Presently used experimental techniques for the characterization of tensile and compressive behavior of active layers in lithiumionbatteries have limitations of different kinds. This is particularly true for measurements of compressive properties.Furthermore, the characterizations of time-dependent stress-strain behavior are largely missing. In order to characterize thestress-strain relationship for a dry cathode active layer in lithium-ion batteries, a mechanical testing method is presented thatpreviously has been applied to the testing of optical fibers. The method is based on U-shaped bending of single-side coatedaluminum foils, which enables separate measurements of tensile and compressive properties. In particular, the method has clearadvantages for measurements of compressive properties in comparison to previously reported techniques. Relaxation experimentsare also conducted in order to characterize the time-dependent properties of the dry active layer and to check if these effectscould explain the measured hysteresis. It is found that the elastic modulus in compression is significantly larger than the elasticmodulus in tension and that the compressive modulus increases with strain level. Contrary, the tensile modulus is approximatelyindependent of strain. Furthermore, hysteresis effects are present at loading-unloading measurements, both for tension andcompression. The low values of the measured elastic moduli show that the electrode properties are largely controlled by thebinder and carbon additives. It is concluded that the development of particle-particle contacts most likely is the reason for thehigher modulus in compression in comparison to tension. The time-dependent effects are significant, primarily for shorter timescales, which explains the relaxation behavior, but they cannot fully explain the hysteresis effects. Most likely non-linear micromechanismsdo contribute as well.

The stress-strain relationship of a dry lithium-ion graphite anode coating has been characterized by a bending test method. The method is based on U-shaped bending of single-side coated electrodes, which enables separate measurements of tensile and compressive properties of the electrode coating. The experiments reveal that the elastic modulus of the anode coating in compression is higher than the elastic modulus in tension and that the compressive stiffness increases with strain level. Contrary, the tensile modulus is approximately independent of strain. The quantitative results for compressive modulus, and in particular the stiffening effect with increasing strain, are believed to be new to the battery research community. The measured stiffness of the anode coating is compared to previously reported results for a cathode coating. It is found that the anode coating is stiffer in compression compared to the cathode coating despite a much larger particle stiffness of the cathode material in comparison to the anode. It is concluded that differences in porosity are the main reason for the observed behavior. The method also successfully captures the hysteresis effects, both in tension and compression, that are present due to the polymeric binder and the evolution of microstructural contacts. Relaxation experiments are as well conducted to characterize the time-dependent properties of the anode coating, and the response is modeled by a Prony series.

A statistical representative volume element for a cathodic NMC active porous materials is developed as a periodic face-centered-cubic structure. Finite element analyses (FEA) are conducted to calculate effective elastic properties and contact force distributions for various numbers of particle–particle and binder-particle contacts. The statistics for the RVE is attained through simulations of stochastic distributions of contact conditions. The variations of effective properties with changing particle–particle and binder-particle contacts are compared to experimental results in the literature. The stiffening behavior at compressive loading is correlated with increasing interparticle connections in the RVE. The average values and the statistical distribution of particle–particle and binder-particle contact forces that result from particle swelling are as well investigated. The significance of contact force distributions, for both particle–particle and binder-particle contacts, on potential particle cracking and binder debonding is addressed. It is noted that particle–particle contact forces appear that are 4–5 times larger than their corresponding average values. Conclusions drawn from differences in average contact forces between particle–particle and particle-binder contacts as well as normalized standard deviations of contact forces are utilized to improve a previously developed analytical model for effective stiffness properties and contact forces. Excellent agreements are found in comparisons to the numerical simulations.