The high energy battery characterization I’ve been publishing about recently isn’t the only in situ technique that can shed light on battery materials. Combining electrochemical techniques with microdiffraction can tell you a lot about the composition of an interface between a battery material and electrolyte. Microdiffraction was developed at the National Synchrotron Light Source (NSLS) by Ken Evans-Lutterodt. It’s a kind of X-ray diffraction or XRD. The beam size is extremely small though, on the micron scale, and that’s why it’s described as “micro.”

We designed a flow cell that allows X-rays to hit an interface that is electrochemically active and has an electrolyte flowing past it. The X-ray beam penetrates the cell through a thin polyethylene window, and here the cell is only 150 μm thick, meaning even relatively low energy photons get through. The beam size is remarkably small: a small oval 2 x 5 μm, and the flow cell can be moved with sub-micron precision to focus the beam on the interface while a metal layer is plating there. The metal diffracts the beam, and an area detector on the other side records circular diffraction patterns. The ring pattern tells you what metals are in the diffraction spot, and their atomic spacing.

Preliminary designs for the small flow cell are shown below in panel (a), all of which ended up being too large or unwieldy in some way. The cell would have to fit in several beamlines at NSLS, and at one in particular it would have to have freedom to rotate 60 degrees without bumping anything. Taking into account the flow tubing and electrical wires, the fit would be tight. The first design, far too long and held together by binder clips, looks comical compared to the final design shown in (b). This cell has a paper-thin flow channel carved with a laser. The channel (c) is placed in the compression rig and screwed together (d).

My lab book from the first visit to the beamline shows what kind of thing I was trying to see. I wrote “Fantastic! See zinc disappear first locally on XRD then globally by E.” (And apparently this first worked at about 7:42 PM.) What’s going on is this: zinc is on the anode tab, much as it would be in a battery, then you dissolve the zinc to discharge the battery. When all the zinc is gone, the cell potential (E) will shoot up to high voltage. But that is macroscopic, meaning an average over the entire anode. By focusing the beam on a particular microscopic spot on the anode, you see it dissolve there first, giving you an insight into that small spot in contrast to the entire anode.

At CUNY we discovered that bismuth can be used to level zinc, but the mechanism wasn’t immediately clear. Using microdiffraction to scan through a zinc layer while it was being plated with bismuth gave a fascinating profile of the two metals, shown below. The XRD signal from the primary reflections of both Bi (012) and Zn (101) were plotted, along with their ratio. The layer was about 52 μm thick. Near the anode tab at 0 μm, the layer was bismuth-poor. Traversing through the layer toward the electrolyte interface, the composition became richer in bismuth, marked by the black arrow. Thus bismuth acts as a metal surfactant on zinc. (Published here.)

At the very edge of the layer (shown by the green arrow), from 35 to 50 μm, the composition was almost entirely bismuth. The reduced bismuth signal there suggested this section could have the structure of a thin bismuth penumbra decorating the denser layer below. Since this data was collected in situ, this decoration might be a temporary structure present during electroplating, which is altered or destroyed by removing the layer from the electrolyte.

We have a new paper out, which describes work using high energy battery characterization to map the insides of batteries without opening them. “Hetaerolite Profiles in Alkaline Batteries Measured by High Energy EDXRD” is in the current issue of the Journal of the Electrochemical Society. The idea is to use X-rays with high energy and high flux to penetrate a thick battery and collect diffraction data. By setting slits to collimate the X-ray beam, a well-defined gauge volume (GV) is examined within the battery, as illustrated below:

Technically this is called energy dispersive X-ray diffraction or EDXRD. For each sample of data, you obtain three pieces of information: 1) Energy of the photons diffracted, 2) Intensity of the photons diffracted, and 3) Physical location inside the battery. This is plotted as a “diffraction contour” with energy on the x-axis, physical location on the y-axis, and log intensity of the diffraction as pixel darkness. As different crystalline materials diffract X-rays of different energy, you get an XRD fingerprint of the materials. Techniques like this are revolutionizing battery analysis, because physically opening a battery results in unacceptable mixing and oxidation.

The plot above shows diffraction contours for four AA sized alkaline batteries discharged at different rates. The anodes are mixtures of zinc and zinc oxide, with higher current leaving more zinc behind. The cathodes are mostly mixtures of Mn₃O₄ and ZnMn₂O₄, which are nearly identical materials called hausmannite and hetaerolite. Even though these materials have structures differing by only 0.04 angstroms, EDXRD distinguishes them via peak splitting, marked above by blue stars.

The interesting thing is that the highest discharge rate (143 mA) has no hausmannite, but is instead split between hetaerolite and a different material, MnOOH. This shows that these two materials are the first to form, and that hausmannite is only a later product. As discussed in another recent paper, we want to know what reaction pathway leads to hetaerolite, because it limits the rechargeable performance of alkaline batteries. The results above provide the best clue we’re aware of, suggesting there is a groutite -> hetaerolite transition.

We have a new paper out, dealing with our long-term effort to develop extremely inexpensive and safe batteries for large-scale use. The full title is “Rechargeability and economic aspects of alkaline zinc-manganese dioxide cells for electrical storage and load leveling.” The idea driving the research is that since DOE cost targets for grid-scale storage are very low ($100/kWh), battery design should begin with the cheapest and safest materials possible, instead of trying to adapt high-power lithium-ion battery designs.

Zinc and manganese dioxide fit that bill, and what the paper shows is that by reducing the depth of discharge (DOD) to some fraction of the full capacity, long cycle life can be achieved at extremely low cost. The plot below shows that at 10% DOD over 3000 cycles are typical, while at 20% DOD it is over 500. These are base designs, focusing on electrode construction, materials, porosity, etc, with no additives or specialty materials. Assuming one cycle a day, this is an 8.5 year lifespan at 10% DOD.

Cycling a fraction of the DOD like this allows a long lifespan, while keeping the cost low. This shallow-cycled Zn-MnO₂ battery comes in very low in lifetime investment cost, 2-5 ¢/kWh/cycle. Using numbers taken from the literature, this is in or below the projected range of vanadium-redox and sodium-metal-halide (ZEBRA) batteries, while being low-temperature and generally lower complexity than these. An extra benefit is that since only a fraction of the Zn-MnO₂ capacity is cycled, the reserve capacity can be used in emergencies.

Over the last few years we’ve developed these types of batteries in many shapes and sizes, supported by DOE’s ARPA-E program as well as other entities like the utility Con Edison. A couple of battery racks in the CUNY lab are shown below. A little over a year ago Urban Electric Power LLC was spun out, and in their battery labs a few blocks away on 127th Street they’ve been developing the batteries for commercialization, and refining the design for higher power and longer life.

Our analysis in the paper addresses evolution of the batteries during their lifetime. If you notice above, at 10% DOD the efficiency falls from near 90% to near 80% during cycling, due to a kind of “formation” of the materials. The analysis, which was Nilesh Ingale’s doctoral thesis, is too dense to describe in full but a combined modeling+experimental effort shows that the MnO₂ active particles in the cathode become coated with hetaerolite (ZnMn₂O₄) and this resistive film drops performance. Below you see this resistivity plotted against cycle number, showing the effect mostly occurs in the first 150 cycles. Preventing this will help efficiency, but could also impact how the batteries are made and lower cost.

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.

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.

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.