| The rapid development of the new energy industry triggered by the oil depletion crisis,lithium-ion batteries commonly used in small portable electronic products have been unable to meet the high-energy requirements of energy storage.The limited energy density of these batteries has hampered their widespread use in a variety of emerging mobile transportation devices.This has led to the worldwide search for new battery technologies that go beyond traditional lithium-ion batteries.Lithium-sulfur batteries are considered to be the most commercial next-generation lithium secondary battery system due to their high theoretical energy density(2600 Wh g-1),theoretical specific capacity(1675 mAh g-1)and environmentally friendly characteristics.However,the practical application of lithium-sulfur batteries still has many difficulties,the two main aspects are:First,the lower active material utilization rate in the electrochemical process is caused by the low conductivity of sulfur and lithium sulfide.Second,shuttle effect of soluble polysulfides result in short cycle life and low coulombic efficiency during the electrochemical process.In order to solve the above problems,herein,based on the functional design of materials and a series of characterization methods,we have prepared specific materials and conducted them to use in lithium-sulfer batteries,which could effectively improve the utilization of active materials and inhibit the shuttle effect.The main research results and conclusions are as follows:Firstly,we synthesized the nitrogen/oxygen heteroatoms doped mesoporous carbon?HDMC?by means of hydrothermal and high-temperature carbonization,and then combined it with sulfur by hot melting method to obtain S@HDMC composite cathode material.It was characterized by XRD,Raman and SEM.The results show that sulfur is uniformly distributed on the HDMC carrier.The electrochemical test results show that the initial capacity of the composite is 802 mAh g-11 at a current density of 0.5 C,and it has a discharge specific capacity of 522 mAh g-11 even at a high current of 5 C.The reasons for the improvement of the electrochemical performance of the composite are as follows:first,the highly conductive HDMC can accelerate the transfer speed of electrons in the electrode material.Secondly,HDMC has a high specific surface area,providing a large amount of activitive sites for the residence of elemental sulfur and filled pores facilitate the infiltration of the electrolyte.Finally,the doped nitrogen/oxygen atoms have strong chemisorption with the polysulfide,and the resulting polysulfide is anchored inside the carrier material.Under the synergic effects of above three points,the cathode material S@HDMC has excellent electrochemical performance.Then,we turned our attention to the design for functionalized membrane.We prepared titanium nitride nanospheres with mesoporous structure by hydrothermal and high temperature sintering.It is evenly coated on a commercial separator by a scrape-coating method and applied to lithium-sulfur battery system.Studies have shown that a strong chemisorption can be formed between titanium nitride and polysulfide,which can effectively alleviate the shuttle effect.More importantly,the highly conductive titanium nitride modified functionalized membrane can act as a current collector to activate and reuse sulfur compounds on the membrane,accelerate the transfer of electrons and ions,and thus effectively improving the cycle stability and rate performance of the battery.In addition,titanium nitride microspheres with a diameter of 500-700 nm can also form a physical barrier to intercept polysulfides,preventing polysulfides from passing through the separator.Therefore,a lithium-sulfur battery based on a mesoporous titanium nitride microsphere separator exhibits excellent electrochemical performance.At a current density of 0.5 C,after200 cycles,the capacity retention rate is as high as 76%,and the current density at 3 C it could still get 672 mAh g-1. |
PDF Full Text Request