Energy

One of the things that was clear to me as we dug into the Al literature and recently funded projects is that nearly everyone (researchers, funding agencies, etc.) has been intoxicated by the promises of ultra-high capacity (2981 mAh/g) and energy density (4140 Wh/kg) for metallic Al. This is not new. Al has been explored as a battery electrode material for literally 170 years. A brief summary of that development is shown in Figure 1. However, no battery has come even close to meeting the capacity and energy density numbers above in a lab cell, let alone a practical one. And the pathway forward is quite unclear. 

In aqueous batteries, Al is either rapidly corroding or catastrophically passivated. In non-aqueous cells, such as those with ionic liquid electrolytes used in aluminum ion batteries (AIBs), the Al complexes in the electrolyte and the overall reaction does not give 3 electrons per Al atom that is assumed in the high numbers above, but actually gets only 3 electrons for every 8 aluminum atoms. Most of the extra Al is not solid, but dissolved in the electrolyte – meaning a lot more weight and volume than “theoretical”. In addition, these AIBs typically use graphite cathodes that need many C atoms per Al, adding mass. The combination of electrolyte mass and cathode mass (in addition to other practical things like packaging, etc.) significantly drives down the achievable energy density to values closer to 50 Wh/kg. And the true limit when all practical components are taken into account is only around 80 Wh/kg. Though these values may be competitive with Pb-acid batteries, they are not competitive with Li-ion batteries at all. And primary chemistries that exist based on aluminum air batteries (AABs) have also only been able to achieve practical energy densities well below 100 Wh/kg, far below the alkaline and LiFeS2 primaries that we can already buy at the drug store. Another aspect where Al-based batteries have failed so far is in their lifetime. State-of-the-art chemistries have very poor long-term operational and shelf stability.