Synthesis and Energy Storage Applications of Iron Sulfides and Reduced Graphene Oxide Composites

硫化鐵/還原石墨烯複合材料的製備及其在能源存儲方面的應用研究

Student thesis: Doctoral Thesis

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Author(s)

  • Hongtao XUE

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date25 Jan 2016

Abstract

Both iron disulfide (FeS2) and iron sulfide (FeS) are well-known as commercial sources of electrode materials for lithium-ion batteries, due to their natural abundance, low cost, high discharge capacity, non-toxicity and environmental benignity. They have excellent performance as positive electrodes for non-rechargeable batteries at ambient temperatures of -40~60°C and rechargeable batteries at high temperature of 375-500°C. However, they have limited reversibility and stability as electrodes for rechargeable lithium batteries at ambient temperature due to the formation of sulfur and polysulfide with poor cyclability. One of the strategies to address this problem is to wrap the electrode materials with a conductive polymer or carbon coating layer. To improve the performance and efficiency of iron sulfides as electrode materials for batteries, it will be necessary to move toward innovative structural design and new material systems.
FeS2 particles wrapped in reduced graphene oxide (rGO) were prepared via a simple solvothermal method. Porous and solid FeS2 particles were prepared by using FeSO4 as iron source, sulfur powder as sulfur source, and triethylene glycol as solvent under solvothermal conditions at 200°C with durations of 4 and 20 hours, respectively. While solid FeS2 particles show diameters of about 2 μm, porous FeS2 particles have smaller diameters of 500–800 nm. When the graphene oxides were added into the synthesis process, both porous and solid FeS2 particles were wrapped in reduced graphene oxide sheets by using the same solvothermal conditions with different reaction durations, respectively.
The electrochemical performance of solid FeS2 microparticles wrapped in rGO sheets as anodes for lithium-ion batteries were evaluated. This composite exhibits significantly enhanced electrochemical cycling stability and rate performance with a capacity of ~970 mAh g-1 after 300 cycles at a current density of 890 mA g-1. Moreover, it retains a capacity of 380 mAh g-1 at a current density of 8900 mA g-1 after 2,000 cycles, indicating its potential as anode material for lithium-ion batteries with long cycle life and high power density. The superior cycling stability and rate performance is due to the excellent conductivity, large effective contact surface area, and superior effective polysulfide preservation of graphene.
The electrochemical performance of porous FeS2 nanoparticles wrapped in rGO sheets as cathodes for lithium-ion batteries was evaluated. This composite exhibits superior cycling stability with a discharge capacity of 435 mAh g-1 at a current density of 1.0 A g-1 after 200 cycles with approximately 80% capacity retention, and rate performance with a discharge capacity of 193 mAh g-1 at a current density of 10 A g-1. It retains a capacity of over 140 mAh g-1 at a current density of 10 A g-1 after 2,000 cycles. Its high performance as cathode can be attributed to the porous nanostructure and synergetic effect between particles and graphene. The porous nanostructure is more easily infiltrated by the electrolyte, enhancing the exchange flux of Li+ across the interface. Graphene can provide continuous conductive paths to reduce the effective electrical and interfacial resistance, also reducing the dissolution of the intermediate polysulfide into the electrolyte which improves its cycle stability.
Porous FeS particles were prepared by using FeCl2 as iron source, sulfur powder as sulfur source, and triethylene glycol as solvent under solvothermal conditions at 200°C with durations of 4 hours. Porous FeS particles show diameters of about 1 μm. The pore sizes are typically in a range of several hundred nanometers. When the graphene oxides were added into the synthesis process, porous FeS particles were wrapped in rGO by using the same solvothermal conditions. The electrochemical performance of porous FeS particles wrapped in rGO as anodes for sodium-ion batteries was evaluated. It exhibits remarkable cycling stability with a capacity of 552 mAh g-1 after 100 cycles at a current density of 100 mA g-1, and rate performance with a capacity of 533 and 307 mAh g-1 at current densities of 1.0 and 5.0 A g-1, respectively. The superior electrochemical properties indicate its potential application as anodes for sodium-ion batteries.