Like Superman, Mark Rodriguez peers inside lithium ion battery with X-ray vision

Editor’s Note: Sandia’s thrust to build a better lithium ion battery is a three-part approach — chemical preparations, characterization, and computer modeling. The Dec. 4 Lab News reported on Tim Boyle’s (1846) efforts to develop the right combination of metals to create the cathode portion of the battery. This story focuses on using X-ray diffraction to see real-time structural changes of batteries as they charge and discharge. A future story will discuss using computer models to determine structural integrity of battery materials.

As part of Sandia’s efforts to build a better lithium ion battery, Mark Rodriguez of Materials Characterization Dept. 1822, along with David Ingersoll and Jill Langendorf of the Lithium Ion Battery R&D Dept. 2521, have developed a method to view real-time changes batteries undergo as they charge and discharge.

Using an X-ray diffractometer that bounces X-rays onto the materials comprising the electrodes, Mark can observe the structural changes in the battery cathode or anode as they occur during battery cycles.

The X-ray diffractometer collects the diffraction pattern during the different stages of charge and discharge of an operating battery specifically designed for X-ray measurements. From this diffraction data, information concerning atomic level changes in the crystal structure can be derived.

"The ability to monitor the changes of the crystal structure during the cycling helps us better understand a battery’s behavior — how the battery functions," Mark says. "And a better understanding of how they work may lead to ideas for improving them."

Sandia researchers have high hopes for the lithium battery, seeing it as the possible battery of the future, one that is light-weight, long-lasting, and environmentally friendly. Lithium ion batteries are already lighter and provide more electricity than non-lithium ion batteries of equal size. Their main drawback is the high cost due to the materials used for the cathode portion of the battery.

Currently, lithium ion batteries are found in laptop computers, camcorders, and other small electronics, but the potential for their becoming the battery of choice for all electronic devices, including batteries for electric cars, is great. The main obstacle to this becoming a reality is finding the right combination of cathode/anode elements — a combination that does not use expensive materials.

The ability to study real-time structural changes of batteries as they charge and discharge gives Sandia scientists a powerful tool to analyze the effectiveness of the new mixtures of cathode metals that are being tested. In addition, understanding changes in the battery materials as they degrade during use may help solve aging problems, resulting in an extended life of the rechargeable batteries.

"Before this X-ray diffraction technique was developed, the only way we could determine the structural and phase changes that take place in a battery was to take it apart and physically look at it after it’s been through a cycle," Mark says. "Now researchers can discharge or charge a battery and watch the changes happen in real time on a computer monitor."

A typical experiment using the X-ray diffraction setup takes about one-and-a-half days. Mark collects about 100 diffraction patterns during that time, with each scan taking about a half hour, going slow enough to view the real-time mechanisms affected.

A lithium ion battery, like all batteries, consists of three basic parts — two electrodes (a cathode and anode) and an electrolyte. Lithium ion batteries use host materials (non-lithium materials that act similar to lithium) for the electrodes to avoid using metallic lithium. Electrochemical reactions at the electrodes produce an electric current that powers an external circuit. During charge and discharge of lithium ion rechargeable batteries, lithium ions are shuttled between the cathode and anode host materials in a "rocking chair" fashion.

In a recent study of batteries prepared with lithium-manganese-oxide cathodes, Mark observed that as voltage is applied to the battery during charging, the cubic lattice structure shrinks. However, the shrinkage does not occur linearly with applied voltage. Instead, it occurs in two distinct voltage regions. Mark uses a computer program to plot these variations, coming up with clear visualization of structural changes during the various stages.

"The diffraction peaks in the patterns move a lot," Mark says. "Our challenge is to correlate changes in the positions of diffraction peaks with the battery’s ability to store and release charge. Once we figure that out, we might better determine which materials are best for a given battery application. It gives us a better analysis tool so that we are not working in the dark."