| Limited lithium resources and high price restrict the further application and development of lithium ion batteries in new energy vehicles and energy storage devices.Sodium-ion batteries,which are rich in resources and cheap in price and have similar working principle to lithium-ion batteries,have become a research hotspot in the battery field in recent years.Cathode material is one of the main factors restricting the development of sodium ion batteries.P2 cathode material has wide voltage window and stable structure.However,large polarization will occur under high voltage,leading to serious battery capacity attenuation.In this paper,P2-Na0.67Mn0.5Fe0.5O2 cathode material is taken as the research object,and the preparation and modification of the material are systematically studied.The specific research contents and results are as follows:P2-Na0.67Mn0.5Fe0.5O2 cathode material was synthesized from cheap Fe and Mn compounds by solid-phase method,and the synthesis conditions of the solid-phase method were investigated.Firstly,the effects of different manganese sources(MnO2,Mn2O3,Mn3O4)on the electrochemical properties of the materials were compared.It was found that the materials prepared with Mn3O4 had better cycling and rate capacities.Then,the influence of different synthesis temperatures(700-1000℃)on the electrochemical properties of the materials was investigated.It was found that the P2 layered structure with high crystallinity could be formed at 900℃,and the cycle retention rate was the highest.The effects of different cut-off voltages on P2-Na0.67Mn0.5Fe0.5O2 cathode materials were investigated.It is found that limiting the charge and discharge in the 2-4 V voltage range can significantly improve the cyclic performance and rate performance of the material.By analyzing the XRD and CV curves,it can be found that the high voltage range of charging to 4-4.3 V will cause irreversible phase transition,which increases the polarization of the material and significantly reduces the stability of the material,which is the main cause of capacity failure.P2-Na0.67Mn0.5Fe0.5O2 cathode material was synthesized by oxalate co-precipitation method,which can make the element mixture more uniform.The influence of pH value,precipitation temperature and feeding mode on the properties of the precursor and the final product was investigated.It was found that the particle size of the precursor prepared was small under the process conditions of pH=4,reaction temperature 30℃,and simultaneous feeding.The precipitation of Fe and Mn is close to the theoretical value.The cathode material prepared by oxalate coprecipitation method has better electrochemical performance,and the capacity retention rate is 60.7%after 50 cycles at 0.2 C rate,and its rate performance is also improved.To curb the material phase transition under the high pressure,the Na0.67Mn0.5Fe0.45Al0.05O2 cathode material was synthesized by co-precipitation method.The capacity retention is 64.7%after 100 cycles at 0.2C rate.Electrochemical analysis shows that aluminum doping can improve the cyclic stability of materials,increase the diffusion coefficient of Na+,improve the rate performance.By CV analysis,it can be found that aluminum doping can reduce the phase transition of the material at 4-4.3V,thus improving the stability of the material.The cathode material Na0.67Mn0.5+xFe0.5-2xCuxO2(x=0,0.05,0.1,0.2)was prepared by co-precipitation method.Cu can undergo redox reaction of Cu2+/Cu3+,which provides certain charge supplement for the material.In addition,according to CV and charge-discharge curves,it can be found that a small amount of Cu(x=0.05)can stabilize the phase transformation of the material at 4.2 V.With the increase of Cu content(x=0.1,0.2),the CV curve and charge-discharge curve change greatly,and the irreversible oxidation peak at 4.2 V is well inhibited.The structural stability of the material is significantly improved.Among the materials,Na0.67Mn0.6Fe0.3Cu0.1O2 has the best electrochemical performance,with the capacity of 103.6 mAh/g at 0.5C rate and 93.4 mAh/g after 100 cycles.The capacity retention rate reaches 90.15%.Meanwhile,the capacity is 74 mAh/g at 5 C rate.Including 41 figures,11 tables and 102 references... |
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