Electrospun Anodes For Flexible Sodium-ion Batteries

As a unique and versatile method,electrospinning has been commonly used to produce one-dimensional(1D)fibers of polymers,metals,ceramics,and composites.These fibers have excellent flexibility and can be used for flexible sodium-ion batteries.Regarding sodium battery materials,alloying-type materials are ideal for sodium anodes because they can accommodate a large amount of sodium at low potential.However,the severe volume expansion of electrodes upon sodiation results in fracture and pulverization,leading to possible electrode instability and failure.In this thesis,we report on the construction of the flexible alloying-type anode materials via electrospinning,including the tin carbon nanofibers(Sn/CNFs)and antimony-bismuth alloy carbon nanofibers(Sb-Bi/CNFs).Their electrochemical performance for sodium storage have been examined.The main contents and conclusions are given as follows:(1)Using electrospinning technology,we have prepared self-supported flexible nanofiber mats,which can be further reduced to Sn/CNFs by subsequent high-temperature annealing.1D nanofiber structures feature a large surface area for the charge transfer reaction and straight paths for electron transport.In addition,the composite with carbon nanofiber structures can wrap the metal material inside the flexible carbon fibers,which greatly alleviates the volume change of Sn metal during the process of(de)sodiation.In this regard,the electrode material exhibits excellent electrochemical performance.When the Sn content is 15%,Sn/CNFs can retain a capacity of 308 mAh g-1 after 100 cycles at a current density of 0.5 C(here 1 C=847 mA g-1),which is-78%of the initial capacity.(2)Considering the synergistic effect of bimetals,we have synthesized Sb-Bi/CNFs flexible anodes via electrospinning.Sb and Bi share the same crystal phase and can form SbBi alloys at any ratio.Sb has a high theoretical specific capacity of 660 mAh g-1,while Bi has a relatively low swelling.In addition,the carbon layer acts as a buffer medium,effectively alleviating the huge volume expansion caused upon the electrode(de)alloying process.Sb1.5Bi/CNFs has shown outstanding sodium storage performance.At rates of 0.5 C,1 C,2 C,5 C,and 10 C,it can afford reversible capacities of 274,259,248,238,and 232 mAh g-1 respectively(here 1 C=500 mA g-1).