Preparation And Coating Modification Of High-capacity Li-rich Layered Cathode Materials For Lithium Ion Batteries

The developments of electric vehicles, smart grid and large-scale energy storagehave put forward high demands on lithium ion batteries (LIBs) with high energydensity and power density. At present, developing new cathode materials with highvoltage and capacity has become the critical factor for further increase in energydensity of LIBs. Li-rich layered cathode materials have been recognized as an idealcandidate of cathode material for the next generation LIBs due to its high capacity,low cost, high safety and environment friendly. However, there are still some keyissues to be addressed before application, such as large initial irreversible capacity,poor rate performance, and capacity fading. In this work, Li-rich layer cathodematerials have been synthesized via co-precipitation followed by calcination, and thepreparation of precursor, optimization of synthesis conditions, and surface coatingmodification are investigated systematically.Li-rich cathode Li1.2Mn0.6Ni0.2O2has been pilot synthesized by co-precipitationfollowed by calcination. The influences of NH3·H2O concentration and stirring speedon the precursor (Ni0.25Mn0.75)CO3are studied, and the electrochemical performanceof Li1.2Mn0.6Ni0.2O2is investigated under25and55oC, respectively. The result showsthat the high concentration leads to the actual composition deviating from thedesigned value. And the fine particles are easily formed at low concentration andresult in low tap density. The final product is spherical particle with average size of8.68μm and tap density of2.06g/cm3. The initial discharge capacity is239.9and298.9mAh/g under25and55oC, respectively, and shows excellent cycle stability.Li-rich cathode Li1.2Mn0.54Ni0.13Co0.13O2has been synthesized via coprecipitationfollowed by calcination. The effects of Li content and calcination temperature on thestructure, morphology and electrochemical properties are investigated. On this basis,Li[Li0.2Mn0.54Ni0.13Co0.13]O2have been pilot synthesized and investigated. The resultshows that the product with Li/M=1.55calcinated at900oC for12h exhibits excellentelectrochemical performance. Its initial discharge capacity is259.7mAh/g andcapacity retention is94.9%after70cycles. The pilot-sample shows high tap density(2.01g/cm3) and good electrochemical properties.To enhance the electrochemical performance, Li1.2Mn0.54Ni0.13Co0.13O2has been surface modified with ZrO2, FePO4, LiF, and Li3PO4, respectively. The effects ofcoating on the structure, morphology, and electrochemical performance areinvestigated. Cyclic voltammetry and electrochemical impedance spectroscopy havebeen employed to further understand the function of surface coating. The result showsthat the electrochemical performance is significantly improved by coatingmodifications. The ZrO2coating improves the cycling performance greatly. Especially,the1wt.%ZrO2-coated sample shows discharge capacity of218.5mAh/g at0.5C andretains207.3mAh/g after50cycles. The integrated electrochemical performance havebeen improved by FePO4coating. The3wt%FePO4-coated sample with a coatinglayer8-10nm delivers an initial discharge capacity of271.7mAh/g at0.05C with ahigh coulombic efficiency of85.1%and still retains203.1mAh/g after100cycles at0.5C. The performance improvement can be attributed to that the physical protectioneffect of the coating layer, which increases the interface stability between cathode andelectrolyte, and restrains the rapid growth of impedance during cycling. The FePO4isamorphous material with electrochemical activity, which is favorable to the Li+diffusion and provides capacity during charge/discharge. LiF, as a coating material,which keeps stable in the non-aqueous electrolyte, prevents the active material fromcorrosion by HF. The coating layer Li3PO4is a solid electrolyte, which can protectelectrode and facilitate Li+diffusion through the interface.