Study On Preparation And Electrochemical Performance Of Mesoporous Carbon/Transition Metal Sulfide (MnS) Composites As Cathode Materials For Lithium-sulfur Battery

In order to enhance the electrochemical performance and sulfur utilization, several sulfide-based cathode composites were prepared by uniformly incorporating sulfur or metal sulfide into a conducting matrix, which include mesoporous composites such as carbon/sulfur composites, metal sulfide/sulfur composites, carbon/metal sulfide nanocomposites and carbon/metal sulfides/sulfur composites.1. Two kinds of mesoporous carbons,OMC1 with a BET surface area of 914.14 m2 g-1 and the OMC2 with a BET surface area of 1115.32 m2 g"1 were prepared by the sol-gel template method combined with spray drying. The resulted OMCl and OMC2 were served as conductive matrix for encapsulating sulfur to form mesoporous carbon/sulfur composites. The Field-emission scanning electron microscopy (FE-SEM) image showed that the OMC2 consisted of some microspheres carbon with size of about 0.36 um and amorphous carbon. The transmission electron microscopy (TEM) clearly showed that these carbon matrixs had very uniform hexagonally ordered pore array structure. At a current density of 100 mA g-1, OMCl/S and OMC2/S composites with a loading of 70 wt.% sulfur had a specific capacity of 1378.8 mAh g-1 and 1381.5 mA h g-1 in the first cycle, after 50 cycles it remained a reversible capacity of 663.4 mA h g-1 and 815.6 mA h g-1, respectively. Their irreversible capacity losses in the first cycle were 270.8 mAh g-1 and 191.5 mA h g-1. And the capacity fading rate of the OMCl/S and OMC2/S composites was 0.80% and 0.63% per cycle, respectively. Besides, the OMC2/S-70 composites exhibit high rate capability. At a current density of 800 mA g-1, the discharge capacity remains at 750 mA h g-1 even after 50 cycles, and the capacity fading rate was only 0.66% per cycle.2. A solvothermal synthetic route has been successfully implemented to prepare mesoporous structure MnS microspheres under mild condition. Then used as substrate for sulfur coating to synthesize mesoporous metal sulfide/sulfur composites (MnS/S) via a simple heat treatment. In this synthetic route, mesoporous silica oxides were used as template. The morphology of the MnS were examined by SEM and TEM analysis. Notably, MnS microspheres were in the range of 0.2um-0.4um, and well-defined mesopores of MnS with diameter of about 3.85 nm were observed. At the current density of 100 and 800 mA g-1, the discharge capacity reached as high as 1156.8 mA h g-1 and 948.9 mA h g-1 at the first cycle, and the reversible capacity of 650.6 mA h g-1 and 351.6 mA h g-1 remained after 50 cycles, respectively. The decay rate of the composites was just 0.80% and 0.63% per cycle, respectively. It really showed that the mesoporous structure of the MnS play a crucial role in alleviating the so-called internal "shuttle mechanism". Therefor, the MnS substrate can effectively improve cycling performance of sulfur electrode.3. Mesoporous carbon (OMC2) with MnS nanoparticles loaded (MnS/OMC2) was obtained via a simple solution-based chemical reaction-deposition method. The SEM and TEM results showed that MnS nanoparticles with diameter of about 40 nm. The electrode with a loading of 50 wt.% MnS (MnS/OMC2) cycled in voltage range from 0.01V-3.0 V exhibited good cycling performance. The results showed that nanosized Mn and Li2S were generated after the initial deep discharge and MnS was reformed during the charge process. At the current density of 100 mA g-1, the discharge capacity reaches as high as 1150 mA h g-1, and remained a reversible capacity of 658 mA h g-1 after 30 cycles. This work provides a new method for the development of lithium-sulfur batteries.