Chunks of this sodium-based salt (left) will only function well in a battery at elevated temperatures, but when they are milled into far smaller pieces (right), they can potentially perform even at low temperatures, making them even more promising as the basis for safer, cheaper rechargeable batteries. Photo: Tohoku University, Japan.By chemically modifying and pulverizing a promising group of compounds, scientists at the US National Institute of Standards and Technology (NIST) have potentially brought safer, solid-state rechargeable batteries two steps closer to reality.
Unlike the traditional liquid electrolytes used in rechargeable batteries, these compounds are stable, solid materials that do not pose a risk of leaking or catching fire. They are based on commonly-available substances known as lithium and sodium closo-borate salts, which are made primarily from hydrogen, boron and either lithium or sodium.
Since first discovering the properties of these compounds in 2014, a team led by NIST scientists has sought to enhance their performance in two key ways: increasing their current-carrying capacity and ensuring that they can operate in a sufficiently wide temperature range to be useful in real-world environments. Considerable advances have now been made on both fronts, according to Terrence Udovic of the NIST Center for Neutron Research, whose team has published a pair of scientific papers that detail each improvement, in Advanced Energy Materials and Energy Storage Materials respectively.
The first advance came when the team found that the original compounds were even better at carrying current with a slight change to their chemical makeup. Replacing one of the boron atoms with a carbon atom improved their ability to conduct ions, which are what carry charge inside a battery. As the team reported in the Advanced Energy Materials paper, this switch made the compounds about 10 times better at conducting.
But perhaps more important was clearing the temperature hurdle. The compounds now conducted ions well enough to operate in a battery – as long as they were in an environment typically hotter than boiling water. Unfortunately, there's not much of a market for such high-temperature batteries. By the time the compounds cooled to room temperature, their favorable chemical structure had often changed to a less conductive form, decreasing their performance substantially.
One solution turned out to be crushing the compound's particles into a fine powder. The team had been investigating particles that are measured in micrometers, but as nanotechnology research has demonstrated time and again, the properties of a material can change dramatically at the nanoscale. The team found that pulverizing the compounds into nanometer-scale particles resulted in materials that could still perform well at room temperature and far below.
"This approach can remove worries about whether batteries incorporating these types of materials will perform as expected even on the coldest winter day," says Udovic, whose collaborators on the Energy Storage Materials paper include scientists from Japan's Tohoku University, the University of Maryland and Sandia National Laboratories. "We are currently exploring their use in next-generation batteries, and in the process we hope to convince people of their great potential."
This story is adapted from material from NIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.