**1. Introduction**

Lithium-ion batteries (LIBs) have been attractive as the most important type of power source for energy-storage system, electric vehicles, and other electronic devices because of their high specific capacities and energy densities [1–3]. Transition-metal oxides (TMOs) with high theoretical energy capacities have been widely applied as replacement anodes for the current graphite of LIBs [4–6]. However, the low intrinsic electric conductivity and the large volume expansion of TMOs during a charge/discharge cycle result in rapid capacity fading, which hinders the commercial application of TMOs for anodes in current LIBs [7,8]. To solve these problems, compositing TMOs with carbonaceous materials has been regarded as a possible solution. Carbon could effectively buffer the stress induced by the large volume change of TMOs during the fast charging–discharging process and improve the electrical conductivity of the anodes [9–11]. Additionally, a carbon matrix could prevent the aggregation of the active materials during repeated cycles by surrounding them, which increases the structural stability of anode materials [12,13]. Therefore, various synthesis strategies for TMOs/carbon composites have been introduced [14–18]. Cho et al. [14] prepared multiroom-structured metal–carbon hybrid microspheres containing empty voids of several tens of nanometers by liquid–liquid phase segregation because of the incongruent melting of the metal salt and dextrin during the spray pyrolysis. The discharge capacity of the multiroom-structured Co3O4–C hybrid microspheres for LIBs at a current density of3Ag−<sup>1</sup> for the 150th cycle was 1243 mA h g<sup>−</sup>1. Zhang et al. [15] also prepared TiO2–graphene composite nanofibers by a simple electrospinning process. The cell assembled with TiO2–graphene composite nanofibers as an anode retained 84% of the reversible capacity after 300 cycles at a current density of 150 mA g<sup>−</sup>1, which is 25% higher than bare TiO2 nanofibers did under the same test conditions. Bhaskar et al. [16] prepared MoO2/multiwalled carbon nanotubes (MWCNTs) composed of

spherical flowerlike nanostructures of MoO2, interconnected by MWCNTs by a one-step hydrothermal route. The one-dimensional electron-transport pathways provided by MWCNTs, which are in direct contact with the MoO2 nanostructures, imparted an improved reversible lithium storage capacity (1143 mA h g<sup>−</sup><sup>1</sup> at a current density of 100 mA g<sup>−</sup><sup>1</sup> after 200 cycles).

Molybdenum oxides are candidate anode materials for LIBs because MoO3 exhibits good electrochemical properties, has a low cost, and is environmentally friendly [19–21]. Therefore, MoO3 nanomaterials with diverse morphologies such as nanoparticles, hollow, nanobelts, naowiles, and porous structures have been prepared. Lee et al. [22] synthesized MoO3 nanoparticles using hot filament chemical vapor deposition method (HFCVD) under an argon atmosphere. Zhao et al. [23] also synthesized MoO3 hollow microspheres by a template-free solvothermal route and subsequent heat treatment in air. The MoO3 hollow microspheres have a relatively high specific surface area. Chen et al. [24] prepared MoO3 nanobelts by a hydrothermal method, in which the morphology of MoO3 nanobelts was affected with the addition of PEG. MoO3−<sup>x</sup> nanowires were prepared by Sunkara et al. [25] in a hot-filament chemical vapor deposition reactor. Ko et al. [26] prepared three-dimensional ordered macroporous structured MoO3 by using a polystyrene bead template via ultrasonic spray pyrolysis.

In this study, low crystalline MoO3/carbon composite microspheres, in which MoO3 nanocrystals were distributed homogeneously in the amorphous C matrix, were directly prepared by one-step spray pyrolysis within several seconds. In here, MoO3 was applied as the host material of carbon microspheres in this process because of its rich chemistry with multiple valence states, low electrical resistivity, high electrochemical activity toward lithium, and affordable cost. The resulting MoO3/carbon composite microspheres worked better in terms of cycling and rate as anode materials for LIBs than did bare MoO3 powders. The simple process introduced in this study is expected to be useful for the large-scale synthesis of TMOs/carbon composite microspheres as practical anode materials for LIBs. Furthermore, the synthesis strategy introduced is generally applied to synthesize various metal TMOs/carbon composites, including NiO, Co3O4, SnO2, and Fe2O3, for a wide variety of applications including energy storage.

#### **2. Materials and Methods**
