Manganese-Based Oxide As Electrode Material For Sodium Ion Battery

With the rapid development of economy and huge consumption of energy,the development and utilization of clean energy is imminent.The large-scale application of clean energy is inseparable from energy storage equipment,so research on energy storage technology with low cost,long life and high energy density becomes extremely important.At present,lithium-ion batteries are widely used in electronic products,electric vehicles and energy storage due to their advantages of high energy density,great safety performance and long cycle life.However,lithium resources are distributed unevenly in the world and low reserved in the earth’s crust.With the growing of demand,the costs are rising rapidly.In this context,Na-ion batteries have attracted extensive attention due to their advantages of abundant resources and low cost,especially in the field of large-scale energy storage.The development of a new Na-ion battery to meet the energy storage requirements has become a hot research topic.In this thesis,a series of investigations is performed on electrode materials of sodium ion batteries:(1)The specific capacity of the conventional layered positive electrode is limited by the content of transition metal with variable valence,while the anionic redox reaction provides the possibility to improve the specific capacity of the positive electrode because of the synergistic redox reaction based on transition metal and anionic oxygen.Na-Li-O、Na-□-O、Na-Na-O、Na-Mg-O、Na-Zn-O have been reported to trigger oxygen redox.The Na-Li-O system can provide up to 270 m Ah g-1 specific capacity,but there is little evaluation of its application in full cells.In this thesis,the effect of Zn content on the electrochemical performance of Na0.67ZnxMn1-xO2(x=0.1,0.2,0.28,0.34)was investigated.It was found that the layered oxide material Na0.67ZnxMn1-xO2(x=0.1,0.2,0.28,0.34)can be used as both positive and negative electrodes.Na0.67Zn0.28Mn0.72O2 showed excellent electrochemical performance and provided a reversible specific capacity of up to 170 m Ah g-1 at the charge-discharge rate of 0.05C(10 m A g-1)when used as a positive electrode.The capacity retention rate of 77.2% was still achieved after 100 cycles.As a negative electrode,Na0.67Zn0.28Mn0.72O2 provides a reversible specific capacity of 76 m Ah g-1 at the rate of 1C.Na0.67Zn0.28Mn0.72O2 also displays excellent capacity retention rate,which can still remain 95% of the initial specific capacity after 100 cycles.Furthermore,we select Na0.67Zn0.28Mn0.72O2 as both positive and negative electrodes to assemble the symmetric full cell utilizing the lattice oxygen redox and Mn4+/Mn3+ redox.The full cell shows good rate performance and cycle stability.The reversible capacity at 0.1C is 86 m Ah g-1,and the capacity retention rate is 75% after 50 cycles.(2)The effect of surface coating on the properties of the P2-Na0.67[Ni0.33Mn0.67] O2 positive electrode was also investigated.P2-Na0.67[Ni0.33Mn0.67] O2 delivers a high specific capacity during the first charge process,which is quite different from most P2-type cathodes,and is considered as a potential cathode material for application.However,when charged to 4.2 V,the reversibility becomes worse and cycle performance deteriorates.In this paper,the P2-Na0.67[Ni0.33Mn0.67] O2 was coated with a layer of Li2CO3 to optimize its electrochemical performance.By regulating the type of precursor,proportion and coating conditions,it was found that: When coated with 20% Li2CO3,P2-Na0.67[Ni0.33Mn0.67] O2 has the best electrochemical performance.Compared with the uncoated original material,the capacity retention rate increases from 47% to 77.5% after 100 cycles.In addition,no obvious phase transition was observed during the first charge and discharge process at the voltage range of 2.5-4.3 V,and the P2 phase maintained very well.