DESPITE their excellent fuel economy, squeaky-clean exhausts and generous government subsidies, electric vehicles have failed to catch on with the motoring public. Fewer than 10,000 pure electrics (less than 0.1% of new-car sales) trickled onto the roads of America in 2012. And that was their best year ever.



Customers continue to be put off by a not unreasonable fear of being stranded by the roadside when the vehicle’s battery pack goes flat, miles from home or the nearest recharging station. Fully charged, pure electrics have a range of typically 75 miles (120km) or so—though Tesla’s $90,000 top-of-the-range Model S, with its 85 kilowatt-hour battery, can officially clock 265 miles on a single charge.



Hybrid cars are a different matter. Having a petrol engine coupled to an electric motor (or two) allows them to recharge their batteries while on the move. Regenerative brakes help the engine-driven generator keep the battery topped up. Their freedom from “range anxiety” has made hybrids like the Toyota Prius a popular choice, especially in California where they enjoy additional subsidies and benefits on the freeways.



Lacking the ability to recharge themselves puts electric vehicles at the mercy of their battery capacity. Invariably, that is a trade-off between weight and cost. In terms of energy density, the most popular battery technology to emerge over the past 20 years is the powerful but temperamental lithium-ion cell—in the news lately for causing fires on board Boeing’s latest passenger jet, the 787 Dreamliner. All 50 Dreamliners currently in service have been grounded as a result, and deliveries of new aircraft halted while air-safety experts try to fathom the cause of the lithium-ion battery fires (see "Difference Engine: An innovation too far", January 28th).



Theory suggests lithium-ion cells should be able to store a little over 400 watt-hours of energy per kilogram. They currently pack about half that and cost around $500 per kilowatt-hour of capacity. A pure electric car like the Nissan Leaf needs 24 kilowatt-hours of electrical storage to travel 75 miles or so on a single charge. That suggests its lithium-ion battery costs around $12,000—in other words, about 40% of the vehicle’s price before tax breaks.



To rid themselves of range anxiety, electric cars need batteries three or four times better. In an ideal world, they would store as much energy per kilogram as the hydrocarbon fuel used in a conventional car. The energy density of petrol is approximately 13 kilowatt-hours per kilogram. After accounting for all the losses in the engine and driveline, the energy available at the wheels for propelling a petrol-engined car is typically no more than 1.7 kilowatt-hours per kilogram of fuel.



As the numbers indicate, the lithium-ion battery—at least in its present form—cannot compete. Some battery researchers believe there are gains still to be had from lithium-ion chemistry. May be. But many are sceptical about claims that a doubling of the cell’s energy density and a halving of its cost is feasible.



But even if the sceptics are proved wrong, a lithium-ion battery offering twice the range would still fall well short of petrol’s performance. And electric-car owners would continue to suffer from range anxiety, even more so than ordinary motorists facing a fuel gauge showing a tank almost empty: the nearest petrol station is usually just a mile or two away; recharging stations for electric vehicles are far fewer and farther apart. At least for electric vehicles, then, lithium-ion would seem to have reached the end of the road.



The only battery technology that comes close to matching the energy density of petrol is lithium-ion’s close cousin, the perpetually promising lithium-air cell. In theory, at least, this can deliver up to 12 kilowatt-hours per kilogram.



Even if only half that were ever achieved commercially, it is not unreasonable to expect something like 1.7 kilowatt-hours per kilogram to be available at an electric vehicle’s driven wheels—given the far greater efficiency of electric motors and their transmission systems. In a “well-to-wheels” comparison, the overall efficiency of an electric vehicle (25%) is twice that of a car powered by an internal combustion engine (12%). The electric vehicle is arguably cleaner, too.



Long before Sony introduced the first commercial lithium-ion battery in 1991, lithium-air was being touted as ideal for electric vehicles. That was back in the 1970s, following the first of the oil shocks that tripled petroleum prices and sent carmakers scurrying for alternative fuels and better energy-storage systems.



It took less than 20 years for lithium-ion, despite its thermal instability problems, to go from powering laptops, phones and other portable devices, to becoming the leading contender for heavy-duty applications such as electric vehicles, plug-in hybrids and even aircraft. So why has it taken twice as long for lithium-air—an even lighter and more powerful storage technology—to make it barely out of the laboratory?



The short answer is the difficulty in building lithium-air cells that can be recharged thousands of times. While the battery in a mobile phone is expected to last no more than two to three years (say, 1,000 recharging cycles at most) before the phone is tossed for a later model, motorists expect electric vehicles to come with at least 100,000-mile warranties on their batteries (say, 3,000 recharging cycles at the very least).



