If you’re reading this article on a laptop computer, tablet, smartphone or similar device, chances are that device is being powered by a lithium-ion battery. Lithium batteries are very lightweight, have extremely high energy density and are rechargeable. With that said, lithium-ion batteries do have limitations and issues. Scientists at Northwestern University in Evanston, Illinois, have been working on a new type of lithium battery that may be able to overcome those limitations and issues – not to mention the potential to power everything from personal devices to cars.
Lithium-ion batteries are popular for a myriad of reasons. Compared to other battery types of the same size, lithium-ion batteries are much lighter, yet have a much higher energy density. As a comparison, an average lithium-ion battery can store 150 watt-hours of electricity in 1 kilogram of battery – A nickel-metal hydride battery of the same weight can store between 60 – 100 watt-hours, and a lead-acid battery only 25 watt-hours. Lithium batteries also hold their charge very well; losing only around 5% of charge per month. Lithium-ion batteries don’t need to fully discharge before you recharge them, because they don’t have the same “memory effect” that some other battery types have, and they can go through hundreds of charging cycles.
As I mentioned earlier, lithium-ion batteries do have their limitations and issues. They start to degrade as soon as they leave the manufacturing facility, and they will only last a few years, regardless of how much you use them. They take much longer to recharge and are also very sensitive to heat which will cause the battery to degrade even faster. If a lithium-ion battery becomes completely discharged, it becomes useless – meaning you have to have a computer-based battery management system in conjunction with it. Then there’s also the issue that has been in the news lately – the rare occurrence of the battery catching fire.
Lithium-ion batteries store and release energy by moving electrons from one side of the battery to the other. The two sides of a battery are known as the electrodes – one electrode is the anode and the other is the cathode. The electrodes are connected with an electrolyte salt solution that contains lithium ions—and that’s how lithium-ion batteries got their name.
When you place a charged battery in a device and turn it on, a chemical reaction begins to produce a stream of positively charged particles called ions as well as negatively charged particles called electrons. Both particles begin to move from the anode to the cathode. The ions move directly through the battery, while the electrons go through the circuit of the device that the battery is connected to. The electrons that travel through the circuit of the device to reach the cathode are what generates the power for your device. Very similar to how the flow of a river provides the power to turn a water wheel. When charging the battery, the electrons move in reverse from the cathode to the anode where they are stored until they need to provide power again.
This usage and charging process creates problems for lithium-ion batteries however, as Jiaxing Huang, Professor of Materials Science and Engineering at Northwestern University explains in a recent report. As lithium gets charged and discharged in a battery, it starts to grow dendrites and filaments, "which causes a number of problems," Huang said. "At best, it leads to rapid degradation of the battery's performance. At worst, it causes the battery to short or even catch fire."
One solution the team explored to get around lithium's destructive dendrites was to use a porous scaffold so to speak. Then during charging, lithium can deposit along the surface of the scaffold, avoiding dendrite growth. There was still a problem, however. As lithium deposits onto and then dissolves from the scaffold support, its volume fluctuates significantly. This volume fluctuation induces stress that could break the scaffold support.
The solution was crumpled graphene balls, which can stack to form a porous scaffold, due to their paper ball-like shape. They not only prevent dendrite growth but can also handle the stress from the fluctuating volume of lithium. Huang discovered these novel ultra-fine particles that resemble crumpled paper balls six years ago. He made the particles by atomizing a dispersion of graphene-based sheets into tiny water droplets. When the water droplets evaporated, they generated a capillary force that crumpled the sheets into miniaturized paper balls.
Compared to batteries that use graphite as the host material in the anode, Huang's solution is much lighter weight and can stabilize a higher load of lithium during cycling. Whereas typical batteries encapsulate lithium that is just tens of microns thick, Huang's battery holds lithium stacked 150 microns high.
For more battery news, check out "Inventor of Lithium-ion Batteries Develops First All-Solid-State Battery cells."