More upcoming work: Probing the materials inside batteries

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This summer we accomplished a lot of further data analysis from a collaboration with researchers at Brookhaven National Lab. We’ve been using cutting edge tools there to identify the materials inside batteries without opening up the battery case, exposing the electrodes to air, or even getting dirty. We published a preliminary paper earlier in 2014, but there is still quite a bit to learn.

BATT FIG

You do this using a technique called EDXRD (or energy dispersive X-ray diffraction). You shine X-ray light with very high energy and very high intensity through the battery. This is called a “white beam” because it has a wide spectrum of wavelengths in it. (That basically means many different “colors” of X-rays. And light with all the colors in it is called “white.”) Some of the light is diffracted by the regularly-occuring patterns of atoms in the battery electrodes, and you set up a detector outside the battery (and several feet away) to measure how much light of each wavelength gets diffracted. Since you’re several feet away and aligned very carefully, you know everything you’re learning pertains to a very small “gauge volume” inside the battery. (In the above cartoon it’s enlarged many times to make it easy to see, but it’s actually cubic microns in size.)

By moving the battery around using a precise x-y-z stage, you can “look around” inside it and see what materials are at every location, provided they’re crystalline enough to diffract X-rays.

AA_capacity1

Take for example a basic rule about batteries: if you discharge them faster you will reduce the capacity you get out of them. The plot above shows discharge curves for two AA alkaline batteries. At a high drain rate of 571 mA you get about 1.7 Ah from the cell, while at 18.1 mA you get double that, about 3.4 Ah. The interesting thing is that these two batteries have entirely different material compositions inside them after discharge. In fact, if you do six different rates, you will get six batteries with six different material profiles in the electrodes. Using a powerful tool like this, you can begin to figure out the extremely complex set of reactions that happen during discharge, which are, believe it or not, largely unknown.

Extracting data from a plot

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Screen Shot 2014-08-23 at 11.02.27 AM

All the time I end up trying to extract data from a published plot, for example with the XRD traces above. My brute force method is to load the plot into some image program, then draw straight lines to all the important features. Up above I’ve drawn lines to peaks L, C, G, and F using 30 degrees as the origin. The line lengths tell you exactly where the peak maxima are, after you normalize them to a line drawn along the axis to get the scale.

Hey it’s a decent method and it works, but I was thinking how useful it would be to have a tool that reads an image file and can spit out the original data as a CSV. Turns out there are a few programs that do exactly that. I haven’t tried WebPlotDigitizer yet, but I will soon. If it’s the answer to all my hopes and dreams I’ll let you know.

A simple fuel cell

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fuel cell drawing edit

When I was a grad student I had to run a lab for undergrads to make their own fuel cells. This was a drawing I made to explain the basic components and how they went together. The metal plates on the outside are the current collectors. At the center is the membrane-electrode assembly or the MEA. It’s a piece of solid electrolyte membrane that feels a bit like a piece of rubber. On each side is a small square of catalyst, usually platinum particles.

I have to tell you, I sort of miss painting platinum onto MEAs. However, I do not miss helping people put these together. Getting all that lined up and sandwiched together is harder than it looks.

A battery scientist’s trivial dilemma

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IMG_20140712_125346

You may find yourself hunting through all the batteries at the drugstore, trying to find an LR44 to buy instead of a 303/357. All because you want to be ‘faithful’ to MnO2. (By the way, this particular day you won’t find one.)

Functionally, these button cells are essentially interchangeable, but they have different active materials inside them. The LR44 is an “alkaline” battery which has the overall reaction:

3 MnO2 + 2 Zn = Mn3O4 + 2 ZnO

The 303/357 is a silver oxide battery having the overall reaction:

Zn + Ag2O = 2 Ag + ZnO

They both give you a potential of about 1.5 V. Actually, the silver oxide battery voltage is a little higher, and its capacity is a bit bigger. But if you’ve been concentrating on MnO2 for a couple years in your work … you know … your loyalty might kick in.