Layer Structured Bismuth Sulfide Composites As Negative Electrode Materials For Sodium-ion Batteries

At present, the sodium ion batteries have drawn increased attention. Transition metal oxides and sulfides as a result of their superior electrochemical properties have been studied as electrode materials in lithium ion batteries and supercapacitors, however, researches on metal oxides and sulfides electrode materals for sodium-ion batteries are still lacking. Bismuth sulfide（Bi₂S₃） is an important semiconductor material. Owing to its unique layered structure, Bi₂S₃ is also studied as an electrode material for lithium-ion batteries, however in the field of sodium-ion batteries, research interest on Bi₂S₃ is still scarse. In this thesis, research work is mainly focused on the synthesis of various types of Bi₂S₃, Bi₂S₃/C and Bi₂S₃@CNT composites, their physical characteristics are analyzed, and electrochemical performances are investigated. The results are briefed as followings:1. Using BiCl₃ as a source of bismuth, and thioacetamide（TAA） as a source of sulfur, nano-sized, disk shaped Bi₂S₃ material are synthesized by hydrothermal method. To improve the stability and conductivity of the material, attempts on synthesis of carbon coated Bi₂S₃（Bi₂S₃/C） and testing of its electrochemical performance for sodium storage are carried out. Compared with pure Bi₂S₃, the carbon coated Bi₂S₃ achieved slightly better performance with increased capacity. The initial discharge capacity of Bi₂S₃/C is 1015 mAh g-1, and the second and third is 845 and 610 mAh g-1, respectively. Bi₂S₃/C shows better cycle stability compared to bare Bi₂S₃, under 0.1C rate, Bi₂S₃/C delivers about 100 mAh g-1 after 50 cycles, while that of bare Bi₂S₃ is only 50 mAh g-1; At a charge/discharge rate of 1C, Bi₂S₃/C material displays a 35 m Ah g-1 capacity after 100 cycles.2. In order to further improve the electrochemical performance of Bi₂S₃, we have designed a Bi₂S₃@CNT nanocomposite as a sodium ion（Na-ion） battery anode material. Through an ultrasonic hydrolysis method, nanoparticle Bi₂S₃@CNT are synthesized. The layer structured Bi₂S₃ provides host sites for insertion of Na-ion and the carbon nanotube（CNT） in the Bi₂S₃@CNT nanocomposite serves as a highly conducting network to promote electron transport. Electrochemical testing results show that the Bi₂S₃@CNT nanocomposite exhibits a high and stable capacity in the 0.01–3 V（vs. Na/Na+） voltage region, notably outperforming the bare Bi₂S₃ material. The Bi₂S₃@CNT nanocomposite as Na-ion anode material is able to discharge 84.4 mAh g-1 capacity at 60 mA g-1 over 60 full cycles.3. For comparison purpose, molybdenum oxides due to their good reported Na-ion staorge properties are studied. In the second part of this thesis, MoO2/C composite materials are explored. Nanosheets of MoO2 coated with amorphous carbon are synthesized by a two step hydrothermal processes followed by thermal reduction. The resulting MoO2/C composite materials exhibit excellent properties for Na-ion storage, retaining a high specific capacity of about 341.1 mAh g-1 over 25 cycles at a 0.05 C charge/discharge rate. At higher rates of 1C, 2C, and 5C, the materials still show high capacities and excellent cycling performance over 115 test cycles. The cyclic voltammetry measurements of the materials at scan rate of 5 mV s-1 shows capacitive performance. The electrochemical results suggest that the MoO2/C composite materials can be potentially used as anode materials for high power Na-ion batteries or high energy density Na-ion capacitor.