Operando identification of the point of spinel formation within batteries

We have a new paper using operando methods to track spinel formation inside batteries while they are discharging. Our work gives important and striking new insight into the MnO2 discharge reaction, by revealing the phase transformations normally hidden within the sealed battery and also pinpointing intermediate phases. We did this using a highly penetrating operando technique, which operates in real time at high battery discharge rate.

The spinels ZnMn2O4 (hetaerolite) and Mn3O4 (hausmannite) are the reason MnO2 cathodes cannot be recharged, and the mechanism by which they form is not agreed upon. One would like to avoid these spinels, and thus it would be great to know how they form. The MnO2 discharge begins as a single-phase proton insertion, written

MnO2 + xH2O + xe → MnO2-x(OH)x + xOH

The end of the reaction is less easy to write. The MnO2 expands as the tunnels in its crystal structure fill up with protons, and at some point phase transformations are triggered, with MnOOH being the first. After that, the results are highly dependent on the work being reported. Some of the most cited results are given below (discharge products listed on the right in bold).

MnO2 discharge mechanism literature

1, 2, and 3 disagree on when Mn3O4 forms, and 4 asserts that it doesn’t form at all and is rather a consequence of taking apart the electrode for analysis. This is important, because Mn(OH)2 can be recharged and is not a problem. 5 includes zinc in the battery, and finds that the zinc and manganese spinels form at different potentials. Clearly the reaction is complicated. (Note that these experiments also differ in electrode construction, type of discharge, and method of observation.)

Gallaway operando battery scheme

This confusion is why we decided to follow this reaction using high-energy, high-flux X-rays, which can penetrate even large batteries and can also be precisely focused on a specific location. The figure above shows X-ray diffraction data collected in the battery cathode, in a slim region directly next to the separator. From porous electrode theory you expect this to be the most active part of the cathode, and thus the fastest to discharge.

gamma-MnO2 tunnels

The type of MnO2 used for the proton-insertion written above can be called several different names: electrolytic manganese dioxide (EMD) which is a classification based on how it is produced; γ-MnO2 which is based on the crystal structure; and ε-MnO2 which is similar to γ-MnO2 but with a subtle difference (that we won’t worry about). For this discussion we’ll use the name γ-MnO2, which is a defected intergrowth of pyrolusite (β-MnO2, which has 1 × 1 tunnels in its crystal lattice) and ramsdellite (R-MnO2, which has 2 × 1 tunnels in its crystal lattice). The picture above shows a simple depiction of γ-MnO2, built of MnO6 octahedra that both corner- and edge-share, making a pattern of tunnels. The 2 × 1 ramsdellite tunnels are colored blue, and the pyrolusite tunnels are colored red.

The long and short of it is this: you would like to maintain this structure while cycling the battery. Protons are inserted into the tunnels during discharge, and on battery charge they are removed. Once this structure starts breaking down, the battery no longer performs in the same way, and won’t recharge.

gamma-MnO2 protons in tunnels

Ramsdellite and pyrolusite lead to different materials when they are fully proton inserted, shown by the following two equations. (α-MnOOH is called groutite, and γ-MnOOH is called manganite. There are quite a few names to remember when dealing with these materials, it is true.)

R-MnO2 + H2O + e = α-MnOOH + OH

β-MnO2 + H2O + e = γ-MnOOH + OH

Both of these are written with the full extent of reaction, or x = 1 in the equation from before. Each formula unit has gained one electron (MnIV became MnIII) and one proton was inserted (O2 became OOH). The protons reside in the tunnels and hydrogen bond to oxygens across the tunnels, and this shears the crystal structure slightly. If the tunnel projection along the c-direction (shown above for both empty and proton-filled structures) is approximated as a parallelogram, the protons make the acute angles slightly smaller. The dotted lines above illustrate that inserted and non-inserted structures do not match up. For example, groutite and pyrolusite cannot fit together in the same phase.

Now we have arrived at the issue: the ramsdellite and pyrolusite tunnels do not fill with protons at the same rate. One fills faster. This means one of the tunnel domains shears before the other. Since they can’t fit together after that, this also shears the crystal apart, which obviously kills any plan to maintain the structure.

gamma-MnO2 lattice shearing

Evolution of the X-ray diffraction pattern inside a discharging battery is shown above in the colorful waterfall plot. The data is collected in a 100 micron wide section directly by the battery separator during galvanostatic discharge at 100 mA. The first appearance of a new crystalline phase is α-MnOOH during the 25th XRD “map” of the cell. This is broken out in the light blue plot on the right, showing the α-MnOOH (400) reflection (with a d-spacing of about 2.66 Å, 1/d = 0.376). The ZnMn2O4 spinel forms directly afterward, and to a great extent. This result was true for every battery discharge rate tested, at every location: the spinel (sometimes ZnMn2O4, sometimes Mn3O4) always immediately followed the the α-MnOOH (400) reflection.

