New cathode materials are currently being created to enhance the capacity of lithium batteries. Among these, multilayer lithium-rich transition metal oxides (LRTMOs) stand out due to their exceptional energy density. Nonetheless, these materials experience a reduction in capacity with each charging cycle due to structural and chemical alterations. A group of researchers from various institutions in China has utilized X-ray techniques at BESSY II to examine these changes with unprecedented accuracy: using a specialized X-ray microscope, they successfully captured morphological and structural variations on a nanometer scale while also shedding light on the associated chemical transformations.
Currently, innovative cathode materials are being developed to boost the capacity of lithium batteries. Multilayer lithium-rich transition metal oxides (LRTMOs) are particularly noted for their high energy density. However, their capacity diminishes with every charge cycle due to alterations in structure and chemistry. Researchers from multiple Chinese institutions have recently employed X-ray techniques at BESSY II to explore these changes for the first time with remarkable precision: utilizing a unique X-ray microscope, they observed structural and morphological developments at the nanoscale and clarified chemical alterations.
Lithium-ion batteries are on the verge of becoming more powerful with new cathode materials. For instance, layered lithium-rich transition metal (LRTMO) cathodes could significantly enhance charging capacity and be utilized in high-performance lithium batteries. However, it has been noted that these cathode materials degrade rapidly, primarily due to the back-and-forth movement of lithium ions during the charging and discharging processes. Until now, the specific changes regarding this degradation were unclear.
To understand these changes, teams from Chinese research institutions applied for beam time at the world’s only transmission X-ray microscope (TXM) at an undulator beamline within the BESSY II storage ring. They utilized 3D tomography and nanospectroscopy to investigate their samples. The HZB-TXM measurements were conducted by Dr. Peter Guttmann from HZB in 2019, prior to the outbreak of the coronavirus pandemic. The X-ray microscopic analysis was further enriched with additional spectroscopic and microscopic evaluations. Following an extensive data review, the results have now been released: they reveal intricate details regarding morphological and structural transformations as well as chemical processes that occur during discharge.
According to Dr. Stephan Werner, who oversees the scientific supervision and further development of the instrument, ‘Soft X-ray transmission microscopy enables us to visualize chemical states within LRTMO particles in three dimensions with high spatial resolution, providing insights into the chemical reactions throughout the electrochemical cycle.’
The findings offer valuable insights into local lattice distortions that are linked to phase transitions and the formation of nanopores. Researchers were also able to locally determine the oxidation states of individual elements. The rates at which charging occurs are crucial in this regard: slower charging promotes phase transitions and oxygen loss, while rapid charging induces lattice distortions and irregular lithium diffusion.
Dr. Werner added, ‘At the TXM, we possess a unique capability: we can provide energy-resolved transmission X-ray tomography. This offers a 3D representation with structural information at each element-specific energy level – energy serves as the fourth dimension here.’
The outcomes from this investigation yield important data for creating high-performance cathodes that maintain long-term stability and resist wear from cycling. Prof. Gerd Schneider, who developed the TXM, noted, ‘The TXM is exceptionally suited for providing new insights into morphological and chemical changes in battery materials through in-operando studies—essentially during the charging and discharging processes.’
