Preparation And Sodium Ion Storage Performance Study Of Manganese-Based Cathode Materials

Currently,mobile phones,laptops and so on have become an indispensable part of our daily life,whose application is inseparable from the support of lithium-ion batteries.However,limited lithium resources make it difficult for lithium ion batteries to meet the larger energy storage demand in the future.Compared with lithium,sodium is abundant and inexpensive.In addition,they are in the same main group and own many similar physical and chemical properties.Therefore,sodium ion battery is a promising alternative to lithium ion battery,which has attracted extensive attentions from researchers.Cathode materials play a crucial role in the development of sodium ion batteries.Manganese-based cathode materials is considered as a promising cathode material because of its high capacity,high working voltage,low cost,environmental friendliness and abundant resources.However,due to the large radius of the sodium ions and some defects of the manganese-based materials,there also exist some problems when manganese-based cathode materials applied in sodium ion batteries,such as insufficient electrochemical performance.These problems are even more pronounced especially at high voltages and high current densities.In this thesis,we systematically studied coating strategies,preparation process,structural characteristics and electrochemical properties of manganese-based cathode materials.?1?Firstly,we prepared Na0.44MnO2 submicron rods with unique 3D S-type tunnel structure by a traditional solid-state reaction method and analyzed the reasons of its poor electrochemical performance in a wide voltage range?2-4.5 V?.Then,we coated it with Al₂O₃ of different amounts.The results demonstrated that 2 wt%Al₂O₃-coated Na_0.44MnO₂ submicron rods cathode materials show the best electrochemical performance at high cut-off voltage?4.5 V?,which deliver an initial discharge capacity of 109.8 mAh g^-1 at 0.4 C and maintain 93.2%capacity retention after 200 cycles.Even at a high rate of 8 C,the high discharge capacity of 97.1 mAh g^-1 can be got,which is much better than that of pristine Na_0.44MnO₂ submicron rods cathode materials.These results indicate appropriate Al₂O₃ coating can improve the electrochemical performance of Na_0.44MnO₂ cathode materials in a wide voltage range?2-4.5 V?.?2?The hexagonal P2-type Na_0.7MnO_2.05 cathode materials have been prepared by a wet chemical method for the first time,and AlPO₄ of different amounts were coated on its surface.The results reveal that the 5 wt.%AlPO₄-coated Na_0.7MnO_2.05cathode materials show the most excellent sodium ion storage performance.At high current density of 1 A g^-1,it delivers discharge capacity of 103.9 mAh g^-1 and high discharge capacity of 96.0 mAh g^-1 also can be got at the 100^th cycle.The corresponding capacity retention is reach to 92.4%after 100 cycles,which is much better than that of pristine Na_0.7MnO_2.05?62.4%?.These data demonstrate that the appropriate AlPO₄ coating can dramatically improve the cycle stability of hexagonal P2-type Na_0.7MnO_2.05 cathode materials.?3?The Na_0.4MnO₂(Na₂Mn₅O₁₀)cathode materials with rectangular tunnel structural hve been synthesized by a sol-gel method.We studied its electrochemical performance in sodium ion batteries in different voltage ranges?2-4 V/2-4.5 V?for the first time.The results show that the Na_0.4MnO₂ cathode materials not only deliver a high initial discharge capacity of 110.3 mAh g^-1 but also own stable cycle performance?keeping 86.6%capacity retention after 100 cycles?at current density of0.1 C and a wider voltage range of 2-4.5 V.In addition,good rate performance(79.3mAh g^-1 at 4 C)also can be obtained.These data are better than that of the electrochemical performance in a voltage range of 2-4 V.Therefore,the Na_0.4MnO₂material is a promising cathode material that can withstand a higher cut-off voltage?4.5 V?.