Preparation Of Highly Stable Superlattice O3-NaMn_1-xAl_xO₂ Cathode Materials And Their Sodium Storage Properties

In recent years,sodium-ion batteries have attracted wide attention of researchers,due to their rich resources,low cost,and similar energy mechanism to lithium-ion batteries.Electrode material is one of the main factors affecting its electrochemical performance.Compared with reported large-capacity,long-cycle performance anode materials,the research on cathode materials with large-capacity,high-rate performance and long-cycle life is the key to high-performance sodium ion batteries.Among many cathode materials,layered O3-NaMnO2 has the advantages of low cost,environmental-friendly,simple-preparation and large capacity.However,due to the existence of Mn3+ Jahn-Teller effect resulting in the lattice distortion,the sensitivity for moisture in the air,and the solubility of manganese ions into the electrolyte,this kind of material exhibits poor cycle life and rate performance.Aiming at the above defects,this paper has been focused on two strategies:the construction of superlattice structure and surface coated by metal oxide,respectively,to modify the O3-NaMnO2 cathode material to remit the lattice deformation of the material and obtain a stable crystal structure,and achieve the improvement of the rate performance and cycle stability.This thesis includes four chapters.The first chapter is the introduction.It systematically introduces the development process,working principle,classification composition and development tendency of sodium-ion batteries,discusses the structure,properties and research progress of layered manganese-based oxide materials,and introduces two strategies to construct the superlattice ordered structure and the formation of coated structure to improve the performance of O3-NaMnO2.At the same time,the topic,research contents,and innovations of the paper have been proposed.Chapters 2 and 3 are experimental parts,which introduce the influence of the two methods:the low-cost and light-weight Al3+ instead of Mn3+ to form a superlattice O3-NaMn1-xAlxO2,and O3-NaMn1-xAlxO2 surface-coated by Al2O3.Chapter 4 summarizes the full text.The main contents are as follows:(1)A highly crystalline and pure phase O3-NaMn1-xAlxO2 material with a superlattice ordered structure were prepared by using one-step solid-phase calcination(at 900? for 15 h under an Ar atmosphere).Through adjusting the atmosphere,temperature and time during the reaction,the optimal experimental conditions for the preparation were confirmed;Through adjusting the ratio of manganese/aluminum in the system,the effect of different aluminum element contents on the crystal structure and morphology had been investigated.The results showed that under the optimized preparation condition,the different-ratios aluminum ions intercalated into the material lattice to form a pure phase material.Considering NaMn0.6Al0.4O2 as an example,the results of structure and morphology characterizations showed that the prepared material was micron-sized particles,and the manganese/aluminum elements were alternately arranged to form a superlattice ordered structure.Then,the sodium-ion batteries have been assembled by employing the 03-NaMn1-xAlxO2,sodium and NaPF6(98%PC:2%FEC)as the cathode,anode and electrolyte for subsequent electrochemical measurements.The results have indicated that when the aluminum content was 40%,the material NaMn0.6Al0.4O2 had the best electrochemical performance.NaMn0.6Al0.4O2 cathode delivered a specific discharge capacity of 160 mAh g-1(at a current density of 20 mA g-1),which was about 98.5%of the theoretical specific capacity.When the current density increased to 1000 mA g-1,the discharge capacity was still 88 mAh g-1,which indicated that the material had excellent rate performance;After 50 cycles at a current density of 500 mA g-1,the capacity retention rate reached 82.4%,which was higher than that of O3-NaMnO2 without a superlattice ordered structure,increased by about 40%.The construction of the layered superlattice with ordered structure provides a new idea for achieving a layered cathode material for sodium-ion batteries with large rate capacity and long cycle life.(2)In order to improve the engineering applications,based on the optimized superlattice NaMn0.6Al0.4O2 material,the Al2O3 coated NaMn0.6AI0.4O2 material was prepared by the stepped ball milling and the subsequent calcination.Through adjusting the amount of precursor nano-Al2O3,the thickness of the coated layer on surface was controlled.The results showed that the material NaMn0.6Al0.4O2@Al2O3(3%)with a thickness of 5-10 nm was prepared when the amount of nano-Al2O3 was 3%.By testing the electrochemical performance of samples with different coated thicknesses,it was concluded that the NaMn0.6Al0.4O2@Al2O3(3%)cathode exhibited the best electrochemical performance at the current density of 20 mA g-1,the specific capacity reached 143 mAh g-1,which was about 90.5%of the theoretical specific capacity;when the current density was increased to 1000 mA g-1,it still maintained the specific capacity of 89 mAh g-1,indicating that the material had excellent rate performance;After 250 cycles at a current density of 500 mA g-1,the capacity retention rate reached 80.8%,which increased 20%more than that of an uncoated superlattice cathode.The above research shows that the surface coating of O3-NaMn0.6Al0.4O2 with inherent-light metal oxide reduces the sensitivity to moisture and the solubility in the electrolyte,further reduces the cost,and provides a new strategy for constructing a low cost and high stability sodium-ion battery cathode materials.