16-08-2018 | | By Rob Coppinger
Electrodes that are made from supermolecular sponges have given Lithium-ion batteries faster recharge rates and higher energy capacities thanks to the mixing of salt with carbon.
The electrodes need a hierarchical three-dimensional structure to deliver the fast recharge and higher capacity characteristics. These 3D structures would be achieved by adding salt to the carbon at an early stage of battery production. The sponge cannot be made with only carbon because while different forms of carbon have excellent electrical properties, such as conductivity, it is very difficult for this element to form an ion retaining three-dimensional structure on its own. This kind of 3D hierarchically organised carbon structure has proven very difficult to grow in a laboratory but is crucial in providing unimpeded ion transport within a battery.
The solution was to add the salt to help grow the carbon-based hierarchical 3D structure from a sponge made with a metal organic framework (MOF) that itself consists of carbon, oxygen and hydrogen. A MOF is a molecularly designed porous material that can have carbon added at a high temperature, 800 degrees Celsius, and higher. “For the demonstration we did 800 [degrees Celsius], but it can be varied. Eight hundred degrees works quite well. The carbon atoms arrange themselves into the hierarchy at high temperatures,” said Tiesheng Wang, a doctoral student in University of Cambridge’s Department of Materials Science and Metallurgy.
The salt reacted with the MOF sponge and turned it from a homogeneous carbon mass to an intricate structure with fibres, struts, pillars and webs. The metal in the MOF acts as a catalyst and the carbon grows around the framework to create the hierarchical structure, which Wang and his colleagues call a nano-diatom. This has a high surface area which is good for ion retention.
While diatoms are actually photosynthesising algae found in seas, a diatom is also a description of its structure, which is very symmetrical. Wang explained that he and his fellow researchers use the term diatom because of the similarity in the hierarchical structure, but not in the chemistry of the natural diatom algae.
Wang was the lead author on a paper published in Journal of the American Chemical Society. The paper set out how the researchers had demonstrated that the salted carbon in Lithium-ion batteries charged-up rapidly and had one of the highest capacities of any battery. The work, which began in early 2017, was undertaken at Cambridge, Queen Mary University of London and at the Max Planck Institute for Solid State Research in Germany.
In the Queen Mary University announcement about the work, its project leader Doctor Stoyan Smoukov, from the University’s school of engineering and materials science, said: “This metamorphosis [of the MOF] only happens when we heat the compounds to 800 degrees centigrade and was as unexpected as hatching fire-born dragons instead of getting baked eggs in the [TV series] Game of Thrones.”
Funding for the research came from the European Research Council and the China Scholarship Council. It was also supported by the UK Centre for Doctoral Training in Sensor Technologies and Applications, which is part of the UK government’s Engineering and Physical Sciences Research Council.