A look at metal-air batteries

Oil consumption, which is mostly used for transportation, accounts for 40% of COemission. A shift towards vehicles running on different energy sources has already begun, and the lithium-ion battery has become the poster-child for that shift. An alternative to lithium-ion batteries, however, could soon become a major part of the electric vehicle industry (1).

Metal-air cells have a metal at the anode and air at the cathode. The air at the cathode is ambient air, which is why many see metal-air batteries as a cut above solar and wind energy because ambient air is always present, unlike sunlight and wind. Almost all metal-air cells have an aqueous electrolyte. During discharge, the metal at the anode is oxidized. Metal ions then move through the electrolyte to the cathode and react with oxygen, forming metal oxides (1). When a cell is charged, the metal ions react with electrons to plate onto the anode (2).

Screen Shot 2017-07-23 at 5.22.47 PM
Metal-air battery reactions. M represents the metal at the anode, M+ represents the metal ions, and MO2x represents the metal oxide (1).

Lithium-air batteries have the highest energy density of the common metal-air batteries (around 3,500 Wh kg-1) (1). Their energy density is also significantly higher than that of Tesla’s current 2170-model lithium-ion battery (roughly estimated to be around 300 Wh kg-1) (3).

While lithium-air batteries, and metal-air batteries in general, exceed lithium-ion batteries in terms of efficiency, they are plagued by some of the same issues as Li-ion batteries. Firstly, an SEI is formed when the anode reacts with the electrolyte. The SEI is necessary for the battery to function in both Li-ion batteries and metal-air batteries, but it also irreversibly lowers battery performance. Secondly, the use of a metal anode leads to dendrite formation. When the cell is charged and metal is deposited back onto the anode, it does not necessarily deposit in the same area from which it was consumed, leading to buildup which can result in a short-circuit. Li-ion batteries do not have this issue because there is no metal plating in either the charge or discharge processes. Another drawback for metal-air batteries that is common among all batteries with an electrolyte is that the ideal electrolyte simply has not been discovered. Different electrolytes stand out by being highly stable, not very volatile (i.e. non-toxic), but few, if any, are ideal in terms of these properties and others that are looked at when evaluating the relative strengths and weaknesses of electrolytes (1).

Lithium-air cell schematic. https://en.wikipedia.org/wiki/Lithium%E2%80%93air_battery

The obvious benefit of metal-air batteries is that their theoretical energy density is ten times higher than that of top-of-the-line lithium-ion batteries. For various industries to benefit from these advantages, the limitations listed above must be addressed. In the meantime, lithium-ion batteries will continue to be the go-to for advanced energy storage and electrical vehicles.

  1. http://large.stanford.edu/courses/2016/ph240/abate1/
  2. http://pubs.acs.org/doi/abs/10.1021/cr030203g
  3. https://www.reddit.com/r/teslamotors/comments/65pt0k/tesla_2170_battery_cell_specifications_calculated/—note that this is not a verified source.

Cover image: https://en.wikipedia.org/wiki/Lithium%E2%80%93air_battery


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