A lithium-ion battery can store more charge in the same amount of space by packing the positive electrode with more lithium. Such a device could theoretically power an electric car 30 to 50 percent farther between charges, but lithium-rich cathodes quickly lose voltage.

Now, researchers have determined how the same chemical processes that give these cathodes their high capacity are also linked to changes in atomic structure that sap performance. The investigation opens new routes for optimizing voltage performance of lithium-rich cathodes by controlling the way their atomic structure evolves as a battery charges and discharges.

Cathodes made by Samsung Advanced Institute of Technology using commercially relevant processes, and SLAC and Stanford scientists at a SLAC Stanford Synchrotron Radiation Lightsource beamline used for battery research. Source: Dawn Harmer/SLAC National Accelerator Laboratoryassembled into batteries similar to those in electric vehicles, were evaluated with different X-ray techniques.

Lithium-rich cathodes are composed of layers of lithium sandwiched between layers of transition metal oxides, such as nickel, manganese or cobalt combined with oxygen. Adding lithium to the oxide layer increases the cathode’s capacity by 30 to 50 percent.

Previous research had shown that several things happen simultaneously when lithium-rich cathodes charge: Lithium ions move out of the cathode into the anode. Some transition metal atoms move in to take their place as oxygen atoms release some of their electrons, establishing the electrical current and voltage for charging, When lithium ions and electrons return to the cathode during discharge, most of the transition metal atoms return to their original spots, but not all of them and not immediately.

With each cycle, this back and forth changes the cathode’s atomic structure as the transition metal migration decreases the potential of the bulk oxygen redox couple. This leads to a reordering in the anionic and cationic redox potentials during cycling as well as stabilization of the oxygen redox couple.

Researchers from Stanford University, SLAC National Accelerator Laboratory, Lawrence Berkeley National Laboratory, Samsung Advanced Institute of Technology in South Korea and Shandong University in China participated in this study, which is published in Nature Communications.