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This work combines experiments and computer models in order to understand the relationships between electrode microstructure and ionic transport resistances so that one may predict cell performance from fundamental principles. A scanning electron microscope (SEM) with focused ion beam (FIB) was used to image sections of commercially made porous electrodes utilizing LiCoO2 active material. The images reveal the existence of discrete porous carbon domains in the microstructure. Further experiments indicated that these carbon domains are highly tortuous and restrict to a large degree the overall ion transport in the cathode. Two types of 3D models for correlating and predicting the electrode microstructure were explored. The first, known as the dynamic particle packing (DPP) model, is based on aggregates of spheres that move collectively in response to interparticle forces. The second is a stochastic grid (SG) model closely related to Monte Carlo techniques used in statistical physics to study cooperative and competitive phase behavior. The models use a small set of fundamental interdomain and bulk interaction parameters to generate structures from a given electrode mass composition and porosity. Both models were able to semi-quantitatively reproduce experimental tortuosity measurements of cathodes at different porosity values.

In this work, we present a mathematical model and associated experiments for describing the performance of porous electrodes under high rates of charge and discharge. By increasing the physical accuracy of porous battery modeling, we hope to enable improved design of cells for high-power applications, such as hybrid and plug-in-hybrid electric vehicles. The model includes an improved accounting of electron transfer between different-size particles or materials, including the conductive carbon additive, as well as a modified Bruggeman relation to handle liquid-phase ion transport through porous electrodes. Both types of resistance, electronic and liquid-phase ionic, are strongly coupled to particle properties, including size and volume-fraction distributions. The model is used to better understand the cause for decreased utilization of active material for relatively highly loaded lithium-ion cathodes at high discharge rates. It was found for Li x CoO 2 cathodes with loading around 1.6 mAh/cm 2 that voltage losses at 1C discharge rate are mostly governed by local interparticle resistances. At 5C discharge rate, diffusional resistance in the liquid electrolyte had the greatest influence on cell performance.

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...