Preparation And Electrochemical Performance Of Iron-based Polyanionic Cathode Materials For Sodium Ion Batteries

Due to its cost and resource advantages,sodium ion batteries are expected to become a key technology for large-scale energy storage applications in the future.The large radius of sodium ions makes kinetic diffusion difficult.In recent years,people have made many attempts in material design and preparation methods.Considering that hard carbon anode materials have stable high specific capacity(>300 m Ahg^-1),but supporting cathode materials are still being explored,the research and development of cathode materials can be regarded as the focus of sodium ion battery development.From the perspective of price and performance,iron-based polyanionic materials will be the potential hope of sodium-ion batteries in the field of large-scale energy storage in the future.Phosphate materials have a 3D large frame structure,good thermal stability and small volume changes during cycling,so they have broad application prospects.The mixed iron-based phosphate Na₄Fe₃(PO₄)₂P₂O₇(denoted as NFPP)based on Fe²⁺/Fe³⁺redox pair shows a high reversible capacity of up to 129 m Ahg^-1 and a high average voltage of 3.2 V.In addition,it has economic and environmental advantages and can be used as a low-cost,non-toxic energy storage electrode material.NFPP with a three-dimensional frame structure faces the same problem of poor conductivity as other polyanionic materials.Na_3.12Fe_2.44(P₂O₇)₂,as a representative of non-stoichio-metric materials in a series of pyrophosphate compounds,has better thermal stability and moderate specific capacity(117.4 m Ahg^-1)than mixed pyrophosphate.However,the sensitivity to moisture and carbon dioxide will affect its cycle stability.In this paper,typical polyanionic iron-based sodium ion battery cathode materials Na₄Fe₃(PO₄)₂P₂O₇ and Na_3.12Fe_2.44(P₂O₇)₂ are the main research objects,and their shortcomings are studied.The specific research content of this article is as follows:(1)PVP-assisted electrospinning method to prepare ultra-small NFPP nanoparticles uniformly embedded in carbon nanoribbons as the electrochemical performance of sodium ion battery cathode materials.Using PVP-assisted electrospinning technology,a composite material with ultrafine NFPP nanoparticles uniformly embedded in carbon nanoribbons was prepared.This material has the advantages of multiple structures.1.Compared with the nanoparticles prepared by pure sol-gel method,the smaller particle size(about 60 nm)shortens the transmission time of Na ions,and the shape of the nanoribbon prevents the self-aggregation of nanoparticles and serious side reactions;2.The special nanoribbon structure not only has the special advantages of one-dimensional nanofibers,but the existence of carbon materials not only improves the intrinsic conductivity of the material,but also enables electrons to be transported between the nanoparticles in three-dimensional directions.3.The thickness of the nanoribbon is in the nanometer range,which facilitates the insertion and extraction of ions,and at the same time greatly shortens the ion diffusion distance of the internal active material.4.One-dimensional nanoribbons are interconnected to form a 3D network structure,which can further significantly improve the transmission of ions and electrons,and prevent NFPP nanoparticles from crushing and agglomeration due to the deintercalation process,thereby reducing the deterioration of contact with the current collector.These advantages improve the electrochemical performance of electrode materials.The specific capacity during charging and discharging at 25?of 0.1 C is 128.6 m Ahg^-1(theoretical capacity 1 C=128.9 m Ahg^-1),the specific capacity at 50 C is 61.2 m Ahg^-1,and the capacity retention rate after 5000 is 72%.At the same time,Na₄Fe₃(PO₄)₂P₂O₇ nanoribbon also shows excellent low-temperature sodium ion storage performance.At-15°C,the discharge capacity of Na₄Fe₃(PO₄)₂P₂O₇ nanoribbon at 0.05 C is 84.5 m Ahg^-1,and shows stable long-term cycle performance(after 700 cycles at 0.5 C,the capacity retention rate is 80.8%).(2)The electrochemical performance of AlF₃ coated Na_3.12Fe_2.44(P₂O₇)₂@CNT double modified sodium ion battery cathode composite material.Use ethylene glycol as the reverse polarity solvent titration method to prepare Na_3.12Fe_2.44(P₂O₇)₂@CNT nanoparticles to improve the conductivity of the material,and then target the nano-Na_3.12Fe_2.44(P₂O₇)₂@CNT pair Moisture(and CO₂)sensitivity leads to poor cycle performance.AlF₃ auxiliary coating is used to improve its cycle stability.Ethylene glycol is used as a reverse polarity solvent to effectively control the phenomenon that the particle size becomes larger or particles agglomerate as the solvent evaporates during the sol-gel synthesis process.Due to the high sensitivity of Na_3.12Fe_2.44(P₂O₇)₂ to moisture and carbon dioxide,AlF₃secondary coating was carried out.Through a series of experiments,the optimal coating amount of AlF₃ was explored,and the 1%AlF₃ coating showed the best battery performance.The initial discharge capacity at 0.1 C is 119 m Ahg^-1;it can still reach55.6 m Ahg^-1 after 5000 cycles at 20 C,and the capacity retention rate is65.9%.Especially in the performance test under high temperature environment(60?),after 6000 cycles at 50 C,the capacity still reached60 m Ahg^-1,the capacity retention rate was 70%,showing excellent electrochemical performance.