Origin of Fast Capacity Decay in Fe-Mn Based Sodium Layered Oxides

摘要

Fe-Mn based layered oxides are recognized as promising cathode materialsfor sodium-ion batteries (SIBs) with high capacities and earth-abundantingredients. However, their real-world applications are still constrained byfast capacity decay accompanied with the requirements of deeper insightsinto the principles behind. Herein, taking O3-Na_xFe_(1/2)Mn_(1/2)O_2 as a classicsample, the capacity fading mechanism of Fe-Mn based layered oxides iscomprehensively investigated through combined techniques. For the firsttime, it is revealed that Fe migration is merely triggered after the oxidation of≈0.3 mol Fe~(3+) based on solid proofs from ex situ X-ray absorption spectroscopyand Moessbauer spectroscopy, which implies the crucial role of the accumulatedstructural distortion induced by Jahn–Teller active Fe~(4+). O3-P3 phasetransition during cycling is obviously constrained along with Fe migration asevidenced by in situ/ex situ X-ray diffraction, well interpreting the intensifiedpolarization and the resulting large capacity loss. More importantly, withinthe desodiation depth (≈80% of sodium extraction) where Fe migration isalmost absent, the capacity fading is dominantly rooted in the Fe~(4+) activatedand Mn-dissolution aggravated surface passivation as confirmed by mass/Xrayspectroscopies and electrochemical analysis. These renewed understandingsof the fast capacity decay in Fe-Mn based layered oxides offer clearerclues for designing desirable cathodes for SIBs.