Effect Of Cathode Supporting Materials And Surface Modification On Electrochemical Performance Of Lithium-sulfur Battery

As one of the most promising battery systems of next generation to replace traditional lithium ion battery,lithium-sulfur battery has shown the advantages of high capacity,high energy density,low cost and so on.However,its practical application is still blocked by the following issues:the first one is the low conductivity of sulfur,which results in the low utilization of the active components due to the hindering of the transmission of electrons and ions.Thus,it is unable to complete the necessary electrochemical reactions.Second,the discharge intermediates Li₂S_x?2?x?8?that generated from the reduction of S₈ to Li₂S is easily dissolved into electrolyte.This shuttle effect will cause the sharp decrease of capacity and the loss of output voltage.Third,Li₂S has a small density,which resulting in the volume expanding?about 80%?during the discharge.This usually cause the structure damage if there is no relief space.What's more,The the serious problem from the cathode will further influence the other components of the cell.Base on the discuss above,this paper aiming at settling the cathodic problems by constructing the reasonable cathode material to improve the electrochemical properties of the cathode.Mesoporous SiO₂ with different morphologies and structures is used as the supporting material for sulfur.Water-bath method was adopted to synthesize mesoporous silica.PH value and template selection can both influence the morphology and pore size of the products.Spherical particles with ordered pore structure,and rod-like,stripe-like,and hexahedron SiO₂ particles with ordered pore structure are prepared.Melt impregnation method is adopted to composite sulfur with SiO₂.After that,weak reduction graphene oxide?RGO?was coated on the S/SiO₂composite to improve its conductivity and conduct as the confine layer for polysulfides.The concept of“layer@adsorbent”architecture is established.The effect of this architecture is studied and verified by testing the electrochemical performance of the RGO@S/SiO₂ cathodes.It is found that mesoporous SiO₂ with the surface-OH groups and enhanced capillary force as well as its solid acidity provides strong adsorption ability to polysulfides and well wettability to sulfur and/or polysulfides,which will bring high reactivity and effective limiting to shuttle effect.Excellent electrochemical performance is obtained on the RGO@S/SiO₂ cathodes.The capacity of 1625,1000,1600 and 650 mAh g^-1 are obtained under 0.2C from the four batteries.After 200 cycles,the capacity can still retain at 1122,800,603 and329.1 mAh g^-1.Metal elements were introduced into ordered mesoporous SiO₂,which give rise to the numbers of the active adsorption sites for polysulfides and the enhanced conductivity owning to the introduction of the nonuniformity in density states.This is proved by theoretical calculation the possible electronic structure of metal elementsof Ti,Al and Sn in the SiO₂ lattice.Among them Ti-SiO₂ structure has the lowest formation energy.The improved cycle stability and sulfur loading and rate capability exhibit the effect of Ti modification in the compound.The rod-like and stripe-like S/Ti-SiO₂ cathodes delivered the capacity of 352 and 443 mAh g^-1 at 0.2C after 1000 cycles,corresponding to 34.2%and 44.1%of the initial capacity,respectively.The capacity decay rates of 0.065%and 0.055%are observed on the two cathodes,respectively.It is found that,Ti modification can not only improve the conductivity,also enhance the adsorption ability of the SiO₂.XPS results prove the existing of Ti-S bonds in the Ti-modified samples,which can limit the dissolution of LiPSs.To verify the universality of this method,Al and Sn modification are conducted on SiO₂ as well.They both exhibit excellent electrochemical performance by retaining the capacity of 944 and 352 mAh g^-1 after 1000 cycles at 0.2C.This demonstrates the generally applicable method can reliably improve the performance of Li-S batteries.Ti₃C₂ MXene?Ti₃C₂ene?which has Ti-O bonds on surface is used as the additive material in cathode of lithium-sulfur battery to improve the electrochemical performance of sulfur.An accordion-like morphological Ti₃C₂ene is fabricated to gain the high sulfur loading.Different sulfur composite methods influence the final performance.The wettability of Ti₃C₂ene depends the functional groups of Ti₃C₂ene.Good electrochemical performance is obtained from the S/Ti₃C₂ene cathode material fabricated by water-bath method,which however,has a lower capacity retention than that by melt-diffusion method.Flash oxidation is conducted on Ti₃C₂ene under different temperature to gain more stable polar Ti-O bonds.Titanium oxides?Ti_x O_y?are in situ generated on the Ti₃C₂ene nanosheets with different oxidized degree.The oxides are transformed from substoichiometric Ti_n O_2n-1 to TiO₂ as the increase of the treating temperature.Improved performance is delivered on the sample oxidized under 500?,which show the most optimum particle size and number.After composed with sulfur,the capacity of 845 mAh g^-1 is obtained after 500 cycles under0.2C.Moreover,the negligible capacity decay of 0.056%,0.083%,0.11%and 0.12%are delivered at high current rate of 0.5C,1C,2C,and 5C.The oxidized material shows improved chemical performance compared with the unoxidized material because of the combination of the high conductivity and surface of Ti₃C₂ene,and the stable adsorption sites from Ti_xO_y.Meanwhile,the interfacial storage of lithium ions is discovered and analyzed.