Next-gen battery electrolyte made from wood offers record conductivity
Today's lithium batteries commonly use a liquid electrolyte to carry ions between the two electrodes, but scientists eyeing solid alternatives see some exciting opportunities ahead. Among them are the authors of a new study who have used cellulose derived from wood as the basis for one of these solid electrolytes, which is paper-thin and can bend and flex to absorb stress as the battery cycles.
One shortcoming of the electrolytes used in today's lithium batteries is that they contain volatile liquids that carry a risk of fire if the device short circuits, and can promote the formation of tentacle-like growths called dendrites that compromise performance. Solid electrolytes, meanwhile, can be made from non-flammable materials, make the device less prone to dendrite formation, and might open up entirely new possibilities around battery architecture.
One of these possibilities relates to the anode, one of the two electrodes, which in today's batteries is made from a mix of graphite and copper. Some scientists see solid electrolytes as a key stepping stone to making batteries work with an anode made from pure lithium metal instead, which could help break the energy-density bottleneck and enable electric cars and planes to travel much farther without charging.
Many of the solid electrolytes developed so far have been made from ceramic materials, which are highly effective at conducting ions but don't stand up so well to stress during charging and discharging owing to their brittle nature. Scientists from Brown University and the University of Maryland sought an alternative to this, and used cellulose nanofibrils found in wood as their starting point.
These wood-derived polymer tubes were combined with copper to form a solid ion conductor boasting a conductivity similar to ceramics and between 10 and 100 times better than other polymer ion conductors. According to the team, this is because the addition of copper creates space in between the cellulose polymer chains for "ion superhighways" to form, enabling the lithium ions to travel with record efficiency.
“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,” said study author Liangbing Hu. “In fact, we found this ion conductor achieved a record high ionic conductivity among all solid polymer electrolytes.”
And because the material is paper-thin and flexible, the scientists believe it will better tolerate the stresses of battery cycling. They also say it has the electrochemical stability to accommodate a lithium-metal anode and high voltage cathodes, or could act as a binder material that encases ultra-thick cathodes in high-density batteries.
“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 study author Yue Qi. “Using materials nature provides will reduce the overall impact of battery manufacture to our environment.”
The research was published in the journal Nature.
Source: Brown University