Heterogeneity of porous electrodes can cause battery failure and performance deficiencies. On the other hand, some types of heterogeneity can improve performance. This study uses a multi-phase smoothed particle (MPSP) model, derived from smoothed particle hydrodynamics (SPH) and which is parameterized and validated by comparison with experimental viscosity, density, electronic conductivity, MacMullin number, and Young’s modulus of electrode films. The MPSP model simulates all major aspects of electrode production: mixing, coating, drying, and calendering, though the focus in this paper (Part 1) is on drying and calendering. Four types of electrodes are included in this study: a graphite anode and three traditional metal oxide cathodes. The model suggests how some types of heterogeneity can form during cathode and anode fabrication. The anode is more susceptible to mesoscale heterogeneities than the cathode due to differences in active particle shape and stiffness. The model and experiments show that regardless of the active material type, calendering increases the variability in electronic and ionic conductivity due to carbon and binder redistribution. This can be explained by means of the proposed multi-phase packing theory. On the other hand, calendering increases mechanical uniformity as also shown by model and experiment.
The drying process of electrodes might seem to be a simple operation, but it has profound effects on the microstructure. Some unexpected changes can happen depending on the drying conditions. In prior work, we developed the multiphase-smoothed-particle (MPSP) model, which predicted a relative increase in the carbon additive and binder adjacent to the current collector during drying. This motivated us to undertake the present experimental investigation of the relationship between the drying rate and microstructure and transport properties for a typical anode and cathode. Specifically, the drying rate was controlled by means of temperature for both an NMC532 cathode and graphite anode. The material distribution was analyzed using a combination of cross-section SEM images and the energy-dispersive X-ray spectroscopy elemental maps. The binder concentration gradients were developed in both the in- and through-plane directions. The through-plane gradient is evident at a temperature higher than 150 °C, whereas the in-plane variations resulted at all drying temperatures. The measurements identified an optimum temperature (80 °C) that results in high electronic conductivity and low ionic resistivity due to a more uniform binder distribution. Trends in transport properties are not significantly altered by calendering, which highlights the importance of the drying rate itself on the assembled cell properties.
In 1980 Steckhan and Schmidt introduced the use of 4,4’,4”-tribromotriphenylamine (1a) and several more highly halogenated derivatives for the electrocatalytic anodic oxidation of organic substrates,1 and a sizeable number of such transformations have been reported.2 The relatively modest oxidation potential of 1 (+0.78 vs Ag/0.1 M AgNO3 reference) imposes restrictions, however, on the range of substrates that can treated in this manner. For this reason, we synthesized a series of triarylamines bearing several electron-withdrawing groups with the expectation that these would be useful for effecting catalytic oxidation of substrates that high oxidation potentials.3 We had found earlier that such substances can be difficult to oxidize anodically.4We have had some success using these more electronegatively substituted triarylamines.5 However, the situation is not as simple as it may appear. For example, the generalization is often made that mediated oxidation can be effected at potentials up to 500-600 mv positive of the oxidation potential of the substrate, but the latter depends, inter alia, upon the rate of further chemical reaction of the oxidized form of the substrate. Other assumptions are that electronegative substituents increase the oxidative power of the triarylamine catalyst, but not any of its other chemical properties and that the amine cation radical is stable on the time scale of the electrolysis. As we will report here, we find that both of these assumptions are often unwarranted for triarylamines. We recently carried out a series of controlled potential electrolyses of a 10:1 mole ratio of trans-stilbene and 4,4’,4”-trimethyl-2,2’,2”-trinitrotriphenylamine (1b) in aqueous acetonitrile. Under these conditions it was found that the initially produced dark blue solution of 1b+ (the cation radical of 1b) quickly turns first violet and then red and much unreacted stilbene remains. The color change demonstrated that 1 + is unstable under these conditions. A subsequent electrolysis of 1 in the absence of stilbene, followed by flash chromatography of the electrolysis mixture afforded a red solid whose formula (from exact mass measurement by mass spectrometry) is C21H16N4O8. We assign structure 2 (a substituted N-phenylphenoxazine) to this substance because a reasonable mechanistic path to it can be written involving three consecutive ECE processes involving nucleophilic attack by water upon 1 +to introduce the oxygen atoms onto the rings.One might conclude from this that the presence of three highly withdrawing nitro groups in 1 + is responsible for its instability by increasing its electrophilicity and that a less highly nitrated derivative would not take this path. This is not so; more recently, we have found that anodic oxidation of the mononitro triphenylamine 1c in aqueous acetonitrile under the same conditions also affords a red substance to which we again assign a phenoxazine structure. Although 1b and 1c both exhibit reversible cyclic voltammograms, this is misleading. In fact, the cation radicals from both 1b and 1c both undergo rather efficient nucleophilic attack by water in experiments of longer time scale. Phenoxazine formation is apparently efficient from 1c even though its oxidation potential is much lower than that of 1b. Decomposition of triphenylamine cation radicals may be a general pattern; the tribromide 1a has also been observed to decompose during preparative scale electrolysis.6 Finally, we note that instability of cation radicals, with its concomitant undesirable impact on electrosynthetic applications, may prove to be problematical in other organic electrocatalytic systems, particularly when they bear one or more strongly electron-withdrawing groups. References 1. Schmidt, S.; Steckhan, E. Chem. Ber. 1980, 113, 577. 2. E.g.: (a) Schmidt, S.; Steckhan, E. Angew. Chem. 1979, 91, 851; (b) Steckhan, E.; Schmidt, S. J. Electroanal. Chem. 1978, 89, 215; (c) Platen, M.; Steckhan, E. Liebig’s Ann. Chem. 1984, 1563; (d) Maissant, J. M.; Bouchoule, C; Canesson, P.; Blanchard, M. J. Mol. Catal. 1983, 18, 189. 3. Wu, X.; Davis, A. P.; Lambert, P. C.; Steffen, L. K.; Toy, O.; Fry, A. J. Tetrahedron 2009, 65, 2408. 4. Halas, S. M.; Okyne, K.; Fry, A. J. Electrochim. Acta, 2003, 48, 1837. 5. Wu, X.; Davis, A. P.; Fry, A. J. Org. Lett. 2007, 9, 5633. 6. Eberson, L.; Olofsson, B. Acta Chem. Scand. 1969, 23, 2355. Figure 1
Alkenes, particularly substituted stilbenes, have long been desirable organic substrates for functionalization via anodic oxidation in the presence of nucleophiles. Though the oxidation potentials of such alkenes were relatively low, addition of electrocatalysts described by Steckhan et al proved necessary for selectively oxidizing substrate. However, the possible electrolytic decomposition of triphenyl-based electrocatalysts is demonstrated in the synthesis and electrolyses of various triarylamines. These reactions were accomplished by mixing triarylamine starting materials with nitrating agents in the presence of a weak acid and the washed products were analyzed via Gas Chromatography and Mass Spectrometry, X-ray crystallography, as well as Nuclear Magnetic Resonance Spectroscopy. The desired nitrated electrocatalysts that exhibited appreciably high oxidation potentials, on the order of 1.28 V versus reference, appeared to be quite susceptible to nucleophilic aromatic substitution by trace water in electrolyte solution. Intramolecular ring closure steadily followed via an ECE mechanism, forming Nphenylphenoxazine derivatives. Although the triarylamines were modified following previously described principles of tuning stability and generating high oxidation potentials by attaching various electron-withdrawing substituents, even the most promising electrocatalyst decomposed into a phenoxazine-based product.
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