The embedded transition metal layered oxide (AMOâ‚‚, A = Li⺠or Naâº, M = transition metal) is a crucial cathode material for lithium and sodium ion batteries. Traditionally, the redox reaction of transition metals was believed to be responsible for charge compensation during ion deintercalation, limiting the capacity of the positive electrode. However, this view has been challenged by the discovery of lithium-rich layered oxides, such as O3-structured Li[Liâ‚“Mâ‚â‚‹â‚“]Oâ‚‚. These materials exhibit an ultra-high reversible specific capacity—over 300 mAh/g—but their capacity cannot be fully explained by transition metal redox alone. Instead, lattice oxygen plays a significant role in the electron-loss process, contributing additional capacity.
This phenomenon isn’t limited to lithium-rich materials; many layered oxides also involve oxygen in charge compensation, offering extra capacity. Yet, understanding how lattice oxygen participates in redox reactions and achieves reversibility remains a major challenge in energy storage research. The key to unraveling this mechanism lies in the relationship between crystal structure and oxygen redox behavior.
Recently, Associate Professor Yan Xiqian and Researcher Hu Yongsheng from the Institute of Physics, Chinese Academy of Sciences, published a groundbreaking study titled “Structure-Induced Reversible Anionic Redox Activity in Na Layered Oxide Cathode†in *Joule*, a top-tier journal in the energy field. Collaborating with researchers from Oak Ridge National Laboratory, Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, and the Stanford Linear Accelerator Center, they used advanced techniques like neutron scattering and synchrotron radiation to investigate the sodium storage mechanism in P3-Na₀.₆[Li₀.₂Mn₀.₈]O₂, a sodium ion battery cathode material.
Their findings reveal that oxygen plays a critical role in the redox process, contributing to the material’s high capacity and stability. By analyzing the structural changes using neutron pair distribution function (nPDF) combined with X-ray and neutron diffraction, the team confirmed that the oxygen redox is reversible, and the bulk phase structure remains stable after charging. This work marks the first time such structural changes related to oxygen charge compensation have been studied in detail.
The research highlights the importance of the P-phase structure, which features a larger interlayer spacing compared to the O3 phase. This allows for tolerance of lattice distortions caused by changes in O–O bond lengths while preventing cation migration into the alkali metal layer. As a result, the layered structure remains intact, enabling reversible oxygen redox and enhancing overall performance.
This study not only clarifies the mechanism behind reversible oxygen redox but also offers new insights for designing high-voltage, high-capacity cathodes for both lithium and sodium ion batteries. It also introduces the powerful tool of nPDF, expanding the scope of structural analysis in energy materials. The research was supported by several national programs, including the 973 Project, the National Science Fund for Distinguished Young Scholars, and the Chinese Academy of Sciences Hundred Talents Program.
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