Design Of Cobalt-carbon Nanostructures For Application In Lithium-sulfur Batteriesmajor:Material Engineering

Energy is the material foundation of human survival during our daily life and production.Sustainable development is the common human goals and aspirations.But the shortage of energy has become one of the most serious problems in today's society.The theoretical energy density of the lithium-sulfur battery is 2600 Wh kg^-1,which is 3 to 5 times of the lithium-ion battery.It is a very promising electrochemical energy storage system and has been paid attention to by many researchers.The insulation of active substances,shuttle effect of polysulfide and active materials'volume fluctuation during charge and discharge process have been restricting the development of lithium-sulfur battery technology.In order to solve these problems,in this paper,we synthesized a kind of S@Co/C nanostructure in order to improve the conductivity by introducing porous hollow carbon structure and metal particles.Then,we synthesized two kinds of sulfur/cobalt oxide/carbon nanostructures to improve the electrochemical property by the synergistic effect of carbon and metal oxide.The main research results are listed as follows:?1?S@Co/C core-shell nanostructure for application in lithium-sulfur batteriesThe Polystyrene?PS?latex was prepared from styrene by emulsion polymerization.Then the PS was used as template to form Co/C nanostructure.The S@Co/C nanostructure electrodes were then formed by mechanical milling and melting-sulfur diffusion combined with elemental sulfur?S?.The S@Co/C nanostructure electrodes exhibited good rating performance.Besides,it get well coulombic efficiency and cycle stability in the charge-discharge cycle test.Under 2 C current density(1 C=1675 mA h g^-1),the apacity retention of S@Co/C nanostructure electrodes is 73.56%after155 cycles test.?2?S@CoO/C core-shell nanostructure for application in lithium-sulfur batteriesThe PS was used as template to form hollow porous carbon nanostructure.Then,the CoO/C nanostructure was synthesized in-situ.The CoO/C nanostructure was combined with elemental sulfur?S?by mechanical milling and melting-sulfur diffusion.The S@CoO/C nanostructure shows good rating performance.And under 2 C current density(1 C=1675 mA h g^-1),the apacity retention of S@CoO/C nanostructure electrodes is 64.08%after 200 cycles test.?3?S@Co₃O₄/C core-shell nanostructure for application in lithium-sulfur batteriesThe Co/C nanostructure was placed in a muffle furnace for oxidation in order to get the Co₃O₄/C nanostructure.Then,the S@Co₃O₄/C nanostructure was combined with elemental sulfur?S?to form the S@Co₃O₄/C nanostructure.The S@Co₃O₄/C nanostructure shows good rating performance.Besides,it shows good coulomb efficiency and its cycling performances are much better than the other two kinds of nanostructures.Under 0.1 C current density,the initial charge specific capacity reached 1360.7 mA h g^-1 and the discharge specific capacity was as high as 1454.5mA h g^-1.And under 2 C current density,the apacity retention of S@Co/C nanostructure electrodes was as high as 91.71%after 120 cycles test.The Co₃O₄ structure can adsorb polysufides chemically and the carbon shell can prevent polysulfides physically in order to inhibit lithium polysulfide dissolution in the electrolyte to prevent the loss of active material.Besides,the S@Co₃O₄/C nanostructure achieved excellent electrochemical performance because the carbon shell boosts the active material conductivity.S@Co₃O₄/C nanostructure provides a reasonable design for large current operating lithium sulfur battery.