Ionic mass transport including electrolyte diffusivity and conductivity depends on the geometric tortuosity of the electrode. This paper compares two experimental methods that determine tortuosity based on diffusivity or conductivity. The polarization-interrupt method previously developed by our group determines tortuosity in terms of effective diffusivity. The blocking-electrolyte method proposed by Gasteiger and coworkers determines tortuosity in terms of effective ionic conductivity and is analyzed using a generalized transmission-line model to account for multiple sources of impedance. Tortuosity of several commercial-quality electrodes was measured using both methods, producing reasonable agreement between the two methods in most cases. The advantages and disadvantages of each method and variables that can affect the accuracy of the measurement, such as electrode wetting and model fitting, are discussed. For particular electrodes, one method may be advantageous or more conveniently applied than the other.
The role of carbon additives in improving the electronic conductivity of composite porous electrodes is well understood. However, there has been little work studying the effect of various carbon additives on effective ionic transport in porous electrodes. This work determines effective ionic conductivities and associated tortuosities of composite cathodes with various types of carbon additive and porosities in an alkaline system. A two-compartment direct-current method was developed to make these measurements and was validated with multiple electrolyte solutions. This experimental method was modeled using COMSOL Multiphysics in order to understand the effect of design parameters on the polarization curve. Empirical correlations were developed to predict the effect of porosity and various carbon additives on tortuosity. As expected, the results show that tortuosity decreases with porosity and increases with carbon amount. Cathodes containing BNB90 and KS6 carbon additives have the highest and the lowest tortuosity, respectively. Furthermore, for cathodes containing BNB90, tortuosity in the direction orthogonal to the direction of compression (in-plane tortuosity) was found to be less than tortuosity in the direction parallel to compression (out-of-plane tortuosity). The performance of batteries is strongly affected by the composition and microstructure of the electrodes because of these variables' connection to transport and reaction conditions. To identify this connection, several works have focused on predicting and understanding microstructure of porous media.1-11 To understand and optimize microstructure of porous electrodes, effective ionic conductivity is an important factor to accompany electronic conductivity. Although there has been much work regarding transport characteristics in lithium-ion batteries, few attempts have been made to describe transport behavior of primary alkaline batteries. This is despite the fact that alkaline batteries represent about a fourth of the worldwide battery market (namely both primary and secondary cells). It is estimated that approximately 6 × 10 9 alkaline and dry batteries are consumed yearly. 12This work focuses on understanding tortuosity and effective ionic conductivity of alkaline electrolyte in porous electrodes. Alkaline batteries use Zn as the anode, KOH as electrolyte, and a mixture of electrolytic manganese dioxide (EMD) and graphite as the cathode. In the cathode, EMD particles are not sufficiently electrically conductive, so the carbon is needed to improve electronic conductivity. 4,7 The carbon additionally acts as a lubricant and binder during manufacturing. However, this additive reduces the capacity of the battery because it occupies a portion of the volume that would otherwise contain active material. In addition, as is shown in this work, carbon additives reduce the ionic transport in the cathode, because when compressed they form dense regions that tend to plug the ioncontaining pores of the electrode. There is a trade-off between ionic and electronic ...
Yearly demand for primary alkaline batteries is around US$5 billion. Even though it is a mature technology, continued performance improvement is still possible. In common with other types of batteries, the performance of alkaline batteries is strongly affected by the composition and microstructure of the electrodes, especially the cathode. Naturally, in trying to optimize an electrode there are trade-offs between the capacity or amount of active material, and the volume fractions of conductive additive (carbon) and pores, which are each needed to transport charge. This work builds on our prior work in directly measuring transport properties of electrodes [1-2]. We focus on understanding ion transport in the alkaline cathode, which is composed of EMD (electrolytic manganese dioxide), graphitic carbon, and electrolyte-filled pores. The effects of varying carbon type, carbon amount, and overall porosity on the ionic conductivity and tortuosity were studied. The correlations between ion transport and the microstructure and electronic conductivity were investigated as well. Tested carbons included a variety of common graphite additives from TIMCAL as well as graphene. Multiple analysis tools were used, including SEM/FIB determination of microstructure, ion-transport experiments, and multi-probe determination of electronic conductivities. It was found that highly compressible expanded graphites like BNB90 actually have a more deleterious effect on overall ionic conductivity that do other additives. This suggests a tradeoff between ionic and electronic conductivity performance in the choice of graphitic additive. Furthermore, the degree of anisotropy of ion transport in the cathode for different conditions was investigated for samples containing EMD and BNB90 additive. Anisotropy of the structure of the graphite, caused by pellet compression, was observed by SEM. It was found that tortuosity is lower and ionic conductivity is higher in the direction orthogonal to the direction of compression. [1] Zacharias et al., J. Electrochem. Soc. 160, A306-A311 (2013). [2] Peterson and Wheeler, J. Electrochem. Soc. 161, A2175-A2181 (2014). Figure 1
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