Investigation of a Fluorine-Free Phosphonium-Based Ionic Liquid Electrolyte and Its Compatibility with Lithium Metal

Abstract

A novel fluorine-free ionic liquid electrolyte comprising lithium dicyanamide (LiDCA) and trimethyl(isobutyl)phosphonium tricyanomethanide (P 111i4 TCM) in a 1:9 molar ratio is studied as an electrolyte for lithium metal batteries. At room temperature, it demonstrates high ionic conductivity and viscosity of about 4.5 mS cm −1 and 64.9 mPa s, respectively, as well as a 4 V electrochemical stability window (ESW). Li stripping/ plating tests prove the excellent electrolyte compatibility with Li metal, evidenced by the remarkable cycling stability over 800 cycles. The evolution of the Li−electrolyte interface upon cycling was investigated via electrochemical impedance spectroscopy, displaying a relatively low impedance increase after the initial formation cycles. Finally, the solid electrolyte interphase (SEI) formed on Li metal appeared to have a bilayer structure mostly consisting of DCA and TCM reduction products. Additionally, decomposition products of the phosphonium cation were also detected, despite prior studies reporting its stability against Li metal.

Designing F/P Hybrid Polymer as Ultrastable Cationic Shielding Interphase for High‐Performance Lithium Metal Batteries

“…X‐ray photoelectron spectroscopy (XPS) was employed to analyze the chemical species after depositing 0.1 mAh cm −2 of Li onto the P−PCy 3 + coated copper electrode. As shown in Figure 3b (top row, before sputtering), typical peaks representing C−F (≈688 eV, F 1s, ≈299 eV, C 1s) and C−P (≈133 eV, P 2p) bonds were observed on top of the lithium deposition, indicating that the F/P polymer coating remains on top of anode regardless of Li deposition [11a,21a] . After argon ion sputtering for 100 s, signals of both C−F and C−P bonds decreased.…”

“…[19] Nevertheless, neutral fluoropolymers have shown limited Li deposition stabilizing effect; only 0.15 mAh cm À 2 of areal capacity was achieved in Li plating/ striping. [15,19] In the meantime, state-of-the-art cationic coatings focus on nitrogen-based cations that could show side reactions such as reduction and deprotonation on imidazoles [20] prior to Li + deposition, while phosphonium cations known to display robust chemical/thermal stability [21] remain unexplored.…”

Eutectic Solution Enables Powerful Click Reaction for In‐Situ Construction of Advanced Gel Electrolytes

“…Instead, stronger signals of C–F bonds (F 1s and C 1s), Li 2 CO 3 (C 1s and O 1s) and LiF (F 1s, 684.8 eV) in the DES‐related system, which belong to the decomposition of lithium salt, were clearly observed. [ 41–45 ] These results indicate the uncoordinated lithium salt in pure DES can be further bound by the polymer network in the GPE system, thus decreasing the side reactions between the electrolyte and the electrode. The B 1s spectrum shown in Figure S8, Supporting Information also confirms a less decomposition of LiDFOB in the PS 1 GPE 80 ‐assembled lithium surface.…”

Lithium Borate Ionic Liquids as Single‐Component Electrolytes for Batteries

“…Another common practice for the prevention of salt concentration polarization in electrolytes for LIBs is the decrease or elimination of solvent in liquid electrolytes, either by preparing highly concentrated electrolytes or by preparing electrolytes based on ionic liquids (in which the anions and cations of the ILs behave as a solvent for the lithium salts). [6,7] However, in both cases, all the problems associated with polarization by lithium salt concentration gradients in the electrolyte persist, because prevention of this type of polarization in the electrolyte requires strict solvent removal and Li + to be the only latently mobile cation in the system. [8] This reasoning inspired the development of some room-temperature (RT) lithium ionic liquids (LiILs) for use as solvent-free liquid electrolytes.…”