Scientists at Brown University and the University of Maryland have discovered that cellulose derived from trees could replace the volatile liquid electrolytes in today’s lithium-ion batteries.
When mixed with copper, cellulose nanofibrils form a layer that conducts ions up to 100 times better than other polymers, which is on a par with ceramics, while being much safer than existing flammable lithium solutions.
Their findings were published in October in the journal Nature.
The researchers said that although solid-state lithium-metal batteries promise both high energy density and safety, existing solid ion conductors fail to satisfy the rigorous requirements of battery operations.
Inorganic (ceramic) ion conductors allow fast ion transport, but their rigid and brittle nature prevents reliable interfacial contact with electrodes. Conversely, polymer ion conductors that are lithium-metal-stable usually provide better interfacial compatibility and mechanical tolerance, but typically suffer from inferior ionic conductivity owing to the coupling of the ion transport with the motion of the polymer chains.
The researchers’ solution is a combination of tree-derived cellulose and copper. They say that through the coordination of copper ions (Cu2+) with one-dimensional cellulose nanofibrils, the opening of molecular channels within the normally ion-insulating cellulose enables rapid transport of Li+ ions along the polymer chains.
Yue Qi, professor at Brown University, said: “The lithium ions move in this organic solid electrolyte via mechanisms that we typically found in inorganic ceramics, enabling the record high ion conductivity.”
Combining the cellulose with copper is the key to this research’s success. Liangbing Hu, professor at the University of Maryland, said: “By incorporating copper with one-dimensional cellulose nanofibrils, we demonstrated that the normally ion-insulating cellulose offers a speedier lithium-ion transport within the polymer chains.”
What results is a thin layer of solid electrolyte that can replace the existing solution of lithium salts dissolved in flammable organic solvents. These solvents are a problem in lithium-ion batteries, where lithium can form dendrites that cause short circuits leading to high heat that can cause fires.
Qi said: “Using materials nature provides will reduce the overall impact of battery manufacture to our environment.”