The Department of Energy wants batteries with five times the energy storage of those we have today. They want them to be five times cheaper and to be ready in five years. Earlier this year the Department’s solicitation for proposals was announced, and now five universities have been chosen for the job along with several national labs and private companies. According to US Energy Secretary Steven Chu, a “Manhattan Project-like atmosphere” is to be fostered. With a funding level of only $ 120 million and no visible enemy at the border, what are the prospects for success?
By most estimates the Manhattan Project — a research program that led to the first atomic bomb — was funded to the tune of $ 2 billion, which today would be around $ 20 billion. The Battery and Energy Storage Hub, as the new project is called, barely scratches the surface of that. Today’s chemical battery technology is fairly mature, and a serious competitor that would be viable on this projected timescale has yet to emerge. Exotic materials like graphene or carbon nanotubes are being explored as anode materials, and seemingly far-out concepts like using viruses to self assemble electrodes have been studied, but these concepts have yet to be proved. To achieve the kind of numbers that the DoE expects, we can only guess at what these folks might may have hiding up their sleeves.
One alternative to Li-ion batteries that has been proposed is an aluminum-ion (Al-ion) battery. Al-ion would have a potential energy density of 1kW-hr/kg compared with 0.4 kW-hr/kg for Li-ion. Aluminum has the advantage of possessing three valence electrons compared to lithium’s single available valence electron. The result is that battery charge/discharge reactions involving aluminum can transfer three times as many electrons and hence triple the current per chemical unit.
Metal-ion batteries still have the drawback that they cannot be discharged to zero. One might have thought that the Tesla roadster would have been designed so that this could never happen, however Murphy’s law recently provided for dramatic headlines as several owners were rumored to have gotten stuck with huge tabs for replacement batteries. Another sore point for metal-ion batteries was highlighted by a recent Chevy Volt crash test report. Three weeks after a rollover test impacted the battery, it caught fire.
At 13kW-hr/kg, gasoline is still a far more attractive option so long as cities have greater troubles to tend to than the associated noise and smog it brings. To really compete today, metal-air batteries (such as Li-air) producing electricity from reaction with atmospheric oxygen are needed. Like a jet engine, they do not need to internally store oxidizer. Their effective energy density is therefore much higher, comparable in fact to gasoline. The major trade-off is the lack of ability to easily recharge electrically. While cities are thinking more critically about electrifying the transportation sector with charging stations and underground wireless induction coils at traffic lights, the battery sector is already moving on — hence the urgency of the new battery mandate.
One challenge to recharging a Li-air battery lies in keeping it protected from the environment. The cathode needs oxygen but it is degraded by humidity. The cathode also needs a huge surface area, which makes designing a compact battery more difficult. This also means that while energy density is high, the power density is typically low, equating to hard limits on the rate at which power can be put in or drawn off the battery. For electric vehicles looking to be quick off the mark, a supplementary supercapacitor charging itself in the background from the battery could be used for those times when the motor needs a lot of current.
Al-air batteries are an attractive technology that has already seen use in military vehicles. Aluminum is a familiar metal, more abundant and less strategic than lithium. As primary cells — i.e. non-rechargeable — the aluminium anode is slowly consumed by its reaction with atmospheric oxygen at the cathode. The cathode, immersed in a water-based electrolyte, converts the aluminum into hydrated aluminum oxide at which point the battery will no longer produce electricity. Physically recycling the aluminum anode from hydrated aluminum oxide is possible but not a process that is envisioned to happen on board a vehicle. Designers also need keep in mind that since metal-ion batteries gain oxygen during their operation, they would be expected to acquire mass as they are depleted.
Next page: What can we do with these new super batteries? Well, make jetpacks, for a start