|Schematic cell configurations for the four types of Li−air batteries. Credit ACS, Lu et al. Click to enlarge.
A new review of Li-air battery technology by a team from Argonne National Laboratory, Beijing Institue of Technology and Hanyang University focuses on the most critical issues that must be addressed for the successful development and commercialization of high energy density Li-air batteries. The review appears in the ACS journal Chemical Reviews.
Li-air batteries are of great interest (as evidenced by more than 300 research papers on the topic in the past 3 years). The Li−air battery potentially offers densities of up to 2−3 kWh/kg on the cell level. A fully developed Li−air battery system (i.e., with the full balance of plant required) is expected to surpass battery technology under development for deployment in the medium term (400 Wh/kg), and meet the requirements for plug-in vehicle applications. However, the reviewers noted,
Development of a practical Li−air battery will involve overcoming many formidable challenges, including the need for a fundamental understanding of Li−O2 electrochemistry, development of new and improved cell materials, and innovation in the critical aspects of cell design. In the past few years, dozens of reviews on the topic of Li−air batteries have been published. These reviews address the technical issues and challenges facing Li−air batteries at the current stage from different perspectives, including the stability of the electrolytes, importance of the air electrode/electro-catalyst, and oxygen-selective membranes. In this review, we mainly focus on the most critical issues that must be addressed, with the hope that it will help to advance a truly rechargeable Li−air battery toward its practical application.… Covering the immense body of all the work published in this field is, however, beyond the scope of this review.—Lu et al.
Li−O2 batteries are based on the oxidation of lithium at the (lithium) metal electrode and reduction of oxygen at the air electrode to induce current flow. On the basis of the oxidation of 1 kg of lithium metal, the theoretical energy density of a Li−O2 cell is calculated to be 11,680 Wh/kg—not much lower than that of gasoline (13,000 Wh/kg). Practically, however, the energy density of the batteries is far less.
Note that the usable energy density of gasoline for automotive applications is approximately 1700 Wh/kg, assuming an average tank-to-wheel efficiency (12.6%) of the US fleet. Fortunately, such energy density accounts only for 14.5% of the theoretical energy content of a fully charged Li−O2 battery, so it is not inconceivable that such a high energy density may be achievable at the cell level, given intensive research effort and long-term development.—Lu et al.
There are four types of Li−O2 batteries are under development, characterized by the type of electrolyte employed: aprotic; aqueous; solid-state; and hybrid aqueous/aprotic. (Earlier post.) All types of Li−O2 systems require an open system to obtain oxygen from the air; Li metal must also be used as the metal electrode to provide the lithium source for all the systems at the current stage.
In the review, the team focused only on the aprotic and aqueous Li−O2 systems, with a particular emphasis on the former since it has dominated the research effort on Li−O2 batteries for the past decade. “Without a doubt,” they concluded, “substantial challenges exist for each component of the aprotic Li-O2 cells.”
A typical aprotic Li−O2 cell consists of a lithium electrode, an electrolyte consisting of dissolved lithium salt in an aprotic solvent, and a porous O2-breathing electrode that contains carbon particles and, in some cases, an added electrocatalyst.
Unlike the aprotic electrolyte, the aqueous electrolyte is limited to acidic or basic solutions only. In the aqueous Li−air battery, electrolyte solvent, e.g., H2O, is not a limiting factor in the cell performance, which is its main advantage over the aprotic system. In addition, the incombustible aqueous electrolytes circumvent the safety issue which is a major concern for the organic electrolytes in an open cell configuration. However, due to the different electrochemical reactions involving Li and O2, the gravimetric and volumetric capacities of an aqueous Li−O2 cell are much lower compared to those of an aprotic cell.
Aprotic electrolyte. Perhaps the greatest challenge at the current stage for aprotic Li-air cells is the search for stable electrolytes, they concluded. While carbonate-based electrolytes have been widely used in most of the initial research work, these electrolytes decompose in the presence of the superoxide radicals. Despite that, many research projects are using them to investigate the catalytic activities of the air electrode materials.
The catalytic activity of the air electrode materials needs to be re-examined in more stable electrolytes, the team suggested. Ether-based electrolytes seem to be relatively stable in the presence of the reduced oxygen species; however, their stability during charge, especially at high voltage, remains unclear.
Lithium salt deserves much more attention, they suggest, since it may have a positive effect on the electrolyte’s stability in aprotic Li−O2 cells.
Without question, searching for a fully stable electrolyte in the oxygen-rich electrochemical environment is the research priority at present. Design of a robust strategy for effectively screening the stability of various electrolytes would be greatly beneficial to the development of a Li−O2 battery for practical application.—Lu et al.
Air electrode. Investigation on how the porous air electrode architecture affects the formation of the discharge product, Li2O2, and the specific capacity of the cell is still of great interest, the team found. Researchers need to understand the key limiting factors to determine the capacity, rate capability, and cycling efficiency of aprotic Li−O2 cells.
Porous carbon with or without additional catalyst is the current choice of the air electrode material, although a few non-carbon air electrode materials have been reported. However, the reviewers noted, the mechanism of Li2O2 growth on the porous air electrode during cell discharge and the subsequent decomposition of Li2O2 on charge is still debatable; this matter needs to be further clarified to develop more efficient catalysts for the aprotic Li−O2 cell.
Li metal electrode. The lithium electrode has been a historic problem in any of the Li battery systems, while the long-term cycling of the Li electrode has yet to be demonstrated.
Controlling reactions of the electrolyte at the Li electrode through suitable membranes or passivation films will be essential for achieving good performance with aprotic Li−O2 cells. These membranes should meet the following criteria, the reviewers said:
block diffusion of oxygen from the air electrode to the lithium electrode;
allow the transport of Li+ to support current flow; and
exhibit excellent mechanical flexibility and stability to be compatible with the mechanical flexibility of the supporting polymer membranes and battery design/processing.
Engineering active membranes with a nanometer-scale thickness could potentially meet these criteria, they suggested.
The researchers also noted that the study of electrolyte stability and electrocatalytic process in the aprotic Li−O2 system will require advanced research tools from both experimental and theoretical modeling are necessary.
Aqueous. For aqueous systems, they found, a better understanding of Li−O2 electrocatalysis is required, since the Li−O2 electrochemistry is unique and different from that of conventional electrocatalysis.
The successful development of any aqueous Li−air batteries severely relies on the prevention of direct contact of the lithium metal electrode with water. The most innovative approach to address this issue is the introduction of Li ion conducting glass ceramics. However, these ceramics are generally fragile and highly resistive at low temperature. Moreover, they may not be very stable in strong acidic or basic media. Future research and development of large and more flexible LiC-GC membranes will be greatly beneficial to the aqueous Li−O2 system. Searching for effective catalysts, in particular, with respect to OER, will be a key challenge for rechargeable aqueous Li−air cells.—Lu et al.
Jun Lu, Li Li, Jin-Bum Park, Yang-Kook Sun, Feng Wu, and Khalil Amine (2014) “Aprotic and Aqueous Li–O2 Batteries,” Chemical Reviews doi: 10.1021/cr400573b