Improved Electrochemical Performance Of Li-S Batteries By Structural Optimization Of Sulfur Electrode And Electrolyte

Batteries with high energy density and high power density are required to satisfy the expanding demand for intelligent electronic equipment and electric vehicles in modern society.Lithium sulfur batteries with sulfur as the cathode and lithium metal as the anode is a very promising candidate due to the high theoretical energy density of 2500 Wh kg-1,which is 5 times more than conventional lithium ion batteries based on intercalation oxide cathodes.Furthermore,sulfur is a low-cost and environmental friendly material,which exists abundantly in the worldwide.Therefore,lithium sulfur batteries have been intensively investigated over the past two decades.However,there are several issues hindering the commercial application of lithium sulfur batteries,such as: poor conductivity of sulfur and its discharge products of Li2S2 and Li2 S,large volume change of the sulfur electrode during cycling process,shuttle effect of the polysulfides and the formation of Li dendrite.Among them,shuttle effect caused by the migration of the soluble polysulfides between cathode and anode is the main reason for low coulombic efficiency and rapid capacity decay of the lithium sulfur batteries using an ether-based liquid electrolyte.Besides,the growth of Li dendrite on the surface of lithium metal due to the unstable deposition of lithium ions may pierce through the separator,resulting in an internal short circuit of the cell and safe concerns.In view of these two issues,the following researches have been investigated.1.Application of diatomite as an effective polysulfides adsorbent to improve the cycling performance of lithium sulfur batteriesDiatomite is a kind of biological sedimentary mineral with abundant natural pores.It is mainly comprised of Si O2 with small concentrations of impurities including Fe2O3,Ca O,Mg O and organics.Owing to the ordered porous structure,large surface area,good chemical stability,excellent sorption ability and cheap price,diatomite has been widely used to adsorb heavy metals and organic chemicals.In this part,the result of the adsorption experiment confirms that diatomite possesses excellent adsorption ability for polysulfides.Hence,diatomite that has abundant natural three-dimensional ordered pores is incorporated into the cathode to trap polysulfides to suppress the shuttle effect and improve the cycling performance of the lithium sulfur battery.The composite cathode(S-DM-AB for short),including sulfur(S),diatomite(DM),and acetylene black(AB)with a weight ratio of 6:3:1 is prepared by an impregnation method.For comparison,another composite cathode(S-AB for short)including sulfur and acetylene black with a weight ratio of 6:4 is also prepared by the same method.The battery with S-DM-AB composite cathode material delivers a discharge capacity of 531.4 m Ah g-1 after 300 cycles at 2 C with a capacity retention of 51.6% at room temperature.By contrast,the battery with S-AB composite cathode material delivered a capacity of only 196.9 m Ah g-1 with a much lower capacity retention of 18.6% under the same condition.The addition of diatomite in the cathode is proved to be a cheap and effective way to improve the life time of lithium sulfur batteries.2.Explore the influence of coverage percentage of sulfur electrode on the cycle performance of lithium sulfur batteries and construct a sealed sulfur electrode with excellent cycling stabilityThe conventional sulfur electrode is exposed to the ether-based electrolyte,resulting in a rapid diffusion of the polysulfides.In this part,we explore the influence of coverage percentage of the sulfur electrode on the cycle performance of lithium sulfur batteries.Four sulfur electrodes covered with composite films with different exposure are prepared and tested in lithium sulfur batteries.The composite film is made of BP2000,nano-Al2O3 and polytetrafluoroethylene(PTFE)with a mass ratio of 2:4:4.The results display that both the capacity retention and coulombic efficiency are improved with the increase of surface coverage percentage among which the cell with the completely sealed sulfur electrode exhibits the best cycle performance.Different from the conventional open structure of the pristine sulfur electrode,the sealed sulfur electrode with 100% coverage was fabricated by placing a piece of sulfur cathode in sandwich between two pieces of composite films followed by sealing the periphery.The discharge capacity of the pristine sulfur electrode deteriorates steeply in the initial dozens cycles,and declines to only 235.5 m Ah g-1 after 500 cycles at 1 C with the capacity decay rate of 0.17 % per cycle.In contrast,the discharge capacity of the sealed sulfur electrode rises in the initial 40 cycles and becomes as high as 1260 m Ah g-1.Subsequently,the discharge capacity decays slowly at a capacity decay rate of merely 0.055% per cycle in the following 460 cycles,reaching 940 m Ah g-1 at the end.The coulombic efficiencies after 500 cycles at 1 C of the pristine sulfur electrode and the sealed sulfur electrode are 85.71% and 99.59%,respectively.Besides,the sealed sulfur electrode could still maintain a super high discharge