Construction,Properties And Mechanism Of High-Performance Sulfur Cathodes Promoted By Synergism Of Intrinsic Electrocatalysis And Confinement Of Carbon-Based Nanocages

Lithium-sulfur(Li-S)battery is one of the most promising secondary battery systems due to its high energy density and low cost.Nevertheless,the main challenges lie in the low sulfur utilization,fast capacity fading,limited power density and low areal sulfur loading and areal capacity owing to the insulating nature of sulfur and Li2S,the remarkable shuttle effect of dissolved lithium polysulfides(LiPS),the sluggish conversion kinetics of LiPS,and the big volumetric change during cycling.In order to solve the above problems,this thesis focuses on the development of high-performance sulfur cathode materials with efficient synergism of electrocatalysis and confinement based on the nitrogen-doped,sulfur-doped and nitrogen,phosphorus co-doped carbon nanocages.The performance and mechanism of these cathodes have been studied in depth.The important progresses of this thesis are concluded below.1.A durable high-power sulfur cathode for Li-S batteries has been developed by employing multifunctional hierarchical nitrogen-doped carbon nanocages(hNCNC)to encapsulate sulfur and serve as interlayer.The highly-efficient electrocatalytic function of N-doped sp2-carbon to lithium-polysulfides conversion reactions is revealed by electrocatalytic experiments and density functional theory simulations(DFT).Filling sulfur into the inner cavity of hNCNC can exert the high efficient synergism with the physical confinement of nanocages,chemisorption and electrocatalysis functions of N-doped sp2 carbon,and the fast charge transfer function of three-dimensional hierarchical structure.Consequently,a smooth "adsorption-transformation" process of LiPS is achieved,and the shuttle effect and polarization effect are effectively suppressed,leading to the durable high-power Li-S batteries.The high power performance with the capacity of 539 mAh g-1 is obtained at an ultrahigh current density of 20 A g-1 for the cathode with areal sulfur loading of 0.8 mg cm-2.The superb durability is demonstrated by 1000 cycles at 10 A g-1 with a retained capacity of 438 mAh g-1.When the areal sulfur loading is increased to 3 mg cm-2,a high capacity of 605 mAh g-1 is still obtained at the high current density of 3 A g-1.This study not only reveals the new function of N-doped sp2 carbon as metal-free catalyst beyond the previously reported chemisorption,but also provides an effective approach to durable high-power Li-S batteries by designing suitable electrocatalytic-active carbon-based hosts.2.Suppressing the LiPS diffusion by the chemisorption of heteroatoms-doped carbon is an effective strategy to improve the performance for Li-S batteries.However,we noticed an abnormal case that the S-doped sp2 carbon can also improve the battery performance despite its weak interaction with the LiPS.Such a disagreement stimulates the further understanding on the mechanism for the performance enhancement.Herein,we use the hierarchical S-doped carbon nanocages(hSCNC)to encapsulate sulfur as the cathode,which demonstrated the superior capacity and rate performance of Li-S battery than the counterpart using the undoped carbon nanocages(hCNC).The performance enhancement mechanism of S-doped carbon was revealed by the combined experiments and theoretical studies:the catalytic effect of S-doped carbon on the LiPS conversions,as confirmed by the higher reduction and lower oxidation onset potentials in the cyclic voltammetry curves,enhances the kinetic process and thereby the conversion efficiency.hSCNC integrates the physical confinement of nanocages and the catalytic conversion function of S-doped carbon,which promotes the performance of Li-S batteries(a retained capacity of 528 mAh g-1 after 600 cycles at 10 A g-1).This study provides an ideological basis for the design and performance optimization of S-doped carbon-based hosts.3.Increasing the areal sulfur loading on the cathode is prerequisite for the high energy density Li-S batteries.However,the suppressed charge transfer and the kinetic reaction process with the increase of electrode thickness result in the decrease of sulfur utilization and capacity,which makes it difficult to meet the requirements in practical applications.In this study,considering the synergetic adsorption and catalysis of N,P co-doped carbon on LiPS,N,P co-doped carbon nanocages(hNPCNC)were introduced in the process of constructing self-supporting high-areal sulfur loading monolithic electrode by in-situ reduction of graphene oxide(GO).The obtained high-areal sulfur loading cathode(S@hNPCNC@rGO)presents unique advantages:1)the hNPCNC promotes the adsorption and conversion of LiPS by physical confinement,adsorption and catalytic function;2)the wrapped rGO further blocked the escaping of LiPS,guaranteeing their recycling within the 3D network structure;3)the monolithic electrode acts as electrolyte reservoir to ensure the intimate contact with active materials;4)no conductive agent,binder and current collector are used,which simplifies the electrode preparation process and is beneficial to improving the energy density of battery.With a high-areal sulfur loading of 6 mg cm-2,the areal specific capacity of Li-S battery retains 4.2 mAh cm-2 after 250 cycles,which is at the top level of the N,P co-doped carbon-based sulfur cathodes and also higher than that of the conventional Li-ion batteries.This study provides a new idea for paving the way to the practical application of Li-S batteries.