Batteries: their past, present and future.

It’s easy to be lulled into thinking that the entire semiconductor industry moves at the hasty clip described by Moore’s Law. But not all technologies are created equal; while semiconductors surge ahead at a compound growth rate of around 41% per annum, battery capacity has only improved at around 6% to 10% year-on-year over the past decade. Not surprisingly, this is of concern when it comes to the growing popularity of mobile computing – after all, your supercharged microprocessor is little more than an expensive lump of silicon without electricity coursing through its silicon veins.

Thankfully those unsung heroes of the semiconductor revolution – materials scientists – are coming to the rescue with new and better battery technologies that should help carry us forward into the next decade and beyond.

Explosive power
Lithium-ion batteries recently captured headlines care of a few unfortunate accidents where a few of them happened to explode. This is because, as they say, there’s no such thing as a free lunch. Lithium-ion happens to be a superb technology when it comes to storing and releasing energy, and this same penchant for power was also its downfall in this case.

Compared to its predecessor, nickel-cadmium, lithium-ion is superior in several crucial ways, primarily energy density; lithium-ion can pack in around double the power per kilogram of nickel-cadmium. This means either twice the battery life from a similar sized cell, or a much smaller and lighter cell with the same battery life, allowing for smaller and thinner devices.

Nickel-cadmium also has the notorious problem of the ‘memory effect’. If the battery isn’t periodically completely discharged and recharged, crystals can form on the surface of the electrodes, degrading the performance of the battery. This is not such a problem if the battery is used in a predictable fashion, but it’s far from ideal for modern portable devices, which tend to be used and charged in fits and spurts. However, lithium-ion doesn’t suffer from the memory effect, and is very tolerant to sporadic use and is happy to be topped up rather than requiring a full discharge/charge each time.

Lithium-ion also has several benefits when it comes to manufacture and disposal, with the materials used being significantly more benign than the heavy metals in nickel-cadmium batteries.

However, lithium-ion isn’t without its bugbears. The batteries are actually so good at releasing energy that precautions need to be taken to ensure the battery doesn’t release it all too quickly. Were this to happen, the temperature inside the battery can rise to dangerous levels, potentially vaporising the volatile liquid electrolyte, thereby risking explosion. As such, lithium-ion batteries feature a number of built-in safety devices to prevent the battery from releasing too much energy too quickly, shutting it down and venting any built up gas if it does so.

These safety devices are generally very reliable, and it takes several of them to fail in succession for the battery to undergo a catastrophic failure. As unlikely as this scenario is, however, it has happened at least a half a dozen times recently, resulting in the recall of millions of Sony-manufactured lithium-ion batteries. This was after it was discovered that a fault in the Sony production line allowed tiny metal particles to contaminate the innards of the battery, and these particles could lead the battery to short circuit and potentially explode.

Another drawback of lithium-ion batteries is their form factor. Lithium-ion cells are manufactured in a cylindrical shape, which limits how small or thin the batteries can be made.

Even with these drawbacks, lithium-ion is still the most popular battery technology for mobile devices today, and is still undergoing development and improvement. Future lithium-ion batteries, such as the third generation batteries demonstrated by Panasonic at the recent CES 2007 show, will feature incrementally improved capacity and, as demonstrated by Toshiba recently, significantly reduced recharge time. Even so, there are other technologies that seek to improve upon and eventually supercede lithium-ion.

Flat out
From around 2000 we started hearing a lot about the rise of lithium polymer batteries, which were developed to address some of the shortcomings of lithium-ion. Lithium polymer batteries use a very similar chemistry to lithium-ion, except they replace the liquid electrolyte with a specially formed dry polymer. The benefit is the polymer can be sandwiched between the electrodes in a very thin and flexible package – as little as 1mm thick.

Lithium polymer also isn’t restricted to the cylindrical form factor of lithium-ion cells, and can be formed into virtually any shape to fit any gap. This made lithium polymer a popular choice for mobile phones and other devices that demanded the thinnest possible battery.

