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Notes on why nanowires might make better batteries

February 22, 2013

As I mentioned in my last post, I wrote a “news story” for a fellowship application about new materials for lithium ion batteries that I really wasn’t happy with. However, I realized after canning the story that the notes I wrote for myself – in the format of a series of questions followed by answers culled from the paper – were actually kind of a neat research highlight on their own.

So, I’ve rewritten them in a readable format, and in reasonably general terms. Take a look – I’d love some feedback on whether this version “works” or not.

1) So what’s the big picture? Who did this experiment, and what did they study?

This paper was on an experiment done by Linsen Li and Fei Meng in Song Jin’s research group at the University of Wisconsin-Madison. Jin’s group investigated whether a new material (nanowires made of a compound called ferric fluoride) could be used to make better cathodes for lithium ion batteries.

2) Don’t we already have good lithium ion batteries? Why do we need to make them better?

Lithium ion batteries have a pretty high energy density. This makes them really useful for portable devices where we want to be able to carry around lots of stored energy without much weight.

However, current lithium ion battery technologies have limited storage capacity. Their capacity is fine for relatively low-power devices like cell phones and laptops, but they generally can’t supply enough energy for large-scale applications like electric vehicles and storage on the power grid.

New materials (like the ferric fluoride nanowires in this paper) have the potential to solve these problems. Using these types of new materials, we might be able to make lithium ion batteries that can store more energy and are more useful for large-scale energy needs.

3) Why is ferric fluoride useful?

In order to store or release energy, lithium ions (and electrons) have to move in and out of electrodes in the battery (these electrodes are attached to the battery terminals, which connect to whatever circuit you’re trying to power).

In conventional lithium ion batteries, one of these electrodes (the cathode) is made of a material called lithium cobalt oxide. Lithium ions can insert themselves, or intercalate, into this material. However, only a certain number of lithium ions can insert themselves before the material fills up, and that limits the storage capacity of the battery.

Ferric fluoride works a little differently. It actually reacts with the incoming lithium ions to form new chemical compounds. It can theoretically react with more lithium ions than the conventional materials can take up. This means that ferric fluoride should have a higher energy density than lithium cobalt oxide does.

Ferric fluoride has two other advantages, as well. First, it’s abundant, and it’s cheaper than lithium cobalt oxide. Second, it’s also really easy to make using simple chemical reactions. This means that ferric fluoride should be a much less expensive material for these batteries.

4) So why did the ferric fluoride have to be made into nanowires?

In order for lithium ions to react with the ferric fluoride, they first has to be able to get to it. If the ferric fluoride is made into really large particles, then it is hard for the lithium ions to react with the stuff in the center. Nanowires are tiny, so almost all of the ferric fluoride is close enough to the surface of the wire that lithium ions can find it and react with it.

Nanowires are also better than other tiny nanostructures for a couple of reasons. First, the materials change size a little bit as the lithium ions and the ferric fluoride react. Nanowires can withstand these shape changes without breaking more easily than other structures can.

And second, the researchers found that bits of iron (yup, the regular old metal) formed in the chemical reaction linked up into long chains within the nanowires. Iron conducts electricity much better than ferric fluoride does, so these bits of connected iron helped transport electrons in and out of the cathode. These wires-within-wires probably wouldn’t have been able to form in other types of nanomaterials.

5) Did it work?

Mostly, yeah. The researchers found that their nanowire batteries had a much higher capacity than batteries made using either normal ferric fluoride nanoparticles or the conventional lithium cobalt oxide. The researchers also found that the chemical reactions on their ferric fluoride cathodes were reversible, which means that it should be possible to repeated charge & discharge these batteries.

But, unfortunately, when they actually tried to repeatedly charge and discharge the batteries, they found that the energy storage capacity dropped the more times the batteries were re-charged. They think that this “capacity fading” came about because not all of the iron metal formed during the discharge reaction converted back to ferric fluoride when the batteries were recharged. This meant that the batteries slowly lost their “active material” and couldn’t store as much energy.

The researchers weren’t sure why this reconversion was incomplete, but think they might be able to tweak the cathode design to minimize loss of the ferric fluoride (perhaps by using even thinner nanowires or including additives to help the reactions go to completion). If they’re successful, these batteries could be good high-capacity alternatives to current lithium ion batteries.

6) Awesome. Anything else we should know?

While this paper focused on a specific cathode design using a specific material, the researchers point out that their nanowire approach should be useful for making cathodes out of other materials, as well. They suggest that simple, controlled syntheses of these types of nanowires could be a good general approach to making cheap, high-capacity batteries.

In this paper, the researchers also only made changes to the cathode material. But batteries are complex systems, and everything has to be perfectly balanced in order to get the best performance. So, it will probably also be necessary to change the anode material or other components in order to optimize these batteries. While that will take a lot of testing and experiments, it means that future batteries using ferric fluoride nanowire cathodes should work even better than the ones the researchers reported in this paper.

7) Where’s the original paper, if I want to read it myself?

Here!

So, that’s it. Thoughts?

Personally, I think I did a better job of explaining the research in this version without getting bogged down in the chemistry. To some extent, had I written it this way the first time around, I might have been able to just remove the questions, re-organize a bit, and ended up with a credible news highlight.

I probably ought to chalk it up to writing this for the second time, and knowing that I needed to consciously avoid the jargon to which this subject lends itself so well. But I think the informal format also lent itself to more informal vocabulary and style (shorter sentences, anyone?), which certainly helped.

What do you think? Is it readable? Any parts that are still super confusing?

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One Comment
  1. Gwen Adams permalink

    Excellent, clear writing, from which even someone with only a rudimentary knowledge of chemistry could glean a mental picture of what you’re talking about. Nicely done!

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