Synthesis And Applications Of Transition-Metal Materials In Lithium-Sulfur Battery Cathode

Lithium-sulfur batteries(Li-S)with active material sulfur as the positive electrode and metal lithium as the negative electrode have ultra-high theoretical specific capacity and theoretical energy density,which are 3 to 5 times that of lithium ion batteries.Thus,Li-S batteries are considered to be a new generation of electrochemical device.However,the practical application of lithium-sulfur batteries is hampered by the problems such as low sulfur utilization,poor rate performance and short cycle life.In response to the problems,this paper proposes a scheme to synthesize multi-functional materials and use them as positive electrodes in lithium-sulfur batteries.Specifically,we designed and prepared two kinds of transition-metal materials to study the synergistic effect of adsorption and catalytic poly sulfide.The main contents of this paper are as following:(1)The Fe-ZIF-8 precursor is obtained by doping trace amount of metal iron source in the ZIF-8.After carbonization,a microporous carbon material with high specific surface area and highly dispersed Fe-N-C can be obtained.The experimental results have shown that carbon materials with high specific surface area and active sites have excellent lithium polysulfide adsorption capacity,which is beneficial to alleviate the "shuttle effect" of polysulfides and volume changes during charge/discharge process.Electrochemical tests show that the S@Fe-N-C composite cathode material has not only a high charge-discharge specific capacity and excellent rate performance,but also better cycle stability at high rates.From 0.1 C to 1 C current rates,with 1178 and 631 mAh g-1,respectively.After 500 cycles of deep charge and discharge at a high rate of 1 C,the Coulomb effect remained nearly 100%,while the capacity only decay to 450 mAh g-1,corresponding to only 0.068%decay rate per cycle.The reason why the S@Fe-N-C composites cathode material has excellent electrochemical performance is mainly due to the synergistic effect of Fe-Nx-Cy on polysulfide adsorption and catalysis,which improves the reaction kinetics and accelerates the solubility by catalytic adsorption on the active site.(2)The ternary hollow NiCo2S4 composites was synthesized using tetragonal Ni-Co acetate hydroxide as a precursor.The elemental sulfur was immersed into NiCo2S4 composites by hot melting method to obtain 83.4%of the high-sulfur S@NiCo2S4 composites.The study has shown that the hollow internal structure of NiCo2S4 composites can be used as a physical barrier to effectively relieve the "shuttle effect" and volume expansion of lithium-sulfur batteries.In addition,the surface of NiCo2S4 has abundant polar sites which can effectively adsorb polysulfide.It also catalyzes the conversion of long-chain lithium poly sulfide into Li2S2/Li2S that is insoluble in the electrolyte,thereby alleviating the "shuttle effect".The experimental shows that S@NiCo2S4 composites has high charge-discharge specific capacity and good cycle stability and has excellent electrochemical performances under high sulfur surface load.Under the rate of 0.1 C,0.2 C,0.5 C,1 C and 2 C,the reversible discharge capacity of the S@NiCo2S4 electrode reached 1140,882,765,710 and 512 mAh g-1,respectively.After 500 deep charge and discharge cycles,the capacity remains 410 mAh g-1.The capacity retention rate is 62.5%and the capacity decay rate per cycle is 0.094%.In addition,after 500 deep charge and discharge cycles,the Coulomb efficiency of the battery remains nearly 100%.On the other hand,when the surface load of the S@NiCo2S4 battery sulfur reach to 2.1 and 4.3 mgsulfur cm-2,respectively,the initial discharge capacity can reach 859 and 587 mAh g-1,respectively.And after 200 cycles of deep charge and discharge cycles,the specific capacity maintained 695 and 492 mAh g-1,respectively,and the capacity retention rates were 80.9%and 83.8%,respectively.