Galvanic Couples in Ionic Liquid‐Based Electrolyte Systems for Lithium Metal Batteries—An Overlooked Cause of Galvanic Corrosion?

Abstract

changes of our transportation and energy storage systems. [1] During recent decades, tremendous efforts were devoted toward the development, establishment, and improvement of electromobility. [2,3] Key requirements to render electromobility more competitive with conventional internal combustion technologies are commonly seen as improvements toward the battery. [4] Whereas the development of conventional lithium ion battery (LIB) technology is already well advanced, improvements toward properties such as energy density, sustainability, and safety remain highly desirable to encourage mass adoption of electromobility. [5] Stateof-the-art LIBs typically contain different types of carbon (e.g., graphitic, hard, soft, or amorphous carbons) and/or silicon as negative electrode materials. [6][7][8] A shift toward lithium metal-based batteries (LMBs) is often considered a desirable option for improvements of overall cell capacities and energy densities due to its high specific capacity (3860 mAh g −1 ) and low standard reduction potential (−3.04 V vs standard hydrogen electrode). [2,9] Nevertheless, the strong benefit of the electrochemical characteristics renders Li metal a key technology for the next evolutional step of batteries which includes the combination of Li with other high capacity materials at the positive electrode side. Novel key technologies like the utilization The breakthroughs in rechargeable lithium metal-anode-based batteries is still challenged by safety and performance limitations. Ionic liquid (IL)-based electrolytes are in consideration for increased safety but their moderate electrolyte performance and high costs still suppress their usefulness in Li metal-batteries. In an effort to deepen the understanding of the limited performance, galvanic corrosion as an electrochemical degradation process is herein identified as a contributing factor toward battery cell deterioration. Four different ILs, based on bis(trifluoromethylsulfonyl)imide in combination with the quaternary ammonium cations N-butyl-N-methylpyrrolidinium, N-methyl-N-propyl-pyrrolidinium, N-butyl-N-methylpiperidinium, and N-butyltrimethylammonium, respectively, are systematically investigated for such corrosive side reactions. The reaction pathways of this commonly neglected phenomenon are found to be both Hofmann-type and reductive eliminations. Supported by headspace-gas chromatography-mass spectrometry, the evolving gaseous reaction products are characterized. With zero resistance ammetry and Li electrochemical dissolution and deposition experiments, the dependency of galvanic corrosion on the presence of the galvanically coupled materials is elucidated. Variation of the lithium bis(trifluoromethylsulfonyl)imide concentration in the electrolytes is shown to influence the extent of detectable degradation products. Based on these findings, the necessity for more sophisticated electrode designs and electrolyte formulations is emphasized.

A Combined XPS and Computational Study of the Chemical Reduction of BMP‐TFSI by Lithium

“…For comparison, we also considered the products of a Hofmann β-elimination following the two-electron reduction of BMP + as suggested by Markevich et al [6] The most favorable reactions with relative energies of À 2.60 to À 2.79 eV (see Table 3) lead to the formation of numerous, mainly volatile products which were identified as reaction products in experimental studies of metallic lithium, or galvanic couples of Li and Cu or Ni, immersed into liquid BMP-TFSI by employing gas chromatography/mass spectrometry. [39,40] These previous studies reported tertiary amines as main decomposition products such as N-butyl-N-methyl-N-but-3-eneamine or N,Ndibutyl-N-methylamine, methyl-and butylpyrrolidin, and (un)saturated hydrocarbons such as butane and butene. Yet, with the experimental procedure used in this work, in which Li is post-deposited on BMP-TFSI multilayers on HOPG, the oneelectron reduction mechanism via radical intermediates appears more plausible.…”

Role of Coatings as Artificial Solid Electrolyte Interphases on Lithium Metal Self-Discharge

“…Several self-discharge studies focus on the corrosion of lithium metal while at rest in contact with the electrolyte either as uncycled bulk lithium or after a single lithium electredepositon. ,,,− Although this is necessary for understanding shelf life stability of the lithium metal anode, the electrochemical cycling of lithium typically causes an evolution of porous morphology and SEI which likely changes the subsequent susceptibility to aging effects when the cell is at rest. In this work, and our previous studies, , we focus on the impact of cycling on self-discharge.…”

A Combined XPS and Computational Study of the Chemical Reduction of BMP‐TFSI by Lithium**

“…For comparison, we also considered the products of a Hofmann β ‐elimination following the two‐electron reduction of BMP + as suggested by Markevich et al [6] . The most favorable reactions with relative energies of −2.60 to −2.79 eV (see Table 3) lead to the formation of numerous, mainly volatile products which were identified as reaction products in experimental studies of metallic lithium, or galvanic couples of Li and Cu or Ni, immersed into liquid BMP‐TFSI by employing gas chromatography/mass spectrometry [39,40] . These previous studies reported tertiary amines as main decomposition products such as N ‐butyl‐ N ‐methyl‐ N ‐but‐3‐eneamine or N,N ‐dibutyl‐ N ‐methylamine, methyl‐ and butylpyrrolidin, and (un)saturated hydrocarbons such as butane and butene.…”