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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Sham, Tsun-Kong

2nd Supervisor

Sun, Xueliang

Co-Supervisor

Abstract

Black phosphorus (BP), as a two-dimensional material, has attracted interest in recent decades due to its unique properties—tunable band gap and high carrier mobility. Specifically, BP shows ultra-high theoretical capacity of 2595 mA h g-1, resulting in high potential practical applications in lithium-ion batteries (LIBs). However, several challenges limit the development of BP in the energy storage field and LIBs: 1) The cost of the current synthesis methods of BP is too high to support extensive research studies and practical applications; 2) The electrical conductivity of BP is insufficient in LIBs; 3) The huge volume change of BP anode materials in LIBs results in rapid fade of the capacity during long cycling; 4) The lack of understanding of the mechanism of the reaction of BP and Li+ in LIBs during cycling hinders the further development of the material. To solve these challenges, this thesis mainly focuses on the design, characterization, and mechanistic understanding of BP-based materials using synchrotron-based techniques along with other techniques. The thesis is arranged as follows.

Firstly, low-cost BP was prepared by the modified chemical vapor transport (CVT) method. The cost of the BP was drastically reduced by replacing the high-cost red phosphorus (RP) with low-cost RP in precursors. The produced low-cost BP exhibits the same purity level, local and electronic structures, as well as the promising hydrogen evolution reaction as high-cost BP.

Secondly, Se-doped BP (SeBP) was prepared by the same CVT method described in the first part. The local structure of SeBP was revealed by the combination of extended X-ray absorption fine structure (EXAFS) and density functional theory (DFT) calculation, indicating the co-existence of substitutional Se and metallic Se in the BP lattice. The bandgap of SeBP declines with the rising Se content, resulting in a significant improvement of electrical conductivity.

In the third part, a nanosized BP-graphite-carbon nanotube (BP/G/CNTs) anode material was prepared by the ball-milling process. The introduction of graphite and CNTs can accommodate the volume expansion of BP and create Li+ transport pathways, respectively. The well-designed BP/G/CNTs delivers high capacity, great rate performance and long cycle stability under high current. In addition, the 3-step reaction mechanism of BP anode material and Li+ during cycling are clearly revealed by the combination of ex-situ XAS, ex-situ X-ray emission spectroscopy (XES), ex-situ X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), operando XAS and operando XRD.

Summary for Lay Audience

Black phosphorus (BP) is a remarkable two-dimensional material in the energy storage field, including hydrogen evolution reaction (HER), lithium-ion batteries (LIBs), etc. However, different challenges hindered the practical applications of BP in energy storage field and LIBs, including the high cost, insufficient electrical conductivity, cracking of BP and the lack of understanding of the mechanism during cycling in LIBs. In this thesis, the four challenges of BP were solved to meet the demand of practical applications.

In Chapter 4, the cost of BP was drastically reduced hundreds of times by using ultracheap red phosphorus as the precursor. The modified synthesis method of low-cost BP shows a conversion ratio competitive with that of the high-cost BP. In addition, the low-cost BP exhibits the same purity level, structures, and promising hydrogen evolution reaction (HER) performance as high-cost BP.

In Chapter 5, the intrinsic electrical conductivity of BP is enhanced by Se doping. The structure of the obtained Se-doped BP was studied by combining the theoretical calculation and the synchrotron-based technique. The results show the co-existence of two phases of Se in the lattice of BP.

In Chapter 6, the cracking of BP anode material was restrained by the specially designed BP-based anode material, BP-graphite-carbon nanotubes (BP/G/CNTs). The well-designed material shows remarkable electrochemical performances in LIBs. In addition, the reaction mechanism of BP and Li+ during cycling was studied by the combination of different synchrotron-based techniques. We also confirmed the importance of operando studies by comparing the results between the ex-situ and operando X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD).

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