The increasing demands for electric vehicles have spurred a worldwide search for next-generation rechargeable batteries with higher energy density. Among various battery systems, the lithium–oxygen (Li–O₂) chemistry offers one of the highest theoretical energy densities, which can reach 3500 Wh kg⁻¹ based on the reaction 2Li⁺ + O₂ + 2e⁻ = Li₂O₂ (E⁰ = 2.96 V vs. Li/Li⁺). The most widely used air electrodes are carbon-based materials because of their large surface areas per weight and high electrical conductivity, which are suitable for promoting the electrochemical formation of Li₂O₂ on the electrode, providing a high gravimetric energy density. For these reasons, carbon-based air electrodes have been intensely researched in recent years, and various types of carbon materials such as porous carbon, carbon nanotubes (CNTs), and graphene have been reported as air electrodes.

Recent studies, however, have shown that parasitic reactions involving carbon-based air electrodes could be one of the factors resulting in the deterioration of the electrochemical cycling of Li–O₂ batteries.[1-3] Upon cycling, carbon can react with Li₂O₂, a discharge product deposited on the carbon electrode, to form insulating Li₂CO₃, which degrades the efficiency and cycle stability. Moreover, carbon itself can be corroded under high oxidizing potential in Li–O₂ batteries and can thus be passivated with carbon-containing byproducts. The damaged carbon surface aggravates the electrolyte decomposition, resulting in the formation of additional byproducts. These byproducts from the electrolyte and carbon electrode can be partially decomposed during charge and typically lead to the evolution of CO₂ gas instead of O₂. As the cycle continues, insulating byproducts are gradually accumulated, which generally results in a high overpotential, low round-trip efficiency, and short cycle life. Therefore, the design of a strategy to reduce side reactions in the carbon electrode is imperative to achieve high electrochemical stability of Li–O₂ batteries.

Our previous work demonstrated that defect sites on carbon materials are more susceptible to reaction with discharge products and are prone to decompose more easily, serving as seeds for side reactions.[3] Thus, a carbon air electrode with higher crystallinity could exhibit more stable cycle properties. However, the preparation of highly crystalline carbon generally requires high-temperature treatment above 2000°C and substantially reduces the surface area, leading to a relatively small discharge capacity. Moreover, the complete removal of the defects in the production of large-surface carbon materials is not trivial. Instead, appropriate passivation of defect sites for a given carbon material may be a suitable approach to fabricate stable carbon-based air electrodes. Selective growth of materials specifically on the defect sites with the minimal amount of coating would be ideal to prohibit the side reactions without excessively increasing the weight of the air electrode. It was previously reported that Al₂O₃ and Pd could be coated on carbon using atomic layer deposition (ALD) to protect the air electrode and reduce the overpotential, resulting in enhanced cycle stability.[4] Nevertheless, the insulating nature of Al₂O₃ caused a large overpotential during cycling, and the high price of the noble metal remains an issue. Moreover, it is not clearly understood how and to what extent these coating materials can aid in the reduction of the side reactions and whether its effect is maintained with extended cycling.

Herein, we demonstrate that the effect of coating is not limited to the protection of the carbon electrode but also remarkably suppresses the decomposition of the electrolyte. These results were analyzed using in situ differential electrochemical mass spectroscopy (DEMS) with labeled isotopic ¹³C air electrode in the electrolyte composed of ¹²C for the Li–O₂ cell. In contrast to the pristine carbon electrode, the coating provided effective protection, which reduced the evolution of undesirable gases from both the electrolyte (¹²CO₂) and carbon electrode (¹³CO₂), with high efficiency of O₂ evolution, resulting in enhanced cycle stability (>100 cycles). Nevertheless, the coating materials were gradually detached from the surface of the carbon materials because of the formation of a discharge product at the interface between the carbon and coating materials during cycling, leading to exposure of the bare carbon electrode and degradation of the Li–O₂ cell. This study suggests that a method to prevent the detachment of the coating material as well as further understanding of the growth mechanism of discharge products, which affects the delamination of the coating, should be sought after in the future.

References

[1] J. Am. Chem. Soc. 2012, 135, 494.

[2] J. Phys. Chem. Lett. 2012, 3, 997.

[3] Chem. Mater. 2016, 28, 8160.

[4] Nat. Commun. 2013, 4, 2383.