High Performance Lithium-ion Battery Cathode And Anode Materials Based On Nitrogen-doped Carbon Modification

After more than 20 years of development,lithium-ion batteries have been widely used in various electronic products due to their advantages of high energy density,long cycle life and high safety.Nowadays,with the rapid advancement of electric vehicles and smart grids as well as other relevant markets,developing batteries with higher power density,longer cycle life,lower price and higher safety has attracted intense interests of many countries.The cathode and anode materials,which are important parts of lithium-ion batteries,play crucial role in determining the overall battery performance.Thus,how to optimize the performance of existing electrode materials and develop new materials with improved properties are of great significance to promote the further progress of lithium-ion batteries.In this thesis,nickel-rich ternary layered oxide cathode LiNi_0.6Co_0.2Mn_0.2O₂?NCM622? and antimony/carbon anode materials are modified based on nitrogen-doped carbon materials and their performance in lithium-ion batteries are systematically studied.The first chapter briefly introduces the development history,working principle,advantages and disadvantages of lithium-ion batteries.Then a summary about the cathode and anode materials as well as the nitrogen-doped carbon materials is presented,among which the structure,reaction mechanism and modification methods of NCM622 and antimony/carbon materials are mainly reviewed.Chapter 2 gives a general introduction of the experimental reagents and equipments used in this thesis.And it detailedly describes the process of electrode preparation and the assembly of CR2032 coin cell.Moreover,the general characterization methods of materials as well as the electrochemical analysis are presented.In chapter 3,we optimize the electrochemical performance of NCM622 by surface modification of nitrogen-doped carbon layers.Although NCM622 has the advantages of relatively high specific capacity and low cost,the structural instability and side reactions with the electrolyte at the surface strictly limit its performance.Here,we introduce a nanoscale nitrogen-doped carbon layer at the surface of the NCM622 particles by using simple mechanical activation and pyrolysis method.Systematic characterizations indicate that the nitrogen-doped carbon layer with thickness of approximately 16 nm is uniformly coated on NCM622,which has been proved beneficial to stabilize the layered structure with less cation disorder and residual lithium at the surface.The coated sample exhibits a capacity retention of 92%after 100 cycles from 3.0 V to 4.5 V at 1 C,and a discharge capacity of 156 mAh g^-1 at 5C.In chapter 4,a nanocomposite of Sb@Sb₂O₃ core@shell particles with 3D porous nitrogen-doped carbon nanosheets is synthesized.Although the theoretical capacity of the antimony-based materials is high,the volume expansion effect during the charge and discharge process results in quick capacity decay.Here we select natural polymer material sodium alginate as the carbon resource and Sb³⁺ complexing agent,cyanamide as the nitrogen source,sodium chloride as the template to synthesize such antimony/carbon composite by freeze drying and sintering under reducing atmosphere.The characterization results show that the specific surface area of the electrode is as high as 839.8 m² g^-1 and the Sb-O-C structure exists between Sb@Sb₂O₃ and nitrogen-doped carbon substrate,both contributing to the excellent electrochemical performance.The initial charge and discharge specific capacities of Sb@Sb₂O₃/NC is 1109 mAh g^-1 and 1810 mAh g^-1,respectively,with coulombic efficiency of 61.3%.Also,it shows a charge capacity of696.9 mAh g^-1 after 500 cycles at 1 C and 458 mAh g^-1 at 5 C.Moreover,the assembled Sb@Sb₂O₃/NC?LiNi_0.6Co_0.2Mn_0.2O₂/NC full battery exhibits a discharge capacity of more than 100 mAh g^-1 after 25 cycles at 0.1 C.At last,in chapter 5,an overview on the originalities and deficiencies of this thesis as well as some perspectives of the future research directions are given.