The electric vehicle revolution is well underway and Ireland is one of many European countries predicting a market for new fully electric vehicles by 2030.
Aside from the continued lack of public charging stations once you drive away from the greater Dublin area, it’s not hard to see the appeal of electric cars. Zero local emissions, a smooth driving experience, far cheaper than a petrol or diesel car, and increasingly impressive driving experiences. Drive an Audi e-tron GT or a Peugeot e-208 and tell me you would go back to burning fossil fuels…
However, the rush to electric is a problem. Chief among them is battery power. Currently, we need very large lithium-ion batteries to provide the range and power that makes electric cars easier and more fun to live with. This need for size means batteries need more raw materials, including many rare-earth metals from countries with questionable human rights and employment records – Congo and China leading the way.
Drexel University, based in Philadelphia in the United States, claims to have made a breakthrough in battery chemistry that has the potential to break us out of this vicious circle of ever-bigger batteries and ever-dwindling natural resources.
It has been known for years that using sulfur in a lithium-ion battery makes it a better lithium-ion battery. The energy capacity for a given weight and size triples and there is the potential for longer life, with greater resistance to damage from constant charging and recharging.
There is another major advantage, which is that sulfur is readily available and can be mined and mined in a much more environmentally friendly way than is the case with other battery materials today.
There has been a consistent and significant problem with the use of sulphur, however. When you run a current through a lithium-sulfur battery, the electricity causes a chemical reaction that creates polysulfides in the battery electrolyte. These polysulfides are so toxic to the battery that a single charge and discharge cycle can completely destroy the battery.
Drexel scientists, however, believe they made a breakthrough, and it was almost accidental. The Drexel team, led by Vibha Kalra, George B Francis Professor in the college’s Department of Chemical and Biological Engineering, was trying to confine the sulfur in an experimental battery to a complex structure of carbon nanofibers. This process apparently didn’t work, but something else did.
Monoclinic sulfur
By passing sulfur through the nanofiber, Kalra’s team had converted it into a variant of sulfur, known as gamma-phase monoclinic sulfur. Not only is this form of sulfur more stable overall, but it also means that the sulfur does not react chemically with the type of carbonate electrolyte commonly used in commercial batteries.
“At first, it was hard to believe this was what we were detecting because in all previous research, monoclinic sulfur was unstable below 95 degrees Celsius,” said Rahul Pai, PhD student and co-author of the research. “Over the last century there have only been a handful of studies that have produced monoclinic gamma sulfur and it has only been stable for 20-30 minutes at most. But we created it in a cathode that underwent thousands of charge/discharge cycles without diminishing performance – and a year later, our examination of it shows that the chemical phase has remained the same.
“When we started testing it started to work well – something we didn’t expect. In fact, we tested it time and time again – over 100 times – to make sure we were really seeing what we thought we were seeing,” Kalra said. “The sulfur cathode, which we suspected would cause the reaction to stop, actually worked incredibly well and it did it again and again.
Drexel’s research seems to indicate that by using this new form of sulfur, a battery could be created that is three times more energy dense than current designs (so you could get three times the range for the same size battery, or the same range of a battery one-third the size of current units), and is more robust when charging, which means battery life could be extended.
After more than a year of testing, the Drexel sulfur cathode remains stable and, as the team reported, its performance has not degraded in 4,000 charge/discharge cycles, which equates to 10 years of regular use.
“While we are still working to understand the exact mechanism behind the creation of this room temperature stable monoclinic sulfur, this remains an exciting discovery and one that could open a number of doors for developing more durable and affordable battery technology,” said Kalra said.
Switching to a sulfur cathode would mean that there would be much less need for battery materials such as cobalt, nickel and manganese. It is these materials that cause so much concern when it comes to the destruction of the environment in mining and the use of child and even slave labor in the mines.
battery design
“Moving away from reliance on lithium and other expensive and hard-to-mine materials from the earth is a vital step in battery development and expanding our ability to use renewable energy sources,” he said. Kalra said. “Developing a viable Li-S [lithium sulphur] battery opens a number of ways to replace these materials.
Other research undertaken by the Massachusetts Institute of Technology (MIT) appears to show similar results for a new battery design that uses a combination of aluminum, sulfur and molten salt instead of lithium. MIT Professor Donald Sadoway said, “I wanted to invent something that was better, much better, than lithium-ion batteries for small-scale stationary storage, and eventually automotive uses. We’ve been experimenting with very high charge rates, charging in less than a minute. The ingredients are cheap and the thing is for sure – it can’t burn.
Although the salt used in the battery chemistry must be kept molten, it is the battery itself, generating heat through charging and discharging, that does this – it does not need heating outside.
While MIT’s design is primarily aimed at large-scale, multi-megawatt storage systems, which act as a buffer for weather-dependent renewables such as wind and solar, Sadoway also says they could be useful in cars. Not like batteries in electric cars, but like storage batteries that charge electric cars.
“The smaller size of aluminum-sulfur batteries would also make them practical for uses such as electric vehicle charging stations,” says Sadoway. “When electric vehicles become common enough on the roads that multiple cars want to charge at the same time, as is the case today with gas pumps, if you try to do it with batteries and you want fast recharging, the amperages are so high that we don’t have this amount of amperage in the line that supplies the installation.
“So having a battery system like this to store energy and quickly release it when needed could eliminate the need to run expensive new power lines to serve these chargers.”
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