| As a kind of important positive electrode materials for Na-ion batteries(NIBs),Na+ ion transition metal layered oxides,NaxMO2,can be easily synthesized and deliver high theoretical capacity.Developing the transition metal layered oxide cathodes with high energy density and excellent cycling performance without using expensive metals(e.g.,Ni,Co,etc.)is able to effectively promote the industrialization of NIBs.O3Na0.90Cu0.22Fe0.30Mn0.48O2 cathode material has promising potential for the commercialization of NIBs due to its low cost and simple synthesis,while the specific capacity and cycling stability are still requiring further improvement.In this thesis,the Cu-,Fe-,and Mn-based(CFM-based)cathode materials were selected to investigate the synthesis and composition optimizations as well as the performance fading analysis.In the second chapter,the effect of different synthesis strategies on the electrochemical performance of CFM-based materials were studied.The relationship between the Na content in the Na layers and the structure/electrochemical performance was correlated,and a universal electrochemical titration method was designed to determine the interlayer Na content in the Na+ ion transition metal layer oxides.The interlayer Na+ ion content of the sample NaxMO2,can be estimated by discharging the original material to obtain the discharge specific capacity.Using this method,we measured the interlayer Na content of several typical O3-phase Na+ion layered oxide electrode materials and compared with the Na occupancy value obtained by X-ray diffraction(XRD)refinement.The results showed high consistency:it is proved that this method can obtain the Na content of O3 layered material reliably and conveniently.Generally,it is found that the CFM-based cathodes synthesized under air with a quench process have the better overall performance.In the third chapter,the modification of the CFM-based cathode materials was studied by doping different elements.The low specific capacity of the O3Na0.90Cu0.22Fe0.30Mn0.48O2 layered cathode material is mainly due to the partial redox reaction of Fe or Cu.In addition,the cycle stability of this material is also inferior due to the Jahn-Teller effect of Mn3+ ion.By mono-and double-doping using Mg2+,Zn2+,Li+,Ti4+,Zr4+ and Sb5+ ions,it is found that the Mg2+-doped O3Na0.90Mg0.08Cu0.22Fe0.30Mn0.40O2 has a higher average discharge voltage,the lowest polarization and the best cyclic performance with a reversible initial capacity of 100 mAh/g and a high initial Coulombic efficiency(ICE)of 96%at 0.2C between 2.4-3.85 V,and 88%of the capacity could be retained after 200 cycles.Furthermore,a capacity retention of 75%could be obtained after 500 cycles at 1C.The average discharge voltage is 3.26 V.The doped Mg2+ion is able to not only increase the electrochemical activity and discharging voltage,but also stabilize the structure and improve the cyclic performance.In the fourth chapter,the mechanism of the capacity decay of the O3Na0.90Mg0.08Cu0.22Fe0.30Mn0.40O2 material at high charging voltage was studied.It is found that the capacity and discharge voltage plateau seriously decayed when this material was charged to 4.2 V.We used the in situ XRD technology to study the structural evolution of this cathode material within different voltage windows.The results showed that the formation of a large number of OP2 phases under high charging voltages might be one of the reasons for the irreversible phase transition and capacity decay.When reducing the charge cut-off voltage to 3.9 V,this material was able to cycle stably.In order to study the structural evolution process before and after cycling,O3-Na0.90Mg0.08Cu0.22Fe0.30Mn0.40O2 was tested by in situ XRD after 100 cycles.The results showed that the material can maintain a reversible phase transition between 03 and P3 even after 100 cycles.The comparison of in situ XRD spectra between Mg2+and Li+doped samples showed that the structural evolution of Na0.90Li0.05Cu0.22Fe0.30Mn0.43O2 during the initial charge and discharge cycle was similar to that of Na0.90Mg0.0sCu0.22Fe0.30Mn0.40O2,which was O3→O3+P3→P3→OP2→P3→O3+P3→O3.The phase transition was also basically reversible.These results revealed that Mg2+doping enhanced the suppression of OP2 phase formation effectively,and improved the reversibility of structural evolution. |
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