Chemists at Stony Brook University and Brookhaven National Laboratory (BNL) have made a new finding about battery performance that points to a different strategy for optimizing cathode materials. Their research, published in Chemistry of Materials and featured in ACS Editors’ Choice, focuses on controlling the amount of structural defects in the cathode material.
“Instead of changing the chemical composition of the cathode, we can alter the arrangement of its atoms,” said corresponding author Peter Khalifah, Professor of Chemistry at Stony Brook with a joint appointment at BNL.
Today, most cathode materials are comprised of alternating layers of lithium ions and transition metals, such as nickel. Within that layered structure, a small number of defects can usually be found. That means atoms from a transition metal can be found where a lithium ion is supposed to be and vice versa.
“You can think of a defect as a ‘mistake’ in the perfection of the material’s structure,” Khalifah said. “It is known that a lot of defects will lead to poor battery performance, but what we’ve come to learn is that a small number of defects should actually improve key properties.”
Khalifah says there are two properties that a good cathode material will have: ionic conductivity (the lithium ions can move well) and electronic conductivity (the electrons can move well).
“The presence of a defect is like poking a hole between the lithium ion and transition metal layers in the cathode,” he said. “Instead of being confined to two dimensions, the lithium ions and electrons can move in three dimensions across the layers.”
To make this conclusion, the scientists needed to conduct high-precision experiments that measured the concentration of defects in a cathode material with far greater accuracy than has ever been done before.
“The concentration of defects in a cathode material can vary between two and five percent,” Khalifah said. “Before, defects could only be measured with a sensitivity of about one percent. In this study, we measured defect concentration with exquisite accuracy—a sensitivity of a tenth of a percent.”
“This work has developed a new way of visualizing structural defects and their relationship to diffraction and scattering strength,” said Saul Lapidus, a physicist in the X-ray Science Division at the Advanced Photon Source (APS) at DOE’s Argonne National Laboratory . “I expect in the future for this technique to be used commonly in the battery community to understand defects and structural characterizations of cathode materials.”