Menage-a-trois no more: new design removes need for conductive additives and polymers in battery electrodes

Battery electrode design has always required a menage-a-trois: an active material or catalyst, a conductive additive, and a polymer binder to keep everyone together.  Unfortunately, constructing batteries this way limits their energy density because it increases material mass and processing steps.  But a new study in Science by Kirshenbaum et al hopes to change this with a cool design idea that completely removes the conductive additive and binder!  Let’s see why we need this additive in the first place and what these authors invented to remove it.

Conventional battery electrodes require that the catalyst, additive, and polymer are brought together to provide fast enough charging and discharging to be technologically useful.  As batteries discharge, lithium (or another cation) leaves the anode through the electrolyte, electrons leave the anode through the external circuit to do work, and the two come back together again at the cathode.

Figure courtesy of Na9234 at wikipedia.org

Figure courtesy of Na9234 at wikipedia.org

To maximize the energy used from this process, ideally all of the electrons would find Li ions as they move into the cathode.   This is never the case because active materials in electrodes are generally poor conductors, so the electrons have difficulty moving through them to find Li sites.  To fix this, scientists developed methods to combine active materials with additional conducting material, like carbon, to create conducting pathways.  But to keep this whole arrangement together, they then had to add a polymer binder that kept the catalyst and conducting materials together on a metal current collector.  Then, electrons enter (or leave, depending on the electrode) through the current collector and use the conducting pathways to find Li ions along the active material.  The setup traditionally looks like this:

Figure courtesy of [2]

Figure courtesy of [2]

That’s all fine and dandy to generate enough power for some applications, but the whole thing begins to seem like a molecular Rube Goldberg machine.  The setup suffers from low energy density because of all the excess material mass.  Also, the full potential of the battery is still not realized because the fabrication process leads to many different electrode domains, some that have sufficient conducting pathways so Li and electrons can meet, and others that don’t.  So improving upon this design could make smaller batteries with the same amount of power and reduce costs.

Kirshenbaum et al have found a way around this design by creating cathodes out of compounds that undergo a chemical reaction upon Li intercalation.  The electrode combines silver (Ag) with vanadium diphosophate (VP2O8).  Before discharging, V has 4+ charge and Ag has 1+ charge.  As the Li intercalates and electrons move into the cathode, the electrons reduce both V to 3+ and Ag to metallic silver.  This chemical reducing process apparently creates a layer of Ag nanoparticles on VP2O8, constructing themselves into a conducting layer for the electrons!  Here’s an SEM image of the electrode – light gray blocks are VP2O8 and the white regions are the Ag conductive chains created upon intercalation and Li reduction:

Figure courtesy of [2]

Figure courtesy of [2]

Detailed experimental tests, discussed in detail in the article, indicate that this method provides a more uniform conductive pathway throughout the electrode.  This reduces the number of different domains I mentioned above that would pop up in conventional design and limit how much of the electrode was actually being used.  So this design ends up solving two problems: reduced the mass by removing the polymer and conductive additive, and found a way to use more of the electrode.

In my book, this is genius – remove what was previously seen as an essential material (conductive additive) by using the Li intercalation (an essential part of discharging process) to replace the role.  A great example of using chemistry to improve the economy of design.  I’m also excited to think about how computational simulations could help move the idea forward (which is what I do for a living, at least right now).  Silver is probably too expensive to use in large-scale production if this type of design becomes commercially viable, but simulations can help to narrow down the chemistry at the molecular level to find the best combination of electrode structure and conductive element to create these conductive layers.

All in all, a great step toward qualitatively thinking about how we construct battery electrodes.  These are the kinds of creative leaps we need to push energy density and discharging/charging rates that will be competitive in the market.

References

ResearchBlogging.org

 

 

 

1)

Kirshenbaum, K., Bock, D., Lee, C., Zhong, Z., Takeuchi, K., Marschilok, A., & Takeuchi, E. (2015). In situ visualization of Li/Ag2VP2O8 batteries revealing rate-dependent discharge mechanism Science, 347 (6218), 149-154 DOI: 10.1126/science.1257289

2)

Dudney, N., & Li, J. (2015). Using all energy in a battery Science, 347 (6218), 131-132 DOI: 10.1126/science.aaa2870

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