Researchers work to increase efficiency of batteries

4 min read

There are fundamental limits to batteries. While they can produce high power or hold large amounts of energy, they are unable to do both.

These restrictions have driven Paul Braun, a professor of materials science and engineering at the University of Illinois, to devise a solution. Having established that the limitations are not based on any fundamental principle, he set about researching how to assemble the important parts of the battery into the right structure. Prof Braun's 'Eureka' moment arrived when he and his team realised they could achieve just that. "That is, to assemble a battery electrode using only simple principles of self assembly into the nearly ideal 3d structure," he asserted. According to Prof Braun, the 3d nanostructure for battery cathodes allows for 'dramatically faster' charging and discharging without sacrificing energy storage capacity. The kind of performance he is talking about could charge mobile phones within seconds, laptops in minutes and defibrillators that do not need to power up before or between pulses. And, said Prof Braun, that's just the tip of the iceberg. Batteries that can store a high amount of energy, release it quickly and rapidly recharge are desirable in a range of applications. Prof Braun established his research group to study lithium-ion and NiMH rechargeable batteries which, when rapidly charged or discharged, can suffer from degrading performance. The team chose a thin film as the active material in the battery. While this enabled speedy charging and discharging, it reduced the capacity to nearly zero as it didn't have the volume to store energy. Wrapping a thin film into a 3d structure creates high active volume and a large current. "By keeping the active layer thin," Prof Braun noted, "the battery can be charged and discharged very quickly. Electron and ion transport in the active layer is typically slow, so when the active layer is thin, the electrons and ions can still get in and out quickly." Prof Braun claims to have developed battery electrodes that can charge or discharge in just a few seconds, 10 to 100 times faster than equivalent bulk electrodes. And, significantly, the devices can perform normally in existing devices. The design process focused on self assembly. "We grew a 3d nanostructured colloidal particle template on a conductive substrate," explained Braun. "Then we filled the spaces in between the colloidal particles with metal. Some of the metal was removed, creating a very open metal foam. Then, the active material was deposited as a thin layer on this foam." The 3d template and substrate effectively consists of a series of tiny spheres packed together to form a lattice. To create such a uniform lattice by other means would be time consuming and unworkable, but Prof Braun's inexpensive spheres settled into place automatically. After the space between and around the spheres has been filled with metal, the spheres were melted, leaving a porous 3d metal scaffolding. The next process involved etching the surface of the scaffold to enlarge the pores and make an open framework – a technique called electropolishing. Finally, the frame was coated with a thin film of the active material. "Key was developing the process to grow the metal foam and deposit the active material," Prof Braun emphasised. "We overcame these issues through careful electrochemistry." The resulting bicontinuous electrode structure has small interconnects allowing lithium ions to move rapidly, while the thin film active material enables rapid diffusion kinetics. So far, the group has experimented with both Li-ion and NiMH rechargeable batteries, but Prof Braun believes any battery material that can be deposited on the metal frame. Prof Braun says the technology could work particularly well in electric vehicles. "You could potentially have vehicles that would charge in similar times as needed to refuel a car with gasoline. If you had five minute charge capability, you would think of this the same way you do an internal combustion engine. You would just pull up to a charging station and fill up." All processes used in the research are also used at large scale in industry, so the technique could be scaled readily for manufacturing. "The next steps are to scale up the process and optimise the batteries for specific applications," Prof Braun concluded. Making batteries last longer Further research is being undertaken at the University of Illinois by Professor Eric Pop, who is leading a project that could enable mobile devices that run longer on lighter, slimmer batteries. His team is developing a form of ultra low power digital memory which, he claims, is 'significantly' faster than similar memory devices. To create a bit, the researchers placed a small amount of pcm in the nanoscale gap formed in the middle of a carbon nanotube. They could then switch the bit 'on' and 'off' by passing small currents through the nanotube. Prof Pop said: "Right now, a smart phone uses about 1W and a laptop runs on more than 25W. Some of that energy goes to the display, but an increasing percentage is dedicated to memory." Several hundred bits have so far been made and tested and Prof Pop is now looking to scale production to create arrays of memory bits that operate together. "Even though we've taken one technology and shown that it can be improved by a factor of 100, we have not yet reached what is physically possible. We have not even tested the limits yet. I think we could lower power by at least another factor of 10." Boosting efficiency Researchers at the Faculty of Applied Sciences at TU Delft are working to increase the efficiency of batteries by adding tiny crystals to solid electrolyte material. According to PhD student Lucas Haverkate, the better the electrolyte – the material between two electrodes – the better efficiently the battery or fuel cell works. The electrolyte, usually a liquid, has to be well enclosed and takes up a relatively large amount of space. But conductivity in solid electrolytes is inferior. Haverkate said: "Solid matter has a network of ions in which virtually every position is taken. This makes it difficult for the charged particles (protons) to move from one electrode to another. It's a bit like a traffic jam. What you need to do is to create free spaces." Haverkate said that, by adding 7 to 50nm nanocrystals of titanium dioxide, protons become attracted and this creates more space in the network. The addition of the crystals appears to improve conductive capacity by up to 100 times.