Hexagonal-WO3 nanorods encapsulated in nitrogen and sulfur co-doped reduced graphene oxide as a high-performance anode material for lithium ion batteries

Nanocomposites consisting of hexagonal WO3 nanorods embedded into nitrogen, sulfur co-doped reduced graphene oxide nanosheets (h-WO3/NSG) have been synthesized through a combination of ultrasonic processing and hydrothermal reactions. The nanocomposite containing 54 ​wt% of (N, S)-doped reduced graphene oxide shows an excellent discharge capacity of 1030 mAh g-1 at the first cycle and high rate capacities ranging from 513 to 150 mAh g-1 at the current density ranging from 100 to 2000 ​mA ​g-1, respectively. In addition, the synthesized WO3/NSG nanocomposite also exhibits remarkable cycle stability with a high discharge capacity of 196 mAh g-1 at the current density of 1500 ​mA ​g-1 for 200 cycles. The excellent electrochemical performance of h-WO3/NSG nanocomposite can be attributed to synergistic contributions from (1) the small size of WO3 nanorods, which increases the contact surface area with the electrolyte, effectively shortening the diffusion length of Li+ ions leading to high Li+ incorporation within h-WO3; (2) the structural support provided by the NSG matrix, which alleviates the large volume change of WO3 nanorods during Li+ insertion/extraction; and (3) the large numbers of surface defects provided by nitrogen and sulfur co-dopants, which serve as channels to improve the Li+ diffusion. Furthermore, electrochemical impedance spectroscopy (EIS) measurement demonstrates that nitrogen, sulfur co-doped reduced graphene oxide can reduce the electrochemical impedance of the electrode, further improving the electrochemical performance of the anode. This study provides important strategies for the design of future high performance anode materials for lithium ion battery.