| Sodium-ion batteries(SIBs)have been promising as an ideal large scale energy storage device to replace lithium-ion batteries in the future owing to possessing the similar working principle to that of lithium-ion batteries,abundant sodium resource reserves,and low cost.The cathode materials,as a key type of components of the SIBs,play a decisive role in the energy density and cycle life of the entire battery.Among SIBs,the layered transition-metal oxides have been attractive as potential cathode materials due to the high discharge specific capacity,simple preparation methods,and environmentally friendliness.However,there are some unavoidable problems in the actual charging and discharging process of these cathode materials,such as the structural collapse caused by complex phase transition,poor rate performance caused by slow sodium ion transport kinetics,and poor stability caused by reaction with substances in the air,all of which are unfavorable for storage and transportation.Therefore,it is of great importance to alleviate the above problems and to advance the further practical applications of layered cathode materials for sodium ion via reasonable cationic doping.In this thesis,two types of O3-type layered transition-metal compounds(Cu-doped O’3-Na3Ni2xCuxSbO6(x=0,0.3,0.5)and Fe/Mg co-doped NaNi0.35Fe0.2Mg0.05Mn0.4O2)were prepared by cationic doping.The morphology,structure and electrochemical performances of the cathode materials were investigated using SEM,TEM,XRD,XPS and electrochemical testing.The main research contents are as follows:(1)Study on the electrochemical performance of copper-doped O’3Na3Ni2SbO6.O’3-Na3Ni2SbO6 cathode material is a kind of layered cathode material with a special honeycomb structure.In order to solve the problem of poor cycle stability caused by the large volume change of partial phase transition during the charge and discharge process,a series of O’3-Na3Ni2-xCuxSbO6(x=0,0.3,0.5)cathode materials doped with different Cu contents were designed and prepared.The Na3Ni1.7Cu0.3SbO6 cathode material with x=0.3 has greatly improved cycle stability and the excellent electrochemical performance.In-situ XRD patterns show that Cu doping promotes the reversibility of O’3 phase transition during the discharge phase,reduces the P’3 and 01 phase transition regions,and eased the volume change of the intermediate phase O1.The doping of Cu element also reduces the charge transfer resistance,increases the diffusion coefficient of sodium ions,and improves the rate performance of the material.The material has a capacity retention rate of 70%after 200 cycles in the voltage range of 2.0 V-4.0 V at a rate of 0.1 C,and a capacity retention rate of 63%after cycling for 300 cycles at a rate of 1 C.Compared with other samples,the capacity retention rate increased by 20%.(2)Study on the electrochemical performance of iron/magnesium codoping O3-NaNi0.5Mn0.5O2.In order to solve the problem of poor cycle stability caused by complex phase transition during the charge and discharge process,a Fe/Mg co-doped O3NaNi0.35Fe0.2Mg0.05Mn0.4O2 layered cathode material was prepared.In situ XRD characterizations demonstrated that the O3-O’3-P3-P’3-P3" complex phase transitions of O3-NaNi0.5Mn0.5O2 material are suppressed,and a highly reversible O3-P3 phase transition is achieved by Fe/Mg co-doping.The doping of electrochemically active element Fe also improves the discharge specific capacity of the material,and the inactive Mg maintains the stability of the material structure.The material has a specific discharge capacity of 129.4 mA h g-1 in the first cycle in the voltage range of 2.0-4.0 V at a rate of 0.1 C,and the capacity retention rate is as high as 86%after 150 cycles at 1 C.Moreover,the Na-ion full battery paired with hard-carbon anode shows excellent electrochemical performance.The working voltage of the full battery is 3.03 V,and the corresponding energy density is 402.3 Wh kg-1,which outperform to reported full battery performance. |
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