Materials News

Electrochemical high-speed AFM dynamically probes fast-charging battery materials

An ongoing challenge for Li-ion automotive batteries is to be able to recharge them quickly, and thus compete with the refueling times of gasoline vehicles. Such fast-charging rates force battery materials to operate at extreme rates. Historically, such high rates were avoided because they led to accelerated degradation and therefore shorter battery life. However, the new targets demand that batteries operate in this regime. Accordingly, there is a renewed scientific interest in examining the fundamental origins of degradation at these faster rates. Since corresponding degradation events are found to be localized, the macroscopic measurements of voltage, current, and temperature do not provide sufficient insights to understand degradation events. In situ or in operando imaging is therefore required to study degradation events while they are happening.

These degradation events are chemical changes such as the formation of a new solid phase due to unwanted side reactions, such as Li plating, and simultaneously result in physical changes. Hence, one can obtain many insights by tracking physical-geometrical evolution during operation. Atomic force microscopy (AFM) is a promising technique to map such geometrical evolution but is constrained by the slower image acquisition times (∼min). An additional challenge is to configure AFM for the unique electrochemical environment of operating cells.

Guangyi Shang, Zhuanfang Bi, and colleagues from Beihang University and Singapore University of Technology and Design have advanced AFM to break these constraints and provide meaningful imaging for fast-charging battery materials. The study was recently published in the Review of Scientific Instruments (doi:10.1063/5.0024425).

The researchers combined advances in AFM in other scientific disciplines to extend the capabilities of AFM to study electrochemical systems. They borrowed high-speed AFM to improve the image acquisition time from a few minutes to a few seconds. In this setting, a smaller cantilever (i.e., AFM probe) is used since it has a higher resonant frequency, lower spring constant and allows for faster scans. The downside of such a cantilever is its small dimensions. The researchers developed a custom optical detection system to accurately detect the displacements of the smaller cantilever.

A large image window was achieved using a scan stage that was actuated by two piezoelectric stacks to provide accurate, fast, and independently controlled motions in x- and y-coordinates (z-coordinate is the sample height to be mapped using the AFM). A custom electrochemical housing was also built to support the AFM tip holder, electrolyte, reference, and counter electrodes. The working electrode—the sample of interest—was mounted on the sample stage and moved relative to the electrochemical housing. The electrochemical housing had a glass window at the bottom for laser optics to detect the cantilever displacement. Such clever instrumentation allowed faster image acquisition as well as a larger image window compared to the traditional electrochemical AFM.

Figure a–c shows example scans of LiMn₂O₄—a positive electrode material for Li-ion batteries. LiMn₂O₄ particles are ∼300 nm in size and are spin-coated on a gold foil. During the experiment, these particles are charged and discharged rapidly (in 100 s each) and 10 AFM scans are captured during each operation. The surface topological changes are compared during charging and discharging (see Figure b–c). For clarity, surface evolution along one coordinate axis is shown, while the images capture a 2.5 × 2 µm² window at 0.75 frames per second.

While discussing the relevance of this study, Jonathan Larson from Lawrence Berkeley National Laboratory (not related to the study) says, “Without question, the advancement of basic energy-storage science stands to greatly benefit from the development of further creative approaches that can expand nanoscopic characterization capabilities of energy-storage interfaces in their native environment. This is acutely true for techniques that can simultaneously access combinations of relevant parameter spaces such as time, space, electrochemical condition, chemical content, electrical properties, pressure, etc.” In his opinion, the group has done a nice job of expanding the electrochemical research community’s characterization toolbox by integrating a handful of these relevant measurement parameters with their recently developed “electrochemical high-speed AFM.” The utility of the new tool to track the nanoscopic structure of electrochemically active material as a function of both time and electrochemical potential is well conveyed in their article, as well as the video provided in the supplemental material. Larson is curious to see the future work that follows.

