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More Reversible Anionic Redox Chemistry for Sodium Storage Triggered By Transition Metal Regulation

As one of the next generation battery system, sodium-ion batteries (SIBs) have been attached great interest for its abundant resources, low cost and good safety. Recently, high energy density cathode materials with anionic redox chemistry in SIBs have become the mainstream. The working potential of oxygen redox in layered transition metal oxide cathodes is about 4.2 V (vs. Na+/Na), which can not only elevate the battery potential but also harvest extra capacity. Thus, reasonably utilizing the oxygen redox can drastically increase the energy density of the batteries. However, the undesired phase transition and structural distortion resulted from oxygen redox usually lead to severe voltage hysteresis and capacity decay of the cathodes. Therefore, it is of great significance to investigate the relationship between the transition metal components and cycling stability of the cathodes, which provides valuable understanding for discovering novel cathodes with more reversible and higher capacity. To improve the structure stability and promote more anionic redox, we have been focusing on tuning the structure of P2-layered cathode materials with the different transition metal ligands such as Al/Ti/Zn-Mn. We have performed in-situ XRD and ex-situ XAS characterizations to investigate the charge compensation mechanism of the cathodes. The O K-edge XAS results indicated O e
 g orbits variation at fully charged state and provided solid evidence of oxygen redox chemistry. Moreover, we combined EXAFS results and DFT calculations to construct the model of local structure distortion. These results provide further understanding of the oxygen redox chemistry mechanism and promote the development of high energy density SIBs.
 
 Acknowledgement
 
 The work was supported by the National Natural Science Foundation of China (No. 52071085).