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X-ray computed tomography (CT) is a promising technique for three-dimensional imaging of batteries. Synchrotrons feature bright x-ray sources that enable micro-scale imaging at a minute time-scale. With hard x-rays the field-of-view is 3 x 3 mm and the x-rays are mostly attenuated by metal current collectors and metal-oxides in the cathode of a battery. Conventional coin cells are not fit for x-ray tomography due to stainless steel being highly x-ray attenuating, even at high energy when it is possible to obtain significant percentage of transmission, view within the coin cell is limited. We demonstrate a design of coin cell with a 5 mm cutout imaging window to enable limited angles x-ray tomography but the resulting images contained high signal-to-noise ratio and the modified coin cells are difficult to handle having electrolyte leaching, electric conductivity and other issues. Several groups have developed a Swagelok cell or an alternative to it, however the design does not allow for high throughput and the compression levels, as well as air-tight sealing present an issue. The current collectors in Swagelok cells have to have graphite inserts, else the alignment of the cell presents an issue because stainless-steel current collector can block the x-ray beam, if the alignment is not perfect. This horizontal configuration is also challenging, as the size of the imaged area is restricted to 3 mm, else the transmission can be an issue, especially for the cathode side because in this horizontal configuration X-rays penetrate 3 mm or more of material from every angle of rotation. We have developed a high throughput pouch cell design for x-ray CT imaging featuring 1 cm diameter active area. The design enables imaging of lithium metal and lithium dendrites, as well as, interfaces in symmetric and actual pouch cells. The pouch cell design does not require precise alignment as imaging is done in a vertical configuration. The pouch is processed in a similar manner as it would, if designed for electrochemical testing. The time-resolved imaging shows the time-sequence of Li plating and stripping as a function of cycling rate. Unlike optical microscopes, that use large distance between the anode and cathode and have limited field-of-view, the pouch cell features micron-scale thicknesses of components and separation distances that are more representative for actual battery design. |
