The shortage of lithium resources, safety and recycling difficulty has focused attention on alternative energy storage devices in recent years. The aqueous zinc-ion battery (ZIB) stands out against such a background because of its earth abundance, safety, and environmental friendliness.1 However, the limited choice of cathode materials hinders the development of advanced high-energy-density aqueous ZIBs. At present, manganese oxide2 and vanadium oxide3 are the two most widely studied zinc-ion battery cathodes, but the migration of Zn2+ in these materials is limited by the strong electrostatic interaction with lattice oxygen ions, resulting in poor reversible capacity. Metal sulfides, instead, may effectively improve the electrochemical performance reversibility of ZIBs. Layered metal sulfides have been extensively studied in monovalent cation (Li+, Na+, K+) rechargeable batteries.4 However, although limited studies with Bi2S35,6 as ZIB cathode material exist, their detailed electrochemical charge storage and transfer mechanisms are not well understood.

In this work, we explore the effect of covalent anchoring Bi2S3 on reduced graphene oxide (rGO) on the stability and cycling performance as a cathode for aqueous ZIBs. During the hydrothermal synthesis, the reduced graphene oxide serves as the nucleation substrate enabling the formation of fine and uniformly sized Bi2S3 grains, Figure 1 (a). Raman and X-ray photoelectron spectroscopy (XPS) confirm the formation of Bi-O-C heterojunctions during hydrothermal synthesis. These oxygen bridges serve as efficient electron transfer channels in the Bi2S3/rGO composite for rapid charge compensation during Zn2+ incorporation/extraction. As a result, Bi2S3/rGO composite shows notably better rate performance and cycling stability compared with pristine Bi2S3. The specific capacity of Bi2S3-rGO8 composite is ~186 mAh g-1 at the current density of 500 mA g-1 after 150 cycles, considerably higher than unsupported Bi2S3. Additionally, the Bi2S3 nucleated on GO with smaller particle sizes can shorten the transport path of zinc ions, which is beneficial for fast charge transfer. Therefore, Bi2S3-rGO8 can deliver more than 100 mAh g-1 at 10 A/g charge/discharge current density, Figure 1 (b).

Also, the zinc storage mechanism was analyzed by X-ray diffraction spectroscopy (XRD) and XPS, indicating a reversible conversion reaction of Zn2+ in the Bi2S3-rGO framework. During discharging, Zn2+ is embedded in Bi2S3-rGO frame to form ZnS and Bi wrapped in rGO. The process is accompanied by the dissolution of bismuth into electrolyte and the formation of (ZnSO4)[Zn(OH)2]3·5H2O (ZHS) on the electrode surface. Inhibition of these two processes may further increase the cycle stability of Bi2S3-rGO. Rotating ring disc electrode (RRDE) measurements, in which we detect dissolved Bi, indicate that Bi dissolution in the electrolyte during charging/discharging is mitigated in Bi2S3/rGO electrode, compared to pristine Bi2S3.