The Synthesis And Performance Research Of Na_2/3Fe_1/2Mn_1/2O₂ Cathode Material For Sodium Ion Batteries

With the wide application of Li ion battery,the storage of lithium resources will not meet the future application of Li ion battery in large-scale energy storage devices.Sodium and lithium are similar in nature to the main group and sodium resources are abundant.Therefore,sodium ion battery is likely to become one of the next generation of energy storage battery system.The performance of the positive electrode material directly determines the performance of the sodium ion battery.At present,there are many positive electrode materials such as excessive metal oxides,polyanionic materials,and fluorides.Among them,Na_2/3Fe_1/2Mn_1/2O₂ cathode material has become one of the most promising materials because of its high specific capacity,low cost and high safety.However,there are relatively few studies on this material.The specific capacity of Na_2/3Fe_1/2Mn_1/2O₂ in current published literature is below 200 mAh·g^-1,which is far from the theoretical capacity of 260mAh·g^-1.In this study,the P2-type Na_2/3Fe_1/2Mn_1/2O₂ cathode material is taken as the object of study.Different preparation methods are used to gradually refine the grain of the material to improve its electrochemical performance,and the effects of different calcination temperature and time on the structure,morphology and electrochemical performance of the material are studied.In this paper,firstly P2-type Na_2/3Fe_1/2Mn_1/2O₂ cathode material is synthesized by a modified solid-state method with organic metal salts as raw materials.XRD studies show that the material is a pure phase below 800°C.When the temperature is higher than 800°C,a part of Na ions are lost and the Iwakiite phase appears.Furthermore,a regular hexagon plate shape for the uniformly distributed particles is revealed by SEM results.The mean size and thickness of the hexagon plates are about 1-4 mm and 200-700 nm,respectively.When the temperature is over 800°C,the particles are agglomerated and the hexagon disappeared.The optimal synthesis condition for the material is calcination at 800°C for 8 h.The electrochemical performance under this condition is the best,compared with the traditional mechanical activation of the material prepared by the calcination temperature decreased to100?.When the charge-discharge voltage range is 1.5-4.3 V,the initial discharge specific capacities of the samples at the 0.1 C and 2 C rates are 230.0 mAh·g^-1and 111.9 mAh·g^-1,respectively.P2 type Na_2/3Fe_1/2Mn_1/2O₂ cathode materials are prepared by spray drying method.The use of liquid phase makes the composition uniformity of synthetic materials better.XRD studies show that the calcination temperature and time have little effect on the purity of the material,and only a small loss of Na ions occurs at 800°C,resulting in a Iwakiite phase.Through SEM studies,it is found that the prepared material is a spherical particle with a diameter of about 5?m composed of primary particles of about 500 nm.Due to the fact that the water absorption of nitrate is strong,no spherical particles are prepared.Samples calcined at 700°C for 8 h have the best electrochemical performance,compared with mechanical activation method of calcining temperature and reduce above 100?.When the charge-discharge voltage range is 1.5-4.3 V,the initial discharge specific capacities of the sample at the 0.1 C and 2 C rates are 217.9 mAh·g^-1and 117.4 mAh·g^-1,respectively.And the material has excellent cycle performance under various magnification conditions,the capacity retention rate is close to 100%.In order to further refine the crystal grains,nano-scale filamentous P2Na_2/3Fe_1/2Mn_1/2O₂ cathode materials are prepared by electrospinning.The XRD results show that the calcined material composition is relatively uniform and crystallized well.When the temperature exceeds 800°C,Iwakiite phase appears,which is consistent with the XRD results of the sample parpered by the mechanical activation method.The TEM and SEM studies revealed that the filamentous fiber with a diameter of 100-200 nm consisting of small hexagonal particles with a diameter of about 50 nm.When the temperature rises to 900°C or the calcination time reaches 10 h,the filamentary mechanism disappears.The sample obtained by calcining at 800°C for 6 h has the most excellent electrochemical performance.When the charge-discharge voltage range is 1.5-4.3 V,the initial discharge specific capacities of the samples at 0.1 C and 2 C rates are 220.9 mAh·g^-1and 138.3 mAh·g^-1,respectively.Although the initial discharge specific capacity of the sample has decreased,its nanoscale particle size gives the material a good rate performance.The exchange current density is an order of magnitude higher(10^-4mA·cm^-2)than other methods.