Preparation Of Microporous Carbon/Sulfur Cathode And Functional Binder For Lithium-Sulfur Batteries

Current commercial lithium-ion batteries（LIBs）owing to their low energy density cannot meet the increasing social demands,so it is very urgent to develop a new high-energy-density energy storage system.Lithium-sulfur batteries are expected to be a promising alternative to LIBs because of their advantages,such as high theoretical capacity and energy density,low cost,safety,and environmental friendship.However,the low conductivity of sulfur,and large volume expansion and serious"shuttle effect"in the cycle process hinder the commercial application of lithium-sulfur batteries.In order to address these issues,this work developed a new sulfur cathode material and designed a novel functional binder to alleviate the"shuttle effect"and increase the utilization of active materials for lithium-sulfur batteries.1.Aiming at the problem of"shuttle effect",we first prepared a nitrogen-doped microporous carbon material（NDMC）as cathode for lithium-sulfur batteries.The carbon material possesses micropores with a unimodal size distribution,and the pore size is mainly concentrated at 0.57 nm.When used as a sulfur host,due to the microporous confinement effect,the S₈ molecules ruptured in heat treatment process cannot re-aggregate at room temperature and can only exist in t he form of small molecules such as S₂ and S₄,resulting in small molecule sulfur cathode(C/S_2-4).In the discharging process,S₄ starts to react to form the final discharge d product Li₂S;during the charging process,Li₂S molecules are oxidized to form S₂ and S₄.Since microporous confinement effect still exists,S₂,S₄ molecules cannot further aggregate to form large S₈ molecules.Therefore,the whole process avoids the formation of Li₂S₆,Li₂S₈ and the occurrence of"shuttle effect".In the carbonate electrolyte,the small molecule sulfur electrode forms a stable solid electrolyte inte rface（SEI）,leading to good cycling stability.At 0.2 C,after 500 cycles,NDMC/S-2 maintains a stable capacity of 902 m A h g^-1 with the coulombic efficiency of 100%.Even at a high rate of 10 C,it still has a high capacity of 511.8 m A h g^-1.2.An aqueous binder,D-PAA/C-EA prepared by deprotonating polyacrylic acid,has many advantages as a binder in lithium-sulfur batteries.（1）D-PAA/C-EA has stronger adhesion strength than PAA,alleviating the volume expansion of the sulfur cathode effectively during the cycle,so as to achieve the purpose of stabilizing the cathode and maintaining the integrity of the electrode.（2）D-PAA/C-EA shows good electrochemical and chemical stability.Cyclic voltammetry test shows that D-PAA/C-EA produces no redox peaks in the working range of lithium-sulfur battery（1.5-2.8 V,vs.Li⁺/Li）;the D-PAA/C-EA film immersed into lithium-sulfur battery electrolyte or polysulfide will not swell;FTIR and ¹H NMR show that D-PAA/C-EA does not react with electrolyte or polysulfide.（3）D-PAA/C-EA has a good anchoring effect for polysulfides,which is beneficial to alleviate the"shuttle effect"and reduce the loss of active materials.（4）The ionic structure in D-PAA/C-EA helps the diffusion of Li⁺and improves battery performance.In the end,the D-PAA/C-EA/S electrode can maintain a high capacity of 716.4 m A h g^-1 with coulombic efficiency of 100%after 300 cycles.Even at a high rate of 10 C,the D-PAA/C-EA/S still shows a high capacity of 638.6 m A h g^-1.When the sulfur loading increases to 3 and 5 mg cm^-2,the capacity of D-PAA/C-EA/S cathode can reach 920.7 and 679.0 m A h g^-1after 100 cycles.