X-ray nanotomography reveals 3D microstructure of graphite anodes for lithium-ion batteries

The optimisation of battery electrode architecture is a key aspect of improving battery performance, provided that precise characterisation of the complex battery microstructure is possible. In this work, X-ray nanotomography [1] was used at beamline ID16B [2] to obtain high-resolution images of the microstructure of graphite battery electrodes, providing 3D analysis and thorough quantification of the electrode/particle inner structure and porosity at the nanoscale.

A crucial step in the production of battery-grade natural graphite for lithium-ion batteries is the spheroidisation process: the morphological change that occurs in the electrode material during cycling or charging/discharging cycles. However, the low yield (30-50%) of this process results in a large quantity of wasted graphite fines that are not suitable for use in lithium-ion batteries due to their small particle size [3]. A method was devised to recycle waste graphite fines via a re-agglomeration process followed by a petroleum pitch coating in order to obtain aggregated graphite particles with sound mechanical strength and battery-suitable size to be used for electrode preparation. A compression step called ‘calendering’ was applied to the electrode’s coating to reduce its thickness and consequently increase its volumetric capacity.

X-ray nanotomography measurements carried out at beamline ID16B provided important microstructural details of the electrode-representative volumes (128 × 128 × 108 µm3 with 50 nm voxel size), along with statistical analysis of ~500 particles imaged in a single measurement. Data acquired on non-calendered and calendered pristine electrodes show that higher electrode density could be reached by calendering the electrode, without considerably affecting the active material accessibility through diffusion in the pore network. Despite the considerable morphological changes, no clear agglomerate fractures were observed, and particle integrity was preserved as individual agglomerate particles could still be distinguished. This highlights the fact that structural integrity is maintained from the electrode scale down to the particle level, and that the calendering process does not compromise the electrochemical performance.

Read more on ESRF website

Image: lectrode and particle porosity evolution with calendering in terms of (a) pore volume fraction and (b-e) microstructure. 3D rendering views of the (b) non-calendered and (c) calendered electrodes and (d,e) corresponding isolated graphite aggregated particles (with cross-section images).

Magnetic vortices come full circle

The first experimental observation of three-dimensional magnetic ‘vortex rings’ provides fundamental insight into intricate nanoscale structures inside bulk magnets, and offers fresh perspectives for magnetic devices.

Magnets often harbour hidden beauty. Take a simple fridge magnet: Somewhat counterintuitively, it is ‘sticky’ on one side but not the other. The secret lies in the way the magnetisation is arranged in a well-defined pattern within the material. More intricate magnetization textures are at the heart of many modern technologies, such as hard disk drives. Now, an international team of scientists at PSI, ETH Zurich, the University of Cambridge (UK), the Donetsk Institute for Physics and Engineering (Ukraine) and the Institute for Numerical Mathematics RAS in Moscow (Russia) report the discovery of unexpected magnetic structures inside a tiny pillar made of the magnetic material GdCo2. As they write in a paper published today in the journal Nature Physics [1], the researchers observed sub-micrometre loop-shaped configurations, which they identified as magnetic vortex rings. Far beyond their aesthetic appeal, these textures might point the way to further complex three-dimensional structures arising in the bulk of magnets, and could one day form the basis for novel technological applications.

Mesmerising insights

Determining the magnetisation arrangement within a magnet is extraordinarily challenging, in particular for structures at the micro- and nanoscale, for which studies have been typically limited to looking at a shallow layer just below the surface. That changed in 2017 when researchers at PSI and ETH Zurich introduced a novel X‑ray method for the nanotomography of bulk magnets, which they demonstrated in experiments at the Swiss Light Source SLS [2]. That advance opened up a unique window into the inner life of magnets, providing a tool for determining three-dimensional magnetic configurations at the nanoscale within micrometre-sized samples.

Utilizing these capabilities, members of the original team, together with international collaborators, now ventured into new territory. The stunning loop shapes they observed appear in the same GdComicropillar samples in which they had before detected complex magnetic configurations consisting of vortices — the sort of structures seen when water spirals down from a sink — and their topological counterparts, antivortices. That was a first, but the presence of these textures has not been surprising in itself. Unexpectedly, however, the scientists also found loops that consist of pairs of vortices and antivortices. That observation proved to be puzzling initially. With the implementation of novel sophisticated data-analysis techniques they eventually established that these structures are so-called vortex rings — in essence, doughnut-shaped vortices.

Read more on the PSI website

Image: Magnetic beauty within. Reconstructed vortex rings inside a magnetic micropillar.

Credit: Claire Donnelly