Research On Sulfur Electrode Additives For Lithium Sulfur Batteries

Owing to the high theoretical specific energy density of 1675 mAh g^-1,environ-mental friendliness and potentially low cost,lithium-sulfur?Li-S?batteries are being pursued as the promising candidate for the next generation electrochemical energy storage systems.However,the way to realize commercialization of Li-S batteries is currently hindered by the rapid capacity degradation,poor cycling stability and serious self-discharge,which all can be ascribed to the sluggish reaction kinetics caused by the poor conductivity of sulfur and the final Li₂S/Li₂S₂ and the shuttle effect of lithi-um polysulfides?LiPSs?during cycling.Aiming at the above problems,the additives of metal inorganic compounds and metal phthalocyanine organic compounds are in-vestigated in this paper to improve the electrochemical performance of Li-S batteries.The metal inorganic compounds explored in this paper were noble metal plati-num,transition metal cobalt and tungsten carbide.The additives were introduced into Li-S system through directly adding into sulfur electrodes and decorating the contact surface of separators?on the sulfur electrodes?.Compared with the former,it can be obtained that the cell with the Pt/C materials decorated separator performed much better when the doped active sites were limited.The cell with PP@Pt separator could maintain the specific capacity of 550 mAh g^-1 even after 400 cycles test at 0.5 C with the average decay rate of per cycle is only 0.078%during the last 300 cycles.The physical and electrochemical analysis futher reveal that the extra layer of Pt/C materi-als could not only mitigate the shuttle effect via chemical adsorption on polysulfide species but also serve as the upper current collector to increase the active materails utilization through the electrochemical catalyst of Pt to promote the redox kinetic.Furthermore,it was found that the transmition metal of cobalt could also improve the electrochemical performance of Li-S batteries.The electrodes with 2%Co doped could maintain the specific capacity of 521 mAh g^-1 after 200 cycles at the rate of 0.5C,exhibiting a 21%enhancement over the bare C-S electrode.Although the effect of Co is inferier than that of platinum,it can be compensated by increasing the amount of addition due to the relatively inexpensive cost,which has a large potential.The additives of metal phthalocyanine compounds?Mpcs?invesgated in this pa-per were mainly about CoPc,FePc and CuPc via uniformly loading on the carbon substrate.A series of experiments revealed that all these Mpcs displayed an improved ability of cycling and rate performance,especially the CoPc additives.The compound materials with 10 wt.%content of CoPc showed the satisfied results.Noticeably,the CoPc/C-S electrode delivered a high initial specific capacity of 1428 mAh g^-1 at the first discharge process.And this specific capacity of CoPc/C-S electrode maintained710 mAh g^-1 even after 400 cycles,showing an 80%enhancement over the bare C-S.The calculate exchange current density,X-ray photoelectron spectroscopy?XPS?tests and the comparative studies on control compounds verify that the effectiveness of the electrocatalyst is closely related to the integrated catalytic Co-N₄ unit.Though the synergetic chemical effect of central cobaltwith sulfur and the surrounding nitrogen with lithium,which make the CoPc could confine the dissolved LiPSs at initial dis-charge process,then catalyze accelerate the transformation of LiPSs,and finally fa-vourably guide the re-deposition of long-chain LiPSs during charging.Thus,the doped CoPc could significantly mitigate the problems that discussed above,namely self-discharge,capacity decay and sluggish kinetics,which are caused by shuttle ef-fect of LiPSs and large area coverage of poorly conducted Li₂S₂/Li₂S passivation lay-er.