Synthesis And Electrochemical Properties Of Silicon Based Materials As Anodes For Lithium-Ion Batteries

Developing a new generation of Lithium-ion batteries(LIBs)with substantially enhanced energy density and safety is an urgent demand for a number of new energy technology applications in portable electronics,electrical vehicles and renewable energy storages.As traditional anode material of LIBs,the theoretical specific capacity of graphite is 372 mA h g-1,which seriously constrains the increasement of energy density.Therefore,it is very critical to develop new anode materials with high specific capacity for LIBs.Silicon(Si)has the highest theoretical specific capacity(4200 mA h g-1),low voltage platform,and abundant reserves,making it one of the best candidates of anode materials for high-energy-density LIBs.However,there is a huge volume expansion in the process of lithiation/delithiation,resulting in poor cycling stability,which limits the practical application of Si-based anodes.By reducing the featured size of Si from bulk to nanoscale,the cycling performance can be effectively improved.However,how to reduce the cost of nano-Si production and to develop a commercially viable preparation method still need to be solved.In addition,nanostructured Si has a low initial Coulombic efficiency(ICE)due to its high specific surface area,and its low tap density restricts the realization of high mass loading and area capacity thus affects the volumetric energy density.In view of the above three problems existing in practical applications of Si-based anode materials,the main research contents and results of this thesis are summerized as follows:1.Preparation of Si/C composite with high mass loading of active materials.We report in situ one-pot synthesis of Si/C composite,where Si nanoparticles are wrapped by graphene-like 2D carbon nanosheets.The raw material is green,cheap and easy to obtain.The method is simple and efficient,which is suitable for large-scale production.The 2D carbon nanosheets can not only accommodate tremendous volumetric expansion of Si nanoparticles during cycling,but also preserve the electronic conductivity of the whole electrode.After 500 cycles at 420 mA g-1,the as-prepared composite anode displays a gravimetric capacity of 881 mA h g-1 with about 86.4%capacity being retained.Even the mass loading reaches to 5.00 mg cm-2,the electrode still has areal capacity of 3.13 mAh cm-2 at 2.10 mA cm-2 over 100 cycles.2.Simple and low-cost preparation of Si material with high ICE.Secondary Si materials of submicron size composed of Si nanoparticles(SS-SiNPs)have been successfully synthesized from millimeter Si by a simple,scalable,and economical high energy mechanical milling method.The tap density of SS-SiNPs is 0.81g cm-3.Millimeter Si(1-3 mm)is used as the raw source because of mature production process and low cost.At the current density of 100 mA g-1,the ICE of the as-prepared SS-SiNPs can reach to 87.03%,which is much higher than those of most nanostructured Si anodes reported(65-85%).The first discharge specific capacity is 3370 mA h g-1,which is very close to the theoretical capacity of silicon(3579 mA h g-1 for Li15Si4 at room temperature).In order to improve the cycle performance,SS-SiNPs,graphite and polyacrylonitrile are mixed and carbonized to obtain Si/C composite.At the current density of 100 mA g-1,the ICE of Si/C composite reaches to 80.8%.After 200 cycles of 500 mAg-1,the discharge specific capacity is 785 mAh g-1,and the capacity retention rate is 68%.3.Preparation of a new anode material named Si diphosphide with high tap density.Si diphosphide(SiP2)is prepared by high energy mechanical milling using millimeter Si and millimeter red phosphorus as raw materials.The SiP2 has a high tap density of 1.19 g cm-3.At a current of 100 mA g-1,the first discharge capacity is 1677 mAh g-1.After coating with graphite,the cycle performance is greatly improved,and a high mass loading is achieved.When the mass loading reaches to 2.97 mg cm-2,the electrode has areal capacity of 2.27 mAh cm-2 over 500 cycles with a capacity retention rate of 72.8%.