Israeli company Phinergy claim to have produced a battery that can power a Citroen C1 for 3000km, and have demonstrated a 1800km drive with a more practical version, three times that available from commercial electric cars. Even more dramatically, the battery weighs just 100km, a fifth of the weight of those in the Tesla Model S.

Metal-air batteries use the oxygen in the air around them, rather than storing it in liquid or solid chemicals. They can store far more energy than most competing technologies. Not needing to contain the oxygen can also cut the weight dramatically – Phinergy claim that 70% of the weight in a conventional car battery is in the cathode, mostly just to store the oxygen.

With such benefits, metal-air batteries have been a topic of research for some time. Lithium-air batteries have theoretical energy per weight almost as high as petrol. However, a range of practical problems have prevented widespread commercialization.

Aluminum-air batteries don't have the same potential energy per kilogram of lithium-air battery, but could theoretically reach energy densities many times better than the lithium-ion batteries that are currently the industry standard.

One of the challenges for metal-air batteries is to capture enough oxygen to provide the power required. Phinergy's porous electrodes have the surface area to allow this. A silver-based catalyst prevents carbon dioxide from permeating the electrodes, a common problem for other experimental versions of this technology that reduces their lifespan to the point of impracticality.

Like anything based on aluminum, there is a lot of embodied energy in Phinergy's batteries, but they are manufacturing their products in Quebec, where the electricity is almost entirely sourced from hydroelectric stations, keeping the carbon footprint small.

The big disadvantage of aluminum-air batteries is that they don't last. The aluminum turns to aluminum-hydroxide. While this can be recycled, it can't be recharged by plugging the battery into a powerpoint. Instead the whole battery will need to be replaced when the aluminum has been used up. Advocates of this system claim that battery swaps can be done quickly and easily.

However, any battery that needs to be replaced so frequently is not only expensive, but runs into the problem that has bedeviled many alternatives to gasoline-powered cars. People are reluctant to buy vehicles that depend on the availability of refueling or replacement stations if these are not available everywhere they might be needed. On the other hand, without a critical mass of owners of suitable vehicles, such stations are not viable.

By extending the capacity of the electric car to drive much further on a single charge, aluminum-air batteries greatly reduce this problem when it comes to quick recharge points, but at the cost of increasing the need to be able to access places where batteries can be replaced.

Phinergy's solution is to use a twin battery solution. A small lithium-ion battery will allow trips up to 50km, more than adequate for most city journeys. The aluminum-air battery will be saved for longer trips, avoiding the need to replace it except where the car is used for frequent long journeys. If the whole battery was aluminum-air the car's maximum range might be as much as 3000km, but with the need to replace the entire engine thereafter.

Even if these problems have been successfully addressed, it remains to be seen whether Phinergy have found a solution to the other major obstacle to aluminum-air batteries, the high cost of the anode.

A zinc-air battery could potentially offer even more advantages but has been hard to mass produce. Phyinergy claim they are on the way to commercializing that as well.