Being able to access the internet on the go is something most of take for granted these days and the benefits are huge and getting bigger all the time. However, this mobile revolution has been hampered at every step by one key problem, battery power. Big bright screens, quad-core processors, Bluetooth connectivity, and GPS positioning are all great but they're also power-hungry.
Our current devices may seem impressive, but with more power things could change radically. Imagine carrying around your main PC in your pocket all the time, a device with the power of a laptop in the palm of your hand; or possibly a return to when phones needed charging weekly rather than daily. And it's not just phones, tablets and laptops either, as better battery technology could revolutionise electric cars, with bigger ranges and less charging.
What we need is simple – more efficient batteries. To be fair this has already taken place. If smartphones still had nickel cadmium batteries instead of the lithium-ion equivalent, we'd either be talking a battery life of a couple of hours, or they'd be considerably heavier and bulkier. Li-ion batteries have up to five times the energy density of Ni-Cad, and batteries are the largest single component of a phone by weight.
If this doesn't provide you with much consolation, you'll be pleased to hear that scientists are far from finished in their quest for the perfect battery. Here we look at the new battery technologies that are currently in the development labs, at a few high performance equivalents that are already available, and at some alternatives that might even make batteries obsolete.
Assault and Battery
To put it simply, a battery is a device that generates electricity from a chemical reaction. Strictly speaking, what we've just described is a cell while a battery is a number of cells, connected together in a single package, to provide a higher voltage than you'd get from one cell alone. So, for example, an AA alkaline 'battery' is actually a cell and generates 1.5V, while a PP3 battery genuinely is a battery as it contains six 1.5V cells, connected in series, to provide 9V. Despite all this, in deference to common usage, we'll continue to talk about batteries to mean either batteries or cells.
The simplest type of battery is a primary battery and is the type that can't be recharged. As electricity is drawn from the battery, the chemical reaction converts the initial chemicals into other, totally different chemicals. In an alkaline battery, the most common type of primary battery in portable electronics gear, zinc (Zn) reacts with manganese dioxide (MnO2) to produce zinc oxide (ZnO) and a different oxide of manganese (Mn2O3). When there's no zinc or manganese dioxide left, the battery is flat and has to be thrown away.
In a secondary battery, that's a rechargeable one, again a chemical reaction takes place as electricity is produced and, again, when all the original chemicals have been converted to something else, the battery is flat. Where it differs from a primary battery is that the chemical reaction is reversible. To reverse the reaction, electricity is put into the battery instead of taken out of it and this process of recharging causes the original chemicals to be regenerated. The Li-ion battery found in most phones, laptops and tablets converts carbon (C) and lithium cobalt oxide (LiCoO2) into lithium carbide (LiC6) and a different lithium cobalt oxide (LiCo2O4) while electricity is being produced. Exactly the opposite reaction occurs during recharging.
Primary batteries might seem outdated compared to their rechargeable counterparts but they are considerably less expensive and commonly they offer a higher energy density – that's the amount of electrical energy that can be produced for a particular weight or volume of battery – which is why they continue to be used. While the drawback of not being able to recharge a primary battery might be acceptable in something like a toy that is used occasionally, the waste of constantly throwing away batteries would be unacceptable in mobile electronics gear. Because of this, the remainder of this article will concentrate squarely on re-useable energy sources.
Li-ion Roar
It's probably inevitable that the first battery developments that are coming our way would be better described as evolutionary than revolutionary. But it pays not to be dismissive of these gradual changes. After all, look how far LCD has come from simple mono displays to 4K monster TVs, those incremental changes can really add up.
Take Amprius, a start-up company based in Silicon Valley that's commercialising the results of research at Stanford University. Their new battery isn't based on some expensive rare earth element, it doesn't digest microbes, and it doesn't charge in 30 seconds flat. Instead it's based on the lithium-ion technology that's been powering our phones for some time now. But their new spin on the technology allows them to pack an extra 25% of energy into the same volume as an ordinary cell with further improvements in the pipeline. Even with today's Amprius batteries, that's an extra two hours per day and, what's more, this is no pie in the sky research. The battery is already in early prodcution and is being tested by manufacturers including Nokia.
^Lithium-ion batteries might represent a huge leap from NiCads but there's plenty more in the technology yet
So what's the secret of Ampirus' success? It's long been known that using a silicon anode, instead of a carbon one, in lithium-ion batteries, can give a huge performance improvement. This is because four lithium ions will bond to each of the atoms in a silicon anode compared to just one for every six carbon atoms in today's graphite anodes.
This improvement comes at a cost though. Because silicon anodes swell and contract as the battery is charged and discharged, they are soon destroyed so instead of the 500 times today's batteries could be charged, only a few cycles would be possible. However, a team of scientists led by Stanford's Yi Cui has discovered that a double-walled silicon nanostructure will last as long as today's batteries and, potentially, could increase battery life to more than 6,000 cycles.
