In the course of the past decade, due to its high theoretical energy density assumed to be comparable to that of gasoline, the lithium-air battery has attracted great attention of many academic and industrial laboratories worldwide. However, the practical development of this battery is still hindered by various issues including (i) the poor reversibility of the lithium-oxygen electrochemical process, (ii) the low energy storage efficiency caused by the rather high polarization resulting in a wide charge-discharge voltage gap, (iii) the reactivity and therefore poor stability of commonly used organic electrolytes both at the lithium side and, in particular, at the oxygen side, and (iv) the sensitivity of the system to CO₂ and H₂O contamination, requiring the use of pure oxygen rather than air as the feeding source.

Recently, some progress was obtained in the understanding of the lithium-oxygen reaction mechanism in nonaqueous media, the identification of a stable electrolyte in the cell environment, and the demonstration of an alternative lithium metal-free concept using SiC composite as anode material. However, the low energy efficiency and severe safety concerns associated with the reactivity of the lithium metal anode remain still unsolved and prevent the practical application of the lithium-oxygen battery technology. Accordingly, the development of a lithium-oxygen cell that combines a sufficiently high energy efficiency and suitable safety even for large-scale devices presents a mandatory step toward its targeted commercialization in future.

Herein, we report a new step forward toward the realization of commercial lithium-oxygen batteries, consisting in the development of a Li/O₂ cell exploiting an ionic liquid (IL)-based (specifically, N-butyl-N-methylpyrrolidinium bis-(trifluoromethanesulfonyl)imide — lithium bis-(trifluoromethanesulfonyl)imide, PYR₁₄TFSI-LiTFSI) electrolyte. This electrolyte was chosen due to its well-known nonflammable nature, high electrochemical stability versus the superoxide formation, and chemical inertness combined with a thermal stability of up to a 300-400 ˚C, that is, features that are expected to address the previously mentioned safety issues.

—Elia et al.

Although others have tested this electrolyte before, the ABILE team modified the ratio of anions and cations to investigate the effect on battery performance. For electrochemical characterization, they fabricated a 2032 coin-cell battery with a lithium metal anode; a sheet of Whatman glass fiber GF/A soaked by the electrolyte as separator; and a GDL-SP electrode (a commercial gas diffusion layer (GDL) coated with carbon black) as cathode.

Cycling tests were carried out within limited capacity regimes of 500 mAh g_carbon^-1 and 1000 mAh g_carbon^-1 applying a specific current of 50 mA g_carbon^-1 and 100 mA g_carbon^-1, respectively. The extended cycling test was performed in the 2.0 V - 3.8 V cut-off voltage range applying a specific current of 100 mA g_carbon^-1.

The researchers fully characterized the resulting battery by electrochemical impedance spectroscopy, capacity-limited cycling, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy.

The basic mechanism of the Li-air (Li-O₂) battery requires highly reversible formation and decomposition of Li₂O₂ at the cathode on cycling. When a Li-O₂ battery discharges, lithium ions from the anode move through the electrolyte to the cathode, reacting with oxygen to form lithium peroxide. Although the process should reverse during charging, the Li₂O₂ can precipitate on the porous cathode, leading to electrode passivation, severe polarization, capacity fading on cycling and premature cell death. (Earlier post.)

Initial testing of the ABILE coin cell showed an overall cell polarization value limited to only 0.6 V—a “remarkably low” value, considering that it was obtained with the catalyst-free GDL-SP electrode.

SEM showed that the formed Li₂O₂ particles are very small—in the order of 200 nm—and thus expected to be easily dissolved and reformed in the course of the charge−discharge cycling process. TEM images showed that the particles formed in the ionic liquid media are one order of magnitude smaller than those formed in a conventional tetraglyme-based electrolyte. XRD patterns clearly showed that the formed lithium peroxide is amorphous when PYR14TFSI-LiTFSI is utilized as electrolyte while it is crystalline when TEGDME-LiCF₃SO₃ is used.

We propose that the large improvement in reducing the cell polarization may be explained in light of the fact that the discharge in the ionic liquid electrolyte leads to the formation of lithium peroxide particles having a much smaller size than that usually observed in other electrolyte media, this finally allowing a facile reconversion and, hence, a low polarization overvoltage. It has to be stressed that this low discharge−charge gap results in an energy efficiency in the order of 82%, that is, a value rarely met in previous Li−O₂ battery studies.

… we may assume that the size of the formed Li₂O₂ particles depends on the diffusion of lithium in the electrolyte media, which in ionic liquids is expected to be lower than in other electrolytes. As a matter of fact, lithium ion diffusion-limited conditions would favor nucleation rather than crystal growth, resulting in the formation of many but smaller Li₂O₂ particles rather than less and bigger particles.

—Elia et al.

The researchers are currently extending their investigation to other ionic liquids to achieve a more detailed understanding of the role of this class of electrolytes in the Li−O₂ battery chemistry.

Although not specifically demonstrated in this work, it is reasonable to assume that the high thermal stability of the IL-based electrolyte can offer the additional bonus of enhancing the safety of the battery. It may then be concluded that the results reported in this work are of importance for the progress of the Li/O₂ electrochemical system by contributing to promote its practical evolution as power source of choice for the sustainable, electrified road transportation.

—Elia et al.

In C&EN, Di-Jia Liu, a chemist who studies Li-air batteries at Argonne National Laboratory, said that the reported efficiency is among the best in the literature and that the energy capacity of the coin-cell battery is also very good. If these two parameters hold up in larger cells and can be sustained for a higher number of charging cycles, he added, Li-air batteries would indeed come closer to practical application.

The team included G.A. Elia and J. Hassoun from the University of Rome – La Sapienza; W.-J. Kwak and Y.-K. Sun from Hanyang University; B. Scrosati from Elettrochimica ed Energia in Rome; F. Mueller, D. Bresser, and S. Passerini from Helmholtz-Institute Ulm and the Karlsruher Institute of Technology; and P. Oberhumer, N. Tsiouvaras and J. Reiter from BMW Group.

Resources

• G. A. Elia, J. Hassoun, W.-J. Kwak, Y.-K. Sun, B. Scrosati, F. Mueller, D. Bresser, S. Passerini, P. Oberhumer, N. Tsiouvaras, and J. Reiter (2014) “An Advanced LithiumAir Battery Exploiting an Ionic Liquid-Based Electrolyte,” Nano Letters doi: 10.1021/nl5031985

• D. Bresser et al. (2014) “Invited Presentation: Air Batteries with Ionic Liquid Electrolytes: The Abile Project” (IMLB 2014)