| This thesis mainly presents our studies on two kinds of spherical carbon materialswith different structures and sizes. The main content includes:(1) the study on thesynthesis and lithium-ion battery performance of a novel hydrogenated carbonnanospheres(HCNSs);(2) the investigations on phase transitions and mechanicalproperties of micron size glassy carbon spheres with amorphous structure under highpressure.The first part of this paper is the synthesis and characterization of hydrogenatedcarbon nanospheres. We report a synthesis of HCNSs via a facile solvothermal routeat low temperatures (60-100oC), using CHCl3as the carbon source and potassium (K)as the reductant. Because of the reducibility of metal potassium, it could selectivelybreak down the relatively lower stable C-Cl bonds of the carbon precursor (CHCl3)and retained C-H bonds, results in the growth of HCNSs. In the carbon precursor(CHCl3), C-Cl bond energy is327kJ/mol but C-H bond energy is411kJ/mol. After aseries of dechlorination and bonding processes in the reaction, large amount ofhydrogenated carbon clusters(HCCs) could be formed, and these HCCs mayaggregate and recrystallize to form the HCNSs. The sketch map for the process ofHCNSs formation is shown in figure1. We studied the structure and electrochemicalperformance of the synthesized HCNSs. Transmission electron microscopy (TEM)showed that the samples have sphere-like morphology and their diameter ranges from40to90nm with a average size of70nm. The HCNSs exhibit an averageBrunauer–Emmett–Teller (BET) surface area of43m2/g, containing a small amount ofmesopores and macropores in the structure. Raman, IR spectroscopy and XPSmeasurements reveal that the sample contains high percentage sp3bonded carbonsand a high degree of hydrogenation of carbons. The HCNSs have a graphite-likeordered carbon structure in spite of their high degree of hydrogenation. Thenanospheres’ sample as an anode material for lithium ion batteries (LIBs) has been studied. It exhibits a high discharge capacity (3539mAh/g in the first cycle,978mAh/g after50cycles) and good cycling stability, demonstrating advantages as apromising candidate for anode materials in LIBs. The high capacity of the HCNSs isdue to their unique nanostructures and high percentage hydrogenation, as well ashydrogenation induced structural defects. Therefore, this hydrogenated carbonnanospheres with unique structure are promising for a high-performance lithium-ionbattery anode material in future.The study of yield strength and phase transition of glassy carbon(GC) microspheresunder high-pressure is the second part of this paper. At room temperature, we useddiamond anvil cells (DAC) studied the yield strength and phase transition of differentsize GC under high pressure by means of in situ high-pressure Raman spectroscopy.For comparison, we used two GC microspheres with different sizes in the experiments,i.e., a big sphere with diameter of30-40um and a small one with diameter below20um.62GPa is the highest pressure reached in our experiments. During the experiment,the large GC sphere began to contact with the upper and lower diamond surfaces atabout25-30GPa, depending on the size of the spheres. Once the contact, besides thecompression from the pressure medium in the chamber, a direct compression from thetwo anvils on the GC spheres occurs. The GC sphere transformed into a transparentcarbon phase above33GPa. This transformation is associated with a change inbonding character of carbon from sp2to sp3hybridization and an increase in hardness.The yield strength of the GC sphere reaches a value of120GPa at a chamber pressureof62GPa, which is comparable to diamond at ambient conditions. The stress inducedby the pressure medium and experimental conditions is important for the observedtransformations of GC under pressure. It is important to note that the phase transformation and hardness increase of GC spheres are closely related to their size.Small GC spheres did not show obvious change in our experiments. Further analysison the results suggests that such difference might be related to different pressureconditions exerted on the glassy carbon spheres with different diameters. |
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