Nowadays, lithium ion batteries (LIBs) play an incomparable role in powering the portable electronic devices, such as laptop/tablet computers, cell phones and digital cameras. However, their potential application in large-scale energy storage systems (e.g. electric vehicles) remains to be a challenge due to the lack of advanced electrode materials, especially cathode materials with higher specific energy/power density. Conversion reaction compounds, such as MFx (M: Fe, Mn, Co, Cu. Ni et al.), possess the advantage of transferring multi-electron per formula unit, thereby exhibiting high theoretical specific capacity. Among them, iron trifluoride (FeF3) is one of the most promising cathode materials, with high capacity of 712 mAh g-1 as well as good thermal stability, attracting ever-increasing interests. However, low electronic conductivity and sluggish kinetics are fatal obstacles to the commercial application of FeF3 as high-capacity cathode material in LIBs. Therefore, the objective of this thesis is to improve the electrochemical performance of FeF3 cathode material by employing different strategies.
Firstly, flower- and cube-like FeF3 micro-particles were fabricated by heat treating the precursors, α- and β-FeF3·3H2O, respectively. Electrochemical performance of FeF3 micro-particles with these two kinds of morphology was estimated by galvanostatic discharge/charge measurement. The electrochemical measurement shows that the flower-like FeF3 powder delivers 170 mAh g-1 of the charge capacity, while the cube-like FeF3 powder exhibits 155 mAh g-1 of the charge capacity, for 20 cycles, at the current density of 20 mA g-1. In contrast, the ball-milled FeF3 particles on the nanometer scale show improved capacity of 270 mAh g-1 for 20 cycles. This comparison sufficiently demonstrates that particle size plays an important role in improving the
capacity. The poor cyclability of both micro- and nano-FeF3 particles could be attributed to the inferior electronic conductivity. In addition, both α- and β-FeF3·3H2O convert to porous α-Fe2O3 with the same shape as its corresponding precursor when they are heated above 500 oC. The as-obtained α-Fe2O3 micro-particles possess hierarchical structure, which are assembled by fine particles with the size of ~150 nm. This unique structure leads to a large increase in the surface area, which is beneficial to the electrochemical reaction with Li ions. Electrochemical measurement demonstrates that the flower- and cube-like α-Fe2O3 hierarchical micro-particles deliver 570 and 675 mAh g-1 at 0.2 C after 100 cycles, respectively, which are both higher than that of 150 nm particles.
Secondly, FeF3 nanocrystals dispersed into a porous carbon matrix (FeF3/C), have been successfully fabricated in a tailored autoclave by a novel vapor-solid (VS) method, which can be generalized to synthesize other metal fluorides. This novel method has several advantages: 1) inhibiting the fast growth of FeF3, 2) being easily scaled up and 3) being extended to fabricate other metal fluorides. Electrochemical measurement demonstrates that the discharge and charge capacities of FeF3/C at the current density of 20.8 mA g-1 still remain 198.9 and 196.3 mAh g-1, with Coulombic efficiency of 98.7%, showing better stability than FeF3, which only delivers 104.0 and 103.7 mAh g-1 after 50 cycles.
Thirdly, graphene-wrapped FeF3 nanocrystals (FeF3/G) were also fabricated by the VS method for the first time. The as-synthesized FeF3/G delivers 155, 113, and 73 mAh g-1 of charge capacity at 104, 502, and 1040 mA g-1 in turn, displaying superior rate capability to the bare FeF3. Moreover, it exhibits stable cyclability over 100 cycles, with the charge capacity of 185.6 and 119.8 mAh g-1 at 20.8 and 208 mA g-1, respectively. The improved electrochemical performance of FeF3/G, together with FeF3/C, could be
ascribed to the buffering effect, high electronic conductivity and large surface area from the graphene or porous carbon. This versatile vapor-solid method and the improved cyclability provide a promising avenue for the application of metal fluorides as cathode materials.
Last but not least, homogeneous LiF/Fe/Graphene nanocomposites as cathode for lithium ion batteries have been synthesized for the first time by a facile two-step strategy, which not only avoids the use of highly corrosive reagents and expensive precursors but also fully takes advantage of the excellent electronic conductivity of graphene. The capacity remains above 150 mAh g-1 after 180 cycles, indicating high reversible capacity and stable cyclability. Both the variation tendency of the capacity and the TEM image of the active material after cycling reveal that the nano-LiF with a size of 100 nm undergoes a pulverization process. The ex situ XRD and HRTEM investigations on the cycled LiF/Fe/G nanocomposites confirm the formation of FeFx and coexistence of LiF and FeFx at the charged state. By modification and optimization, the combination of nano-LiF with ultrafine Fe anchored on graphene sheets could open up a novel avenue for the application of metal fluorides as cathode materials for LIBs.
| Date of Award | 2 Oct 2013 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Chi Yuen CHUNG (Supervisor) |
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