Synthesis and Electrochemical Performance of Sulfur Cathode Materials with High Stability
Supervisor:li wen cui
Lithium-sulfur(Li-S)batteries have attracted increasing attention as next-generation energy storage devices for vehicles,unmanned aerial vehicle and grid-scale stationary storage due to their high energy density of 2600 Wh kg-1,environmental friendliness and low cost.Despite these considerable advantages,the practical realization of Li-S batteries is hindered by several issues.First,the intrinsic insulation of sulfur and lithium sulfides leads to poor rate performance.Second,Li-S batteries suffer from severe capacity degradation during cycling,mainly caused by the dissolution of intermediate polysulfides and volume expansion of S cathode.Third,in high-loading Li-S batteries,a significant amount of solid sulfur species will precipitate on the electrode/electrolyte interface during charge/discharge process.Since the solid sulfur species are highly insulating,the ion transportation channels in electrodes are likely to be blocked,which will result in poor sulfur utilization and severe shuttle effect.To mitigate these challenges,design of novel sulfur cathodes is particularly attractive,which an solve the problems from the origin.Herein,my research work has focused on designing and synthesizing a series of sulfur cathodes with greatly enhanced electrochemical performance to solve the problems mentioned above.We had developed an electrolysis approach to prepare carbon-sulfur hybrid and investigated its electrochemical performance.Then,a paragenesis BN/CNTs hybrid was designed as a promising sulfur host to improve the cycle stability of sulfur cathode.The paragenesis BN/CNTs hybrid exhibited high conductivity and high adsorption capacity of polysulfide.When used as hosts for sulfur cathode,the BN/CNTs-sulfur cathode exhibited a prominent cycling stability and high rate performance.At last,3D self-supporting porous carbon hosts with N,O-doping were designed and synthesized as working electrodes for sulfur electrodeposition to prepare sulfur cathodes.The obtained sulfur cathodes show high areal sulfur loading,high areal capacity and good cyclic stability.Specifically,the work includes the following parts:(1)The sulfur-carbon hybrid cathodes prepared by the methods including mechanical mixing,melt-diffusion strategy and solution-based synthesis,generally contain some non-conductive sulfur agglomerations due to insufficient incorporation of sulfur inside the pores of carbons.To address this problem,an electrolysis approach was developed to prepare carbon-sulfur hybrid by using monolithic carbon as the working electrode and sodium polysulfide solution as electrolyte.The monolithic carbon consists of homogeneously interconnected nanosheet units,which produces conductive network and abundant unblocked macropores in the carbon framework.The conductive network facilitates electron transmission and the abundant unblocked macropores allow easy diffusion of the electrolyte,which are beneficial to electrolysis.During electrolysis,the polysulfide ions in the electrolyte could access the pores and active sites under the electric field and capillary forces,and once polysulfide ions reached these places,they were converted into sulfur by losing two electrons.The sulfur gradually filled the pore space with increasing time of electrolysis.Such sulfur is confined in the pores with small size and electrically connected to the carbon matrix in nature,leading to high utilization of the ectrochemically incorporated S.Thus,the sulfur-carbon hybrid showed excellent electrochemical performance,including good rate capability(4 C,652 mA h g-1),and good cycling stability(maintain 612 mA h g-1 after 500 cycles at 0.5 C).(2)The BN/CNTs hybrid with paragenesis structure was prepared by using the mixture of boric acid,urea and cobalt acetate as precursor via one-step co-pyrolysis method.Such paragenesis structure remarkably lowered the contact resistance between non-conducting BN and conductive CNTs,and the O dopant in the BN narrowed the bandgap of BN,both giving the BN/CNTs hybrid a high conductivity of 1884 S m-1.In addition,p-BN/CNTs has a high surface area of 168 m2 g-1 and pore volume of 0.33 cm3 g-1,which is higher than that of most polar metallic compounds.The high surface and the O doping enable BN/CNTs hybrid to adsorb polysulfides with a superior capacity,which can reduce the "shuttle effect".Noticeably,the sulfur in the p-BN/CNTs is monoclinic phase and highly dispersed due to the strong interaction between p-BN/CNTs and the sulfur,which can improve the untilization of sulfur.As a result,the BN/CNTs-sulfur cathode delivered high specific capacity(1374 and 1068 mA h g-1 at 0.2 C and 1 C,respectively),superior cycling stability(a constant coulombic efficiency of～99%and a decay rate of 0.045%per cycle along with 500 cycles at 1 C)and excellent rate capability(840 mA h g-1 at 4 C).(3)A self-standing conductive and robust carbon network(CNTs/N-PCNF),built by welding CNTs with 0,N-doped hollow carbon nanofibers together,was designed as an ideal sulfur host to improve the area capacity of sulfur cathode.In such cathode construction,the CNTs and N-PCNF were welded together,not only offered enough interconnected and continuous open channels to facilitate fast electrolyte ion transportation but also built the robust and highly conductive skeleton.Moreover,the large surface area,hollow structure and O,N-doping of the N-PCNF ensured a uniform sulfur distribution,high area sulfur loading and strong absorb-capability of LiPSs.It had been found that the electrolysis approach is superior to the popular melt-diffusion method for the preparation of high-performance S-C composites.Thus,the self-supporting CNTs/N-PCNF was directly used as a working electrode to prepare sulfur cathode,combining the advantages of CNTs/N-PCNF and electrolysis approach.The sulfur loading in CNTs/N-PCNF-S electrodes can be tuned from 4.7 to 10.2 mg cm-2 by changing the electrolysis time.The cathode with an area sulfur loading of 4.7 mg cm-2 showed a high specific capacity of 1362 mA h g-1,corresponding to an areal capacity of 6.4 mA h cm-2.After 100 cycles at 0.2 C,a specific capacity of 1076.5 mA h g-1 is still retained.Even with a high areal sulfur loading of 10.2 mg cm-2,the cathode showed a high areal capacity of 11.5 mA h cm-2 at 0.1 C.