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DALLAS (SMU) – Lithium-sulfur batteries have never lived up to their potential as the next generation of renewable batteries for electric vehicles and other devices. But SMU mechanical engineer Donghai Wang and his research team have found a way to make these Li-S batteries last longer – with higher energy levels – than existing renewable batteries.  

The research team has been able to prevent Li-S batteries from producing an unwanted side effect known as polysulfide dissolution that appears over time, shortening their lifespan.

“This breakthrough could lead to more durable, long-lasting batteries,” said Wang, the Brown Foundation Chair of Mechanical Engineering and Professor of Mechanical Engineering at SMU Lyle. His research focuses on the design and synthesis of nanostructured functional materials and energy storage technologies like Li-ion batteries and also beyond Li-ion technology.

A published in the journal Nature Sustainability shows that the team’s newly developed hybrid polymer network cathode allows Li-S batteries to deliver over 900 mAh/g (milliampere-hours per gram mass), compared to the typical 150-250 mAh/g capacity in lithium-ion batteries. That means it has a much higher amount of electrical energy it can preserve.  

“It also offers excellent cycling stability – outperforming conventional lithium-sulfur batteries,” Wang said.

Cycling capacity measures the number of times a battery can charge and discharge before its capacity degrades sharply. A higher cycling capacity means a longer-lasting battery.  

Assisting Wang with designing the cathode were researchers from Pennsylvania State University, Pacific Northwest National Laboratory, Brookhaven National Laboratory, University of Illinois at Chicago and the Argonne National Laboratory.

A cost-effective solution that delivers more energy

What makes Li-S batteries so promising as a source of renewable energy is that they’re more cost-effective and can hold more energy than traditional ion-based rechargeable batteries.   

But there is a key problem with these batteries.  

“Over the years, the battery community has struggled to mitigate the negative effects of polysulfide dissolution,” Wang said. 

All batteries have a positive terminal and a negative terminal. Inside the battery, the chemical reaction that is continuously happening between these two terminals provides power or electricity to the battery. 

In the case of Li-S batteries, a sulfur-containing positive electrode or terminal called a cathode is paired with a lithium metal negative electrode called an anode. In between those components is the electrolyte, or the substance that allows ions to pass between the two ends of the battery.

Yet, sulfur is far from an ideal material for an electrode. 

When lithium ions bind with sulfur atoms at the cathode, they create soluble polysulfide molecules that drift into the electrolyte, causing degradation of the cathode and reducing the battery's ability to endure multiple charging cycles. This is known as polysulfide dissolution.

Wang and his team have found a way to fix this issue by using what they called a hybrid polymer network cathode.

Schematic of the proposed sulfurized inorganic-organic hybrid polymer network (S-HYB), where sulfur chains are immobilized by multiple covalent tethers and the Li2S discharge products are regulated by on-site atomic adsorption and gathered within the inorganic–organic S-HYB. 

“Our cathode uses multiple sulfur bonding tethers, atomic adsorption, and fast Li-ion/electron transport at the molecular level,” Wang explained. “This combination allows for real-time re-bonding and adsorption of any unbound sulfur species, thus effectively eliminating soluble polysulfides and extending the battery's cycle life.”

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