Solid-state lithium-ion batteries are a hallowed goal in energy: they’d be more powerful – and safer – than current commercial liquid-based models. But researchers are still looking for solid materials that can conduct electricity well and integrate with the rest of the battery.
One such substance has been found by a team of US materials scientists, and has an unlikely source: cellulose, taken from trees.
Described in a paper in Nature, the material is made from combining cellulose nanofibrils (or polymers), extracted from wood, with copper ions.
The resulting material is very good at conducting lithium ions, and as thin and flexible as a sheet of paper. As well as being used as an electrolyte in batteries, the researchers say that the material could be used in sensors and information-processing devices.
Most previous attempts to make solid-state battery conductors have either focussed on polymers like cellulose, which conduct poorly but integrate well with other battery components, or on inorganic materials like ceramics, which conduct well but often don’t meet the physical requirements for batteries. This cellulose and copper ion method combines the two properties, making something flexible and conductive.
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“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,” explains Professor Liangbing Hu, from the department of materials science and engineering at the University of Maryland, US, and co-author on the paper.
He adds that the material is better than any previously recorded polymer conductor: “We found this ion conductor achieved a record high ionic conductivity among all solid polymer electrolytes.”
The researchers also used computer modelling to determine how the material transported lithium ions, explaining why it was so much more conductive than its counterparts.
“The lithium ions move in this organic solid electrolyte via mechanisms that we typically found in inorganic ceramics, enabling the record high ion conductivity,” says co-author Professor Yue Qi, of the school of engineering at Brown University, US.
The researchers hope that the material advances solid-state batteries another step towards the market. While current commercial lithium-ion batteries work well, their liquid electrolytes are usually highly flammable materials, and they also make the batteries more prone to short-circuiting.
“Using materials nature provides will reduce the overall impact of battery manufacture to our environment,” says Qi.