LiMn₂O₄ is electrochemically charged-discharged (= cyclic voltammetry) and in situ atomic force microscope (AFM) scans are carried out simultaneously. (a) The electrochemical scan sweeps 0–1 V in 100 s, that is, at a 10 mV/s scan rate; subsequently, the voltage is swept back to 0 in the next 100 s. Ten AFM scans are taken during (b) charging and (c) discharging. The larger scanning window ∼micron² and smaller acquisition time ∼1 s for an electrochemical system are key breakthroughs of the new technique. The numerals in (b) and (c) correspond to voltage states in (a). Credit: Zhuanfang Bi and Guangyi Shang.

The researchers are excited about various future investigations for this technique. Other positive electrode materials such as lithium iron phosphate and lithium nickel manganese cobalt oxide will be studied, particularly over larger voltage windows where they have been known to undergo morphological changes. Another interesting possibility is to study negative electrode materials like graphite and silicon, which are known to show side reactions and volume expansion. Such in situ imaging studies will finally allow researchers to verify degradation predictions of physics-based battery models, especially at faster rates relevant to next-generation batteries.

Trigonal prismatic cage molecule enables new type of 3D covalent organic framework

Porous crystalline solids are made up of molecules connected in a network such that there is a high degree of porosity. Think of tinker toys: large, complex node molecules are coordinated to smaller linking molecules, forming two- or three-dimensional (3D) arrays. The chemistry of these molecules allows materials to be rationally designed with specific—even dynamic—structures and functionalities. Covalent organic frameworks (COFs) are a type of porous crystalline solid that, to date, have mostly been realized as stacks of two-dimensional layers. In an article published in a recent issue of the Journal of the American Chemical Society (doi:10.1021/jacs.0c07732), researchers at the University of Liverpool, UK, describe their work synthesizing a new dynamic organic cage-based 3D COF.

COFs are a cousin of the more well-studied metal–organic frameworks (MOFs), which have a wealth of 3D structures. Researcher Marc Little talks about starting with that family resemblance: “Our approach took inspiration from the three-dimensional secondary building blocks that have been used to synthesize MOFs and enabled the synthesis of topologically diverse MOF nets.” Specifically, molecules with trigonal prismatic coordination greatly expanded the variety of 3D MOF structures, so the researchers worked to design a similar molecule for COFs. They named their bespoke molecule Cage-6-NH₂ for the pendant amine group at each of the triangular prism’s six vertices and the empty cage at its center. Paired with the linear linking molecule 2,5-dihydroxyter-ephthalaldehyde (DHTPA), Cage-6-NH₂ resulted in 3D-CageCOF-1, the first example of a 3D COF with acs topology, a designation which describes a hexagonal arrangement of linked triangular prisms.

The unique porosity and chemistry of COFs and MOFs allow them to accommodate guest molecules for a variety of purposes. But, according to Little, 3D-CageCOF-1 did this in a surprising way: “Qiang Zhu, a PhD student working on the project, found that by loading the pores with different solvents, including dimethylformamide, the COF structure changed.” Another PhD student, Xue Wang, built computer models to study the structure of 3D-CageCOF-1. “Using a combination of powder x-ray diffraction and our structural models, we were able to determine that the COF structure expanded to accommodate the solvent molecules in the pores,” Little says. Moreover, this change was reversible. 3D-CageCOF-1 returned to its small-pore phase when the guest molecule was removed via thermal treatment. “Although three-dimensional COFs and MOFs are known to exhibit reversible dynamic behavior, we were surprised to find that our COF switched from a small-pore structure to a large-pore structure as we filled the pores with solvent,” Little says.

Scheme for the synthesis of 3D-CageCOF-1 from Cage-6-NH₂ and 2,5-dihydroxyterephthalaldehyde, which can be topologically represented as a triangular prism and a linear strut, respectively. Model atom colors: C, white; N, blue; O, red. H atoms are omitted for clarity. Credit: American Chemical Society.

With potential for applications in sensing, capture, and catalysis, COFs and their MOF cousins make for a rich and exciting field of research. The Liverpool team is hopeful that further research will build on this work, using the researchers’ approach to synthesize more diverse COF topologies and enable new functionalities. With the trigonal prismatic cage molecule, and its pendant amine groups, 3D-CageCOF-1 takes a big step toward achieving that goal.

