Visualizing the Operation of a Lithium-Ion Battery
Have you ever had a laptop computer whose battery became swollen over time? It doesn’t take a specialist to tell you that the battery shouldn’t poke out of the computer’s casing. Understanding why and how lithium-ion batteries change size during use, however, presents a challenge for scientists trying to build longer-lasting batteries, which would perform for longer stretches of time between recharging than today’s batteries.
Just as water undesirably stresses pipes as it expands in freezing temperatures, the insertion of lithium ions can give rise to unwanted forces in battery electrodes. These forces can pulverize a battery electrode, leading to a loss of electrical contact and a battery that doesn’t work.
A new technique developed by researchers at the Nanostructures for Electrical Energy Storage (NEES) enables real-time monitoring of battery expansion and contraction and the resulting internal stress. Compared to similar methods, the technique represents a platform to rapidly study and screen materials being considered for lithium-ion batteries.
Next-generation batteries containing alloys of lithium with silicon, tin, aluminum, etc., as the negative electrode, or anode, can store more charge for a given mass than current technology, meaning these batteries would last longer than today’s batteries. The downside to these new materials is that they significantly expand and contract as lithium ions are inserted and removed, respectively.
In a project led by Reza Ghodssi at NEES, researchers developed an optical-based method to detect changes in the volume of a silicon anode as lithium ions moved in and out. These changes in size are related to stress in the active battery material.
The new method involves forming a battery underneath a miniature drum. As lithium ions are cycled in and out of a silicon anode, the resulting expansion and contraction of the silicon is measured from the constructive and destructive interference of light incident and reflected off the drum. A control experiment allowed the correlation to be made between the deflection of the drum and the stress in the battery.
The optical deflection technique reveals the reversible and irreversible deformations associated with lithium ion insertion and extraction from silicon anodes. Such mechanistic insights are critical to understanding and taming the expansion/contraction of lithium-alloy materials.
Because lithium compounds are air-sensitive, previous optical-based measurements of battery expansion and contraction have been performed in a glove box. The NEES team, however, modified the casing of a standard battery with a transparent window to perform measurements at ambient conditions on batteries assembled in a glove box, greatly simplifying the process.
While the researchers focused on silicon anodes in their initial work, the drum design is suited to study other materials whose size changes with electrochemical processes. Furthermore, their procedure allows many drums to be fabricated at once, potentially allowing for combinatorial studies of battery materials.
Pomerantseva E, H Jung, M Gnerlich, S Baron, K Gerasopoulos, and R Ghodssi. 2013. “A MEMS Platform for In Situ, Real-Time Monitoring of Electrochemically Induced Mechanical Changes in Lithium-Ion Battery Electrodes.” Journal of Micromechanics and Microengineering 23(11):114018. DOI: 10.1088/0960-1317/23/11/114018