Structure Of Sn/Si-based Compositions And Their Electrochemical Performance As Li-ion Anodes

It is of great significance to explore lithium ion batteries(LIBs) with higher capacity, higher energy density, and better thermal stability, which have been primary power sources for protable electronic devices, and also widely used in hybrid electric vehicles(HEVs) and electric vehicles(EVs). Alloying anodes, such as tin(Sn) and silicon(Si) are promising negative electrodes for LIBs, because of their high theoretical capacity and low average potential. However, these alloying anode materials suffer large volume expansion during lithiation, which can leading to rapid capacity fade. In order to enhance electrochemical performance for Sn/Si-based alloy anodes, we have synthesized Sn-C composites by a novel ball milling method, studied their structural stability during lithiathion combinated experimental and simulated results, and also explored a new Si-B anodes for LIBs in this thesis.Sn-C composites have been prepared by an efficient synthesis method, namely dielectric barrier discharge plasma assisted milling(P-milling). Since the melting point of Sn(231.9°C) is low, Sn particles can be refined efficiently by the synergy effects of plasma heating and impact stress in P-milling. Also, the very rapid heating can cause Sn explosion to release the thermal stress and expansion, and the crystalline nature of graphite could be maintaind, which results in nanosized Sn homogenously dispersed in a graphite matrix to form a micor-nano particle. Therefore, P-milled Sn-C has a lower specific surface area than Sn-C prepared by conventional ball milling(C-milling). During the first lithiation process, the electrolyte decomposition on the electrode surface would comsume less Li+ to form solid-electrolyte-film. Meanwhile, the absolute volume expansion of nanosized Sn is small and graphite can buffer it efficiently. As a consequence, P-milled Sn-C composite anodes deliver much higher reversible capacity and better cyclability than that prepared by C-milling.In order to further improve the cyclability of Sn-C composites, an advanced nanocomposite of tin oxide-coated tin in graphite(Sn@Sn O_x/C) has been synthesized with a one-step of dielectric barrier discharge oxygen plasma-assisted milling(O2-P-milling). The resulting composite possesses a unique microstructure, where Sn nanoparticles coated by an ultrathin amorphous/ nanocrystalline Sn O_x layer are homogeneously embedded within a graphite matrix. During the first discharge, Li2 O generated from the reaction between Sn O_x and Li+, can accommodates the volume expansion of Sn. Therefore, the Sn@Sn O_x/C nanocomposites display superior electrochemical performance to P-milled Sn-C composites. The Sn@Sn O_x/C50 nanocomposite exhibits a high capacity retention of 395.7 m A h/g after 100 cycles.The plastic strain, the total displacement and the volume expansion distribution of the SnC electrode after full lithiation of Sn have been simulated using the finite element analysis method. Based on the experimental results, the fading mechanism of Sn in the Sn-C anodes has been discussed, and the formation and growth of Sn whisker have also been studied. Lastly, Sn particle size and matrix dimension has been optimized for Sn-C composites. The simulation result shows the largest displacement happens on the surface between Sn and the matrix in Sn-C compositions after Sn particle is fully lithiated. As Sn particle decreases, the total displacement would be smaller and matrix suffers less damage. Moreover, the stiff Li2 O layer can buffer volume expansion of Sn particles. Therefore, double-coating Sn@Sn O_x/C composites perform more stable microstructure, and higher capacity retention of Sn compared with single-coating Sn-C composite.A new Si-based system, Si1-x Bx thin films with 0.071 ≤ x ≤ 0.950, has been explored using combinational sputtering. All Si1-x Bx film compositions have an amorphous structure consisting of amorphous Si and B phases, as characterized by X-ray diffraction. In Li cells, it is found that all of the Si is active and the B is inactive with lithium. Nevertheless, the Si1-x Bx films have similar characteristics as pure Si film electrodes in Li cells, except with reduced capacity due to the added B. Voltage shift could be observed in Si1-x Bx films when the B content increases, which may be induced by stress-voltage coupling between active Si and inactive B phases. As x increasing in Si1- x Bx, average lithiation and delithiation voltages show a negative/positive shift rate of-0.040 and 0.141, respectively. Nevertheless, the voltage shift rate is small compared with other Si-TM(transition metal), which would induce shorter capacity truncation of Si. Therefore,the reversible capacity of Si1- x Bx can be released 1170 m Ah/g when x = 0.8.Amorphous Si1-x Bx composites have also been prepared by ball milling. B addition is benefit for the amorphization of Si phase. The Si1-x Bx electrodes coating are made containing graphite and then calendered, which can increased energy density comapred to a pure alloy coating. The Si0.40B0.60 composite electrode shows the best electrochemical performance, with an initial coulombic efficiency of 80.9%, and volumetric specific capacity of 1702 Ah/L. After 50 cycles at 0.2 C rate, Si0.40B0.60 electrode exhibits a high capacity retention rate of 96.9% to the first reversible capacity. Based on the cell stack energy density model, the full cell volumetric energy of the Si0.40B0.60 composite electrode is predicted 30% higher than that of gain compared with the graphite anode. Therefore, Si0.40B0.60 is expected to be an ideal choice for new generation anode for LIBs.