Preparation Of Transition Metal Nitrides Entrapped In N-doped Porous Graphitic Carbon And Investigation Of Catalytic Conversion Mechanism Of Sulfur Cathode

Commercial lithium-ion batteries are limited by the theoretical lithium storage capacity of cathode materials,which is difficult to meet the requirements of large-scale energy storage system development.Therefore,the developing of new high specific energy system is one of the research hotspots currently.Sulfur（S₈）has an ultra-high theoretical specific capacity(1675 m A h g^-1).The theoretical energy density of lithium-sulfur（Li-S）battery obtained by combining the S₈ cathode with lithium metal anode(3860 m A h g^-1)can reach 2600 W h kg^-1.Meanwhile,S₈ also has the advantages of abundant natural resources,low price,and environmental friendliness.Therefore,Li-S batteries are considered as one of the most promising energy storage systems in the next generation.However,the practical application of Li-S batteries is still constrained by some inherent problems.And these problems mainly focus on the sulfur cathode,including 1)the insulation and volume expansion of the cathode;2)the shuttle effect of the lithium polysulfides（Li PSs）;3)the sluggish electrochemical reaction kinetics.In order to solve the above problems,3D porous graphitic carbon was studied with the aim of improving its adsorption and electrocatalytic activity in this work.And the 3D N-doped porous graphitic carbon composites that hybridized with a series of transition metal nitrides（TMN@N-PGC）were obtained by a compounding strategy.The obtained TMN@N-PGC were then used as sulfur host materials.Firstly,the 3D carbon substrate can provide conductive skeleton for electron transmission in the sulfur cathode,which can effectively improve the conductivity of the sulfur cathode.Secondly,the 3D carbon substrate can also provide interconnected channels for electrolyte to infiltrate cathode and facilitate the lithium-ion diffusion.TMNs with polar surfaces can effectively anchor polysulfide by enhanced chemical adsorption,thereby alleviating the shuttle effect of Li PSs.In addition,TMNs also have high electrocatalytic activity,which can promote the conversion of sulfur species and improve the kinetics of sulfur cathode.The main research results achieved in this paper are as follows:1.A“one-pot”strategy was proposed.And the N-doped porous graphitic carbon that is hybridized with oxygen-doped titanium nitride（O-Ti N@N-PGC）was synthesized by using this strategy.The enhanced chemical adsorption of polar O-Ti N makes the maximum adsorption capacity of O-Ti N@N-PGC to Li₂S₆ 6.5 times that of PGC.Simultaneously,the transfer rate of Li⁺in the cathode is greatly improved due to the excellent electrocatalytic effect of O-Ti N.Thus,the initial discharge specific capacity of the S/O-Ti N@N-PGC cathode can reach 1408 m A h g^-1 at 0.1 C.The residual discharge specific capacity is still 513 m A h g^-1 after 1000 cycles,and the corresponding average capacity decay rate is only 0.049%per cycle.2.The above“one-pot”strategy was extended to the preparation of various TMNs.Three transition metals,Mo,Nb,and Zr,which are in the same period,were taken as the research objects.The metal salts with highest valance of the three transition metals were selected as the metal sources.N-doped porous graphitic carbon composites loaded with Mo₂N,Nb N,and Zr N were obtained by adjusting the thermal treatment temperature and the nitrogen-containing starting material.Comparing the preparation processes of the three composites,it was found that TMN formation was correlate with the oxidation states of the metal cations.The transition metal ions with relatively high oxidation state(e.g.,Mo⁶⁺)can be readily reduced to the corresponding nitride.By contrast,the reduction of metal cation with lower oxidation state(e.g.,Zr⁴⁺)to the corresponding nitride requires relatively harsh conditions.3.The electrocatalytic activities of Mo₂N,Nb N,and Zr N for sulfur cathode reactions were investigated in terms of electrocatalytic conversion of polysulfides and detachment of Li₂S from TMNs.Both the experimental results and DFT theoretical calculations show that Nb N facilitated the Li₂S₂-to-Li₂S conversion as well as Li₂S detachment at the same time.Thus,Nb N@N-PGC composite is concluded as the best sulfur host material for Li-S batteries among the three composites tested.As a result,the Li-S battery prepared with S/Nb N@N-PGC cathode can maintain a reversible discharge specific capacity of 521 m A h g^-1 at a high current density of 6 C.The batteries with S/Nb N@N-PGC cathodes can work stably over 1300 cycles under a long-term cycling performance measurement at a small current density of 0.5 C,and the corresponding average capacity decay rate is only 0.030%per cycle.Even at a high sulfur loading of 6.7 mg cm^-2,S/Nb N@N-PGC cathode can work stably after 100 cycles with a high capacity retention（over 88%）.