Pore-Scale Micro-Structural Analysis of Electrode Conductance in Metal Displacement Batteries
Authors: Godinez Brizuela Omar Emmanuel , Niblett Daniel , Einarsrud Kristian Etienne 
Published: July 2022

Metal displacement batteries (MDBs), or liquid metal batteries, are an emerging technology with significant potential in providing high capacity, low-cost energy storage solutions, capable of addressing many of the challenges associated with storing energy from renewable sources. The key characteristic of metal displacement batteries is that at least one of the electrodes is in liquid state and a molten salt is used as an electrolyte.
Since its original proposal in the 1960’s liquid metal batteries have re-emerged in recent years and different battery chemistries and designs have been explored, including Ca-Bi, Na-Sb, and many others [2,10]. Recently, Na-Zn liquid metal batteries have been studied as an alternative to other configurations, showing significant potential in achieving good performance for large-scale energy storage, while avoiding the high cost associated with some electrode materials such as Nickel or Lithium [7,8].
In past years, alternatives to all-liquid cells have emerged in the form of designs where the cell materials are a mixture of solids and liquids. Examples of this include the commercially available Zebra battery, where a Na-NiCl electrode pair is used [1,6]. These designs offer some of the advantages of all-liquid cells, while simultaneously mitigating many of the disadvantages of handling and operating a very high temperature system. Na-Zn have also been proposed for solid cathode designs, taking advantage of the lower cost of Zn over Ni [4,3].
The cathode in these designs is composed of a porous structure, within which multiple chemical species can co-exist. Electrolyte components share the space with metal deposits,salt crystals, and other electrochemical reaction products. As a result, the micro-porous structure of this composite system is an important factor in determining the performance of the cell, as the spatial distribution of different materials can have an impact on the effective conductivity of the electrode [5,9]. The porous structure hosts complex multi-component mass transfer phenomena as well, potentially having an impact on the mass-transfer overpotential of the cell.
This work aims to study the impact of the microstructural properties of the solid electrode in a liquid displacement battery, and their importance to the effective conductivity of the system. We have developed a computational tool that enables us to create randomized microstructures in 3D, representing the electrode-electrolyte assembly. We are able to preserve the desired physical characteristics by using target pore-size distributions and volume fraction input as seed parameters. We use this tool to generate representative structures and analyze their effective bulk conductivity by solving Laplace’s equation over the resulting domain, accounting for the different local conductivity of each material.
This methodology is applied to a novel Na-Zn cell in order to assess the importance of the pore-scale properties of the cathode, as well as its material components, including solid Zn metal, solid NaCl deposits, and molten salt components. It is expected that different material arrangement configurations will induce heterogeneous current distributions in this system. Furthermore, the ionic composition of the electrolyte would be different at different charge levels, leading to additional variation through its charge/discharge cycle.
Using this methodology, the range of different electrode phase configurations produced during operation can be studied in the absence of microstructure imaging data. A representative elementary volume for the Zn electrode assembly is analyzed to determine the best approach for up-scaled performance predictions of the Na-Zn cell. With this method, it is possible to acquire data to elucidate desirable or undesirable electrode structure properties of this system, providing insight which can be used for improving manufacture and operation of the cell. [1] Dustmann. “Advances in ZEBRA batteries”. J. Power Sources (2004).
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Figure 1