When a lithium-ion battery is overstuffed with lithium, that is, when more lithium is added, the battery will have the ability to store more charge in the same space, theoretically powering an electric car to run 30%-50% percent more between charges. However, such lithium-stuffed cathodes tend to rapidly lose voltage, and many years of investigations could not yet answer why.
Following analyzes of the challenge from different points of view, scientists from Stanford University, two Department of Energy national labs, and the battery manufacturer Samsung developed a comprehensive idea of the way in which the same chemical processes providing the cathodes with the higher capacity are also related to alterations in atomic structure that reduce the performance.
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, Graduate Student, Stanford University and Siebel Scholar who headed the research
Michael Toney, a co-author of the paper, who is a distinguished staff scientist at SLAC National Accelerator Laboratory, stated that “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.”
The outcomes of the study have been published in the Nature Communications journal on December 12th, 2017.
The team investigated the cathodes by adopting different X-ray methods at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL) and Lawrence Berkeley National Laboratory’s Advanced Light Source (ALS). Theorists at the Berkeley Lab’s Molecular Foundry, headed by David Prendergast, also took part in the study and assisted the researchers to understand what to search for and to explain their outcomes.
The cathodes were developed by Samsung Advanced Institute of Technology by adopting commercially applicable processes and were assembled inside batteries which were identical to the ones used in electric vehicles.
This ensured that our results represented an understanding of a cutting-edge material that would be directly relevant for our industry partners.
William E. Gent
Being an ALS doctoral fellow in residence, Gent took part not only in the experiments but also in the theoretical modeling for the research.
Like a Bucket Half Empty
Batteries convert electrical energy to chemical energy for storage. There are three fundamental parts in a battery: the anode, the cathode, and the liquid electrolyte between these electrodes. When a lithium-ion battery is charged and discharged, lithium ions move forward and backward between the anode and the cathode electrodes, and they get immersed into the electrode materials.
If an electrode can absorb and release more ions in relation to its size and weight (a factor called capacity), it can store more energy and a battery can be smaller and lighter, thus reducing the size of batteries and allowing electric cars to travel more distance between charges. “The cathode in today’s lithium-ion batteries operates at only about half of its theoretical capacity, which means it should be able to last twice as long per charge,” stated 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.
Professor William Chueh, Investigator, The Stanford Institute for Materials and Energy Sciences (SIMES), SLAC
Similar to prevalent cathodes, lithium-rich cathodes are formed by sandwiching lithium layers between transition metal oxides layers, where transition metal oxides are elements such as manganese, nickel, or cobalt combined with oxygen. When lithium is added to the oxide layer, the capacity of the cathode is increased by 30%-50%.
Connecting the Dots
According to Chueh, earlier studies have demonstrated that various things simultaneously happen upon charging lithium-rich cathodes—lithium ions leave the cathode and reach the anode. Certain transition metal atoms take up the place of the lithium ions in the cathode. At the same time, oxygen atoms liberate a portion of their electrons, initiating the voltage and electrical current for charging.
During the discharge cycle, the electrons and lithium ions return to the cathode, and a majority of the transition metal interlopers return to their actual positions, though not all return and not right away. After each cycle, this backward and forward movement of electrons and ions alters the atomic structure of the cathode. Chueh stated that it is similar to a bucket being transformed into a smaller and a somewhat different bucket.
“We knew all these phenomena were probably related, but not how,” stated Chueh. “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.”
At SLAC’s SSRL, Toney and his team adopted different X-ray techniques to carefully ascertain the way the chemical and atomic structure of the cathode changed when the battery charges and discharges.
Another significant tool was soft resonant inelastic X-ray scattering (RIXS), which acquires atomic-scale information related to the electronic and magnetic characteristics of a material. A state-of-the-art RIXS system that started to function at ALS last year has the ability to scan samples considerably faster than earlier.
“RIXS has mostly been used for fundamental physics,” stated ALS researcher Wanli Yang. “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.”
The researchers are already involved in applying the basic understanding gained by them to develop battery materials with the potential to attain their theoretical capacity and not lose voltage in the due course.