Synthesis Of Nitrogen-doped Hierarchical Porosity Carbons And The Application For Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries are one of the most promising candidates for high energy secondary lithium batteries.Using sulfur as cathode materials can theoretically deliver a high capacity of 1675 mAh g^-1.In addition,sulfur has the merits of low cost,environment benignity,and safety.However,practical application of Li-S batteries is still hindered by several existing drawbacks including insulating nature of pristine sulfur and Li₂S,the volume expansion,and“shuttle effect”caused by the high solubility of lithium polysulfides in the electrolyte.Structrual design and suface modicaiton of carbon materials as sulfur host is the main approach for enhancing the electrochemical performance of Li-S batteries.It is believed that the cabon materials with hierarchical porosity（i.e.,micropores,mesopores,and macropores）possess both high specific surface area and large pore volume,which is benieficial for high efficiency utilization of sulfur.Moreover,nitrogen doping might endow carbon materials with surface polarity besides enhancing their conductivity,which helps to suppress the dissolution loss of polysulfides by strong chemical adsorption.Herein,we designed and prepared two kinds of nitrogen-doped carbon materials with hierarchical porosity and investigated the electrochemical performance of the corresponding sulfur/carbon cathode materials.The main works and results are summarized as follows:1.Ordered mesoporous carbons（OMC）was first synthesized by a soft-template route,which was then activated in a ammonia flow to obtain the target carbon material with high specific surface area,large pore volume,and high nitrogen content.The effect of activation temperature（700°C-1000°C）and duration（1-15 h）on the structure and chemical composition of carbon materials was investigated in detail.It clearly exhibits that ammonia activation enables the hierarchically porous carbon with relatively high porosity(specific surface area up to 1565 m² g^-1;pore volume up to0.88 cm³ g^-1)and nitrogen content（up to 4.4 wt%）.Those prepared carbons were then used as the sulfur hosts in lithium-sulfur batteries.The composite delivered an initial discharge capacity of 1095 mAh g^-1 at a current density of 335 mA g^-1.It still preserved a capacity of 708 mAh g^-1 after 100 cycles with a capacity retention ratio of 65%.This should be attributed to the hierarchical porosity of micro/mesoporous synergy,and high nitrogen content.It has a stronger physical and chemical adsorption on polysulfide,which can effectively inhibit the"shuttle effect"and improve the electrochemical performance of the lithium-sulfur batteries.2.We designed and synthesized a novel tube-in-tube hollow porous carbon material（MWNT-HC）for impregnation of sulfur.The MWNT-HC was synthesized by sequential depositon of SiO₂ and resorcninol/formaldehyde（RF）resin onto carbon nanotubes,followed by carbonation of the resin and removal of the SiO₂.Typical MWNT-HC material has a high specific surface area of 967 m² g^-1 and a large total pore volume of 1.93 cm³ g^-1.After nitrogen-doping modification by a post-synthesis pathway,the nitrogen content of MWNT-HC increased from 1.2 wt%to 4.2 wt%.The resultant carbon material was then used as the sulfur hosts in lithium-sulfur batteries.The composite delivered an initial discharge capacity of 1272 mAh g^-1 at a current density of 335 mA g^-1 and it retained a capacity of 924 mAh g^-1 after 50 cycles.The hollow porous carbon material is not only easy to synthesize,but also has a high specific surface area and large pore volume structure.It is also expected to be used in other energy storage devices such as supercapacitors and fuel cells.