Mechanical deformation in battery electrodes incites degradation and capacity loss by causing cracks in the active material, electrical isolation of particles from the current collector, and additional surface area available for electrolyte decomposition. Surface (stress) and bulk (strain) mechanical changes occur asynchronously, and increasing our understanding of their individual contributions to deformation can aid in the design of more robust and longer-lasting batteries. One method with which to compare the complex interplay between stress and strain is electrochemical stiffness. Electrochemical stiffness is the voltage-dependent ratio of the stress derivative to strain derivative. It offers insight into the rate at which competing surface and bulk processes are occurring during electrochemical cycling. In this talk we discuss electrochemical stiffness measurements obtained for LiMn2O4 (LMO) and LiFePO4 (LFP), cathode materials chosen because of their wide-spread commercial use and differing intercalation mechanisms and crystal structures. Electrochemical stiffness measurements for LMO reveal asynchronous stress and strain behavior. An electrochemical stiffness peak shows that stress dominates prior to LMO delithiation, which suggests the existence of surface morphological changes as delithiation begins. Likewise, the stress derivative with respect to potential for LFP also shows an increased rate of stress change just prior to delithiation. To probe this behavior, we performed in-situ X-ray reflectivity (XRR) measurements on an indium tin oxide (ITO) model cathode to elucidate changes in surface composition as a function of lithiation state.

In-situ electrochemical stiffness in Li-ion composite cathodes

254th National Meeting (2017)

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