During the charge / discharge the cathode and anode undergo an expansion and contraction.
As you charge a cell it expands, when you discharge a cell it contracts and as the cell ages over its lifetime we see a continuing cell expansion. Thus the cell expansion can be divided into:
- Reversible cell expansion
- Irreversible cell expansion
This expansion and contraction puts a significant mechanical strain on the materials, thus over time they fracture and hence degrade. One option is to increase the stiffness of the particles, but perhaps a more interesting approach is to increase the elasticity.
Lim et al [1] implemented a multi-walled carbon nanotube (MWCNT) on the surface of battery electrode materials.
Bare NCM exhibited a surface displacement of approximately 50 nm in its radius, whereas the 0.5 wt % CNT coated EF-NCM showed a maximum surface displacement of about 5 nm.
However, the 0.1 wt % CNT coated EF-NCM exhibited a significantly larger maximum surface displacement of approximately 30 nm, which is markedly increased compared to 0.5 wt % CNT coated EFNCM due to the insufficient CNT framework.
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Stallard et al [2] reviewed the experimental data to give cell designers an insight into the micromechanical behaviour and properties of cathode materials and to provide a toolkit for understanding how fracture resistance can be improved.
They note: The extent of microcracking can be controlled via a variety of different particle-based strategies, the relevance of which depends upon if fracture arises from nonuniform lithiation due to fast charging or anisotropic swelling of primary grains. These strategies include limiting active material strains through control of the voltage or composition, decreasing the size of primary or secondary particles and enhancing their ionic conductivity.
Song et al [3] show that cathodes with thick coating have wider and deeper cracks. Thinner cathode coatings can withstand larger tensile and shear stresses compared to thicker cathode coatings, thus thicker cathode coatings experience more severe damage.
References
- Lim, Jong-Heon, Kim, Jaehyun, Oh, Jiwoong, Kwon, Jaesub, Lee, Kyoung Eun, Lee, Youngsu, Park, Seongeun, Lim, Jun, Shin, Dongwook, Jo, Changshin, Kim, Yong-Tae, Moon, Janghyuk, Hersam, Mark C., Park, Kyu-Young, Enhancing Mechanical Resilience in Li-Ion Battery Cathodes with Nanoscale Elastic Framework Coatings, ACS Nano, American Chemical Society, 2025
- Joe C. Stallard, Laura Wheatcroft, Samuel G. Booth, Rebecca Boston, Serena A. Corr, Michaël F.L. De Volder, Beverley J. Inkson, Norman A. Fleck, Mechanical properties of cathode materials for lithium-ion batteries, Joule, Volume 6, Issue 5, 2022
- YuJie Song, Jun Wang, LiHong Liang, Thickness effect on the mechanical performance of cathodes in lithium-ion batteries, Journal of Energy Storage, Volume 86, Part B, 2024
See also: Chemistry Mechanics