[en] Hydrofluoroethers (HFEs) have been adopted widely as electrolyte cosolvents for battery systems because of their unique low solvating behavior. The electrolyte is currently utilized in lithium-ion, lithium–sulfur, lithium–air, and sodium-ion batteries. By evaluating the relative solvating power of different HFEs with distinct structural features, and considering the shuttle factor displayed by electrolytes that employ HFE cosolvents, we have established the quantitative structure–activity relationship between the organic structure and the electrochemical performance of the HFEs. Moreover, we have established the linear free-energy relationship between the structural properties of the electrolyte cosolvents and the polysulfide shuttle effect in lithium–sulfur batteries. These findings provide valuable mechanistic insight into the polysulfide shuttle effect in lithium–sulfur batteries, and are instructive when it comes to selecting the most suitable HFE electrolyte cosolvent for different battery systems. (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

[en] Despite the exceptionally high energy density of lithium metal anodes, the practical application of lithium-metal batteries (LMBs) is still impeded by the instability of the interphase between the lithium metal and the electrolyte. To formulate a functional electrolyte system that can stabilize the lithium-metal anode, the solvation behavior of the solvent molecules must be understood because the electrochemical properties of a solvent can be heavily influenced by its solvation status. We unambiguously demonstrated the solvation rule for the solid-electrolyte interphase (SEI) enabler in an electrolyte system. In this study, fluoroethylene carbonate was used as the SEI enabler due to its ability to form a robust SEI on the lithium metal surface, allowing relatively stable LMB cycling. The results revealed that the solvation number of fluoroethylene carbonate must be $\ge <!--\; ???\; -->1$ to ensure the formation of a stable SEI in which the sacrificial reduction of the SEI enabler subsequently leads to the stable cycling of LMBs. (© 2020 Wiley‐VCH GmbH)

[en] Highlights: • Acrylonitrile is revealed as an excellent electrolyte additive for lithium metal rechargeable batteries. • Polyacrylonitrile solid electrolyte interphase via electropolymerization improves the properties of lithium plating-stripping. • Excellent battery performance in 0.4Ah Li||LiNi_0.6Mn_0.2Co_0.2O₂ pouch cells using crylonitrile additive is demonstrated. We report acrylonitrile (AN) as an effective additive in carbonate-based electrolytes to enable uniform and dense lithium (Li) deposition and to improve the coulombic efficiency of Li metal anode. Our electrochemical, spectroscopic, and theoretical study reveal that AN is cathodically electropolymerized on the Li surface prior to the electrochemical decomposition of the electrolyte during Li deposition. The resultant polyacrylonitrile artificial solid electrolyte interphase enables uniform nucleation and growth of Li deposition with significantly reduced side reactions. The effectiveness of the AN additive is demonstrated in 0.4 Ah Li||LiNi_0.6Mn_0.2Co_0.2O₂ pouch cells (using 50-μm Li anode, 3 mAh cm⁻² cathode areal capacity, and 4 g Ah⁻¹ electrolyte) with excellent cycle stability under realistic charge-discharge condition.

[en] Highlights: • Introduce a new single-solvent electrolyte system comprising LiFSI and TFPMS for very stable cycling of high-voltage LIBs. • Unlike α-fluorinated sulfone, LiFSI-TFPMS system at normal salt concentration is compatible with the graphite anode. • The LiFSI-TFPMS electrolyte enables the formation of a robust SEI by the sacrificial decomposition of LiFSI. • The LiFSI-TFPMS system outperformed many reported electrolytes for high-voltage lithium-ion system. A new single-solvent electrolyte system comprising lithium bis(fluorosuflonyl) imide (LiFSI) and β-fluorinated sulfone (TFPMS) was designed to enable very stable long-term cycling of high-voltage lithium-ion batteries. Compared to other fluorinated solvents such as α-fluorinated sulfone (FMES) and fluorinated carbonate (FEMC), which are prone to reduction on the graphite anode, the LiFSI-TFPMS system displayed outstanding compatibility with graphite. While regular carbonate and sulfone from the LiFSI electrolyte system are compatible with the graphite anode, their high solvating power not only induces severe corrosion on the aluminum cathode current collector at high voltage, but also renders a low aggregation level at a normal salt concentration (about 1.0 M), resulting in the formation of an unstable solid-electrolyte interphase (SEI) on the graphite anode. Owing to the low solvating power of TFPMS, the aggregation level of the LiFSI-TFPMS system is relatively high even at normal salt concentration, which not only facilitates the formation of a robust SEI by the sacrificial decomposition of LiFSI, but also suppresses the aluminum corrosion of the LiFSI electrolyte system at high voltage. Together with the high intrinsic anodic stability of TFPMS, the superior cycling performance of graphite||LiNi_0.6Co_0.2Mn_0.2O₂ cells was achieved by employing the non-flammable LiFSI-TFPMS single-solvent electrolyte system.

[en] Highlights: • Reveal the relationship between the solvating power of electrolyte solvents and the fading mechanism of high-voltage LIBs. • A LFER was discovered between the solvating power of the electrolyte and the extent of cathode parasitic reactions at high voltage. • There was no evidence correlating the solvation ability of electrolyte with the decay of the high-voltage cathode LNMO. • The major decaying mechanism of the NMC/graphite cell cycled at a high voltage (> 4.5 V vs. Li/Li⁺) is solvation-driven. Understanding the decaying mechanism in lithium-ion batteries (LIBs) is critical to establishing a stable electrolyte system. Despite the advent of various novel electrolyte solvents designated for high-voltage LIBs, their working principles are not fully understood. Currently, oxidative decomposition of electrolytes is believed to be the major cause of capacity fade, and tremendous effort has been devoted to discovering a new electrolyte with enhanced anodic stability. However, the oxidative decomposition process cannot solely explain the rapid decay of some electrolyte systems with intrinsic high anodic stability when used with a high-nickel layered oxide cathode such as LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622). In this report, a study of the quantitative structure-activity relationship was conducted to deepen the mechanistic understanding of the decay in high-voltage LIBs. The results obtained from the newly introduced molecular pair analysis and linear free-energy relationship (LFER) studies were highly consistent with the solvation-involved decaying mechanism in a high-nickel layered oxide cathode cycling at high voltage (> 4.5 V vs. Li/Li⁺). There was no evidence correlating the solvation ability of electrolyte solvents with the decay of a high-nickel layered oxide cathode cycling at a relatively low voltage (vs. Li/Li⁺), nor with the high^-voltage spinel cathode LiNi_0.5Mn_1.5O₄ (LNMO). Undoubtedly, the unveiled mechanistic insight provides a critical guideline for the development of an appropriate electrolyte system targeting different high-voltage cathode materials.