In situ tracking of ion insertion in iron phosphate olivine electrodes via electrochemical quartz crystal admittance

LiFePO₄ is one of most promising cathode materials for lithium-ion batteries (LIBs) due to its superior rate handling ability, moderate cost, low environmental hazards, and safe long-term cyclability. In addition to the electrochemical information on the charge and discharge process, electrochemical quartz crystal admittance (EQCA) of LIB electrodes provides direct access to potential-driven frequency shifts (Δf_exp) and changes of the resonance peak width (ΔΓ) due to Li-ions insertion/extraction. It is not only possible to monitor mass changes of the electrode, but the two parameters Δf_exp and ΔΓ also reflect mechano-structural changes caused by hydrodynamic solid-liquid interactions from the operation of a LIB. Applying a suitable model that takes into account such interactions, potential-induced changes of the effective thickness and permeability of the composite electrode have been determined. The latter shows that ion insertion/extraction results in a nonuniform deformation of the electrode. Using EQCA as a unique mechanical probe for insertion-type electrodes, the dynamic effect of the local host environment on Na ⁺-ions insertion/extraction has been studied in a mixed solution of Li- and Na-salts. As a highly reliable and quantitative tool, EQCA may enable a broader understanding of coupled electrochemical and mechanical events in LIB during their long-term operation. This includes information about the distortion/deformation of the electrode intercalation particles and the entire composite electrode under polarization. Also, EQCA can help to clarify the role of polymeric binder in the composite electrodes as the factor stabilizing long-term cyclability of Li-ions batteries.