Courtesy Chevrolet Corp.
Electric cars are great emissions-wise, we all know that. But they still have an Achilles heel: their range. This is partly due to the limits of energy density of the ubiquitous lithium-ion battery. But researchers in Australia have come up with a sweet solution – add sulphur, then sprinkle on a shake or two of sugar. By James Teo.
Melbourne, Australia. September 2021. Most electric vehicles (EVs), use lithium-ion batteries invented and commercialised in the 1970s. They are relatively light, compact and best of all can be charged quickly – and thousands of times. But they have a few disadvantages. They are made with toxic and exotic materials such as lithium, cobalt, nickel and manganese that are expensive, and sometimes of dubious origin. They explode if you are not nice to them. And they are very low on the list of energy density materials (the amount of power they can store in a given space - dynamite is particularly good at that).
Petrol isn’t bad – its energy density is 47.5 MJ/kg so a fully filled car tank has the same energy content as a thousand sticks of dynamite; enough to easily take it many hundreds of kilometres and fill up anywhere.
Monash Energy Institute team have invented a battery they believe could revolutionise vehicle travel. Courtesy Monash University.
However, a typical lithium-ion battery in contrast has a paltry 0.3 MJ/kg, meaning even the best, most expensive (and heaviest) electric cars can only manage an average of a couple to three hundred kms at best.
A solution has been hovering on the sidelines for a while: lithium sulphur batteries. They offer a much higher energy density; scientists at Melbourne’s Monash University say they can get up to five times the energy density of lithium ion batteries from their experimental lithium sulphur power packs of a similar weight.
“The problem has been that, in use the electrodes deteriorated rapidly, and the batteries broke down,” they explain, pointing out that although lithium-ion can easily see between 500 and 1,500 charge cycles, lithium-sulphur ones became useless after 50 charge cycles.
There were two reasons for this, say the team, made up of lead author Professor Mainak Majumder and researchers Mahdokht Shaibani, Matthew Hill and Yingyi Huang at Monash Energy Institute. Firstly, the positive sulphur electrode suffers from substantial expansion and contraction. This causes any casing to bulge and potentially break – useless in cars and phones alike. Also the negative lithium electrode becomes quickly contaminated by sulphur compounds stopping the charge from flowing.
So the Monash team looked at solutions, firstly to the expansion problem. They built a lithium sulphur battery on a springy matrix of carbon which could expand and contract during charging and running, without affecting the casing. This instantly boosted the durability of the batteries to hundreds charge cycles. Next they looked at the build-up of sulphides that caused a drop off in conductivity. And that’s when sugar offered a sweet solution.
The new battery is way more powerful than tradition lithium ion, and can be recharged hundreds of times. Sweet Solution. Courtesy Monash University.
While researching the problem of sulphur degradation, Huang came across a forty-year old agricultural research paper, which mentioned that sugar helped stop sulphur breakdown in soil compounds.
"She found (the mention) and said, 'Hey, let's try this',” explains Prof. Majumder. “We tried it and lo and behold it worked.”
Next, the team introduced a glucose-based additive into its springy matrix cathode – and it worked. The glucose stabilised the sulphur and prevented it from clogging the lithium electrodes, giving a much longer working life.
Tests on the new construction with its dose of sugar showed the new design had a charge-discharge life of at least 1000 cycles, while at the same time still holding way more capacity (two to five times more energy/kilo) compared to equivalent lithium-ion batteries of the same weight.
“This means each charge lasts longer, extending the battery’s life,” says Huang. “And manufacturing the batteries doesn’t require exotic, toxic, and expensive materials either.”
Prof. Majumder is bullish about the new battery design. “In less than a decade, this technology could lead to vehicles like electric buses and trucks that can travel from Melbourne to Sydney without recharging,” he says. “It could also enable innovation in delivery and agricultural drones where light weight is paramount.”