C L E A N E N E R G Y T R A N S I T I O N A N D S U S T A I N A B L E T E C H N O L O G I E S

S C I E N T I F I C H I G H L I G H T S

9 6 H I G H L I G H T S 2 0 2 3 I

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

X-ray phase-contrast nanotomography was used at beamline ID16B to highlight the impact of graphite electrode calendering and particle coating on the microstructure of a graphite electrode, and its related electrochemical performance. The results could aid in the development of more efficient 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. X-ray phase-contrast nanotomography provides unique capabilities for in- depth 3D internal structural analysis of the complex architecture of battery electrodes. 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: in which the graphite flakes are mechanically shaped into nearly spherical particles. 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 (Figure 74a). Data acquired on non- calendered and calendered pristine electrodes (Figure 74b and c) 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 (Figure 74d and e). 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.

Complementary post-mortem measurements performed on dismantled electrodes (retrieved after 200 cycles) compared graphite anodes of non-coated aggregated graphite particles with pitch-coated ones. Phase-contrast images revealed the presence of an important Solid Electrolyte Interface (SEI) decomposition layer formed during cycling (Figure 75b and c). At the electrode scale, the volume fraction of bulk SEI formed was larger for the

Fig. 74: Electrode 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).