Controllable Synthesis And Interface Regulation Of Prussian Blue Analogues As Cathode Materials For Sodium Ion Battery

Due to the abundant sodium resources and low cost,sodium ion battery(SIB)shows great application prospects in large-scale energy storage,low-speed transportation and5G base station construction.However,it is difficult to develop the suitable host material because of the large radius and molar mass of Na⁺ion.Fortunately,Prussian blue analogues(PBAs)become the valuable cathode materials for SIB due to their 3D open framework and large interstitial sites.However,PBAs are very sensitive to Fe(CN)₆vacancy and crystal water in the framework and lattice change during charge and discharge process.Thus,the results of practically electrochemical measurements show large deviation from theoretical prediction.This paper mainly focuses on the internal relation of PBAs crystal and interface structure and sodium storage performance.The structure stability of PBAs materials is modified by reducing crystal defects.Besides,the matching electrolyte modification and the reinforcement of PBAs interface stability are carried out to improve the electrochemical performance of PBAs.The main researches and results of this paper are as follows:In order to solve the problems of many Fe(CN)₆ vacancies and crystal water and low content of Na⁺ion in nickel-based Prussian blue analogue(NiHCF),low defects and Na-rich rhombohedral NiHCF(r-NiHCF)is synthesized by low-temperature controlled chemical co-precipitation method.The sodium storage performance and mechanism of r-NiHCF are studied.The r-NiHCF shows high capacities of 53 mAh g^-1 at 40 C and cyclability of 5000 and 10000 cycles at 2 and 10 C,respectively.The synchrotron radiation XAS and in-situ XRD techniques are used to analyze the local and long-range crystal structure evolution of r-NiHCF upon sodium ion migration.The crystal water migration during cycling and Na⁺ion migration path are studied by FTIR tests.The experimental results show that the obtained excellent performance of r-NiHCF is mainly related to the reduction of Fe(CN)₆ vacancies and crystal water and small change of ionic radius during low spin Fe ion redox.In order to solve the problems of low cyclability and rate capacity caused by the high content of crystal defects and large lattice change during cycling for iron-based Prussian blue analogues,high crystallinity iron-based Prussian blue analogue(HQ-MnCoNi-PB)with low content of Fe(CN)₆ vacancies and crystal water is prepared by combining technique of complexing agent and Mn,Co and Ni co-doping at 0?.The capacity retention of HQ-MnCoNi-PB is up to 81.1%after 500 cycles at 1 C,because of the low reaction rate after using complexing agent and crystal structure regulation by co-doping technique.The synchrotron radiation XAS and in-situ XRD measurements are used to analyze the local and long-range crystal structure evolution of HQ-MnCoNi-PB upon sodium ion migration.The crystal water migration is studied by FTIR tests.After 1 wt%Al Cl₃ is used as electrolyte additive,the capacity retention of HQ-MnCoNi-PB after 500cycles at 1 C is further increased to 92.2%,and even the long lifetime of 1500 cycles at1 C can be achieved.The experimental results show that the synergistic effect of Mn,Co and Ni co-doping can be well played to regulate the lattice to effectively improve the cycling stability.The crystal water is repeatedly extracted and inserted during charge and discharge process,and they can be captured by 1 wt%Al Cl₃ electrolyte additive to eliminate the negative effect of crystal water on electrochemical performance and obtain the further improvement of cycle lifetime for HQ-MnCoNi-PB.This research first proposes that the modification of PBAs electrochemical performance can be conducted by dealing with crystal water from the view of electrolyte.In order to solve the problems of easy oxidation of Fe²⁺,difficult preparation of Na-rich rhombohedral phase and its fuzzily fading mechanism,Na-rich rhombohedral iron-based PBA(r-PBA)is synthetized by optimizing process of the usage of inert atmosphere and reductant with the suppressed Fe²⁺oxidation.The capacities and rate capability of r-PBA material are excellent with the achievement of 119 and 75 mAh g^-1 at 1 and 10 C,as well as 85.5%capacity retention after 200 cycles at 1 C.The strategy of in-situ solidification of liquid electrolyte is used by adding 5wt%Al Cl₃ lewis acid to catalyze fluoroethylene carbonate(FEC)polymeration,in which can effectively reduce the interface side reactions between cathode and electrolyte to improve the interface stability of r-PBA.The resulting capacity,rate capability and cyclability can be significantly improved.The r-PBA can show 123 and 88 mAh g^-1 at 1 and 10 C,respectively,and deliver 116 mAh g^-1 at 2 C with an ultra-long lifetime of 4000 cycles.The experimental results show that the performance deterioration of r-PBA should be mainly dominated by the r-PBA interface stability.The strategy of in-situ solidification of liquid electrolyte to reinforce the r-PBA interface can well improve the cycling reversibility for rhombohedral material.Thus,this research also shows that the interface stability of Prussian blue analogues can play a vital role during long-term cycling.