Improved electrochemical performance of lithium-sulfur batteries by crosslinked polymer layers

Abstract

Lithium sulfur (Li-S) battery is new generation system. Sulfur is widely known as a high theoretical capacity (1672 mAh g-1) and high theoretical energy density (2600 Wh kg-1). The attractive features of sulfur are low cost, abundant resources and nontoxic. Sulfur (S) is utilized as a cathode material and Li metal is an anode in Li-S cells. Since Li metal has high theoretical capacity of about 3860 mAh g-1 and the most electropositive (-3.04V versus standard hydrogen electrode), a high energy density can be achieved. During the discharge process, elemental sulfur (S8) electrochemically reduces to soluble long-chain polysulfides and the resulting polysulfides can be dissolved into the electrolyte. Dissolved long-chain lithium polysulfide can diffuse to the Li anode and short-chain intermediate species (insoluble Li2S2 and Li2S) may deposit on the anode, leading to the formation of unstable and non-uniform solid electrolyte interphase (SEI) layer. It can cause considerable capacity fading and safety concern related to the dendritic Li generated by non-uniform current distribution of the Li anode. These are important issues about thermal stability in all battery systems. In this study, we aim to understand thermal properties of sulfur cathodes and improve electrochemical performance of Li-S cells. In chapter II, we investigate exothermic peaks for sulfur cathode according to different depth of discharge and fully charge step compared to delithiated lithium metal oxide cathode in a Li-ion battery by using the DSC technique. The exothermic peak of lithiated and delithiated sulfur cathode in the battery is considerably reduced at around 360 oC. Also surface changes of the sulfur cathode were clearly demonstrated by ex-situ XPS technique during the different depth of discharge and fully charge processes. The thermal reaction between lithium metal and sulfur generated catastrophic exothermic heat in the presence of the ether-based electrolyte, but, the mixture of Li metal and lithium sulfide (Li2S) showed greatly reduced exothermic peak. In chapter III, we demonstrate the positive impact of the protective film on electrochemical properties of lithium metal anode in Li-S cells. Li metal, which is very reactive anode material, readily undergoes the reactions with polysulfides dissolved from the sulfur cathode. It is expected that the introduction of a protection layer based on the crosslinked gel polymer (semi-IPN structure) prevents unwanted reactions with polysulfide. Li-S cells without the protection layer show significant overcharge behavior during 10cycles, while the cell with protection layer effectually mitigates the overcharging.