Preparation And Electrochemical Performance Of Carbon-sulfur Composite Cathode Materials For Lithium-sulfur Batteries

Lithium–sulfur?Li–S?battery has a high theoretical energy density of 2600Wh·kg^-1 and high theoretical capacity of 1675 mAh?g^-1 which are much higher than the commercial Li–ion batteries.Sulfur is the cathode material of Li–S batteries,which has advantages of abundance,low price,environmental friendliness and Li-S battery is a promising energy storage device.Despite these promising advantages,several drawbacks still limit the practical application of lithium-sulfur battery.The main suppression factors for commercial lithium-sulfur battery are as follows:?i?low sulfur utilization due to the ionic and electrical insulation of sulfur and its discharge products?Li₂S and Li₂S₂?;?ii?the so-called“shuttle effect”,in which the polysulfides dissolve into the electrolyte and shuttle between the Li anode and sulfur cathode,thereby resulting in loss in active materials,low coulombic efficiency,and poor cyclability;?iii?the volume of sulfur expands during the lithiation process,which greatly decreases the cycle life of the lithium-sulfur battery.In this paper,carbon-based materials were used to design and prepare a series of carbon/sulfur composites,and their electrochemical properties were studied.The main research contents of this paper has the following aspects:?1?Cattail derived activated carbon?CAC?is prepared by pyrolysis of natural cattail following with KOH activation.Research results shows that the CAC sample has a high BET surface area of 1913.7 m²·g^-1 and pore volume of 0.893 cm³·g^-1,which is beneficial to effectively hold sulfur and restrain the shuttling effect of polysulfides during the galvanostatic charge/discharge processes using as the cathode matrix for the rechargeable Li-S battery.The pore size distribution of the CAC sample is about 2 nm,which makes the nonconductive sulfur being loaded into the pores of the carbon matrix in highly dispersed state and facilitates electrons transports.The S/CAC composite material shows an excellent cycle performance and a good rate capability when used as the cathode material in Li-S battery.At 200 mA·g^-1,the maximal discharge capacity of 1255.1 mAh·g^-1 and 663.3 mAh·g^-1 after 100 cycles,can be obtained respectively.Different rates test shows that S/CAC composite has good cycle stability and a stable 611.2 mAh·g^-1 discharge capacity at 1C rate.Compared with sulfur electrode,the electrochemical performance of S/CAC has been significantly improved.?2?Preparation of carbon nanotubes by magnesiothermic reduction of carbon dioside and acid-leaching the reaction products.Research results shows that,the CNTs sample has a high BET surface area of 1407.9 m²·g^-1,pore volume of 2.172cm³·g^-1 and the pores are mainly larger mesopores.As a positive electrode for lithium-sulfur batteries,carbon nanotubes can be well dispersed and loaded with elemental sulfur and can better restrain the shuttling effect of polysulfides.Therefore,S/CNTs composites have a good cycle performance.At 200 m A·g^-1,S/CNTs composites shows an initial discharge capacity of 1384.9 mAh·g^-1 and 607.9mAh·g^-1 after 100 cycles,the average capacity decay rate per circle reaches 0.56%.CNTs as a carrier for active material sulfur enables batteries to have a high specific discharge capacity and good cycle performance.Compared with sulfur electrode,the electrochemical performance of S/CNTs has been significantly improved.