Silicon Based Anode Materials Prepared By RF Plasma And Their Electrochemical Performance For Li-ion Batteries

Li-ion batteries (LIBs) have many advantages such as high energy density, long cycle life, safe and reliable. They have been widely applied in the fields of portable electronic devices, gradually developing to electrocars or energy storage. High energy density LIBs become one of the important development directions. Traditional LIBs anode material of graphite (theoretical specific capacity,372 mA h g-1), is difficult to meet the ever-growing demands for high energy and power density. For anode materials, silicon stands out as the most promising material for the next generation LIBs due to its known highest theoretical capacity (4200 mA h g-1) and appropriate voltage platform. However, silicon as anode material is severely hindered by low electric conductivity and the huge volume changes during lithium insertion/extraction process, which cause the silicon particles dramatically pulverized and eventually lead to the capacity rapidly fading. These disadvantages severely limit the application. To solve the above problems, highly dispersed nano silicons were prepared by plasma and their electrochemical performance for lithium-ion batteries were researched in this dissertation.The main innovative results are listed as follows:(1) Morphology controlled 0D silicon nanospheres (OD-SiNSs) were synthesized by RF plasma with a size distribution of 50-100 nm. They had good dispersion and could be well dispersed in solution and resin. By analysis the influence of reaction parameters on crystal morphology and particle size through thermodynamics and growth kinetics, the Si crystals growth in plasma were thought to the VS nuclear mechanism and including three stages:supersaturated, nucleation and growth. In the growth stage, high cooling rate was helpful to gain well dispersed, smooth, compact SiNSs.(2) Silicon nano-spheres synthesized by RF plasma were used as anode for LIBs and the electrochemical performance was studied in detail. The SiNSs kept good dispersion and stability as electrode, and exhibited no break at fully discharged with a volume change of 270%. Good dispersion and stability made its electrochemical performance improved a lot with high specific capacities and cycle stability. For SiNS electrode, the initial specific capacities was 2388 mA h g-1, the initial coulombic efficiency (ICE) was 71%. After 5 cycles, the CE (coulombic efficiency) stabilized at about 99%. Importantly, after 50 cycles, the capacity of SiNS electrode was still 500 mA h g-1, which is greatly superior than that of bulk silicon.(3) To further improve the electrochemical properties of SiNSs synthesized by RF plasma, porous carbon (PC) was introduced and SiNS/PC composites were synthesized, with Si nanospheres dispersing uniformly in carbon matrix. PC wrapped in the surface of Si and formed a closed core-shell structure. The thickness of carbon layer is～10 nm. Compared with SiNSs, the obtained composite materials had two main advantages: abundant pore structure supplied room to absorb the volume expansion and relieve the volume effect; Core-shell morphology could avoid the immediate contact of SiNS and electrolyte. During cycles, the particles expanded from 43 nm to 52 run, with a volume change of only 174%. Therefore, its electrochemical performance exhibited remarkable improvement with an initial specific capacities of 2510 mA h g-1. Importantly, the SiNS/PC electrode still exhibited high capacity of 778 mA h g-1 after 100 cycles, corresponding to 2.1 times that of graphite anode. Also, SiNS/PC composites exhibited super rate capacity and could remain stable at high current density. Even at 4200 mA g-1, the capacity reached to 445 mA h g-1, corresponding to 1.2 times that of graphite anode.(4) 3D porous and spherical Si/C micro-/nano-spheres were further designed and fabricated by spray drying procedure to improve the structure stability. The SiNS/C composite exhibits spherical shape with a diameter of 3 μm.～50 nm Si/C microsphere constituted the skeleton structure, and the carbon matrix acted as binder or conductor, enhanced the mechanical stability and made a 3D conductive network that improves the overall conductivity. During cycles,-50 nm Si/C sphere only experienced a small volume variation (-68%) and the secondary particles were almost no obvious change. The obtained :omposite materials exhibited ultra-high ICE of 88%, improved capacity of 2059 mA h g-1 The overall cycle curve was steady and the capacity kept at 1500 mA h g-1 after 50 cycles, and the retention rate was 73%. The cycle performance of the silicon anode materials were further improved.(5) Compared with SiNSs, silicon nanowires (SiNWs) had the advantage of reduced volume change in axial and improved Li-ion transportation in radial direction. ID SiNWs were fabricated by strengthening the temperature holding time of plasma, with a diameter of 30-50 nm and length of few hundred nanometers to microns. On this basis,3D porous wool-ball like SiSW/C spheres were designed. During cycles, the volume changes of Si were obsorbed by the pores between SiNWs, and the microstructure could keep stable after long cycles (300 cycles). As anode for LIBs, the specific capacity was 1206 mA h g-1, with an ICE of 94%, and the capacity kept at 620 mA h g-1 after 300 cycles. The cycling curves tended towards stability and the capacity retention is about 87% from 100th cycle to 300th cycle.