Gongwei Wang

Exploring polycyclic aromatic hydrocarbons as an anolyte for nonaqueous redox flow batteries

Nonaqueous redox flow batteries (RFB) can potentially achieve high energy density due to the extended operating voltage windows and redox-active material candidates. However, the development of reversible anode materials with a low redox potential and high solubility is still one of the main challenges. Here, we systematically explore polycyclic aromatic hydrocarbons (PAHs) and their corresponding radical anions (PAH˙n−) as anode redox-active couples with a combination of experimental and computational methods. The results reveal that naphthalene and its radical anion (Nap/Nap˙−) are a promising anode redox-active couple. Paired with catholytes separately containing ferrocenium hexafluorophosphate (FcPF6) and TEMPO, the resulting RFBs can provide theoretical maximum energy densities of 39 W h L−1 and 208 W h L−1, respectively, which are much higher than that of a traditional all-vanadium flow battery (25 W h L−1). As proofs of concept, both static-mode and flow-mode of the as-proposed RFBs are assembled and can deliver dozens of consecutive charge–discharge cycles.

Dual carbon-protected metal sulfides and their application to sodium-ion battery anodes

Metal sulfides are considered as promising anode materials for sodium ion batteries owing to their good redox reversibility and relatively high theoretical capacity. However, their cycle life and rate capability are still unsatisfactory because of poor conductivity and a large volume change during the discharge/charge processes. A facile method for preparing dual carbon-protected metal sulfides is reported. Metal diethyldithiocarbamate complexes are used as precursors. The synthesis only involves a co-precipitation of metal diethyldithiocarbamate complexes with graphene oxide and a subsequent thermal pyrolysis. As an example, N-doped carbon-coated iron sulfides wrapped in the graphene sheets (Fe1−xS@NC@G) are prepared and used as the anode material for a sodium ion battery. The as-synthesized Fe1−xS@NC@G electrode exhibits a high reversible capacity (440 mA h g−1 at 0.05 A g−1), outstanding cycling stability (95.8% capacity retention after 500 cycles at 0.2 A g−1), and good rate capability (243 mA h g−1 at 10 A g−1). Coupled with a Na3V2(PO4)2@C cathode, the full battery exhibits a high capacity retention ratio of 96.5% after 100 cycles and an average output voltage of ca. 2.2 V. More importantly, the proposed synthesis route is universal and can be extended to fabricate diverse transition metal sulfide-based composites with a dual carbon-protected nanostructure for advanced alkali ion batteries.