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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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Posted By Graphene Council, The Graphene Council, Friday, February 22, 2019
Updated: Friday, February 22, 2019

Koffi Pierre Yao, a new assistant professor of mechanical engineering at the University of Delaware, is uncovering novel insights about what happens inside the batteries that power our smartphones, laptops, and electric vehicles. He plans to use this knowledge to develop faster-charging batteries that make electric vehicles the go-to automobiles for drivers.

Several of today’s electric vehicles, such as the Tesla Model 3 and Nissan Leaf, run on lithium-ion batteries. But it takes inconveniently too long to recharge those vehicles when you can fill up your gas tank in the time it takes to pick up gas-station coffee. In a lithium-ion battery, positively charged lithium ions move through the electrode to deliver energy.

Scientists all over the world do time-consuming research on lithium-ion batteries in an attempt to optimize these power units. “Usually people will make an electrode, test it, make another one, test it, and so on, and it’s kind of a serial process,” said Yao.

Instead, Yao uses physical probes to look inside batteries while they work and develop a direct physical understanding of how lithium ions flow within batteries. When a battery is charging, the lithium flows unevenly in a way that’s difficult to measure. Yao started working on this while he was a postdoctoral associate at Argonne National Laboratory (ANL), a position he held from 2016 until 2018, when he joined UD’s faculty.

In a new paper published in Energy & Environmental Science, a journal published by the Royal Society of Chemistry, Yao describes how he and his colleagues at ANL used X-rays to get a micron-scale movie of how lithium distributes within the electrode while lithium-ion batteries are running.

“We put an industrial-grade battery under an X-ray beam and mapped the distribution of the lithium within the electrodes,” he said.

Yao and his colleagues knew that the lithium did not distribute homogeneously. Imagine a group of people running through a small doorway. It takes time for people to spread out into the interior of the room; therefore, there will be crowding at the entry point. That’s similar to how lithium moves through the electrode. Still, Yao and his colleagues were surprised at the extent to which lithium scattered inhomogeneously.

The goal is to use this knowledge to reduce testing time and speed up the research and development (R&D) process for these batteries.

In another new paper published in Advanced Energy Materials, Yao describes how he and his colleagues used X-rays to quantify the activity in a silicon-graphite electrode. Cell phone batteries typically contain graphite, but silicon offers some potential benefits over graphite.

“We’re interested in silicon because it can increase the capacity of the electrode by a factor of 10 compared to graphite,” he said. However, silicon is less stable than graphite and degrades faster, so a blend of the two may prove to be a viable solution. “Some of the lithium goes into the graphite, and some goes into the silicon,” he said.

Yao and his colleagues sought to discover exactly where the lithium ions traveled within this blended electrode.

“It’s something people haven’t previously been able to do in the literature,” Yao said. “We provide a clear picture of which of silicon and graphite plays host to lithium at any point in time. Now we can go forward and manipulate this pattern to stabilize the cycling.” This knowledge can help Yao in his quest to design novel particles to make faster-charging and higher energy batteries.

At UD, Yao plans to expand upon his research on batteries with his colleagues at the Center for Fuel Cells and Batteries and more. Yao received his master’s and doctoral degrees in mechanical engineering from the Massachusetts Institute of Technology (MIT) and his bachelor’s degree in mechanical engineering at UD. As an undergraduate at UD, he was mentored by Ajay Prasad, Engineering Alumni Distinguished Professor and Chair of Engineering, who introduced him to electric cars and electrochemistry, and the science behind them.

Tags:  Batteries  Graphene  Koffi Pierre Yao  Li-ion Batteries  Lithium  University of Delaware 

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