Preparation And Electrochemical Performance Of Molybdenum-based Mediators For Lithium-sulfur Batteries

Lithium-sulfur batteries with high theoretical energy density are one of the most promising battery systems for next-generation energy storage systems.However,the practical application of lithium-sulfur batteries still faces many problems,such as the shuttling behavior of lithium polysulfides and their slow conversion kinetics.The introduction of mediator materials that can modulate the physical or chemical behavior of sulfur species in the anode is an effe ctive way to solve these problems.Molybdenum-based materials show high catalytic activity in reactions such as hydrodesulfurization and electrochemical oxygen reduction,and hold a great potential to enhance the reaction kinetics of lithium-sulfur battery cathodes.In this thesis,different molybdenum-based mediators are designed and synthesized.The working mechanisms of the prepared mediators in lithium-sulfur batteries are investigated through a combination of theory and experiment.MoN@graphene（MoN@G）mediator is constructed and the effect of its use as cathode surface modification layer on the electrochemical performance of lithium-sulfur batteries is investigated.It is found that MoN@G can reduce the charge transfer resistance of cathodes,accelerating the kinetics of cathode electrochemical reactions.The cell exhibites a discharge capacity of 606 m Ah g^-1 at a current density of 2 C.Meanwhile,MoN@G can effectively adsorb polysulfide s and improve the cycling performance of batteries.The average capacity decay rate of the battery during 1000 charge/discharge cycles is only 0.029%.In addition,the electrochemical test results show that MoN@G has a catalytic effect on the decomposition reaction of Li₂S.First-principle calculations reveal that the decomposition energy barrier of Li₂S on the MoN surface is only 0.22 e V,which is significantly lower than that on the graphene surface（1.88 e V）.The mechanism by which MoN promotes the decomposition of Li₂S is that 1)MoN can covalently bind Li₂S and reduce the charge density of S atoms in Li₂S,thus weakening the S–Li bonding（S-Li bond length increases from 2.1（?）to 2.4（?））;2)lithium ions generated by the decomposition of Li₂S can migrate rapidly on the MoN surface,which is conducive to the continuation of the decomposition reaction.Mo₅N₆@graphene（MNNP@G）mediator is prepared by a solvothermal route combined with high-temperature nitridation.Its structure and morphology are characterizated and it is found that Mo₅N₆ nanoparticles in MNNP@G are uniformly loaded on graphene.The electrochemical test results show that Mo₅N₆can effectively catalyze the conversion reactions of lithium polysulfides.The results of first-principle calculations reveal that Mo₅N₆ has an intrinsic facilitation effect on the cathode discharge process of lithium-sulfur batteries.The nucleation and growth processes of Li₂S on MNNP@G surface are systematically investigated by a combination of Li₂S deposition test,electrochemical impedance spectroscopy test and electron microscopy technique.The results show that MNNP@G can guide three-dimensional deposition of Li₂S,thus effectively reducing the electrode polarization and improving the utilization of active material s.The battery still delivers a high discharge capacity of 954 m Ah g^-1 at a current density of 2 C and can stably work for 1000 cycles.B-doped MoS₂@carbon nanotube（B-MoS₂@CNT）mediator is designed and constructed.The surface electronic structures of MoS₂ and B-MoS₂ are investigated by first-principle calculations.The results show that the unoccupied orbitals on MoS₂ surface are parallel to the basal plane and cannot form an effective overlap with the occupied orbitals of the surface adsorbed species.B doping endows MoS₂with unoccupied orbitals outward perpendicular to the basal plane which can form a maximum overlap with the occupied orbitals of lithium polysulfide s in the form of"head-on".Both electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy confirm that B doping enhances the interaction between MoS ₂ and sulfur.Adsorption tests confirm that B doping enhances the adsorption capability of MoS₂ on lithium polysulfides.In addition,compared with MoS₂,the adsorbed lithium polysulfides on B-MoS₂ surface have a greater lithiation tendency,indicating that B doping can enhance the facilitation effect of MoS₂ on the cathode discharge process.Meanwhile,B doping reduces the Li₂S decomposition energy barrier on MoS₂ surface,which improves the catalytic effect of MoS₂ on the cathode charging reactions.The electrochemical test results show that the battery has fast redox reaction kinetics and excellent cycling stability,exhibiting a discharge capacity of 1229 m Ah g^-1 at a current density of 0.2 C and still a reversible capacity of 711 m Ah g^-1 at a high current density of 5 C.The average capacity decay rate during 1000 charge-discharge cycles is only 0.020%.