Electrospun carbon nanofibers/nanotubes for Li-based batteries

Abstract

Considerable attention has been paid to rechargeable lithium ion batteries (LIBs) because of their high energy density and long cycle lifetime. However, exploring and developing novel electrode materials with sufficiently high energy and power density to meet the requirements imposed on application of LIBs in high-power devices such as electric vehicles (EV) and hybrid electric vehicles (HEV) remains a challenge. The current commercially available anode materials employed in LIBs are graphites due to their long lifespan, low cost and low electrochemical potential with respect to lithium metal. However, the practical use of carbon in the application of LIBs in EV and HEV has been restricted by its low storage capacity (372 mAh/g), the limited rate performance, the internal short-circuiting associated with the formation of dendritic lithium because of its working potential of around 0 V versus Li+/Li. To address these problems, novel carbonaceous-based anode alternatives need to be developed. This thesis focuses mainly on developing novel electrospun carbonaceous nanomaterials in an effort to formulate carbon-based anode materials with high capacity, excellent rate capability, and long cycle life. The objective also involves establishing the relationship between electrospinning conditions, post-treatments, porous and/or hollow structure, surface area, morphology of the resulting materials and their electrochemical performance. Additionally, nanostructured sulfur cathode in porous hollow hybrid carbon is also investigated. By in situ formation and electrospinning together with thermal and acid treatments, the hollow graphitic carbon nanospheres (HGCNs) in amorphous carbon nanofibers (ACNFs) are fabricated, and the prepared nanomateirals show enhanced conductivity, more extra sites for Li+ storage, and a better ability for withstanding large volume expansion and shrinkage during the Li insertion and extraction procedure. As a result, the ACNFs/HGCNs display a high reversible specific gravimetric capacity of ~750 mAh g⁻¹ and volumetric capacity of ~1.1 Ah cm⁻³ with outstanding rate capability and good cycling stability. In order to increase the density of HGCNs in the resulting materials, a novel triple-coaxial electrospinning method is employed to prepare amorphous carbon nanotubes decorated with HGCNs (ACNHGCNs). As anodes, these ACNHGCNs display a very high reversible specific capacity of ~969 mAh/g at a current density of 50 mA/g, which is nearly 2.6 times the theoretical capacity of graphite (372 mAh/g), high volumetric capacity of ~1.42 Ah/cm³ and good cycling stability. Carbon nanotubes (CNTs) and CNFs represent ingenious high-capacity carbon anode materials for LIBs. By a novel in situ chemical vapor deposition method, activated N-doped hollow CNT/CNF composites are prepared having a superhigh specific Brunauer-Emmett-Teller (BET) surface area of 1840 m2 g⁻¹ and a total pore volume of 1.21 m³ g⁻¹. As an anode, this material has a reversible capacity of ~ 1150 mAh g⁻¹ at 0.1 A g⁻¹ after 70 cycles. At 8 A g⁻¹, a capacity of ~320 mAh g-1 fades less than 20 % after 3500 cycles, which makes it a superior anode material for a LIB. A novel method to control Ni-induced graphitization is exploited by diffusing Ni nanoparticles from the graphitic carbon spheres into N-doped amorphous carbon nanofibers, which turns amorphous carbon into graphitic carbon and produces a hollow-tunnel structure in electrospun carbon/Ni nanofibers. After a combination of KOH activation and acid treatment, activated N-doped hollow-tunneled graphitic carbon nanofibers (ANHTGCNs) are obtained. It can be demonstrated that ANHTGCNs can be excellent anode materials for LIBs, displaying a superhigh reversible specific capacity of ~1560 mAh g⁻¹ and a remarkable volumetric capacity of ~1.8 Ah cm⁻³ at 0.1 A g⁻¹ with outstanding rate capability and good cycling stability. Significant challenges for the commercialization of lithium-sulfur battery include its rapid capacity fading and low power capability. Encapsulating the sulfur in pores of small volume of a porous carbon material alleviates this problem. A carbon-sulfur nanoarchitecture is prepared by encapsulating sulfur in porous hollow CNTs@CNFs. As a cathode, this material with 55 wt.% sulfur shows a high capacity of ~ 1313 mAh g⁻¹ at 0.2 C, and maintains ~ 700 mAh g⁻¹ at 1 C after 100 cycles and 430 mAh g⁻¹ at 5 C after 200 cycles, which makes it a superior cathode material for a rechargeable Li-S battery.