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by Peter Ruegg for ETHZ News
Zurich, Switzerland (SPX) Apr 11, 2013
ETH-Zurich researchers use x-ray tomography to screen lithium ion battery electrodes and can reconstruct the microstructure in high resolution. This helps to understand the discharging and charging process better and develop optimised electrodes.
Mobile phone batteries that last longer, car batteries that enable you to drive further, storage that accumulates a lot of energy from wind and solar generators: many applications require better batteries. The research essentially focuses on three aspects here: researchers want to increase the energy density - in other words store more energy in a smaller battery.
They are also looking to improve the discharging and charging speed by changing and controlling the material, shape and size of the electrochemically active particles and the structure of the battery electrodes in a targeted fashion. And scientists are working on the durability of the battery in general by trying to understand the degradation mechanisms that shorten the life of batteries.
Martin Ebner, a doctoral student from the group headed by Vanessa Wood, a professor at the Department of Information Technology and Electrical Engineering, has been examining the issue of the discharging and charging speed. In order to understand what influences it, he has been researching the microstructure of the electrodes of commercially available and home-made lithium ion batteries. Knowing this also enables us to understand the charging and discharging mechanism better and endeavour to produce optimised electrodes with more efficient batteries in mind.
Hard-to reach microstructure scanned
"This radiation, which can be produced at the Swiss Light Source at the Paul Scherrer Institute, is very bright and spectrally pure. This allows many high-resolution experiments in a short space of time," says Ebner. It only took around five minutes to study a sample on the TOMCAT beamline as opposed to up to five hours on conventional devices. This meant that Ebner could x-ray many electrode material samples produced under different conditions.
Using the hundreds of gigabytes of data that the x-ray tomography generated, the electroengineer was ultimately able to reconstruct the three-dimensional electrode structure. His paper was recently published in the journal Advance Energy Materials and the raw data of the sixteen cathodes studied deposited in a freely accessible open-source database.
Small particles on boundary layer
The size, distribution and configuration of the particles, however, have a major influence on a battery's discharging and charging speed. Smaller particles form a compact structure while the structure in large particles tends to be looser and thus provide more pore space. The porosity of the material ultimately determines the battery's energy density and the speed at which the lithium ions surge through the electrodes during charging or discharging.
The flow behaviour of the lithium ions can be described by what is known as tortuosity - the value that indicates the degree of a structure's twistedness. To put it simply, the more twisted the path of the ions through the electrode, the more slowly the battery is charged or discharged and the greater the tortuosity.
Graphite plates hamper ion flow
Depending on the direction from which the ions hit the graphite plates, the tortuosity can be very high. In order to flow around the tiles, long paths are required, which vastly reduces the discharging and charging speed. Lengthwise, however, the lithium ions cross the graphite without any major detours. The analyses reveal that graphite electrodes already exhibit direction-dependent differences in path length of over 300 per cent with a porosity of forty per cent.
The tortuosity of graphite electrodes might be improved through the use of round graphite particles. The drawback here is that up to seventy per cent of the valuable raw material is wasted during production - one reason why many battery manufacturers still use plate-shaped graphite as an anode material.
Optimising established technique
One idea is to rely on the self-organisation of the materials used. However, the criterion is and will remain whether the method is feasible and affordable for industry. "We mustn't forget that a battery is a mass product that needs to be producible in large quantities," says Ebner.
How lithium ion batteries work
However, these travel in the electrolyte fluid inside the battery. The process is reversible: lithium ion batteries can be recharged with electricity. In most lithium ion batteries these days, the plus pole is composed of the transition metal oxides cobalt, nickel and manganese, the minus pole of graphite. In more powerful lithium ion batteries of the next generation, however, elements such as tin or silicon may well be used at the minus pole.
Ebner M, Geldmacher F, Marone F, Stampanoni M, Wood V. X-Ray Tomography of Porous, Transition Metal Oxide Based Lithium Ion Battery Electrodes. Advanced Energy Materials 2013. Article first published online: 13 MAR 2013. DOI: 10.1002/aenm.201200932
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