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Graphene sponge helps lithium sulphur batteries reach new potential

Posted By Graphene Council, The Graphene Council, Friday, May 3, 2019
Updated: Wednesday, May 1, 2019
To meet the demands of an electric future, new battery technologies will be essential. One option is lithium sulphur batteries, which offer a theoretical energy density more than five times that of lithium ion batteries. Researchers at Chalmers University of Technology, Sweden, recently unveiled a promising breakthrough for this type of battery, using a catholyte with the help of a graphene sponge.

The researchers' novel idea is a porous, sponge-like aerogel, made of reduced graphene oxide, that acts as a free-standing electrode in the battery cell and allows for better and higher utilisation of sulphur.

A traditional battery consists of four parts. First, there are two supporting electrodes coated with an active substance, which are known as an anode and a cathode. In between them is an electrolyte, generally a liquid, allowing ions to be transferred back and forth. The fourth component is a separator, which acts as a physical barrier, preventing contact between the two electrodes whilst still allowing the transfer of ions.

The researchers previously experimented with combining the cathode and electrolyte into one liquid, a so-called 'catholyte'. The concept can help save weight in the battery, as well as offer faster charging and better power capabilities. Now, with the development of the graphene aerogel, the concept has proved viable, offering some very promising results.

Taking a standard coin cell battery case, the researchers first insert a thin layer of the porous graphene aerogel.

"You take the aerogel, which is a long thin cylinder, and then you slice it - almost like a salami. You take that slice, and compress it, to fit into the battery," says Carmen Cavallo of the Department of Physics at Chalmers, and lead researcher on the study. Then, a sulphur-rich solution - the catholyte - is added to the battery. The highly porous aerogel acts as the support, soaking up the solution like a sponge.

"The porous structure of the graphene aerogel is key. It soaks up a high amount of the catholyte, giving you high enough sulphur loading to make the catholyte concept worthwhile. This kind of semi-liquid catholyte is really essential here. It allows the sulphur to cycle back and forth without any losses. It is not lost through dissolution - because it is already dissolved into the catholyte solution," says Carmen Cavallo.

Some of the catholyte solution is applied to the separator as well, in order for it to fulfil its electrolyte role. This also maximises the sulphur content of the battery.

Most batteries currently in use, in everything from mobile phones to electric cars, are lithium-ion batteries. But this type of battery is nearing its limits, so new chemistries are becoming essential for applications with higher power requirements. Lithium sulphur batteries offer several advantages, including much higher energy density. The best lithium ion batteries currently on the market operate at about 300 watt-hours per kg, with a theoretical maximum of around 350. Lithium sulphur batteries meanwhile, have a theoretical energy density of around 1000-1500 watt-hours per kg.

"Furthermore, sulphur is cheap, highly abundant, and much more environmentally friendly. Lithium sulphur batteries also have the advantage of not needing to contain any environmentally harmful fluorine, as is commonly found in lithium ion batteries," says Aleksandar Matic, Professor at Chalmers Department of Physics, who leads the research group behind the paper.

The problem with lithium sulphur batteries so far has been their instability, and consequent low cycle life. Current versions degenerate fast and have a limited life span with an impractically low number of cycles. But in testing of their new prototype, the Chalmers researchers demonstrated an 85% capacity retention after 350 cycles.

The new design avoids the two main problems with degradation of lithium sulphur batteries - one, that the sulphur dissolves into the electrolyte and is lost, and two, a 'shuttling effect', whereby sulphur molecules migrate from the cathode to the anode. In this design, these undesirable issues can be drastically reduced.

Tags:  Aleksandar Matic  Battery  Carmen Cavallo  Chalmers University of Technology  Graphene  Li-ion Batteries  Lithium 

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New research uses graphene sensors to detect ultralow concentrations of NO2

Posted By Graphene Council, The Graphene Council, Wednesday, April 10, 2019
Updated: Wednesday, April 10, 2019
The research, as published in ACS Sensors, was led by an international collaboration of scientists from Linköping University, Chalmers University of Technology, Royal Holloway, University of London and the University of Surrey.

The findings demonstrate why single-layer graphene should be used in sensing applications and opens doors to new technology for use in environmental pollution monitoring, new portable monitors and automotive and mobile sensors for a global real-time monitoring network.

As part of the research, graphene-based sensors were tested in conditions resembling the real environment we live in and monitored for their performance. The measurements included, combining NO2, synthetic air, water vapor and traces of other contaminants, all in variable temperatures, to fully replicate the environmental conditions of a working sensor.

Key findings from the research showed that, although the graphene-based sensors can be affected by co-adsorption of NO2 and water on the surface, at about room temperature, their sensitivity to NO2 increased significantly when operated at elevated temperatures, 150 °C. This shows graphene sensitivity to different gases can be tuned by performing measurements at different temperatures.

Testing also revealed a single-layer graphene exhibits two times higher carrier concentration response upon exposure to NO2 than bilayer graphene — demonstrating single-layer graphene as a desirable material for sensing applications.

Christos Melios, a lead scientist on the project from NPL, said: “Evaluating the sensor performance in conditions resembling the real environment is an essential step in the industrialisation process for this technology.

“We need to be able to clarify everything from cross-sensitivity, drift in analysis conditions and recovery times, to potential limitations and energy consumption, if we are to provide confidence and consider usability in industry.”

By developing these very small sensors and placing them in key pollution hotspots, there is a potential to create a next-generation pollution map – which will be able to pinpoint the source of pollution earlier, in unprecedented detail, outlining the chemical breakdown of data in high resolution in a wide variety of climates.

Christos continued: “The use of graphene into these types of gas sensors, when compared to the standard sensors used for air emissions monitoring, allows us to perform measurements of ultra-low sensitivity while employing low cost and low energy consumption sensors. This will be desirable for future technologies to be directly integrated into the Internet of Things.”

NO2 typically enters the environment through the burning of fuel, vehicle emissions, power plants, and off-road equipment. Extreme exposure to NO2 can increase the chances of respiratory infections and asthma. Long-term exposure can cause chronic lung disease and is linked to pollution related death across the world.  

Figures from the European Environment Agency also links NO2 pollution to premature deaths in the UK, with the UK being ranked as having the second highest number of annual deaths in Europe. In 2014, 14,050 deaths in the UK were recorded as being NO2 pollution related, 5,900 of which were recorded in London alone1.

When interacted with water and other chemicals, NO2 can also form into acid rain, which severely damages sensitive ecosystems, such as lakes and forests.

Existing legislation from the European Commission suggests hourly exposure to NO2 concentration should not be exceeded by more than 200 micrograms per cubic metre (µg/m3) or ~106 parts per billion (ppb), and no more than 18 times annually. This translates to an annual mean of 40 mg m3 (~21 ppb) NO2 concentration2

In central London, for example, the average NO2 concentration for 2017 showed concentration levels of NO2 ranged from 34.2 to 44.1 ppb per month, a huge leap from the yearly average.

These figures show there is an urgent need for a low-cost solution to mitigate the impact of NO2 in the air around us. This work could provide the answer to early detection and prevention of these types of pollutants, in line with the government’s Clean Air Strategy.

Further experimentation in this area could see the graphene-based sensors introduced into industry within the next 2–5 years, providing an unprecedented level of understanding of the presence of NO2 in our air.

Tags:  Chalmers University of Technology  Christos Melios  Graphene  Linköping University  National Physical Laboratory  Royal Holloway  Sensors  University of London  University of Surrey 

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