Abstract:

As a potential substitute for commercial lithium ion batteries (LIBs), sodium ion batteries (NIBs) have attracted increasing interest during the last decade. However, compared to the LIBs, the sluggish kinetics of sodium ion diffusion in NIBs due to its larger ionic radius results in deteriorated electrochemical performances, which hinders the future development and application of NIBs. Therefore, exploring anode materials that exhibit a novel kinetic mechanism is desired. Recently, extremely rapid kinetics has been realized by introducing the pseudocapacitance effect into battery systems; this effect generally refers to faradaic charge-transfer reactions, including surface or near-surface redox reactions, and fast bulk ion intercalation. To obtain a pseudocapacitance effect in battery systems, the critical step involves the rational design of a two-dimensional structure with a high conductivity. In this regard, the bimetallic sulfide thiospinel NiCo₂S₄ stands out by virtue of its high conductivity (1.25 × 10⁶ S·m^-1) at room temperature, which is at least two orders of magnitude higher than that of the oxide counterpart (NiCo₂O₄). Herein, NiCo₂S₄ hexagonal nanosheets with a large lateral dimension of ~2 μm and thickness ~30 nm have been successfully synthesized through coprecipitation followed by a vapor sulfidation method. As the anode material in NIBs, the NiCo₂S₄ nanosheets deliver a reversible capacity of 387 mAh·g^-1 after 60 cycles at a current density of 1000 mA·g^-1. Additionally, the NiCo₂S₄ nanosheets exhibit high reversible capacities of 542, 398, 347, 300, and 217 mAh·g^-1 at the current densities 200, 400, 800, 1000, and 2000 mA·g^-1, respectively. Ex situ X-ray diffraction analysis has been employed to reveal that the sodium ion storage process is a result of a combined Na⁺ intercalation and conversion reaction between Na⁺ and NiCo₂S₄. Further quantitative analysis of the kinetics has verified the extrinsic pseudocapacitance mechanism of the Na⁺ storage process, in which the capacitive contribution enlarges as the current density increases. The observed capacitive contribution of NiCo₂S₄ electrode is as high as 71% at a scan rate of 0.4 mV·s^-1. This is closely attributed to the modified thin-sheet structure of NiCo₂S₄ and hybridization with graphene that account for the superior high-rate performance with long-term cyclability. These intriguing results shed light on a new strategy for the structural design of electrode materials for advanced NIBs. Moreover, this vapor transformation route can be extended to the preparation of other transition metal disulfides with high electrochemical activities, such as FeCo₂S₄, ZnCo₂S₄, CuCo₂S₄, etc.

Key words: Sodium ion battery, NiCo₂S₄ nanosheets, Anode material, Sodium storage capalibity, Pseudocapacitance