Study Of Electrochemical Properties Of Silicon-based Anode Materials For Lithium-ion Batteries

As one of the most promissing anode material for lithium-ion batteries, silicon-based material had received considerable attention for its huge lithium storage capacity, slightly higher discharge platform compared with carbon materials, and abundant reserves. However, its commercial use has been hindered by poor cycling performance and large initial irreversible capacity loss. Firstly, the electrochemical properties of bulk and nano-silicon particles were characterized. And then, the failure mechanism of electrochemical performance and fading mechanism of capacity for silicon material with different structure were explored by studying the relationship of composition, structure and electrochemical properties. On the basis, starting from the direction of the silicon-carbon composite, the Si/Graphite/C composites were prepared using ultra-fine grinding and pyrolysis method. The influences of graphite substrate, proportion of silicon/carbon, and pyrolysis temperature on electrochemical performance of Si/Graphite/C composites were systematically reported. The electrochemical properties of Si/Graphite/C composites were studied. The Si/Graphite/C composites exhibited excellent electrochemical performance compared to nano-silicon material.The electrochemical performance of the bulk silicon particles was characterized. The lithiation/delithiation capacity was2726/1716mAh/g with only62%coulombic efficiency in the1st cycle. The lithiation/delithiation capacity decreased to below100mAh/g after9th cycle. The potential difference between lithiation/delithiation process gradually increased with cycling, from0.26V in the2nd cycle to0.46V in the9th cycle, which indicated that the electrochemical reaction polarization of the silicon electrode gradually increasing with cycling.The SEM and EDS analysis of the bulk silicon electrode were carried out before and after cycling. During the lithiation process, the bulk silicon particles experienced volume expansion, cracking and pulverization. The volume expansion reduced the electrical connection between silicon particles and conductive carbon materials. The thickness of the electrode increased from60μm to90μm before and after cycling with polarization of the electrode gradually increasing. The cracking and pulverization of silicon particles lead to the loss of electrical connection between some small silicon particles and conductive carbon materials, therefore, the lithiation reaction of the silicon material could not completely carried, and the lithiation capacity was low. During the delithiation process, the silicon particles experienced volume shrinkage which also reduced the electrical connection between silicon particles and conductive carbon materials, accordingly the lithium could not completely take off from the silicon materials in the delithiation process. During subsequent cycling, the silicon particles continuous experienced volume expansion, cracking and pulverization which destroyed conductive network of the electrode. The thickness of the electrode increased to150μm leading to the increase of the electrode polarization. The lithiation/delithiation capacity decreased continuously which cycling. Due to smaller polarization, the volume expansion, cracking and pulverization of the silicon particles close to the surface of the electrode were especially serious compared to the silicon particles close to the copper foil. A slit with width of60μm appeared between the surface and the bottom of the electrode after9th cycle. The surface of the electrode completely broke away from the electrode which lesd to the failure of the electrode.The electrochemical performance of nano-silicon particles using CMC/SBR composite binder was characterized. The lithiation/delithiation capacity was4105/3201mAh/g with78%coulombic efficiency in the1st cycle. The delithiation capacity after30th cycling was2221mAh/g. Compared to electrode using PVDF binder, the electrochemical performance using CMC/SBR composite binder was significantly improved.The XRD and SEM analysis of the nano-silicon electrode were carried out before and after cycling. During the lithiation process, the nano-silicon particles experienced only volume expansion, without particle cracking and pulverization. The diameter of the nano-silicon particle increased from70nm to100nm after30th cycling. The volume expansion led to a variation of the electrode structure in thickness and porosity, and the destruction and regeneration of the SEI film on the particle surface. The cycling performance of electrode continuously decreased. Compared to the PVDF binder, the using CMC/SBR composite binder could reduce the whole electrode structure change, facilitate the generation of stable SEI film, and then the electrochemical performance of the electrode was greatly improved.A new type of pyrolytic-carbon coated Si/Graphite/C composites preparation process was developed. The influences of graphite substrate, proportion of silicon carbon, and pyrolysis temperature on electrochemical performance of Si/Graphite/c composite were systematically researched. When using AGP-8carbon matrix,0.5:9.5silicon/carbon ratio,3%amount of pyrolytic carbon, with pyrolysis temperature of650℃. the delithiation capacity of the Si/Graphite/C composites was535mAh/g with81.3%coulombic efficiency in the1st cycle, and the delithiation capacity was482mAh/g with90.1%capacity retention rate after40th cycling. There was a0.45V platform belonging to the silicon material on the Si/AGP8/C composite voltage-capacity curve. The increasing of the Si/AGP8/C composite platform during the late lithiation process was beneficial to the safety problem by avoiding the formation of lithium dendritic crystal.The SEM, XRD and TEM analysis were carried out to characterize the structure of Si/AGP8/C composite. The nano-silicon particles were loaded on the surface of carbon particles with conductive carbon network between particles. The carbon matrix could buffer the destroy of electrode structure due to the volume expansion of silicon particles. The pyrolytic carbon coating furtherly improved the properties of the material. The prepared Si/AGP8/C composites exhibited excellent electrochemical performance.