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 (Δfexp) 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 Δfexp 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 EQC...