Design Of Metal And Eutectic Accelerators And Their Applications For High Performance Lithium/Sodium Sulfur Battery

Sulfur has the highest theoretical specific capacity(1675 mAh g^–1)in the solid state cathodes,and possesses the advantages of natural abundance,low price and environmental friendliness,which make sulfur cathodes receive wide attentions.Lithium–sulfur and sodium–sulfur batteries have theoretical energy density up to 2600and 1247 Wh kg^–1,respectively,and are the most promising candidates in the next generation battery systems.However,the development of lithium/sodium sulfur batteries have been greatly hindered due to the insulation of sulfur and the solubility of intermediate polysulfides.The insulation of sulfur leads to limited activity and sluggish reaction kinetics,thus accelerating the reaction kinetics is an effective solution.The solubility of the polysulfide intermediates leads to the dissolution and loss of active material,resulting in low Coulombic efficiency and shuttling effect.This thesis mainly focus on solving the issues of slow reaction kinetics and the loss of polysulfide intermediates.The reaction kinetics and reaction pathway are controlled by designing appropriate accelerators and selecting different sulfur cathode materials to accelerate reaction kinetics and reduce or inhibit the dissolution of lithium polysulfides.The catalytic mechanism,reaction pathway and electrochemical performance are investigated.The main results and conclusions of this thesis are summarized as follows:1.A readily available cobalt–carbon nanofiber（Co–CNF）current collector is designed to construct a high sulfur–loading and high performance cathode is also investigated the catalytic effect of cobalt metal on the redox kinetics of sulfur cathode.It is shown that the Co–CNF promotes electron and Li–ion transfer,leading to fast sulfur cathode redox reaction,which mitigates the loss and shuttling of polysulfides.Furthermore,cobalt metal facilitates the uniform nucleation of lithium sulfide on the surface of the carbon nanofiber.The Co–CNF with 4.6 mg cm^–2 sulfur loading delivers 970 mAh g^–1(4.5 mAh cm^–2)at 0.1 C and 670 mAh g^–11 at 1 C,respectively.Even when sulfur loading reaches as high as 9.6 mg cm^–2,the reversible capacity of 730 mAh g^–1(7 mAh cm^–2)is obtained.The Co–CNF/Li₂S₆ cathode shows a degradation rate of only 0.06%per cycle over 300 cycles.This work demonstrates that cobalt metal catalysts can be used to simply and efficiently build high performance Li–S batteries cathodes.2.Tellurium is used as eutectic accelerator and is doped into the sulfrized polyacrylonitrile to obtain a modified sulfur cathode(Te_0.04S_0.96@pPAN),which has a unique reaction pathway and faster reaction kinetics,resulting in excellent electrochemical performance in both carbonate and ether electrolytes.Such cathode can be operate with the“dissolution–deposition”mechanism without polysulfide dissolution.Sulfurized polyacrylonitrile（S@pPAN）is suggested to involve S3-4 and soluble intermediate Li2S3-4,thus a fast and one–step reversible conversion to insoluble Li2S2 and Li2S can prevent polysulfide dissolution.Te is uniformly distributed through Te–S bonding and accelerate the reactivity while contributing capacity.Te0.04S0.96@pPAN cathodes deliever high capacities of 1507 and 861 mAh g^–1 at 0.1 and 10 A g^–1,and stable cyling over 600 cycles in ether electrolyte（0.05% decay per cycle）.Good performance is also demonstrated with 3.11 mg cm^–2 sulfur loading and a electrolyte sulfur ratio6 of uL mg^–1.This work shows Te_0.04S_0.96@pPAN cathode operates under“dissolution–deposition”without polysulfides dissolution.3.Tellurium–doped modified Te_0.04S_0.96@pPAN is used as cathode for room temperature Na–S battery,which significantly accelerates reaction kinetics and improves sulfur utilization,resulting in high performance room temperature sodium–sulfur batteries compatible both with carbonate and ether electrolytes.Te_0.04S_0.96@pPAN delivers capacities of 1236 and 629 mAh g^–1,1111 and 601 mAh g^–1 at 0.1 and 6 A g^–1 in carbonate and ether electrolytes,respectively.Furthermore,UV–vis spectra and shuttle current test both reveal diminished sodium polysulfides in ether electrolyte,attributing to the fast kinetics enabled by Te–doping.More importantly,spectra and electrochemical analysis demonstrate a two–step reaction pathway in which Na₂S₃ and Na₂S are the main intermediate and final discharge–product,respectively.This is the first time that sulfurized polyacrylonitrile–based sodium sulfur batteries is reported to exhibit compatibility in both carbonate and ether electrolytes.4.The SEI membrane is generated in situ to obtain the sulfur cathode operating under quasi–solid state reaction mechanism,and sulfur cathode is modified by using tellurium as eutectic accelerator,which effectively improves the reaction kenitics of sulfur cathode under the quasi–solid state reaction mechanism.Te forms molecular–level dispersion with sulfur and provides capacity with its own electrochemical reaction.Electrochemical studies have revealed that Te–doping can reduce the reaction overpotential,accelerate the transport of lithium ions and reduce the reaction resistance,all of which are beneficial for the reaction kinetics.XPS and electrochemical impedance analysis show that Te–doping leads to the formation of SEI film with higher lithium–ion conductivity and smaller SEI impedance.1 wt% of Te–doping CMK–3S1 cathode delivers 645.9 mAh g^–1 at 1.5 C,and shows a specific capacity of 726.9 mAh g^–1 over 200 cycle at 0.2 C.This work presents a bifunctional eutectic accelerator Te to enhance the performance of lithium–sulfur batteries in quasi–solid state reaction mechanism with the perspective on increasing reactivity and SEI conductivity.