A team of scientists from the Chemical Sciences and Engineering division of the U.S. Department of Energy’s Argonne National Laboratory has successfully employed a state-of-the-art X-ray technique to observe the movements of components within a battery cell while it charges and discharges.
This breakthrough study is marking one of the first times that such movements are being directly observed at an incredibly fine scale of one-millionth of a meter.
Competition to be the Best
An array of battery types are currently contending for the title of next-generation batteries. As electric vehicles and grid energy storage are key elements in the efforts to mitigate climate change, sustainable batteries assume a significant role. Among these, the lithium-metal battery appears to be a promising contender.
The lithium-metal battery differs from lithium-ion batteries by replacing the graphite anode, a positive electrode, with a lithium-metal anode, which serves as the negative electrode. Although the lithium metal anode provides a tenfold increase in energy storage capacity compared to graphite anodes, it experiences decreased performance after undergoing multiple charge-discharge cycles.
The researchers used non-destructive, X-ray tomography, to capture 3D images of the battery components in action. They were able to see how the lithium metal anode expands and contracts during charging and discharging, leading to swelling and shrinking that can damage the battery over time. Subsequently, the team observed the accumulation of detrimental materials, such as lithium dendrites, on the surface of the anode. A monumental finding, given the accumulation of these materials, can cause short circuits and reduce battery performance.
“An ideal lithium battery would reversibly deposit and strip a uniform layer of lithium over thousands of charge-discharge cycles,” said Abraham. ?“Such a battery would store and deliver charge for many years.”
During the charging of a lithium metal battery cell, lithium ions undergo migration from the cathode, pass through the electrolyte, and eventually reach the anode. As the lithium atoms get deposited on the anode surface, it expands. Upon discharge, the anode contracts as lithium is removed. This repetitive expansion and contraction are considered normal.
Due to its high reactivity, lithium metal rapidly reacts with the electrolyte in its surroundings when deposited on the anode, resulting in the formation of a solid electrolyte interphase layer on the lithium metal surface. However, this interphase layer can pose a problem if it becomes excessively thick. Additional lithium deposition can lead to further reaction and interphase layer formation. Leading to, the formation of small pores, resulting in an increase in the interphase volume and surface area, effectively altering the design and function.
“If you’re trying to design batteries for phones, computers, and cars, you don’t want the irreversible expansion of the anodes to be too much,” said John Okasinski, an Argonne physicist in the X-ray Science division and one of the study’s authors. ?“That could cause problems with other components in the device.”
The Power of Energy-Dispersive X-ray Technology
By using the energy-dispersive X-ray diffraction technique, the researchers were able to observe the changes in the atom arrangements of the cell’s materials at a high resolution. In turn, enabling theteam to track the movements of the lithium metal anode, cathode, and separator during charging and discharging.
This non-destructive method can be used to study the behavior of batteries over multiple charge and discharge cycles. Essential for understanding the long-term performance and stability of lithium-ion batteries, which are used in a wide range of applications.
In the future, researchers aim to use this tool for various applications:
- Measuring the amount of reversible lithium plating and stripping
- Measuring the amount of lithium metal permanently lost to the interphase layer
- Detect if the battery components are exerting too much pressure on the separator
What does this all mean?
Ultimately, the data can aid in the selection of the most efficient electrolytes for future batteries. It can also help in identifying any issues that may obstruct the movement of lithium ions and determining the concentration gradients of lithium in the cathode.
“It’s really exciting to be able to visualize these movements,” said Daniel Abraham, a senior materials scientist in Argonne’s Chemical Sciences and Engineering (CSE) division and one of the study’s authors. ?“Other researchers have previously guessed that these movements happen, but they’ve not been able to show them in such fine detail.”