Effect Of Cationic Fe-site Substitution On The Electrochemical Performance Of Na₂FePO₄F Cathode Material

Due to the abundance and low cost of sodium,sodium-ion batteries have attracted more and more attention in large-scale applications.At present,the primary task is to explore a new generation of cathode materials with excellent performance.Na2FePO4F is the preferred cathode material for sodium-ion batteries because of its stable crystal structure and outstanding electrochemical performance,as well as its two-dimensional Na ion diffusion channels.However,the low ionic mobility and low conductivity of polyanions lead to the decrease of their lifetime and discharge capacity at high current.Therefore,the poor electrochemical properties of polyanions are the main reasons for the stagnation of cathode materials for sodium-ion batteries.These factors also limit the practical application of materials.In order to improve the performance of Na2FePO4F,the methods of surface coating,ion doping and material nanocrystallization can be used.In this thesis,glucose,citric acid and ascorbic acid were used as carbon coating.After comparing the electrochemical performance,ascorbic acid was used as carbon source.On the basis of the preparation of Na2FePO4F/C,the Fe-site substitution was carried out by traditional solid-state reaction.The Na2Fe1-xMgxPO4F/C,Na2Fe1-xCoxPO4F/C and Na2Fe1-xNixPO4F/C cathode materials are successfully prepared,It was observed that the cycle life and discharge capacity at high current were significantly improved after doping Mg,Co and Ni,especially for Na2Fe1-xMgxPO4F/C,but the electrochemical performance of Na2Fe1-xMgxPO4F/C could not be fully optimized.Therefore,Na2Fe1-x-yMgxNiyPO4F/C cathode materials were prepared on the basis of 15%Mg doping.The structure of the material was stabilized and the conductivity was improved by doping Mg and Ni elements in the Fe site.The electrochemical performance of the material was verified by the volt-ampere characteristic test and charge-discharge test.Compared with Na2FePO4F/C,in the case of Na2Fe0.85Mg0.15PO4F/C is further doped with Ni,Na2Fe1-x-yMgxNiyPO4F/C shows a stable discharge ability,and Na2Fe0.8Mg0.15Ni0.05PO4F/C can still maintain 91.3%of the initial specific capacity after 50 cycles at the rate of 0.1 C.At the same time,the multiplying power is greatly improved.Optimized Na2Fe0.8Mg0.15Ni0.05PO4F/C sample can deliver a specific discharge capacity of 53 mAh/g at 5 C rate.As a low-cost cathode material,this study provides more possibilities for the development of sodium-ion batteries.