Stresses in composite electrodes and their effect on battery design

Currently, the capacity of lithium-ion battery is mainly limited by its cathode material. Research efforts are underway to find high energy density alternatives to LiCoO2. Li-excess layered compounds are promising options, but their mechanical and electrochemical behavior needs to be understood completely before using them in place of LiCoO2. The stress and strain fields induced in the electrodes during battery operation are one of the main driving forces behind the mechanical degradation of composite electrodes (both anode and cathode). The substrate curvature method was used to measure the stress evolution in composite cathodes in a full cell configuration. It was observed that the composite cathode is subjected to tensile stresses during delithiation (battery discharge) with a peak stress reaching 1.4 MPa and compressive stresses during lithiation (battery charge) with a peak value reaching 8.4 MPa. It was also observed that the stress evolution indicated some features that correspond to phase changes in cathode particles; they were repeatable over several cycles and appear to occur approximately at a particular state of charge. Hence, in addition to providing valuable data that helps in predicting life of composite electrodes, this simple stress measurement technique could complement the elaborate and time consuming X-ray based experimental techniques in understanding the structural changes in cathodes. Finally, a simple semianalytical method was developed which, by using measured stress data, was able to estimate the pressure that a commercial Li-ion battery (jelly roll configuration) casing could undergo as a function of state of charge