As an alternative energy storage technology to the predominantly employed lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) have revitalized increasing scientific attention for large-scale applications, mainly because of the low cost, large resource availability, and similar chemistry of sodium with lithium. However, the main bottleneck to the commercialization of SIBs is the limited choice of anode materials. It is well known that Na ions are about 55% larger in radius compared with Li ions, which significantly hinders the utilization of well-developed electrode materials in LIBs owing to the insufficient interlayer spacing. Therefore, it is difficult to find suitable materials that can accommodate Na ions and allow reversible ion insertion/deinsertion. Up to now, various alternative materials, including carbon, layered metal oxides, alloy-based materials and metal chalcogenides, have been explored as potential anodes for SIBs. Although some encouraging progress have been made, developing appropriate electrode materials with high capacity and good reversibility is less successful and still require further research.

We demonstrate a simple solvothermal method to in situ decorate 2D CoS nanoplates on rGO nanosheets. The resulting CoS@rGO hybrid architecture offers unique characteristics, which is needed for advanced anode. Benefiting from the novel structure and improved electric conductivity, CoS@rGO hybrid composite exhibits high capacity (540 mAh g^-1 at 1 A g^-1), superior rate capability (636 mAh g^-1 at 0.1 A g^-1 and 306 mA h g^-1 at 10 A g^-1), and ultra-long cycle life (420 mAh g^-1 at 1 A g^-1 after 1000 cycles) as anode for SIBs. These results endow CoS@rGO composite as advanced anode for SIBs. Remarkably, a full cell, which is based on CoS@rGO hybrid anode and electrospun Na₃V₂PO₄@carbon (NVP@C) cathode, has been assembled and manifests high capacity and outstanding cycle stability, indicating its huge potential as promising anode for SIBs industry.

CoS@rGO hybrid composites has been prepared by a one-pot solvothermal strategy using Co(CH₃COO)₂ (Co(Ac)₂), thiourea (Tu), and GO as precursors in ethanol medium at 180 °C for 12 h. As shown in the schematic illustration in Figure 1a, in situ directly growth of CoS@rGO composites is induced by the heterogeneous nucleation of CoS on graphene. During the reaction process, GO provides large amounts of defects/functional groups as nucleation sites for in situ growth of CoS nanplates. The presence of carboxyl and hydroxyl groups on the GO sheets can make the thiourea grafted on the GO through surface functional groups, as the amino groups in thiourea are activated by the C=S bonds. Moreover, Co²⁺ could incorporate on the GO sheet by electrostatic force. As a consequence, the absorbed Co²⁺ reacts with the gradual release of S^2- ions deriving from the decomposition of Tu to form CoS nuclei that are tightly anchored onto graphene sheet with well dispersion. Meanwhile, GO sheets are reduced to rGO sheets with ethanol medium as a mild reductant. Thus, the continuous solvothermal reaction resulted in oriented alignment of CoS nanoplates grown on rGO sheets. This unique structure is expected to provide high accessibility to the electrolyte and fast sodium-ion transport pathways.

In summary, the novel CoS@rGO hybrid composites with precisely controlled unique configurations have been successfully developed by an efficient in situ solvothermal technique. The well-defined CoS nanoplates with a thickness of around 10 nm are uniformly grown on rGO frames with strong adhesion, which provides structually stable host for Na-ion intercalation and deintercalation. Suprisingly, as anode for SIBs, an impressive high specific capacity (540 mAh g^-1 at 1 A g^-1), excellent rate capability (636 mAh g^-1 at 0.1 A g^-1 and 306 mA h g^-1 at 10 A g^-1), and extraordinarily cycle stability (420 mAh g^-1 at 1 A g^-1 after 1000 cycles) has been demonstrated by CoS@rGO for sodium storage. The 2D conductive framwork of rGO, ultrathin feature of CoS nanoplates, as well as the unique nanoarchitecture with enhanced electrolyte penetration, are responsible for the outstanding electrochemical performances of the CoS@rGO composite. Such intriguing electrochemical properties of CoS@rGO make it a promising anode materials for advanced SIBs.

Acknowledgements

The authors are thankful for the support by funding from the National Research Foundation, Clean Energy Research Project (Grant Number: NRF2009EWT-CERP001-036) and TUM CREATE center for Electromobility.