I was looking though some research photos and I found this nice example of why post mortem battery analysis is better done by some X-ray technique. Once you start opening things up, you mix the materials around, you oxidize them, etc, unless you have a very good method, which is probably tedious.
We just had a paper published in The Journal of Materials Chemistry A about some research done at Brookhaven National Lab. It’s cool and new because it uses extremely high-power X-rays that can penetrate thick materials, even metals. The technique was developed to find points of strain inside high-performance materials like turbine blades. We use it to do the same thing, but inside batteries. And not just small batteries, but very thick ones, like D-cell batteries, which are an inch or two across.
Inside the battery, the X-rays bounce off crystal faces of the materials, and because of that you can know things about how far apart the atoms are. A D-cell has zinc at its center (anode) and manganese dioxide around its outside (cathode). The lines in the image above are like fingerprints of these materials. (And the numbers like (002) refer to the crystal faces themselves.)
Another cool thing about this technique is that it is very fast. You can scan the battery in a few minutes. This means that as it’s charging and discharging you can watch the materials changing in real time, inside the sealed battery. Basically this is what we do in the paper, shown below, seeing some things no one has ever been able to see before (except by cracking a battery open after cycling it, which can sometimes be effective, but not always). Brookhaven (on Long Island, in New York) is one of the only places in the world you can do this.
I got an email from a producer for National Geographic a few weeks ago, and they wanted to record me explaining how a citrus battery works. It’s for a new science/comedy show that comes out next year called Duck Quacks Don’t Echo.
You put nails made out of two different metals into some acidic fruit, like an orange. If one is zinc and one is copper, you essentially make a zinc-hydrogen cell. The battery half-reactions are 1) zinc electrodissolution (anode):
Zn → Zn2+ + 2e–
And 2) hydrogen formation (cathode):
2H+ + 2e– → H2
The zinc electrodissolution obviously happens on the surface of the zinc nail, and releases electrons. These electrons are at a low potential and want to flow to someplace at high potential. The hydrogen formation has a higher potential, and occurs when the protons (H+) in the fruit acid meet the electrons at the copper nail to form hydrogen gas (H2).
So if the two nails are connected, electrons will want to flow from the zinc nail to the copper nail. In between these two nails you place something like a cell phone. Flowing electrons are electricity, and so when they flow through the phone charger it’s electricity to charge the phone. This is how batteries work.
The press office at Brookhaven National Lab did a nice story about our research to study why batteries fail and what “failure” even means from a materials science perspective. Batteries are designed to be good at providing power, but unfortunately that makes it difficult to see inside them and understanding how the reaction rate, potential field, and concentrations look at any given time. Consider this: a battery might cycle, charging and discharging, perfectly normally for a year, and then suddenly “fail.” What caused that failure to occur? A slow change over time? Some sudden trigger? Something in between?
My colleagues Can Erdonmez (BNL), Dan Steingart (Princeton), and I have been designing experiments cycling alkaline batteries under several different conditions while viewing their material compositions using beamline X17B1 at the National Synchrotron Light Source. Some batteries are monitored under normal operating conditions. Other batteries are aged. And some undergo extreme conditions such as extremely high rates. You definitely see a lot of interesting things happen to the battery active material over time as a function of location in the cell. It’s well known that at high current the material nearest to the battery separator will shoulder the majority of the reaction, but now, using this powerful tool, we can observe this happening directly in a real battery.
I’ll post more on this after we publish some of the results. In the meantime we have some conference talks scheduled.
I’m getting a new website started, quietly posting things until I point my URL to it. How about a greatest hit from last year: riding CUNY’s battery-charging bike at the 2012 ARPA-E Energy Innovation Summit. You can find me in this photoset alongside other luminaries like Bill Clinton, Bill Gates, and Nancy Pelosi.
The bike charges the 4 flow-assisted Ni-Zn batteries on the table. Each battery is 35 Wh, so that’s 0.14 kWh total. The two battery electrodes are actually zinc (Zn) and nickel oxyhydroxide (NiOOH), giving the cell a nominal potential of about 1.8 V. The benefit of these batteries is that they last a long time, being able to cycle at 100% capacity more than 3000 times. There are a few tricks to doing this. One is to have the electrolyte flowing, which is why we call it a “flow-assist” battery.