Energy Density is a Harsh Taskmaster
There’s no way around this one. Energy density is probably the greatest challenge in the electrification of transportation. This applies to EVs but is even more critical to airplanes, as they are more sensitive to added mass. The real problem is that as a fuel, petrochemicals are actually really, really, good in terms of energy density. For reference, jet fuel has a specific energy of about 11.9 kWh per kilogram (kWh/kg). Gasoline is about 12.9 kWh/kg and diesel is 13.3 kWh/kg. Lithium Ion (Li-ion) batteries come in between 100 and 243 Wh/kg. To put it another way, the worst petrochemical (jet fuel) has 48 times more energy per unit of mass than the best Li-ion battery. In practical terms, that means that for every kilogram of fuel you are looking to replace, you need between 48 and 54 kilograms of battery. Keep in mind that this is specific energy measured at the cell level, without any of the additional packaging, wiring, and cooling capacity that has to be built into the battery pack. When these factors are considered, EV batteries wind up having between 100 and 168 Wh/kg.
There is a weight savings from electric motors, which are significantly lighter than an internal combustion engine (ICE). An installed Rotax 912, a common Light Sport ICE, weighs in at about 64 kilograms, while an equivalent electric motor only weighs 11 kilograms. But this weight savings isn’t nearly enough to offset the massive battery weight relative to standard fuel tanks.
Will energy density improve? Yes, but the consensus on that rate of improvement seems to be five- to eight-percent per year and research suggests that we may be approaching the limits of the current technology. There is promise in solid-state battery technology. Solid-state batteries replace the liquid or gel electrolyte in the battery with a solid electrolyte. This change offers much better packaging, cooling, and energy density capability than traditional Li-ion batteries. Solid-state batteries do exist and will provide a major step forward. The catch is that to build them in large scale is astronomically expensive. Therefore, the widespread application of solid-state batteries will likely take many years, even with massive private and public research efforts.
The top battery is an 18650 Li-ion cell (AA battery shown for scale). 18650s are used in Tesla Model S and Model X battery packs.
Chemistry, Cobalt, and Capitalism
If you’re in the battery business, chemistry is the name of the game. Batteries of all kinds use chemistry to store electricity and the exact nature of that chemistry can have significant effects on the performance of the batteries. That’s why there’s so much focus on battery research. Any potential gains could have huge economic benefit to those who discover and commercialize them. But that search to find even better chemistry can lead to interesting materials. And those materials have concerns of their own.
One key material is cobalt. Cobalt is a metal that is used in a number of applications, notably Li-ion batteries. Battery manufacturers have been working to reduce the amount of cobalt in their cells because it is expensive and prices have been climbing. Cobalt also requires significant processing as it is very rare in its pure form. Cobalt’s supply chain is also a concern. The majority of cobalt is mined in the Democratic Republic of Congo as a byproduct of copper mining. About half of the cobalt supply is refined in China. An expensive metal with massively increasing demand and a geopolitically sensitive supply chain is another challenge for electrification.
Then there’s the industry’s dirty little secret. None of the headline-grabbing car makers actually make their own batteries — not even Tesla and its Gigafactory. Battery cells for Tesla’s Model S and Model X vehicles are made by Panasonic in Japan, and its Model 3 battery cells are made by Panasonic in the Gigafactory in Nevada. This approach is not unique to Tesla; it’s actually standard practice in the industry. GM, Hyundai, Daimler, Ford, and Volkswagen buy cells from LG Chem and BMW buys cells from Samsung.
This industry practice has an interesting aspect for us in GA. It is a potentially tremendous benefit in that all of the research and development in battery technology can be easily transferred. This is a massive game-changer. In the electrical space, any advancement made by any battery manufacturer can be directly dropped in. That’s huge. But on the flip side, it also means that you are competing for cells with all of these other users.
So where does that leave us? About five years ago, I would have said that we were very much in the experimental/proof of concept phase. Today, we are beginning to see the transition to the potential to field truly functional GA airplanes with electric propulsion. Let’s take a look at two projects and where they stand right now.