Preparation And Properties Of Cathode For Lithium-sulfur Batteries

Sulfur has a specific energy density of 2600 Wh/kg in theory and the raw materials which are cheap,easy to be obtained and friendly to environment,are expected to become a high energy storage cathode material for lithium battery system.However,the elemental sulfur as a lithium-sulfur battery cathode material has a certain degree of constraints.On the one hand,elemental sulfur is electron and ion insulation which leads to the poorly electrochemical performance;On the other hand,in the discharge process,polysulfide can be easily soluble in electrolyte which results in "shuttle effect" and the loss of active substances.In order to improve the properties above,we study the influence of different proportions of active materials,conductive agents and adhes ives on the performance of the batteries in this paper.The active material was prepared by the method of ball milling,which the mass ratio of sublimed sulfur and acetylene black was 7:3.Acetylene black and polyvinylidene fluoride(PVDF)were acted as conductive agent and binder,respectively.Five different proportions of active material were prepared.Electrochemical tests show that the highest initial discharge capacity of cathode material with proportion of 7:1:2 reaches 1019.0 mAh/g at the current rate of 0.1C,and after 100 cycles,the value is 547.2 mAh/g with capacity maintain of 53.7% among the five different ratios.The morphologies of sulfur/carbon composites are affected by the preparation methods.Mechanical ball milling can only achieve a s imple mixture of nano-carbon and large particles of sulfur.The particle size of sulfur particles is above 10μm,and the particle spacing is relatively large.Heat treatment can make the sulfur distribute in the pores and particle gap of nano-carbon particles,getting small and high dispersion of sulfur/carbon composites.The particle spacing becomes smaller and charge transfer impedance decreases which will show better electrochemical performance.In the first discharge,the specific capacity of sulfur/carbon composites treated by mechanical ball milling method is 1002.4 mAh / g at the current rate of 0.1 C when the doping content of Nd2O3 is 5wt% and after 100 cycles the value is 547.2 mAh/g with capacity maintain of 53.7 %.The best property is obtained when the doping amount of Nd2O3 is 5wt %.However,the weak adsorption force and poor conductivity of CeO2 and TiO2 lead to the relatively bad performance of lithium-sulfur battery.In the first discharge,the specific capacity of sulfur/carbon composites with the 5wt% doping content of Nd2O3 is 1222.4 mAh/g at the current rate of 0.1 C and after 100 cycles the value is 547.2 mAh/g with capacity maintain of 53.7%.The best property is obtained when the doping amount of Nd2O3 is 5wt %,consistent with mechanical ball milling.Because Nd2O3 has a strong adsorption of elemental sulfur,it can be tightly adsorbed around the carbon forming a good conductive network,which can greatly reduce the transmission resistance of lithium ions and increase the specific capacity of sulfur cathode.In addition,the strong adsorption of polysulfides reduces its solubility in the electrolyte and improves the cycle stability.