The Design And Electrochemical Performance Of Cathode Materials For Lithium-Sulfur Batteries

The research of Lithium-Sulfur?Li-S?batteries has become increasingly sought-after in recent years due to the high specific capacity(1675 mA h g^-1).In spite of the appealing feature,the problems of high overpotentials and low utilization of active materials caused by low conductivity of sulfur still cannot be neglected.Besides,the intermediate polysulfides generated during charging and discharging are highly soluble in the ether electrolyte,resulting in the poor cycle life,low efficiency,and serious self-discharge.Therefore,a strategy is needed to be developed to improve the utilization of active materials and inhibit the diffusion of polysulfides.My research work focuses on the design and synthesis of cathode materials for Li-S batteries,trying to solve the problems of Li-S batteries from the root cause.We demonstrate the reasonable design and preparation of carbon-based composite containing chitosan?CTS?and highly dispersed nanosized sulfur?denoted as CTS/C-S?for Li-S batteries by a precipitation method.Then,to further improve the performance of lithium-sulfur batteries,we prepared small particles of sulfur under the action of CTS,which were then coated with graphene oxide?GO?,the CTS/S@GO cathode exhibited outstanding cycle stability and discharge specific capacity.Specifically,my thesis work involves following aspects:?1?The CTS/C-S hybrid nanosized sulfur was preapred by a precipitation method making use of the alterable solubility of chitosan?CTS?in aqueous solution.CTS is used as a capping agent for sulfur particles and an inhibitor to limit the size of sulfur particles during synthesis.And CTS with numerous amine groups?-NH₂?and hydroxyl groups?-OH?can provide plenty of binding sites for polysulfides to prevent the diffusion of polysulfides.In addition,KB and CNTs act as the three-dimensional conductive channels to accelerate the electro transfer.Thus,the prepared CTS/C-S cathodes deliver an initial specific capacity of 1141.6 mA h g^-1 under 0.5C(1 C=1675 mA g^-1),and a capacity of 842.3 mA h g^-1 is remained even after 300 cycles,which is⁵0%better than that of the C/S cathodes,indicating that the presence of CTS in CTS/C-S cathodes can enhance the performance of Li-S batteries.?2?The CTS/S@GO hybrid was prepared by coating GO on sulfur particles.GO coating layers are important to accommodating volume expansion of the coated sulfur particles during discharge,trapping soluble polysulfide intermediates,and improving the conductivity of sulfur particles.The CTS/S@GO hybrid shows high stability and high reversible capacity up to¹100mA h g^-1 under 0.5 C over more than 100 cycles,and a capacity of 799 mA h g^-1 is remained even after 300 cycles at 1 C,showing excellent cycle stability.