Research On Functional Cathode And Separator Composite Materials For Lithium?sulfur Batteries

Lithium?sulfur?Li?S?battery,notable for its extremely high theoretical specific capacity(1675mAh g^?1),abundance available and environmental friendly,has attracted increasing attention as next-generation energy storage devices for electric vehicles.However,broad adaption of Li?S batteries has been hampered by various obstacles.Li?S battery suffers from insufficient capacity retention,limited charge?discharge Coulombic efficiency and severe self-discharge,resulting from the migration and diffusion of soluble polysulfide intermediates?Li₂S_x,4?x?8?.In order to solve these issues,researchers have studied the structural design and preparation methods of cathode materials,and introduced a variety of materials compositing with sulfur.However,the traditional porous carbon materials have only weak physical adsorption to the Li₂S_x.Rational design of cathode and separator materials to restrain the“shuttle effect”of polysulfide is a great need for high-performance lithium–sulfur batteries.The main results of this work are as followed:In the first part,we prepared three-dimensional hierarchical porous TiO₂/graphene aerogel?TiO₂/GA?composites as promising anchoring materials for sulfur species via a one-pot hydrothermal process.Specifically,we adjusted the TiO₂ content in aerogel.The one containing15.3 wt%TiO₂ exhibits a superior boost in specific capacity,rate capability and cyclability.The factors contribute to the high performance attributing much to the strong interfacial chemical and physical interactions of TiO₂/GA with Li₂S_x.Moreover,further improved sulfur utilization is achieved by adding glucose to adjust the surface area and pore size distribution of the aerogels.At a high rate of 2 C(1 C=1675 mA g^?1),an initial discharge capacity up to 1011 mAh g^?1 is obtained with a decent cyclic efficiency and 100%coulombic efficiency over 500 cycles.In the second part,metal organic framework?MOFs?-derived TiO₂ embedded porous carbon nanocomposites were prepared as a host of sulfur to realize functionalization of cathode materials for Li?S battery.After high temperature pyrolysis,TiO₂/C composite still maintains a good disk-like structure,indicating its excellent chemical stability.TiO₂ nanoparticles embedded in the carbon matrix provide a large number of adsorption sites for interfacial chemical capture of Li₂S_x.The carbon embedding can effectively hinder the growth of TiO₂ nanoparticles,while increases the conductivity of electrode.The TiO₂/C/S cathode achieves a highly reversible capacity from 1281mAh g^?1 to 780 mAh g^?1 over 100 cycles at 0.5 C with capacity retention of 61%.Moreover,the lithium ion diffusion coefficient in the electrochemical reaction has been increased.In the third part,the traditional polyolefin separators were functionally modified by using metal organic framework materials?MOFs?with highly developed pore structure and extremely large specific surface area.MIL-125?Ti?-PP/PE separator was successfully prepared with ion selective function.It can allow free access of lithium ions and prevent the Li₂S_x from getting through the separator to arrive the lithium anode surface leading to side reactions.At the same time,MIL-125?Ti?also has adsorption capability for Li₂S_x,which can synergistically inhibit the"shuttle effect"caused by polysulfides.Attributed to the combination of MIL-125?Ti?,the obtained separator exhibit outstanding cycle performance from 1218 mAh g^?1 to 726 mAh g^?1 at 0.2 C after 200 charge?discharge cycles.This approach shows the way forward to practical use of Li?S batteries.