Synchrotron X-ray characterizations of black phosphorus: preparation, doping and applications in energy storage
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.