Elucidation of Anionic and Cationic Redox Reactions in a Prototype Sodium-Layered Oxide Cathode.
Chen ChengSiyuan LiTiefeng LiuYujian XiaLo-Yueh ChangYingying YanManling DingYue HuJinpeng WuJinghua GuoLiang ZhangPublished in: ACS applied materials & interfaces (2019)
The ever-increasing demand for large-scale energy storage has driven the prosperous investigation of sodium-ion batteries (NIBs). As a promising cathode candidate for NIBs, P2-type Na2/3Ni1/3Mn2/3O2 (NaNMO), a prototype sodium-layered oxide, has attracted extensive attention because of its high operating voltage and high capacity density. Although its electrochemical properties have been extensively investigated, the fundamental charge compensation mechanism, that is, the cationic and anionic redox reactions, is still elusive. In this report, we have systematically investigated the transition metal and oxygen redox reactions of NaNMO nanoflakes using bulk-sensitive soft X-ray absorption spectroscopy and full-range mapping of resonant inelastic X-ray scattering from an atomic-level view. We show that the bulk Mn3+/Mn4+ redox couple emerges from the first discharge process with the increment of inactive Mn3+ upon cycling, which may have a negative effect on the cyclability. In contrast, the bulk Ni redox mainly stems from the Ni2+/Ni3+ redox couple, in contrast to the conventional wisdom of the Ni2+/Ni4+ redox couple. The quantitative analysis provides unambiguous evidence for the continuous reduction of the average valence state of Mn and Ni over extended cycles, leading to the voltage fading. In addition, we reveal that the oxygen anions also participate in the charge compensation process mainly through irreversible oxygen release rather than reversible lattice oxygen redox. Such understanding is vital for the precise design and optimization of NaNMO electrodes for rechargeable NIBs with outstanding performance.
Keyphrases
- transition metal
- metal organic framework
- ion batteries
- high resolution
- electron transfer
- magnetic resonance
- reduced graphene oxide
- gold nanoparticles
- magnetic resonance imaging
- working memory
- gene expression
- computed tomography
- mass spectrometry
- genome wide
- solar cells
- atomic force microscopy
- high intensity
- solid state
- room temperature
- quantum dots
- energy transfer