The electric vehicle revolution is well under way and Ireland is among a host of European nations planning for an all-electric new vehicle market by 2030.
Aside from a continuing lack of public charging points once you get away from the greater Dublin area, it’s not hard to see the appeal of electric cars. Zero local emissions, a smooth driving experience, cheaper to run by far 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’d go back to burning fossil fuels…
However, the rush to electric is causing problems. Chief among them is battery supply. Currently, we need very big lithium-ion batteries to provide the sort of range and power that make electric cars easier and more pleasant to live with. That need for size means that batteries need more raw materials, many of them rare-earth metals which come from countries with dubious human rights and employment records – Congo and China chief among them.
Drexel University, based in Philadelphia in the US, claims to have made a battery chemistry breakthrough that has the potential to untether us from that vicious cycle of ever-bigger batteries and ever-dwindling natural resources.
It’s been known for years that using sulphur in a lithium-ion battery makes for a better lithium-ion battery. The energy capacity for a given weight and size trebles and there is the potential for longer service life, with more resistance to damage from constant charging and recharging.
There’s another major benefit, which is that sulphur is readily available, and it can be extracted and mined in ways that are much more environmentally friendly than is the case for other, current, battery materials.
There has been one consistent and significant problem with using sulphur, though. When you run a current through a lithium-sulphur battery, the electricity causes a chemical reaction that creates polysulphfides in the battery’s electrolyte. These polysulphides are so poisonous to the battery, that just one cycle of charging and discharging can destroy the battery completely.
Scientists at Drexel, though, think they’ve made a breakthrough, and it was almost an accidental one. The Drexel team, led by Vibha Kalra, George B Francis chair professor in the college’s department of chemical and biological engineering, were trying to confine the sulphur in an experimental battery within a complex carbon nanofibre structure. That process, apparently, did not work but something else did.
Monoclinic sulphur
By passing the sulphur through the nanofibre, Kalra’s team had converted it into a variant of sulphur, known as monoclinic gamma-phase sulphur. Not only is this form of sulphur more stable overall, it also means the sulphur does not chemically react with the sort of carbonate electrolyte commonly used in commercial batteries.
“At first, it was hard to believe that this is what we were detecting because, in all previous research, monoclinic sulphur has been unstable under 95 degrees Celsius,” said Rahul Pai, a doctoral student and co-author of the research. “In the last century, there have only been a handful of studies that produced monoclinic gamma sulphur and it has only been stable for 20-30 minutes at most. But we had created it in a cathode that was undergoing thousands of charge/discharge cycles without diminished performance – and a year later, our examination of it shows that the chemical phase has remained the same.”
“As we began the test, it started running beautifully – something we did not expect. In fact, we tested it over and over again – more than 100 times – to ensure we were really seeing what we thought we were seeing,” Kalra said. “The sulphur cathode, which we suspected would cause the reaction to grind to a halt, actually performed amazingly well and it did so again and again.”
Drexel’s research would seem to indicate that, by using this new form of sulphur, a battery could be created that’s three-times more power-dense than current designs (so you could either have three-times the range for the same size of battery, or the same range from a battery one-third the size of current units), and is more robust when charging, meaning that battery life could be extended.
After more than a year of testing, the Drexel sulphur cathode remains stable and, as the team reported, its performance has not degraded in 4,000 charge/discharge cycles, which is equivalent to 10 years of regular use.
“While we are still working to understand the exact mechanism behind the creation of this stable monoclinic sulphur at room temperature, this remains an exciting discovery and one that could open a number of doors for developing more sustainable and affordable battery technology,” Kalra said.
Switching to a sulphur cathode would mean that there would be much less need for battery materials such as cobalt, nickel and manganese. It’s these materials that cause such concern when it comes to environmental destruction in mining, and the use of child and even slave labour in mines.
Battery design
“Getting away from a dependence on lithium and other materials that are expensive and difficult to extract from the earth is a vital step for the development of batteries and expanding our ability to use renewable energy sources,” Kalra said. “Developing a viable Li-S [lithium sulphur] battery opens a number of pathways to replacing these materials.”
Other research undertaken by the Massachusetts Institute of Technology (MIT) seems to show similar results for a new battery design which uses a combination of aluminium, sulphur and molten salt in place 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 ultimately for automotive uses. We did experiments at very high charging rates, charging in less than a minute. The ingredients are cheap, and the thing is safe – it cannot burn.”
Even though the salt used in the battery’s chemical make-up needs to be kept molten, it’s the battery itself, generating heat through charging and discharging, that does that – it doesn’t need any outside heating.
While the MIT design is primarily aimed at large-scale, multimegawatt storage systems, which act as a buffer for weather-dependent renewables such as wind and solar power, Sadoway also says that they could be useful in cars. Not as batteries in electric cars, but storage batteries that charge up electric cars.
“The smaller scale of the aluminium-sulphur batteries would also make them practical for uses such as electric vehicle charging stations,” Sadoway says. “When electric vehicles become common enough on the roads that several cars want to charge up at once, as happens today with gasoline fuel pumps, if you try to do that with batteries and you want rapid charging, the amperages are just so high that we don’t have that amount of amperage in the line that feeds the facility.
“So having a battery system such as this to store power and then release it quickly when needed could eliminate the need for installing expensive new power lines to serve these chargers.”
