In Situ Neutron Powder Diffraction Using Custom-made Lithium-ion Batteries

The overall goal of this procedure is to prepare a rechargeable electrochemical cell suitable for in situ neutron diffraction experiments. This is accomplished by first preparing a negative electrode of lithium foil, two separators and an aluminum or copper sheet. The second step is to spread a slurry of positive electrode material over the aluminum or copper sheet that will act as a current collector of the cell.

Then add wires to the electrodes. Next, stack the separators and electrodes in alternating layers, then roll the layers into a cylinder. The final steps are to insert the rolled cell into a vanadium can add electrolyte and seal the can with the wires accessible ultimately in situ neutron diffraction.

Data collected with the electrochemical cell is used to show the realtime structural changes of multiple battery components during electrochemical cycling or battery function. The main advantages of using in-situ neutron diffraction experiments are that the entire bulk of the battery is probed in a non-destructive manner with multiple components investigated simultaneously, and a potential for following the structural changes of lighter elements such as lithium and oxygen. The U core chemical cells enable this experiment and allow researcher to use gram scale synthesized material related to the significantly larger quantities required in the construction of the commissure cells.

Generally speaking, individuals who are new to the technique will struggle with constructing a high quality in situ cell that both enables the collection of high quality neutron diffraction data while optimizing electrochemical properties so often there is a balance that needs to be struck between the electrochemical properties and being able to collect high quality neutron diffraction data Begin by first preparing the positive electrode. The positive electrode material mixture has been made ahead of time and contains polyvinyl di fluoride carbon black, and an active material for the current collector. Use 20 micron thick aluminum or copper foil, cut to about 200 millimeters by 70 millimeters.

Obtain a clean, flat, smooth glass surface larger than the current collector sheet and lay it flat to adhere the foil. Apply a few drops of ethanol to the surface, then place the current collector over the drops. Add a few more drops of ethanol on top of the foil.

Smooth the foil so there are no creases. Obtain a hemisphere shaped puddle of the positive electrode slurry and place it on one end of the foil using a notch bar with a notch height of one to 200 micrometers. Spread the slurry over the current collector.

After the slurry has been spread over the collector surface, it will be dried in a vacuum oven. Gently remove the foil from the glass surface and transfer it to the oven for drying overnight at elevated temperature After overnight drying, place the collector and positive electrode onto a flat surface. To continue cut out a region of the electrode to match the dimensions of the foil of the negative electrode.

The region should include a one centimeter tab of uncoated collector foil at one end. In addition, cut a piece of the current collector material of the same size. Take the coated collector to a flat plate press.

Place it on the press coated side up and cover it with paper. Apply a pressure of 100 kilonewtons for 30 to 60 minutes to improve battery performance. After retrieving the coated collector from the press, move it and the uncoated collector material to a scale.

Measure the mass of each and compare them. Use the difference to determine the mass of the electrode mixture, and from that, the mass of the active material, the various cell components and cell casing should now be gathered. The casing is a tube made completely of vanadium, which is used because it produces virtually no signal in neutron powder diffraction data.

The tube is sealed at one end and the open end has a rubber stopper with notches to allow wires through. Next work with the foil that will be used for the negative electrode. For experiments, the electrode is made from lithium metal foil and worked with in a glove box.

For this demonstration, aluminum foil is used. Choose the thickness to minimize neutron absorption and use a sheet appropriate to the casing here. 120 millimeters by 35 millimeters.

With the foil ready, select the separator for the cell. Two polyethylene sheets are used in this video. They are cut to be slightly larger than the negative electrode about 140 millimeters by 40.

Next, move the current collector and positive electrode to a flat work surface. After laying it flat, obtain an aluminum rod or wire with a diameter of less than two millimeters, and place it on the uncovered end of the collector. Roll the foil of the tab tightly around the rod.

Do not roll the positive electrode region. Move on to work with the negative electrode, metal foil, and a piece of copper wire and roll the two in a similar manner. Here are the completed electrodes.

The final components needed before cell construction are the electrolyte solution for the battery and dental wax. For sealing the casing, the casing positive electrode separators, electrolyte and wax are ready to be transferred to an argonne filled glove box for cell construction. For this video, the cell construction will take place outside of the glove box before proceeding.

Ensure the work surface is non-metallic Start cell construction by stacking the components beginning with one of the separator strips on top of the separator. Place the current collector with the positive electrode facing up and the aluminum rod at one end. Next place the second strip of separator.

Finally put the negative electrode on top with its wound copper wire at the same end as the aluminum rod. Now beginning at the end of the stack with the aluminum and copper wire. Start rolling all the layers together tightly while rolling.

Maintain the alignment of the layers and ensure that the two electrodes do not come in contact. Once the rolling is complete, check the outside piece of separator. It should completely wrap around the roll so the electrodes are covered and will not touch the vanadium housing.

Insert the rolled stack into the vanadium can with wires protruding from the top, the copper wire and aluminum rod should extend two to three centimeters beyond the opening. Add electrolyte dropwise to the can so that a total of 1.5 milliliters is used. After adding the electrolyte seal, the can with the rubber stopper and melt dental wax over its top and around the plastic sheath of the wire.

Once the can is sealed, the construction is completed. Store the cell horizontally and allow it to age for 12 to 24 hours. This in situ neutron powder diffraction pattern data shown in black crosses is from a cell containing LSTN copper and lithium.

Prior to cell discharge, the cell was modeled using multi-phase refinement and the fit is shown as the red line through the data. The green curve at the bottom shows the difference between the data and the model. The vertical bars are reflection markers for lithium in blue, copper in red and LSTN in black.

The neutron wavelength was about 1.4 angstrum. This data is for the 1 1 5 reflection in LSTN during cycling. In the first frame is the full width at half maximum in degrees.

In the second is the peak height in arbitrary units. In the third is the scattering angle. All are given as a function of time.

Note that the width measure and the peak height change opposite to each other as the lithium content varies. The observed reversible broadening indicates the potential of two-phase behavior and may also be indicative of reversible symmetry. Lowering Additional plots show the variation of the lattice parameter and the battery potential as a function of time.

The tan bar’s highlight regions during which the discharge voltage was below one volt. This correlates with the onset of the two-phase region. The red bar highlights the interval during which the cell was allowed to relax and its potential to equilibrate.

The most important aspect of the procedure is to remember to ensure that there is good contact between the different battery layers, while also ensuring that the either electrode do not come into contact with each other or with the cell casing. After watching this video, you should have a good understanding of how to construct a high quality rolled in situ electrochemical cell for the use in neutron diffraction experiments. Good luck.