As Professor Cui described, the new double-walled silicon nanotube anode is made by a clever four-step process: "Polymer nanofibers (green in the diagram) are made, then heated … until they are reduced to carbon (black). Silicon (light blue) is coated over the outside of the carbon fibers. Finally, heating in air drives off the carbon and creates the tube as well as the clamping oxide layer (red)". Because silicon oxide is such a tough ceramic material, the outer oxide layer keeps the outside wall of the nanotube from expanding, so it stays intact.
^Anodes made from double-walled silicon nanotubes provide much improved lithium-ion cells (Image: Hui Wu, Stanford, and Yi Cui)
There's even more to come though, with Yi Cui also involved in early work on a lithium anode, which will apparently bring the best possible performance from a Li-ion battery. "Of all the materials that one might use in an anode, lithium has the greatest potential. Some call it the Holy Grail," said Yi Cui. "It is very lightweight and it has the highest energy density. You get more power per volume and weight, leading to lighter, smaller batteries with more power."
Giving examples of how the new battery technology could be used the team said it could increase battery life and bring down costs. Phone battery life could be doubled or tripled and electric cars with a range of 300 miles could cost just $25,000 (£14,700).
True Capacity
An important characteristic of a battery is its energy density, that is how much electrical energy can be produced per gram or per cubic millimetre of battery. Things aren't that simple though because, in many cases, it's just not possible to extract all the energy that is, at least in theory, trapped inside a battery.
First, manufacturers usually quote an ideal case scenario which involves extracting the energy at a given rate. If you have a power-hungry device that has a greater rate of energy use, you'll end up with less energy out of the battery. Alkaline cells are particularly bad in this respect which means that, while they are quoted as having a higher energy density than common rechargeable batteries, in practice it can be lower. An AA cell, for example, might provide 3W hours of energy if you draw just 25mA of current, but at 500mW you might get less than half as much.
Second, the capacity often depends on the temperature so, if you're using kit outside in cold weather, you could find battery to be disappointing. In fact, the capacity of an alkaline cell can drop five-fold in going from +10°C to -20°C. Something we've experienced with smartphones in very cold conditions.
Finally, while some battery technologies maintain their voltage until they're almost flat, with others, and again alkaline batteries are especially poor, the voltage gradually drops as they're discharged. This was particularly noticeable with old style torches that used filament bulbs, as they got progressively dimmer as the battery became exhausted. Modern electronic gear can better cope with this because today's power supplies can operate over a broad range of voltages but, even so, they'll eventually come to a point at which energy still remains in the battery but the voltage is too low to do anything useful with it. Batteries that maintain their voltage as they're discharged, only dropping when they're almost flat, will allow better use to be made of their energy.
^The amount of energy you get out of a battery depends on how you use it
Better Batteries
It seems likely that batteries based in lithium-ion technology will continue as the dominant force in portable power for many years to come. That isn't preventing far-sighted scientists from looking for its successor, though.
Intriguingly, one hot area of research for portable power of the future isn't a battery at all, although most people will probably view it as such. Instead of storing chemical energy which is converted to electrical energy when power is required, in this solution the energy is genuinely stored as an electrical charge. Electronic components that do exactly this have been around for over a century, in fact your smartphone and PC motherboard will contain loads of them.
The component in question is of course a capacitor but, for ordinary electronic applications, the amount of charge they can store is minute. An up-and-coming breed of super-capacitor, otherwise known as the electrochemical capacitor, promises to provide much greater levels of storage than ordinary capacitors. Even so, that energy density suffers by comparison to the best batteries on offer but they do have other advantages as Dr. Pooja M. Panchmatia of the Department of Chemical Sciences at the University of Huddersfield described.
"Electrochemical capacitors have unusually high energy densities when compared to common capacitors as well as very high specific short duration peak power (i.e., rapid energy delivery). Batteries have significantly higher energy densities than electrochemical capacitors with lower short duration peak power.
"For example, Li-ion batteries have specific energy densities of hundreds of Watt hours per kg, with electrochemical capacitors having specific energy densities ranging from several to tens of Watt hours per kg. However, the short duration peak power of an electrochemical capacitor exceeds 1,000 Watts per kg, which is an order of magnitude better than that of typical Li-ion batteries."
When we bear in mind that it's also possible to charge super-capacitors much more quickly than batteries, perhaps in seconds, we can see another reason for the intense research interest in this technology.
This level of interest in super-capacitors doesn't mean that true batteries, the ones that rely on electro-chemical reactions, have come to the end of the line except for incremental improvements to lithium-ion technology. Dr Panchmatia outlined several technologies that might just provide that much sought-after combination of high energy and power densities, low cost and safety.
