Time to go at full charge?

Li-ion batteries rule the world of mobile devices, however, they are far from perfect. To fully understand how to achieve higher capacity and longer lasting life needs nothing less than an entirely new approach. Luckily Dr Vijay Kalyanaraman, has just the thing as she explores x-ray ptychography

Li-ion electrode components are made from micron-sized particles and often nanometer-sized particles. During the battery operation, the electrode components undergo irreversible changes chemically, structurally and morphologically, which are hard to pinpoint with presently available tools.

A central mechanism that governs all Li-ion batteries is the shuttling of Li-ion between cathode and anode during a charge/discharge cycle – this is the same mechanism responsible for these irreversible changes in the electrodes. Watching the battery at work could reveal the mechanisms underlying such changes. Although the widely used tools – such as transmission x-ray microscopy (TXM) and scanning transmission microscopy (STXM) ­– are suitable to study batteries while being charged and discharged, they don’t offer sufficient spatial resolution.

Figure 1 shows a micrometer-sized particle swelling and breaking apart. What you see is Ge particles that form the anode in a germanium (Ge) based battery imaged by TXM in tomography mode (3D). As the particles that make up the electrode pulverize, they lose electrical contact with each other sabotaging battery capacity and lifetime. Ge based anodes in a Li-ion rechargeable battery could offer four times higher power than the currently used anodes. But this irreversible damage of electrodes prevents us from taking advantage of Ge based Li-ion batteries.

[caption id="attachment_65956" align="aligncenter" width="200"] Figure 1 Ge nanoparticles as the electrode of LiGe battery, before lithiation (a), after lithiation (b) andafter delithiation (c) from Energy and Environmental Science, 2014, 7, 2771. DOI: 10.1039/c4ee01384k[/caption]

What’s the crack?

During charge cycling, lithium is released from the cathode and it alloys with germanium at the anode (forming Li15Ge4) - lithiation. Since each Ge atom takes up almost four lithium ions, the volume of agglomerates increase enormously, causing certain regions to thin down and the shuttling of lithium back to the cathode. This causes a fracture at those thinner regions.

Figure 2 shows a series of images, during lithiation and delithiation, of the cathode in a Li-ion battery that are currently used in electric vehicles. What you see are agglomerates of LiFePO4 nanoparticles that form the cathode in lithium iron phosphate based batteries. The images are obtained by (2D) imaging with TXM and are colour coded to distinguish between iron atoms in two chemical states. Fe2+ (as LiFePO4) is red and Fe3+ (as FePO4) is green. Red hue upon lithiation, gradually turns into green; the reverse happens upon delithiation.

[caption id="attachment_65957" align="aligncenter" width="200"] Figure 2 chemical imaging of LiFePO4 electrode with TXM. Fully charged - green: fully discharged – red, ChemElectroChem 2015, 2, 1576-1581[/caption]

What story does this figure convey? In lithium iron phosphate batteries, while we charge our devices at the power outlet, the cathode, lithium iron phosphate, (shown in the picture) releases Li-ions and electrons that then find their way to the anode (Li –metal) through the electrolyte. While we use the device, Li-ions and electrons revert to LFP electrode – discharging. According to this mechanism, complete discharge of the battery should result in complete red hue and fully charged battery should be all green. But this is not the case. We see some regions not releasing lithium and certain regions that did not receive lithium back, indicating inhomogeneous charging and discharging.

Why is the charging/discharging not homogenous? The above only hints of the role pore structure plays within the agglomerate while charging/discharging. Both phenomena ­– mechanical cracking and inhomogeneous charging/discharging ­– reduce the capacity as well as lifetime of a battery. Materials scientists working on this need sophisticated tools to help them watch the electrode at work with nanoscale resolution to identify suitable electrode pore structures that support uniform charging.

Looking closer into the region of interest to study individual nanoparticles is not possible with TXM. This is because x-rays coming out of object are diffracted around the objective lens aperture edges, making the image blurred. X-rays don’t bend easily and therefore it is harder to focus them. Extremely complicated lens designs are required to rectify the simple objective lens. STXM is an improvement, but still suffers from low resolution as the spatial resolution is determined by the spot size.

Similarly it is difficult to perform operando experiments using electron probes for transmission electron microscopy, as this requires thinner samples. Scanning electron microscopy provides smaller spot size but is limited to surface level.

What we need is a microscope with a resolution of around 1­–10 nm; to look at individual nanoparticles (high spatial resolution) and that can reveal structural, morphological and chemical changes during battery charging/discharging.

