Offering such benefits as extended usage and increased portability, lithium-ion batteries have become a key component in consumer electronics and other important devices.
Two researchers at Iowa State are looking at ways to improve the technology within the batteries by providing more insight into how silicon could be integrated as an electrode material.
Valery Levitas, Schafer Professor and faculty member of aerospace engineering and of mechanical engineering, and his Ph.D. student Hamed Attariani have published the paper “Anisotropic compositional expansion and chemical potential for amorphous lithiated silicon under stress tensor,” in Scientific Reports, a new research journal from the Nature publishing group.
In their paper, the researchers suggest an unexpected solution to the basic and applied problems that arise when using nanoscale silicon anodes within lithium-ion batteries.
The use of silicon in place of the commonly used commercial graphite as anode offers more capacity, but the material’s tendency to expand (by 334%) when integrated with lithium results in huge stresses that create fractures in the anodes.
Because of this problem, many researchers have been looking for ways to decrease these stresses, also referred to as stress relaxation.
Levitas says most research in this area has been founded in the plasticity theory, which describes plastic flow and stress relaxation when stresses exceed the yield strength.
“This approach is unrealistic because the yield strength of the lithiated silicon is much higher than the generated stresses,” Levitas explains.
That’s why he and Attariani tried a new approach, suggesting that stress relaxation is due to anisotropic, or directionally dependent, compositional straining that occurs during the insertion-extraction of lithium at any stresses.
With this approach, the researchers predicted that oscillating lithiation-delithiation could significantly reduce stresses in this situation. Their theory was confirmed by known experiments.
The stress relaxation mechanism they suggest also leads to a new concept and expression for chemical potential under general stress conditions, as well as predictions of unexpected phenomena.
While their work has lead to a better understanding of how silicon may be integrated into lithium-ion batteries, Levitas says the approach can also be applied to large compositional deformation and stress relaxation for any material systems, as well as for chemical reactions and melting under nonhydrostatic conditions.