Design,Synthesis And Application Of Sodium Storage Function Oriented Iron-Based Phosphates Cathode Materials

Due to the advantages including abundant sodium resources and low cost,sodium-ion batteries（SIBs）are one of the most promising candidates for the large-scale energy storage application.Cathode material is an important component of SIBs,which has a decisive influence on the electrochemical performance.Among the various cathode materials for SIBs,iron-based phosphates have the the advantages of abundant resources,low cost,and good thermal and structural stability,thus holding high promise to become low-cost,high-safety,long-life and high-rate cathode material for SIBs.However,the low electronic conductivity of phosphates and the low electrochemical activity of some thermodynamically stable iron-based phosphates results in that the development of high-performance iron-based phosphate cathode materials still have many challenges.To solve these problems,we used amorphization,crystalline phase regulation,ion doping and carbon coating strategies to enhance the electrochemical performance of iron-based phosphates including NaFePO₄,KFe PO₄ and Na₄Fe₃（PO₄）₂（P₂O₇）,and analyzed the structure-performance correlation and performance optimization mechanism.Some significant research results were obtained and the detailed are summarized briefly as follows:（1）In order to address the low electrochemical activity of thermodynamic stable NaFePO₄ phase,we used high-energy ball-milling technology to realize the amorphization of maricite NaFePO₄/C composite,improving the electrochemical activity.By tunning the ball-milling time,a series of carbon coated nanocrystalline/amorphous NaFePO₄ composites with different degree of amorphization.Electrochemical performance test results indicate the positive relationship between the sodium storage capacity and amorphization degree of carbon coated nanocrystalline/amorphous NaFePO₄ composites.The optimized carbon coated nanocrystalline/amorphous NaFePO₄ composites（NFP-15）exhibit the sodium storage capacity of 115 m Ah g^-1 with capacity retention as high as 91.3%after 800cycles.In addition,based on the X-ray absorption near edge structure and Raman spectra,the structural model of amorphous NaFePO₄ was preposed,revealing the structural origin of the enhanced electrochemical activity of amorphous NaFePO₄.After amorphization,ordered chains of egde-sharingFeO₆ octahedra transfor into disordered chains of various egde-sharing or corner-sharingFeO_n polyhedral,which is the key to attain highly enhanced electrochemical activity.（2）A new metastable crystal phase of KFe PO₄ has been found,and the formation conditions of the metastable crystal phase of KFe PO₄ were investigated.The metastable crystal phase of KFe PO₄ could be obtained in the anealing temperature range of 500-700°C.The metastable phase will transform to thermodynamically stable monoclinic phase when anealing temperature reached or exceeded 700°C,but the kinetics of the phase transition is slow at 700°C.Moreover,the three-dimensional porous carbon network encapsulating metastable KFe PO₄ nanocrystalline composites ware successfully prepared by a simple sol-gel metal and following anealing process.As cathode mateials for SIBs,the composites displayed a reversible capacity of 105m Ah g^-1 and thecapacity retention of 67.2%after 5000 cycles at 1000 m A g^-1.The kinetics analysis results indicate that metastable KFe PO₄ possesses higher sodium-ion diffusivity compared to thermodynamically stable phase,which is responsible for the significantly improved electrochemical performance of metastable KFe PO₄/C composites.Morover,the large specific surface area and short ion diffusion distance of nanocrystals and the fast electronic transport network provided by three-dimensional porous carbon network are also farvorable for achieving excellent electrochemical performance.（3）The Mg-doped Na₄Fe₃（PO₄）₂（P₂O₇）/C composites with different doping content（0%,5%and 10%）were prepared by the sol-gel method and following anealing process.Among these composites,5%Mg-doped Na₄Fe₃（PO₄）₂（P₂O₇）/C composite（NFPP-Mg5%）displayed the best rate performance.Even at the high current density of 20 A g^-1,NFPP-Mg5%still delivered a capacity of about 40 m Ah g^-1.Meabwhile,NFPP-Mg5%also displays excellent cycling stability.After 10000cycles at 5 A g^-1,the capacity of NFPP-Mg5%remained at about 64 m Ah g^-1,corresponding to the capacity retention rate of 87.6%.As the best of our knowledge,that is the best cycling performance for Na₄Fe₃（PO₄）₂（P₂O₇）-based cathode materials.The kinetics analysis results demonstrate the higher sodium-ion diffusivity and lower interface charge transfor resistance of NFPP-Mg5%,which are responsible for the better rate performance of NFPP-Mg5%.It’s worth noting that NFPP-Mg5%at high mass loading still exhibits excellent electrochemical performance.At mass loading of24.5 mg cm^-2,an areal capacity of about 2.6 m Ah cm^-2 can be achieved.Besides,full sodium-ion battery based on NFPP-Mg5%cathode and hard carbon anode displays a dishcarge capacity of 85 m Ah g^-1 with average discharge voltage of 2.85 V at 500m A g^-1.These results demonstrate that NFPP-Mg5%is low-cost,high-safety,long-life and high-rate cathode material for SIBs with high application potential.