Paul van Gerven
17 February 2020

Adding to much of the research progress in recent years, scientists from Australia’s Monash University have ticked yet another box for getting promising lithium-sulfur battery technology ready for commercial application.

Power your smartphone for five continuous days or drive your electric vehicle for a thousand kilometers without having to ‘refuel’. The battery technology making this and many other things possible is right around the corner, as “Monash University researchers are on the brink of commercializing the world’s most efficient lithium-sulfur battery, which could outperform current market leaders by more than four times,” reads a press release from the Australian research institute. The technology isn’t quite ready for prime time, though.

A lithium-sulfur battery doesn’t differ all that much from a regular lithium-ion battery. It’s mostly a matter of swapping materials. For the cathode, out goes cobalt oxide (the most often-used cathode material) and in goes sulfur. Anode-wise, modern lithium-ion batteries use graphite, but metallic lithium is often preferred as the anode for lithium-sulfur batteries.

Sulfur has two things going for it: it’s dirt cheap and it can store a lot of lithium. On paper, that should translate into reasonably priced batteries that can store a lot of energy per unit of weight. Theoretically, they’re able to store up to five times as much energy per kilo as conventional lithium-ion batteries.

In practice, using ‘just’ sulfur as the cathode isn’t viable, however. For one, it’s a poor conductor of electricity. Additionally, it easily cracks under the mechanical stress of repeatedly loading and unloading lithium. To address these issues, the sulfur needs to be processed and mixed with a conductor, which adds cost and negatively affects the specific energy (energy stored per unit of weight). But that doesn’t mean lithium-sulfur couldn’t still outperform lithium-ion, in the end.

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Commercially viable

The Australian researchers, along with colleagues from Germany and Belgium, appear to have spotted a promising way to make a robust sulfur electrode economically. They use common ingredients – sulfur, conducting carbon and a binder – and cook them up in a straightforward manner. Of particular importance is mixing in a minimal amount of binder, resulting in a rather ‘loose’ mesh. This accomplishes two things: it increases the sulfur surface available for lithium ions to penetrate and, upon penetration, helps to accommodate the electrode’s mechanical expansion.

Tests confirm the durability of the new type of electrode, which retains its performance after 200 charge-discharge cycles. This number in itself isn’t remarkable, given that some designs have made it to well over 1000 cycles. Crucially, however, those records – as is often the case – were obtained in industrially irrelevant setups. The Australian team worked with thick electrodes featuring high sulfur loadings, two characteristics that are required to make a commercially viable battery.

Electrodes
Scanning electron microscope images of four differently-made sulfur electrodes before (A, C, E and G) and after (B, D, F and H) an intense cycling regime. The electrode in the top two pictures was made according to Monash’s new recipe, which is most successful in keeping the sulfur particles together. Credit: Science Advances/Monash

Lithium stabilization

Unfortunately, durability isn’t the only problem associated with lithium-sulfur batteries. First, sulfur electrodes tend to ‘leak’ lithium-sulfur compounds into the electrolyte, which wreak all kinds of havoc in a battery. Furthermore, lithium-sulfur, like many types of lithium-based batteries, has been plagued by dendrite formation. These spikes emanating from the anodes eventually short out the battery. And, finally, degradation of the lithium anode also limits battery lifetime.

Other research groups have made good progress on solving these issues. Interlayers and coatings on the battery separator negate sulfur leakage and electrolyte additives prevent dendrites from rearing their ugly heads. These solutions are fully compatible with Monash’s cathode, lead researcher Mahdokht Shaibani e-mails Bits&Chips.

The lithium electrode remains a major obstacle, however. “Whilst we stabilized the sulfur cathode, we believe a similar sort of breakthrough on stabilization of the lithium electrode is vital to increase the lifetime of this next-generation of battery technology to the required levels,” writes Shaibani.

Give it a few more years.