Preparation And Electrochemical Performance Investigations Of Nano-Structured Zn₂SnO₄ Materials

The growing requirement for high-power and high-capacity lithium ion batteries (LIBs) in the emerging technologies (e.g., electric vehicles and hybrid electric vehicles) has prompted tremendous research efforts towards developing high-performance electrode materials.Traditional commercial graphite electrodes could not meet the demand of consumers because of its low theoretical capacity (372 mAh/g-1) and poor high-rate performance. On one hand, efforts are needed to improve the efficiency of the use of traditional energy sources, on the other hand, we also need to actively develop new energy technologies and application of green environmental protection. Therefore, developing alternatives with excellent performance is greatly important for the improvement of LIBs.Zn2SnO4 has attracted much attention due to its high theoretical capacity and high electrical conductivity. However, huge volume change during the Li+alloying and dealloying processes for Zn2SnO4 will result in their pulverization and exfoliation from the current collector, which leads to fast capacity fading, thus limitating their commercial application. In this thesis, we aim at utilizing simple methods to design nanostructured electrode materials with high performance for the LIBs, in which the preparation of nanostructured Zn2SnO4 is the main point. We respectively synthesized hierarchical Zn2SnO4 hollow nanospheres and Zn2SnO4/Mn3O4 composite material, and their electrochemical performances were studied. In addition, ZnSn(OH)6 used as anode materials were firstly studied for the electrochemical properties as lithium ion batteries. The main results are listed as follows:(1) Hierarchical ZnSn(OH)6 hollow nanospheres were synthesized by a simple hydrothermal method using ammonia, PAA, Zn(CH3COO)2·2H2O and Na2SnO3·4H2O as the raw materials, in which ammonia has been utilized to produce hydroxyl anions. we studied the morphology of the product under different amounts of PAA in detail; The morphological evolution process of this organized product has been investigated by examining different reaction intermediates such as hydrothermal time during the synthesis. After calcination in air atmosphere, we obtained hierarchical Zn2SnO4 hollow nanospheres with the morphology of maintained. It is demonstrated that tuning of the structure and the pore size of the products is very significant in electrochemical capacitor. When the amount of PAA is 0.75g, the products have the best electrochemical properties. When the current density was increased to 1A g-1, the electrode can still retain a discharge capacity of 442.8 mAh g-1 for the 60th cycle. In addition, the electrochemical properties of the as-obtained hierarchical ZnSn(OH)6 hollow nanospheres were firstly studied, which exhibit a reversible capacity of 741.9 mAh g-1 after 1000 cycles.(2) 1D Mn3O4/Zn2SnO4 hierarchical composites have been synthesized for the first time through a convenient one-step hydrothermal method, which consisted of Zn2SnO4 nanoneedles attached on Mn3O4 nanorods. Through comparing and analyzing the crystal structures of MnO2 and Mn3O4/Zn2SnO4, it is evidenced that the formation of products is driven by the lattice match growth between cubic Mn3O4 and Zn2SnO4. Li-ion battery testing is given to demonstrate that 1D Mn3O4/Zn2SnO4 hierarchical composites show excellent capacity retention, superior cycling performance and high rate capability. Specifically, the obvious improvement of the electrochemical performances might be attributed to the following four reasons:(i) its unique 1D structure facilitates the electrons transport during the processes of charging and discharging; (ii) the Zn2SnO4 nanoneedles are strongly anchored on the Mn3O4 nanorods to form an unique branched structure with good stability and integrity; (iii) the large degree of porosity of the composites enhances the electrolyte and Mn3O4/Zn2SnO4 electrode contact area and the open space between neighboring Zn2SnO4 nanoneedles, which allows for easy diffusion of the electrolyte. Morever, on the basis of the lattice matching theory proposed above, we also synthesized Mn3O4/ZnFe2O4, Mn2O3/CoFe2O4 and Mn2O3/NiFe2O4 composite structures, and their electrochemical performances were also studied.