Flexible Carbon Fibers Composite Electrode For Sodium Ion Batteries

Ever-growing energy storage and consumption have led to a global concern over sodium ion batteries(SIBs),which have been considered as an alternative rechargeable batteries of lithium ion batteries as the next generation power batteries,grid-scale energy storage systems,and energy storage devices for consumer electronics,for the reason of their low cost,potentially high electrochemical performance and competitive safety performance.The emergence of wearable and portable electronics has witnessed the rapid advancement of flexible energy-storage devices in the past decade,particularly the flexible SIBs due to their characteristic advantages.However,the development of SIBs is blocked by electrode materials with low electrochemical performance.Hence,it is vital to fabricate the flexible electrode with suitable potential window,large reversible specific capacity and superior structure stability.In this thesis,flexible SIBs have been deeply investigated in terms of preparation of electrode materials and fabrication of flexible electrodes and devices.At first,a flexible electrode with new and characteristic composite structure is present.In detail,the hollow NiS2 nanospheres are encapsulated inside the porous carbon fibers(NIS2(?)PCF).Typically,acid-treated carbon fibers were soaked in Ni(NO3)2 solution,then it was treateded by high-temperature reduction to get the NicPCF,and finally the NiS2(?)PCF was obtained after a sulfuration treatment.The structure,morphology,and chemical composition of the materials had been investigated,and electrochemical measurements had been performed as well in which the NiS2cPCF was used as the SIBs anode.The NiS2(?)cPCF electrode delivered an exceptionally electrochemical performance including high reversible discharge capacity(733 mAh g-1 at 0.1 C),good rate performance(42%capacity retention from 0.1 to 10 C),and excellent long-term cycling stability(76%retention over 5000 cycles at 5C).And these can be attributed to the advantage of the fantastic composite structure.In detail,three-dimensional(3D)porous(with multi-scale pores)and loosely stacked structure can not only accommodate the significant volume expansion but also prevent active mass from aggregating during cycling sodiation/desodiation process.Moreover,the unique architecture provides 3D conductive path and highly accessible surface area for electrons and electrolyte ions,respectively,which is critical important for high capacity and outstanding rate capability.Additionally,for comparison,NiS2 on surface of PCF(NiS2@PCF)electrode was prepared.After that,a cable-type flexible sodium battery was assembled to demonstrate the application of NiS2cPCF electrode in flexible electronics.The NiS2(?)cPCF anode is rounded by metal sodium with a flexible separator sandwiched in between and a heat-shrinkable tube is used to assembly encapsulate the device.Furthermore,the influence of bending test on the electrochemical performance of NiS2(?)cPCF based SIB was investigated.As a result,the excellent performance not only shows the huge potential of NiS2(?)PCF as anode for flexible SIBs but also offers a new material-design paradigm toward novel fiber electrodes embedded with active materials.