Construction Of Lithium-Sulfur Battery Cathodes With Molybdenum-Sulfur Compounds And Their Sulfur Fixation Mechanism

Lithium-sulfur batteries have become a research hotspot for energy storage devices due to their high specific energy density(～2600 Wh kg-1),non-toxicity,abundant storage capacity,and low cost.However,the development of lithium-sulfur batteries is still limited to some technical obstacles,including incomplete utilization of sulfur as the cathode active material,rapid capacity fading,and low Coulombic efficiency,which seriously hinder the electrochemical performance of lithium-sulfur batteries.These problems are mainly due to the following points:① During the discharge process,the sulfur ring is opened and converted into polysulfides(Li2Sn,n=4～8)soluble in the electrolyte.Under the action of concentration polarization,these polysulfides shuttle through the separator to the negative electrode and generate "shuttle effect",which leads to the loss of active material in cathode material.② Sulfur is non-conductive and has low ionic conductivity,which hinders the transmission of electrons.③ Due to the "shuttle effect",polysulfides can react with metal lithium,causing the battery discharges itself,and at the same time destroying the solid electrolyte film(SEI)on the surface of lithium metal,reducing the conduction of ion and inhibiting the intercalation/deintercalation behaviour of lithium-ion.The insulating properties of sulfur and the shuttle effect of polysulfides are the key issues hindering the commercialization of lithium-sulfur batteries.Therefore,finding suitable cathode materials is the fundamental way to solve the current problems.In this paper,different kinds of metal-sulfur-based nanocomposites and molecular cluster materials are designed as sulfur hosts.On the one hand,they can effectively inhibit the shuttle effect of polysulfides,and on the other hand,they can accelerate the catalytic conversion of polysulfides and lithium sulfide.To improve the capacity and cycle life of lithium-sulfur batteries.The details are as follows:(1)High-density amorphous MoS3 nanomaterials were prepared by a simple acid precipitation method.Compared with the two-dimensional multilayer MoS2,the amorphous MoS3 is composed of one-dimensional Mo with sulfide and disulfide ligand bridges,and MoS3 has more active sites to inhibit the shuttle of polysulfides and improve the capacity and cycle life of lithium-sulfur batteries.Compared with the pure S electrode material,the MoS3/S modified cathode material exhibits excellent electrochemical performance and high specific capacity(the initial capacity is 934.2 mAh g-1 at 0.1C),excellent rate performance(the capacity is 410 mAh g1 at 5C),and low capacity decay rate(the capacity decay rate is 0.082%after 300 cycles at 1C,the capacity decay rate is 0.127%after 500 cycles at 2C).(2)Metal cobalt clusters doped MoS2 with sulfur defects tube-in-tube nanostructure CNT/MoS2-Co was designed by electrospinning method.The CNT/MoS2-Co tube-in-tube nanomaterials can physically confine the shuttle of polysulfides through unique structures,and can also trap polysulfides through the direct chemical interaction between MoS2-Co and polysulfides.Among them,CNT can improve the electron transport rate.The sulfur-defective MoS2 doped with metallic Co clusters enhances the catalytic conversion from soluble polysulfides to insoluble lithium sulfides.The CNT/MoS2-Co@S composite cathode material exhibits excellent electrochemical performance,showing high specific capacity(initial capacity is 1372.3 mAh g-1 at 0.1C),excellent rate performance(the capacity is 634.4 mAh g-1 at 5C),and a low capacity decay rate at a large rate(the capacity decay rate is 0.050%after 1000 cycles at 5C).The pouch cell was assembled with CNT/MoS2-Co@S composite cathode material also exhibited excellent electrochemical performance(the initial capacity is 650 mAh g-1 at 0.2 C).(3)(NH4)2Mo3S13 molybdenum-sulfur molecular clusters were synthesized as sulfur host materials.In Li-S batteries,(NH4)2Mo3S13 can provide a certain capacity contribution,leading to higher volume/mass energy density.There is a strong chemical interaction between(NH4)2Mo3S13 molecular clusters and polysulfides,which can effectively inhibit the shuttle of polysulfides,and(NH4)2Mo3S13 molecular clusters can oxidize lithium sulfide and reduce the polarization during the charge and discharge process.The(NH4)2Mo3S13/S composite cathode material shows excellent battery performance,with high initial specific capacity(1125.2 mAh g1 at 0.1C),excellent rate performance(the capacity is 494.9 mAh g-1 at 5C),and a low capacity decay rate(the capacity decay rate is 0.051%after 400 cycles at 1C).(4)(NH4)2Mo2S12 molybdenum-sulfur molecular clusters were prepared as sulfur host materials.The half-cell assembled with pure(NH4)2Mo2S12 molecular clusters and Li sheets exhibits a high specific capacity(326 mAh g-1 at a current density of 100 mA g-1).Using(NH4)2Mo2S12 molecular clusters as sulfur host materials can improve the gravimetric/volumetric energy density of lithium-sulfur batteries.The interaction between(NH4)2Mo2S12 molecular clusters and polysulfides can inhibit the shuttle effect of polysulfides,and the oxidation of lithium sulfide by(NH4)2Mo2S 12 molecular clusters can promote the charging process of the battery and reduce polarization.(NH4)2Mo2S12/S composite cathode material shows excellent electrochemical performance,high initial capacity(1448 mAh g-1 at 0.1 C),excellent rate performance(the capacity is 798 mAh g-1 at 3C),good cycle stability(the capacity decay rate is 0.065%after 600 cycles at 3C).The(NH4)2Mo2S12/S composite cathode material also exhibited good battery performance at high and low temperature(-20℃,the initial capacity is 273.2 mAh g-1 at 1C;60℃,the initial capacity is 1218.9 mAh g-1 at 1C),the pouch cell with(NH4)2Mo2S12/S composite further proves its practical application.(5)A novel oxygen-bridged and polysulfuryl hetero-bridged binuclear molybdenum-sulfur nitroso compound(Mo2S11N2O3)3-was prepared as a sulfur host material.On the one hand,it can inhibit the shuttle of polysulfides,and can catalyze the reduction of polysulfides and the oxidation of lithium sulfide,thereby reducing the polarization of the battery,and finally achieving the purpose of improving the specific capacity and cycle life of lithium-sulfur batteries.The battery assembled with(Mo2S11N2O3)3-/S composite showed excellent electrochemical performance,with high initial specific capacity(1391.3 mAh g-1 at 0.1C),low capacity decay rate(the capacity decay rate is 0.082%after 400 cycles at 1C),and long cycle life(the initial capacity is 647.7 mAh g-1 after 500 cycles at 2C).