New cathode materials are being developed to further increase the capacity of lithium batteries. Multilayer lithium-rich transition metal oxides (LRTMOs) offer particularly high energy density. However, their capacity decreases with each charging cycle due to structural and chemical changes. Using X-ray methods at BESSY II, teams from several Chinese research institutions have now investigated these changes for the first time with highest precision: at the unique X-ray microscope, they were able to observe morphological and structural developments on the nanometre scale and also clarify chemical changes.
Lithium-ion batteries are set to become even more powerful with new materials for the cathodes. For example, layered lithium-rich transition metal (LRTMO) cathodes could further increase the charge capacity and be used in high-performance lithium batteries. However, so far it has been observed that these cathode materials ‘age’ rapidly: the cathode material degrades as a result to the back-and-forth migration of lithium ions during charging and discharging. Until now it was unclear what specific changes these would involve.
Teams from Chinese research institutions have therefore applied for beam time at the world’s only transmission X-ray microscope (TXM) at an undulator beamline at the BESSY II storage ring to investigate their samples using 3D tomography and nanospectroscopy. The HZB-TXM measurements were performed by Dr. Peter Guttmann, HZB, back in 2019, before the coronavirus pandemic. The X-ray microscopic analysis was then supplemented by further spectroscopic and microscopic examinations. After careful evaluation of the extensive data, the results are now available: they provide detailed information on changes in the morphology and structure of the material, but also on chemical processes during discharge.
‘Soft X-ray transmission microscopy allows us to visualise chemical states in LRTMO particles in three dimensions with high spatial resolution and to gain insights into chemical reactions during the electrochemical cycle,’ explains Dr Stephan Werner, who is responsible for the scientific supervision and further development of the instrument.
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Image: The left side of the figure shows nanotomography images of an LRTMO particle taken at the TXM of BESSY II before the first charging cycle (top) and after 10 charging cycles (bottom). In the simulation (right side), the isolated pores are highlighted in light blue. After 10 charging cycles, the number of pores and cracks has significantly increased.
Credit: HZB