Direct Measurements of Effective Ionic Transport in Porous Li-Ion Electrodes

Abstract

Detailed battery models require mass-transport resistance parameters such as ionic conductivity and salt diffusivity. Effective transport properties can be related to the tortuosities of the porous layers containing the electrolyte. Nevertheless, relatively few direct tortuosity measurements have been performed for cathodes and for anodes. Tortuosities of several Li-ion cathode and anode films were determined using a previously developed polarization-interrupt method. Also, a new and more robust procedure using liquid gallium to delaminate electrodes from aluminum current collectors was developed and validated. This method was shown to be superior to the previous mechanical removal procedure, especially with regard to repeatability. Multiple experiments were performed to assess the effect of the carbon and binder amounts and porosity on electrode tortuosity. Results are well fit with a modified Bruggeman-type function, showing as expected that tortuosity is inversely related to porosity. Additionally, increasing the amount of carbon and binder increases the electrode tortuosity due to small particles plugging the pores.There is much interest in the development of lithium-ion batteries for use in high-power applications, while currently they are used mainly in smaller consumer electronic devices. One way to improve these batteries is to understand the sources of resistance within the electrodes and to be able to quantify and minimize those resistances. Much work has been done to describe the transport, kinetic and thermodynamic properties associated with porous electrodes. [1][2][3] Our research program has endeavored to improve the realism of porous electrode models for Li-ion batteries through careful consideration of ionic and electronic transport processes. 4-6 As porous electrodes are multiphase structures with irregular pores and particle shapes, a fully realistic 3D model of ionic transport in the pores is quite challenging. 6 For coarse-grained models, this complexity can be reduced by describing ionic transport in the porous electrode in terms of one effective geometric parameter, tortuosity (τ). 7-9 Tortuosity accounts for the fact that pores are not straight nor of uniform cross section. It is a collective property of the system, and is influenced by the size and shape distributions of the active material and additives, and mesoscopic heterogeneities. Tortuosity is assumed to affect liquid-phase diffusivity and conductivity similarly: 10where is the liquid volume fraction, or porosity, and κ eff and κ are the effective and intrinsic ionic conductivities, respectively. Similarly, D eff and D are the effective and intrinsic electrolyte diffusivities. The term tortuosity has more than one working definition in the scientific literature; Eqs. 1 and 2 are used to define tortuosity in this work. In order to reduce the complexity of calculating and predicting effective transport properties, a long-standing practice is to relate them solely to the porosity. A Bruggeman-type relation is often used:where the scal...

Micro-Four-Line Probe to Measure Electronic Conductivity and Contact Resistance of Thin-Film Battery Electrodes

“…The method could be adapted to measure localized ionic conductivity. 15 Additional lines would make the probe more robust mechanically in the case of lost contact with a rough material to be interrogated and would enable additional electrical measurements during a single contact with the sample that improve the estimation capability of the measurement. Moreover, the spatial variability of electrodes could be correlated with microstructural determination and mesoscale models of electrode performance.…”

Direct Measurements of Effective Electronic Transport in Porous Li-Ion Electrodes

“…The commercial cathodes were made under contract by Saft American, and more fully described elsewhere. 5 They have a fixed baseline composition (corresponding to composition 1 in Table I) and were made from a single production run. The cathode roll was then calendered to four different porosities to our specifications.…”

“…Cathode fabrication.-The cathode fabrication is based off previous work 5,11 though some changes were made. The fabrication procedure described here is designed for single-sided LiCoO 2 cathode sheets with loading around 14.85 mg cm −2 active material.…”

“…This procedure is performed for both the commercial and laboratory-made films using liquid gallium as described in Ref. 5, with the following modification. In order to increase the cathodes' mechanical stability during measurement of electronic conductivity, the exposed surface of the cut square electrode is adhered to a piece of glass prior to removal of the current collector.…”

“…Under transient conditions the overall electrode impedance is a compli- cated function of electronic and ionic conductivity, exchange-current density, and double-layer capacitance. 12 However, ionic conductivity would not be expected to have a large effect on the overall measured conductivity for transient wet experiments, because ionic conductivity is in general much smaller than electronic conductivity for these materials; 5,11 and in the experiments in which salt was absent, ionic conductivity would be zero. Nevertheless, for our purposes it is important to fully eliminate or decouple these other factors and measure only the electronic conductivity under wet conditions.…”