Preparation Of Sulfur Cathodes For Lithium/Magnesium-Sulfur Batteries And Study On Performances Of Electrode/Electrolyte Interface

Sulfur is a promising cathode material for next generation secondary battery due to its advantages of high theoretical specific capacity,low cost and environmental friendliness.However,sulfur has some disadvantages,including its electronic insulator and the shuttle effect caused by its reaction intermediates.Metals Li and Mg are ideal anodes for sulfur battery due to their high capacity and low reduction potential,but the metals/electrolyte interface is unstable.In this dissertation,the sandwich-like composite cathode materials are prepared to improve electronic conductivity and cyclic stability of sulfur electrode,and the compatible solid electrolyte interface film is constructed by adding electrolyte additives to enhance the interface compatability of metal anodes?Li and Mg?.The main work is as follows:?1?A novel composite,sulfur supported by multi-walled carbon nanotubes and coated with polyaniline?denoted as MWCNTs-S@PANI?,is prepared as cathode of lithium-sulfur battery.MWCNTs-S is prepared by loading sulfur on MWCNTs via chemical deposition and coated with polyaniline via in situ polymerization.The physical and electrochemical performances of the resulting MWCNTs-S@PANI are investigated by nitrogen adsorption-desorption isotherms,X-ray powder diffraction?XRD?,thermogravimetric analysis?TGA?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,electrochemical impedance spectroscopy?EIS?,and charge/discharge test.It is found that MWCNTs-S@PANI exhibits good cycling stability and rate capability compared to MWCNTs-S as cathode of lithium-sulfur battery.MWCNTs-S@PANI delivers an initial discharge capacity of 970.8m Ah/g at 0.2 C and maintains 545.5 mAh/g after 205 cycles,but only 921 mAh/g and 353.4m Ah/g for MWCNTs-S,respectively.?2?Another composite?CG-S@PANI?,sulfur?S?loaded in curved graphene?CG?and coated with conductive polyaniline?PANI?,is proposed as cathode of lithium-sulfur battery.CG is prepared by splitting multi-wall carbon nanotubes and loaded with S via chemical deposition and then coated with polyaniline via in situ polymerization.The physical and electrochemical performances of the resulting CG-S@PANI are investigated by nitrogen adsorption-desorption isotherms,XRD,TGA,TEM,EIS,charge-discharge test,self-discharge test and electronic conductivity measurement.CG-S@PANI as cathode of lithium-sulfur battery delivers an initial discharge capacity of 851 mAh/g at 0.2 C with a capacity retention of over 90%after 100 cycles.This nature is attributed to the co-contribution of CG and conductive PANI to the concurrent improvement in electronic conductivity and chemical stability of sulfur cathode.?3?A novel spherical composite?PCNS-S@PANI?,sulfur?S?loaded in porous carbon nanospheres?PCNS?and further coated with conductive polyaniline?PANI?is prepared as cathode of lithium-sulfur battery.PCNS is prepared by convenient and controllable hydrothermal synthetic route and loaded with S via chemical deposition and then coated with conductive polyaniline via in situ polymerization under the control of ascorbic acid.The physical and electrochemical performances of the resulting PCNS-S@PANI are investigated by SEM,XRD,XPS,nitrogen adsorption-desorption isotherms,TGA,electronic conductivity measurement,galvanostatic charge-discharge test and EIS.It is found that mesoporous PCNS and conductive PANI are able to absorb and confine sulfur,provide electronic conductive network and improve the cyclability and rate performance of lithium-sulfur battery.PCNS-S@PANI delivers a discharge capacity of 881 mAh/g at 0.2 C with a capacity retention of 72%after 100 cycles and a rate capacity of 324 mAh/g at 2 C.?4?Electrolyte additives are proposed to improve the performance of lithium-sulfur battery.Threekindsofadditives?LiNO₃,Ferrocene?denotedasFe1?,Bis?pentamethylcyclopentadienyl?iron?denoted as Fe2??are brought into the 1.0 M LiTFSI-DOL+DME?Volume ratio 1:1?based electrolyte.Cyclic voltammetry?CV?,Li deposition/stripping andcharge/dischargetestareperformedtoinvestigatethe electrochemical performances of lithium-sulfur battery.SEM and Raman spectrum are simultaneously performed to demonstrate the effect of additives on morphology and structure of the electrodes.The results show that the additives can obviously improve the interface compatability of lithium metal and decrease the overpotential of Li deposition/stripping,especially for Fe1 and Fe2 additives.?5?Electrolytes that are able to reversibly deposit/strip Mg are crucial for rechargeable Mg batteries.The most studied complex electrolytes based on Lewis acid-base chemistry are expensive,difficult to be synthesized,and show limited anodic stability.Conventional electrolytes using simple salts such as Mg?TFSI?₂ can be readily synthesized,but Mg deposition/stripping in these simple salt electrolytes is accompanied by a large overpotential due to the formation of a surface layer on the Mg metal with a low Mg ion conductivity.Here theoverpotentialforMgdeposition/strippinginasimplesalt Mg?TFSI?₂-1,2-dimethoxyethane?DME?,electrolyte is significantly reduced by adding a small concentration of iodine??50 mM?as an additive.Mechanism studies demonstrate that a Mg ion conductive solid MgI₂ layer is formed on the surface of the Mg metal and acts as a solid electrolyte interface?SEI?.With the Mg?TFSI?₂-DME-I₂ electrolyte,a very small voltage hysteresis is achieved in a Mg-S full cell.?6?Electrolytes containing additives can not only improve Mg deposition/stripping overpotentials,but also benefit to the sulfur cathode performances of magnesium sulfur battery.To solve the issue of interface compatibility of sulfur cathode/electrolyte and the dissolution of polysulfides,three kinds of additives?I₂,Fe1,Fe2?are brought into the Mg?TFSI?₂-DME based electrolyte.CV and charge/discharge tests are performed to study the effects of additive concentrations on the performance of sulfur cathode.It is shown that sulfur cathode contributes the major capacity,and one pair of redox peaks at 1.4 V and 1.6 V can be observed,corresponding to reduction of sulfur and oxidation of magnesium sulfide,respectively.The contribution of additives to the capacity increases with increasing the concentration of the additives.The discharge capacity and cycling stability of sulfur cathode are improved.The discharge capacity is kept 178.6,216,and 292 mAh/g,corresponding to high concentration of 10 mM I₂,Fe1,and Fe2,respectively after 100 cycles at 168 mA/g.