Synthesis And Electrochemical Performance Of Na₂Fe_1-xMn_xPO₄F For Sodium-ion Batteries Cathodes

To meet the demand for large-scale energy storage systems such as electric vehicles and smart grid applications,new generation of secondary batteries with low price and high efficiency is urgently required.Sodium-ion battery(SIB)is the most promising candidate due to its abundant raw materials,environmental-friendly and comparable performance in comparison to Li-ion battery.Currently,the large-scale application of SIBs still lacks the key electrode materials with high specific capacities,superior rate capabilities,and prominent cycling stabilities.In particular,the search of high-performance cathode has been long stagnated.Polyanionic fluorophosphates Na2MPO4F(M=Mn,Fe),a kind of novel cathode materials with high capacities and high safety,have received great interests due to their highly competitive structural advantages and predictable application prospects.Given the poor electrochemical activity of Na2MnPO4F and the low electrode potential of Na2FePO4F,in this paper,the research works focus mainly on the modification of materials with a synergetic strategy of nanostructure design,conductive decoration,and elecment doping.With these efforts,it is expected to provide valuable experimental and theoretical support for the development and application of high-performance SIBs.Firstly,the best synthesis process of highly electrochemical acitivity Na2MnPO4F/C composite was explored by spray-drying technique to overcome the preparation issue and poor reactivity facing Na2MnPO4F electrode material.Meanwhile,the relationship among the structure,morphology,and electrochemical performance of the desired composite was effectively constructed by combining physical characterization and electrochemical testing.It was revealed that Na2MnPO4F/C composite prepared at 650℃ for 6 h exhibited the highest Na+ diffusion coefficient of about 6.35 × 10-16 cm2/s over other samples,and delivered an intial discharge capacity of 52.1 mAh/g with Coulombic efficiency of 72.5%at 0.05C.This is due to the fact that the synthesis conditions are conducive to ensuring the purity and crystallinity of the target phase;On the one hand,the experimental conditions can help to limit the size of a single particle and prevent the over growth and agglomeration of products.On the other hand,the biggest feature of spray drying is that the raw materials are mixed evenly,so the heat treatment temperature is low.The pure phase product can be obtained at relatively low temperature,which avoids the impurity of target product decomposition when sintering at high temperature.The test results indicate that the spray drying technology is a simple,rapid and effective method for the preparation of high activity Na2MnPO4F electrode materials.Secondly,the Na2FePO4F/C composite with high rate capability and long cycling performance was designed by spray-drying technique with the purpose of addressing the poor electroconductivity and low material utilization facing Na2FePO4F cathode.Electrochemical tests showed that the optimum synthesis product displayed a high reversible capacity of 96 mAh/g at 0.1C,and meanwhile,delivered an intial discharge capacity of 67.4 mAh/g with capacity retention of 85.2%after 50 cycles at 1.0C.Furthermore,on the basis of the kinetic analysis,the cycling performance degradation of the desired Na2FePO4F/C composite can be attributed to the soar of ohmic and charge transfer resistance and the dramatic drop of Na+ diffusion coefficient upon cycling,which provided an important basis and theoretical support for the structure design and performance optimization of SIB cathode materials.Particularly,this work implies that the spray drying method is expected to simultaneously solve the disadvantages of Na2MnPO4F and Na2FePO4F electrode materials.Thirdly,a series of fluorinated polyanionic compounds Na2Fe1-xMnxPO4F/C(0<x<1)were prepared by spray-drying technique based on the synergetic strategy of nanostructure design,conductive modification,and element doping,and hereby,the relationship among x value,phase structure,and electrochemical performance of the obtained composites was clarified in detail.It turned out that the voltage polarization of the resulting composites with high Mn content is larger than those of the as-prepared materials with high Fe content.Particularly,the desired electrode of Na2Fe0.6Mn0.4PO4F/C showed a modest operating voltage,and meanwhile,displayed the lowest charge transfer resistance(242.2 Ω)and the highest Na+ diffusion coefficient(1.13 × 10-15 cm2/s),suggesting the superior rate capability and cycling stability.Furthermore,the results of EIS recorded from Na2Fe0.6Mn0.4PO4F/C electrode during cycling revealed that the reduction of its charge transfer resistence in high charge state and/or low discharge state was expected to further promote the electrochemical performance.More importantly,this work reveals that the spray drying technology can effectively achieve the composite modification of electrode materials,which provides a new strategy and/or idea for the development of polyanionic fluorophosphates cathode materials.Finally,Na2Fe0.6Mn0.4PO4F was taken as an example,to be clarified the regulation rules and effects of various carbon sources,including oxalic acid,ascorbic acid,citric acid,and glucose,on the phase structure,morphology,and electrochemical performance of micro-nanostructured Na2Fe0.6Mn0.4PO4F/C composites via spray-drying technique,based on the influence of carbon sources on the synthesis and performance of polyanionic compounds.As a result,the Na2Fe0.6Mn0.4PO4F/C electrode prepared with ascorbic acid as carbon source showed the lowest charge transfer resistance(188.6 Ω)and the highest Na+diffusion coefficient(2.68 × 10-15 cm2/s).Meanwhile,it delivered a reversible capacity of 95.1 and 48.1 mAh/g at 0.05 and 1.0C,respectively,and maintained a high capacity of 55 mAh/g with capacity retention of 91.7%after 100 cycles at 0.5C.Undoubtedly,these results show that the optimization of carbon source can significantly improve the electrochemical performance of nanocomposites,and the resulting product is expected to be a promising SIB cathode material with high energy/power density.