Atomic scale evaluation of lithium-ion batteries

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2015

In situ transmission electron microcopy was used to understand how lithium-ions work at the atomic scale. 

KEY CONCEPTS
• In situ transmission electron microcopy is used to evaluate the impact that lithiation has on a single-crystal, zinc-antimony nanowire to provide an atomic scale examination of lithium-ion batteries.
• Insertion of lithium-ions into the zinc-antimony nanowire causes atoms to engage in a shuffling process leading to a layered structure.
• The placement of more lithium-ions leads to increasing strain, which causes a reduction in battery performance and eventually failure.

THE PUSH TO DEVELOP A SAFER LITHIUM-ION BATTERY IS ONGOING due to the indication that this technology may prove to help improve the efficiency of automobiles to the point where they can meet the ambitious fuel economy standards required in the future. But lithium-ion batteries have performance problems related to safety concerns that might lead to smoke and fire.

In a previous TLT article, a new approach was developed to minimize the chance of thermal runaway that can lead to safety problems (1). A gummy electrolyte was prepared as a hybrid-containing liquid and solid electrolytes. During normal operation, the liquid electrolyte facilitates the migration of ions, and the solid ensures that no battery leakage can occur. But if the temperature starts to rise, the gummy electrolyte will melt and form a non-conductive layer on the electrodes to, in effect, shut down the lithium-ion battery.

The instability seen in lithium-ion batteries needs to be further clarified at the atomic scale. Reza Shahbazian-Yassar, Richard & Elizabeth Henes Associate Professor in Nanotechnology and adjunct associate professor of materials science and engineering at Michigan Technological University in Houghton, Mich., says, “A key aspect of understanding how lithium-ion batteries work at the atomic scale is to understand the electrochemically driven crystalline phase transitions that take place. This process leads to complex charge-discharge performance cycles.”

Shahbazian-Yassar considers these transitions to be the way electrodes (particularly anodes) share lithium-ions back and forth so that electronics such as cell phones and laptops can be powered. The currently accepted material used in anodes is graphite. But crystalline materials show more promise.

Shahbazian-Yassar says, “Crystalline materials have the potential to function as more effective electrodes because they can use more lithium-ions in performance cycling, which will improve battery efficiency. During this cycling, it is noted that the crystalline material can change to a different structural phase as the lithium-ions are moving.”

A better understanding is needed about these crystalline transitions to assess how they might impact the performance and safety of a lithium-ion battery. Research has now been conducted to understand this process on the atomic scale.

ATOMIC SHUFFLING
Shahbazian-Yassar and his research associates used in situ transmission electron microscopy to study the impact that lithiation has on a single-crystal, zinc-antimony nanowire. He says, “We chose to work with a zinc-antimony nanowire because past work has shown that antimony functions as a very effective electrode because it can accommodate a lot of lithium-ions without much fracture or failure.”

The zinc-antimony nanowire is prepared through a chemical vapor deposition method. Depending upon processing, the nanowire can have a diameter ranging from 25 nanometers up to 250 nanometers with a length of 5 to 10 microns. 

A small-scale nanobattery incorporated into the electron microscope was used to induce lithium-ions into the zinc-antimony nanowire. Shahbazian-Yassar says, “Initially, we started with a one-layer structural arrangement of atoms in the zinc-antimony nanowire in an AB configuration. As lithium-ions started to be inserted into the nanowire, we observed that the atomic layers started to move around in a shuffling process where they will switch locations with each other more and more in a rearrangement process needed to accommodate more and more lithium-ions.”

The net result is the creation of a sandwich structure in the zinc-antimony nanowire, as reflected in Figure 1. This is reflective of a large degree of contractions and expansions occurring as lithium-ions are added. 


Figure 1. A snapshot image shows how the addition of lithium-ions into a zinc-antimony nanowire creates a layered structure through a shuffling process. The colors reflect different degrees of strain, with the bright yellow regions showing the highest strain. (Courtesy of Michigan Technological University)

Shahbazian-Yassar says, “Figure 1 is a snapshot image showing the amount of strain that is placed on the atoms in the nanowire as they are undergoing shuffling. The colors reflect different degrees of strains in specific regions of the nanowire. A very yellow color represents the highest strain observed.”

As more lithium-ions are accommodated in the layered structure, the increasing strain leads to a reduction in battery performance and eventually failure. Shahbazian-Yassar says, “We attempted to put as much lithium into the nanowire as possible to create sufficient electrode strain, leading to the development of fracture.”

The researchers observed that the atomic order in the nanowire changed from an AB to an ABC orientation. This is reflected in the phase transformation from a hexagonal crystal to a cubic structure. The change in crystal structure is the most important technical observation seen by the researchers, according to Shahbazian-Yassar.

The net result of lithiation is to destabilize the structure of the crystalline zinc-antimony electrode. Shahbazian-Yassar says, “We have now been able to visualize the insertion of lithium and have shown that the instability correlates with crystalline strain. If lithium-ions continue to be added, eventually the nanowire will break up and then fail.”

Shahbazian-Yassar hopes the spectroscopic technique developed in this work will help with development of a more suitable electrode that can better handle the infusion of lithium-ions. Future work will involve evaluating other types of crystalline electrodes.

Additional information on this research can be found in a recent article (2) or by contacting Shahbazian-Yassar at reza@mtu.edu.

REFERENCES
1. Canter. N. (2014), “Safer lithium-ion batteries,” TLT, 70 (5), pp. 10-11.
2. Nie, A., Cheng, Y., Zhu, Y., Ardakani, H., Tao, R., Mashayek, F., Han, Y., Schwingenschlogl, U., Klie, R., Vaddiraju, S. and Yassar, R. (2014), “Lithiation-Induced Shuffling of Atomic Stacks,” Nano Letters, 14 (9), pp. 5301-5307.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.