Correlating Structure Change With Magnetic Ordering and Spin Fluctuation During Delithiation of Transition Metal Layered Oxide

Current Li-ion battery technology relies on Li⁺ insertion/extraction coupled with electron gaining/loss at both cathodes and anodes. Although extensive efforts have been devoted to studying the structure change of cathode materials with Li⁺ extraction/insertion, changes in the magnetic properties arising from the accompanying redox processes have been largely overlooked. Here, we systematically investigate both the structure evolution and magnetic-property changes during the delithiation of the representative layered oxide LiCoO₂, by combining the operando synchrotron-based x-ray diffraction with dedicated magnetic measurements. We construct a magnetic phase diagram as a function of Li content x in Li_xCoO₂, which closely mirrors the corresponding structure evolution diagram. The results reveal a series of complicated magnetic transitions upon Li extraction: paramagnetic → antiferromagnetic → paramagnetic → diamagnetic → paramagnetic. Moreover, the variation in the effective magnetic moment of Co⁴⁺ is strongly correlated with local structural changes within the CoO₆ octahedra, indicating that the Co⁴⁺ spin-state fluctuation may play an important role in the structure evolution and electrochemical performance. These findings help close a critical gap in understanding structure-magnetism coupling during the electrochemical cycling and may inspire the design of new layered oxide cathodes from a spin-electronics perspective.