Preparation Of Composite Micro-Nanofibers By Electrospinning And Their Application In Lithium-sulfur Batteries

Lithium-ion batteries have been widely used in many areas over the past decades,such as portable electronics,electric vehicles,and communications equipment.However,due to the low theoretical specific capacity,lithium-ion batteries cannot meet the requirements of today’s large-scale energy storage systems and electric vehicles.Therefore,high specific energy lithium-ion batteries have become the development trend of energy storage technologies in the future.Lithium-sulfur（Li-S）batteries are considered as one of the promising candidates for the next generation of high energy density energy because of their high theoretical specific capacity(1675mAh g^-1),high theoretical energy density(2600 Wh kg^-1),low cost and nontoxicity.However,Li-S batteries still face some challenges before they can be commercialized:（1）the poor electron and ion conductivity of nature sulfur and its Li₂S₂ and Li₂S products,which restricts the rate performance of the lithium-sulfur batteries;（2）the shuttle effect caused by polysulfides（Li₂S_x,4≤x≤8）,which would be dissolved in the organic liquid electrolyte,leading to poor utility of the active materials and fast capacity fading;（3）the huge volume expansion during charging and discharging give rise to electrode structural damage,thus affecting the cycling life of the batteries.Therefore,the main content of this thesis is as follows:Molybdenum dioxide-carbon composite nanofibers（MoO₂-CNFs）were synthesized based on electrospinning and the influence of heat treatment process on the diameter and morphology of MoO₂-CNFs was studied.MoO₂-CNFs were prepared by electrospinning of high molecular weight PAN and phosphomolybdic acid（PMA:H₃PO₄·12MoO₃）solution using N,N-dimethyl formamide（DMF）as solvent.The diameters of the as-prepared MoO₂-CNFs could be controlled by varying the calcining temperature.Furthermore,the as-prepared MoO₂-CNFs were used as an additive to prepare sulfur/MoO₂-CNFs composites cathodes for lithium-sulfur（Li-S）battery to explore the battery electrochemical performance and cycle performance.Compared with pure sulfur electrode,the composites cathodes could improve the electrochemical performance.The porous structureβ-Mo₂C-carbon nanofibers（β-Mo₂C-CNFs）were prepared as sulfur-coated carriers by electrospinning method as the cathode material of lithium-sulfur batteries,which provides a good electrochemical reaction interface for redox reactions.Compared with pure sulfur electrode,the cathode with S/β-Mo₂C-CNFs can provide a larger reaction interface and promote electron transfer.In addition,S/β-Mo₂C-CNFs composites can effectively increase the lithium ion diffusion coefficient of the electrode,reduce the interface resistance,and improve the electrochemical performance of the composite electrode.Because lithium polysulfide would be dissolved in liquid electrolyte and shuttle to the anode,which is the so-called“shuttling”effect of the polysulfide,leading to the formation of the insoluble and insulating Li₂S₂/Li₂S on the face of lithium anode and sulfur cathode,resulting in a low coulombic efficiency and poor cycle life of Li-S batteries.Therefore,in order to prevent the polysulfide from being dissolved.MoO₂-CNFs were prepared by electrospinning as an interlayer of lithium-sulfur battery,which was inserted between the cathode and the separator.The MoO₂-CNFs interlayer could suppress the“shuttling”effect and improve the utilization of the active materials and the cycling life of the batteries.The S/MoO₂-CNFs/Li battery with 2.5mg cm^-2 sulfur mass loading delivered a high initial areal capacity of 6.93 mAh cm^-2(1366 mAh g^-1)at current density of 0.42 mA cm^-2,and reversible area capacity of the cell was maintained at 5.11 mAh cm^-2(1006 mAh g^-1)over 150 cycles.Even at highest current density(8.4 mA cm^-2),a reversible capacity of 865 mAh g^-1 can still be achieved.Excellent electrochemical performance can be attributed to the three-dimensional network structure formed by MoO₂-CNFs and high specific surface area,contributing to ion transfer and electron conduction.