As part of this general research, a team from the University of Illinois at Chicago (UIC), which includes researchers from Argonne National Laboratory (ANL), Michigan Technological University and the University of Bath, has been developing a cathode based on manganese dioxide.
“For the tunnels to survive for long lasting function, they need support structures at the atomic scale,” said Reza Shahbazian-Yassar, an associate professor at UIC. “We call them tunnel stabilisers and they are generally big positive ions, like potassium or barium.”
Because they are positively charged, tunnel stabilisers and lithium ions should repel each other. However, the team has shown, using an electron microscope with atomic level resolution, that certain large ions can hold the tunnels open, while allowing charge-carrying ions to pass through the electrode easily and quickly.
Yifei Yuan, an ANL researcher, said: “We saw that, when you add a tunnel stabiliser, the tunnels expand, their electronic structures change and such changes allow the lithium ions to move in and out, around the stabiliser.”
According to Shahbazian-Yassar, the presence of potassium ions in the tunnels improves the electronic conductivity of manganese dioxide and the ability of lithium ions to diffuse quickly in and out of the nanowires.
“With potassium ions staying in the centre of the tunnels, the capacity retention improves by half under high cycling current, which means the battery can hold on to its capacity for a longer time,” he concluded.