We took this “operando” X-ray diffraction data and combined it with a proven mathematical model for cylindrical Zn-MnO2 batteries. The model allowed us to calculate the local reaction extent at every point, a radius-dependent value of x, written xr. The phase transformation to α-MnOOH alway occurred at xr = 0.79, regardless of the discharge current or location in the battery. (It was actually a range that spanned xr = 0.78-0.81, but 0.79 was by far the most common value.) It was reliable that at xr = 0.79, α-MnOOH formed and then spinel soon followed. This implies that α-MnOOH, which has sheared apart from the rest of the crystal structure, is the precursor to spinel, and that spinel formation is not potential-dependent, contradicting the conclusions of refs. 4 and 5 above. Mn(OH)2 never formed, showing that in this situation Mn3O4 was clearly preferred, contradicting refs. 1-3.

After the full analysis the new insights into γ-MnO2 proton insertion were:

  • At all locations in the cathode, well-formed α-MnOOH occurred after insertion of 0.79 H+ per Mn atom (xr = 0.79).
  • Well-formed γ-MnOOH was never observed, despite a substantial fraction of pyrolusite in the starting material.
  • Mn(OH)2 did not form, due to the high mass loading of γ-MnO2 used. The Mn(OH)2 formation mechanism requires a higher amount of conductive surface.
  • Insertion of 0.79 H+ correlated to 104% of the ramsdellite tunnel capacity (0.76), although the ramsdellite was not fully-filled when the α-MnOOH phase was detected (i.e. the α-MnOOH was non-stoichiometric). The formula of the newly-formed α-MnOOH could not be precisely calculated, but was estimated to be greater than α-MnOOH0.88.
  • Spinel, either ZnMn2O4 (near separator) or Mn3O4 (cathode interior), formed immediately following α-MnOOH in all cases.
  • Spinel formed at the expense of α-MnOOH, confirming α-MnOOH is the reactant.
  • The bottom line, informing battery engineering with MnO2 materials chemistry: avoid the α-MnOOH phase transition, and the battery will remain rechargeable.

 

XANES tomography from NSLS

battery-figure-720px

A group I’ve worked with before at Brookhaven National Lab have just published a paper looking at LiFePO4 de-lithiation visualized by XANES tomography. The cut-away view above shows that the Li initially transports preferentially from the left side, along the y-axis. This is followed by the z-direction, and then finally isotropic de-lithiation in the shrinking core at the end. (The red arrows in the bottom row illustrate this.) The paper can be found here.

New paper: operando methods to pinpoint a phase change

Gallaway JOPS 2016 TOC figure

We have a paper in the newest volume of the Journal of Power Sources. Like much of my recent work, this is about collecting localized diffraction data inside a battery while it cycles. This let us see the sudden phase change from MnO2 to α-MnOOH, and also see that this precipitates a sudden conversion to spinel. (Which is bad.) I’ll have a full write up soon.

For now check out my new publications list for this and more.

New PNNL paper on zinc/manganese oxide energy storage

I’ve had quite a few people email asking what I think about the new paper Reversible aqueous zinc/manganese oxide energy storage from conversion reactions in Nature Energy, by authors at PNNL. Understandable, since I spend a lot of my time talking about Zn and MnO2 for electrical storage. Fair!

PNNL-zinc-manganese-oxide-battery

First: unfortunately the work is being marketed (by PNNL) with a terrible graphic of a Platonic ideal supergreen™ battery that sits in a sunlit field and emits rays of light that save the world, but that’s pretty standard for battery research these days. Once the PR department gets ahold of it, you’re waist-deep in pictures of suns, windmills, iPhones, and Teslas. Most people, even most scientists, don’t understand the many levels of hierarchy involved in battery design and engineering, so I try to overlook these kinds of silly Photoshop excursions.

