Study On Morphology And Structure Regulation And Electrochemical Performance Of Lithium Rich Manganese Based Cathode Materials

Li-rich layered oxide（LR）is an appealing candidate of high energy density and low cost cathode material for next generation lithium ion batteries（LIBs）,due to its high capacity and low cost.To solve the problems of low conductivity,poor rate capability and voltage attenuation of LRs,LRs with different morphologies,compositions and structures are synthesized by using blend and surface modification approaches,to study the relationships among morphologies,crystal structures,compositions,local fine structures and electrochemical properties of LRs.A controllable co-precipitation method is reported to synthesize Mn_4/6Ni_1/6Co_1/6CO₃ precursor and novel hierarchical double-shell hollow spherical Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ combining with calcination method.The formation mechanism of the structure and its effect on the electrochemical performance of LR are studied.NH₄HCO₃-assisted strategy promotes the growth of precursor,and fast growth weakens the particle internal crystallinity as compared to that of surface,which thus causes heterogeneous contraction and the corresponding morphological conversion process among the micro-spherical solid structure,the core-shell structure and the target hierarchical double-shell hollow spherical structure.Compared with the solid spherical structured LR,hierarchical double-shell hollow spherical structured LR exhibits remarkable electrochemical performance.As a result,the resultant LR presents a high capacity of 223,196,170 and 132 m Ah g-1 at 1,3,5 and 10 C,respectively,and a high capacity retention of 87.6% after 400 cycles at 3 C.The double-shell hollow spherical porous structure improves the rate performance of LR by facilitating efficient Li-ion transport,while the reliable structural stability hierarchical micro-/nano-structure and uniform multielemental components enhance the cycle stability of LR.An approach is reported to fabricate layered-spinel 3D LR hierarchitectures,by using a hydrothermal,high temperature sintering and components controllable method.The formation mechanism of the 3D hierarchitectures and the relationship between multicomponent and electrochemical performance are studied.A transformation mechanism of 1D solid nanorods to 2D plane to 3D hollow structure is proposed to explain the growth of hierarchical β-Mn O2 precursor.The layered-spinel LR has three phases: a monoclinic C2/m Li₂MnO₃,a rhombohedral R-3m LiTMO₂ and a cubic Fd-3m spinel.XRD Rietveld refinement results show that the cell volume of LR increases with the spinel components.LR with a moderate amount of spinel component delivers a capacity of 251,227,202,155 m Ah g-1 at 1,2,5,10 C,respectively,and the Li-ion diffusion coefficient can reach to 7.74×10-13 from 1.69×10-13 cm2 s-1.The unique 3D hollow hierarchical structure effectively shortens the pathways of Li-ion transfer.Moreover,spinel component introduces more effective 3D Li-ion diffusion channels by increasing the unit cell volume.An approach is reported to design and synthesize high-loading and free-standing CNT/spinel mixed conductive layer modified LR,by using a vacuum filtration and sintering method.CNT can improve the electronic conductivity of LR and effectively solve the problem of cracking of LR and breaking away from the conductive agent during charging and discharging cycles.Monoclinic Li₂MnO₃ phase transforms to cubic-spinel phase in the CNT matrix during the sintering process caused by the thermal reduction of Mn element.Spinel component can improve the Li-ion diffusion coefficient of LR to 1.08×10-12 from 2.53×10-13 cm2 s-1,which presents a high capacity of 304 m Ah g-1 at 0.1 C with a coulombic efficiencies of 91.7%,remains high capacities of 266,238,215,166 m Ah g-1 at 1,3,5,10 C,respectively,and achieves a high capacity retention of 87.7% after 400 cycles at 3C.Electrochemical performance improvement is due to the enhanced electronic conductivity by the CNT material which has a long-term,continuous three-dimensional electronic transmission network.Spinel component can introduce more effective 3D Li-ion diffusion channels to improve the ionic conductivity.A hydrothermal method is reported to regulate and control the surface structure of LR.XRD Rietveld refinement method,synchrotron radiation X-ray imaging,X-ray absorption fine-structure and STEM microscopy are used to research the relationship among surface atomic scale crystal structure,local fine-structure and electrochemical properties of LR.Hydrothermal treatment can enlarge the c-axis and cell volume of LR.Ni is found to be enriched at the surface in regions perpendicular to the（200）facet,which causes a phase transformation from layered phase to cubic rock-salt phase,generating about 3 nm Ni/Li mixed layer in the surface.Hydrothermal treatment can effectively eliminate the Ni/Li mixed layer,and thus the Ni/Li mixed ratio can be reduced from 3.52% to 1.35%.The optimal LR presents a high capacity of 301 m Ah g-1 at 0.1 C with a coulombic efficiencies of 90.7%,remains high capacities of 248,201,163,133 m Ah g-1 at 1,3,5,10 C,respectively,and the capacity can be improved to 227 from 158 m Ah g-1,while the average discharge potential can be improved to 3.41 from 3.16 V after 200 cycles at 1C.Hydrothermal treatment can improve the rate performance and cycle stability of LR by suppressing the Ni/Li mixed layer,and increasing the lithium diffusion layer spacing and unit cell volume.