Research Of Synthesis Process,Structure And Thermal Stability For NaNi_1/3Fe_1/3Mn_1/3O₂ Cathode Material Based On In Situ Synchrotron X-Ray Techniques

With the rapid development of new energy in recent years,the limited lithium resources and high price are the bottlenecks of lithium-ion batteries for large-scale application.We have to find and develop a new type of power source that is rich,high energy density,and can be rapidly industrialized to partially replace lithium-ion batteries.A sodium ion battery is the perfect candidate which meets all of the above requirements.Sodium is very abundant in the earth.In addition,sodium is located directly below the lithium element in the periodic table and has very similar chemical properties to lithium.This makes the operation mechanism of the sodium ion battery the same as that of the lithium ion battery,Therefore,the development and research of a commercially available sodium ion battery is of great strategic importance.Researchers have already seen the potential of sodium-ion batteries and have conducted long-term research and tracking.Although the properties of sodium-ion batteries are similar to lithium-ion batteries,there are still a lot of differences because the sodium ion is 25%larger than lithium ion.Such as Na hardly intercalates into the graphite while Li can work well,because the graphite is stressed when some Na intercalates into it and the Na-GICs are unstable.Also,the coordination of the transition metal and oxygen in the layered oxide cathode material is different.Therefore,we still need to conduct in-depth systematic research on sodium ion batteries,it will provide a theoretical basis for designing optimized materials and improving the electrochemical performance of sodium ion batteries.A cathode material is the key in a battery,which determines the energy density and electrochemical performance,layered transition metal cathode material is a promising candidate.Based on the background of the above,this work aims to choose a promising sodium ion battery cathode material as the object,investigate the synthesis process,working mechanism and the thermal stability by advanced research techniques,and develop a high capacity,long cycle life,excellent ratio of sodium ion battery cathode material based on the results of the study.The main research contents are as follows:The hydroxide precursor was used to prepare the O3 phase layered NaNi_1/3Fe_1/3Mn_1/3O₂ cathode material by solid state method.The in-situ XRD technique was used to investigate the phase transition and lattice parameters evolution during solid state reaction.It was found that the best calcination condition is 850?for 20 hours.Based on this result,we synthesized NaNi_1/3Fe_1/3Mn_1/3O₂ material and characterized the structure?morphology and electrochemical performance,which showed a high capacity of 136.4 mAh/g and a high 80%capacity retention after 600cycles in a full cell,in addition to the excellent rate performance with a capacity of 96mAh/g at a 5C high current discharge rate.In situ XRD and TXM-XANES technique was established to investigate the structural evolution of cathode materials for sodium-ion batteries.The structural evolution depends on the cutoff voltage.Reversible evolution of the O3-P3-P3-O3hexagonal phase occurs when the cutoff voltage is set to 4.0 V.When the cut-off voltage is set to 4.3 V,the O's monoclinic phase appeared after charging over 4.0V.TXM-XANES mapping shows a few mixed-phase zones,revealing the existence of a substitutional solid solution in Na-poor Na_xNi_1/3Fe_1/3Mn_1/3O₂.The Ni²⁺/Ni³⁺redox couple is active in the initial charge stage,both Ni and Fe are electrochemically active,and the redox couples of Ni³⁺/Ni⁴⁺and Fe³⁺/Fe⁴⁺are mainly present in the charge voltage range from 4.0 to 4.3 V.We have further used accelerating rate calorimetry,scanning electron microscopy,and operando synchrotron high-energy X-ray diffraction to investigate the thermal stability of charged Na_xNi_1/3Fe_1/3Mn_1/3O₂ cathode material at both cell and component level.It is found that the thermal decomposition of desodiated Na_xNi_1/3Fe_1/3Mn_1/3O₂ is a redox reaction that can be facilitated with the presence of either a reductive environment,such as electrolytes,or a strong oxidative environment that can result from a higher degree of desodiation.We also have used the in-situ coherent diffraction imaging technique to investigate the structure evolution at the single crystal level.Combine with normal in situ XRD technique,we found that large volume change during the cycle process and In situ coherent XRD showed the crystal cracked during the formation and get a biphase?O/P?materials after formation,this also a new discover which showed the difference between sodium ion batteries and lithium ion batteries.