Lithium-sulfur solid-state batteries promise fire safety at high performance

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A ‘cupcake’ approach may have brought the commercialization of lithium-sulfur solid-state batteries one step closer.

Researchers from Singapore’s A*Star NanoBio Lab (NBL) have designed a hybrid, quasi-solid electrolyte featuring a liquid-infused porous membrane which they say offers higher conductivity and stability properties than alternatives. The results have been published in Nano Energy.

The Singapore group claim the material improves the fire safety of lithium batteries while maintaining high performance. Lithium-ion battery fires, especially in South Korea, have prompted the industry to monitor which battery chemistries and technologies can be used safely in various applications. Solid, or semi-solid-state devices have been studied as a potential solution to fire safety concerns. The electrolyte used in conventional lithium-ion batteries is highly flammable and organic. When coupled with thermally and mechanically unstable electrode separators, battery fires have resulted, especially in large scale applications. Removing the organic electrolyte, however, has proven difficult as its properties enable lithium-ion batteries to perform at high energy density.

The Singapore team said previous research into solid-state batteries had shown improved safety properties but resulted in poor electrode and electrolyte contact, as well as limited ionic conductivity, inhibiting performance. The NBL researchers claim their electrolyte addresses stability and safety concerns while maintaining commercially viable battery performance.

Electrolyte

“Hybrid, quasi-solid electrolytes, comprising both liquid and solid components have emerged as a practical compromise to obtain safer batteries while maintaining good performance,” said Jackie Ying, head of the NBL research team. “However, the high resistance of the solid component has thus far limited the performance of such batteries. To overcome this, we have re-engineered the microstructure of the solid component. Our solution eliminates electrolyte leakage and is thermally and mechanically stable.”

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The electrolyte is made of Li7La3Zr2O12 (LLZO) sheets, which form a porous membrane. The researchers’ paper states LLZO was chosen for its high ionic conductivity and excellent chemical and electrochemical stability. Handling issues during battery assembly were addressed as the electrolyte is non-rigid and thus will not crack easily. The LLOZ-based, semi-solid electrolyte is also said to be stable over a wide voltage range, meaning it could be used with different lithium battery electrode materials, including high-voltage cathodes.

Cupcakes and puff pastry

The NBL group have already produced a trial lithium-sulfur battery with the electrolyte. The device reportedly showed high capacity, fast charge and discharge capability and polysulfide shuttling control, which stabilized performance. “In tests, the novel electrolyte achieved remarkable rate capability (of around 515 and around 340 mAh/g [milliamp Hours per gram] at 1 and 2 degrees Celsius, respectively) at 1.5 mg/cm2 loading density,” the group stated. “This is among the highest known performance achieved by lithium-sulfur hybrid, quasi-solid batteries.”

Image: Agency for Science, Technology and Research/The porous structure of the membrane sheets provide favorable electrical properties.

Image: Agency for Science, Technology and Research/ NanoBio Lab

The team developed a ‘cupcake’ method of synthesizing the LLZO sheets used to construct the electrolyte’s framework. The researchers dissolved metal precursors and sucrose in water with the solution then heated in a furnace to make a cupcake of the brown material left when the water evaporated. The cupcake was then heated at high temperatures to separate it into sheets. Puff pastry, you might also say.

“Our 3D-sheet-based framework was found to be crucial for optimal battery performance,” said Ying. “Moreover, our system demonstrated outstanding stability under extreme temperatures. These results illustrate the excellent potential of our sheet-based structure as a framework for other semi-solid lithium batteries.”

In January, researchers from Japanese tech company Kyocera and U.S. start-up 24M announced they will start commercial production of residential energy storage batteries featuring a semi-solid electrolyte. That new tech was said to offer a 40% price advantage in manufacturing costs and the production process reportedly makes it possible to manufacture electrodes four to five times quicker than the industry standard, reducing the need for copper, aluminum and a separator, to offer further favorable pricing and an energy density several times higher than rival products. 24M claimed last year those benefits would add up to a capital requirement half that of conventional lithium-ion energy storage products. The start-up had already claimed an energy density of 350 Wh/kg with its SemiSolid cell architecture by that point.