Research On Positive Cathode Material And Polymer Electrolyte For High Performance Lithiun-sulfur Battery

Elemental sulfur as a cathode material for lithium-sulfur batteries has a high theoretical specific capacity(1675 mAh g^-1)and theoretical specific energy(2600 Wh kg^-1),and it is considered to be one of promising candidites for the next generation of high energy density secondary batteries.Due to the electronic insulation properties of elemental sulfur and lithium sulfide,and the"shuttle effect"of soluable polysulfides which are discharge intermediate products,the practical specific capacity and cycle performance of lithium-sulfur batteries are inferior.To address abrove issues,tremendous research effort has been paid in suppressing shuttle effect by preparation of novel cathode material,optimization of electrolyte,modification of separator.In this work,functional carbon-sulfur cathode and composite polymer electrolyte membrane were designed and prepared to improve the electrochemical kinetics of the electrode and relieve the polysulfide shuttle.The N atoms and CoS₂ nanoparticles in functional carbon entrap polysulfide in cathode by chemical absorption,leading improvement of cycle stability,and replacement of liquid electrolyte with solid polymer electrolyte fundamentally suppress shuttle effect.The specific work mainly has the following aspects:?1?Sulfur composite materials with polyhedral structure were prepared with a precursor of MOFs.Functional Carbon host?NC-Co?with nitrogen-doped-Co nanoparticles and carbon-porous structure was prepared by carbonization of metal organic framework.With the assistance of porous structure and CoS₂ nanoparticles,the Li-S batteries with NC-CoS₂ cathode can deliver as high as 1510 and 1150 mAh g^-1 at current density of 0.1 and 0.5 C,and a capacity of 960 mAh g^-1 is retained after cycling 100 times at 0.5 C.Additionally,the NC-CoS₂ cathode shows high initial discharge capacity of 920 mAh g^-1 at 2 C,and about 835 mAh g^-1 has been reserved after 100 cycles.The results indicate that the CoS₂ nanoparticles play a crucial role in improving the performance of batteries owing to their catalysis to polysulfide conversion.?2?High-performance inorganic/polymer composite solid electrolyte?PI-PEO electrolyte?were prepered by pouring a mixture of PEO,lithium salt,and LATP nanoparticles into a porous polyimide?PI?membrane.The electrochemical characteristics of the device with and without an interlayer?CNT-PEO?between the positive electrode and the electrolyte membrane were compared.Consequently,The solid state Li-S batteries?SSLSB?assembled can deliver 1212 mAh g^-1 at 35?and0.1 C,and 800 mAh g^-11 is remained after 100 cycles.Even the current density is increased to 0.5 C,a capacity of 970 mAh g^-11 can be delivered,and 600 mAh g^-1 is remained after 150 cycles at 35?.Furthermore,we increase sulfur content to 60%,and the results indicate that initial discharge capacity of SSLSB with CNT interlayer/PI-PEO reaches 750 mAh g^-1 at 0.05 C and 35°C,and it can remain 83.75%capacity after 320 cycles.The result indicate that the CNT-PEO interlayer can not only be used to reduce the interface resistance of the sulfur positive electrode,but also fix the polysulfide on the electrolyte side,thereby avoiding the diffusion and migration of soluble polysulfide ions to the lithium negative electrode.?3?Super P which shows excellent electtonic conductivity is chosen to replace CNT to prepare functional interlayer,namely Super P-PEO,and it show excellent electrochemical performances.The results showed that the inexpensive and easy-to-prepared Super P-PEO membrane can also capture soluble polysulfide and provide framework for transportion of electrons and ions,leading to alleviation of loss of active material,and improvement of the cycle stability.The solid polymer Li-S batteries assembled by SP interlayer/PI-PEO electrolyte and C/S?33%S loading?diliver 1190 mAh g^-1 in the initial discharge at a current density of 35?and 0.1 C,and 1145 mAh g^-1 at 0.5 C.After 100 cycles,the capacity can still remain at 600 mAh g^-1.The initial discharge capacity is 1190 mAh g^-1,when the sulfur content increases to 50%,and it can still be maintained at 690 mAh g^-1 after 100 cycles.