Wednesday, 22 June 2016: 09:10
Grand Ballroom (Hyatt Regency)
Li-S battery has attracted intensive attentions during the past decades as a strong candidate that may overcome the limit of current lithium-ion battery technology. In spite of its promises, a number of challenges still remain unsolved hindering the commercialization of Li-S batteries in near future, especially for the EV applications. It is generally accepted that the technical problems of Li-S battery are related with the intrinsic nature of conversion reactions, like low conductivity, poor diffusivity and significant morphology change, along with the unique properties of sulfur chemistry which include high solubility and strong reactivity of polysulphide species in organic solvents. However, fundamental mechanisms behind the limited performance of Li-S batteries are not fully understood and remain controversial according to the analytical methods and the specific experimental conditions. Since the charge-discharge electrochemical reaction in Li-S batteries involves complicated chemical reactions in bulk electrolyte, ex-situ analysis does not reflect the actual situation inside the battery, due to the rapid re-equilibrium of the reaction species during the sample preparation process and even during the observation period. Moreover, high volatility of elemental sulfur (S8) often leads to the misinterpretation of characterization results which are obtained in the vacuum condition or with high temperature treatment.
In this study, Li/S cells were investigated with operando techniques using Scanning Electron Microscope (SEM), Ultraviolet-Visible absorption spectroscopy (UV-Vis) and Raman spectroscopy. All-solid-state polymer electrolytes were employed to design the operando cells, which allowed a real time observation of SEM images and a speciation by UV-VIS and Raman without needs for the solvent dilution. The operando SEM successfully visualizes the cross-section of Li/S polymer cell and reveals the evolution of morphology changes with cycles, as well as the aging mechanism including the migration of sulfur species and irreversible loss of active materials out of the cathode electrode. Appearance of elemental sulfur is clearly observed beyond 2.3V in the middle of charge process. The non-destructive operando UV-VIS spectroscopy demonstrates the evolution of characteristic absorption bands with progress of discharge and charge, and shows the accumulation of soluble sulphides in the electrolyte, not only high order polysulfides but also less soluble Li2S, over cycling. The inverse relationship between the intensity of S42- and S62- bands in UV-VIS spectroscopy implies that the charge and discharge reactions take place through different pathways; the formation of S42- is prominent during discharge and shuttle reactions, while S62- is more prevailing during charge.