Lithium polymer batteries are also not quite as susceptible to the explosive dangers of conventional lithium-ion batteries. However, they do have their limitations. Early lithium polymer batteries suffered from high internal resistance, which limited the amount of power they could release in bursts, making them less suitable for demanding tasks such as powering a notebook. Recent designs have improved, and Toshiba now uses lithium polymer batteries in its Portégé R200 – a notebook that is only 9.9mm thick – with the battery located under the palm rest.

However, even with the improvements in lithium-ion and lithium polymer batteries over the past few years, they still haven’t been able to achieve the holy grail of powering a notebook for a full day. To reach that milestone, we have to turn to alternative technologies.

Next generation
Experimentation with new materials suitable for batteries never stops, with the zinc-silver combination being touted as the combination that could finally push beyond the limits of lithium-based batteries.

The biggest advantage of zinc-silver batteries is that they tout double the energy by weight compared to lithium-ion. With such a high energy density, zinc-silver may be able to power your average notebook for up to twice as long – closing on the target 12-16 hour mark – or could allow for even smaller and lighter devices.

Furthermore, zinc-silver cells feature a water-based electrolyte that isn’t flammable, which makes them considerably safer than lithium-ion batteries. The materials are also non-toxic and can be fully recycled – assuming the infrastructure is in place to do so.

That’s not to say zinc-silver is without its shortcomings. First, and most obvious, is the cost of silver. Even though the materials can be recycled, the initial cost is still likely to be higher than lithium-ion.

There’s also a problem with the zinc being highly soluble in the electrolyte. When the battery is recharged, the dissolved zinc reforms on the electrode in the form of small spikes, called dendrites. These spikes can then damage the internals of the battery, such as by puncturing the separator membrane between the two electrodes. Without compensating for this effect, a zinc-silver battery might have only a dozen or so recycle charges in it before it fails.

There are several companies actively pursuing zinc-silver technology at the moment, such as the California-based Zinc Matrix Power, which has developed a new technique to prevent the formation of dendrites by placing the electrodes within a special polymer. Zinc Matrix Power has already released prototype batteries to hardware manufacturers for testing. CEO, Ross Dueber, suggests we should see batteries using its technology some time around the end of 2007 or early 2008. Analysts at Gartner are a little more pessimistic, stating it could be 2010 or later before zinc-silver takes off.

Alcohol powered
When talking about the holy grail of batteries, it’s hard not to mention fuel cells. Fuel cells share some similarities with batteries, but they differ in that they don’t just store an electrical charge in a chemical state, but they consume fuel, converting it into electricity. A fuel cell works by allowing protons from the fuel to pass through a special membrane, which is also an electrical insulator. This forces the freed electrons to pass around the membrane through an electric circuit, thus producing a current. The principle has been proven, and fuel cells can potentially offer incredibly high efficiencies – as high as 80 percent in some cases.

The fuel cell design that shows the most promise for portable devices is called direct methanol fuel cell (DMFC). Not surprisingly, the fuel used is simple methanol, or wool alcohol. The waste product of a DMFC is carbon dioxide and water, although the water is usually recycled internally to be mixed with the methanol on injection into the cell.

According to Garnter’s latest figures, fuel cells could potentially offer up to 10 times the life of a lithium-ion battery, although the fuel cell needs to be replenished with methanol when it runs out. This raises the question of how to refuel it, with several companies, such as UltraCell, suggesting small methanol fuel cartridges. Another issue is the lifetime of the fuel cell itself, as the methanol has the tendency to degrade the membrane after only a few uses.

Fuel cells are currently under development by swathe of technology companies, including Toshiba, Sony, Hitachi, NEC, Intel and others. The first generation of fuel cell devices are likely to be external chargers for mobile devices, with internal fuel cells coming in the more distant future.

However, it’s important to note that this doesn’t mean batteries are going to disappear altogether. Fuel cells output a constant wattage, but are unable to provide ‘peak’ power, such as when a notebook boots or the hard drive spins up. As such the fuel cell will likely work in tandem with a battery – much like in a hybrid car like the Toyota Prius – constantly recharging the battery while the battery handles the varying load of the device.

The first fuel cell chargers are already on the market, albeit in small numbers and with a high sticker price. We can expect to see more throughout 2007, with internal fuel cells appearing in the next few years.