|Advanced iron-based anode materials for high performance rechargeable batteries
|Chen, Guohua (ME)
|Storage batteries -- Materials
Anodes -- Materials
Hong Kong Polytechnic University -- Dissertations
|Department of Mechanical Engineering
|xiv, 128 pages : color illustrations
|To meet the increasing energy demand for electric vehicles, portable electronics and stationary storage systems, it is imperative to develop high performance and inexpensive electrode materials for rechargeable batteries. Recently, iron-based compounds, as one of typical conversion type anode materials, have been explored as promising anodes for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) because of their high theoretical capacity and low cost. Nevertheless, several issues such as large volume change and low conductivity impede their application. Thus, some strategies need to be developed to improve their performance. In the present study, iron sulfides and iron phosphosulfides, were synthesized as anodes for rechargeable batteries. Their performance was then further optimized by polymer/carbon coating or heterostructure construction.
First, polypyrrole (PPy) coated Fe7S8 nanoparticles on carbon cloth (PPy- Fe7S8@C) was fabricated by a vulcanization treatment and a chemical polymerization process. When utilized as anodes for SIBs, PPy-Fe7S8@C showed a superior rate capability and cycle retention to Fe7S8@CC. At a current density of 1 A g-1, a high specific capacity of 569 mAh g-1 could be achieved between 0.1 and 3.0 V after 100 cycles. At a high current density of 4.25 A g-1, a capacity of 327 mAh g-1 could be retained even after 1000 cycles. Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetric (CV) analyses revealed that PPy coating could decrease the charge-transfer resistance and enhance the pseudocapacitive behavior of anodes for SIBs. Besides, Scanning Electron Microscopic (SEM) images after redox cycling demonstrated the protective effect of PPy shell during cycling, which improved the stability of iron sulfides.
Then, a heterostructure of Fe7S8 and FePS3 (Fe7S8/FePS3) was synthesized via a facile two-step strategy. The product showed a nanoflower-like morphology and the phase boundary between Fe7S8 and FePS3 could be observed. When applied as anode materials for SIBs, they exhibited high capacities and good rate performance. A high initial charge capacity of 737 mAh g-1 could be delivered at a current density of 0.2 A g-1. When the current density increases to 5 A g-1, a reversible capacity of 395.9 mAh g-1 could still be retained. In-situ X-Ray Differaction (XRD) tests revealed the phase transitions of Fe7S8/FePS3 anode for SIBs during the discharge/charge process.
Moreover, CV at different scans and EIS tests illustrated that the good storage performance of Fe7S8/FePS3 could be attributed to the pseudocapacitive behavior and smaller charge-transfer resistance.
To further improve the cycling stability of Fe7S8/FePS3 for SIBs, carbon was coated on its surface (Fe7S8/FePS3@C) through the pyrolysis of polydopamine. Compared to Fe7S8/FePS3, Fe7S8/FePS3@C delivered a better stability and rate performance for SIBs (463 mAh g−1 at 0.5 A g-1 after 200 cycles). Furthermore, when Fe7S8/FePS3@C was employed as anode and Na3V2(PO4)3 as cathode, a full sodium ion cell could be assembled. It could deliver a capacity of 351 mAh g-1 at 0.3 A g-1 in the 40th cycle between 0.4 and 3.8 V. Further kinetic analyses illustrated that carbon coating improved electrochemical properties of the anode by enhancing its electrical conductivity and pseudocapacitive behaviors.
Finally, Fe7S8/FePS3 was utilized as anode materials in LIBs and it exhibited a high initial capacity, good cycle stability and good rate performance. Fe7S8/FePS3 delivered a high capacity of 664 mAh g-1 at 1 A g-1 even after 500 cycles. In-situ XRD and ex-situ X-ray Photoelectron Spectroscopy (XPS) results showed that the Fe7S8/FePS3 was irreversibly transformed into iron sulfides and phosphides along with the breakage of the P-S bonds in the initial cycle. Moreover, it is proved that the good lithium storage performance of Fe7S8/FePS3 is also relevant to the pseudocapacitive behavior and small charge-transfer resistance.
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