Preparation Of Iodine-Doped Graphene And Lithium Storage Properties

Nowadays,lithium-ion batteries?LIBs?has been the most widely used energy storage technology due to their long cycle life,light weight,memoryless effect and wide operating temperature range.In recent years,although the rise of electric vehicles?EVs?further stimulates the development of LIBs,the low theoretical capacity(372mAh g^-1)of commercial graphite anode limites its further applications in the powering batteries.In order to boost the rapid development of the EVs market,it is essential to develop new anode materials with the high energy and power densities to replace the traditional graphit anode.Among all kinds of the LIB anode materials,graphene is considered as the promising anode material to develop the adcanced LIBs due to its fast lithium-ion diffusivity,extraordinary electronic conductivity,outstanding theoretical capacity,flexible porous construction and high specific surface area.Because heteroatom-doping can effectively improve the electrochemical performance of graphene anode,and the doping position and content play an important role in its Li-storage properties,it is of great significance to investigate that how to effectively enhance the Li-storage performance of graphene by precisely control the doping position and content of heteroatoms.Therefore,based on the previous report that edge-iodization can improve the electrochemical performance of graphite anode in LIBs,we further study the respective effect of iodization-position and iodine-doping content on the Li-storage performance of graphene anode,thus providing the foundation for the controllable preparation of high-performance heteroatom-doped graphene anode materials.The thesis mainly includes the following three aspects:?1?The theoretical calculations were used to study the effect of center-and edge-iodization on the electrochemical performance of graphene anode for the first time.The results demonstrate that the electronic conductivity and charge density of graphene can distinctly be enhanced after edge-or center-iodization.Simultaneously,it can also be found that compared with edge-iodization,center-iodization can further improve electronic conductivity and charge density of graphene,suggesting that as the LIB anode materials,center-iodizated graphene?CIG?anode possesses the better Li-storage performance than that of edge-iodizated graphene?EIG?anode.?2?We successfully prepared the CIG and EIG materials by the one-pot hydrothermal method and ball-milling approaches,respectively,and studied the effect of iodization-position on the Li-storage performance of graphene anode in LIBs for the first time.The experimental results reveal that the CIG anode exhibites more excellent electrochemical performance than that of EIG,still delivering a specific capacity of1121 mAh g^-1 after 180 cycles at the current density of 0.5 A g^-1,which is about 2.6times higher than 434 mAh g^-1 of EIG.Even after rate capability measurements,the CIG electrode can still remain a high specific capacity of 749 mAh g^-1 with the ultralow decay of 0.01%per cycle after 300 cycles at the current density of 1 A g^-1.The high specific capacity,excellent rate capability and long cycle life of the CIG anode are mainly ascribed to the following three aspects:Firstly,the high charge density and vast active defects of CIG effectively increase the active-site area of the lithium storage,thus offering the high specific capacity for it.Secondly,the high electronic conductivity of CIG distinctly facilitates the solid-state transport of lithium ions and electrons in the active material,thus leading to the high-rate capacity for it.Thirdly,the stable construction of CIG greatly restrains the collapse and aggregation of graphene during the repeated Li⁺insertion/deinsertion process,thus resulting in the excellent cycling stability for it.?3?We employed a one-pot hydrothermal method to prepare the iodine-doped graphene samples(I-RGO_0.0025,I-RGO_0.01 and I-RGO_0.02)with different doping content by adjusting the amount of NaI,and studied the effect of doping content on the electrochemical performance of the iodine-doped graphene anode in LIBs.The experimental results indicate that although the higher doping content can create more active defects for iodine-doped graphene to store lithium ions,it also impedes the transport of lithium ion in the active material,thus decreasing the Li-storage capacity.Therefore,the I-RGO_0.01 anode exhibites the best Li-storage properties,still remaining a specific capacity of 958.1 mAh g^-1 after 390 cycles at the current density of 1 A g^-1,which are 1.7 and 1.4 times higher than 568.8 mAh g^-11 of I-RGO_0.0025 and 708.1 mAh g^-1 of I-RGO_0.02,respectively.More importantly,even at the current densities of 2 and4 A g^-1,the I-RGO_0.01 anode still delivers the high-rate capacities of 525.8 and 343.1mAh g^-1 after 1045 and 2000 cycles with the low decay rates of 0.032%and 0.024%,respectively.The high specific capacity,excellent rate capability and long cycle life of I-RGO_0.01 highlight its potential applications as the promising anode material in next generation high-performance LIBs.