The Regulation Effect Of Metal Nitride And Sulfide For Intermediate In Lithium-sulfur Batteries

Lithium–sulfur（Li–S）batteries attracted much attention due to their high theoretical energy density(2500 Wh kg^-1).Furthermore,sulfur has many advantages such as natural abundance,low-cost and environmental friendliness.All these factors make Li–S batteries more competitive to the current LIBs.Nevertheless,the practical applications of the Li–S batteries are still restricted by the inherent defects of cathodes:（1）the poor conductivity of S and Li₂S that decreases the utilization of the active materials.;（2）large volume variation（over 80%）caused from the different density of S and Li₂S poses a big challenge to the stability of the cathode structure;（3）the shuttle effect caused by the dissolution and migration of intermediate long-chain polysulfides（Li₂S_n,4≤n≤8）decreases the coulombic efficiency.In order to address the above issues,this dissertation designs several novel cathode structures to control the diffusion and conversion of the intermediate polysulfides during the electrode process.The main research contents and experimental results are as follows:（1）Biomimetic root-like carbon coated TiN/C porous nanofibers are synthesized via the electrospinning technique and are used as a freestanding sulfur host to realize a fast trapping-diffusion-conversion of polysulfides for high areal loading Li-S batteries.As a result,the average capacity decay rate is found to be 0.1%per cycle at 4 mg cm^-2.Even at 6 and 8 mg cm^-2,the good cycle stability can be maintained.When the sulfur areal loading is high 10.5 mg cm²,the high areal capacity of 790 mA h g^-1 at 0.2C,corresponding to 8.4 mA h cm^-2,which is almost two times more than the current Li-ion battery(4 mAh cm^-2).（2）Core-shell C@MoS₂ hollow spheres are synthesized and used as sulfur host.In this system,the hollow carbon spheres can provide a microregion for the conversion reactions of the sulfur species;the inner MoS₂ nanosheets can further limit the diffusion of polysulfides.At 2C current,the discharge capacity is found to be 706 mA h g^-1.The average capacity decay rate is found to be 0.1%per cycle during 500 cycles.