Advanced Cathode And Anode Materials For Lithium Ion Batteries: Controllable Synthesis, Surface Modification, Structure Characterization And Their Electrochemical Performances

Regarding of the promising applications of energy storage and mobilepower systems, lithium ion batteries have received much attention recently.To date, there are lots of bottlenecks on the state of art of methods forsynthesizing electrode materials with high performance, such as expensiveraw materials, complex preparation procedures, long production period andenvironmental pollution, etc. Hence, synthetic processes, which are simple,inexpensive, environmentally friendly, and with high production efficiency,continue to be a hot topic in this field.In this dissertation, microwave heating has been introduced to improvethe traditional hydrothermal and sol-gel synthetic methods. A series ofLiFePO4materials with different morphologies have been controllablysynthesized through microwave-assisted hydrothermal method. An optimumsynthetic process has been finally determined through comparing themorphologies and electrochemical performances of LiFePO4materialsobtained by microwave-assisted hydrothermal method, hydrothermal andsol-gel synthetic methods. A microwave-assisted hydrothermal approach combined with carbothermal reduction, which is simple, environmentallyfriendly, and with high production efficiency, has been developed tosynthesize monodisperse porous LiFePO4/C microspheres. Ferric citrate andlithium dihydrogen phosphate are adapted as starting materials. Theas-received microspheres show meatball-like morphology aggregated by thecarbon-coated LiFePO4nanoparticles of50-150nm, and possess the diameterrange of1.0-1.5μm, high tap density of1.2-1.3g cm-3, and mesoporouscharacteristic with Brunauer-Emmett-Teller (BET) surface area of30.6m2g-1. The Li-ion diffusion coefficient of the LiFePO4/C microspherescalculated from the cyclic voltammetry (CV) curves is~6.25×10-9cm2s-1.The electrochemical performance can achieve about100and90mAh g-1at5C and10C charge/discharge rates, respectively. The capacity retention at10C rate over100cycles is98%and the average coulombic efficiency is99%,implying the possibility of high power applications. As cathode material, theas-prepared LiFePO4/C microspheres show excellent rate capability and cyclestability, promising for high power lithium-ion batteries.The influnces of nonstoichiometric synthesis on composition,morphology and electrochemical performance of the as-prepared LiFePO4materials synthesized by microwave-assisted hydrothermal and sol-gelmethods have been investigated systematically. By introducing a properexcess of lithium content into the starting materials, Li1.08FePO4, which wasdemonstrated to be the optimized composition, exhibited the highest ratecapacity. This improvement was attributed to the absence of impurity phaseor Li vacancy in Li1.08FePO4composite. The results indicate thatnonstoichiometric synthesis can control the components of LiFePO4cathodematerial obtained by microwave-assisted hydrothermal process, henceeffectively improving the electrochemical performance.The effects of carbon and Li3PO4coating on electronic and ionicconductivity of LiFePO4material have been investigated. Proper amount of carbon coating can effectively enhance both of the electronic and ionicconductivities for LiFePO4/C microspheres, hence improving the ratecapability and cycle stability as cathode material. Li3PO4coating on LiFePO4material obtained through magnetic sputtering can improve the ionicconductivity. Proper amount of Li3PO4coating as well as carbon coating canimprove the electrochemical performance.Core/shell structured SnO2/C microspheres and porous structuredSnP2O7/C microspheres have been synthesized through a one-pot rapidmicrowave-assisted hydrothermal treatment, which is a template-free methodwith environmental friendly and cheap starting materials. The growthmechanism of the core/shell structured SnO2/C microspheres during themicrowave hydrothermal process has been studied through analyzing themorphology evolution of the products at different stages of reaction.According to TEM images, there are lots of SnP2O7particles dispersed insidethe porous structured SnP2O7/C microspheres. Due to the stable structure,both of these two materials show excellent cycling performances.The microwave-assisted hydrothermal method for synthesizingLiFePO4/C microspheres and tin/carbon composites possess the advantagesof high production efficiency, low cost, and environmentally friendly, hencemaking it a promising method for mass producing high-performanceelectrode materials of lithium-ion batteries. This method can also be appliedfor synthesizing other nano-materials.