Composite of Vn/Carbon Nanofibers As a Protective Layer on Cathode for High Performance Li-S Batteries

Abstract

With increasing demand for large scale energy storage systems (ESS), Li-S battery is regarded as a potential candidate for ESS, due to its high theoretical energy density and natural abundance of the cathode material. However, for conventional Li-S batteries, sulfur cathode has suffered from many issues such as low intrinsic electrical conductivity and instability leading to the insufficient utilization of sulfur. In particular, the dissolution of polysulfide intermediates into electrolyte results in the rapid capacity fading of the batteries. Therefore, for the development of the highly efficient Li-S battery, it is essential to improve the conductivity of the sulfur based cathode and maintain/reuse the soluble polysulfide within the cathode structure. To address these issues, some researchers used protective layer composed of carbon, metal oxides or polymers on the sulfur cathode. According to previous studies, carbon based materials as a protective layer on cathode reduced the charge transfer resistance of the sulfur cathodes by acting as an upper conducting network and localized the soluble polysulfides during cycling. This strategy is regarded as a simple and effective solution to make high capacity rechargeable Li-S batteries. In addition, other group demonstrated that Pt as an electrocatalyst could help to convert polysulfide deposits back into soluble long-chain polysulfide, and hence enhanced reaction kinetics and retained high Coulombic efficiency. Catalytic role of vanadium nitride, VN resembles that of more precious Pt. VN has desirable advantages for a sulfur cathode: (1) strong chemical adsorption of polysulfides on it, leading to suppressing the shuttle effect, (2) its high electrical conductivity, and (3) good catalytic activity for enhancing red-ox reaction kinetics. In the present work, we fabricated a composite of VN/carbon nanofibers (VN/CNFs), as a protective sulfur electrode on cathode for Li-S cell, using electrospinning method. The morphology and chemical composition of electrospun VN/CNFs were characterized by SEM, TEM, and XPS, respectively. The Li-S cell with VN/CNFs protective layer exhibited excellent electrochemical performance in terms of specific capacity, cyclability, and rate capability. Owing to the strong and synergic polysulfides anchoring effect of the VN/CNFs, a high initial discharge capacity of 1,480 mAh g-1 and a Coulombic efficiency of ~98% were measured at a rate of C/2, when used in Li-S cell. Further, the Li-S cell exhibited a long-term cyclability: 82% capacity retention even after 200 cycles. Figure 1