NiCo_2S_4六角片作为钠离子电池负极材料的电化学性能及储钠动力学

通过共沉淀以及后续的气相硫化成功制备了横向边长约为2μm,纵向厚度约为30 nm的NiCo_2S_4六角片,并研究了其作为钠离子电池负极材料的电化学性能。电化学性能测试结果显示在1000 mA·g~(-1)的电流密度下,NiCo_2S_4电极循环60次后仍然可保持约387mAh·g~(-1)的可逆比容量。此外,NiCo_2S_4电极还具有良好的倍率性能,在200、400、800、1000和2000mA·g~(-1)的电流密度下,容量分别为542、398、347、300和217mAh·g~(-1)。通过进一步动力学机制分析发现,NiCo_2S_4电极的良好的倍率性能得益于其二维片层状结构诱导产生的赝电容。上述结果表明,NiCo_2S_4纳米六角片是一种极具潜力的钠离子电池负极材料。

Sodium Ion Storage Performance of NiCo2S4 Hexagonal Nanosheets

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.