Screening of Coatings for an All-Solid-State Battery Using In Situ Transmission Electron Microscopy

Electrode and electrolyte coatings are critical to ensure all-solid-state battery stability and long-term performance. Our screening method, based on a simple in situ TEM study helps us to determine the ideal coating material, coating thickness, single-layer or multi-layer coating, as well as base coating procedures. The coatings are applied on silicon nanoparticles.

The well-known drastic volume change of silicon during the lithiation and delithiation allows us to track lithiation and delithiation through the coating at relatively low magnification and thus, at a low electron dose rate, which ensures no electron beam damage. Today, Dr.Junbeom Park, a postdoctoral researcher from my group, will demonstrate the procedure. Dr.Janghyun Jo, a postdoctoral researcher from ER-C Institute, will also help demonstrating the procedure.

To begin, prepare a half-cut TEM grid by placing the three-millimeter TEM grids with lacy foam on a clean glass slide. Then, cut the TEM grid into half-cut grids with a razor blade. Next, disperse the titanium dioxide-coated silicon nanoparticles into acetone and drop cast onto one of the half-cut TEM grids with a pipette.

Cut a tungsten wire using a nipper into small pieces with a length of 0.5 to 1 centimeter. After mixing the two components of conductive glue on the clean slide glass, glue the tungsten wire on the half-cut grid with conductive glue. Then, cure the conductive glue by drying it at room temperature in a safe place for four hours.

Cut the tungsten wire using a nipper into small pieces with a length of approximately two centimeter and mount the tungsten wire on the electropolishing machine. Mix 50%of 1.3 moles per liter of sodium hydroxide and 50%ethanol in a 10-milliliter beaker. Set the proper movable range of a counter electrode to carry the electrolyte from the beaker.

Apply the voltage until the tungsten wire is cut into two pieces, resulting in two sharp tungsten needles. Then, load the prepared tungsten needle on the probe head. Insert the tungsten needle-loaded probe head in situ TEM holder with the drop-casted, half-cut TEM grid and small glove bag into the argon glove box.

Scratch the lithium metal with the prepared tungsten needle probe head and mount the lithium-loaded tungsten needle to the in situ TEM holder. Put the assembled in situ TEM holder into a small glove bag. After closing the small glove bag, remove it from the glove box.

Seal around the empty TEM goniometer with a large glove bag and put the closed small glove bag containing the assembled in situ TEM holder into the large glove bag. Pump and purge the large glove bag with an inert gas more than three times. Then, open the small bag and insert the assembled in situ TEM holder.

Note that the slight air exposure forms a lithium oxide layer on the lithium. This lithium oxide layer acts as the solid electrolyte. Next, connect the cables between the in situ TEM holder, its control unit, and the voltage current source.

Find the half-cut TEM grid. Then, move the goniometer to place the grid to the eucentric position of the TEM. Next, find the tungsten needle with the lithium oxide-coated lithium.

Run TEM stage wobbling. After locating the needle to eucentric height by coarse movement of the in situ holder, move the needle close to the grid by coarse XY movement of the holder, and as the needle comes closer to the grid, magnification is increased. Move the needle forward to the grid to make physical contact between the lithium oxide-coated lithium and the titanium dioxide-coated silicon nanoparticles by the fine movement of the piezo probe head of the in situ holder.

Set appropriate magnification and electron beam dose rate. Then, apply a voltage between the silicon nanoparticles and lithium using the current voltage source and capture the image series to record the lithiation and delithiation processes through the coating layer. First, load the TEM image.

Then, select the polygon selection tool and draw a polygon to the target particle. Then, measure the area of the drawn polygon and compare the measured area among various TEM images to estimate the change in the area at different points during the lithiation and delithiation process. TEM images of lithiation on titanium dioxide-coated silicon particles were obtained.

The five-nanometer coating shows that significant expansion occurred in the whole area and the coating was not broken during huge expansion. In the case of 10-nanometer coating, a relatively small expansion occurred even for a longer lithiation time and the coating was broken after two minutes. During lithiation, the five-nanometer coating case showed around twice areal expansion, while the 10-nanometer coating case showed only 1.2 times areal expansion, demonstrating that the expansion rate was six times faster in the case of the five-nanometer coating case.

Appropriate thickness of lithium oxide layer is required for a successful experiment, thus, controlling the amount of air exposure is a critical step. Following this procedure of micro-battery fabrication inside the helium-sodium ion propagation through electrode and electrolyte interfaces can also be visualized. This method of analyzing the suitability of particular coatings based on a simple in situ TEM will certainly fast forward the selection of ideal anode, cathode, and electrolyte coatings and thus the commercialization of all-solid-state batteries.