Study On Separator Modification And Electrochemistry For Lithium-sulfur Batteries

With the severe pollution of the global environment and the rapid consumption of fossil energy,the development of new and clear energy sources has become the general trend.At present,lithium ion batteries,which are widely used in mobile phones and other portable devices,have disadvantages such as high cost and high environmental toxicity.Therefore,looking for new batteries with low toxicity,low cost,long life,high capacity and energy areal density is an important direction for the development of electrochemical energy storage.In recent years,lithium-sulfur batteries have become research hotspots in the field of chemical energy storage because of their high theoretical capacity(1675 mAhg^-1)and theoretical energy density(2600 Whkg^-1).However,a few issues,such as the electronic and ionic insulation of elemental sulfur and the final discharge product,and the diffusion of polysulfides during the charging and discharging process,lead to extremely low utilization of active materials and poor cycle life of lithium-sulfur batteries,which limit its commercialization.In order to overcome these problems,herein,based on the functional design of separator,we have prepared special materials to use in lithium-sulfur batteries.By modifying the separator,the shuttle effect is significantly suppressed.In addition,the interface structure and internal impedance of batteries are also modified,which effectively improves the capacity and safety performance.The main findings and conclusions are as follows.First,we synthesize nitrogen-deficient carbon nitride?D-C₃N₄?by magnesium thermal denitrification technology.From the characterization test,we know that the synthesized material has a massive structure,large specific surface area,and good conductivity.The D-C₃N₄ and polyvinylidene fluoride?PVDF?areevenly mixed and then coated on a separator to obtain the D-C₃N₄ separator.Electrochemical test results show that at a current density of 0.5 C(1 C=1675 mAh g^-1),the initial capacity of the cell with D-C₃N₄ separator is as high as 1247 mAh g^-1.After 200 cycles,it remains 817 mAh g^-1.Even at 5 C,the discharge specific capacity is still 437 mAh g^-1.The reasons for such a large increase in electrochemical performance are as follows:first,the magnesium reaction reduces the nitrogen content in carbon nitride.The conductivity of the materials improved,and the electron transfer rateis enhanced.Secondly,D-C₃N₄ has a high specific surface area and ample voids,which are conducive to the infiltration of the electrolyte and also provide a large number of active sites for polysulfides.Finally,the nitrogen atoms and polysulfides in the material have a strong adsorption effect,which can effectively suppress the shuttle effect.Subsequently,we introduce carbon nanofibers containing MnS sulfiphilic sites?MnS/CNFs?synthesized by an electrospinning technique as a flexible interlayer for lithium-sulfur batteries.On the one hand,sulfiphilic MnS particles can capture polysulfides by chemisorption and significantly promote the electrochemical conversion of polysulfides.On the other hand,the structure of conductive carbon nanofibers can be used as a physical barrier to intercept polysulfides.More importantly,electrospinning synthesis is a simple and effective technique that can be considered as the primary choice for short-term,low-budget and mass production.The cell with MnS/CNFs has good rate and cycle performance at room temperature.When the sulfur load is as high as 5 mg cm^-2,the capacity reaches 714 mAh g^-1 after200 cycles at 0.5 C.More importantly,even at high temperature of 55 ^oC,the initial capacity of the cell canreach 1080 mAh g^-1 with the retention of 80%after 100 cycles.At low temperatures,it also has good electrochemical performances.This work provides new method for achieving low self-discharge and wide temperature application of lithium-sulfur batteries.