Preparation And Electrochemical Properties Of Silicon Suboxide Based Anode Materials For Lithium Ion Batteries

Graphite is the commonly used anode material for commercial lithium ion batteries (LIBs). The low specific reversible capacity (LiC6:372mAhg-1) of graphite cannot meet the increasing demands for the next generation LIBs with high performance, Thereby, it has become extremely urgent to develop advanced anode materials with higher specific capacity to replace the current graphite-based anodes.In this thesis, silicon suboxide based anode materials with improved electrochemical properties were prepared through the approaches of nanoengineering, surface coating and element composition tuning. Sol-gel. colloidal and combustion routes were employed to synthesize kinds of SiOx based composites with core-shell, micro/nano and glass-like structures. The microstructure of SiOx was investigated by FT-IR, HRTEM, STEM, NMR and XPS. The effect of microstructure of SiOx on the electrochemical properties was investigated and the enhancement mechanism of electrochemical properties was discussed. The obtained SiOx based composites show superior electrochemical properties.The detailed research contents are as follows:(1) Amorphous SiO2/C composite. Amorphous SiO2/C composite was synthesized via a carbon coating of silica aerogel precursor. The effects of mechanical milling on the microstructure and electrochemical properties of SiO2/C composite were investigated. The results reveal that the crystallinity of SiO2has an important influence on its electrochemical activity. Crystallized SiO2has poor electrochemical activity toward lithium ion storage due to the strong Si-O bond and the amorphization of SiO2is conducive to good electrochemical activity. Mechanical milling is employed to cause SiO2amorphization to possess good electrochemical activity. The reversible capacity of the amorphous SiO2/C electrode retains600mAhg-1at0.1Ag-1for100cycles and480mAhg-1at0.5Ag-1. The coating carbon layer can not only accommodate the volume expansion occurred during discharge/charge process, but also increase the electronic conductivity. The preparation route is facile and cost effective.(2) Core-shell SiOx/C composite. Active SiO2colloidal particles were prepared via a modified colloidal route. Carthermal reduction occurs during heat-treatment process at a relatively low temperature between pyrolyzed carbon and SiO2colloidal particles, reducing the atomic ratio of O/Si. The obtained SiOx/C composite with the atomic ratio of O/Si of ca.0.88exhibits good electrochemical activity. The excess carbon component was coated on the surface of SiOx nanoparticles, forming a core-shell nanostructure. The microstructure results show that the core-shell SiOx/C composite presents a well-distributed nanostructure composing of SiOx nanoparticles coated with a thinner carbon layer. The particle size is ca.100nm and the thickness of carbon layer is3-6nm. The electrode delivers a stable reversible capacity of ca.820mAh-1over100cycles, and exhibits excellent rate capability. The XPS result shows that there are different chemical states of Si in SiOx. The existence of Si0, that is Si in the composite, is conducive to high lithium storage. The content of Si is ca.2.04%. The nano-scale feature, low O/Si ratio of SiOx and the coating carbon layer ensure the excellent electrochemical performance of SiOx/C. The stable electrochemical impedance of different cycles and the homogeneous cycled electrode morphology indicate the good structural stability of SiOx/C electrode. The volumetric productivity of colloidal route is much higher than that of conventional wet chemistry route, which makes this preparation route be adopted for tunable large-scale production of high-capacity SiOx/C composite anode.(3) Micro/nano structural SiOx/C composite. Micro/nanostructural SiOx/C composite was synthesized via a citric-nitrate auto-combustion method. The effects of heat treatment atmosphere on the elemental composition, microstructure and electrochemical properties were investigated. The gas generated during combustion process makes the composite form a loose structure, which can provide void space to accommodate the volume expansion during discharge/charge process, thus preserving the structural integrity of electrode materials. The obtained SiOx/C composite heat treated under Ar atmosphere shows hierarchical structure. The micro-sized irregular particle is composed of a large number of primary particles with diameter of10nm. Carbon coating and post heat-treatment can further reduce the O/Si ratio, which is favorable for enhancing the electrochemical activity. The SiOx/C composite exhibits excellent electrochemical properties, which delivers a specific capacity of ca.700mAhg-1even after200cycles.(4) Glass-like SiOx/C composite. Glass-like SiOx/C composite was synthesized via a sol-gel route. The choice of silicon source and the tuning of heat-treatment temperature can avoid the generation of Si-C bond in the composite. The heat-treatment temperature has strong impact on the structure and electrochemical property of the synthesized composite. The O/Si ratio decreased with increasing heat-treatment temperature. SiC component is generated at high heat treatment temperature, which reduces the specific capacity of the composite. The two phases, SiOx and C, mix each other at nanoscale, forming a homogeneous interdispersion structure. The NMR result shows that different types of atomic building units exist in the SiOx phase. The average coordinated oxygen number of Si decreases with increasing heat-treatment temperature. The carbon phase can build a fast electron transport highway, promoting the uniform distribution of stress during the electrode reaction process. The micro-sized composite particles exhibit a dense feature, leading to a high tap density, which is desired for building high volumetric energy density batteries. The glass-like SiOx/C composite exhibits excellent electrochemical properties, which delivers a specific capacity of ca.810mAhg’1even after100cycles. Down-sizing of particles can alleviate the capacity-climbing phenomenon at initial cycles and improve the rate-capability of SiOx/C electrode. The ball-milled sample delivers a reversible capacity of674mAhg-1at0.8Ag-1, which is much higher than that of the original one, increased by18.5%.