Thoughts About Glass Electrolytes

19 Mar 2017

In a recent episode of one of my favorite podcasts, the Skeptic’s Guide to the Universe, they discussed a recent paper from the John Goodenough lab. I figured I’d take a look at this paper and see what’s inside. They propose a type of battery where there’s a lithium-metal reduction on both sides of the cell. Overall, the electrochemical results they show are impressive: very high capacities with good cyclability at a decent cell potential (between 2.5 and 2.7V); though not quite as impressive as the press-release makes them sound. However, I’m not clear on how they think this reaction actually happens. Many of the clarifying details are missing, which makes it hard to properly evaluate. Regardless of the electrochemical mechanism, though, if the results can be reproduced reliably, then great!

There’s really two parts to this paper. The first is the use of a solid, glassy electrolyte to overcome the limitations of using metallic lithium anodes. Dendrite formation is a major problem and, as such, modern rechargeable batteries use graphite, resulting in a lower energy density and cell potential. Their strategy also avoids the flammable, toxic liquid electrolyte, which would be nice. If the results are reproducible, this technique could be combined with existing cathodes to give a nice increase in energy density. To be clear, though, the idea of using solid or polymer electrolytes in not new.

The second part seems to be a fundamentally new mechanism for constructing batteries. Based on the schematic they’ve drawn in figure 4, it looks like they’re oxidizing lithium metal at the anode, and then reducing lithium metal at the cathode. The anode part is fine; that’s the standard discharge reaction for lithium-metal batteries. However, having the same reduction/oxidation (“redox”) half-reaction taking place at both the cathode and the anode would give a cell potential of 0V; that is, the battery wouldn’t be able to store any energy. They claim that:

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.

The implication here is that the potential of the cathode redox couple is raised by the presence of copper. It’s not clear to me why the Fermi level¹ of plated lithium would be the level of the current collector. It is true that the chemical potential of metallic lithium would be higher than that of metallic copper, and if they were proposing an alloying mechanism, then the Fermi level would be influenced by the copper. However, they’re claiming that the lithium plates on to the copper. Presumably, anything beyond the first several atomic layers of lithium wouldn’t really be influenced by the presence of copper underneath. A simple test would have been to replace the copper with aluminum and see what affect this has on the potential, but they didn’t perform this experiment.

It’s also not clear to me what the role of the sulfur, ferrocene or manganese-oxide redox center is. The idea that these redox centers are not reduced during the reaction suggests that they serve a catalytic role, which the authors confirm in the introduction. These redox centers therefore can’t influence the thermodynamic potential of the cell, only the kinetic barriers that must be overcome to initiate redox². This would effectively raise the potential needed to charge the cell, but lower the potential achieved on discharge, and so doesn’t explain the asymmetry between the cathode and anode potentials.

Having said all that, the images they show combined with the extremely high full-discharge capacities do seem to confirm that they have moved all the lithium through the glass from one electrode to the other. If this can be reproduced, then I have an opportunity to improve my understanding of some of these foundational electrochemical principles.

There are a few other things I think are missing from the paper. First, it’s unusual that no cyclic-voltammetry (CV) was done. CV is an electrochemical technique that sweeps across a range of potentials, and the current response at each point gives information about which redox processes are occurring as well as the inherent kinetic barriers in the system. It’s a standard technique for characterizing new materials that operate by known mechanisms, let alone an entirely new mechanism such as this one.

The press release claims

high volumetric energy density and fast rates of charge and discharge

neither of which are reported in the paper. The paper does claim “high volumetric energy density” (Joules per mL) in describing previous work on sodium anodes, but never actual reports the values they achieved, instead opting for gravimetric energy density (Joules per gram). To be fair, volumetric energy density is hard to calculate for a system like this, and gravimetric values are far more common. Regarding their rate capability, they show reversible electrochemistry data at 0.25C and 0.1C rates, which would corresponding to charging your car in 4 and 10 hours respectively³. Not bad by any means, but I would characterize this as “moderate”. The authors even describe this towards the end as “The cell voltages and rates are acceptable.”, which I agree with.

In summary, the results look promising but I await replication and better description of what’s actually happening. For a paper that sounds as revolutionary as this one does, it’s noteworthy that it’s only 6 pages and has only 5 references, 3 of which are from the authors’ own research group. This is not the first paper to report “a safe, low-cost, lithium or sodium rechargeable battery of high energy density and long cycle life.” but somehow those claims never seem to be as transformative as promised. From the paper:

All that remains to be optimized is the thickness of the solid glass electrolyte, the loading and choice of the redox center in the composite cathode to provide a required voltage, and optimization of the rate of ion transfer across the cathode/electrolyte interface to obtain a desired rate performance of the stored electric power.

So what, about 5-10 years maybe…?

Footnotes

1) The Fermi Level for a species can be (non-rigorously) described as the work needed to add an electron to a material. Since battery chemistry is all about moving electrons around, Fermi levels are a key concept for understanding the underlying thermodynamics.

2) This is similar to how enzymes can only influence the rate of a bio-chemical reaction, not the underlying thermodynamic states; a second, higher-energy, reaction (eg ATP –> ADP) must be coupled to the reaction of interest in order to reverse the reaction arrow.

3) This description needs a large asterisk next to it. The authors only charged the cell partially in 10 hours but also have a significantly larger capacity than conventional materials, so it’s difficult to compare this in a “your car charges in 4 hours” kind of way.