In situ tracking of hydrodynamic and viscoelastic changes in electrophoretically deposited LiFePO4 electrodes during their charging/discharging
Electrophoretically deposited (EPD) lithium ferrophosphate (LFP) electrodes (LiFePO4) containing either soft Mg(OH)(2) or rigid PVdF binders have been fabricated and tested in Li2SO4 aqueous solution. The use of Electrochemical Quartz-Crystal Microbalance with dissipation monitoring (EQCM-D) was shown to be extremely advantageous to distinguish between the effectively viscoelastic and rigid states of LFP and LFP/PVdF electrodes, respectively, based already on the raw EQCM-D (i.e. recording resonant frequency and resonance width changes of the electrode on multiple overtone orders). The approach that we developed for testing composite battery and supercapacitor electrodes is quite general, and includes mechanical characterizations of the electrodes in air, in contact with liquids and electrolyte solutions, and most importantly, during combined electrochemical and mechanical characterization of battery electrodes subjected to Li-ions insertion/extraction. A new theory of hydrodynamic admittance of porous semispherical bumps has been developed and successfully applied for the characterization of rigid porous LFP/PVdF composite electrode in its both intercalated and deintercalated states. We show that the extended Voight-type viscoelastic model describes quantitatively the intercalated and deintercalated states of LFP electrode coating containing soft Mg(OH)(2) binder. The approach based on non-gravimetric application of EQCM-D developed in this work is unique and quite promising for in-situ mechanical characterization of a large variety of battery and supercapacitor electrodes for energy-storage devices.