Researchers discover pathway to optimize performance of lithium-rich cathodes; potential boon for EVs
Lithium-rich layered oxide cathodes are of significant interest because their specific capacities often exceed 200 mAh g-1 at high operating voltages over 3.5V, in contrast to the performance of their conventional layered counterparts with capacities of ~145-165 mAh g-1. The higher energy density could theoretically power an EV 30-50% further between charges.
However, most of these Li-rich materials suffer from voltage drop and capacity fading during cycling, thus limiting their use. Years of research have not been able to pin down why this occurs. Now, researchers from Stanford University, two Department of Energy national labs and the battery manufacturer Samsung have created a comprehensive picture of how the same chemical processes that give these cathodes their high capacity are also linked to changes in atomic structure that sap their performance. The team’s open-access paper appears in Nature Communications.
This is good news. It gives us a promising new pathway for optimizing the voltage performance of lithium-rich cathodes by controlling the way their atomic structure evolves as a battery charges and discharges.—William E. Gent, a Stanford University graduate student and Siebel Scholar who led the study
It is a huge deal if you can get these lithium-rich electrodes to work because they would be one of the enablers for electric cars with a much longer range. There is enormous interest in the automotive community in developing ways to implement these, and understanding what the technological barriers are may help us solve the problems that are holding them back.—Michael Toney, a distinguished staff scientist at SLAC National Accelerator Laboratory and co-author
The researchers studied the cathodes with a variety of X-ray techniques at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL) and Lawrence Berkeley National Laboratory’s Advanced Light Source (ALS). Theorists from Berkeley Lab’s Molecular Foundry, led by David Prendergast, were also involved, helping the experimenters understand what to look for and explain their results.
The cathodes themselves were made by Samsung Advanced Institute of Technology using commercially relevant processes, and assembled into batteries similar to those in electric vehicles.
This ensured that the results would be directly relevant for our industry partners, Gent said. As an ALS doctoral fellow in residence, he was involved in both the experiments and the theoretical modelling for the study.
The more ions an electrode can absorb and release in relation to its size and weight—its capacity—the more energy it can store and the smaller and lighter a battery can be, allowing batteries to shrink and electric cars to travel more miles between charges.
Current Li-ion cathodes operate at only about half of their theoretical capacity, said Stanford Professor William Chueh, an investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC.
But you can’t charge it all the way full. It’s like a bucket you fill with water, but then you can only pour half of the water out. This is one of big challenges in the field right now – how do you get these cathode materials to behave up to their theoretical capacity? That’s why people have been so excited about the prospect of storing a lot more energy in lithium-rich cathodes.—William Chueh
Like today’s cathodes, lithium-rich cathodes are made of layers of lithium sandwiched between layers of transition metal oxides—e.g., 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, Chueh said: Lithium ions move out of the cathode into the anode. Some transition metal atoms move in to take their place. Meanwhile, oxygen atoms release some of their electrons, establishing the electrical current and voltage for charging.
When the lithium ions and electrons return to the cathode during discharge, most of the transition metal interlopers return to their original spots, but not all of them and not right away. With each cycle, this back and forth changes the cathode’s atomic structure.
We knew all these phenomena were probably related, but not how. Now this suite of experiments at SSRL and ALS shows the mechanism that connects them and how to control it. This is a significant technological discovery that people have not holistically understood.—Willliam Chueh
At SLAC’s SSRL, Toney and his colleagues used a variety of X-ray methods to make a careful determination of how the cathode’s atomic and chemical structure changed as the battery charged and discharged.
Another important tool was soft X-ray RIXS (resonant inelastic X-ray scattering), which gleans atomic-scale information about a material’s magnetic and electronic properties. An advanced RIXS system that began operation at ALS last year scans samples much faster than before.
RIXS has mostly been used for fundamental physics. But with this new ALS system, we wanted to really open up RIXS for practical materials studies, including energy-related materials. Now that its potential for these studies has been partially demonstrated, we could easily extend RIXS to other battery materials and reveal information that was not accessible before.—ALS scientist Wanli Yang
The team is already working toward using the fundamental knowledge they have gained to design battery materials that can reach their theoretical capacity and not lose voltage over time.
The research was funded by the DOE Office of Energy Efficiency and Renewable Energy’s Vehicle Technologies Office and by Samsung Advanced Institute of Technology Global Research Outreach Program.