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A new catalyst for lithium-air batteries

Scientists in the United States demonstrated a lithium-air battery with improved energy and stability performance, thanks to the use of molybdenum triphosphide as a catalyst for both charge and discharge reactions.

Main building at IIT’s campus in Chicago.

Image: Joe Ravi/CC by SA 3.0

Lithium-oxygen, or lithium-air batteries are one of many pathways to improving on today’s energy storage technologies. Lithium, and other metal-air batteries are favored in research for their potential for high energy densities, however low efficiency and poor cycle life have proved tough obstacles to overcome in developing the technology.

One way to boost the performance is using a catalyst to speed up reactions at the electrodes. However, finding a material that can speed both the charge and discharge mechanisms presents a further challenge, and many of the working catalysts reported to date rely on prohibitively expensive materials including platinum and gold. “…designing a highly active catalyst that can minimize the energy barriers—excess input energy—to form and decompose Li2O2 nanoparticles at the cathode is a key challenge for the development of this technology,” state scientists led by the Illinois Institute of Technology (IIT) in a recently published paper.

And the group at IIT set out to design such a catalyst. In previous research, trimolybdenum phosphide (Mo3P) has shown promise as a catalyst for similar reactions, so evaluating its performance in a lithium-air battery served as a starting point. The group fabricated a custom design three electrode cell, and then a full Li-air battery. The work is described in full in the paper Kinetically Stable Oxide Overlayers on Mo3P Nanoparticles Enabling Lithium-Air Batteries with Low Overpotentials and Long Cycle Life, published in Advanced Materials.

With the Mo3P catalyst, the battery operated at discharge and charge overpotentials of 80 and 270 millivolts (mV), among the lowest reported for a lithium-air battery. And the battery also retained close to 100% of its initial performance after 1200 cycles.

Oxide layer

A close analysis of the performance revealed that during cycling a stable layer of lithium carbonate (Li2CO3) formed around the anode, preventing other unwanted reactions with components both in the air and in the electrolyte. And the catalyst also formed a layer of molybdenum oxide (MoO), which further enhanced the battery’s performance.

After 1000 cycles, however, the battery began to lose performance, which the group attributes to lower activity in the electrolyte. They are convinced, however, that with further work the Mo3P catalyst could be a promising development for energy storage. “The developed surface reconstructed Mo3P catalyst with kinetically stable oxidized overlayer offers significant promise in the advancement of sustainable energy storage systems,” the paper concludes, “owing to its unique electronic and structural properties discovered in our Li–air battery cell.”

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