Influence of the Binder on Lithium Ion Battery Electrode Tortuosity and Performance

Abstract

The electrochemical performance of porous graphite anodes in lithium ion battery applications is limited by the lithium ion concentration gradients in the liquid electrolyte, especially at high current densities and for thick coatings during battery charging. Beside the electrolyte transport parameters, the porosity and the tortuosity of the coating are key parameters that determine the electrode's suitability for high power applications. Here, we investigate the tortuosity of graphite anodes using two water as well as three n-methyl-2-pyrrolidone based binder systems by analysis of symmetric cell impedance measurements, demonstrating that tortuosities ranging from ∼3-10 are obtained for graphite anodes of similar thickness (∼100 μm), porosities (∼50%) and areal capacity (∼3.4 mAh/cm 2 ). Furthermore, selected electrodes with tortuosities of 3.1, 4.3, and 10.2 were cycled in cells with reference electrode at charging C-rates from 0. Understanding and predicting rate limitations in lithium ion batteries with porous electrodes requires profound knowledge of not only the electrolyte transport parameters (i.e., transference number, diffusion coefficient, conductivity) and the thermodynamic factor, but also the porosity and the tortuosity of the electrodes. The tortuosity of the electrode is particularly critical because the effective electrolyte conductivity and the effective diffusion coefficient in the electrolyte directly scale with the inverse of the tortuosity, so that care has to be taken to develop electrodes with minimized tortuosity. Using experimental approaches 1-4 or 3D tomography, 5,6 it was shown that the shape of active material particles distinctly influences the electrode tortuosity. For a given active material, however, electrode tortuosity and rate capability can also be improved by the design of the electrode layer such that short diffusion distances can be obtained across the electrode. For example, improved performance was demonstrated for graphite anodes when the platelet graphite particles were aligned normal to the current collector surface by means of a magnetic field 7 or when silicon/graphite anode electrodes were laser structured. 8 In this study we focus on the role of the electrode composition, specifically the role of the binder, on its tortuosity as well as on its implication for battery performance. While in the literature the link between binder and electrochemistry is frequently studied empirically using rate capability tests 9 and long-term cycling experiments, 10-13 we focus on the correlation of electrode tortuosity with binder content/type and its effect on rate capability.In the following, the tortuosity of graphite anodes with different binders, different binder contents, and different amounts of conductive carbon additive will be determined by electrochemical impedance spectroscopy (EIS) of symmetric cells using the transmission line model approach. 3,14 In the first part of our analysis we will demonstrate the effect of binder and conductive carbon additives on electrode t...

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