Cathode/electrolyte interface-dependent changes in stress and strain in lithium iron phosphate composite cathodes

Dynamic interactions between electrode and electrolyte species during electrochemical cycling have an impact on the interfacial kinetics and mechanical stability of the electrode materials. In this study, we employ in-situ stress and strain measurements to investigate potential-dependent mechanical changes in lithium iron phosphate (LiFePO₄) cathodes during cyclic voltammetry in LiPF₆ and LiClO₄-containing electrolytes. Analysis of the stress and strain derivatives in LiClO₄-containing electrolytes exhibits single peaks during both lithiation and delithiation that coincide with LiFePO₄ phase transformations. An additional feature in the stress and strain derivatives is observed in LiPF₆-containing electrolytes at the onset of the delithiation process. The current peak splitting in LiPF₆ is larger than in LiClO₄, and electrochemical impedance spectroscopy measurements show higher impedances in LiPF₆ versus LiClO₄-containing electrolytes in lithiated LiFePO₄. The larger current peak splitting and higher impedance in LiPF₆ electrolytes suggest the potential-dependent growth of a thick and resistive cathode/electrolyte interface (CEI) layer on LiFePO₄ cathodes. We hypothesize that kinetic limitations in Li⁺ transport through the CEI leads to additional stress and strain development at the electrode surface.