"In recent years, metal-air and metal-sulphur batteries have generated great interest in the research community", she said. "For example, Zinc-air batteries combine atmospheric oxygen and zinc metal in a liquid alkaline electrolyte to generate electricity with a by-product of zinc oxide. When the process is reversed during recharging, oxygen and zinc metal are regenerated. Zinc-air batteries are attractive because of the abundance and low cost of zinc metal, as well as the non-flammable nature of aqueous electrolytes which makes the batteries inherently safe to operate."
However, it appears we're not going to be seeing this technology in our smarthpones anytime soon. "It remains a grand challenge to develop electrically rechargeable batteries, with the stumbling blocks being the lack of efficient and robust air catalysts, as well as the limited cycle life of the zinc electrodes", she warned. "Zinc-air batteries have been commercialized only for small medical and telecommunication applications", she said. "Similar constraints remain with the Li-sulphur and the Li-air batteries, which have not yet been commercialized."
^Zinc Air batteries might be used mostly in hearing aids today but all that could change
Intriguingly, Dr Panchmatia even suggested that something as simple as making batteries a different shape could reap major benefits. "Further research into the redesign of the batteries has also gained momentum. Researchers from the Pacific Northwest National Laboratory claim to increase battery power by 30% by simply changing the shape of the battery from cylindrical to planar. Additionally new materials using elements of the periodic table that are readily available such as sodium and nickel are also being sought and even commercialized by General Electric".
Fuelling the Future
Invariably, when conversation turns to innovative new energy sources, sooner or later the fuel cell crops up. So it's somewhat surprising, perhaps, to learn that the fuel cell was invented over 150 year ago. Despite their long heritage, however, and while fuel cells have been use used for decades in niche applications like submarines and spacecraft, they are still to make a significant impact on everyday life. Yet, potentially, they have a lot to offer.
Like primary batteries, the chemical reaction that generates electricity in a fuel cell cannot be reversed in the cell. Where it differs from a non-rechargeable battery, however, is that the supply of chemicals used in the cell can be replenished. In the first fuel cells, the chemical reaction was between hydrogen, which is the fuel supplied to the cell, and oxygen taken from the air to produce water which is discharged into the atmosphere as steam. Commonly, and especially where the fuel cell is designed for use in portable equipment, hydrogen is not used as the fuel because other substances are more easily and safely handled. Instead, various chemical compounds that are a source of hydrogen – often methanol or a hydrocarbon such a liquefied butane – are used instead.
The amount of heat that can be generated by burning the butane gas that's often used in space heaters and barbecues is testimony to the energy that's locked away in its chemical bonds. In fact the energy density of many hydrocarbons can be twenty times that of today's lithium ion cells so the benefit of being able to use this technology for powering mobile devices is huge. Despite all this, fuel cells remain elusive as a power source for electronic equipment, although a pending product launch provides a glimpse of what the future may hold.
Lilliputian Systems will shortly start to ship Nectar 3.0, the latest incarnation of its fuel cell-based portable power source, which is intended to recharge batteries in mobile electronic equipment, via their USB ports, without access to mains power. It's fairly compact, weights 200g, and can supply 55 Watt hours per cartridge which, the company claims, is sufficient to charge a typical smartphone ten times. The hot-swappable fuel cartridges, or pods as they call them, weigh just 35g. Unfortunately, this level of convenience doesn't come cheap. The basic unit is priced at US$299 with cartridges coming in at US$10 each.
^Lilliputian Systems' Nectar 3 promises almost unlimited power but at a cost
If a portable fuel cell-based power source is good, having the fuel cell built into the equipment is surely better since you don't have a separate box of tricks to carry around. Several companies including Toshiba and Panasonic have done just that, demonstrating various prototype laptops, some as long as eight years ago. With an internal fuel cell, power is supplied via a small plug-in fuel cartridge which provides plenty of power and, in so doing, will make larger screens, faster processors and many of the other benefits of desktops available to laptop users.
None of these became commercial products with cost and convenience being key issues, we all have a large, uninterrupted supply of electricity in our home, and having a device run off another fuel source would probably require us to maintain a supply of it. It remains to be seen whether these obstacles can be overcome and this 176 year old technology eventually becomes the ultimate enabler of the 21st century's love affair with ubiquitous electronics.
Generate your Own Energy
The most common method of generating a small amount of electrical power on the move is via a hand-cranked generator with dozens of companies now having jumped on the bandwagon. This approach was first adopted by Trevor Baylis, of wind-up radio fame, and his company Freeplay are still major players in the market. Their Freecharge 12V is one of the more universal wind-up chargers available. In generating power at 12V you can charge any equipment for which you have an adapter for a car cigarette lighter socket, and are not restricted to USB-powered gear. Realistically, though, it's only practical to use it with smartphones and similar small items of electronic gear as you'd have to wind forever to power a laptop for more than the odd minute. A few years ago Freeplay had a pedal-powered charger for use with more power-hungry equipment but, presumably, most people weren't prepared to have a good workout just to inject enough energy for five minutes of operation into a laptop battery.
