Mechanical property measurements of heterogeneous materials by selective nanoindentation: Application to LiMn2O4 cathode

Mechanical properties of composite materials for application as electrodes in batteries have been measured by means of selective statistical nanoindentation. The sample is strongly heterogeneous, as it consists of LiMn2O4 particles, carbon black and PVDF embedded in a soft and compliant epoxy matrix. For comparison, a similar composite sample of SiO2 particles in epoxy was prepared. The difference in terms of elastic modulus between the matrix and the particles is of one order of magnitude. Structural compliance and edge effects induce inconsistent tests which in return cause spurious measurements. A 2-step filtering method has been designed to overcome this problem. This automated method consists in identifying spurious tests and withdraw them from the final statistical analysis. First, nonquadratic load versus displacement curves are filtered out. Then, the Joslin-Oliver analysis is used to filter out tests with an apparent structural compliance. The method greatly improves the noise to signal ratio. After deconvolution, the E-modulus of the silica particles was measured as 69.8 GPa (±1.2). It shows the reliability of the method. 105 GPa (±7.5) was found for the LiMn2O4 particle E-modulus. After pile-up correction, the real E-modulus of LiMn2O4 particles is estimated to be 13% smaller. The developed method has been demonstrated to be an effective tool to investigate mechanical properties of composites.