capacity of ~ 800 m Ah g-1 after 1000 cycles at 2 C with capacity retention over 77%.The ex-situ field emission scanning electron microscopy(FE-SEM),energy dispersive spectroscopy(EDS),ultraviolet-visible spectroscopy(UV-Vis)and nitrogen sorption measurements reveal that the sealed structure with narrow and irregular interstitials effectively suppresses the diffusion of soluble polysulfides out of the embraced environment with virtually no effect on lithium ion conduction.The present study demonstrates the effectiveness of the sealed structure for achieving robust electrochemical performance of lithium sulfur batteries.3.Improved cycling stability of sulfur electrode by a Li-Nafion-supported sealed configurationNafion is a perfluoro ionomer which possesses a perfluoronated side chain and a polytetrafluoroethylene skeleton.Nafion film can allow the positively charged cations to pass and block the migration of the negatively charged anions.In this part,a pristine sulfur electrode is sandwiched in between two Li-Nafion-supported composite films followed by totally sealed the periphery.The Li-Nafion-supported composite film is made of BP2000,Li-Nafion and PTFE with a mass ratio of 1:3:6.The dense Li-Nafion-supported sealed configuration acts as the effective physical barrier to hinder the diffusion of the polysulfides.Besides,the Li-Nafion uniformly distributed in the sealed configuration could electrostatically reject the negatively charged polysulfides due to the sulfonic acid groups of Nafion,further preventing the polysulfides from migrating out of the sealed sulfur electrode and passing through the separator to react with the lithium metal.The excellent cycling performance of the battery is obtained by this delicate designed Li-Nafion-supported sealed sulfur electrode.The discharge capacities of the pristine sulfur electrode at 0.2 C after 100 cycles,0.5 C after 200 cycles and 1 C after 300 cycles are 308.6 m Ah g-1,243.1 m Ah g-1 and 414.6 m Ah g-1,respectively.The corresponding capacity retention rates are 25%,21.1% and 39.2%.In contrast,for the Li-Nafion-supported sealed sulfur electrode,the discharge capacities at 0.2 C after 100 cycles,0.5 C after 200 cycles and 1 C after 300 cycles rise to 803.1 m Ah g-1,732.4 m Ah g-1 and 728.9 m Ah g-1,respectively.And the corresponding capacity retention rates of the Li-Nafion-supported sealed sulfur electrode increase to 81.4%,81.7% and 93.6%.The rate cycling performance also demonstrates the excellent electrochemical reversibility of the Li-Nafion-supported sealed sulfur electrode.Furthermore,compared with the nano-Al2O3-supported sealed configuration,the Li-Nafion-supported sealed configuration could further suppress the shuttle effect to enhance the cycling performance,especially at a low C-rate.After 30 cycles at 0.1 C,the discharge capacity of the Li-Nafion-supported sealed sulfur electrode is 1201.6 m Ah g-1 with the capacity retention as high as 89.8%.In contrast,the discharge capacity of the nano-Al2O3-supported sealed sulfur electrode rapidly decays to 952.4 m Ah g-1 with the capacity retention of only 62.4%.4.A safe and long life lithium metal-sulfur battery enabled by a single ion conducting battery structureSulfur cathode pairing with lithium metal anode results in lithium sulfur batteries with high energy density.Although the cathode has been intensively studied and obtained effective progress,safety issues of the lithium dendrite haven't been tackled yet.Indeed,increasing the transference number of lithium ion up to unit can avoid the formation of ion depletion layer at where dendrite grows.However,applying this method to stabilizing the lithium metal for lithium sulfur battery has rarely been reported.In this study,lithium 4-aminophyenylsulfonyl(trifluoromethylsulfonyl)imide(Li ATFSI)is side-chain grafted with poly(ethylene-alt-maleic anhydride)(PEMA)to immobilize the anions,leading to cations transferring only.PEMA-graft-Li ATFSI polymer blended with poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP)to form a gel single ion conducting polymer electrolyte membrane that is used as the electrolyte as well as the separator for battery test.The ionic conductivity and lithium ion transference number of the gel single ion conductor at 25 ? are 0.21 m S cm-1 and 0.94,respectively.The gel single ion conductor can tolerate the galvanostatic cycling(±10 m A cm-2)for as long as 1600 hours in the symmetric cell,still maintaining with the metallic luster of lithium metal.But the lithium tested in the commercial dual ion electrolyte corrodes seriously and some coarse granules formed on the uneven surface of lithium metal.The comparison experiment confirms the necessity and importance of single ion conducting for the high energy density lithium metal secondary batteries.Finally,a long-term single ion conducting lithium sulfur battery using S@PAN as the cathode is demonstrated,delivering a stable discharge capacity of ~780 m Ah g-1 for 1000 cycles at 1 C with no capacity decay compared to the reversible discharge capacity of the second cycle,which is superior to the cycling performance of the battery using the commercial dual ion electrolyte with the discharge capacity of only 443.3 m Ah g-1 after 1000 cycles at 1 C.