| In recent years,due to the growing problem of the greenhouse effect and the depletion of natural resources,many countries have begun to vigorously develop renewable energy.Expensive lithium-ion batteries are not suitable for large-scale energy storage systems.However,sodium element with similar physical and chemical properties to lithium are widely distributed in the crust and the ocean.Therefore,the application of sodium ion batteries in large energy storage systems has begun to attract researchers’ attention.At present,the research on sodium ion batteries is still in the initial stage.Among them,transition metal layered oxides are the most important type of cathode materials in sodium ion batteries.These materials have the characteristics of high specific capacity,easy synthesis,and low price.However,they have the disadvantage of poor cycle stability.The tunnel phase Na0.44MnO2 has a unique tunnel crystal structure composed of MnO6 octahedron and MnO5 quadrangular pyramid.This crystal structure is very conducive to the insertion and extraction of Na+.And this large tunnel structure can also withstand the volume strain caused by structural changes during the charge and discharge process,so the tunnel phase Na0.44MnO2 has good cycle performance.The tunnel phase Na0.44MnO2 has a theoretical capacity of 121 mAh/g.However,due to the large particles synthesized at high temperature,the electrochemical activity is low.Generally,the reversible specific capacity is only 92 mAh/g and the rate performance is poor.Some researchers have synthesized nanowires or nanorods in the tunnel phase by methods such as high-temperature hydrothermal,thermal polymerization and electrostatic spinning to increase their specific capacity.These methods are too low for large-scale production,and the nanoparticle powder has a very low tap density,which greatly reduces the volume energy density of the batteries.Therefore,in this paper,Na0.5MnO2-xFx of F-doped tunnel-layered intergrowth structure was synthesized by a simple and large-scale solid-state sintering method,and its energy storage mechanism and electrochemical performance were investigated through a series of characterization methods.This has a reference role for the development of future sodium ion batteries cathode materials.The main research contents of this article are as follows:In this paper,pure tunnel phase NMO and NMO@F-x with different F doping amounts were prepared by a simple and large-scale solid-state sintering method.The paper analyzes the effects of different proportions of NaF content on the crystal structure,particle morphology and electrochemical properties of Na0.5MnO2-xFx in the precursor.The experimental results show that tunneling and layered symbiotic Na0.5MnO2-xFx can be generated at 20%NaF in the precursor.The Na0.5MnO2-xFx of the tunnel and layered symbiotic phase has a high reversible specific capacity,excellent rate performance and excellent cycle stability.It has a reversible specific capacity of 141.3 mAh/g at a current density of 0.5 C,and there is almost no capacity attenuation after 200 cycles.Under the high current density of 5 C,it has a reversible specific capacity of 120 mAh/g,and it still has 80%capacity retention rate after 800 cycles.In this paper,Na0.44MnO2-xFx with pure tunnel structure was also synthesized by the sol-gel method for anode materials of aqueous sodium ion batteries.The effect of different temperatures on the structure of Na0.4MnO2-xFx under the same F doping amount was studied.By controlling the sintering temperature to avoid the formation of layered sodium manganate,a pure tunnel phase Na0.44MnO2-xFx was obtained for aqueous sodium ion batteries.While greatly improving its rate performance,it can still maintain good cycle stability. |
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