Second: the innovation of the paper is that they are making a rechargeable Zn-MnO2 battery in a mildly acidic electrolyte, and getting good cycle life. The way they’re doing this is by using α-MnO2 as their cathode active material. MnO2 comes in several polymorphic forms, some of which you can see below. (I adapted this figure from a paper by Poinsignon.) They are built from MnO6 octahedra, but can distinguished by the tunnel structures in the crystal.

MnO2 types fig small

A lot of my recent work has focused on the polymorph γ-MnO2, which is an intergrowth of (a) and (b) above. The PNNL work makes an interesting discovery about α-MnO2: they see the α-MnO2 going through a conversion reaction to MnOOH, which is somewhat unexpected. As you can see in the figure above, α-MnO2 is usually thought of as a host structure, to intercalate guest ions (like Ba2+). They then see that the surface of the MnOOH is coated with a large flake-like material that originates with the sulfate electrolyte, ZnSO4[Zn(OH)2]3 · xH2O. In this respect, the reaction is a bit reminiscent of a lead-acid battery, which also involves a sulfate film.

The paper is very interesting in that it provides unexpected evidence of α-MnO2 acting in the manner of a conversion reaction. (And that’s why that term is important and shows up in the paper’s title.) Also the zinc hydroxyl sulfate flake-film is a tantalizing look at what could be a very complex cathode reaction. And I’m a sucker for complex electrochemical reactions, as I hope you know. The test bed for the research was a CR2032 form factor, which is the kind of battery that goes in my running watch. So, the picture the PR machine and the science press are painting (with that world-saving battery up above) is a bit overblown, but the electrochemistry research underpinning this paper is extremely interesting, and I hope to see more.

Manganese dioxide: the almost perfect cathode

The year is ending, and I’m wrapping up some researching findings for publication. And some exciting news: once the new year starts I’m headed to NSLS-II to use the cutting edge submicron resolution spectroscopy beamline for some experiments on a brand new battery chemistry.

First I’d like to pause to reflect on what I’ve spent the last couple years on: shallow-cycled manganese dioxide (MnO2) as a cathode material. Specifically, why we need it:

  • It’s extremely cheap
  • It’s extremely safe
  • It’s found all over the world
  • It works in aqueous (water-based) batteries

These facts make it ideal for an emerging battery market, which is large-scale, grid-level battery storage for buffering solar and wind power. This storage market will be fundamentally different than the last major new market, which emerged in the 90s: that for portable computing and electronics. Portable computing requires high energy density, with cost and safety being secondary. In contrast, the planet-wide battery deployment needed for a green (i.e. solar) future will live and die on battery cost and safety, with energy density being secondary.

MnO2 the almost perfect cathode

This leads to the almost in the title of this post: MnO2 is perfect for grid-scale battery storage (it even has high energy density) except its crystal structure breaks down at the end of discharge. This is shown in the X-ray diffraction data above. The three prominent MnO2 reflections shift to lower values of 1/d as the electrode discharges, because protons are inserted into the lattice causing it to expand or dilatate. Reaching the end of discharge, these reflections spread out and become dull (or somewhat amorphous) as other manganese oxides form, including the major discharge product groutite (α-MnOOH). Upon charge, groutite is converted back to MnO2, and the lattice shrinks as protons are de-inserted. We’ve gotten the MnO2 lattice back, but the problem is the “other” manganese oxides that formed at the end of discharge and are irreversible. Some reflections of hausmannite (Mn3O4) are now found in the electrode. Haumannite is highly resistive, and if too much forms, the electrode will fail. It doesn’t take many cycles. A few like this will do.

With funding from ARPA-E we developed a Zn-MnO2 battery that can cycle thousands of times, by limiting the discharge depth and not going all the way to 0.9 volts, as was done above. (That of course raises cost, but since zinc and manganese dioxide are cheap, the economics still come out in your favor.) Even then, we found that Mn3O4 (and its relative ZnMn2O4) still ends up forming around the MnO2 particles, giving them a resistive coating. Thankfully this doesn’t kill the battery, but it does mean there is a limit to how deeply you can discharge this way. However, by doing some very fundamental forensic-type experiments, resolving the manganese oxide crystal structures within undisturbed batteries, we learned something interesting. There are some important differences between the manganese oxides you see in operando (like those above) and the ones in a battery that has aged a while. In other words, the Mn3O4 seen above isn’t completely formed yet, and that suggests that perhaps its formation can be reversed, at least before too much time has passed. And that … is what we’ve been trying. Stay tuned for more on that.