Study On The Variable Transition Metal Compound Carrier To Enhance The Electrochemical Performance Of Lithium Sulfur Batteries

Li-S batteries mainly sufferred from the shuttle effect,this paper made improvements by choosing a series of transition metal compound with variable metal element including LiMn₂O₄,VPO₄,VOPO₄ and?-MoO₃.These compounds all had efficient adsorption of polysulfide and could restrain effectively the shuttle effect.In addition,these compounds had a stable molecular structure during platform voltage in Li-S battery,and could function as the storage substrate of the active sulfur.In chapter of the experiment with LiMn₂O₄,UV/vis absorption spectra suggested Li₂S₆ was adsorbed by LiMn₂O₄,FTIR spectroscopies indicated hypsochromic shift in LiMn₂O₄/Li₂S₆ was contributed to presence of complexation between LiMn₂O₄ and Li₂S₆.The results demonstrated that LiMn₂O₄ had strong adsorption of polysulfides.Combining XPS measurements result,a possible adsorption mechanism was speculated that part Mn⁴⁺on the surface of LiMn₂O₄ was reduced and transformed Mn³⁺and Mn²⁺.At the same time,some S₆^2-was oxidized and transformed Li₂S_6-x-x which was adsorbed on S/LiMn₂O₄electrode.In addition,S/LiMn₂O₄ electrode could still exhibited 512.2 mA h g^-1 at 1 C at300^th cycles,exceeding 392.6 mA h g^-1 of S/C electrode.And the diffusion coefficient of S/LiMn₂O₄ electrode was determined to be 1.41×10^-13 cm² s^-1,better than 1.22×10^-13cm² s^-1 of S/C electrode,indicating good lithium ion conductivity of S/Li Mn₂O₄ electrode.Therefore,as good lithium ion conductor,spinel LiMn₂O₄ could remarkably promote electrochemical performance of Li-S batteries.In chapter of the experiment with VPO₄ and VOPO₄,the cell with SVOC and SVC electrode in aspect of electrochemical performance showed an improved cycling stability,rate performance,and discharge capacity.The cell with SVOC?or SVC?electrode could exhibit discharge capacity up to 1044.9?or 1261.3?mA h g^-1 at 0.1 C rate and 535.5?or508.9?mA h g^-1 after 500 cycles at 2 C rate.The adsorption experiment and UV-vis absorption spectra showed that VPO₄ and VOPO₄ can effectively adsorb Li₂S₄ and Li₂S₆.Infrared spectra indicated that VPO₄/Li₂S₄,VPO₄/Li₂S₆,VOPO₄/Li₂S₄ and VOPO₄/Li₂S₆curve occurred shift due to the existence of chemical adsorption between VPO₄?or VOPO₄?and lithium polysulfides,these results strongly suggested that VPO₄?or VOPO₄?had strong adsorption effect on polysulfides.Moreover,according to the contrast analysis results of XPS,including S 2p,V 2p,P 2p,O 1s,it could be seen that there existed obvious chemical interaction between VPO₄?or VOPO₄?and lithium polysulfides.Thanks to rich oxygen atoms on VPO₄ and VOPO₄ molecular surface,which could form a strong adsorption with lithium polysulfides,so that inhibited effectively the shuttle effect,and made the utilization of active sulfur effectively enhancing,finally improved the electrochemical performance of lithium sulfur batteries.In chapter of the experiment with?-MoO₃,the cell with S/??-MoO₃+CNTs?electrode could exhibit discharge capacity up to 1151.7 m A h g^-1 at 0.1 C rate and 84.0%capacity retention after 400 cycles at 0.5 C rate.On further studies of the mechanism,UV-vis absorption spectrum,FTIR spectroscopy,Raman analysis and XPS study were applied,and results revealed that?-MoO₃ could effectively alleviate the migration of polysulfides via anchoring polysulfides with the Li bond.The adsorption energy that Li₂S₄ adsorbed on the surface of?-MoO₃?001?was-7.11 eV,and the binding energy that Li₂S₆ adsorbed on the surface of?-MoO₃?001?was-7.39 eV by DFT calculation and analysis.It indicated the interaction between Li₂S₄?or Li₂S₆?and?-MoO₃,which made the molecular structure of?-MoO₃ changed after the adsorption,and the Li-O bond played a major role in it.The DFT calculation showed that both Li₂S₄ and Li₂S₆ could bond with the?-MoO₃,thus effectively adsorbed the soluble polysulfides and enhanced the electrochemical performance of the lithium sulfur battery.Compared to the complicated sulfur cathode design,our work was simple in experiment,presenting the potential of practical utilization and scale-up production.