| Lithium sulfur?Li-S?battery is the next generation high specific energy storage device because of its advantages of high specific capacity,low sulfur price of active material and wide source.However,the low conductivity of the elemental sulfur,the dissolution and diffusion of polysulfide,the expansion of the electrode volume and the lithium dendrite,seriously restrict the practical application of lithium sulfur battery.Based on the large specific surface area and unique pore structure of metal organic frameworks?MOF?,the surface of MOF was modified as precursor of carbon materials.The carbon materials doped with nitrogen and oxygen were prepared by direct carbonization at high temperature to prepare carbon/sulfur composite positive materials loaded with sulfur active compounds.This article focused on the effects of MOF-derived carbon materials doped with different atoms on the performance of lithium-sulfur batteries.The specific research contents are as follows:?1?The preparation of nickel based metal organic framework?Ni-MOF?.The nickel based metal organic framework was prepared by hydrothermal method was used as precursor to prepare carbon materials with different structure morphology under high temperature in argon atmosphere.The effects of pH value,carbonation temperature and activation conditions on the performance of the battery were investigated.The results showed that adding a proper amount of ammonia in the preparation of MOF could increase the discharge capacity of the battery,but the addition of ammonia water would lead to the opposite effect.The battery capacity was higher when the calcination temperature was 900?.The battery with graphene as the conductive agent had better charge discharge characteristics.?2?The properties of Ni-MOF and its derived porous carbon/sulfur positive electrode modified by furfuryl alcohol?FA?.In order to improve the surface properties of MOF derived carbon and increase the interaction force of porous carbon and polysulfide,FA?furfuryl alcohol?containing oxygen functional groups?FA?was used to modify Ni-MOF,and the effect of oxygen atoms doped on the surface of porous carbon derived from MOF on the performance of the battery was investigated.The results of electrochemical performance test showed that the first discharge capacity of FA/C-Ni-S positive electrode was 1145.1 mAh·g-11 at the ratio of 0.1 C which was increased 55.6mAh·g-11 to the undoped FA(1089.5 mAh·g-1).The sulfur utilization ratio is 68.4%.After 100 cycles of charge and discharge,the specific capacity could still maintain 364.7 mAh·g-1.At the same time,the oxidation peak and the reduction peak current of the cyclic voltammetry curve increased obviously.The reduction peak shifted right and the oxidation peak shifted left,indicating that the polarization of the FA/C-Ni-S positive electrode decreased during the charge discharge process.This was mainly due to the change in the polarity of the electrode surface after doping FA,which increased the force of carbon materials and polysulfide,inhibited the dissolution of polysulfide into the electrolyte and reduced the loss of the active sulfur.?3?Ethylenediamine?EDA?modified Ni-MOF was used to prepare nitrogen doped porous carbon/sulfur?EDA/C-Ni-S?cathode.The carbon/sulfur composite cathode was prepared by Ni-MOF doping with ethylenediamine and introducing sulfur into carbon materials after high temperature carbonization.The experimental results showed that the specific capacity of the EDA/C-Ni-S first discharge capability was 1218.9 mAh·g-1,the sulfur utilization ratio was72.8%,the capacity was 439 mAh·g-11 after 100 cycle and the capacity retention rate was 36%which were obviously improved than?C-Ni-S?undoped EDA anode and better than FA/C-Ni-S anode doped FA.In order to compare the effects of doping nitrogen and oxygen atoms,the Starcide BK?TNO?was selected which was high symmetry and with the same oxygen and nitrogen content in the oxygen and nitrogen co doping experiments. |
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