Sulfidation of Electrospun Vanadium Oxide Fiber Mats for Lithium-Ion Battery Electrodes

Abstract

Energy storage plays a fundamental part in fulfilling the necessity of a more efficient, renewable, and environmentally friendly energy supply chain. Lithium-ion batteries are currently the most widely portable storage devices and typically employ transition metal oxides or dichalcogenides as electrode materials that promote redox reactions with lithium ions by either insertion or conversion mechanisms. For the latter, metal sulfides are prominent electrode materials owing to their high electrical conductivity and redox reversibility and high specific capacity. Vanadium sulfides exist in a range of crystal structures that can promote insertion of different ions in both anode and cathode side. Besides, they can also take part in conversion mechanisms in lithium-sulfur batteries. However, slow diffusion of lithium ions through the structure and high volumetric expansion compromises the average performance and stability of these materials. One strategy commonly adopted is the synthesis of hybrid nanostructured particles that combine a large surface area with short ion and electron transport pathways. The presence of conductive carbon in the hybrid material constrains the material expansion and ensures a conductive network during the conversion mechanism. Compared to other synthetic approaches, electrospinning enables the production of fibers constituted of nanosized structured materials. The continuous network of fibers forms free-standing mats that can be directly used as electrodes with no post-processing, coating or binder additives. Such mats allow continuous electron transport across the material while the synthesis itself promotes the homogeneous distribution of the components along the fibers, preventing particle agglomeration. In this work, vanadium oxide (VOx) mats are produced by electrospinning and converted to vanadium sulfide by thermal sulfidation process. The VOx fibers were prepared by the previously optimized electrospinning process.[1] The fiber mats were thermally treated under H2S/Ar atmosphere in different conditions to convert VOx into VSx. Lower synthesis temperatures promote a high degree of conversion while keeping smaller VSx domain size,and a higher interfacial contact between the carbon and the VSx phase. The VSx/C hybrid mats were directly applied as Li-ion battery electrodes and tested in different potential ranges to evaluate conversion and insertion mechanisms. The electrodes were paired with Li-ion discs in standard LP30 electrolyte using custom-built cell using spring-loaded titanium pistons.[2] The different VSx fibers were tested regarding redox profile, rate handling, and cycle stability. For the fibers with smaller VSx domain size, capacities up to 900 mAh/g were achieved when evaluating the whole active potential window, yet rate handling was compromised. When restricted to intercalation side capacities ranging of 150-280 mAh/g were observed, with good rate handling up to 2 A/g and full recovery of the initial capacity.