Abstract
The Li-S battery is a promising next-generation technology due to its high theoretical energy density (2600 Wh kg−1) and low active material cost. However, poor cycling stability and coulombic efficiency caused by polysulfide dissolution have proven to be major obstacles for a practical Li-S battery implementation. In this work, we develop a novel strategy to suppress polysulfide dissolution using hydrofluoroethers (HFEs) with bi-functional, amphiphlic surfactant-like design: a polar lithiophilic “head” attached to a fluorinated lithiophobic “tail.” A unique solvation mechanism is proposed for these solvents whereby dissociated lithium ions are readily coordinated with lithiophilic “head” to induce self-assembly into micelle-like complex structures. Complex formation is verified experimentally by changing the additive structure and concentration using small angle X-ray scattering (SAXS). These HFE-based electrolytes are found to prevent polysulfide dissolution and to have excellent chemical compatibility with lithium metal: Li||Cu stripping/plating tests reveal high coulombic efficiency (>99.5%), modest polarization, and smooth surface morphology of the uniformly deposited lithium. Li-S cells are demonstrated with 1395 mAh g−1 initial capacity and 71.9% retention over 100 cycles at >99.5% efficiency—evidence that the micelle structure of the amphiphilic additives in HFEs can prohibit polysulfide dissolution while enabling facile Li+ transport and anode passivation.
Highlights
Over the past two decades, tremendous effort has been invested in developing clean and renewable energy technologies that may address urgent environmental concerns surrounding fossil fuels
The ethylene oxide (EO) moiety is lithiophilic and incompatible with fluorinated solvents, able to coordinate with Li+, while the fluorocarbon moiety is lithiophobic but fluorophilic, which is more likely to associated with fluorinated solvents like tetrafluoropropyl ether (TTE)
We have reported a novel strategy to suppress polysulfide dissolution in Li-S cells utilizing an HFE with bi-functional and amphiphilic structure similar to that of a surfactant
Summary
(Fan et al, 2018) In order to satisfy global demand for high-capacity energy storage, new chemistry and electrode materials have been proposed such as the lithium-sulfur (Li-S) battery (Song et al, 2013a; Yin et al, 2013; Hietala et al, 2018; Fang et al, 2019). This redox couple has a theoretical energy density of 2600 Wh kg−1, and material costs are expected to be low due to the worldwide abundance of sulfur, which makes Li-S stand out as a promising nextgeneration storage solution. These strategies include physically and/or chemically confining polysulfide intermediates within a hierarchical matrix (Wang et al, 2015; Wu et al, 2015) or crosslinked organic structure (Li et al, 2019), blocking contact between polysulfide and electrolyte by building protective layers/shells on the cathode surface (Hu et al, 2016; Wu et al, 2018), forming stable/protective SEI layers on the anode surface using electrolyte additives (e.g., LiNO3) (Aurbach et al, 2009), and developing template configurations that allow solid-state conversion of nanoconfined sulfur (S2−4) (Xin et al, 2012; Li et al, 2014)
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