Improving Electrochemical Performances Of P2-Type Layered Cathode Materials By Ion Doping And Surface Modification For Sodium-Ion Batteries

The rapid development of smart products and electric vehicles have greatly increased the demand on secondary-ion batteries.So far,lithium-ion batteries still play an important role in secondary-ion batteries.However,the insufficient supply of lithium resources and the increasing production cost have not met the increasing demand for energy.Sodium-ion batteries(SIBs)have been regarded as the most promising alternative to replace lithium-ion batteries,owing to its advantageous characteristics of rich sodium resources,low price,and similar working principle to lithium-ion batteries.Among the components of SIBs,the cathode materials play a decisive role in battery performance.Layered oxide cathode materials have received intensive attention from the viewpoint of academic and industry due to their advantages of simple preparation method,high capacity,and high voltage.Nevertheless,layered oxides have still suffered from the common problems.The structural changes are caused by the complex and irreversible phase transitions,and finally causing structural collapse due to the large radius of Na+inserted and extracted between layers.In addition,the cathode material and the electrolyte are in direct contact during the battery operation,leading to the side reactions occuring at the interface,and causing the metal ions to dissolve and structural damage.Therefore,controlling the main structure and interface modification through reasonable measures is the key to the development of high-performance cathode materials.In this thesis,to solve the problems of the complicated high-voltage phase transition,the Na+/vacancy ordering and the instability of the interface,Mg/Ti co-doped P2-Na0.67Ni0.28Mg0.05Mn0.62Ti0.05O2 and Al-doped ZnO coated of P2-Na2/3Ni1/3Mn2/302,were prepared by the Mg/Ti co-doping and the surface modification of Al-doped ZnO,respectively.The following two main parts have been presented in the thesis.(1)The improved electrochemical performances of P2-Na0.67Ni0.28Mg0.05Mn0.62Ti0.05O2 via Mg/Ti co-dopingTo allieviate the structural damage and slow kinetics caused by the unfavorable phase transition under high voltage and the Na+/vacancy ordering under low voltage of P2-Na2/3Ni1/3Mn2/302,P2-Na0.67Ni0.28Mg0.05Mn0.62Ti0.05O2 was prepared through rational Mg/Ti co-doping.Mg/Ti co-doping suppresses the unfavorable P2-O2 phase transition and facilitates the transforms it into more reversible P2-OP4 phase transition,which significantly improves the structural stability of the cathode material.In addition,Mg/Ti co-doping enables the interlayer space larger,changing the ratio of Nae and Naf between layers,and also suppresses the Na+/vacancy ordering,which helps to increase the Na+diffusion coefficient by an order of magnitude.Consequently,P2-Na0.67Ni0.28Mg0.05Mn0.62Ti0.05O2 exhibits a specific capacity of 82.7 mAh g-1 over 2.0-4.35 V at a current density of 0.1C for 100 cycles,while P2-Na2/3Ni1/3Mn2/3O2 only has a capacity of 49.8 mAh g-1,P2-Na0.67Ni0.28Mg0.05Mn0.67O2 has a capacity of 71.5 mAh g-1,and P2-Na0.67Ni0.33Mn0.62Ti0.05O2 has a capacity of 62.7 mAh g-1.Therefore,the multi-ion co-doping indeed plays a synergistic role in improving the electrochemical performance.(2)The boosted electrochemical performance of P2-Na2/3Ni1/3Mn2/302 via Al-doped ZnO surface encapsulationThe side reaction caused by the contact with the electrolyte during the long cycles,which damages the structure of P2-Na2/3Ni1/3Mn2/3O2.The low conductivity of P2-Na2/3Ni1/3Mn2/3O2 also leads to poor rate performance.The aluminum-doped zinc oxide(AZO)coated P2-Na2/3Ni1/3Mn2/3O2 was prepared by using the sol-gel method,and the electrochemical performance were optimized by controlling the synthesis temperature.The electrochemical conductive AZO coating significantly reduces the charge transfer resistance at the interface and increases the Na+diffusion rate.As a result,the material calcined and coated at 500℃ exhibits an excellent Na+diffusion rate of～10-11 cm2 s-1,and retains 82.6%of the initial capacity over 2.5-4.15 V at a current density of 5C for 500 cycles.The in-situ XRD patterns prove that the AZO coating effectively relieves the lattice stress of P2-Na2/3Ni1/3Mn2/3O2 during the cycles.The volume change of the crystal unit cell after one cycle is only-0.7%,which is close to "zero strain".Importantly,the AZO coating effectively inhibits the generation of surface cracks during the long cycles,reducing the side reactions between the active material and the electrolyte,which significantly improves the structural stability.