Electrochemical Stability of Zinc and Copper Surfaces in Protic Ionic Liquids.

Ionic liquids are versatile solvents that can be tailored through modification of the cation and anion species. Relatively little is known about the corrosive properties of protic ionic liquids. In this study, we have explored the corrosion of both zinc and copper within a series of protic ionic liquids consisting of alkylammonium or alkanolammonium cations paired with nitrate or carboxylate anions along with three aprotic imidazolium ionic liquids for comparison. Electrochemical studies revealed that the presence of either carboxylate anions or alkanolammonium cations tend to induce a cathodic shift in the corrosion potential. The effect in copper was similar in magnitude for both cations and anions, while the anion effect was slightly more pronounced than that of the cation in the case of zinc. For copper, the presence of carboxylate anions or alkanolammonium cations led to a notable decrease in corrosion current, whereas an increase was typically observed for zinc. The ionic liquid-metal surface interactions were further explored for select protic ionic liquids on copper using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to characterize the interface. From these studies, the oxide species formed on the surface were identified, and copper speciation at the surface linked to ionic liquid and potential dependent surface passivation. Density functional theory and ab initio molecular dynamics simulations revealed that the ethanolammonium cation was more strongly bound to the copper surface than the ethylammonium counterpart. In addition, the nitrate anion was more tightly bound than the formate anion. These likely lead to competing effects on the process of corrosion: the tightly bound cations act as a source of passivation, whereas the tightly bound anions facilitate the electrodissolution of the copper.

Fluorinated Boron-Based Anions for Higher Voltage Li Metal Battery Electrolytes.

Lithium metal batteries (LMBs) require an electrolyte with high ionic conductivity as well as high thermal and electrochemical stability that can maintain a stable solid electrolyte interphase (SEI) layer on the lithium metal anode surface. The borate anions tetrakis(trifluoromethyl)borate ([B(CF3)4]-), pentafluoroethyltrifluoroborate ([(C2F5)BF3]-), and pentafluoroethyldifluorocyanoborate ([(C2F5)BF2(CN)]-) have shown excellent physicochemical properties and electrochemical stability windows; however, the suitability of these anions as high-voltage LMB electrolytes components that can stabilise the Li anode is yet to be determined. In this work, density functional theory calculations show high reductive stability limits and low anion-cation interaction strengths for Li[B(CF3)4], Li[(C2F5)BF3], and Li[(C2F5)BF2(CN)] that surpass popular sulfonamide salts. Specifically, Li[B(CF3)4] has a calculated oxidative stability limit of 7.12 V vs. Li+/Li0 which is significantly higher than the other borate and sulfonamide salts (≤6.41 V vs. Li+/Li0). Using ab initio molecular dynamics simulations, this study is the first to show that these borate anions can form an advantageous LiF-rich SEI layer on the Li anode at room (298 K) and elevated (358 K) temperatures. The interaction of the borate anions, particularly [B(CF3)4]-, with the Li+ and Li anode, suggests they are suitable inclusions in high-voltage LMB electrolytes that can stabilise the Li anode surface and provide enhanced ionic conductivity.

Spectroscopic and Computational Study of Boronium Ionic Liquids and Electrolytes.

Boronium cation-based ionic liquids (ILs) have demonstrated high thermal stability and a >5.8 V electrochemical stability window. Additionally, IL-based electrolytes containing the salt LiTFSI have shown stable cycling against the Li metal anode, the "Holy grail" of rechargeable lithium batteries. However, the basic spectroscopic characterisation needed for further development and effective application is missing for these promising ILs and electrolytes. In this work, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and density functional theory (DFT) calculations are used in combination to characterise four ILs and electrolytes based on the [NNBH2 ]+ and [(TMEDA)BH2 ]+ boronium cations and the [FSI]- and [TFSI]- anions. By using this combined experimental and computational approach, proper understanding of the role of different ion-ion interactions for the Li cation coordination environment in the electrolytes was achieved. Furthermore, the calculated vibrational frequencies assisted in the proper mode assignments for the ILs and in providing insights into the spectroscopic features expected at the interface created when they are adsorbed on a Li(001) surface. A reproducible synthesis procedure for [(TMEDA)BH2 ]+ is also reported. The fundamental findings presented in this work are beneficial for any future studies that utilise IL based electrolytes in next generation Li metal batteries.

The interaction of ethylammonium tetrafluoroborate [EtNH3+][BF4-] ionic liquid on the Li(001) surface: towards understanding early SEI formation on Li metal.

The electrode cyclability of high energy density Li-metal batteries can be significantly improved with the use of ionic liquid (IL) based electrolytes, which can ameliorate device issues through the suppression of dendrite initiation and propagation. This enhancement is often attributed to the formation of a stable solid electrolyte interphase (SEI) layer between the electrode and the electrolyte. In this paper, we have modelled the adsorption of the IL ethylammonium tetrafluoroborate [EtNH3+][BF4-] on a Li(001) surface, using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations to capture the initial stages of the SEI layer formation, and gain a greater insight into the stability of [EtNH3+][BF4-] on a lithium surface. Eleven unique minimum energy structures of the [EtNH3+][BF4-] pair adsorbed on the Li(001) surface were found, having binding energies between -1.80 eV to -1.58 eV. The interface between the electrolyte molecules and electrode surface were stabilized by the formation of Li-F bonds between the anion and Li surface leading to formation of Lix-BF4 clusters, where x = 2-4. This was accompanied by a transfer of charge from the lithium surface to the cation and anion. The thermal stability of the IL was investigated via AIMD simulations, and the IL was found not to spontaneously dissociate on the surface at room temperature or at an elevated temperature of 157 °C within the examined simulation time of 4.64 ps, with Lix-BF4 clusters forming early into the simulations (<1 ps). These findings provide useful information for future development of Li-metal batteries.