Ask any user of a mobile phone, digital camera or laptop what new feature they would like, and the answer coming back is likely as not to be "longer battery life". For several years now that longevity has been provided by so-called lithium-ion cells, which have largely replaced nickel-cadmium by virtue of their higher energy density and lower weight. They also do away with NiCad's "memory effect" - recharging from partial discharge does not reduce capacity - and they hold their charge well when idle.
Li-ion has been so successful that, as its price has come down, to about $400/kWh today, this technology is even making electric cars a practical reality. But as Boeing's ongoing struggle to overcome a couple of in-service battery fires aboard 787s demonstrates, safety is the dark side of Li-ion. Watching its rival's difficulty, Airbus has reverted to Ni-Cad late in the development of its A350 - but stresses that Li-Ion may come back into the reckoning.
In short, for price and performance, Li-ion is today unbeatable, and as aircraft designers strive to replace hydraulic systems with lighter, easier to service electric alternatives the demand for higher-performance batteries will only grow. But with several exotic alternatives in development promising up to 10 times more energy density and greater safety, is Li-ion going to be first choice for the next generation of aircraft?
The all-electric Nissan Leaf carries 24kWh of power in its Li-ion batteries, 12 times as much as a Boeing 787
Quite possibly. According to a paper by Lux Research of Boston, next-generation batteries will make their debut in military applications around 2020, with consumer electronics following with strong demand around mid-decade. But, in transportation, Li-ion "will prove very tough to displace".
Lead analyst Cosmin Laslau told Flightglobal that of the four promising Li-ion replacements - lithium-air, lithium-sulphur, solid-state and zinc-air - only solid-state offers a serious alternative for aerospace in the foreseeable future. The technology is extremely attractive, as by combining separators and electrolyte into a single ceramic or polymer, energy density is greatly boosted and safety is increased. This technology does away with the liquid, lithium-based electrolyte that makes Li-ion batteries comparatively delicate and accounts for much of their fire risk - even in small-scale devices such as mobile phones.
There are "big" technical hurdles to putting solid-state batteries into commercial use, but Laslau believes these can be overcome. However, cost could prove to be an insurmountable obstacle. Where Li-ion costs around $400/kWh today, solid-state runs in the millions, a cost which Laslau, not surprisingly, describes as "off the scale".
He reckons, though, that efforts to develop high-throughput, roll-to-roll production processes may well bear fruit. If they do, solid-state batteries could be cost-competitive with Li-ion by 2024 at about $200/kWh. Production at the moment, he notes, is by vacuum deposition, in very small volume under laboratory conditions.
As for the other alternative technologies, aircraft applications look unlikely, at best. Lithium-air promises the greatest energy density, but is still in the laboratory stage of development and it will take another 10 to 15 years to scale it up to useful capacity. Lithium-sulphur also promises great energy density, but both of these types will rely on as-yet undeveloped materials to be practical. Neither type will be ready for use before the 2020s, so they will almost certainly not be available for next-generation aircraft projects.
In any case, adds Laslau, both types rely on pure lithium, making them very dangerous and thus reliant on high-performance coatings to keep this highly reactive metal out of contact with air or water. The risk of wear, degradation or accidental damage makes them unsuitable for aviation.
Zinc-air batteries are well-known today - as the coin cells used in hearing aids, for example - but rechargeability has been elusive.
Meanwhile, Li-ion is not standing still. The energy-carrying advantages of the alternatives will look relatively less attractive in ten years, when their cost and reliability starts to make them contenders for Li-ion's market share.
Laslau's expectation is that military applications, with their "more esoteric requirements" will lead the way to next-generation battery use in other areas. One unmanned aircraft maker, for example, wants to increase range by a factor of 10 without increasing vehicle weight; cost, clearly, is less of an issue than in civil aerospace or consumer electronics.
But ultimately, if aerospace wants to move beyond Li-ion it is going to have to get its battery show on the road - literally. Automotive battery requirements sound a lot like aviation's needs: light weight, high energy density, reliability over many charge/discharge cycles, low-to-zero fire risk and crash resistance. The big difference is volume, which, as Laslau points out, translates into cost-effective product development. Among car makers, he says, "everybody" is working on solid-state technology, but no company developing battery technology can afford to do it based on prospective sales to aviation. Thus, the batteries that will be used in future aircraft will probably be derived from the units that will power electric cars, especially when they move to solid-state.
To underscore just how urgent it is to develop volume production techniques for solid-state car batteries, Laslau observes that the all-electric Nissan Leaf carries 24kWh of power in its Li-ion batteries - about 12 times as much as a 787.