A nanoscale look at how lithium batteries work

Dr. Jörg Schwartz, joerg.schwartz@photonics.com

A new type of scanning probe microscopy that can visualize electrochemical strain has enabled scientists to study the movement of ions in the cathode material of lithium-ion batteries, an approach that not only offers a better understanding of the ions’ mechanisms but that also could help improve their performance.

The technique, called electrochemical strain microscopy (ESM), was developed by researchers at the Center for Nano-phase Materials Sciences at the US Department of Energy’s Oak Ridge National Laboratory (ORNL).

A new technique called electrochemical strain microscopy maps how lithium ions flow through a battery’s cathode material. This 1 x 1-μm composite image demonstrates how regions on a cathode surface display varying electrochemical behaviors when probed with ESM. Courtesy of Oak Ridge National Laboratory.

Lithium-ion batteries have a number of advantages over other types of rechargeable batteries, including a good capacity-to-weight ratio, no memory effect and a slow loss of charge. They are not only highly popular for consumer electronics products, but also critical parts for future electric cars or as buffers for renewable yet noncontinuous energy sources such as solar cells.

Their main components are the anode and cathode electrodes separated by an electrolyte. The lithium ions move from the anode to cathode through the electrolyte during charge and discharge, producing electric work. The movement of lithium ions into and out of electrodes is central to the charge capacity and to the power of lithium-ion batteries; therefore, the processes of insertion (or intercalation) and extraction (de-intercalation) of ions are areas of active research.

The researchers say that, although the process has been extensively studied at the device level, it remains virtually unknown at the nanoscale level of grain clusters, single grains and defects.

One method used to date is atomic force microscopy (AFM) to study how the surface morphology of the electrodes changes while the battery is charging or discharging. Static strains can be derived from this and electronic currents mapped across the electrode surfaces. However, a dynamic study of the intercalation processes, strain charge and ion transport at the level of single-grain boundaries and dislocations in the electrodes is not possible with standard AFM alone.

This is exactly what the researchers have done with ESM, which they reported on in Nature Nanotechnology 5, pp. 749 to 754, published online Aug. 29, 2010. By using the tip of an AFM to concentrate an oscillating electric field onto the cathode of a lithium-ion battery, they triggered lithium ions to intercalate and de-intercalate in a small volume underneath the biased tip. This resulted in periodic changes of the cathode volume and a strain at its surface. The strain was then measured by the same AFM tip, leading to a map of the lithium intercalation and transport processes.

The method showed highly anisotropic expansion of the battery with sub-100-nm resolution, which they correlated to local lithium diffusivity and to the microstructure of LiCoO2, one of the most used cathode materials for lithium-ion batteries. “We can provide a detailed picture of ionic motion in nanometer volumes, which exceeds state-of-the-art electrochemical techniques by six to seven orders of magnitude,” researcher Sergei Kalinin said.

“Very small changes at the nanometer level could have a huge impact at the device level,” added his colleague Nina Balke. “Understanding the batteries at this length scale could help make suggestions for materials engineering.”

Although the initial focus was on lithium-ion batteries, the team expects that its ESM technique can be applied to measure other electrochemical solid-state systems, including other battery types, fuel cells and similar electronic devices that use nanoscale ionic motion for information storage.