Determination of the tortuosity and contact resistances in thick graphite anodes via electrochemical impedance spectroscopy

Developing thick electrodes is vital to achieving the requirement of next-generation of lithium-ion batteries (LIBs) with high energy density. However, the thick electrode suffers from severe polarization, which makes the investigation of its internal impedance and dynamics crucial. Herein, the three types of resistances (ionic transmission resistance, ohmic resistance, and contact resistance) dominated by the composition and microstructure of thick graphite anodes are systematically explored by utilizing electrochemical impedance spectroscopy measurements on the symmetrical cell. Firstly, the relationship between tortuosity (τ) calculated from ionic transmission resistance and porosity (ε) of electrodes is studied, which conforms to the deformed Bruggeman equation (τ=fε−α). And the effect of different morphologies of mesocarbon microbeads (MCMB) and flake graphite (FG) on tortuosity is analyzed, then the mechanism of improving the dynamics of thick graphite anodes through constructing vertical electrolyte channels is clarified. Secondly, the ohmic resistance produced by electronic transmission and by ionic transmission are quantitatively characterized separately. Thirdly, the factors affecting the contact resistance of electrodes are also discussed in detail, revealing that the contact area and electronic resistivity govern the contact resistance. This work contributes to preparing the thick electrodes with low resistances.