An interdepartmental research team at Rice University in the U.S. has developed a soft porous, and flexible material that selectively allows the movement of lithium ions and could help build better solid-state batteries in the future.\u00a0<\/p>\n
Solid-state batteries are considered the future of energy storage as they rely on a solid electrolyte, which is safer and has higher energy densities. The liquid electrolyte in conventional batteries is prone to catching fire, and replacing it with one in solid state makes it less flammable.\u00a0<\/p>\n
Moving lithium ions is only a part of the challenge in solid-state batteries; how the material behaves can be important too. Metal-organic frameworks (MOFs), a type of porous crystalline material, are usually used as solid-state electrolytes since they contain tiny, ordered channels that can either hold or move specific ions. This is a much preferred quality of an electrolyte material, but MOFs are rigid by nature, which can affect how they can remain in contact with electrodes.\u00a0<\/p>\n
Porous, flexible MOF<\/p>\n
The team of researchers from the departments of Chemical and Biomolecular Engineering, Chemistry and Materials Science and Nanoengineering, as well as the Advanced Materials Institute at Rice University, developed ZnBTCA that uses an aliphatic linker that makes the MOF softer and more adaptable.\u00a0<\/p>\n
\u201cMOF electrolytes are often designed primarily around how effectively they transport ions,\u201d said Zina Deriche, a graduate student at Rice who was involved in the research. \u201cHere, we\u2019re showing that the mechanical properties of the framework can also be an important part of the design.\u201d<\/p>\n
Both Zn and the aliphatic linker it is made from are abundant and hence low-cost components. In addition, the negatively charged structure makes it easier for positively charged\u00a0lithium ions<\/a>\u00a0to move through its channels.\u00a0<\/p>\n \u201cBringing 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,\u201d added<\/a> Stavroula Alina Kampouri, assistant professor of chemical and biomolecular engineering at Rice in a press release.\u00a0<\/p>\n Overcoming short circuits<\/p>\n The charging and discharging process inside a lithium-ion battery can create unwanted pathways through an electrolyte, raising concerns about short circuits. When they incorporated a flexible polymer into the electrolyte and placed it between lithium metal electrodes, the cells operated stably for over 300 hours, even as current increased during testing, without any short-circuiting.\u00a0<\/p>\n The researchers also replaced the sodium ions in the structure with lithium. They found that 95 percent of the mobile charge-balancing ions were lithium, and yet the structure did not break apart. This shows that mechanical flexibility, too, alongside chemical composition, pore structure, and charge, can be a design tool for researchers working on MOF electrolytes.\u00a0<\/p>\n \u201cTo our knowledge, ZnBTCA is the first MOF electrolyte built from an aliphatic linker with a flexible carbon-chain backbone,\u201d said Sibani Lisa Biswal, chair of chemical and biomolecular engineering, at Rice. \u201cWhat this work shows is that we can think beyond just the chemistry and pore structure of these materials. Mechanical flexibility can also become a design tool for developing MOF electrolytes for\u00a0solid-state bat<\/a>teries<\/a>.\u201d<\/p>\n