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This section presents the manufacturing process and assembly method for half cells, me-chanically assembled full cells, and fully printed cells. The compositions of the cathode and anode inks are also presented. All cells were assembled in coin cells for more uniform man-ufacturing, but the makeup of each type of cell differs. Half cells consist of two symmetric electrodes that represent one set of the redox reactions that comprise the full cell chemistry, typically reactions associated with a metallic anode material.

6.3.2.1 Cathode and Anode Ink Compositions

Table 6.4 presents the cathode and anode ink compositions used for this chapter. Inks were made as discussed in Chapter 4.

Table 6.4: Cathode and Anode Ink Compositions

6.3.2.2 Half Cells and Mechanically Assembled Full Cells

Half cells and mechanically assembled full cells were assembled for electrochemical testing and cycling. For half cells, both electrodes were 250µm Zn foil (Goodfellow) that was cut into 1cm² squares. For mechanically assembled full cells, 1cm² MnO2 cathodes were printed on stainless steel foil (as described in Chapter 4) and cut out with scissors, leaving a roughly 0.5mm border of stainless steel foil around the printed electrode. The same Zn foil as used in half cells was used as the anode. These cells were assembled in coin cells and immediately sealed after assembly. Coin cell assembly is discussed in detail in Section 6.3.2.4.

6.3.2.3 Fully Printed Cells

For fully printed cells, a doctor blade was used to cast the GPE solution directly on top of the printed cathode layer. Work described in this chapter was performed by a manually controlled offset doctor blade with the same aluminum plate setup as described in Chapter 3. For each batch of cells, 8 cathodes were printed on a 0.001 inch thick stainless steel foil substrate with the same size Kapton stencil as described in Chapter 3. The cathode was fully dried in air at 80 C before the first layer of electrolyte was printed. A wooden applicator (Puritan) was used to place enough electrolyte to cover all electrodes at the top of all cathodes, and the doctor blade was manually pulled forward to cover all electrodes.

Multiple passes with increasing doctor blade offset height were required in order to ensure full coverage of the cathode layer. In order to ensure cohesion between printed GPE layers, drying times were controlled to ensure good mixing between previous and currently printed layers.

Once the final layer of GPE was fully dried, a similar Kapton stencil was placed directly on top of the electrolyte, and the anode slurry was printed. No additional tape or adhesive was used to affix this final stencil as the adhesion between the Kapton and the electrolyte was adequate to prevent stencil movement. The doctor blade was separated from its offset holder such that the blade was in direct contact with the stencil, again similar to the meth-ods described in Chapter 3. Care was taken to align the anode stencil directly above the cathode below the printed electrolyte layers. Separate stencils were used in order to avoid

contamination between the cathode and anode inks. Table 6.5 presents doctor blade heights and drying times for both electrode layers and all GPE layers. All drying was performed at 80 C.

Table 6.5: Fully printed cell printing procedure

Component Layer Number Doctor Blade Height Drying Time [m]

Cathode – stencil 20

Once the anode layer was fully dried, the cells were allowed to cool back to room temper-ature. The multiple cast layers of GPE resulted in a mechanically sound layer that allowed for a free standing cell without the need for external pressure to maintain contact between the electrode and electrolyte layers. A razor blade was used to carefully cut out each cell from the surrounding excess polymer, leaving a roughly 1mm excess border of GPE around the electrode stack. The cell was then peeled off the foil substrate, being careful to minimize any bending stresses to the cell, and sealed in a coin cell.

6.3.2.4 Cell Assembly

All cells were assembled and sealed in 2032 coin cells (MTI Corp.) for electrochemical testing and cell cycling. The use of a reuseable two-electrode Swagelok cell was initially investigated, but the resulting electrode contact and pressure were found to be too inconsistent for reliable use compared to sealing in coin cells.

Cell assembly varied slightly based on the type of cell and GPE casting method. For half cells and mechanically assembled full cells, cells were assembled directly within the coin cell, building the cell as part of the coin cell assembly process. For fully printed cells, the entire cell was mechanically free standing after printing and removal from the stainless steel foil substrate, so coin cell assembly was much simpler and consisted only of additional spacers and springs.

For half cells, one Zn foil electrode was first placed down, followed by the solution cast disc of GPE, followed by the second Zn foil electrode. For mechanically assembled half cells, the printed MnO2 cathode was first placed, followed by the solution cast disc of GPE, followed by a Zn foil electrode.

Additional spacers and springs were used within coin cells in order to properly match cell thickness to the required coin cell can and lid thicknesses for proper crimping and sealing. A second spacer was used for half cells and fully printed cells prior to any electrode placement

in order to ensure good electrical contact between the positive electrode and the coin cell housing. This spacer was not used for mechanically assembled cells because the additional thickness of the stainless steel foil resulting in poor crimping during the sealing process.

Care was taken to ensure all components were properly centered and aligned within the cell stack in order to avoid electrical shorts that could arise from contacting spacers, overhanging electrodes, or misaligned GPE layers.

Table 6.6 presents the order of components for half cells, mechanically assembled (MA) full cells, and fully printed (FP) cells. Figure 6.4 visually shows the order of cell assembly by type. All spacers and springs were made from stainless steel.

Table 6.6: Coin cell components

Half Cell MA Cell FP Cell

Top coin cell lid coin cell lid coin cell lid

spring spring spring

0.5mm spacer 0.5mm spacer 0.5mm spacer 1cm² zinc foil 1cm² zinc foil

GPE GPE fully printed cell

1cm² zinc foil printed MnO2 cathode

0.5mm spacer 0.5mm spacer

Bottom coin cell can coin cell can coin cell can

Figure 6.4: Order of components inside coin cells for half cells, mechanically assembled full cells, and fully printed cells.

After cells were fully assembled, a multimeter was used to test series resistance for half cells and open circuit potential for mechanically assembled full cells and fully printed cells in order to ensure cycleability. The coin cells were then sealed with a hand powered coin cell press which crimped the outer lip of the coin cell can around the plastic ring on the lid, sealing the components inside and preventing further movement. The cells were then allowed to rest for 24 hours before beginning testing.

6.3.3 Scanning Electron Microscopy

Scanning electron microscopy was used to investigate the microstructure of the cast polymer and GPE from varying drying temperatures and environments. A tabletop scanning electron microscope (Hitachi TM-1000) was used to take all SEM images. After punching out the polymer or GPE discs, a razor blade was used to cut out a sample towards the center of each disc. These samples were then mounted on aluminum SEM stages (Ted Pella) with double-sided adhesive carbon tape.

The chamber atmosphere was pumped down to vacuum prior to engaging the electron beam. Because of the low conductivity of the polymer, the automatic focus and brightness/-contrast settings could not be reliably used. Furthermore, it was observed that the electron beam itself degraded the polymer if focused on one area for too long, so speed was required in order to capture images that accurately represented the polymer and GPE microstructure.