Structural Design,Modification And Electrochemical Performance Of Carbon Nanofiber/Sulfur Cathode

Sulfur is considered as a promising secondary battery electrode material due to its low-cost,rich resources,environmentally friendly,high theoretical specific capacity(1675 mAh·g^-1)and high energy density(2500 Wh·kg^-1),etc.But there are still many shortcomes which hinder its practical application:1.The poor electrical conductivity;2.During the charging and discharging process,the density of sulfur will change,causing volume expansion and active material shedding;3.The polysulfide,the reduction products of sulfur,diffusing into the electrolyte during charge and discharge process and causing the“shuttle effect”;4.Power state of sulfur,needing extra conductive additives and binders.In order to relieve the above problems,highly conductive carbon nanofibers are used as freestanding substrate for sulfur in this thesis.Furthermore,some methodes including structural design,transition metal oxide modification and hetero-atom doping were used to alleviate the above problems,which can improve the electrochemical performance of carbon nanofibers/sulfur electrodes.The main content as following aspects:（1）Four kinds of carbon nanofibers with different structures were designed and prepared:porous carbon nanofibers（PCNFs）,thin-wall porous carbon nanofibers（TW-PCNFs）,porous solid-core carbon nanofibers（PCNFs-SC）and hollow porous carbon nanofibers（HPCNFs）.The effects of structure on electrochemical performance were studied in detail.Due to the large specific surface area of HPCNFs,the infiltration of the electrolyte was increased,which shorten the electron transport path and improve the electrical conductivity of the electrode;while the large pore volume is beneficial to alleviate the volume expansion of sulfur and improve the cycle performance.The HPCNFs/S electrode possesses the highest initial specific capacity(806 mAh·g^-1)and the best capacity retention（49%after 150 cycles）at a current density of 0.1 A·g^-1.Based on above research,the effect of sulfur loading on electrochemical performance of the HPCNFs/S electrode was studied.The results show that if too much sulfur is loaded,the sulfur will agglomerate and reduce the conductivity of the material;if too little sulfur is loaded,the active material cannot provide sufficient capacity.When the mass ratio of HPCNFs and S is 1:1,the HPCNFs/S-1 composite electrode exhibits the highest specific capacity(872 mAh·g^-1 at 0.1A·g^-1)and the best cycle performance（54%after 150 cycles）.（2）In order to improve the cyclic capacibility of the electrode,HPCNFs was modified by metal transition oxide.TiO₂ and Mn O₂ nanoparticles were added to the electrospun precursor solution,TiO-HPCNFs and Mn O-HPCNFs composites were obtained after high temperature reduction.The strong polar interactions of XO（X=Ti,Mn）and polysulfides can improve the adsorption of polysulfides and limit“shuttle effect”.The results show that,the lithium-sulfur battery with TiO-HPCNFs/S as the positive electrode shows 900 mAh·g^-1 and 64%capacity retention after 150 cycles at a current density of 0.1 A·g^-1.Compared with HPCNFs/S electrode（54%after 150 cycles）,the cycle performance of the TiO-HPCNFs/S has been greatly improved.（3）To further enhance the conductivity and electrochemical performance of the TiO-HPCNFs/S composite,B@TiO₂-HPCNFs,P@TiO₂-HPCNFs and BP@TiO-HPCNFs were prepared through coaxial electrospinning and chemical vapor deposition,using boric acid and red phosphorus as boron source and phosphorus source,respectively.Boron doping can move the Fermi level of carbon to the conduction band,and also form a synergistic effect with nitrogen doping in PAN-based carbon fibers to enhance the conductivity and polysulfide adsorption of material.Finally,the lithium-sulfur battery using B@TiO₂-HPCNFs/S as the positive electrode shows the best electrochemical performance,which shows initial capacity of 1070 mAh·g^-1 and 761 mAh·g^-1 after150 cycles（71 retention）at a current density of 0.1 A·g^-1.