^The Freecharge 12V will allows you to power just about anything by cranking its handle
Hand cranking and pedalling aren't the only technologies for producing human-generated power but all methods tend to involve carrying out some repetitive action, often for a protracted period of time. Shaking is another option although, to date, this has tended to be used as the power source of battery-less LED torches as opposed to charging separate electronic gear. Torches that require a handle to be squeezed periodically are also available.
Charging on the Go
Already you can buy solar cells to power your hand-held devices but, realistically, these are never going to be particularly effective with typical British weather. There are also devices that will turn a modest amount of shaking or cranking into electrical power for emergency use but scientists are also looking for ways to keep your gear charged up, without access to a power socket, and with no deliberate action on your part.
Just walking around is an inefficient process because a lot of the energy we use doesn't translate to forward motion. Acoustic energy is lost as the click of our footsteps on the pavement, for example, and no doubt we heat up the ground just by pounding it with our feet. These might sound like infinitesimally small losses but, even so, scientists are finding ways of harnessing some of the energy we waste in walking to power our electronic gear.
Of the many techniques that have been used, one of the latest employs the effect that gives us static shocks in hot and dry weather. Called the triboleteric effect, this process generates a static charge when two dissimilar materials – for example our hands and a cat's coat – rub against each other.
What Professor Zhong Lin Wang and his team at the Georgia Institute of Technology have done is to arrange for two sheets of material rub against each other as a result of walking. While, initially, only minute amounts of energy could be produced, by using nano materials the area of contact between the two sheets was increased 1,000-fold leading to the potential of 33W per square metre. Keeping a smartphone's battery constantly topped up is, therefore, entirely feasible.
^Using the triboelectric effect, researchers at Georgia Tech have salvaged the energy lost in walking
Ann Makosinski, a 15-year old student from Victorian Vancouver, has used a different method to provide battery-less power to a torch with the potential for powering other electronic kit. The development, which gained her a prize in Google's annual international science fair, generates electrical energy from body heat. Her device uses Peltier tiles which produce power from the temperature difference between one side of the tiles, which is in contact with the user's hand, and the other side which is in contact with the air. At present it generates a fairly small 24 lumens, but work is being done to improve this into the hundreds.
^Makosinski's award-winning torch generates power from body heat
The Outer Limits
While we've been looking at the main contenders for future battery technology, there are some more unusual research projects that are worth a mention; as seemingly off-the-wall developments do occasionally make huge breakthroughs.
Take, for example, researchers at Virginia Tech who have created a battery that generates electrical energy from sugar using a chemical reaction that parallels the way living organisms obtain energy from food. Their creation is a type of fuel cell, so it's charged by adding a cartridge of sugar whenever its power output dwindles but, as its inventors point out, unlike the fuels used in other fuel cells, sugar is neither explosive nor flammable. An energy density ten times greater than today's best batteries is claimed.
Going one further, in that they use real living organisms rather than just mimicking their chemical behaviour, scientists at the University of Newcastle have turned their attention to generating energy from bacteria. The team have created a microbial fuel cell that uses the Bacillus stratosphericus bacteria which is found in large quantities 30km above the Earth's surface but, for this application, was extracted from the Wear Estuary. However, it's rather early to get your hopes up about the possibility of a bacterial-powered tablet. Although the University has doubled the energy density previous available from microbial fuel cells, 200mW per cubic metre isn't exactly huge. We can imagine that having to carry the fuel cell in a large backpack wouldn't be too popular.
Another unlikely sounding possibility is a battery made out of cloth and a specially formulated ink but this is exactly what scientists at Stanford University have been fine tuning for several years. The batteries can store about three times more energy than ordinary lithium ion cells but this isn't the main benefit on offer. Instead, its developers say that it will be key to the up-and-coming wearable electronics revolution. So cloth batteries won't be produced as components in their own right but will be printed onto garments, alongside other printable electronic components. The batteries can be printed in any colour, they can be washed, and they'll even continue to work if stretched to twice their original length.
^Scientists a Stanford University have used special ink to print batteries onto fabric
No End in Sight
While lithium-ion technology represents a huge improvement over earlier generations of batteries, indications suggest that we're still a long way from the theoretical limits of portable power. Coupled with the massive enhancements we've seen in the electrical efficiency of processors in recent years –and with further gains pretty much assured – the mobile revolution shows no signs of running out of steam. So are we going to see smartphones lasting a week on a single charge in the next decade? Surely only the brave would bet against such an eventuality.