Preparation and Electrochemical Performance of Zinc and Manganese Oxides/Carbon Composites

Author:Wang Dong

Supervisor:wen guang wu


Degree Year:2018





The rapid development of portable electronic devices and electric vehicles has prompted extensive research interestes in pursuing advanced anode materials for lithium-ion batteries(LIBs)with high energy/power densities and low cost.Zn-Mn mixed oxides(ZMMO)have been regarded as alternative anode materials owing to their high specific capacity,natural abundance,and environmental friendliness.Moreover,the operating voltage of ZMMO is lower than those of Co-or Fe-based oxides.However,ZMMO anode still suffers from low electronic conductivity and large volume expansion during lithiation/delithiation.Nanostructure engineering,surface modification,and hybridization with conductive materials have been employed to improve the lithium storage properties of ZMMOs.Nevertheless,several issues including low initial Coulombic efficiency and limited cycling life still remain unresolved.Moreover,current synthetic strategies are always low yield and high cost.In this work,we rationally designed and prepared several ZMMOs/carbon hybrids with optimized structure and composition in order to improve the lithium storage properties.The corresponding chemical composition,microstructure,formation process,and electrochemical peroformance were systematically characterizated and studied.The main work is listed as follows:Zn Mn2O4/NG nanosheets with“sandwich-like”structure were prepared via a one-step solvothermal process.Zn Mn2O4 nanocrystals with the size of 10-12 nm are chemically anchored on N-doped graphene nanosheets with high uniformation.Graphene oxide(GO),p H value,and the solvent play key roles on the formation of the Zn Mn2O4/NG nanosheets.The unique two-dimensional(2D)architecture,ultrathin Zn Mn2O4 nanoparticles,flexible and conductive graphene,and strong interaction between them endow the Zn Mn2O4/NG electrode with abundant active sites,rapidions/electronstransportation,enlargedcontactareaof electrode/electrolyte interface,and robust structure.When evaluated as anodes for LIBs,the Zn Mn2O4/NG nanosheets manifest excellent lithium storage properties in view of superior rate capability(500 m Ah g-1at 3200 m A g-1),and good cycling performance(747 m Ah g-1at 500 m A g-1after 200 cycles).Zn O-Mn O/GF composites were synthesized by a facile hydrothermal process of Znx Mn1-x-x CO3/GF and subsequent calcination treatment.Calcination temperature and graphene are two key factors on the formation of bimetallic monoxides(Zn O-Mn O).Zn O-Mn O/GF electrode shows superior lithium storage properties compared with Zn Mn2O4/GF in view of higher initial Coulombic efficiency,higher electrochemical reversibility,and rate capabilities,which should be attributed to the reduced formation of irreversible Li2O during the first cycle and the more porous structure of Zn O-Mn O particles.Ex-situ SEM,TEM,and EIS investigations demonstrate the excellent structural stability of the Zn O-MnO/GF electrode.During repeated charge/discharge processes,the Zn O-Mn O/GF electrode become more and more dense,while the Zn O-MnO microparticles crack into nanocrystals among the graphene nanosheets.Furthermore,we prepared Zn O-Co O/GF,Ni O-Co O/GF,and(Fe O)0.333(Mn O)0.667/GF composites under different calcination temperatures.In comparison with Zn Co2O4/GF,the Zn O-Co O/GF also manifests higher initial Coulombic efficiency and higher specific capacity.Hierarchically porous Zn-Mn-BTC hollow nanodisks constructed by parally stacked 2D subunits were prepared via a coprecipitin process at room temperature.Znx MnO@C hollow nanodisks,in which ultrafine Znx Mn O particles(4-8 nm)are uniformly distributed in porous carbonaceous matrix,were obtained by annealing Zn-Mn-BTC in Ar gas without obvious structural deterioration.The high specific surface area,abundant pore channels,unique hierarchical hollow structure,and nanosized metal oxides of Znx Mn O@C could effectively promote the electrolyte infiltration,shorten the transportation length of Li+,and provide abundant electrochemically active sites.Moreover,the continuous and conductive carbon could not only improve the electronic conductivity but also accommodate the volume expansion of Znx Mn O nanoparticles during lithiation/delithiation process.Benefitting from the structural and compositional advantages,the Znx Mn O@C hollow nanodisks electrode shows excellent electrochemical performance in view of high specific capacity(1050 m Ah g-1at 0.1 A g-1after 200 cycles),superior rate capability(330 m Ah g-1at 10 A g-1),and remarkable cycling performance(547 m Ah g-1at 2 A g-1after 1000 cycles).Kinetics analysis shows that the lithium storage behavior of the Znx Mn O@C electrode is mainly capacitive-controlled.A simple and powerful synthetic method for the mass and economical production of Zn O-Mn O/NC nanosheets was developed.The whole process involves the formation of viscous gel via a liquid-phase sol-gel reaction and rapid thermal treatment in Ar gas.By integrating the advantages of both liquid-phase and gas-phase methods for the synthesis of 2D materials,the as-synthesized 2D Zn O-Mn O/NC nanosheets possess large lateral size up to hundreds of micrometers and nanometre thickness.A new“gel-blowing”mechanism is proposed to explain the formation process of 2D nanosheets.The amount control of reactants and rapid calcination treatment play important roles on the formation of 2D structure.When evaluated as anodes for LIBs,the 2D Zn O-Mn O/NC nanosheets manifest superior cycling stability(543 m Ah g-1at 5 A g-1after 1400 cycles)and remarkable rate capability.Kinetic analysis indicates that the 2D Zn O-Mn O/NC nanosheets electrode shows enhanced pseudocapacitance behavior compared with the bulk Zn O-Mn O/NC electrode.Furthermore,various 2D metal oxides/carbon composites(Fe3O4/NC,Mn O/NC,and Zn O-Znx Fe3-x-x O4/NC)and 2D metal oxides nanosheets(Fe2O3,Mn3O4,Zn Mn2O4,Zn O/Znx Fe3-x-x O4,(Cox Mn1-x)Fe2O4,and(Znx Mn1-x)Fe2O4))were also easily prepared by this new synthetic strategy.