Lithium-air batteries, with a theoretical gravimetric energy density of ∼3500 Wh/kg, are of great interest as next-generation energy storage systems that would enable, among other things, much longer range in EVs.

Lithium-ion rechargeable batteries are based on a pair of intercalation electrodes. On charging, lithium ions move from the cathode through the electrolyte and insert into the anode; discharging reverses the process.

Challenges facing the commercialization of Li-air batteries are both scientific and engineering, and include a lack of understanding of major limitations in the reaction mechanism, electrolyte instability, poor cycle life and rate capability, and low round-trip efficiencies largely resulting from high over-potentials on charge. (Earlier post.)

Furthermore, it is not yet known how the presence of the non-O₂ components of air affect the reaction mechanism in Li−air batteries because most previous studies have been conducted in a pure O₂ atmosphere, under the assumption that the other components of air will be less important in the operation of the battery. To develop a Li−“air” battery technology usable at ambient conditions, it is critical to elucidate the effects of the other constituents of air (N₂, Ar, H₂O, and CO₂) on the operations of the Li−air battery.

Supposing that the moisture is removed by using water-proof films (which is known to fatally deteriorate electrolyte and lithium anode), CO₂ should have the most influence on the chemistry of the Li−air cell among the various constituents of air. Although N₂ and noble gases such as Ar are more abundant in ambient air, the conventional cathode voltage range of ∼3 V cannot activate electrochemical reactions involving these gases and Li...In contrast, CO₂ is known to be much less inert than N₂ or Ar, and it can therefore undergo electrochemical reactions with Li (some involving O₂ as well).

...The difference in chemical stability implies that there is always a thermodynamic driving force to convert Li₂O₂, the desired discharge product of a Li−air cell, into Li₂CO₃ in the presence of CO₂. This observation has also led people to believe that the irreversible formation of Li₂CO₃ might limit the cyclability of Li−“air” batteries.

Moreover, the high solubility of CO₂ gas in organic electrolytes (∼50 times more soluble than O₂) results in the major possibility of CO₂ being incorporated in battery reactions, despite its low concentration in ambient air. Thus, to further the development of Li−air battery technology, it is critical to understand the reactions involving CO₂ and the chemistry of Li₂CO₃ within a Li−air cell.

—Lim et al.

The team from the Korea Advanced Institute of Science and Technology (KAIST) and Seoul National University investigated the reaction mechanisms in the Li–O₂/CO₂ cell under various electrolyte conditions using quantum mechanical simulations combined with experimental verification.

They found that “the subtle balance” among various reaction pathways influencing the potential energy surfaces can be modified by the electrolyte solvation effect. A low dielectric electrolyte tends to primarily form Li₂O₂, while a high dielectric electrolyte is effective in electrochemically activating CO₂, yielding only Li₂CO₃.

Their most surprising finding was that a high dielectric medium such as DMSO can result in the reversible reaction of Li₂CO₃ over multiple cycles.

This is of vital importance because the superior thermodynamic stability of Li₂CO₃ leads to its formation being unavoidable in an environment containing CO₂. Moreover, the realization of cell cycling based on the stability of Li₂CO₃ seems to help attain a more stable cyclability for Li−air cells.

On the basis of our systematic investigation of the reaction chemistry of CO₂ within a Li−air battery cell combined with the idea of “reaction pathway leveraging using dielectric media”, we suggest that the use of a high dielectric electrolyte may help to preserve the reversible reaction of Li₂CO₃ by electrochemically activating CO₂. However, we should note that the electrolytes with the highest dielectric constants are usually either protic or carbonate-type; the former is not suitable for Li−ion chemistry and the latter has stability problems. Thus, DMSO might be optimal. In addition, our findings might further open up the new possibility for a novel rechargeable Li−O₂/CO₂ battery based on the single discharge product of Li₂CO₃, which has proven advantageous with regard to cyclability.

—Lim et al.

Resources

• Hyung-Kyu Lim, Hee-Dae Lim, Kyu-Young Park, Dong-Hwa Seo, Hyeokjo Gwon, Jihyun Hong, William A. Goddard, III, Hyungjun Kim, and Kisuk Kang (2013) Toward a Lithium–“Air” Battery: The Effect of CO2 on the Chemistry of a Lithium–Oxygen Cell. Journal of the American Chemical Society doi: 10.1021/ja4016765