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

Layered transition metal oxide-Na<i>_x</i>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 Jahn-Teller distortion associated with Mn³⁺/Mn⁴⁺redox are yet to be overcome.^2,3 To improve the electrochemical performances, various cations (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[Fe_0.50Mn_0.50]O₂(NFMO) material and co-substituted Li⁺and Cu²⁺in the transition metal layer to create a composition of Na[Li_xCu_yFe_0.50-(x+y)Mn_0.50]O₂ (NCLFMO) and compared their electrochemical performances. Both materials crystalize in a rhombohedral structure and Rietveld refinement and TEM analyses suggest an enhanced O-Na-O distance after the substitution. The NFMO and NLCFMO cathode display a capacity of 135 and 121 mAh g^-1 in the potential window of 4.0-2.0 V <i>vs </i>Na⁺/ Na⁰ at 0.1 C rate, with an average voltage (V_avg) 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 the 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 along with a suppressed Mn⁴⁺/Mn³⁺redox activity. Additionally, chemical co-substitution significantly improves capacity retention, with NCLFMO retaining 98% of its initial discharge capacity after 500 cycles compared to 52% for NFMO. Faster Na⁺-ion diffusivity from the galvanostatic intermittent titration technique (GITT) and reduced cell resistance from impedance spectroscopy support the enhanced performances of NCLFMO cathode. <i>In-situ</i> XRD measurement reveals a reversible O3-P3-O3 phase transition during cycling and <i>ex-situ</i> 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 fabricated, delivering a capacity of 105 mAh g^-1with a V_avg of 3.15 V in the potential window of 4.0-1.5 V at a 0.1 C rate. In conclusion, the (Li⁺ + Cu²⁺)-co-substitution strategy improves the overall performance of the Fe-Mn-based layered oxide cathode, paving the way for developing better cathodes for commercial SIBs.<br/><br/><b>References</b><br/>[1] J. Y. Hwang, S. T. Myung, Y. K. Sun, <i>Chem. Soc. Rev.</i> <b>2017</b>, <i>46</i>, 3529.<br/>[2] S. Komaba, N. Yabuuchi, T. Nakayama, A. Ogata, T. Ishikawa, I. Nakai, <i>Inorg. Chem.</i> <b>2012</b>, <i>51</i>, 6211.<br/>[3] 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, C. Delmas, <i>J. Mater. Chem. A</i> <b>2015</b>, <i>3</i>, 10976.<br/>[4] L. Yang, X. Li, J. Liu, S. Xiong, X. Ma, P. Liu, J. Bai, W. Xu, Y. Tang, Y. Y. Hu, M. Liu, H. Chen, <i>J. Am. Chem. Soc.</i> <b>2019</b>, <i>141</i>, 6680.<br/>[5] G. Singh, N. Tapia-Ruiz, J. M. Lopez Del Amo, U. Maitra, J. W. Somerville, A. R. Armstrong, J. Martinez De Ilarduya, T. Rojo, P. G. Bruce, <i>Chem. Mater.</i> <b>2016</b>, <i>28</i>, 5087.<br/>[6] L. Wang, Y. G. Sun, L. L. Hu, J. Y. Piao, J. Guo, A. Manthiram, J. Ma, A. M. Cao, <i>J. Mater. Chem. A</i> <b>2017</b>, <i>5</i>, 8752.<br/>[7] A. Ghosh, R. Hegde, K. Kumar and P. Senguttuvan, <i>J. Am. Chem. Soc.</i>, submitted (2024).