Investigation of the Inner Structures of ZEBRA Cells with a Microtomograph

Abstract

Sodium/nickel cells have high energy density and are developed as a power source for electric vehicles. A microtomograph was used in order to determine the inner structure of a commercial ZEBRA cell after fabrication. The tomograms represent the absorption density in a slice 0.05 mm thick. The different gray values of the tomogram allow the identification of different material classes: air, sodium, alumina or sodium chloride, porous nickel chloride, and solid nickel. It was found that the structure of the cathode of nickel chloride is dependent on the charging state and the height inside the cell. Tomographic imaging of such cells may serve the purposes of imaging of material changes at operating temperatures and quality control during fabrication. The sodium/nickel chloride cell has a theoretical energy density of 790 Wh/kg and prototype cells have been developed at different locations.1,2 The ZEBRA batteries have been developed for more than 10 years and their specific energy is now 3 100 Wh/kg and specific power higher than 150 Wh/kg, which is more than sufficient to power traffic compatible electric vehicles.Tokoi et al. 4 showed that computed tomography may be used to image the material distribution inside a sodium sulfur battery cell, and they extracted quantitative structural data of the battery state at room temperature. Since their batteries have a rather large diameter of 64 mm, they need a 6 MeV linear accelerator in order to penetrate the cells. In a second article they show, 5 how in situ tomography may quantify the density changes of the sulfur electrode during several charge-discharge cycles at the operating temperature of 350°C.The quality control of cells in a series production is an important issue for quality. Conventional nondestructive examination methods like ultrasound, eddy current, or shadow radiography may be used to investigate the critical regions after production, but they need a well-trained eye to judge possible defects. The tomograms of the ZEBRA-type sodium nickel chloride cells show virtual cross sections, which are easier to analyze, and the defect classification of the cells may be done with sophisticated image processing programs. ZEBRA BatteryA single ZEBRA cell of the type SL09 has a square steel casing with 37 mm base length ͑Fig. 1͒. The inner structure in the present ML-type cells was changed in order to achieve higher power density: the alumina cross section now has a cloverleaf form and the inner nickel profile is replaced by two wires. However these differences change the radiographic properties of the cells only slightly, and the results of this work would be correct for ML-type cells also. The interior of the SL09 cell is described in Fig. 2. A beta alumina tube is contacted by thin steel strip springs. At the center of the alumina tube two solid nickel collectors hold a carbon felt which divides the tube interior in two half cylinders. These half cylinders are filled with porous nickel, salt granules, and NaAlCl 4 electrolyte. After heating the cell above 270°C it ...

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An X-ray Tomographic Study of Rechargeable Zn/MnO2 Batteries

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Recent developments in X-ray diffraction/scattering computed tomography for materials science

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