Study On Surface Modification Of Prussian Analogue Cathode Material And Its Sodium Storage Mechanism

The working principle of sodium ion battery is similar to that of lithium ion battery.Sodium ion battery have great application prospects in the field of large-scale energy storage because of the abundant sodium element in the earth and low cost.At present,developing high-performance cathode materials is the key to the further development of sodium ion batteries.As the most potential cathode material for sodium ion batteries,prussian blue analogue material（PBs）has an open three-dimensional skeleton and porous ion channels,allowing the rapid migration of sodium ions.However,a large number of vacancies in PBs will cause lattice distortion and collapse in the process of Na⁺deintercalation,and its combined water will have a side reaction with organic electrolyte,thus affecting the electrochemical stability of the material.In addition,the low conductivity is not conducive to the high rate performance of the material.In order to solve the above problems of PBs in this work,the high crystalline Na_xFe Fe（CN）₆material with excellent cycle stability was prepared by introducing ascorbic acid with strong reducibility in precipitation environment,and the material was surface modified by metal oxides and three-dimensional porous conductive carbon networks.The growth mechanism and sodium storage mechanism of PBs with high rate performance and long cycle life were explored.The specific research and conclusions are as follows:（1）Na_xFe Fe（CN）₆ material with high crystallinity was successfully prepared by coprecipitation method assisted by sodium citrate and ascorbic acid.On the basis of adding 15 g of sodium citrate and 6 mmol of ascorbic acid,Na_xFe Fe（CN）₆ shows the highest specific discharge capacity at 0.2 C.Its first specific discharge capacity can reach 112.7 m Ah g^-1.After 160 cycles,the reversible specific capacity is still 109.02m Ah g^-1 with the capacity retention of 96.73%.The capacity retention of Na_xFe Fe（CN）₆ material prepared with 9 mmol ascorbic acid after 160 cycles at 0.2 C is up to 100%.The addition of ascorbic acid obviously improves the cycle stability of the material,which may be due to the mixed growth of some ascorbic acid in the crystal,and the strong reducibility of ascorbic acid inhibits the electrochemical activity of partical Fe^2+/3+,so that some iron ions can play a certain supporting role and enhance the structural stability of the material during charging and discharging.（2）ZnO shells with different thicknesses were successfully coated on the surface of Na_xFe Fe（CN）₆ material by wet chemical method,which improved the electronic conductivity and electrochemical stability of the composite material.The Na_xFe Fe（CN）₆@ZnO material with 4 wt.%ZnO shows excellent electrochemical performance.The specific discharge capacity in the first cycle at 1 C is 105.2 m Ah g^-1,and the capacity retention after 300 cycles is 81.10%.The coupling of ZnO and Na_xFe Fe（CN）₆ improves the conductivity of the material and stimulates the electrochemical activity of low spin state Fe^2+/3+in the material.ZnO coating layer can also prevent the direct contact between Na_xFe Fe（CN）₆ material and electrolyte,inhibit the occurrence of side reactions on the surface of the material,and improve the electrochemical stability of the electrode.In addition,the ZnO shell does not participate in electrochemical reaction,but can play a role of mechanical support,effectively improving the cycle life of the battery.（3）Na_xFe Fe（CN）₆ particles were in-situ grown in the pores of three-dimensional porous N-doped conductive carbon networks（NC）by single iron-source method,and Na_xFe Fe（CN）₆/NC materials with high capacity and long cycle stability were obtained.The specific discharge capacity of Na_xFe Fe（CN）₆/NC at 0.2 C can reached 117.02 m Ah g^-1 in the first cycle.After 400 cycles,the reversible specific capacity is 94.49 m Ah g^-1 with the capacity retention of 81.12%.Its specific discharge capacity at 5 C is still80.04 m Ah g^-1.The graphitization degree of N-doped conductive carbon networks is high,which enhances the conductivity of the composite material.The three-dimensional porous structure of the carbon networks allows rapid ion transport and storage of electrolyte.Moreover,the ultra-thin and flexible carbon skeleton effectively buffers the volume expansion of active materials during cycling.