Nano-structured Carbon Materials For High Performance Lithium-sulfur Batteries

Lithium-sulfur(Li-S)batteries have been considered as most promising candidates for the next generation lithium-ion batteries,as sulfur features high specific energy density(2600 Wh kg-1),high theoretical capacity(1675 mAh g-1),cost nontoxicity,and effectiveness.However,the broad application of Li-S batteries is limited by many persistent issues,for instance the low electrical conductivity of sulfur and its discharge product,the diffusivity of polysulfide intermediates in organic electrolytes and high solubility,and the related side reaction named "shuttle effect",as well as volumetric expansion caused by the intercalation of Li into the sulfur.Therefore,enhancing the conductivity of the sulfur cathode,improving the tolerance for volume expansion,limiting polysulfide diffusion,and maintaining the soluble polysulfides within the cathode region are the key points for the development of Li-S batteries.The oriented-mesoporous-carbon with ?-Al2O3 modification is synthesized via carbonization of a glucose-urea-resin-filled MOF MIL-53 with nano-CaC3 and double templates of NaCl-KCl eutectic.The NaCl-KCl eutectic and glucose-urea resin fill the tunnels in the MIL-53 to prevent the combination and tunnel collapse during carbonization.The nano-CaCO3 stays out of the tunnels to avoid forming sealed tunnels.The synthesized oriented-macroporous-carbon displays outstanding polysulfide absorption capability,attributed to the co-existence of active O and N sites and its ultrahigh specific surface area for polysulfide absorption.The resultant sulfur electrode exhibits much higher discharge capacity,outstanding rate performance,and long life.An initial capacity of 1626 mAh g-1 at 0.05 C and a rate capacity as high as 430 mAh g-1 at 10 C have been obtained.A reversible capacity of 850 mAh g-1 is retained after 300 cycles at a discharge rate of 0.2 C.A green and scalable preparation method of porous carbon is developed for high performance lithium-sulfur battery via carbonization of biomass.The unique architecture of the carbon obtained from the carbonization of watermelon rind(WR)offers much higher specific surface area to facilitate polysulfide adsorption in Li-S battery.Derived from citrulline containing in WR,active site for polysulfide absorption is created after pyrolysis.The abundant micropores and the created S-C-N bondage contribute the superb polysulfide absorption ability of not only Li2S4 but also Li2S3 unlike other carbon materials.The resultant pressed Li-S battery shows excellent cycleability.A capacity as high as 1,030 mAh g-1 is retained after 100 cycles at 0.2 C charge-discharge rate.A uniform porous carbon is prepared by carbonization of a freeze-dried mixture obtained by flash freeze of a suspension containing starch and nano-CaCO3(template).We first find that sulfide heaving occurs at cathode,attributed to the repeated sulfur segregation and increasing presence of interparticle sulfide during cycling.Sulfur segregation arises from the electro-osmotic drag during Li2S delithiation.Lithiation of the segregated S forms interparticle sulfide,repelling neighboring particles to create cavities within electrode layer.Exerting an external pressure on cathode can effectively suppress sulfide heaving,thereby achieving high and stable areal capacity and energy density.The cathode using S-loaded porous carbon containing 90 wt.%of S demonstrates an energy density of 11.7 mWh cm-2 and retains 10.5 mWh cm-2 after 300 cycles at 0.2 C under an external pressure of 600 Kg cm-2.When increasing S-loading up to 15.1 mg cm-2,a stable areal energy density of 19.2 mWh cm-2 is reached,which is 1.7 times as high as that of commercialized LIBs,presenting an important progress to practical application of high energy Li-S battery.