Failure Analysis of Batteries Using Synchrotron-based Hard X-ray Microtomography

The overall goal of this procedure is to visualize the interior of a model battery composed of two lithium metal electrodes separated by a polymer electrolyte membrane. This is accomplished by first solvent casting a polymer electrolyte film. The second step is to assemble the samples by sandwiching a piece of the polymer electrolyte between two lithium metal electrodes and vacuum sealing in an airtight pouch.

Next, the samples are galvan statically cycled until they fail by short circuit or as long as desired. The final step is to reduce the sample size, remove the nickel tabs, and then image by hard x-ray micro tomography. Ultimately, x-ray micro tomography imaging is used to show morphological changes like dendrite growth in the lithium metal electrodes and polymer electrolyte as a function of cycling.

The main advantage of this technique over existing methods like electron microscopy, is that the resulting images show the interior of the electrode and electrolyte layers revealing structures that are hidden using traditional imaging techniques Before beginning this procedure, synthesize polystyrene, block polyethylene oxide, copolymer, or SEO using onic polymerization as referenced in the text protocol. Perform all sample preparation in an Argonne glove box where the water and oxygen levels are controlled and remain less than five parts per million dissolve. 0.3 grams of polymer in anhydrous and methyl two pyro or NMP with dry lithium brif fluoro, methane sulfonamide, or L-I-T-F-S-I salt.

Use an L-I-T-F-S-I salt to SEO mass ratio of 0.275 and an NMP to SEO mass ratio of 13.13. This quantity of polymer will yield a membrane large enough to make approximately 20 samples. Eat the solution to 90 degrees Celsius on a hot plate and allow it to stir until the polymer dissolves.

Cast all of the polymer and salt mixture onto an approximately 15 by 15 centimeter square piece of nickel foil. Using a doctor blade loosely. Cover the film with aluminum foil and allow it to dry on the casting plate at 60 degrees Celsius overnight.

After drying, peel the film from the nickel foil and allow it to dry further under vacuum at 90 degrees Celsius. Wrap the resulting freestanding film in nickel foil and store it inside an airtight box in the glove box for later use. Use a seven 16th inch diameter round metal punch to cut out two lithium metal electrodes from a roll of 99.9%pure battery grade lithium metal foil.

Then use a half inch diameter metal punch to cut out a piece of polymer electrolyte film sandwich the polymer electrolyte film between the two lithium metal electrodes and press the nickel tabs onto the electrodes. Vacuum seal the sample in an airtight pouch made of polypropylene and nylon lined aluminum. One of the lithium electrodes is easily swapped with a cathode.

If one wants to study a full battery to perform symmetric cell cycling. Place the vacuum sealed sample into an oven held at 90 degrees Celsius and cycle using electrochemical cycling equipment. Heat the sample during cycling to achieve reasonable ionic conductivity through the electrolyte membrane for safety, ensure that the sample does not approach the lithium metal melting point of 180 degrees Celsius.

Pass a current density of 0.175 milliamps per square centimeter through the sample for four hours and follow with a 45 minute rest. Next, pass a current density of negative 0.175 milliamps per square centimeter through the sample for four hours and follow with a 45 minute rest. Repeat this cycling routine as many times as desired.

After the symmetric cell is cycled, bring it back into the glove box and remove it from its pouch. Use a one eighth inch metal punch to cut out the center portion of the cell vacuum. Seal the center portion of the cell in pouch material and remove it from the glove box for transport to the synchrotron facility.

Once at the beam line, use polyamide tape to affix the sample to the sample stage. Place the metal marker roughly in the center of the sample to mark the location around which the sample will rotate. Once aligned, mount the sample onto the rotating stage in the beamline hutch.

For imaging, use 20 kilo electron volt x-rays to image the sample with an exposure time optimized for the system. Optimize the exposure time by balancing the scan time and the number of counts per image. Estimate the total scan time by multiplying the exposure time by the number of images collected here.

Use an exposure time of 300 milliseconds resulting in a scan time of five to 10 minutes. Measure the pixel size associated with the optical lenses at the beginning of every beam time shift position and align the sample on a rotation stage with respect to the detection system so that it remains in the detector's field of view as it rotates through 180 degrees. Position the sample as close to the detector as possible while ensuring that the sample does not hit the detector at any rotation angle.

Once aligned, perform a scan consisting of 1025 images collected over sample rotations between zero and 180 degrees. A series of bright field and dark field images is automatically collected during the scan. Collect bright field images by moving the sample out of the field of view.

Additionally, collect dark field images by taking images while the beam is off. Use these to normalize the sample images for in homogeneous illumination sill later response and CCD camera response Representative Galvan static cycling results for a lithium metal polymer electrolytes symmetric cell are shown here in order to apply an ionic current of 0.175 milliamps per square centimeter. A voltage of about 0.07 volts is required after cycling.

The sample is imaged using hard x-ray micro tomography. The schematic shown here illustrates how the sample is positioned with respect to the incident. Beam images are collected continuously as the sample is rotated 180 degrees.

The radiograph image shown here is an example of one of the thousands of images taken of the sample as it was rotated. The brightness of the pixels in the radiograph is proportional to the amount of x-rays transmitted through that region of the sample. The image shown here is a cross-section slice through the reconstructed dataset for the sample shown previously after reconstruction.

Fine features like the lithium metal dendrite indicated on the left side of the image are visible. A 3D rendering of this lithium dendrite is made from the stack of reconstructed x-ray images. The top and bottom electrodes are made transparent to allow one to see the morphology of the lithium dendrite shown in orange puncturing the polymer electrolyte shown in light blue After its development.

This technique paved the way for researchers in the field of electrochemical engineering to explore the reason for battery failure.