| Due to the abundant and non-polluting sources of singlet sulfur,which has a high theoretical specific capacity(1675 m Ah·g-1)as a battery cathode,lithium-sulfur batteries show a large potential for development in the field of energy storage devices.However,monolithic sulfur as the cathode of lithium-sulfur batteries suffers from the disadvantages of large volume expansion,poor electrical conductivity and polysulfide dissolution during the reaction process,which limits the large-scale application of lithium-sulfur batteries.Cotton straw is abundant,cheap and readily available,and can be applied to the preparation of cathode materials for lithium-sulfur batteries through carbonisation modification.In this study,cotton stalks were used as a biomass carbon source to prepare carbon materials by activator modification,metal sulfide coating and biological pretreatment,followed by composite materials with molten sulfur to improve the electrochemical performance of lithium-sulfur batteries.The main conclusions obtained are as follows.（1）Using cotton stalk as raw material and KOH as activator,sulfur carbon composites were prepared by controlling alkali carbon ratio,activation temperature and sulfur content.The results show that high graphitization degree,large specific surface area and pore volume can alleviate the volume expansion of sulfur,improve the conductivity of sulfur and adsorb polysulfides,so as to improve the electrochemical performance of lithium sulfur battery.When the ratio of alkali to carbon is 4:1,the ratio of sulfur to carbon is 2:1 and the activation temperature is 800°C,the prepared electrode AC800-14-S-67%（sulfur load is66.8%）has the best electrochemical performance.When the cycle test is carried out at 0.1 C current density,the specific discharge capacity of the first cycle is 1468 m Ah·g-1.After 1100 cycles,the capacity remains at 787 m Ah·g-1.At the same time,it also shows the best rate performance in the charge discharge test under high current.（2）ACF-12N-100%Mo S2was synthesised by hydrothermal reaction using an activated carbon material（AC800-14）as a substrate,and then ACF-12N-100%Mo S2-S material was prepared with molten state sulfur.Mo S2 can enhance the adsorption of polysulfide and hinder further capacity decay,but its conductivity is poor,so combining it with carbon material can reduce the negative effects.While both the thiourea,the raw material for the synthesis of Mo S2,and the added nitrogen-containing compounds contain nitrogen,which introduces nitrogen to the prepared material and thus can improve the electrochemical performance of the battery.The results show that the AC800-14-S-67%electrode has an initial cycling capacity of 915.3m Ah·g-1 at 0.5 C and a capacity of 485.6 m Ah·g-1 after 100 cycles.At a nitrogen concentration of 33%and a carbon to Mo S2 ratio of 1:1 and a carbon to sulfur ratio of 1:2,the prepared ACF-12N-100%Mo S2-S electrode（with a sulfur loading of 58.16%）,with an initial discharge capacity of 953.2 m Ah·g-1 at 0.5 C and a discharge capacity of 694.8 m Ah·g-1 after 100 cycle tests,performed significantly better than the AC800-14-S-67%electrode.（3）To further improve the battery performance,cotton straw was subjected to biological pretreatment,KOH activation and Mo S2 coating to investigate the effects of various factors on the material structure and battery electrochemical performance.The results showed that the Cel0.3-Ye7-ACF-N/Mo S2-S material electrode（67.165%sulfur loading）prepared under optimal conditions（cellulase 0.3%,yeast dosage 7%）of biological pretreatment,KOH activation（4:1 alkali to carbon ratio）and Mo S2 coating（1:1 carbon material to Mo S2 dosage ratio）,with a sulfur ratio of 1:2.The discharge capacity under 0.5 C was 1102.1 m Ah·g-1and remained at 783.3 m Ah·g-1 after 100 cycles,indicating that the pretreatment with cellulase and yeast can increase the number of pores in the carbon material,while achieving endogenous and homogeneous doping of the material and improving the electrochemical performance of the cell. |
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