Emerging solution

An emerging technique, called x-ray ptychography, is able to tackle the issue of resolution. It has proven potential to reveal bulk material structure at nanoscale resolution. Invented by Walter Hoppe, a German Physicist, the name derives from the optical configuration used in the experimental set up. It means interference between two Bragg reflections (Figure 3) from coherently illuminated overlapping spots.

[caption id="attachment_65958" align="aligncenter" width="200"] Figure 3. Bragg reflection[/caption]

Ptychography combines the strengths of two different techniques – coherent x-ray diffraction and scanning x-ray microscopy. Deeper probing of micrometer-sized agglomerates requires strongly penetrating x-rays especially at the higher energy end of the spectrum. The good news is that modern synchrotrons are capable of producing intense tunable x-rays over a wide range both soft (up to 2-5 KeV) and hard x-rays (2-5 KeV and above).

X-rays must also be coherent – both temporally and spatially because again we have a hurdle -related to our ability to extract the information on the material structure. There is this famous phase problem that is yet to be solved. In simple

terms, when x-rays encounter an obstacle let’s say an atom or in our case a group of atoms (nanoparticle), the photons get scattered. While the intensity of the scattering waves can be measured, information on their phase change after scattering is lost. Subsequently, the information on the structure of the nanoparticle is lost. But if the phase of the probe x-rays are fixed and known, then the phase change caused by the scatterer can be evaluated.

Monochromators are used to obtain temporally coherent x-rays while confining the x-rays by pinholes or slits makes them spatially coherent. We can look into the material in detail to distinguish structures that are even 2nm (soft x-ray ptychography) and 10 nm (hard x-ray ptychography) apart. Thus with coherent xraysthe spatial resolution is not an issue anymore.

The ptychographic trick

There is no way to obtain the information that is lost. Then, what is the Ptychographic trick? Scanning the sample with precisely controlled motors and 2D detection combined with iterative algorithms reveals the phase change of scattered x-rays indirectly.

Ptychography measures diffraction patterns from two individual points on the sample (A and B for example) with the illumination overlapping (“fold”) anywhere between 5 -50%. With the help of iterative algorithms, phase information in the overlapping region, C can be guessed from diffraction pattern at A and diffraction pattern at B, individually. Here spot B serves as a feedback center while guessing the structure from diffraction pattern at A and vice versa. This way errors occurred while assuming the structure at the spot A can be eliminated. More spots may improve accuracy.

We should note that the region C contains crucial information. Hence knowing the exact position of A and B which the precision controlled motors provide is the key to obtain accurate structure at those points. Structure of the material at the spot A and B is thus imaged. By scanning the material spot by spot the entire material can be imaged with nm accuracy (theoretically 10 nm – 0.1nm, depending on the x-ray energy).

Essentially, x-ray ptychography is nothing but STXM operating in ptychography mode. In STXM, incoherent x-rays pass through the object and a 1D detector, at each scanned spot, collects just the amplitudes of the diffracted waves. On the other hand, in ptychography, coherent x-rays pass through the object and diffracted waves from overlapping regions form an image on a 2D detector. Thus ptychography is able to offer much better spatial resolution.

In terms of the demonstrated capabilities of x-ray ptychography; with (soft) x-ray ptychography, chemical and structural information can be extracted even for thick samples (micrometer), with unprecedented spatial resolution. Soft x-rays are most desirable in detecting elements with lower atomic mass such as carbon, nitrogen and oxygen where conventional fluorescence techniques fail due to low fluorescence yields.

In ptychography, one can obtain something called an optical density image that reveals the morphology of the nanoparticles ­– such as cracks. It has been shown to be very powerful in revealing cracks in a sample of lithium iron phosphate compared to STXM. Ptychography images indicate multiple particles and cracks while STXM could only show a blurred image. And combining chemical imaging with an optical density map can be even more revealing.

Since a ptychography set-up is very similar to that of STXM – modifying already existing STXM layout at the beam lines with new hardware is sufficient. Effort is required, however, to create suitable software to collect diffraction data and reconstruct the diffraction patterns so the material structure can be evaluated, but this is achievable.

3D structure determination is another attractive feature of x-ray ptychography that is emerging. Overall x-ray ptychography is a boon to nanoscience.

Author:

 Dr Vijayalakshmi Kalyanaraman is a chemistry teacher, science communicator and a material scientist based in California