Flexible porous material may improve solid-state batteries

Rice University researchers have developed a relatively soft, flexible material that can selectively transport lithium ions, offering a new approach to porous solid electrolytes that combine efficient ion movement with better mechanical adaptability inside a battery. The research was published in Chemical Science.

The material, called ZnBTCA, is a metal–organic framework, or MOF: a type of porous crystalline material built from metal atoms connected by organic molecules. MOFs contain tiny, ordered channels that can be tailored to move or hold specific molecules and ions, making them attractive candidates for battery electrolytes. Many MOFs explored as battery electrolytes are built from relatively rigid aromatic linkers. ZnBTCA takes a different approach: it uses a flexible aliphatic linker that makes the framework itself softer and more mechanically adaptable. The material is also built from relatively abundant, low-cost components, including zinc and an inexpensive aliphatic linker.

“MOF electrolytes are often designed primarily around how effectively they transport ions,” said Zina Deriche, a graduate student in chemical and biomolecular engineering at Rice. “Here, we’re showing that the mechanical properties of the framework can also be an important part of the design.”

Solid-state batteries replace the flammable liquid electrolytes used in conventional lithium-ion batteries with solid materials, potentially improving safety and energy density. For many MOF electrolytes, however, their relatively rigid crystalline structures can make it harder to maintain close contact with electrode surfaces. The challenge is to preserve efficient, selective lithium-ion transport while making the material more mechanically adaptable.

The Rice team designed ZnBTCA with both challenges in mind. Its flexible molecular building blocks make it relatively soft, while its negatively charged structure favours the movement of positively charged lithium ions through its channels.

“What is exciting is that the same material brings together two useful properties,” said Stavroula Alina Kampouri, a corresponding author of the study. “Its flexible building blocks make the framework softer and more adaptable, while its negatively charged structure promotes selective lithium-ion transport. Bringing these properties together gives us a new way to design MOF electrolytes by considering how ions move and how the material behaves mechanically at the same time.”

After exchanging sodium ions in the material for lithium, the researchers found that about 95% of the mobile charge-balancing ions were lithium while the framework remained structurally intact. Tests also showed that lithium ions carried about 79% of the ionic current through the material.

Short-circuiting is an important concern in lithium-metal batteries, where uneven lithium growth during repeated charging and discharging can eventually create unwanted electrical pathways through the electrolyte. The team incorporated ZnBTCA into a solid electrolyte membrane and placed it between two lithium-metal electrodes. The cells operated stably for nearly 300 hours as the researchers progressively increased the current with no evidence of short-circuiting under the conditions tested.

The results suggest that mechanical flexibility could become another design tool for researchers developing MOF electrolytes, alongside factors such as chemical composition, pore structure and charge.

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