Year

2015

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Lithium-ion batteries (LIBs) have been widely used since the early 1990s as the power sources for the small electronic devices that are ubiquitous in our daily lives, due to their high energy densities. With the increasing demand for Li commodity chemicals and geographically-constrained Li mineral reserves, however, LIBs are confronting a huge challenge to satisfy the demands of the ever growing electronics market and the markets for electric vehicles (EVs) or hybrid electric vehicles (HEVs), and stationary energy storage systems. Rechargeable sodium ion batteries (NIBs), therefore, are currently regaining interest for use in large-scale applications because of their huge advantages in terms of low cost and the abundance of sodium resources. My doctoral work focuses on the sodium-storage properties of anode materials, including reduced graphene oxide, tin oxide/reduced graphene oxide composite, molybdenum disulfide/carbon composite, molybdenum disulfide/reduced graphene oxide composite, and iron disulfide/carbon composite. The obtained active materials were characterized for their physical characterization and electrochemical properties. The corresponding sodium-storage mechanisms were studied as well.

Reduced graphene oxide (RGO) was fabricated via the facile Hummers’ method, which achieved by exfoliating the graphite through oxidation by a strong acid. The obtained RGO is a compound of carbon, oxygen, and hydrogen in the form of C-C, C=O and C-OH species. With large d-spacing, high surface area, high conductivity, and high density of cavities and/or holes and/or defects in the graphene nanosheets, its structural properties result in excellent cycling stability for sodium ion batteries. The reaction mechanism between sodium and the graphene layer is capacitive behavior, showing outstanding cycling stability without capacity decay over 1000 cycles. Excellent rate capability is delivered due to its adsorption mechanism, and the reversible capacity is as high as 217.2 mAh g-1at 0.2 C, which slightly decreases to 95.6 mAh g-1 when the applied current rate increase to 5 C.

Tin oxide/ reduced graphene oxide (SnO2/RGO) nanocomposite was prepared via a simple hydrothermal method, and it possesses a lacunose structure with large amounts of buffering space between the small SnO2 nanoparticles and nanovoids amongst the agglomerated SnO2 clusters. This unique nanostructure is able to tolerate the large volume expansion during charge/discharge processes, and is favourable to fast Na+ diffusion. Furthermore, the synergistic effects due to RGO in the composite are capable of increasing the overall conductivity of the electrode. SnO2/RGO thus shows good cycling performance (330 mAh g-1 after 150 cycles) and outstanding rate capability. The mechanism is deduced to be a reversible alloying/dealloying reaction between tin and sodium, while RGO contributes to the capacity via the absorption reaction.

Exfoliated molybdenum disulfide (E-MoS2) was prepared by chemical exfoliation, and the E-MoS2/C composite was further fabricated by the hydrothermal method. EMoS2/ C composite shows a graphene-like structure, with an enlarged interlayer distance of 0.64 nm (d-spacing of 0.61 nm for pristine MoS2) and higher conductivity. When it was applied as anode material in sodium-ion batteries, different electrolytes were used to optimize the electrochemical properties. The composite exhibits a high capacity of 400 mAh g-1 at 100 mA g-1 over 100 cycles in electrolyte consisting of 1.0 M NaClO4 with propylene carbonate / ethylene carbonate and 5 wt. % fluoroethylene carbonate additive (PC/EC + 5 wt. % FEC). Meanwhile, the anode was tested at different cutoff voltages, which correspond to different sodium-storage mechanism, thereby showing various cycling performance. Furthermore, ex-situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were conducted to investigate the conversion reaction mechanism between sodium and MoS2.

Exfoliated molybdenum disulfide / reduced graphene oxide (MoS2/G) composite was synthesised by attachment of exfoliated MoS2 nanosheets onto a graphene nanosheets matrix. The MoS2/G composite has a graphene-like structure with interlayer spacing of 0.69 nm, so that it was first investigated as an anode for sodium-ion batteries. The reaction mechanism between MoS2 and Na was clearly confirmed by ex-situ XRD, and it is supposed to be an intercalation reaction at first discharge process, with a following conversion reaction for the subsequent cycles. The reversible capacity was 313 mAh g-1 at 100 mA g-1 after 200 cycles. Due to the voltage hysteresis in the conversion reaction, the MoS2/G composite can be applied in sodium-ion pseudocapacitors. The NanMoS2/G electrodes from the sodiation of MoS2 were utilized to construct symmetric sodium-ion pseudocapacitors. The full cell exhibits outstanding cycling performance of ~ 50 F g-1 during prolonged 2000 cycles at 600 mA g-1.

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