Preparation And Properties Of Lithium-rich Layered Transition Metalinlay Lithium Oxide

Layered Lithium rich type lithium-embedded transition metal oxide is a new type ofcathode material based on solid solution for lithium ion batteries. It has attracted intenseresearch for its ultra high capacity and capacity density. However, the poor conductivitylimited its further development. Therfore, this dissertation intends to improve the performancethrough optimizing synthesis method, dopping modification, coating modification andnano-crystallization.Firstly, Layered Lithium rich cathode material Li1.2Mn0.54Co0.13Ni0.13O2was preparedwith solid state method, coprecipitation method, sol-gel method, hydrothermal method andOrganic coprecipitation. The materials were characterized by X-ray diffraction (XRD),scanning electron microscop (SEM), galvanostatic charge-discharge cycle tests (C&D), etc. Itis shown that the as prepared lithium rich materials with above mentioned methods havelayered crystal structure as α-NaFeO2. Some superlattice peaks can be found in XRD patternsin the range of2θ=20°~25°, indicating the existence of Li2MnO3. The particle size of solidstate method and coprecipitation method produced samples are uniform. Sol-gel method,hydrothermal and Organic coprecipitation method produced samples take on smaller particlesize and smaller spacing between particles. The specific discharge capacity of solid statemethod, coprecipitation method, sol-gel method and Organic coprecipitation producedsamples is as high as250mAh/g, and that of hydrothermal method produced samples isaround230mAh/g.F-which has stronger electronegative than O2-, was selected as the doping ion at anionsites. Based on that, the Layered Lithium rich cathode materials Li1.2Mn0.54Co0.13Ni0.13O2arefurther co-doping modified with the combination of Zr/Cl and Fe/F. The doping schemeoptimization were discussed based on the comprehensive characterization including XRD,SEM, C&D, cyclic voltammetry (CV) and AC impedance (EIS). Among a series F dopedmaterials Li1.2Mn0.54Ni0.13Co0.13O2-xFxprepared with organic coprecipitation preparation route,better rate capability can be achieved when the doping amount is x=0.01. It retained a specificdischarge capacity of50mAh/g at a current density of500mA/g, which is much higher thanthe undoped sample. As for the Fe/F co-doped Li1.2Mn0.54-xNi0.13-xCo0.13-xFe3xO2-yFymaterials, the best performance belongs to Li1.2Mn0.50Ni0.09Co0.09Fe0.12O1.95F0.05(x=0.04, y=0.05), whichretained a specific discharge capacity of more than140mAh/g at a current density of300mA/g. The Fe/F co-doped materials Li1.2Mn0.54-xZrxNi0.13Co0.13O2-yClyhave a particle size inthe range of100~250nm and take on fine layered structure. All the peaks in XRD patterns areattributed to the typical peaks of a hexagonal α-NaFeO2structure with R-3m space group,except for the superlattice peaks of Li2MnO3. With the Zr/Cl doping amount increase, severalweak diffraction peaks in the range of2θ=25°~35°can be detected, which should beattributed to ZrO2. The extra peaks mean that the Zr can’t totally enter the crystal lattice athigher doping concentration. When cycled at low current,Li1.2Mn0.50Zr0.04Ni0.13Co0.13O1.92Cl0.08(x=0.04, y=0.08) shows higher reversible capacity. Aspecific discharge capacity of234mAh/g is presented even after25cycles. On the other hand,when the doping amount is controlled at x=0.02, y=0.04, the best rate capability is presented,that is150mAh/g at a rate current of250mA/g.Some special shell materials, including electrolyte stable Al2O3, another kind of highcapacity cathode material LiV3O8, another high voltage cathode material spinelLiMn1.5Ni0.5O4, and graphene/polyaniline composite material with excellent conductivity arechosen to be coated on the particle surface of Li1.2Mn0.54Ni0.13Co0.13O2, and the coatingmodified materiasl are characterized with XRD, SEM, C&D, CV and EIS techniques. Al2O3coated material remain the same crystal structure as he pristine material, but the particlespacked more closely leading to shorter distance between particles. Since electrolyte inertoxide shield the malignant interaction between material and electrolyte, the cycle stability ofmaterial Al2O3coated materials are improved effectively. The coating modified materialretains a specific discharge capacity of240mAh/g after25cycles, which is95.1%of its initialcapacity, while the pristine material can remain only82.3%of its initial capacity. There is noextra differaction peak can be detected in the XRD pattern of LiV3O8coated material,indicating the amorphous coating layer. Both3wt%and5wt%LiV3O8coated materialspresent specific discharge capacity of80mA/g higher than the pristine sample at rate currentof500mA/g. The LiMn1.5Ni0.5O4coated material remain the same layer structure asLi1.2Mn0.54Ni0.13Co0.13O2, and have a specific discharge capacity of70mA/g higher than thepristine one at a current density of200mA/g. Graphene/polyaniline composite layer coated Li1.2Mn0.54Ni0.13Co0.13O2keep the layered structure, while the rate capability is muchimproved due to the outstanding conductivity of the composite shell layer.Then nanao sized Li1.2Mn0.54Ni0.13Co0.13O2particles are prepared through successivesolid state, sol-gel and hydrothermal procedure separately, using MnO2nanowires presynthesized by hydrothermal method as the precursor. The as prepared material has finelayered strucuture and uniform particle size of around100nm. The nano material producedthrough a successive sol-gel process has a high reversible capacity, while sample from thesuccessive hydrothermal process take on the best rate capability.