Reaction Mechanism Study By Synchrotron Radiation Spectroscopy And Performance Improvement Of Mg/S Battery

Mg/S battery is a promising battery system because of its high theoretical energy density,high safety and low cost.Although a lot of efforts have been dedicated to improve its performance,Mg/S battery is still in its very early stage due to lack of mechanistic study of sulfur reaction pathway.Understanding the reaction mechanism in Mg/S battery is crucial to address the irreversible issues and unlock the conversion of this promising chemistry into usable technology.In this thesis,in-situ synchrotron X-ray absorption spectroscopy(XAS)tool was applied to systematically study the decay mechanism of Mg/S battery in Mg(diglyme)2[HMDSAlCl3]2 electrolyte.Guided by the experimental results from in-situ XAS study,we have proposed three effective strategies to improve the performance of Mg/S battery.This thesis includes four sections as follows:(1)Synthesis of mono-Mg2+ electrolyte Mg(diglyme)2[HMDSAlCl3]2.Compared with bulky binuclear cation,mono-Mg2+ salt is beneficial for the transport of Mg2+in electrolyte and at the electrolyte-electrode interface.However,in conventional Cl-containing electrolyte,it is easy to form bulky[Mg2(μ-Cl)3(solvent)n]2+ cation.This cation complex consists of two octahedrally coordinated Mg atoms bridged by three chlorides.Such bulky cation will cause low ion conductivity and high polarization voltage.In this work,Mg2+ coordination was pre-protected by diglyme solvent to form mono-Mg2+ ion.The as-prepared electrolyte is capable of highly reversible Mg electrodeposition,eduring high current density and enabling a stable electrochemical potenstial window.When mono-Mg2+electrolyte Mg(diglyme)2[HMDSAlCl3]2 was used in Mg/S cells,Mg/S cells show a good cycling stability.(2)In-situ XAS study of the capacity decay mechanism in Mg/S battery.Mg/S battery features a high theoretical energy density,a dendrite-free Mg anode and a low material cost.However,an understanding of the reaction mechanism of sulfur cathodes in Mg cation environment is still lacking,which has severely hindered effective measures to be proposed for performance improvement of Mg/S.In this project,in-situ XAS was applied to study the reaction mechanism in Mg/S battery.We find that the first discharging process in Mg/S battery can be divided into three stages:1.Formation of high order MgSx(MgS8,MgS4)intermediates at a fast reaction rate,because both MgSs and MgS4 are soluble in the electrolyte.2.Reduction of high order MgS4 to Mg3S8.3.Further reduction of low-order Mg3S8 to MgS.The third stage is much more sluggish because the reaction occurrs in the solid phase rather than in the liquid phase.The discharge products Mg3S8 and MgS cannot be reversed back to high order polysulfides or elemental S as revealed in the in-situ XAS results,which is the devil root that causes the capacity decay in Mg/S battery.Based on our experimental results,TiS2 was applied to active MgS and Mg3S8,resulting in improved reversibility.(3)YCl3 additive in electrolyte of Mg/S battery.According to the fundamental study of Mg/S battey by in-situ XAS tool,we concluded that MgS and Mg3S8 are the main reason to cause the capacity decay in Mg/S battery.It is critical to find an effective way to decompose the inactive products of MgS and Mg3S8.In this project,we tried to apply rare earth chlorides as additive in electrolyte.Yttrium chloride was selected as potential candidate because of its low standard reduction potential and trace water "getter" features.When coupled with magnesium polysulfide cathode,the new electrolyte greatly improved the performance of Mg/S battery.(4)Ag and Cu current collector in Mg/S battery.This project proposed an effective strategy to improve the performance in Mg/S battery.Usually,pyrolytic carbon was used as a current collector in Mg/S battery.Because pyrolytic carbon does not have catalytic effect,the Mg/S battery shows short cycling life and fast capacity decay.When Ag and Cu current collectors were applied to replace pyrolytic carbon current collector,improved cycle life and higher discharging capacity can be seen in Mg/S batteries.In-situ and ex-situ XRD,ex-situ XPS,ex-situ XAS were applied to study the reaction mechanism in Mg/S battery when Ag and Cu current collectors were utilized.