Synthesis And Modification Of Nickel-based LiNi_1-2xCo_xMn_xO₂Cathode Materials For Lithium-ion Batteries

Layered LiNi1-x-yCoxMnyO2materials have been considered as a promising cathode material for lithium ion batteries in that it owns the advantages of LiNiO2, LiCoO2and LiMnO2. However, there are still some obstacles limiting its applications, such as the loss of capacity during cycling and insufficient rate capability. In this thesis, the Nickel-based LiNi1-2xCoxMnxO2(x=0.01and0.02) cathode materials have been synthesized by different methods, and modified to improve its electrochemical performance.Layered LiNi1-2xCoxCoxMnO2(x=0.01and0.02) cathode materials have been synthesized by spray drying and co-precipitation methods. The influences of synthesis conditions and methods on the structural and electrochemical performance of LiNi1-2xCoxMnxO2materials have been investigated. Rietveld refinement showed that the cation mixing degree of LiNi1-2xCoxMnxO2materials prepared by spray drying method were5.5%(x=0.01) and4.3%(x=0.02), LiNi1-2xCoxMnxO2materials prepared by co-precipitation method exhibited lower degree of cation mixing, which were3.8%(x=0.01) and3.1%(x=0.02). Electrochemical test and electrochemical impedance spectroscopy (EIS) showed that LiNi1-2xCoxMnxO2materials prepared by co-precipitation method exhibited lower charge transfer resistance and better electrochemical performance than sample prepared by spray drying method. Take LiNi0.8Co0.1Mn0.1O2materials as example, the discharge capacities of sample prepared by spray drying method were194.4,177.8,166.5,153.5and127.5mAh g-1at0.1C,0.5C,1C,2C and5C between2.8and4.3V. Capacity retentions after50cycles were87.8%,82.9%and80.3%at0.5C,1C and2C, respectively. LiNi0.8Co0.1Mn0.1O2materials prepared by co-precipitation method were186.6,173.7,165.7,155.8and138.6mAh g-1at0.1C,0.5C,1C,2C and5C between2.8and4.3V. Capacity retentions after50cycles were91.1%,86.5%and83.8%at0.5C,1C and2C, respectively.LiNi1-2xCoxMnxO2/RGO (Reduced Graphene Oxide) composites were prepared by a simple spray drying method. XRD showed that composites possessed a typical hexagonal structure. Raman and FTIR confirmed the existence of RGO in the composites. SEM and TEM verified that LiNi1-2xCoxMnO2particles were wrapped with RGO sheets. LiNi0.6Co0.2Mn0.2O2/RGO composite displayed improved electrochemical performance than pristine LiNi0.6Co0.2Mn0.2O2materials. However, LiNi0.8Co0.1Mn0.1O2/RGO composite prepared by spray drying method showed worse electrochemical performance. Thus, using LiNi0.8Co0.1Mn0.1O2and RGO as raw materials, composite with RGO content of2%was prepared by physical mixing method. The composite displayed an initial discharge capacity of192.5mAh g-1, and its electrochemical performance was better than that of pure LiNi0.8Co0.1Mn0.1O2materials. EIS showed that the RGO can greatly reduce the charge transfer resistance, improving the kinetic behaviors during charge/discharge process and affecting the electrochemical performance.Fluorine substituted LiNi1-2xCoxMnxO2-zFz cathode materials have been synthesized by calcining NH4F with corresponding LiNi1-2xCoxMnxO2materials at a relatively low temperature. The structure, ionic valency and electrochemical performance of LiNi1-2xCoxMnxO2-zFz materials have been investigated. LiNi1-2xCoxMnxO2-zFz(0<z<0.06) materials possessed a typical hexagonal structure without impurity. However, the degree of cation mixing increased with the increase of fluorine contents. XPS proved that the valence of transition metals changed with fluorine substitution, which caused the change of lattice constant. Although the fluorine substituted materials showed somewhat lower initial discharge capacity, it exhibited improved cycling performance compared with the pristine material. XRD and TEM provided evidences that fluorine substitution protected the electrode from HF attack and maintained the structure stability of electrode. As shown in EIS, Rct values of fluorine substituted samples were reduced compared with pristine electrode. The small charge transfer resistance suggested that a small amount of fluorine substitution can improve the kinetic behaviors during charge/discharge process, affecting the electrochemical performance.