Preparation Of Flexible Polyacrylonitrile-based Carbon Nanofiber Membranes For Sodium Storage

The development of wearable devices will provide us with a more convenient life experience.In order to meet the flexibility requirements when the device is bending,it is necessary to design a flexible energy storage device that matches it.The advantages of sodium mine reserves and cost make sodium ion batteries one of the candidates for flexible energy storage equipment.As the indispensable part of a sodium ion battery(SIB),the development of anode materials has attracted more attention.Among them,the hard carbon material is advantageous for sodium ion storage due to its wide graphite spacing and turbostratic layer stacking.However,the low density and low surface loadinglimit the performance of sodium ion batteries in portable devices.Moreover,the powdered carbon anode materials mixed with binders cannot afford bending and folding strength.In the current research of hard carbon anode materials,all above problems remain to be solved.In addition,the study of sodium storage mechanism can guide the design of flexible anode materials to meet the performance requirements.In this paper,a series of flexible carbon nanofiber membranes were prepared from polyacrylonitrile with different fillers by using electrospinning technology.In the first part,the melamine was selected as the nitrogen-rich precursor filler and the carbonization process was adjusted to prepare ultra-flexible carbon nanofibers membranes with high nitrogen content(15 a.t%)and crimped carbon nanofibers morphology.The doping of heterogeneous elements and their effect on sodium storage performance were explored.As the disorder degree of hard carbon increases,the behavior of sodium storage is dominated by adsorption and insertion mechanism.The electrochemical performance tested under 1 A g-1 showed excellent long cycle stability,.Even the areal density reaches 8 mg cm-2,the material still had an areal capacity of 0.82 mAh cm-2.These excellent properties are due to the highly disordered carbonaceous structure in the carbon membrane and the rich active sites provided by the high pyrrole nitrogen content in the carbon skeleton.In the second part,different pore-forming agents were chosen to design flexible carbon nanofiber membranes with hieratical pore structure,and the effect of pore structure on sodium storage performance was compared.Wen PMMA was selected as the pore-forming agent,the carbon nanofibers exhibit a hieratical pore structure with tow-like cross-linking lap morphology.The pore size distributions shows the micropores around 0.6 nm and 1.2 nm and the mesopores around 15 nm,which providing a specific surface area up to 522.97m2 g-1.The tubular mesoporous channel shortens the ion transmission path,while the micropores provide more desorption sites for sodium ions.The unique multi-level pore structure allows the material to obtain high rate performance and cycle performance.When tested at 1 A g-1,the carbon anode revealed almost 100%capacity retention rate,and still had a specific capacity of 120 mAh g-1 after 1200 cycles.