Nitridation-boosted V eg occupation of a VN@CNT flexible electrode for high-rate Zn-ion hybrid supercapacitors

Abstract

Flexible zinc-ion hybrid supercapacitors (f-ZHSCs), with their inherent safety, combine the advantages of the high power density of supercapacitors and the high energy density of zinc-ion batteries, making them a promising energy supply device for wearable and implantable devices. However, commonly used rigid cathode materials and fracture-prone metallic current collectors encounter significant challenges, such as inadequate flexibility and compromised cycling stability, which impede the further development of f-ZHSCs. Herein, we design a free-standing flexible membrane electrode VN@CNT for f-ZHSCs through a nitridation strategy. Soft X-ray absorption spectroscopy (s-XAS) reveals the boosted occupation of electrons in the V e_g orbital 3d_x²−y² state after nitridation, leading to enhanced metallicity and conductivity. As a result, the VN@CNT flexible electrode exhibits an excellent specific capacitance of 314.44 F g⁻¹ at 0.5 A g⁻¹. Moreover, it demonstrates exceptional rate capability, retaining 80.17% of its capacitance at a high current density of 10 A g⁻¹ compared to that of 0.5 A g⁻¹. Importantly, it also shows excellent flexibility, enduring bending angles of 0 to 180° and showing no detectable degradation in capacitance after 1200 bending cycles. By design and in-depth study of the local structure of the flexible electrode, this work provides insight into the development of flexible electronics.