Preparation And Lithium Storage Performance Of CMFs-based Metal Oxides/graphene Composites

With the shrinkage of fossil fuels and the emergence of climate problems such as greenhouse effect,the demand for clean and renewable energy or energy storage devices is ever-growing.As green energy storage devices,lithium-ion batteries(LIBs)have been used in a wide range of electronic devices and electric cars since its first appearance in the 1990s.However,the current commercial graphite anode material has gradually failed to meet the growing requirements for the energy and power density due to its low theoretical capacity(372 m Ah g^-1)and inferior rate performance.Therefore,it is significant to develop new alternative anode materials to enhance the performance of LIBs.Metal oxides are potential anode materials due to their high theoretical capacity,abundant reserves and environmentally friendly.However,the large volume change during the charge/discharge process and low electrical conductivity deteriorate their lithium storage performance,thus restricting the practical application of metal oxides anodes.In this thesis,bimetal(mixed)oxides/reduced graphene oxide(r GO)nanocomposites are designed and synthesized using cyanometallic frameworks(CMFs)/graphene oxide(GO)composites as precursors by a facile two-step thermal treatment method.The as-prepared materials are characterized by XRD,SEM,TEM,Raman,TGA,BET,XPS,and so on.The lithium storage performance of the materials is systematically studied,and the relationship between the composition and structure of the composites and the lithium storage properties is revealed.The main research contents are as follows:(1)Ni Fe-Ni Fe₂O₄/r GO composites are synthesized by a facile cyanometallic framework precursor strategy.The compositions and microstructures of the Ni Fe-Ni Fe₂O₄/r GO composites are systematically adjusted to realize the controlled synthesis.In the composites,the Ni Fe/Ni Fe₂O₄ nanoparticles with adjustable mass ratio of Ni Fe₂O₄ to Ni Fe alloy are homogeneously deposited on the reduced graphene oxide(r GO)sheets.As anode materials for LIBs,the optimized Ni Fe-Ni Fe₂O₄/r GO composite displays remarkably enhanced lithium storage performance with an initial specific capacity as high as 1362 m Ah g^-1at 0.1 A g^-1 and a decent capacity retention of ca.80%after 130 cycles.Besides,the composite can deliver a reversible capacity of 550 m Ah g^-1 at 1 A g^-1 after 300 cycles.During the charge-discharge cycles,the aggregation of Ni Fe-Ni Fe₂O₄ nanoparticles and the structure collapse of the electrode can be well alleviated by r GO sheets.Moreover,the conductivity of the electrode can be significantly improved by the well-conductive Ni Fe alloy and r GO sheets.All of these contribute to the improved lithium storage performance of Ni Fe-Ni Fe₂O₄/r GO composites.(2)Mn₃O₄-Fe₃O₄/r GO composites are successfully prepared by a two-step thermal treatment of the Mn₃[Fe(CN)₆]₂/GO precursor.In the composites,Mn₃O₄-Fe₃O₄ nanoparticles with a size of 300?500 nm were uniformly distributed on the surface of r GO sheets,and the r GO content can be systematically adjusted.The synergistic effects between Mn₃O₄,Fe₃O₄ as well as r GO remarkably enhance the electrochemical lithium storage performance of the composites.Especially,the r GO can not only act as a conducting matrix,but also buffer the volume expansion and avoid the aggregation of Mn₃O₄-Fe₃O₄ nanoparticles during the charge/discharge process.The optimized Mn₃O₄-Fe₃O₄/r GO composite exhibits a high reversible capacity of 1013 m Ah g^-1 after 100 cycles at 0.1 A g^-1,good rate capability of 510m Ah g^-1 at 3.0 A g^-1 and long cycle stability(804 m Ah g^-1 after 500 cycles at 0.5 A g^-1).(3)Novel Ni-Ni O-Mo O₂/r GO composites are prepared by a simple two-step thermal treatment of the Ni₂[Mo(CN)₈]/GO precursor.In the composites,Ni-Ni O-Mo O₂ nanoparticles with a size of 60 nm are uniformly distributed on the surface of r GO sheets.When used as anode material for LIBs,the Ni-Ni O-Mo O₂/r GO composite shows excellent cycle performance(discharge capacity up to 910 m Ah g^-1after 220 cycles at 0.5 A g^-1)and rate capability(discharge capacity of 534 m Ah g^-1 at3 A g^-1).The Ni metal and r GO in the composites can improve the electrical conductivity of the composites.Especially,the introduction of r GO can prevent the nanoparticles from aggregation,improve the specific surface area of the composites,and alleviate the volume change of the electrode during the charge/discharge process,thus resulting in an excellent lithium storage performance.