Mechanically flexible V₃S₄@carbon composite fiber as a high-capacity and fast-charging anode for sodium-ion capacitors

Abstract

Hybrid Na-ion capacitors (NICs) have received considerable interests owing to their low-cost, high-safety, and rapidly charging energy-storage characteristics. The NICs are composed of a capacitor-type cathode and a battery-type anode. The major challenge for NICs is to search for suitable electrode materials to overcome the sluggish diffusion of Na⁺ in the anode. Herein, ultrafine vanadium sulfide is encapsulated in carbon fiber (V₃S₄@CNF) as a self-supported electrode by electrospinning and in situ sulfurization. The carbon cladding and one-dimensional (1D) nanofiber network-like structure could alleviate the volume expansion of V₃S₄ during Na⁺ de-/intercalation process. Consequently, the V₃S₄@CNF anode exhibited a pseudocapacitive sodium storage in terms of large Na⁺-storage capacity (476 mAh·g⁻¹ at 0.1 A·g⁻¹), high-rate capability (290 mAh·g⁻¹ at 20.0 A·g⁻¹) and excellent cycling stability (95% capacity retention for 1500 cycles at 2.0 A·g⁻¹) in Na half-cells. By employing V₃S₄@CNF as the anode and the activated carbon (AC) cathode, the as-assembled NICs could deliver a high energy density of 110 Wh·kg⁻¹ at a power density of 200 W·kg⁻¹. Even at a high power of 10,000 W·kg⁻¹, the specific energy is still up to 42 Wh·kg⁻¹.

Graphical abstract

摘要

混合钠离子电容器因其具有成本低、高安全性和快速充放电等特点而, 备受研究者关注。混合钠离子电容器一般由电容型正极和电池型负极组成，其发展最大的难点在于要克服负极侧缓慢的钠离子扩散动力学。在此，我们通过静电纺丝以及后续的碳化、原位硫化，将超细硫化钒颗粒封装在碳纤维的碳基质中，形成一种柔性自支撑电极材料V₃S₄@CNF。这种独特的碳纤维包覆作用能够很好的缓解电极循环中V3S4的体积膨胀，使得V₃S₄@CNF电极具有提升的电化学性能。在钠离子半电池的测试中，V₃S₄@CNF负极展示出高的储Na⁺容量（在0.1 A·g⁻¹电流密度下容量476 mAh·g⁻¹）、高的倍率性能（在20 A·g⁻¹电流密度下容量为290 mAh·g⁻¹）和长的循环稳定性（在2 A·g⁻¹电流密度下稳定循环1500圈）。最后，用V₃S₄@CNF负极和活性炭正极组装的混合钠离子电容器，在200 W·kg⁻¹的功率密度下输出了110 Wh·kg⁻¹的高能量密度，即使功率增加到10000 W·kg⁻¹，也能提供42 Wh·kg⁻¹的能量密度。