Synthesis And Characterizations Of Li-excess Layered Cathode Materials For Li-ion Battereis

Recently, a solid solution system, Li2MnO3-LiMO2, which could be consideredas a Li-excess layered material, has become a new promising lithium-ion batterymaterial due to its extraordinarily high capacity, nontoxicity, low cost and high safety.Although a series of important progresses have been made in terms of synthesis,structure and electrochemical properties, it still needs an intensive study ofmechanism of material aging in electrolyte, the effects of aging on electrochemicalproperties and the relationships between structure transition during charge-dischargeprocess and electrochemical dynamics properties. In this thesis, we made an intensivestudy on the above problems and obtained important results as following:Firstly, Li-excess layered material Li1.18Ni0.15Co0.15Mn0.52O2was synthesized viaa sol-gel method and the optimum synthesis condition was determined. X-rayphotoelectron spectroscopy (XPS) indicated that Ni, Co and Mn are in2+,3+and4+,respectively. The electrochemical properties were studied in the temperature range of-20~60oC. The first discharge capacity was only111.8mAh g-1at the current densityof20mA g-1at-20oC due to the insufficient electrochemical reactions; while thecapacity could reach306.8mAh g-1at60oC. But the cycle stability at hightemperature became worese because of the activation of a great number of Mn4+ionswhich are Jahn-Teller active and easily dissolable in electrolyte.In situ synchrotron X-ray diffraction, electrochemical impedance spectroscopy(EIS), and the galvanostatic intermitten titration technique (GITT) were applied tostudy the relationships between structural changes and electrochemical kinetics of theLi-excess material during the first charge. It turned out that the layer character increases on the removal of Li ions, with a decreasing Li+/Ni2+disorder degree. TheLi+diffusion coefficient (DLi+) decreased to10-16cm2s-1rapidly with the release ofoxygen and changes of surface structure. At the same time, the activation energy ofsurface charge-transfer increased. Thus, it is the poor electrochemical dynamicsproperty that limits the electrochemical properties of Li-excess layered materials.Subsequently, we left the materials ageing in electrolyte to reveal the ageingeffects on crystal structure, surface structure and electrochemical property. The crystalstructure of the material was not changed during the aging process. The surface of thepowders is severely etched in the LiPF6based electrolyte. This causes the dissolutionof some transition metals (especially Ni and Co) and the formation of a spinel-likesurface phase. Also, some by-products resulted from electrolyte decompositiondeposit on the surface of the material. The changes in surface structure and surfacechemistry hinder the electro-chemical kinetics of the electrode. As a result, theelectrochemical properties of Li-excess layered cathode, especially for the oxygenloss process, are severely retarded. This causes gradual activation of the Mn4+ionswhich results in continuous increase of the discharge capacities in the initial severalcycles, leading to the increase of capacity in the first few cycles.Finally, we took advantages of MWCNT to improve the electrochemicalproperties of Li1.18Ni0.15Co0.15Mn0.52O2. It turned out that the conductivity of materialwas improved by the MWCNT, leading to a improved electrochemical property. Thecomposite material with3%MWCNT delivered169.3mAh g-1,242.5mAh g-1,302.8mAh g-1at-20oC, room temperature and60oC, respectively, with excellent rateperformance and cycle stability.