Studies On Performance Improvement Of Sodium-Iron-Based Polyanionic Cathode Materials

Nanostructured sodium-iron-based polyanion compounds are promising in achieving the goal of low cost,high energy and power densities for sodium ion batteries,due to their ease of preparation and high working voltage.The specific crystal structures of these materials have exhibited three-dimensional sodium ion transport channels that effectively enhences the diffusion of sodium ion,thus reaction kinetics can be improved and high rate performance and long cycle life be achieved.However,owing to the poor conductivity and low coulombic efficiency of these materials,cycle stability is affected.Also due to the instability of ferrous ions in air,the sodium-iron-based polyanion materials are easy to be oxidized and susceptible to deterioration in the storage process,which can lead to decrase of electrochemical performance.To tackle the above problems,this work focuses on structural design and performance optimization of three sodium-iron-based polyanion materials by systematically characterizing their phase structure and measuring their electrochemical performance,the following mian results have been achieved.A new porous Na3.16Fe2.42(P2O7)2 with carbon nano fiber composite cathde material(NFP@C)has been studied.The sodium ferric pyrophosphate compound material was synthesized with improved morphological structure,thus high energy-density sodium storage performance was achieved.X-ray photoelectron spectroscopy(XPS)revealed that there was existence of chemical bondings between the carbon fiber substrate and bulk materials.The porous NFP@C shows better cycle and capacity performance than the bare NFP.In particular,the specific capacity of porous NFP@C at 0.1 C can reach 104.3 mAh g-1,and that at 20 C is 69.5 mAh g-1.After 1000 cycles at 5 C the capacity is still 74.6 mAh g-1,corresponding to a capacity retention of 85.3%.The results show that the improvement of the rate performance is related to the porous nano-structure of NFP@C which ensures the rapid diffusion of sodium ion,carbon fiber can improve the conductivity of the composite material.The possible side reactions between electrode surface and electrolyte are also effectively inhibited by the carbon coating,thus enhancing the long cycle performance of the electrode material.Since the sodium ferrous sulfate compounds possess characteristics of high working voltage as cathodes of sodium ion batteries,a novel stoichiometric sodium ferrous sulfate Na6Fe5(SO4)8(NFS)cathode material was prepared.The multi-channel NFS@C composite material was synthesized by adding different mass percentages of carbon nanotubes(CNTs).The emphasis was put on how the in-situ introduction of CNTs affects the electrochemical characteristics of NFS.The results of charge/discharge teast show that the composite(NFS@C)with 5wt.%carbon nano fiber demonstrates the best electrochemical performance.Its working voltage is as high as 3.7 V(vs.Na+/Na),and at 0.1 C the reversible initial specific capacity is 110.2 mAh g-1.After 1000 cycles at 2 C,the capacity still remains 87.1 mAh g-1,indicating excellent cycling stability.The improvement of the performance of NFS@5%C electrode is due to the three-dimensional network channels provided by CNTs for bulk materials.Charge transfer and ion conduction inside the particles of bulk materials is enhanced.In addition,it effectively inhibits the volume expansion and contraction of materials during the charging and discharging process,therefore structural stability of the materials is improved.A new type of fluorinated sodium ferrous sulfate Na3Fe2(SO4)3F(NFSF)electrode material was prepared by means of anion doping and modified by carbon nanofibers.According to the first principle calculation,sodium ions diffusion path in the crystals of the NFSF material is along the plane of a-axis and c-axis,and the lowest diffusion energy barrier is 0.5 eV.The charge/discharge test results indicate that the prepared NFSF@C material possesses a high working voltage of 3.8 V(vs.Na+/Na).The specific discharge capacity at 0.1 C is 102.1 mAh g-1,and after 1100 cycles at 2 C,the capacity still remains at 70 mAh g-1.It is believed that the addition of fluorine promotes a charge difference and increases working voltage of the material.In addition,NFSF nanoparticles are connected in series by the carbon nanofiber,which plays a role of charge transfer bridge,and effectively enhances the conductivity as well as the sodium ion diffusion rate,which also leads to improvement of the rate performance and long cycle life of the materials.Full-cell batteries were constructed and their electrochemical performance was evaluated in detail.Using hard carbon(HC)as anode and NFP@C,NFS@5%C,and NFSF@C as cathodes,full-cell sodium ion batteries were assembled.The charge and discharge mechanism of the batteries was explored and its electrochemical performance was studied.Through testings of the full cells,the electrochemical results show that the NFP@C//HC batteries have good cycle stability,the capacity remains stable after 150 cycles at 0.5 C,and its working voltage is 2.5 V(vs.Na+/Na).The NFS@C//HC cells show a high working voltage of 3.6 V(vs.Na+/Na),the sodium storage capacity at 0.1 C is 94.5 mAh g-1,the energy density is close to 350 Wh kg-1,even at a high rate of 2 C,its capacity is still 61.8 mAh g-1 after 1000 cycles,which shows a good cycle stability.For the NFSF@C//HC cell,its capacity retention rate is 80%after 1200 cycles.