Synthesis And Electrochemical Properties Of Tin-based High-capacity Anode Materials For Energy Storage

Secondary rechargeable batteries are playing an important role in the clean energy conversion and storage. During the past two decades, lithium-ion batteries（LIBs） have been widely used in the portable devices. Graphite, as the widely-used anode material for LIBs, has disadvantages of low theoretical capacity of 372 m A h g-1, as well as the safety issues related to the formation of lithium dendrites, which could strongly limit its prospect of application in the electrical vehicles and large-scale energy storage. Note that the situation is more severe in sodium-ion batteries（SIBs）, since the radius of Na+（1.02 ?） is nearly 1.34 times larger than that of Li+（0.76 ?）, making it difficult for Na+ to be reversibly inserted and extracted from graphite. Hard carbon could delvier the discharge capacity of about 300 m A h g-1, which is lower than that of graphite in LIBs. Thus, it is higly expected to design and sythesis of new anode materials for both LIBs and SIBs. Element Sn attracts much attention due to the low working potential below 1 V and high theoretical capacity of 944 and 847 m A h g-1 when alloying with lithium and sodium, respectively. Howerver, the unavoidable volume expansion during the alloying process would lead to the capacity fading and pulverization of the electrode. To solve these issues, we design and sythesis several tin-based nanocomposites by taking advantage of the alloying reaction between Sn and Li+/Na+ to acquire high revesible capacity and superior rate performance in our doctorate dissertation. The research results are listed as follows:Firstly, Sn-Fe-C nanocomposite was synthesized vial a sol-gel method followed by chemical vapor deposition process. Electrochemical active Sn, Sn O2, Fe3O4 and inactive Fe-C compounds were inserted into the carbon matrix with unique 2D layered structure, which was induced by the catalytic effect of Fe. The active particles deliver high capacity, while inactive ones and carbon matrix could restrict the volume expansion and the corresponding crack and pulverization of the electrode. Additionally, the carbon matrix could accelerate the transmission of electron, facilitating high rate performance of anode materials. The Sn-Fe-C nanocomposites delivered a high capacity of 800 m A h g-1 at the current density of 0.1 A g-1 with a capacity retention of 96.9% after 50 cycles. Furthermore, the anode exhibited a high discharge capacity of 400 m A h g-1 at the current density of 1 A g-1.Secondly, we extended our research area into SIBs. The nanocomposites consisting of SnSe₂ and three kinds of carbonaceous materials, such as Super P（SP）, carbon nanotube（CNT） and reduced graphene oxide（RGO）, were prepared via a two-step ball milling method. Scanning electron microscope（SEM） and transmission electron microscope（TEM） shows that SnSe₂ and carbon materials were uniformly distributed. But the dimensionality and structure of conducting network made a great impact on its manifested performance. SnSe₂/RGO delivered the highest discharge capacity of 400 and 145 m A h g-1 at the current density of 0.05 and 5 A g-1, respectively, which was better than those values of SnSe₂, SnSe₂/SP and SnSe₂/CNT. The main reason could be attributed to the improved conductivity by RGO as well as buffering the volume expansion and holding the integrity of electrode. Cyclic voltammetry（CV） and ex situ XRD patterns showed that Na15Sn4 and Na2 Se formed when discharged to 0.01 V and SnSe₂ with low crystallinity formed when charged to 3.0 V.Then, we adjusted the proportion of Sn/Se and synthesized Sn Se/RGO nanocomposite in order to acquire high theoretical capacity, which took less reaction time with higher crystallinity. X-ray photoelectron spectroscopy（XPS） showed that Sn Se and RGO were closely connected via a Sn-O-C bond, which make it more effective during the charging/discharging process. The nanocomposite showed better performance within a narrow window of 2⁰.01 V. Sn Se/RGO delivered 495 and 260 m Ah g-1 at the current density of 0.1 and 10 A g-1, respectively. The capacity retention at 1 A g-1 after 120 cycles was achieved as 98%. Electrochemical impedance spectroscopy（EIS） shows that the compositing with RGO improved the conductivity and restrict the increase of impedance during cycling. Ex situ SEM images presented visual evidence of the restrained expansion and the improved integrity of electrode. The crystal phase of Sn Se maintained after 50 cycles according to the ex situ XRD pattern, which well explained the good cyclic stability of Sn Se/RGO electrode.Finally, amorphous Sn₂P₂O₇ is synthesized, followed by mixing with RGO. The amorphous Sn₂P₂O₇/RGO material showed high discharge capacity（460 m A h g-1）, super rate performance（165 m A h g-1 at the current density of 10 Ag-1） and long cycle life（15000 cycles at the current density of 2 A g-1）, which was much better than that of the crystalline Sn₂P₂O₇/RGO. The improved rate performance could be attributed to the decreased particle size and diffusion length of sodium ion by amorphization as well as improved electronic conductivity by compositing with RGO. The nonmetallic matrix would buffer the volume change of alloy, which contributed to a better cycle stability. Additionally, the sodium storage mechanism was studied via ex-situ XRD and FTIR.In this dissertation, to alleviate the effect of volume expansion and pulverization of the electrode during the electrochemical cycles, we synthesized several high-performance anode materials by adjusting the element proportions, compositing manner and carbonaceous species. Moreover, we focused on the studies of their energy storage mechanism, which was constructive to understand the electrochemical behavior of the anode materials with（de）alloying mechanism and served as theoretical and technical guidance for the design and synthesis of new anode materials for LIBs and SIBs.