| Electrochemical energy storage technology is an important means to balance energy supply and consumption.Driven by the goal of "Carbon Neutral",its performance improvement is becoming more and more important.Due to the abundant raw materials and low cost,sodium ion batteries are expected to have a wider range of applications in the field of energy storage,but they have adverse effects such as(de)intercalation of sodium ions on structural damage.The use of rare earth elements with a larger ion radius to replace traditional transition metals can obtain a more stable materials structure and improve battery performance.In this paper,the sodium ion layered oxides with rare earth elements(Lu,Yb)as the key components were prepared and successfully applied to sodium ion batteries.The main research contents are as follows:(1)The pure-phase layered NaLuO2 materials were prepared by solid-phase synthesis,and it was characterized by XRD,SEM,XPS,etc.It was found that the synthesized material belongs to the space group R3m.The morphology of NaLuO2 is a regular polyhedron with a diameter of 1 μm,its crystallinity is good and its structure is complete.A sodium ion battery was assembled with NaLuO2 as the cathode,and its electrochemical performance was studied and analyzed.After research,it was found that every 20 charge-discharge cycles of NaLuO2 battery would regularly appear"capacity mutation" phenomenon.The specific capacity would increase suddenly(Charge specific capacity up to 1100 mAh·g-1/Discharge specific capacity up to 400 mAh·g-1),and the specific capacity of charging and discharging will drop rapidly in the several cycles.The specific capacity is 50 mAh·g-1 during stable charge-discharge cycles.The characterization found that before and after the mutation cycle,the element content in the plate changed significantly.It indicated that an irreversible reaction and deposition occurred on the counter electrode.After analysis,the sudden increase in specific capacity in this particular cycle comes from the partially reversible electrochemical reaction between the electrolyte and the material.The reversible reactions provide additional specific capacity,and the products of the irreversible reactions deposit and cause changes in the elements on the plate.(2)In order to further explore the application of rare earth elements in sodium ion batteries.NaYbO2 material was prepared by using Yb which is also a lanthanide element instead of Lu in the same process.The structure and morphology of NaYbO2 were similar to NaLuO2.NaYbO2 was used as the cathode of sodium ion battery.The research found that during the charge-discharge cycle,the specific capacity of NaYbO2 battery will increase with the increase of the number of cycles.Charge-discharge specific capacity increased from initial 60 mAh.g-1 to 70 mAh·g-1(60 cycles),and the coulombic efficiency of the battery can still be maintained at～99%during this process.After analysis,the increasement of specific capacity comes from the co-(de)intercalation behavior of cationic and anionic in the sodium ion battery during the cycle.During the electrochemical process,the energy storage is not only provided by the conventional cationic(Na+)(de)intercalation with the charge compensation of cationic reversible redox,but also achieved by the release and uptake of anions(ClO4-)accompanied by an anionic(O2-/O-)reversible redox.Furthermore,the results show that the anion-participating electrochemical process is a reversible process,The reversible process provides additional specific capacity for the battery.The cationic and anionic co-(de)intercalation mechanism could extend to more anions. |
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