Preparation And Sodium Ion Storage Properties Of Iron-Based Polyanionic Cathode Materials

In order to achieve the strategic goal of " carbon peak,carbon neutrality " as soon as possible,it is imperative to build electrochemical energy storage devices with new energy as the main body.Lithium-ion batteries,as one of the most mature energy storage systems since its commercial application,have successfully changed the way we live,learn and work.However,the limited and unevenly distributed lithium resources along with huge demands have led to a sharp rise in price.Sodium-ion batteries are regarded as one of the ideal supplements to the next generation of largescale energy storage technology due to the abundant resources,low cost and similar working principle to lithium-ion batteries.In recent years,iron-based polyanionic cathode materials have attracted more and more attention from researchers.In addition to the advantages of low cost,this kind of materials has excellent electrochemical performance due to its stable structure and small volume change,which is expected to be applied in the field of large-scale energy storage.Among them,the iron-based mixed polyanion compound Na4Fe3(PO4)2P2O7(hereinafter referred to as NFPP)has the advantages of long cycle life,three-electron transfer,high operating voltage and theoretical specific capacity due to the stable framework structure of(PO4)3-and the induction effect of(P2O7)4-.However,the material still has the following issues: On the one hand,in the previous reports,there is always maricite-type Na Fe PO4 impurity in NFPP materials,which limited the diffusion of sodium ions,and seriously affects the rate performance of NFPP.On the other hand,the energy density of battery based on NFPP material is low,which seriously restricts their commercial application prospects.Therefore,it is urgent to develop novel highperformance iron-based polyanionic cathode materials and build high-energy density sodium-ion batteries to meet the needs for large-scale energy storage.In order to overcome the above issues,firstly,we carry out the structure design of the material,and pure phase NFPP cathode material was synthesized successfully by sol-gel method,realizing the breakthrough of electrochemical performance on the material level.And the relationship between material structure and performance is studied in depth.Then,based on thick electrode design,we obtained a thick electrode film with high areal capacity,which improved the energy density of the battery,and further proved the application prospect of the new cathode material in the field of largescale energy storage.The main research contents are listed as follows:In order to solve the issue of the existence of Na Fe PO4 impurity in NFPP materials,we learnt from the molecular configuration design concept of the lithium-rich material x Li2 Mn O3·(1-x)Li Mn O2 to regulate the ratio of Na Fe PO4 to Na2 Fe P2O7,and successfully obtained the pure phase material Na3.4Fe2.4(PO4)1.4P2O7.Benefiting from the elimination of the negative effects of the impure phase,the kinetic performance of the material has been greatly improved,and thus obtains impressive electrochemical performance.The capacity retention of Na3.4Fe2.4(PO4)1.4P2O7@C composite is higher than 90 % after 10000 cycles at a high current density of 20 C.Subsequently,we use in-situ XRD and Synchrotron-based XANES to explore the structural evolution and charge compensation mechanism of the reaction process,and further reveal the relationship between the structure and electrochemical performance.Finally,we evaluate the low temperature performance of the material.At-20 °C,the specific capacity can still reach 83.0 m Ah g-1,which further proves the excellent structural stability and electrochemical performance of the material.Then,in order to improve the energy density of sodium ion batteries,we prepared“NFPP/carbon nanotube” thick electrode film by combining NFPP material with carbon nanotube conductive adhesive.When the mass loading is increased to 40.1 mg cm-2,the thickness can reach 511 μm,and the areal capacity is increased to 3.72 m Ah cm-2.By exploring its electrochemical performance and diffusion kinetic mechanism,we find that a unique 3D porous network structure is formed between NFPP material and carbon nanotubes,which makes the thick electrode highly conductive,thus obtain excellent electrochemical performance.In addition,we use NFPP material as the cathode and hard carbon as the anode to construct the “NFPP @C//Hard Carbon” full battery,further demonstrated the potential of NFPP cathode materials in practical applications.