Investigation Of Metal Oxide/graphene As Anode Materials For Sodium Storage

lithium often has fatal disadvantages including the low content in the earth and high price,which restricts the development of Li-ion batteries(LIBs)in the future.Generally,the physical and chemical properties of sodium are similar to that of lithium,and its resource are very affluent and widely distributed in relative to Li.On base of the above characteristics,Na-ion batteries(NIBs)have shown a strong competitive advantage in large-scale energy storage systems.However,NIBs are often lack of cathode and anode materials with high safety and excellent performance.Thus,it is urgent to develop new electrode materials for NIBs.Transition metal oxide materials often have high theoretical capacity and suitable sodium storage potential.Nevertheless,metal materials often have the problems of poor conductivity and prone to volume expansion during charging and discharging.Doping of heteroatoms can create defects and oxygen vacancies in materials is an effective means to enhance its conductivity has been widely reported.In addition,reduced graphene oxide(RGO)has received extensive attention from researchers due to its unique layer structure,chemical stability,high specific surface area and high electrical conductivity,which is very suitable for compounding with metal oxide materials.Therefore,this work mainly improves the electrochemical performance of Fe₂O₃and Ti O₂by doping of heteroatoms and hybridization with carbon materials.The main research works are as following:(1)To buffer the volume change and facilitate the ion/electron transportation of Fe₂O₃during cycling,Fe₂O₃/SNRGO composites was prepared by microwave method.The doping of N and S heteroatoms on the graphene enhances the connection between graphene nanosheets and Fe₂O₃nanoparticles,resulting in superior sodium storage properties.The uniform embedding of Fe₂O₃nanoparticles into the surface of SNRGO hybrid nanosheets can not only alleviate the face-to-face restacking of SNRGO nanosheets,but also reduce the size of Fe₂O₃particles.As the buffer layer,SNRGO can accommodate the volume change and inhibit the structure of Fe₂O₃nanoparticles from collapsing during the Na⁺insertion/extraction.Benefitting from the synergistic effect between the SNRGO and Fe₂O₃,the as-prepared Fe₂O₃/SNRGO exhibits the high reversible capacity of 467 m Ah g^-1at 0.1 A g^-1after 50 cycles,as well as outstanding rate performance(200.2 m Ah g^-1at 2A g^-1).(2)To solve the problems of Ti O₂electrode materials,such as short cycle life and intrinsic poor conductivity,N-doped modified graphene/mixed crystal phases Ti O₂composites(Ti O₂/NRGO)have been prepared by microwave-assisted method.The numerous spherical hybrid anatase/rutile Ti O₂nanoparticles evenly anchored on NRGO nanosheets and the Ti O₂nanoparticles is approximately 20-50 nm.The ions/Na⁺diffusion rate can be increased by the crystal interface effect of mixed crystal phases Ti O₂.In addition,the doping of nitrogen further improves the sodium storage properties of the composites.With this strategy,the as-prepared Ti O₂/NRGO composites delivers remarkable reversible capacities(223.3 m Ah g^-1at 0.1 A g^-1over 50 cycles)and outstanding rate capability(101.1 m Ah g^-1at 2 A g^-1)for NIBs.(3)This paper reports a simple sol-gel–microwave two-step route to synthesize Fe-doped anatase Ti O₂/N-doped modified graphene(FTi O₂/NRGO)composites with superior performance in NIBs.The doping of low-valence Fe³⁺can generate vacancies in the oxygen sub-lattice of anatase Ti O₂,thereby improving electronic conductivity.Furthermore,the introduction of N-doped graphene can further enhance the structural stability of the composites and facilitate the ions/Na⁺diffusion.With this strategy,the FTi O₂/NRGO electrode delivers outstanding reversible capacities(161.3 m Ah g^-1at 0.1 A g^-1over 300 cycles)and superior rate performance for NIBs(102.1 m Ah g^-1at 2 A g^-1).