Researchers aim at developing better battery by peering into atom-sized tunnels

In a recent research, scientists have used a special electron microscope with atomic-level resolution to show that certain large ions can hold the tunnels open so that the charge-carrying ions can enter and exit the electrode easily and quickly. Earlier, battery researchers seeking improved electrode materials have focused on "tunneled" structures that made it easier for charge-carrying ions to move in and out of the electrode. 

The findings are published in Nature Communications. "Significant research has been done to increase the energy density and power density of lithium ion battery systems," said Reza Shahbazian-Yassar. The current generation is useful enough for portable de vices, but the maximum energy and power that can be extracted is limiting, he said.

"So for an electric car, we need to increase the energy and power of the battery -- and decrease the cost as well," he added. The team has focused on developing a cathode based on manganese dioxide, a very low cost and environmentally-friendly material with high storage capacity. Manganese dioxide has a lattice structure with regularly spaced tunnels that allow charge carriers -- like lithium ions -- to move in and out freely.

"But for the tunnels to survive for long-lasting function, they need support structures at the atomic scale," Shahbazian-Yassar said. Adding, "We call them tunnel stabilizers, and they are generally big, positive ions, like potassium or barium." But the tunnel stabilizers, being positively charged like the lithium ions, should repel each other.

"If lithium goes in, will the tunnel stabilizer come out? The research community was in disagreement about the role of tunnel stabilizers during the transfer of lithium into tunnels. Does it help, or hurt?, " Shahbazian-Yassar said. The study also represented the first use of electron microscopy to visualize the atomic structure of tunnels in a one-dimensional electrode material -- which the researchers say had not previously been possible due to the difficulty of preparing samples.

It took them two years to establish the procedure to look for tunnels in potassium-doped nanowires of manganese dioxide down to the single-atom level. Yifei Yuan was then able to use a powerful technique called aberration-corrected scanning transmission electron microscopy to image the tunnels at sub-?ngstrom resolution so he could clearly see inside them and he saw they do change in the presence of a stabilizer ion. 

"It's a direct way to see the tunnels and we saw that when you add a tunnel stabilizer, the tunnels expand, their electronic structures also change, and such changes allow the lithium ions to move in and out, around the stabilizer," Yuan said. The finding showed that tunnel stabilizers can help in the transfer of ions into tunnels and the rate of charge and discharge.

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 center 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 said.