Electrolyte Solutions for Rechargeable Li-Ion Batteries Based on Fluorinated Solvents

Abstract

Electrolyte solutions based on fluorinated solvents were studied in high-voltage Li-ion cells using lithium as the anode and Li 1.2 Mn 0.56 Co 0.08 Ni 0.16 O 2 as the cathode. Excellent performance was achieved by replacing the conventional alkyl carbonate solvents in the electrolyte solutions by fluorinated cosolvents. Replacement of EC by DEC and by their fluorinated counterparts FEC, 2FEC, and fluorinated ether (F-EPE) considerably improved the cycling behavior of the cells charged up to 4.8 V. The improvement achieved is attributed to formation of a stable and protective surface films on the cathode particles due to unique surface reactions that are enabled by the nature of the fluorinated solvent molecules. The surface films formed on the lithiated transition metal oxide cathodes isolate the active mass, which is highly reactive toward the electrophilic alkyl carbonates, from continuous detrimental reactions with solution species. The positive effect of fluorinated electrolyte solutions on the performance of high-voltage cathodes was exploited in experiments with full graphite−Li 1.2 Mn 0.56 Co 0.08 Ni 0.16 O 2 cells. Excellent cycling performance was recorded (1000 cycles) with solutions containing 2FEC, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE) and 1% TMSP, tris(trimethylsilyl)-phosphate (TMSP), which also provided very good results with Li−Li 1.2 Mn 0.56 Co 0.08 Ni 0.16 O 2 cells in shorter experiments. The extraordinary electrochemical stability of this electrolyte solution makes it a suitable candidate for other high-voltage cathode materials. KEYWORDS: lithium-ion batteries, Li 1.2 Mn 0.56 Co 0.08 Ni 0.16 O 2 /graphite cells, fluorinated solvents, fluoroethylene carbonate (FEC), difluoroethylene carbonate (2FEC), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (F-EPE), tris(trimethylsilyl)-phosphate (TMSP), Mn dissolution

Identifying Pitfalls in Lithium Metal Battery Characterization

“…In the first part of this study, experiments were carried out with a series of symmetric (Li||Li) cells to obtain a comprehensive baseline and understand the influence of electrolyte amount in the cell, current density applied, and amount of cycled lithium on the lifetime and polarization induced in each of two electrolyte formulations. The first electrolyte, in the following referred to as ‘carbonate’ electrolyte, is a mixture of fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) (v : v=1 : 1) with 1 M of LiPF 6 salt, known to be effective for the suppression of dendritic growth and short circuits [30] . The second electrolyte, in the following simplified to ‘ether’ electrolyte, is a mixture of 1,2‐dimethoxyethane (DME) and 1,3‐dioxolane (DOL) with 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), as is typically used in lithium‐sulfur cells, which function well for long‐term cycling [16] .…”

High Energy Density Rechargeable Batteries Based on Li Metal Anodes. The Role of Unique Surface Chemistry Developed in Solutions Containing Fluorinated Organic Co-solvents

“…Hence, not all fluorinated organic solvents can contribute to LiF formation on the electrodes’ surfaces in Li batteries. Only specific fluorinated solvents can generate surface LiF moieties, in which the position of fluorine and the molecular structure can play vital roles in their involvement in the desirable surface chemistry. ,− The most promising cycling results were obtained with fluorinated cyclic organic carbonates − ,,,,,− and fluorinated ethers. − Extremely high anodic stability, good low-temperature performance, and excellent safety characteristics were reported for non-flammable electrolyte solution containing mono-fluorinated ethylene carbonate (FEC) and highly fluorinated organic ethereal co-solvents. − …”

“…In Li-sulfur batteries, the sulfide anions formed by sulfur reduction are considered also as strong nucleophiles and participate in reaction with FEC to form a protective SEI . In addition to FEC, trans -difluoro­ethylene carbonate (DFEC) was also recognized as a co-solvent whose surface reactions may affect very positively the passivation of anodes and cathodes in Li batteries. ,,, …”

4-Hydroxy-2-Butanesulfonic Acid Gamma-sultone as a Bifunctional Electrolyte Additive for LiCoO2/Graphite Batteries With Enhanced Performances

“…Previously, some high-voltage solvents, such as fluoroethylene carbonate (FEC), sulfolane (SL), , and adiponitrile (ADN), have been confirmed to evidently strengthen the stability of electrolyte. The improvements can be ascribed to the reason that the functional groups such as −F, −SO–, and −CN are more electronegative than carbonyl groups, so their HOMO energy are higher than traditional carbonate solvents, resulting in a better electrochemical stability to resist the oxidation of the electrolyte at high-voltage. , However, the universal disadvantage of above-mentioned high-voltage solvents is that the viscosity of the electrolyte seems relatively large, which will hinder the diffusion of Li + in the electrodes surface, leading to a rapid decay in the cycle performance of LIBs.…”

“…Previously, some high-voltage solvents, such as fluoroethylene carbonate (FEC), 12 sulfolane (SL), 13,14 and adiponitrile (ADN), 15 have been confirmed to evidently strengthen the stability of electrolyte. The improvements can be ascribed to the reason that the functional groups such as −F, −SO−, and −CN are more electronegative than carbonyl groups, so their HOMO energy are higher than traditional carbonate solvents, resulting in a better electrochemical stability to resist the oxidation of the electrolyte at high-voltage.…”