Preparation Of Porous Carbon Matrix Composites And Their Applications In Lithium-sulfur Batteries

Nowadays,there are more and more urgent needs for advanced energy storage technology,such as low emission electric vehicle and smart grid,storage system,which has initiated the exploration of new battery system beyond traditional lithium ion chemistry.Therefore,rechargeable lithium metal batteries have attracted great interest because of their high energy potential in secondary batteries.Because of its inherent high theoretical energy density,it has become a promising candidate for various lithium-based batteries.The high energy density of Li-S battery is mainly determined by two factors.First,the lithium metal anode has the highest specific capacity of all solid electrode materials(3860 mAh g^-1)and the lowest potential?-3.04V vs standard hydrogen electrode?.The theoretical specific capacity of the light cathode active substance?S8?is 1675 mAh g^-1,which is much better than that of the most advanced anodes of lithium-ion battery.Besides high energy density,attractive features such as low cost,non-toxic,and rich sulfur also bring additional benefits to the commercial market.In this paper,the lithium sulfur batteries are studied from the aspects of ternary composites and porous carbon materials.?1?High-energy-density rechargeable lithium-sulfur?Li-S?battery is a kind of potentially chemical energy storage source because of its low cost,abundant resources and non-pollution.Herein we present a fabrication of a nanosheet structured electrode through coating elemental sulfur on the graphene nanosheets decorated with NiFe2O4 nanocrystals?NiFe2O4/graphene?.In this structure,graphene nanosheets work as a conducting additive and have a large specific surface area.The NiFe2O4 nanocrystals anchored on the grapheme nanosheets acts as adsorbing materials for dissolved polysulfide.The hybrid material delivers balanced reversible specific capacity of 781 mAh g^-1 after 100 cycles at 0.2C,together with better rate performance and Coulombic efficiency.?2?In this work,we successfully prepared nitrogen-doped porous carbon nanosheets?NPCN?by using low-cost raw materials?monosodium glutamate?via a facile carbonization approach without any activation process.The obtained NPCN-800 possesses a special micro/mesoporous structure,with a relatively high BET surface area and appropriate nitrogen doping.By employing the NPCN-800 as a reservoir to impregnate sulfur as the cathode material of lithium-sulfur batteries,the resulting sulfur/carbon composite with 65.5 wt%sulfur content exhibits an initial discharge capacity of 1070 mAh g^-1 at 0.2C and enhanced capacity retention of 680 mAh g^-1 after 150 cycles.Moreover,a capacity of 452 mAh g^-1 at 0.5C is achieved over 200 cycles with 98.6%Coulombic efficiency.The superior electrochemical property is attributed to the synergistic effect of the hierarchical porous structure?physisorption?and N-doping?chemisorption?.?3?In this paper,we selected goose hair as a low-cost,environmentally friendly precursor to prepare in situ nitrogen-doped porous biological carbon material?NPC?by activation with KOH.The morphology and microstructure of NPC were adjusted by activation pyrolysis and recombination with sulfur at different mass ratios.The NPC substrate prepared at 850? carbonization has a two-dimensional porous structure,a high specific surface area of 3245 m2 g^-1 and an appropriate pore size distribution.Because of the special physical structure and the inherent synergistic effect of N doping,the utilization ratio of active sulfur and the electron ion transfer rate in the materials are improved,and the shuttle behavior of soluble lithium polysulfide is also suppressed.Therefore,NPC:S=1:2 composites have good electrochemical performance,showing a high initial discharge capacity of 799 mAh g^-1 at 0.2?,and maintaining a capacity of 665 mAh g^-1 after 250 cycles.