If maintained properly, lithium-ion batteries can manage that without too much difficulty. As they stand, lithium-air batteries cannot. After 40 years of development, the life of rechargeable lithium-air cells tends to be measured at best in terms of a few hundred, rather than several thousand, charging cycles.



The key to lithium-air’s high energy density is the way it dispenses with the heavy metal cathode of its lithium-ion cousin, and uses instead a lightweight porous electrode made of carbon that breathes oxygen from the air. On discharge, an electrolyte grabs positively charged lithium ions from the anode and transports them to the cathode, where they combine with the oxygen to form lithium peroxide. Meanwhile, electrons flow in an external circuit from the anode to the cathode, powering an electric motor or other equipment on the way.



Though theory says it should, the lithium peroxide is reluctant to do the reverse (replate the anode with lithium metal and return the borrowed oxygen to the air) when a current is applied externally—ie, when charging, rather than discharging, the cell. Why that should be so has stumped researchers for decades, and is one of the reasons why lithium-air has failed to fulfill its promise.



Lately, however, unsuspected reactions have been detected between the lithium ions and the electrolyte as well as with the carbon in the cathode. Such clues have helped researchers choose different electrolytes, membranes and electrode materials that promise to endow lithium-air batteries with far longer recharging lives.



No question there has been as much hype as hope among lithium-air’s hard-core supporters. But those championing rival chemistries—including zinc-air as well as lithium-ion—have been guilty of belittling lithium-air’s genuine advances, while making pie-in-the-sky promises of their own.



Inevitably, the back-biting is all about money for research—especially from the ARPA-E programme established recently by the Department of Energy (DoE). Endowed with $400m of economic-stimulus money, ARPA-E is a clone of the Pentagon’s hugely successful Defence Advanced Research Projects Agency (DARPA). For over half a century, DARPA has made a name for itself by backing bold ideas facing entrenched opposition and long gestation periods. The internet was one of the agency’s early successes. The F-117 stealth fighter was another.



Last November ARPA-E announced some 66 grants worth a total of $130m for “transformational” changes in energy research. The largest award, amounting to $4.5m, went to PolyPlus Battery Company, a 30-person firm in Berkeley, California, that spun out of Lawrence Berkeley National Laboratory in the 1990s. Earlier last year PolyPlus also won a $9m grant from the DoE’s Advanced Manufacturing Office, to build a pilot line for making its special “protected lithium electrodes”. The process encases the lithium anode with a ceramic membrane that allows lithium ions to pass through but is impervious to liquids or gases.



Protecting the lithium electrode this way changes everything. For a start, it allows freer use of aqueous electrolytes. Normally, water is the last thing that should be allowed near lithium. As readers may recall from their school chemistry class, the metal ignites on contact and burns furiously. For decades, non-aqueous electrolytes (mainly carbonates, ethers, esters and other organic compounds) were used in lithium batteries for that very reason. Lack of an efficient electrolyte has been another reason for lithium-air's slow development.



The protection technology has even made it possible to use seawater instead of air as a source of oxygen—like the gills of a fish. With an eye on the marine market, PolyPlus has already built a lithium-seawater battery that puts out 1,300 watt-hours per kilogram—three times more than lithium-ion’s theoretical maximum. The company’s long-term goal is to use this know-how to advance its own lithium-air system. PolyPlus’s first commercial applications are destined for consumer electronics. Later, when the the number of recharging cycles is high enough, the plan is to tackle the electric-vehicle market.



PolyPlus will not be alone. Last month, Toyota agreed to share its battery know-how with BMW, in exchange for a steady supply of modern diesel engines from 2014 onwards. The collaboration includes the development of an advanced lithium-air battery as well as a new fuel-cell for vehicles Toyota plans to start selling in 2015.



Meanwhile, IBM’s Battery 500 Project, launched in 2009 at the company’s Almaden research laboratory in San Jose, California, aims to develop a lithium-air battery capable of propelling a car 500 miles on a single charge. Like PolyPlus, much of the research has focused on finding the most appropriate electrolyte and developing a membrane that can filter pure, dry oxygen from the atmosphere.



IBM is not doing this out of idle curiosity. Like everyone else bidding for a piece of the business, it truly believes there will be a seriously large market for advanced traction batteries once they have the storage density to rival petrol and are capable of providing years of trouble-free motoring. When will that be? The consensus is early in the next decade.