4 ms·

There are a couple things wrong with what you said, here's just a clarification after I've read the paper and some of the references a few times. 1) The cross

• logfromblammo 10y ago
  1. I assumed it would be a cylindrical cell, or a sandwich, with the copper conductor common to both sides. The cathode was described as an ink, with sulfur as one component. The copper is not part of the cathode, but a means to conduct electrons to and from it. I didn't see anything about how this cathode was processed, so I assumed it had a gel-like consistency, and the individual components retain some physical mobility. The charge carriers would be in aqueous solution, and the liquid solute would be stabilized by support molecules to keep it in place. It might need to be liquid at first to make a good connection between electrolyte glass and copper conductor, then dries out. Even ink that feels dry to the touch can retain enough water to still be considered aqueous chemistry, right? A block of firm gelatin is still mostly water. Maybe the cathode is like that? Perhaps it is dried entirely and sealed against moisture. In that case, I'm not sure what the atoms in the cathode are doing. 2. The cathode is physically very thin, according to the diagrams I have seen. There's no room for very much of the lithium/sodium to enter it. If the lithium exited the electrolyte as ions, it could plate at the cathode, but I think most of the ions remain on the cathode side of the electrolyte glass, as close as they can get to it, then get forced to take back an electron on the cathode side. Once neutralized, they are immobilized in the glass. 3. You need something present to pull the electrons off the lithium/sodium. It can't send its own extra through the electrolyte that does not conduct electrons, only ions. The lithium is only mobile through the electrolyte when ionized. If the lithium were in direct contact with the copper, it could ionize there, but then what does the sulfur do? 4. The atomic/ionic radii in pm are Li+ 90, S 102, Na+ 116, Li 134, Na 154, S-- 170. If the mean aperture diameter in the glass is between 180 and 268 pm, lithium ions would be mobile through it as neutral atoms are immobile, and at that size neutral sulfur could enter it, but ionized sulfur could not. If the mean aperture diameter is between 232 and 308 pm, the same is true for sodium and sulfur. I know there are some macroscopic minerals useful in chemistry (i.e. zeolites) for their aperture size in relation to molecular dimensions, but I don't know how precisely we can tune man-made materials. It seems likely that we could still overproduce with an imprecise technique and then select the appropriate materials through binning, though.
  • tcpekin 10y ago
    Sure, all valid assumptions, but a lot of these are discussed in the paper. If you let me know your email, I can send you a copy. But in response to your points/anyone else who is interested, 1) It's a button or coin cell [1] (halfway down). So the conductor is the steel casing, the e- travel as you put a wire from either side wrapping around the cell. The copper current collector is only on one side. You're right in that they don't really fully describe the cathode in the methods portion. They simply state the cathode consists of a redox center (S or MnO2) embedded in electrolyte and carbon in contact with the copper, which is pressed against the glass electrolyte. It could be a slurry that they press, or something that has water, but that is never mentioned. I think the problem is that nobody really knows what the atoms at the cathode are doing. I would be hesitant to say anything is aqueous, but I guess there is the possibility. 2) Cathode is reported at 0.06 mm. They show SEM pictures of Li plated on the cathode, and directly call the cell a metal plating cell. From their conclusion, "With the Li-glass and Na-glass electrolytes, we have demonstrated in this paper one possible new strategy in which the cathode consists of plating the anode alkali-metal on a copper–carbon cathode current collector at a voltage V > 3.0 V." Additionally, from the results, they "examined the electrodes with the naked eye and with SEM EDS analysis, as shown in Fig. 2, which indeed shows lithium plated on the cathode current collector and no evidence of metallic lithium remaining on the stainless steel at the anode or the anode side of the electrolyte after full discharge of the lithium anode." 3) The e- are pulled off by the voltage applied during charging with any voltage higher than the work function of whatever state the plated Li is in (I think), causing a charge imbalance and Li ionization. This is balanced by the ionized Li moving back through the electrolyte to the anode side. The role of the sulfur is debated and I would say not understood, but they claim it's a redox center, where redox does not actually happen. Here is the key part: We therefore conclude that the sulfur acts as a redox center determining the voltage of the cell at which electrons from the anode reduce the Li+ at the electrolyte/cathode interface to plate lithium rather than reducing the sulfur, so long as the voltage remains above 2.34 V; below 2.34 V, the S8 molecules are reduced to Li2Sx (1 r<= x <= 8) and the lithium on the anode becomes exhausted after 28 days in the cell of Fig. 1. The cell reaction was no longer reversible after this full discharge. At voltages V > 2.34 V, the cell is rechargeable and the sulfur is not reduced. The Fermi level of the lithium plated on the carbon–copper composite cathode current collector is determined by the Fermi level of the cathode current collector, whereas the Fermi level of the lithium anode remains that of metallic lithium, but the cell voltage is determined by the energy of the redox couple of the unreduced redox center That is the part that I think is confusing a lot of people, myself included, on how that happens exactly. They have some evidence that supports their claim, but I'm sure that's the next paper, how this actually works. 4) With a cathode that is 60 microns thick that they dry from a slurry, there is no real basis for "apertures" through the thickness, especially in a disordered glassy structure. Instead, ion transport is through vacancy mediated diffusion. This does have a dependence on atomic radii, but from what I'm reading, is more dependent on the atom species being ionic as opposed to neutral. Again though, the important portion of the finding is that the electrolyte is solid, not liquid, and therefore during lithium deposition on either the cathode or anode, ions cannot easily preferentially grow dendritic structures as opposed to evenly plating the anode/cathode. I would agree that more characterization needs to be done of the role o the components during all stages of charging and usage. Comparing a recharged and emptied batteries in a microscope would be helpful, as well as more careful chemical analysis as to what is actually going on. [1] http://batteryuniversity.com/learn/article/types_of_battery_cells http://batteryuniversity.com/learn/article/types_of_battery_...