3D printed colloidal microswimmers with complex shapes propelled catalytically

Algae, bacteria, or spermatozoa exhibit complex chiral motion, self-propulsion, alignment, and collective behaviors. These swimming motions are predicted to be intimately connected to their shape. Although synthetic colloidal microswimmers such as active rods and dumbbells have already shown interesting behavior in their ability to mimic biofilm formation by self-propulsion and self-assembly, they have still a rather simple shape. To explore further the diversity of motions and their relation to the shape of microparticles, the research team of Daniela Kraft from Leiden University in The Netherlands is using three-dimensional (3D) printing to fabricate active microswimmers with intricate shapes including spirals, helices, starships, and boats at the submicrometer scale (Figure a). The results are published in a recent issue of Soft Matter (doi:10.1039/D0SM01320J).

Typically, the team used two-photon polymerization, which is a 3D printing method similar to photolithography, but that uses femtosecond lasers operating at a wavelength of ∼780 nm instead of UV light. As a result, a resolution down to 100 nm can be attained. Kraft’s team used a commercial photoresist to print microparticle arrays onto fused-silica substrates at an approximate rate of 2–5 min per 100 particles, depending on their shape and volume. Before their removal from the substrate, the microparticles were coated by sputtering a 5-nm-thick layer of Pt/Pd. This coating was used as a catalytic path for self-propulsion. Indeed, when suspended in water in the presence of hydrogen peroxide, a catalytic reaction occurred that liberated oxygen and water and caused the propulsion. Finally, sonication detached the particles from the substrate and centrifugation was used to concentrate them.

The researchers found coupling between translational and rotational velocities in clockwise and anticlockwise helices of 10 µm and 4 µm length and diameter, respectively. In these microswimmer geometries, the active Pt/Pd layer was deposited at one end, propelling the particle from behind. To follow their motion, the microswimmers were suspended in an aqueous solution containing 10% hydrogen peroxide, and they were observed under an optical microscope. Their motion was followed using a particle tracking algorithm Trackpy. After examining a large batch of individual particles, it was found that their translational velocity was approximately 2.13 times higher than their rotational velocity (Figure b). Each particle had a slightly different speed due to inhomogeneities in coating and dimensions at the nanometer scale.

(a) Electron micrograph of a 3D printed boat-shaped microswimmer. (b) Translational velocity as a function of the rotational velocity for active clockwise helical microswimmers with a Pt/Pd path deposited at their end. Trajectories in 10% hydrogen peroxide of anticlockwise helices coated with Pt/Pd at their end (c) and on their side (d). Credit: The Royal Society.

“The coupling between the translational and rotational velocities is exciting from a fundamental perspective, but also from an applications perspective, as it enhances propulsion. Or, in other words, the particles swim faster,” says Kraft.

Furthermore, the location of the Pt/Pd layer could be controlled due to the printing method. During 3D printing, the orientation of the particles on the substrate can be selected. For example, helices coated on a side had a very different motion to those coated at an end (Figure c–d). Although it is still challenging to scale up the 3D printing to generate large numbers of microparticles, the results presented in this study open new capabilities to investigate the active motion of microswimmers and their behavior in dilute or concentrated suspensions. They may provide new strategies to develop drug delivery systems or to better understand biological systems like biofilm formation and growth.

“Such understanding is especially exciting as it may allow for mobility in otherwise challenging conditions such as in viscous or complex media,” says Ahmet Demirörs, senior scientist at ETH Zürich, Switzerland, a specialist in colloidal self-assembly and who did not take part in this study. “Furthermore, the programmability of the motion path could be also diversified and combined with external fields controls, for example by coating the particles with magnetic layers. This could enhance further the level of path control for delivery applications.” Future work by Kraft and her team will focus on understanding how shapes affect the motion and interactions of these particles, to compare with biological microswimmers and ultimately to propose design principles for smart self-propelled microrobots.