Preparation Of Two-Dimensional Porous Anode Materials Using Graphene Oxide For Li-ion Batteries

At present,commercial lithium-ion batteries?LIBs?applied common graphite as anode material,which cannot satisfy the increased demands owing to the limited specific capacity and poor rate performance.Therefore,it's urgent to design and develop new anode materil with high performance.Graphene,as a novel carbon nanomaterial,owing high electrical conductivity,large specific surface area and good chemical stability,is an ideal candidate as anode material for LIBs.So graphene has been paid more attention in the world.Although the research of graphene-based electrode materials has made important progress,as an emerging field,a series of key scientific problems need to be solved,such as the composition and structure of the material,electrochemical performance and storage mechanism.Especially,continuous innovation researches are also needed to promote the technological progress and make full use of the structural characteristics of graphene for higher electrochemical performance.Therefore,it has become a hot spot and important development direction in the field of LIBs that the research of novel nanostructured electrode materials based on graphene with excellent electrochemical performance.In this thesis,using graphene oxide?GO?as starting material,we synthesized a series of novel two-dimentional porous structured anode materials with excellent electrochemical perormance from the following several aspects,preparation of porous graphene,structure desing of porous graphene electrode and synthesis of two-dimentional porous micro-nano structured metal oxides using GO as oriented template,etc.This thesis provides innovative experimental ideas for fabricating novel structured electrode material based on graphene for LIBs.The detailed contents are listed as follows:1.A simple method of preparation of porous graphene was proposed.The mixture of metal salts and GO was treated at high temperature under the protection of inert gas atmosphere.The porous graphene was obtained after removing the metal nanoparticles with acid.Nickel was chose as research system,and the influence of GO/Ni,holding time and type of metal salts for preparing porous graphene were studied.The electrochemical performance and possible mechanism of porous graphene were also investigated.The results demonstrate that when the temperature reaches up to above 700?,metal nanoparticles can react with graphene forming pores.Nickel,iron,cobalt and copper can also be used to prepare porous graphene.The size of the pores and pore density on the basal plane of graphene sheets could be readily controlled by the ratio of the reactants,the heating temperature and time,different metal salts.The porous graphene electrode tested as anode materials exhibite high electrochemical performance with a first charge specific capacity of 933 mAh g^-1 at current density of 50 mA g^-1,which is more than twice that of reduced graphene oxide.2.On the basis of metal etching method for preparing porous graphene,two-dimentional porous structured graphene film was fabricated to investigate the effects of electrode material structure on the electrochemical performances.Three-dimensional hierarchical porous graphene/Co aerogel was prepared by coupling a hydrothermal method and thermal treatment.Graphene nanosheets interconnect to form conductive nteworks,and cobalt nanoparticles not only react with carbon atoms of graphene to form nanopores on the graphene nanosheets,but also increase the conductivity of the aerogel.With efficient ion and electron transport pathways as well as high packing density(0.38 g cm^-3),the compressed porous graphene/Co electrode delivers a gravimetric capacity of 900 mAh g^-1 and a volumetric capacity of 358 mAh cm^-3 at current density of 50 mA g^-1.Furthermore,flexible,free-standing and porous graphene/Ni film with vertical nano-channels inside is prepared by combining vaccum filtration and thermal treatment.Due to its unique interior channel architecture which is favorable for fast ion transport,the film electrode exhibits higher capacity,much better rate capability and good cycling stability.3.Metal oxides with novel structure were prepared by using GO as a template.The mixture of metal salts and GO was treated at high temperature in air to prepare two dimensional porous micro-nano structured metal oxides.NiO was initially selected as the typical research system.The influence of GO/Ni,annealling temperature,holding time for morphologies and structures of the products were studied.The electrochemical performance and possible universality of this reaction were also investigated.The results demonstrate that uniform two dimensional porous micro-nano NiO can be obtained when GO/Ni was 10/1 and annealing at 600?for 3 h.The NiO shows good electrochemical performance due to its unique structure.A series of metal oxides?NiO,Fe2O3,Co3O4,Mn2O3 and NiFe2O4?with the similar nanostructure were investigated using this simple method.4.The micro-nano ZnCo₂O₄-graphene film was fabricated via the two steps.First,the ZnCo₂O₄ precursors homogeneous mixed with GO and graphene nanosheets in solution,followed by vacuum filtration.Subsequently,thermal treatment in air was performed.GO was used as a template to prepare the two-dimensional micro-nano structured ZnCo₂O₄ and ensure the homogeneous distribution of the ZnCo₂O₄ in the film.As expected,the as-prepared flexible free-standing film used as binder free anode shows high capacity and excellent cycling stability.The spesific capacity of the composite film eletrocde is 1255 mAh g^-1 at 0.1 A g^-1.The capacity is as high as 980 mAh g^-1 after 1000 cycles at 2 A g^-1.Using this micro-nano ZnCo₂O₄-graphene film as anode and LiFePO4 as cathode,we further assembled a thin,lightweight and flexible full LIB,which also exhibits outstanding electrochemical performances.Its capacity is 140 mAh g^-1 at 0.5 C,and the capacity retention is 87% after 100 cycles at 2 C.