One of the downsides of Lithium-ion batteries is that they can be unsafe if stressed, meaning they have to be treated with care and respected. If they are stressed by an internal short, excessive heat, or physical damage, for example, then they can catch fire or even explode. Using individual cells in large configurations can set off a chain reaction and create a fire that is difficult to deal with. The main cause of that fire hazard is the organic solvents in the electrolyte. Electrolytes are used to conduct the lithium ions between the battery’s anode and the cathode. They are usually made from lithium salt dissolved in a liquid organic solvent. At high currents, tiny filaments of lithium metal, called dendrites, can form in the electrolyte leading to short circuits. In addition, liquid electrolytes are made with flammable and toxic chemicals, which can catch fire. The cathode in the battery also usually contains oxygen, which can be released under certain conditions. The oxygen helps feed the fire and provides the extra element needed to make the fire self sustaining.
One way scientists have tried to make lithium batteries safer is to use a solid substance instead of a liquid electrolyte to take away the accelerant from the battery. The problem is that these solid state electrolytes are usually made from ceramic materials which are thick, rigid and brittle. Stresses during manufacturing as well as charging and discharging can lead to cracks and breaks, which decreases the battery’s lifetime. The latest researchers who have tried to solve this problem are from Brown University and the University of Maryland, and they have made a breakthrough with a substance derived from trees.
The team has developed a solid ion conductor that combines copper with cellulose nanofibrils, which are polymer tubes derived from wood. The researchers claim that the new material has an ion conductivity that is 10 to 100 times better than other polymer ion conductors and it can be used as either a solid battery electrolyte or as an ion-conducting binder for the cathode of an all-solid-state battery. The new material is thin and flexible like paper, with an ion conductivity is on par with ceramics.
Yue Qi, a professor at Brown’s School of Engineering and Qisheng Wu, a senior research associate at Brown, performed computer simulations of the microscopic structure of the copper-cellulose material to understand why it is able to conduct ions so well. It showed that copper increases the space between cellulose polymer chains, which normally exist in tightly packed bundles. The expanded spacing creates what amount to ion superhighways through which lithium ions can pass relatively unimpeded.
The new material can also be used as a cathode binder for a solid-state battery. In order to match the capacity of anodes, cathodes need to be substantially thicker. That thickness can compromise ion conduction, reducing efficiency. In order for thicker cathodes to work, they need to be encased in an ion-conducting binder. Using their new material as a binder, the team demonstrated what they believe to be one of the thickest functional cathodes ever reported.
The research at Brown University was supported by the National Science Foundation.