University of Michigan Research Changes Narrative on Battery Cracking
New research indicates that lithium-ion battery cracking could benefit battery performance.
A known “issue” in lithium-ion batteries is the propensity for electrodes to experience cracking. In recent history, many of the industry’s largest battery manufacturers have been exploring means of preventing this cracking, believing that it leads to aging and degradation.
Degraded battery. Images used courtesy of Adobe Stock
However, University of Michigan researchers claim cracking may actually be good for the battery.
Cracking in Lithium-Ion Batteries
Cracking in lithium-ion batteries refers to the formation of fractures or cracks within the electrodes of the battery, specifically within the materials that make up the anode or cathode. This phenomenon has several causes.
During the charging and discharging cycles of a lithium-ion battery, the movement of lithium ions between the anode and cathode causes the active materials in the electrodes to expand and contract. This repeated expansion and contraction lead to mechanical stresses that can cause cracks in the electrode materials. This process is further complicated by the uneven distribution of lithium ions within the electrode material, known as inhomogeneous lithiation, which can cause localized stress points and lead to cracking.
Particle fracture and SEI growth in a lithium-ion battery. Image used courtesy of Edge et al.
The specific materials used in the electrodes also play a role in their propensity to crack. Some materials may be more prone to this due to their crystal structure or bonding characteristics. Extreme high and low temperatures can exacerbate the cracking phenomenon, with high temperatures accelerating material degradation and low temperatures increasing brittleness.
Finally, the manufacturing process of lithium-ion batteries can introduce imperfections such as voids, impurities, or uneven coatings, which can act as initiation points for cracks.
Is Battery Cracking Bad?
Cracking in lithium-ion batteries is historically considered detrimental due to its impact on the battery's performance, efficiency, and longevity.
When cracks form within the electrodes, particularly in the anode or cathode, they can disrupt the uniform movement of lithium ions during charging and discharging cycles. This disruption decreases the battery's ability to hold a charge, reducing its overall capacity.
The mechanical stresses that cause cracking are often a result of repeated expansion and contraction of the active materials in the electrodes. Over time, these stresses can lead to wear and degradation of the materials, further shortening the battery's lifespan. Cracking can also create pathways for unwanted chemical reactions between the electrode material and the electrolyte, forming solid-electrolyte interphase (SEI) layers. These layers can cause additional mechanical stresses, exacerbating the cracking problem.
In some cases, cracking can even pose safety risks. If the cracks become severe enough, they can lead to internal short circuits or other malfunctions, potentially causing the battery to fail or, in extreme cases, catch fire.
A New Perspective on Battery Cracking
Recently, researchers from the University of Michigan revealed that rather than being solely detrimental, cracks in the positive electrode of lithium-ion batteries can actually reduce battery charge time.
The research team focused on the positive electrode, or cathode, composed of trillions of microscopic particles of materials commonly used in more than half of all electric vehicle batteries. The conventional belief was that smaller cathode particles would charge faster than larger ones due to a higher surface-to-volume ratio, allowing lithium ions to diffuse through them more quickly.
Design and fabrication of multi-electrode arrays. Image used courtesy of Min et al.
However, the University of Michigan's research found the charging speed of individual cathode particles did not depend on their size. Instead, the cathode particles were cracked and had more active surfaces to take in lithium ions on their outer surface and inside the particle cracks.
The researchers employed a neuroscience-inspired technique to measure individual cathode particles' charging speed. They used a custom-designed chip with up to 100 microelectrodes and inserted the particles into a device typically used by neuroscientists to study how individual brain cells transmit electrical signals.
The experiment revealed that larger particles behave like a collection of smaller particles when they crack, providing more surface area for the lithium ions to interact. This unexpected behavior led the researchers to conclude that cracks in the cathode materials could have an upside, potentially reducing the charging time for electric vehicles.