The Li-O₂ (or Li-air) battery is a promising next-generation energy storage system, especially for electric vehicles, due to its exceptionally high theoretical energy density. Unlike typical Li-ion batteries which operate based on an intercalation mechanism into electrode frameworks (with redox reactions of heavy transition metal components), Li-O₂ batteries feature distinct surface chemistry involving the formation and evolution of lithium-oxides on an oxygen electrode. However, one of the challenges with the Li-O₂ battery is the solid, insulating lithium peroxide (Li₂O₂) products typically formed on the oxygen electrode during discharge, often deactivating the surface of the electrode and catalytic sites. This, in turn, results in insufficient round-trip efficiency and poor cyclability.

… a critical step toward advancing cell performance is to develop highly efficient catalytic electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), corresponding to the formation and decomposition of Li₂O₂ products, respectively.

Tailoring functional catalysts through decoration on carbonaceous oxygen electrodes affords numerous catalytic sites with high surface area and enables facile charge transfer to and from the products, thereby decreasing the substantial overpotentials between discharging and charging. … Although optimization of oxygen electrode morphology by using low-dimensional catalyst supports has been attempted to facilitate the decomposition of products formed on the catalyst surface, most catalytic sites on the oxygen electrode are gradually buried by solid products during discharging. Subsequently, electron transport through the insulating Li₂O₂ products become difficult, causing a large ohmic loss and a corresponding increase in the overpotential during charging.

Therefore, to effectively preserve catalysts and avoid deactivation after discharge tailored design and effective positioning of catalyst materials on the oxygen electrode are required for the further improvement of Li-O₂ battery systems. In this work, we introduce a new architecture of an oxygen electrode employing a functional catalytic polymer-based membrane to enhance the efficiency of interfacial reactions in Li-O₂ batteries.

—Ryu et al.

The catalytic membrane consists of uniformly distributed discrete Pd nanoparticles (NPs) on a 1D polyacrylonitrile (PAN) nanofiber scaffold. This was inserted between the oxygen electrode and separator as an interlayer. The Pd/PAN catalytic membrane offers a number of benefits:

• The insulating polymer membrane scaffold preserves catalytic activity against the formation and growth of thick Li₂O₂ products from the oxygen electrode during discharging.

• The catalytic membrane facilitates oxygen evolution by maintaining active Pd NPs (charging) due to the electrically insulating nature of the PAN scaffold.

• The geometric positioning of catalysts on the polymer scaffold instead of on the conducting electrode substrates prevents direct electrochemical nucleation of Li₂O₂ products and enables facile charge transfer near the interface between the electrode and electrolyte.

Furthermore, Ru NP-decorated multiwalled carbon nanotubes were used as oxygen electrode catalysts in concert with the catalytic membrane to increase the number of total catalytic sites.

Cycle performance of MWCNT, MWCNT-Pd/PAN membrane, Ru/MWCNT, and Ru/MWCNT-Pd/PAN membrane under specific capacity limit of 1000 mAh/gcarbon in a voltage window between 4.35 and 2.35 V at a current density of 500 mA/g. Credit: ACS, RYu et al. Click to enlarge.

The team investigated the electrochemical performance of Li-O₂ cells with and without the Pd/PAN membrane using pristine multi-walled carbon nanotubes (MWCNT) as well as Ru/MWCNT materials loaded on Ni mesh substrates as oxygen electrodes.

• Pristine MWCNT and Ru/MWCNT electrodes with a catalytic membrane showed higher first discharge capacities of 3000 and 2650 mAh/g_carbon, respectively, compared with pristine MWCNT (2815 mAh/gcarbon) and Ru/MWCNT (2425 mAh/g_carbon) electrodes without the catalytic membrane.

• The pristine MWCNT and Ru/MWCNT electrodes with a catalytic membrane exhibited lower overpotentials during charging than those without a catalytic membrane.

• A Ru/MWCNT electrode without the catalytic membrane started to lose efficacy after 37 cycles; a Ru/MWCNT electrode with a polymer catalytic membrane maintained a capacity value of 1000 mAh/gcarbon for 60 cycles without any capacity loss. The pristine MWCNT with a catalytic membrane exhibits a stable cycling performance up to 25 cycles compared with only 18 cycles without a catalytic membrane.

The degree of improvement in the cycling performance for the pristine MWCNT is less than that for the Ru/MWCNT electrode, which emphasizes that the combination of the catalyzed oxygen electrode and the mesoporous catalytic membrane maximizes the performance of the Li-O₂ battery.

—Ryu et al.

Proposed reaction schematic of an oxygen electrode without and with a catalytic membrane during discharging and charging. Credit: ACS, Ryu et al. Click to enlarge.

This cell design strategy using a functional interlayer can be applied to diverse combinations of catalysts and polymer nanofiber membranes and also provides new insight into the geometric positioning of catalytic sites in the oxygen electrode of Li-O₂ batteries.

—Ryu et al.

Resources

• Won-Hee Ryu, Forrest S. Gittleson, Mark Schwab, Tenghooi Goh, and André D. Taylor (2014) “A Mesoporous Catalytic Membrane Architecture for Lithium–Oxygen Battery Systems” Nano Letters doi: 10.1021/nl503760n