Capturing surface interlayer cation migration in Na-0.6[Li0.2Mn0.8]O-2 layered cathode materials for sodium battery

Anionic redox of charge compensation is an effective strategy to release the excess capacity of the cathode material for sodium ion batteries. Nonetheless, the development of cathode materials with this electrochemical reaction mechanism faces great challenges in terms of voltage attenuation and irreversible loss of capacity. As a result of the complexity of its structure, the failure mechanism is not well understood. Here we conduct STEM coupled with EELS on a typical anionic redox material of sodium ion batteries (Na-0.6[Li0.2Mn0.8]O-2) after the electrochemical cycle. Surprisingly, it was found that the transition metal ions on the surface of the particles migrated into the alkali metal layer, forming a spinel structure, and resulting in a decrease in the valence of the transition metal on the surface. The possibility of this kind of migration is confirmed, and the most likely migration path is calculated by DFT. Combining the electrochemical curve and XPS spectroscopy, we found the evolution of the charge transfer mechanism, which gradually changes from anion to cation with the progress of the electrochemical cycle. The structural reconstruction of the particle surface and the evolution of the charge transfer mechanism lead to the voltage hysteresis and capacity loss of the cathode. The new failure mechanism provides an important basis for solving the problems of this type of material for sodium cathode.