Self-Discharge Induced Stress and Associated Mechanical Damage in Na-Ion Battery Electrodes

Background Self-discharge is known to cause chemical degradation in batteries; however mechanical behavior of electrode materials during self-discharge process has not been explored. Objective This study aims to investigate potential evolution and stress development in electrodes due to self-discharge process, and to assess the effect of self-discharge induced stresses on the electrode performance. Methods Germanium thin film electrodes were cycled against Na under galvanostatic and self-discharge conditions. A multibeam optical sensor (MOS) integrated with a custom electrochemical cell was used to carry out operando stress measurements during galvanostatic and self-discharge processes; a subsequent SEM analysis of the samples was carried out to assess the influence of stresses on mechanical integrity. Results During ~ 165 h of self-discharge, the potential evolved continuously from a fully sodiated state of 5 mV to 1.2 V vs. Na/Na + , indicating continuous loss of stored sodium-ions from Ge. As a result, electrode stresses evolved continuously throughout the OCP, changing from a compressive value of—0.5 GPa at the start of OCP to a tensile stress value of 0.71 GPa, i.e., a total stress change of 1.2 GPa without any external agency applying forces on the electrode. A prolonged (i.e., more than ~ 100 h) exposure to high tensile stress causes electrode fracture (mechanical damage). Conclusions Self-discharge not only causes irreversible electrochemical parasitic losses but also promotes mechanical damage in high volume expansion electrodes if not addressed properly. Hence, to develop high performance electrodes for rechargeable batteries, electrochemical measurements should be coupled with mechanical characterization.