at a high discharge rate of 10,000 mA g(-1).

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