| As the increasing proportion of Li-S batteries in secondary battery research,it is widely regarded as one of the powerful candidates for the next generation of high-energy rechargeable lithium batteries.However,owing to some inevitable issues such as poor conductivity of active substances,high solubility of polysulfides,defects in the electrochemical properties of Li-S batteries has been caused.Focus on the above issues,the article prepared a composite material via introducing nitrogen element and transition metal into the carbon material.These materials have superior conductive ability and strong polarity,which can absorb polysulfides,resulting in the improvement of the battery properties.The composites are applied to decorate cathode materials for Li-S batteries.Our principal research details are as follows:(1)Mn and N were introduced into graphene by low-cost formamide as the reaction medium,and the Mn-NG composite with many active sites was fabricated by a simple hydrothermal method.The coral-like morphology of Mn-NG provides a favorable structure for the efficient transportation of lithium ions and electrons.The Li-S batteries with Mn-NG modified positive electrode have the first discharge specific capacity of 938.2 m Ah·g-1,and the average decay rate of specific capacity during 200 cycles is 0.22%per cycle,while that of the undoped sample is 0.33%.The result indicates that the introduction of Mn and N enhance the utilization of active materials in lithium-sulfur batteries.The active sites of Mn and N in the composite material have better catalytic properties,which can promote the reaction dynamics of lithium polysulfide conversion.Thus the performance of the Li-S batteries is enhanced effectively.(2)Co and N doping graphene(Co-NG)composites with different ratios were fabricated by solvothermal route.The morphology of Co-NG provides a favorable space for the volume expansion of the active material sulfur during the reaction process,preventing the pulverization of the positive electrode.The results show the differences of electrochemical properties of composites at different doping concentrations,but they are all significantly improved comparing to the undoped samples.The first charge-discharge capacity of the battery reaches 955.7 m Ah·g-1 for 0.02 mol·L-1 concentration of Co,which has the highest cycle stability.As a contrast experiment,the discharge specific capacitance of the undoped battery is only 77%for the Co-NG battery.The active sites provided by Co and N in Co-NG can improve the reaction kinetics of lithium polysulfide conversion.Thereby curtailing the shuttle effect of the battery,and decreasing the damage of active material and the accumulation of passivation coating on the negative electrode.(3)Based on the above two parts of the experiment,we studied the composite of different transition metals and graphene using this approach,and the transition metals used are Fe,Ni,Mo,Zn and bimetal Fe/Co.It is demonstrated that the solvothermal method is not only suitable for single transition metal material doping graphene,but also can realize the synthesis of bimetal doping graphene(Fe Co-NG).In this metal-doped composite material,the electrochemical performance of Ni-NG is more outstanding than Fe-NG,Mo-NG,and Zn-NG.It keeps good electrochemical stability in the long-cycle test and the capacity retention is 46.18%after 200 cycles,which is 1.4 times that of the undoped sample.The capacity retention of the bimetal doped composite Fe Co-NG is 38.7%,which is 1.16 times that of the undoped sample.We speculate that the lithium ion diffusion coefficient of different transition metal doped materials is different,which leads to differences in their electrochemical performances. |
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