Minimizing Long-Chain Polysulfide Formation in Li-S Batteries by Using Localized Low Concentration Highly Fluorinated Electrolytes

Abstract

Electrolytes that suppress the formation of long-chain polysulfides and limit the sulfur (S) cathode dissolution and polysulfide shuttle effect, while maintaining high active material utilization and long cycle life, have long been sought for advancing Li-S battery applications. Here we demonstrate that localized electrolytes based on fluorinated ethers with the total salt concentration of only 0.1 M show remarkable performance characteristics. By systematically reducing the fraction of sulfolane (SL) as a cosolvent with 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (HFE), we reveal a dramatic change in the cell discharge behavior. The higher voltage ∼2.4 V plateau corresponding to the formation of long-chain polysulfide shrank, while the medium voltage ∼2.1 V plateau corresponding to the transformation to short-chain polysulfides almost disappeared. Additionally, a third low voltage plateau (∼1.9 V) emerges as S transform directly to a short chain polysulfides and Li2S, which have significantly lower solubilities. This plateau extended with increasing HFE and became a dominant process. When compared with the traditional dimethoxyethane/1,3-dioxolane (DME/DIOX) electrolyte solvent mixture, the reported electrolytes offer substantially higher capacity and coulombic efficiency due to decreased solubility for Li+ in the HFE, causing a shift to favoring short chain length polysulfides in the discharge reaction.

Locally Concentrated Ionic Liquid Electrolytes for Lithium‐Metal Batteries

“…Due to the sluggish Li + transport, the specific capacity delivered in the neat ILE is low, and the discharge profile does not show any flat voltage plateau. When the LCILEs are employed, the discharge profiles exhibiting a slope around 2.1 V and a plateau slightly below 2.0 V are different from those observed with 1 M LiTFSI in DME/DOL, but similar to those reported from locally concentrated electrolytes based on organic solvents [85, 88] . Rebecca et al.…”

“…When the LCILEs are employed, the discharge profiles exhibiting a slope around 2.1 V and a plateau slightly below 2.0 V are different from those observed with 1 M LiTFSI in DME/DOL, but similar to those reported from locally concentrated electrolytes based on organic solvents. [85,88] Rebecca et al recently reported that the difference between the discharge profiles obtained with a conventional ether electrolyte and locally concentrated sulfolane electrolytes originates from a minimised long-chain polysulphides formation with the addition of hydrofluoroether co-solvents in the locally concentrated electrolytes. The poor solubility of Li + in the HFE leads to a shift to favouring short chain length polysulphides in the discharge of sulphur.…”

“…Nonetheless, the notorious shuttle effect of polysulphides, i.e., the dissolution and migration of polysulphide, leads to rapid capacity fading and low CE of Li−S batteries [82–84] . With the conventionally used ether‐based electrolytes, e.g., 1 M LiTFSI in dimethoxyethane/1,3‐dioxolane (DME/DOL), the discharge profile of Li−S batteries shows two plateaus at ≈2.4 and 2.1 V vs. Li/Li + , which correspond to the lithiation of long‐chain polysulphides and its further transformation to shorter chained polysulphides, eventually Li 2 S, respectively [85] . It has been revealed that the solubility of polysulphides, particularly the long‐chain ones, in TFSI − ‐based ILEs is suppressed as compared with conventional ether‐based electrolytes, which leads to improved cyclability and CE accompanied with when the ILEs are employed [86, 87] .…”

Recent Progress of Anode Protection in Li–S Batteries

“…Glaser et al used 1,1,2,2‐tetrafluoroethyl 2,2,3,3‐tetrafluoropropylether (HFE) as a cosolvent to sulfolane (SL) to make a localized low concentration electrolyte (LLCE) with 0.1 m LiTFSI. [ 37 ] They observed that increasing the HFE changed the redox reaction on the cathode side. In short, the S was being lithiated to Li 2 S in a solid‐state reaction, significantly decreasing polysulfide dissolution.…”

Facile and Cheap Carbonized Cotton Cloth Interlayer Endows Lithium-Sulfur Batteries with High Performance in All Climates

“…The volume expansion of sulfur toward Li 2 S 2 /Li 2 S gives rise to the structural damage of the cathode. , The shuttling effect of polysulfides during charging and discharging leads to capacity loss and further lifespan decrease . Researchers have tried to solve these problems through sulfur host regulation, electrolyte engineering, separator modification, and self-standing interlayer insertion. − Among these, the self-standing interlayer introduction stands out to be simple and effective because it provides not only considerable physical entrapment for polysulfides but also further chemical adsorption and conversion after further modification. − Therefore, the material selection of the interlayer is of great importance.…”