The overall goal of this experiment is to use a dual beam-focused ion beam system to prepare electrochemically active, all solid-state, thin film nanobatteries for in situ electrochemical cycling. This method can help answer key questions in the solid-state electrochemistry field, uncovering the nature of buried solid-solid interfaces, including their thermodynamic and electrochemical stability during cycling. The main advantage of this in situ technique is that the nanobattery is never exposed to external factors, allowing an unhindered view of the dynamic processes limiting solid-state battery operation.
The Tarascon group's preparation of the first thin film battery cross-section inspired us to go further and fabricate an electrochemically active nanobattery that enables in situ characterization. Begin by preparing a sample for the experiment. This is an example of a thin film battery used for the protocol.
The active layers of the battery are in a two-millimeter-diameter region on an aluminous substrate. Details of its structure are provided in this schematic. The substrate is aluminum oxide.
Above that, is a platinum cathode current collector, followed by a lithium cobalt oxide cathode. Next, there is a lithium phosphorus oxynitride electrolyte, an amorphous silicone anode, and a copper anode current collector. The experiment requires a dual scanning electron microscope and focused ion beam instrument.
It should be equipped with a micromanipulator. Another required instrument is a low-current potentiostat. Connect the cathode lead of the potentiostat to the stage through a shielded electric feedthrough.
Internally, connect the feedthrough to the stage with a shielded wire with an exposed tip. Use the potentiostat in constant current mode to perform a low-current noise test of the feedthrough cir. On reviewing the data, the current resolution should be less than a picoamp, as with this example.
Now connect the anode lead of the potentiostat to the micromanipulator ground cable. Here are the electrical connections at this point in the protocol. Note the sample is not yet mounted.
Introduce a copper TEM lift-out grid into the chamber. The grid should be conductive and have fingers to mount the nanobattery onto. When first mounted, the grid's fingers should be parallel to the electron beam.
In this setup, there is a 52-degree angle between the electron and ion beam. Apply double-sided carbon tape on a 25-millimeter scanning electron microscopy stub, and then mount the battery on the tape, aluminous side down. Apply conductive tape to electrically connect the current collector to the stub.
At this point, mount the stub with battery into the dual beam setup. Here are the stub and battery in the chamber, ready for the experiment. To continue, begin pumping the system down.
When ready, turn on the electron and ion beams. When loaded, the battery's current collector is normal to the electron beam direction. Tilt the sample so that the ion beam is normal to the battery's current collector.
Set the ion beam voltage and the ion beam pixel dwell time for the duration of the experiment. Begin with the current collector. Select a 20 micrometer by two micrometer region.
Then, use the focused ion beam to deposit 1.5 to two micrometers of organometallic platinum. Next, work with the thin film battery stack around the platinum deposit. Prepare to mill a rectangle with a depth one micrometer below the active film layer.
Use a step-pattern cross-sectional focused ion beam milling option to mill. Mill a symmetric region on the other side of the platinum deposit to define the nanobattery. Next, use a cross-sectional cleaning procedure in both regions to clearly expose the layered structure.
Now, tilt the battery back so its current collector faces the electron beam. The side of the nanobattery is now accessible to the ion beam. Use rectangular undercuts created in parallel J-cuts to isolate most of the nanobattery.
Then, rotate the battery 180 degrees to expose the back of the battery. Repeat the milling steps for the undercuts to isolate the bottom and sides of the nanobattery. For the next steps, rotate the sample by 180 degrees again.
At this point, insert the micromanipulator set at the park position. Slowly move the micromanipulator to the platinum on the nanobattery, and bring it into contact. Prepare to fix the micromanipulator to the platinum region of the battery.
Use the ion beam to deposit platinum to fix the two. Once the micromanipulator is attached, work to remove the nanobattery from the sample. Identify the last connected portion of the nanobattery for milling.
Ion mill this region to separate it from the sample. Raise the nanobattery vertically with the micromanipulator. Move the nanobattery to the copper lift-out grid.
Place the nanobattery in contact with the lift-out grid, and prepare to mount it there. Use the ion beam to deposit two micrometers of platinum to fix it in place. This is a different view of the nanobattery on the lift-out grid.
Next, work to detach the micromanipulator. Arrange to mill the connection between it and the nanobattery. Ion mill away the connection between the micromanipulator and the nanobattery.
Move the micromanipulator to leave a mounted nanobattery attached to the copper grid. For the next steps, tilt the stage so the ion beam is parallel to the nanobattery cross-section. Near the mounted edge of the nanobattery, use a cleaning procedure over a five-micrometer-wide section.
The cleaning procedure is to expose a clear view of the electrochemically active layers. Next, prepare to create electrical contact between the cathode current collector and the copper grid. With a focused ion beam, deposit 500-nanometer-thick platinum to connect the two.
These images show the effect of the deposition step from a different angle. Here is the nanobattery before the deposition. The platinum is evident in this image from after the deposition.
Prepare to use the ion beam to remove a three-micrometer-wide segment of the anode, anode current collector, and electrolyte. Removing the segment isolates these elements from the copper grid. This is an alternate view of the nanobattery after this step.
Isolating the anode and anode current collector before making electrical contact is the most crucial step described in the protocol. Without appropriate connection and isolation, the nanobattery will be shorted and will not cycle. Next, tilt the stage to use the ion beam to carefully apply rectangular cleaning cross-sections to the sides of the nanobattery to remove redeposited material.
After cleaning, the individual layers are distinctly visible. Engage the micromanipulator, and put it into contact with the platinum above the anode collector. Deposit 0.2-nanometer-thick platinum with the ion beam to connect the micromanipulator and current collector.
Now, at the potentiostat controls, set the current parameters and run the potentiostat in galvanostatic cycling mode to perform in situ cycling. This curve represents the electrochemical charging profile of a focused ion beam-fabricated nanobattery with a current density of 50 microamps per square centimeter, showing a capacity of 12.5 microamp hours per square centimeter. A different curve with a higher current density of 1.25 milliamps per square centimeter results in a capacity of 105 microamp hours per square centimeter.
Both curves demonstrate a 3.6-volt plateau. Here are the nanobattery charging and discharging profiles for a current density of 60 microamps per square centimeter. The charge capacity was limited to 30 minutes.
The discharge capacity was limited to two volts. This demonstrates a reversibility of around 35%Once mastered, this technique can be done in three hours if it is performed properly, though keep in mind that the length of cycling can extend as long as desired. While attempting this procedure, it is important to remember to avoid unnecessary imaging with either the E-beam or the ion beam, as this can damage the device.
Having demonstrated the fabrication of an electrochemically active nanobattery, similar nanobatteries may be transferred to an in situ TEM cycling holder for more comprehensive analysis of the interfaces during cycling. After watching this video, you should have a good understanding of how to extract a cross-section of a thin film battery and how to fabricate an electrochemically active nanobattery within a dual beam FIB.
