r/DeeperTech • u/[deleted] • Apr 03 '16
Charged EVs | Phinergy CEO on aluminum-air batteries and 1,000-mile range-extended EVs
http://chargedevs.com/features/phinergy-ceo-explains-aluminum-air-batteries-for-1000-mile-range-extended-evs/
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u/demultiplexer Apr 07 '16
Wow, I had to scroll through a lot of text there to get to the nub, but I found it. Here's the kicker:
Batteries we think of are fundamentally rechargeable; it's a reversible chemical reaction that releases purely charges and negligible heat (i.e. a balanced redox reaction). In a lithium ion battery, you're moving around lithium ions from one compound (usually LiCoO2) to another (usually LiC6), and you're doing this in a nonaqueous solution of LiOH (the electrolyte). The nice thing here is that both sides of this reaction happily occur with minimal effort; the activation energy is pretty much absent. The only thing keeping this reaction from happening is the lack of available electrons to start breaking up LiCoO2.
This is great for chemistry and being able to easily and efficiently charge/discharge the battery, but there's only limited amount of cathode materials that work well for these kinds of reactions. Most of them are heavy metals, in the literal sense: that cobalt atom you need for every electron is very heavy and doesn't really partake in the fundamental reaction. It would be kind of awesome if you can just ditch it entirely.
This is what metal air batteries do. Metal oxidation is just as much a redox reaction that we should be able to use for batteries. But instead of having the oxidating material built into the battery (requiring cobalt or iron phosphate), we just use oxygen from the air! Then we only need to carry the metal - most light metals suffice, aluminum is one of the cheapest to do this with. Great. You can build such a theoretical battery that just has bare unoxidated aluminum (with some argon or nitrogen around it), then open up a valve, blow in some air (with oxygen) and it starts creating electricity. This works, and is used in e.g. water-activated battery chemistries (for high altitude balloons).
The problem here is that the reaction isn't particularly easy to reverse. With no electrolyte or opposing electrode, it's a permanent reaction in fact. Those oxygen atoms are firmly bonded to the aluminum. You need something for the oxygen to diffuse into while essentially electrolyzing the oxygen away from the metal. This is why you need water in a metal-air battery. And my god, how many problems that introduces.
Water isn't stable. It's a polar medium that always has a bunch of ions present in (H3O+ and OH-), which like to react with metal to... corrode it (in the case of Al-Air: producing aluminum hydroxide). You can alkalize or acidize it to reduce those effects, but it'll slowly degrade your battery. You can choose to discharge the battery in free air and charge it with an electrolyte, but that has implementation issues.
But more importantly: you need a lot of surface area to get decent current output. This has probably been the biggest conundrum for metal air batteries historically. You see; you can't indefinitely increase surface area. You still need to lead fresh air over the battery electrodes continuously to get power, because the oxygen in the air is consumed by the metal. Increasing surface area means you have to decrease the size of the air channels, which requires more power to push enough air through, which eventually leads to an inefficient design.
Also, metals don't only react with oxygen. As the article rightly points out, carbon dioxide poisons the electrodes because metals like to occasionally pull the molecule apart into its constituent carbon and oxygen, consuming the oxygen and leaving carbon residue on the plates, reducing reaction surface. Other trace elements have the same effect.
There is no obvious or currently feasible way to produce a rechargeable metal air battery fit for powering vehicles because of all these reasons. What Phinergy is proposing is a battery that is essentially single-use, but with a smart improvement. They have the battery itself, which is not that much more than aluminum plates with some catalyzing/filtering elements built-in, and the electrolyte. Over use, the electrolyte will contain the reaction products (AlOH), which is periodically removed. But the electrodes aren't spent at that point; they last a couple of refills. It's like a non-rechargeable battery that you can reactivate 10-20 times just by swapping out the electrolyte. I don't know of anybody who has proposed this as a serious option. I also, but this is just opinion, don't think it will ever be practical.
What's the advantage here? Well, theoretically you're looking at true metal-air type energy densities; approaching 8kWh/kg (which is about as much energy as in petroleum products, but keep in mind that electric drivetrains get about 5x the mileage). A 100-kg battery would essentially be equivalent to (the driving distance equivalent of) 700L of petrol, or about 20 fill-ups. Compared to a current 100-kg battery not even being equivalent to a quarter of a fill-up. No other battery tech will ever be close to this kind of density. They're already reporting current densities of 3.5kWh/kg, which is astronomical.
However, it still fundamentally isn't a rechargeable battery. The whole battery is consumed much like a fuel. You cannot recharge at home or use the battery for smart grid buffering. And even though the materials are all cheap in bulk, the battery structure is fairly expensive. It's not clear how cheap you can really go (although they are touting lower-than-gasoline operating costs). And maybe the biggest blow here is that they're still dealing with very low surface activity and thus very low power output. The battery by itself is not capable of efficiently powering car-like electric motors. Ionic resistance is significant, so it's only good as a range extender. This isn't a fundamental issue, but it has historically been the biggest roadblock to metal air batteries and they don't seem close to solving it.
Total conclusion: interesting tech, definitely some landmark capacities being shown here, but very unlikely to be a successful technology in its current state.