In a lithium ion cell, the active materials that store the lithium on the positive and negative sides of the cell are contained in porous electrode sheets that are wound or stacked, depending on the type of cell. By volume, these electrodes are about 30% pore space, which is infilled with liquid electrolyte. The 70% solid content of the electrodes consists of the micrometer-sized active particles (about 95% by mass) and a conductive additive and polymer binder phase (about 5% by mass). Conductive additives (such as carbon black) provide paths for electrons to reach the current collector. The polymeric binder provides mechanical stability of the active materials and conductive additive in the electrodes and the adhesion of the electrode to the current collector.

During each charge and discharge cycle, active materials in the cell lithiate and delithiate, leading to volume expansion and contraction of the active material particles and the electrodes themselves. These volume changes are particularly dramatic for high energy density active materials such as silicon that undergo an alloying reaction with lithium. Repeated volume changes can cause the carbon-black and binder phases to detach from the active material, leading to loss of electrical contact and electrochemical activity. At the cell level, we observe this loss of electrochemical activity as a loss in capacity.

In our recent paper in Nature Communications, we report how we quantified the detachment of the carbon black and binder from active materials for the first time [1].  We developed a number of sample preparation techniques and used correlative imaging, combining information from 2D cross sectional data from scanning electron microscopy with 3D transmission x-ray tomography performed at the Advanced Photon Source at Argonne National Laboratories to obtain the distance between the carbon black and binder domain and the active particle as a function of particle size and cycle number.

We combined this quantitative visual data on detachment with electrochemical performance data to develop and validate a model that links properties of the active materials and electrode (such as active particle size and volume expansion, particle-binder adhesion, electromechanical properties of the porous electrode, and solid electrolyte interphase growth rate) to the detachment probability of the carbon-black and binder phase from the active particles and the observed capacity fade of the cell. This model can be used to develop design guidelines for the types of binders and elasto-mechanical properties of the electrodes that should be targeted in next generation lithium ion cells.

References:

[1] S. Müller, P. Pietsch, B.-E. Brandt, P. Baade, V. De Andrade, F. De Carlo, V. Wood. "Quantification and modeling of mechanical degradation in lithium-ion batteries based on nanoscale imaging" Nature Communications, 9, 2340 (2018).

Additional References:

P. Pietsch, V. Wood. "X-ray Tomography of Lithium Ion Batteries: A Practical Guide" Annual Reviews of Materials Research 47, 451-479 (2017).

Group Website: www.made.ee.ethz.ch