Study on advanced electrolyte for improving electrochemical performance of lithium-sulfur batteries

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Study on advanced electrolyte for improving electrochemical performance of lithium-sulfur batteries
Yeon, Jin-Tak
Choi, Nam-Soon
Issue Date
Graduate School of UNIST
Lithium-sulfur (Li-S) batteries have been drawing attention as one of the high density energy storage devices due to their high theoretical capacity (1,672mAhr g-1), high theoretical energy density (2600Wh kg-1), which is 3 to 5 times higher than that of Li ion batteries based on intercalation reactions, eco-friendliness and low cost. In spite of these advantages, there are many problems that hinder practical applications. The challenges are attributed to the solubility of the polysulfide ions (Sn2-) formed on electrochemical reduction of S8 or on electrochemical oxidation of insoluble sulfides. In the first discharge step, electrochemical conversion of S8 to form S42- occurs through a sequence of soluble molecular poly-sulfides. The formation of insoluble Li2S2 is hindered and conversion of Li2S2 to Li2S as the last discharge step is the most difficult. In addition, Li metal as an anode in Li-S batteries is problematic when it is contact with any kind of liquid electrolyte solution, because of it`s high reactivity. Li metal would result in poor cycling efficiencies due to the severe growth of the SEI layer and Li dendrite formation. Moreover, insulating products layer such as Li2S and Li2S2 on Li anode can be formed by the reaction of Li and soluble polysulfide intermediates Li2Sn (4≤n≤6), which are diffused from the cathode. This study is concerned with the understanding and the improvement of Li-S battery. In order to understand basic operation mechanism of Li-S battery, structural evolution of sulfur cathode and lithium anode was investigated by using Raman spectroscopy and X-ray diffraction during discharge and charge. With based on such understanding, the improvement of Li-S battery was also performed by using protection layer with FEC based electrolytes . The effect of solvents on the discharge behavior of Li-S cells was investigated by ex situ Raman spectroscopy and X-ray diffraction. Lithium polysulfide species formed in a sulfur cathode during cycling are characterized by Raman experiments for the first time and their structures are examined with regard to three different electrolytes at fully charged and discharged states. Moreover, ex-situ Raman studies give the evidence for the formation of lithium polysulfide on a Li metal anode by shuttle phenomena and the coexistence of soluble lithium polysulfide with elemental sulfur even after full charge. It was found that 1,3-dioxolane (DOX)/1M LiTFSI facilitates the migration of soluble lithium polysulfide toward a lithium anode and initiates a polysulfide shuttle causing a considerable capacity loss in Li-S cells. Raman results and cycling tests using an air-tight cell demonstrated that tetra(ethylene glycol) dimethyl ether (TEGDME)-based electrolytes hindered the significant overcharge and led to the formation of Li2S2 contributing to high discharge capacity through further electrochemical reduction to Li2S. In addition, the impact of a fluoroethylene carbonate (FEC) solvent on the electrochemical performance of Li-Li and Li-S cells was investigated. To confirm the effects of FEC on electrolyte decomposition and cell resistance, the surface chemistry and impedance of a Li electrode cycled in electrolytes with and without a FEC solvent were investigated using attenuated total reflectance–Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectrometry (ToF-SIMS), and electrochemical impedance spectroscopy. It is found that the protective layer with FEC hinders the migration of soluble lithium polysulfides toward a Li metal electrode and results in the suppression of overcharging of Li-S cells.
Battery Science & Technology
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