| The rapid development of modern society has stimulated the demand for the energy storage devices with high energy density in electric vehicles,aerospace and military,etc.However,the traditional lithium-ion batteries(LIBs)are approaching their theoretical energy limit.Lithium-sulfur(Li-S)battery,with extremely high theoretical specific energy and energy density(2600 Wh·kg-1 and 2800 Wh·L-1,respectively),abundant raw material and low cost,is becoming an important research field which is widely concerned by scientific and industrial communityHowever,many challenges of Li-S batteries always hinder their further development especially for commercialization.The effective electron/ion transfer is hindered due to the electronic/ion insulation nature of sulfur powder(S)and lithium sulfide(Li2S),the discharge product,resulting in very low utilization of the active material.In addition,the huge volume change(up to 80%when lithiation)leads to the break of electrode materials and seriously damages the intact structure of cathode.Another serious problem is the dissolution of lithium polysulfide,the intermediate products during Li-S battery cycling,in ether electrolytes.The dissolved polysulfide can easily diffuse from cathode to anode and are reduced on the lithium surface(i.e.the shuttle effect),which results in the rapidly decay of battery capacity and the decrease of Coulombic efficiency.The process of producing polysulfide intermediate products during the charging/discharging of Li-S battery is quite complex and the shuttle effect of polysulfide involves all parts of the battery,including cathode,anode,separator and electrolyte.The shuttle effect is always an important factor that limited the long-term stable cycle of Li-S batteries.In view of the above problems,herein we design and complete several effective methods to inhibit the shuttle effect of polysulfide from the perspectives of cathode material,lithium metal anode,electrolyte and interlayer.Specific research contents are as follows:The sulfydryl-functionalized carbon/sulfur cathode composite material(SCBC/S)is synthesized.The cyclic ?-octasulfur(S8)is fixed on the sulfydryl-functionalized carbon fiber(SCBC)in the form of covalent bond,which can effectively reduce the generation of soluble long-chain polysulfide during battery discharge.Electrochemical tests show that SCBC/S cell can cycle 500 times stably at the current density of 1 C with an extremely low capacity decay of 0.025%each cycle.Density Functional Theory(DFT)calculation finds that the S-S bond near the middle region of covalent sulfur chain has longer bond length and lower bond energy,which means that it is more prone to the formation of short-chain sulfide.In situ UV/Vis and X-ray absorption spectrum(XAS)show no long-chain polysulfide signals in the electrolyte,confirming that the covalent sulfur cathode limits the formation of long-chain polysulfide.A polymer layer is constructed on lithium surface(TG-Li)through the spontaneous reaction between tween-20 molecule and lithium metal,which can effectively prevent the reduction of polysulfide on the surface of active lithium metal.In addition,the polyethylene oxide structure in the polymer layer can not only compatibly contact with PEO electrolyte,but also provide higher ionic conductivity than traditional SEI.The resultant Li-S batteries using PEO based polymer electrolytes exhibit high reversible capacity of 1051.2 mAh g-1 at 0.2 C and show increased cycle life to 500 times.The results of in situ XRD and gas pressure test reveal that TG-Li can inhibit the decomposition of electrolyte and thus improve the safety of battery.XPS with Argon-ion sputtering shows that the polymer interfacial layer can greatly block the diffusion of polysulfideMolecularly imprinted polymers(MIPs)are prepared and used as the interlayer between cathode and separator to realize efficient identification and domain fixation to polysulfide and thus contribute to the reducing of polysulfide concentrations that dissolved in electrolyte.Employing MIPs as interlayer,the assembled Li-S battery(S/MIPs)shows high initial specific capacity of 1059 mAh g-1 at 0.5 C,and the capacity remains around 826 mAh g-1 after 150 cycles,which illustrates its advantage over traditional battery.According to the in situ UV/Vis analysis of electrolyte during the cycle process of S/MIPs battery,it is proved that MIPs material effectively limits the continuous dissolution and diffusion of Li2S8 in electrolyte and suppresses the shuttle effect significantly.New dimethyl trisulfide(DMTS)electrolyte system is constructed to realize a brand-new reaction mechanism of Li-S battery,where the intermediate product of lithium polysulfide in traditional ether electrolytes is replaced with dimethyl sulfides,and the shuttle effect is inhibited.In addition,DMTS can participate in the electrochemical reversible reactions and increase battery capacity.Using DMTS electrolyte system,the Li-S battery presents the high initial specific capacity of 1497.3 mAh g-1 and the total capacity can reach to 8.67 mAh when the sulfur load is increased to 8.12 mg cm-2.At the same time,the capacity retention of the battery using DMTS electrolyte is 84.1%after 500 cycles at 2 C rate.The liquid nuclear magnetic(NMR)characterization and in situ UV/Vis spectrum analysis of the electrolytes at different discharge voltages indicate that sulfur is involved in the radical exchange reactions of electrolytes and thus no information of polysulfide can be found. |
PDF Full Text Request