Designed Synthesis And Electrochemical Performance Of Manganese-based Electrode Materials For Lithium-ion Batteries

Lithium-ion batteries have been widely used in our daily life,which have shown great potential for application in electric-vehicles and large energy storage systems.Compared with commercial LiCoO₂ cathode and graphite anode materials,manganese-based oxide cathode?LiMn₂O₄?and anode?MnO_x,ZnMn₂O₄?have attracted extensive research due to their advantages of abundant resources,low cost,safety,environmental protection,and high specific capacity for application.However,to meet the applied requirement of high energy/power density batteries in the future,their cycling life and charge-discharge performance at large current densities need to be further improved.In this paper,various manganese oxide electrode materials were prepared.The relationship between morphology,microstructure and electrochemical properties was studied.Moreover,the electrochemical activity and cycling stability have been obviously enhanced by controlling the particle-size and designing reasonable morphology and microstructure.The main research contents are as follows:1.The homogeneous submicron MnCO₃ precursors were synthesized by a surfactant-assisted precipitation method,and thus the submicron LiMn₂O₄ particles cathode materials were obtained by molten salt impregnation after thermal decomposition of the precursors.Polyethylene glycol?M=1000?was used as the surfactant to control the precipitation reaction of MnCO₃ precursors.The influence of the addition amount on the particle-size,size distribution,crystallinity of the precursors and corresponding products,as well as the relationship with the final electrochemical properties were analyzed.The tests show that using 10%of the total quality of reactants,the final LiMn₂O₄?LMO-10%?product owns an average of only250 nm in particle-size,a uniform size distribution and the highest crystallinity.The discharge capacity is 125 and as high as 100 mAh g^-1 at 1 and 50 C rate,respectively,with the capacity retention of 74%after 1000 cycles at 20 C.Compared with micro-sized particle synthesized by traditional solid-phase method and many other morphologies materials,LMO-10%has faster Li⁺diffusion rate and better structural stability.Its excellent electrochemical performance is attributed to the higher crystallinity,small and uniform size of submicron particles.2.In order to improve the Li⁺diffusion rate and cycling stability of manganese oxide anode materials,a porous micro-/nanostructure was designed and synthesized on the basis of controlling the particle-size.The submicron MnCO₃ precursors were synthesized by an ethylene glycol assisted precipitation method.By changing the pyrolysis conditions of the precursors,the composition and micro-/nanostructure of the four porous MnO_x submicro-spheres were studied,and the corresponding electrochemical properties were tested.The results show that the average discharge capacities of MnO₂,Mn₂O₃,Mn₃O₄ and MnO were as high as 912,1100,1182 and944 mAh g^-1 at the current density of 0.1 A g^-1,respectively.The discharge capacities of 1208,1156,1147 and 972 mAh g^-1 were maintained after 200 cycles at0.2 A g^-1,with capacity retention of 123%,119%,94%and 99%,respectively.Compared with the traditional graphite anode and many large particles anode materials,this porous micro-/nanostructure formed by weight loss of the pyrolysis can improve the diffusion rate by shortening the migration path of Li⁺and electron,and keep material particles from collapse and crush,thus achieving high rate and cycling performance.3.Porous micro-/nanostructures with special morphologies were fabricated to further improve the electrochemical performance of manganese oxide anode.The spherical,peanut-like,rice-shaped and cubic Zn_1/3Mn_2/3CO₃ micron or submicron precursors were synthesizes in ethylene glycol,water/triethanolamine,triethanolamine solvent and water-n-pentanol-CTAB-n-hexane microemulsion system using solvothermal and microemulsion method,respectively.Four corresponding porous micro-/nanostructures of ZnMn₂O₄ anode materials were obtained by pyrolysis of the precursors.The action-mechanisms of medium systems on forming of the precursors were analyzed,and the influence of final morphologies and microstructure on their electrochemical properties was systematically studied.The results show that formation of the precursor precipitate is greatly influenced by the active groups or the molecular structures of the solvent medium and reaction systems.The tests show that the discharge capacity of spherical ZnMn₂O₄ is 411mAh g^-1 at 4 A g^-1,and retaining 799 mAh g^-11 after 200 cycles at 0.5 A g^-1.The discharge capacity of peanut-like ZnMn₂O₄ is 569 mAh g^-1 at 4 A g^-1,and retaining1256 mAh g^-11 after 600 cycles at 0.5 A g^-1.The discharge capacity of rice-shaped ZnMn₂O₄ is 571 mAh g^-1 at 5 A g^-1,and retaining 663 mAh g^-11 after 550 cycles at a large current density of 2 A g^-1.The discharge capacity of cubic ZnMn₂O₄ is 585mAh g^-1 at 8 A g^-1,and retaining 1138 mAh g^-11 after 1000 cycles at 6 A g^-1.Due to the inherent physicochemical coordination and complementarity of the bimetallic elements,ZnMn₂O₄ has more potential with higher electrochemical properties than mono-metal MnO_x anode materials.By contrasting the morphologies and electrochemical properties of several porous micro-/nanostructure ZnMn₂O₄,it is found that,compared with the traditional spherical morphology,the peanut-like,rice-shaped and cubic secondary particles have larger specific surface area and pore volume,which can further improve the function of porous micro-/nanostructure in promoting Li⁺and electrons diffusion rate,and buffering the volume transformation.Cubic secondary particles with higher frame strength can effectively cooperate with porous micro-/nanostructure to resist the volume transformation.Therefore,in order to improve the electrochemical properties of the target electrode materials,particle-size control,structure design and morphology design can be three complementary modification ideas.