Lithium-ion batteries are like paper clips. What is chemo-mechanical degradation, which appears in both headphones and electric vehicles?
Everyone knows that batteries inevitably degrade with use, but for a long time no engineer could explain exactly how small microscopic changes turn into a constant loss of capacity over time.
A new study in Science now points to an important piece of the puzzle: the mechanical “breathing” of batteries, that is, the repeated expansion and contraction of their internal components with each charge and discharge cycle.
The research, developed by a team from the University of Texas at Austin, Northeastern University, Stanford University and Argonne National Laboratory, describes a phenomenon known as chemo-mechanical degradation. In simpler terms, when a battery charges, lithium ions enter the electrodes; when it discharges, they come out. This chemical migration generates physical stress on materials. Over thousands of cycles, this effort accumulates and causes internal deformations that weaken the cell to the point of failure.
“With each ‘breath’ of the drums, there is a certain degree of irreversibility”explains Yijin Liuassociate professor at the University of Texas at Austin and leader of the study. A, in an article dedicated to the study, compares the phenomenon to what happens with a paper clip, which is repeatedly bent until it breaks, a single load may not cause visible damage but continuous repetition ends up leaving marks.
The great novelty of the work is the identification of an internal mechanism called “deformation cascades”. The electrodes are composed of hundreds of thousands of densely packed microscopic particles, and the team observed that these particles do not react uniformly to electrochemical stress. Some move quickly, while others remain relatively stable.
This discrepancy creates localized stresses that, over time, favor the formation of cracks and structural damage. When fractures begin, electrical pathways degrade and loss of performance tends to accelerate.
To reach these conclusions, the researchers turned to advanced “operating” imaging techniques, which make it possible to observe materials while they are working, and not just after they fail. Among the methods used are in-service transmission X-ray microscopy and 3D X-ray laminography, which make it possible to see the inside of intact batteries during actual charge and discharge cycles, without cutting or physically altering them.
The phenomenon was initially detected unexpectedly when the team was analyzing tiny batteries in wireless earphones: the same mechanical stresses arise in both small cells and larger batteries.
The implications go far beyond cell phones. Degradation is a central obstacle for and for grid storage systems associated with renewable energy, which rely on batteries capable of supporting many years of cycles. Batteries with a shorter lifespan increase costs, generate more waste and delay the energy transition.
By identifying how and where mechanical stress accumulates, the study also points to clues to improve the design: apply controlled pressure to cells to limit deformation and redesign electrodes so that particles move more homogeneously, reducing the likelihood of deformation cascades.
