Limits of batteries

Electric cars are meant to solve many environmental problems: Assuming they're charged with 'green' electricity from sources like wind, solar or hydropower, they'll be practically emission-free - apart from the large amounts of energy involved in the manufacturing process for vehicles of any kind, of course, whether fossil-fueled or battery-powered. E-cars reduce noise pollution, too, as they glide along streets almost silently. And they're fun to drive, with better acceleration than regular cars, and often better handling.

But driving e-cars has a drawback: Constant worry about how much charge is left in the battery. Once it's empty, if there's no recharging station in the area, the fun is over, and calls to a towing service are next on the menu.

The two main limiting factors on the attractiveness of e-cars are both related to their batteries. How much driving range is achievable with a fully charged battery? And what's the availability of nearby recharging posts - especially the fast-charging high-voltage DC systems that can recharge an e-car's batteries several times faster than the typical AC recharger in the garage at home? These are questions every prospective e-car driver asks. Let's take a look at the state of play.

How good do batteries have to get before electric cars can prevail?

Six factors are critical to the success or failure of battery-powered cars. First, there's batteries' energy density, i.e. how much electricity they're able to store per cubic cm or per kg. Second, there's charge-cycle durability, i.e. the number of charge cycles a battery can go through before it wears out. Third, there's charging time. Fourth, there's robustness in the face of temperature extremes - i.e. the question of whether a battery will continue to function well during a ski trip in the mountains, or on a hot summer day. Fifth, there's the availability of fast-recharging stations. Sixth, there's the issue of price. The batteries are, at present, by far the single most expensive component of fully electric cars.

How far can I drive with a battery-powered vehicle?

The smallish e-cars currently available on the market (real e-cars that are purely battery-powered - we're not talking about hybrids) have ranges of 150 to just over 200 kilometers (93 to 124 miles). In principle, it would be possible to extend e-car ranges simply by doubling or tripling the mass of batteries they carry. However, this would make them prohibitively expensive. Moreover, batteries are very heavy. Adding more battery mass would require the cars' frames to be heavier-duty. But the idea behind most e-cars is to be small and light, suitable for use in city traffic, not to be huge and heavy.

As it happens, Daimler recently presented a prototype e-truck for urban cargo delivery applications, dubbed the "urban eTruck." It has up to 200 km range. The battery alone weighs two and a half tons. On the plus side, the truck's electric motor is considerably lighter than that of a comparable diesel-engine truck.

Which batteries dominate the market?

Nearly all modern rechargeable batteries, whether in mobile phones, laptops or cars, use variations of lithium-ion technology. The term encompasses a wide variety of battery types in which lithium, an alkali metal, occurs both in the positive and negative electrodes, as well as in the liquid between the electrodes - the 'electrolyte.' The negative electrode is typically made primarily of graphite. Depending on which additional materials are present in the positive electrode, one speaks, for example, of lithium cobalt dioxide batteries, lithium titanate batteries or lithium iron phosphate batteries.
There's also a special type called the 'lithium polymer' battery. Here, the electrolyte is a gel-like plastic. These batteries are currently the most powerful ones on the market. They attain energy densities up to 260 watt-hours per kilogram. Other lithium-ion batteries offer a maximum of 140 to 210.

How do various battery types compare?

Lithium-ion (Li-ion) batteries are quite expensive, due to the high market price of lithium. But they have many advantages compared with older types, such as lead-acid batteries or those based on nickel. For one thing, lithium-ion batteries are suitable for fast recharging. That is, a car can be charged using a normal AC current in about two to three hours. At special DC-current fast-charging stations, they can be charged in under an hour.

Older battery types don't have these advantages, and they can store much less power: Nickel batteries achieve an energy density of only 40 to 60 watt-hours per kg. Lead-acid batteries, including conventional car batteries, have an energy density of only about 30 watt-hours per kilogram. But they're much cheaper and can last for many years.

How long do modern batteries last?

Older types of batteries had the disadvantage of a so-called 'memory effect.' It was particularly strong in nickel batteries. If one recharged a device - such as a cordless screwdriver or a laptop - at a point when the battery was still half full, its storage capacity was irreparably diminished. So before each new recharging cycle, the battery's charge had to be fully used up.

A 'memory effect' of this kind would be disastrous for electric cars, because e-cars should always be recharged when they happen to be near a convenient parking spot with an available recharging post - not when the battery has been freshly drained.

Lithium-ion batteries don't have a memory effect. Manufacturers claim Li-ion batteries can survive up to 10,000 charging cycles and a 20-year service life-span. In reality, users often find laptop batteries need to be swapped out after five to seven years. External factors, such as extreme temperatures or an accidental deep discharge or overcharging can damage a battery. Precise and responsive charging-system electronics are important for preventing damage.

Is a super-battery on the way?

Forschungszentrum Jülich is a leading applied sciences research center with nearly six thousand staff, encompassing everything from plant science and supercomputing through to magnetic resonance tomography and climate modeling. It's located in a small German town about an hour's drive west of Cologne near the Belgian border. One research group on the Jülich research campus is working on the development of 'silicon-air' batteries, which could present a major breakthrough if they can be made to work.

The idea is not entirely new. There had previously been work done on 'lithium-air' batteries, in which the positive electrodes were composed of a nano-scale carbon grid. The electrode itself doesn't directly participate in the electrochemical process; instead, it only serves as an electrical conductor on the surface of which oxygen is reduced.

Silicon-air batteries, first discovered by Yair Ein-Eli of Israel's Technion Institute of Technologyy, function on the same principle, but they have a major comparative advantage: Silicon, unlike lithium, is very common and very cheap. Most sand is made of quartz, which is largely silicon oxide. Processes for refining silicon from sand are also already well-developed, since silicon is routinely used in the semiconductor industry.

In addition to the potentially low manufacturing cost due to the relatively cheap core materials, the performance numbers of silicon-air batteries seem, at first glance, very attractive: They can attain volumetric energy densities that are three to 10 times as big as those of the best current-generation Li-ion batteries.

Unfortunately, silicon-air batteries remain a long way from commercial readiness. The biggest challenges are around their poor durability - so far, they can withstand significantly less than 1,000 recharging cycles.

Experiments done by the researchers at Jülich do offer some hope: They've found that the durability of the batteries can be substantially increased by regularly pumping in fresh electrolyte solution composed of potassium hydroxide dissolved in water. Even then, however, the durability of silicon-air batteries is only a fraction of that of today's li-ion batteries.