ROLE OF CATHODE ARCHITECTURE IN CONVERSION REACTION CHEMISTRIES
4.4 Performance of Architected Electrodes
Figure 4.10 shows these CVs performed at a slow scan rate of 0.01 mV s-1. The large peak to peak separation suggests that this redox couple is not truly reversible, but quasi-reversible in a similar vein as the CVs of Figure 2.1. As this is a fairly complex redox process involving the growth and dissolution of solid species, one would expect a quasi-reversible CV. For the case of nickel nanoparticles, the peak separation is 200 mV and centered around the potential observed in the GITT of Figure 3.4 for an all nitrate electrolyte. When considering the thorough characterization of nitrate reduction with a nickel nanoparticle cathode in Chapter Three, we can be confident that this quasi-reversible couple can be attributed to the reversible growth and dissolution of Li2O through nitrate reduction.
The peak to peak separation is much larger in the thin film and lattice cases, consistent with the expectation for a much smaller surface area electrode in a quasi-reversible system (the nanoparticle electrode has a surface area 2–3 orders of magnitude larger than the thin film and lattice ones).In particular, the cathodic scan for nitrite oxidation has a much larger overpotential than the anodic scan for nitrate reduction, but there is a slowly increasing current across the cathodic scan. This wide peak separation and small peak current suggests that the thin film and lattice electrodes will not be able to achieve a large current on discharge. In addition, if the discharge capacity in such a system were proportional to the cathode surface area, then we would expect a negligible absolute capacity. This all points to these low surface area electrodes being poor candidates for a cathode in this system.
Galvanostatic Discharge
Having an understanding of the rate capability of the low surface area electrodes, we attempt a galvanostatic discharge. The CVs of Figure 4.10 give a sense of the achievable currents for each cathode, and after testing a range of values, a current of 25 µA was selected for the lattice cathode. In order to fairly compare across electrode structures, the nanoparticle cathode was discharged at the same rate of 25 µA, and the thin film cathode was discharged at 10 µA so that the current normalized by electrochemical surface area would be the same. These discharges are shown in Figure 4.11.
Considering the CV behavior of the lattice and thin film electrodes, one might have expected their discharge performance to be significantly worse than expected.
However, the discharge capacity of the lattice and nickel cathodes is quite similar at the same current, clearly pointing to the role of structure in discharge capacity
Figure 4.11: First discharge of nitrate reduction on different cathodes, all of which are 1 cm in diameter. Discharged at 25 µA for the lattice and nanoparticle cathodes, and 10 µA for the thin film cathode.
for a chemistry such as nitrate reduction. Before more thoroughly discussing the different normalization schemes available to compare these discharge capacities, we first want to see if any Li2O is actually growing within the lattice structure. As before, the cathode was extracted from heated batteries inside an argon-filled glovebox and rinsed with NMA. This process was extremely delicate for the lattice electrodes, and few survived, disallowing a thorough discharge product characterization. However, enough were salvaged to allow Figure 4.12, SEM micrographs of a discharged lattice.
Figure 4.12: Nitrate reduction discharge product on lattice electrode, by SEM.
Discharge conditions similar to those of Figure 4.11 (scale bar: (A) 15 µm, (B) 20 µm, and (C) 10 µm).
Note two observations from the discharge product. First, it is growing within
the lattice structure, confirming that the pre-wetting of the electrode successfully allowed the infiltration of electrolyte. Second, the morphology of the discharge product is octahedral, suggesting that this is in fact Li2O growing according to its Wulff construction as observed for the nanoparticle cathodes in Figure 3.10. While this is not an exhaustive confirmation of the nitrate reduction chemistry, it does seem to confirm the growth of Li2O.
Having established that the lattice electrode performed as desired and accommodated the growth of Li2O, we can turn back to the discharges of Figure 4.11. There are several questions of interest, and the best way to address them is to normalize the capacity in a few different ways: by projected electrode area to get a sense of cell level capacity, by mass of nickel in a similar vein to the lithium-oxygen system, and by the true surface area of nickel to see its interplay with electrode structure. The projected electrode area is simply the area of the 1 cm diameter electrodes, 0.785 cm2. The mass of nickel was simply measured for the nanoparticles, and for the thin film and lattice it was estimated from the film thickness, and in the case of the lattice, the calculated volumes in SOLIDWORKS. These masses were too small (on the order of 100 µg) to measure accurately on a balance, but the substrate was too heavy to use a micro-balance. The surface areas of nickel for the thin film and lattice were taken from the CV measurements earlier in this chapter, and for the nanoparticles, the BET value mentioned in Chapter Three was used.
Thin film Lattice Nanoparticle electrode electrode electrode (10 µA) (25 µA) (25 µA)
Areal capacity 1.7 4.1 5.1
(mAh cm-2projected)
Gravimetric capacity 3400 8200 790 (mAh g-1nickel)
Surface capacity 8700 8300 79
(mAh m-2nickel)
Table 4.2: Normalized discharge capacity of nitrate reduction on different cathodes, all of which are 1 cm in diameter.
These normalized capacities are displayed in Table 4.2, and they indicate a few things. First, the lattice electrode has an areal capacity comparable to the nanoparti- cle one, and both of these values are similar to those found in commercial lithium-ion batteries. This clearly demonstrates the role that "useful" surface area plays in de-
termining the capacity in these types of systems. Second, the thin film and lattice electrodes have gravimetric values so large that they lose meaning. By architecting an electrode comprised of a thin film wrapped around a polymer scaffold, one can design an electrode with an arbitrarily small amount of nickel and achieve an inflated capacity value, a problem in the lithium-oxygen community which has thankfully started to be addressed in recent years. Finally, the surface capacities of the thin film and lattice electrodes are not only much larger than that of the nickel nanoparticles as discussed earlier, but they are also remarkably similar in value. This suggests that if one designs a sufficiently open pore structure, the capacity will in fact scale with surface area.