Polar Metal Oxides As The Additives In Sulfur Cathode For High Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries are one of the most promising next-generation energy storage systems due to their high theoretical energy density,low cost and environmental benignity.However,commercial application of Li-S batteries is still limited by insulating nature of S and Li2S,shuttle effect of polysulfides,volumetric expansion of sulfur cathode.Li dendrite and corrosion.In this paper,we studied systematically the effects of polar metal oxides as the additives on the electrochemical performance of sulfur cathodes.Combined with material characterization,theoretical calculation and electrochemical testing,the possible roles of metal oxides on retarding the polysulfide shuttling were analyzed.Main conclusions are summarized as follows:(1)A strategy has been developed to mediate the polysulfides redox reaction by introducing surface oxygen vacancies on MnO2.A series of oxygen-dificient MnO2 nanospheres were synthesized and used as the sulfur hosts.The results showed that the oxygen vacancies could enhance the polar interaction between MnO2 and polysulfides.Moreover,surface oxygen vacancies could promote the conversion of soluble polysulfides into insoluble thiosulfate and polythionate.Such a chemical conversion strategy could fundamentally limit the shuttling of polysulfides,as well improve the utilization of active materials and extend cycling life.The resulting cathode could achieve a high reversible capacity of 551 mAh g-1 at 2 C over 400 cycles.The concept of surface chemistry engineering paves the way for the future application of metal oxides in sulfur cathodes.(2)A series of mesoporous carbon hollow spheres/metal oxide/sulfur composite cathodes were developed for high-performance Li-S batteries.It was found that most of the metal oxides including La2O3,Al2O3,Ceo2 and Nb2o5 could effectively improve the electrochemical performance of sulfur cathode.Especially,Nb2o5 could serve as the electrochemical catalyst,which significantly promote the reduction kinetics of polysulfides and deliver a high reversible capacity of 721 mAh g-1 at 5 C.However,MoO3 accelerated the deposition of Li2S/Li2S2 to form a thick passivation layer of insulating soilds,which increases electron transfer and ion dilffusion resistance and shortens the second discharge platform.Thus,rational selection and design of metal oxide is vital to develop high performance Li-S batteries.