Optimizing O3-Type Fe-Mn-Based Cathodes for Na-Ion Batteries: Unveiling the Synergistic Impact of Chemical Co-Substitution

Abstract

Layered transition metal oxide-Na x MO2 (M = transition metal) is the most widely explored class of cathode materials in lithium and sodium-ion batteries (LIBs and SIBs).1 The replacement of expensive Co and Ni with cheaper elements like Fe and Mn is appealing, which can reduce the overall cost of the battery. Despite the higher operating voltage of Fe3+/Fe4+ and the large specific capacity of Mn3+/Mn4+, issues like irreversible Fe-migration and J. T. distortion of Mn3+/Mn4+ are yet to be overcome.2,3 To improve the electrochemical performances of layered oxide cathodes, various cationic substitutions (e.g., Li+, Cu2+, Mg2+, Zn2+, and Ti4+) are partially substituted in the place of M.4–6 Along this line, in search of better Fe/Mn-based cathodes, we selected an O3-type Na[Fe0.50Mn0.50]O2 (NFMO) material and co-substituted Li+ and Cu2+ in the transition metal layer to create a composition of Na[LixCuyFe0.50-x-yMn0.50]O2 (NCLFMO) and compared their electrochemical performances.. Both materials crystallize in a rhombohedral structure and Rietveld refinement and TEM analyses suggest an enhanced O-Na-O spacing after substitution. The NFMO and NCLFMO cathodes display a capacity of 135 and 121 mAh g-1 in the potential window of 4.0–2.0 V at 0.1 C rate, with an average voltage (Vavg) of 2.63 V and 3.31 V respectively, which leads to an increased energy density of 400 Wh kg-1 for NCLFMO.7 Following substitution, the capacity contributions of Mn and Fe become 20.2% and 79.8%, respectively, instead of 49.8% and 50.2% in NFMO. Clearly, it indicates a pronounced Fe4+/Fe3+ redox activity with a suppressed J. T. distortion for Mn4+/Mn3+ redox. Chemical co-substitution significantly improves capacity retention, with NCLFMO retaining 98% of its initial discharge capacity after 500 cycles, compared to 52% for NFMO. Better rate capability, faster diffusion coefficient of Na+ ions from GITT, and reduced cell resistance from impedance spectroscopy support the enhanced performances of NCLFMO. In-situ XRD measurement reveals a reversible O3-P3-O’3-P3-O3 phase transition and ex-situ x-ray absorption spectroscopy analysis confirms a prominent Fe4+/Fe3+ redox with a suppressed Mn4+/Mn3+ redox during charge-discharge cycling. More significantly, NCLFMO remains air-stable, retaining its structure and electrochemical properties after exposure to air for 30 days. Finally, a full cell with precycled HC is constructed, delivering a capacity of 105 mAh g-1 with a Vavg of 3.15 V in the potential window of 4.0-1.5 V at a 0.1 C rate. In conclusion, the (Li + Cu)-co-substitution strategy improves the overall performance of the Fe-Mn-based layered oxide cathode, paving the way for developing better cathodes for commercialized SIBs. References J. Y. Hwang, S. T. Myung, and Y. K. Sun, Chem. Soc. Rev., 46, 3529–3614 (2017).S. Komaba, N. Yabuuchi, T. Nakayama, A. Ogata, T. Ishikawa and I. Nakai, Inorg. Chem., 51, 6211–6220 (2012).B. Mortemard De Boisse, J. H. Cheng, D. Carlier, M. Guignard, C. J. Pan, S. Bordère, D. Filimonov, C. Drathen, E. Suard, B. J. Hwang, A. Wattiaux and C. Delmas, J. Mater. Chem. A, 3, 10976–10989 (2015).L. Yang, X. Li, J. Liu, S. Xiong, X. Ma, P. Liu, J. Bai, W. Xu, Y. Tang, Y. Y. Hu, M. Liu and H. Chen, J. Am. Chem. Soc., 141, 6680–6689 (2019).G. Singh, N. Tapia-Ruiz, J. M. Lopez Del Amo, U. Maitra, J. W. Somerville, A. R. Armstrong, J. Martinez De Ilarduya, T. Rojo and P. G. Bruce, Chem. Mater., 28, 5087–5094 (2016).L. Wang, Y. G. Sun, L. L. Hu, J. Y. Piao, J. Guo, A. Manthiram, J. Ma and A. M. Cao J. Mater. Chem. A, 5, 8752–8761 (2017).A. Ghosh, R. Hegde, K. Kumar and P. Senguttuvan, J. Am. Chem. Soc., submitted(2023).