The stress-strain relationship for a dry cathode active layer in lithium-ion batteries has been characterized by a mechanical testing method that previously has been applied to the testing of optical fibers. The method is based on U-shaped bending of single-side coated aluminum foils, which enables separate measurements of tensile and compressive properties. In particular, the method has clear advantages for measurements of compressive properties in comparison to previously reported techniques. It is found that the elastic modulus in compression is significantly larger than the elastic modulus in tension and that the compressive modulus increases with strain level. Contrary, the tensile modulus is approximately independent of strain. Furthermore, hysteresis effects are present at loading-unloading measurements, both for tension and compression. Relaxation experiments are conducted in order to characterize the viscoelastic properties of the active layer and to check if these effects could explain the measured hysteresis. The viscoelastic response is modeled by a Prony series. The low values of the measured elastic moduli show that the electrode properties are largely controlled by the binder. Furthermore, the stiffening effect at increasing compressive loading indicates that the evolution of particle-particle and particle-binder contacts are of importance. The viscoelastic effects are significant, primarily for shorter time scales, but they can not fully explain the hysteresis effects. Most likely non-linear micro-mechanisms do contribute as well.
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