The Synthesis Of Iron-based Aerogel Cathode Material Based On Sodium Alginate And Its Sodium Storage Performance

With the continuous development of technology and society,due to the excellent electrochemical performance,lithium ion secondary batteries have been widely used in various fields such as portable electronic devices and electric vehicles.However,the lack of lithium reserves can not satisfy the demand of large-scale power storage.In addition,the lithium ion batteries also have high production costs and insufficient safety.So it is extremely urgent to develop new energy storage and conversion technologies.Therefore,sodium ion batteries（SIBs）,which have similar working principle with lithium ion batteries,have received widespread attention.Sodium and lithium belong to the same main group and have similar chemical properties.Moreover,sodium is abundant in reserves,low in cost and high in safety,indicating that sodium ion batteries are quite potential alternative to lithium ion batteries.At the present stage,compared to the well-developed anode materials,it is imperative to develop positive electrode material with rapid Na⁺intercalation/deintercalation kinetics and high specific capacity for SIBs.Among the reported SIBs cathode materials,iron-based polyanionic compounds have been extensively studied for their advantages,such as low cost,excellent thermal stability,high safety and environmental friendliness.These compounds have various crystal structures and provide open channels for Na⁺transmission.The series of tetrahedral or octahedral anion structural units in polyanionic compounds are connected by strong covalent bonds and form a three-dimensional network structure.The stable polyanion framework can enable the material to overcharge resistance and exhibit excellent cycling stability during charge/discharge process.However,the electron conductivity of the polyanionic cathode material is generally low,which results in poor charge/discharge properties at high current densities.Therefore,it is imperative to seek a simple and efficient method for effectively improving the electronic conductivity of iron-based polyanionic compounds.In this paper,a series of iron-based polyanionic compounds were prepared by simple ion-exchanged method,freeze-drying and carbonization processes using the unique structure of sodium alginate macromolecules.And we studied their properties as cathode materials for sodium ion batteries:（1）Na_3.12Fe_2.44（P₂O₇）₂/r-GO aerogel was synthesized using sodium alginate as a template and carbonization at 600℃in a molar ratio of Na:Fe:P=3.12:2.44:4.Iron ions collide with four fourα-L-guluronic acid segments in sodium alginate to form an“egg box”structure,and sodium ions are electrostatically immobilized by carboxyl inβ-D-mannuronic acid segment,which can effectively avoid surface oxidation reaction with H₂O and CO₂.The“egg box”structure is transformed into metal/carbon core shell structure after carbonization,and the carbon skeleton of sodium alginate is thermally pyrolyzed into three-dimensional porous aerogel.When evaluating it as cathode materials for SIBs,the reversible capacity was 116.1 mAh g^-1 at 0.1 C.When the current density is 20 C,the sample still has a reversible capacity of 72.3 mAh g^-1 after 5000 cycles,and the capacity retention rate is88.82%.（2）NaFePO₄/r-GO aerogel was synthesized using sodium alginate as a template and carbonization at 600℃in a molar ratio of Na:Fe:P=1.8:1:1.Scanning electron microscope images show that the NaFePO₄（NFP）nanoparticles are uniformly encapsulated in three-dimensional porous carbonaceous aerogel.When evaluating it as cathode materials for SIBs,the sample exhibits a reversible capacity of 134.8 mAh g^-1 at 0.1 C.When the current density was 1 C,the initial discharge capacity of NFP/r-GO is 115.6 mAh g^-1,and after 300cycles the reversible capacity is still 113.2 mAh g^-1,which the capacity retention rate is97.92%.At a current density of 10 C,the NFP/r-GO still have a reversible capacity of 79.7mAh g^-1 after 3000 cycles,and the capacity retention rate is 85.73%.（3）Na_2+2xFe_2-x（SO₄）₃/r-GO aerogel was synthesized using sodium alginate as a template and carbonization at 350℃in a molar ratio of Na:Fe:S=2:2:3.When evaluating it as cathode materials for SIBs,the sample exhibits a reversible capacity of 84.2 mAh g^-1 at 0.1C.When the current density is 1 C,The initial discharge capacity is 73.8 mAh g^-1,and the NFS/r-GO exhibits a reversible capacity of 63.3 mAh g^-1 after 200 cycles,and the capacity retention rate was 85.77%.