Chemically Boosting Intercalation Voltage and Cycling Stability of Layered Na–Fe–Mn–O Cathode for Na-Ion Batteries through Li/Cu Cosubstitution

Iron-manganese-based layered Na-ion cathodes are appealing for building low-cost Na-ion batteries. However, their practical realization is hindered by the lower intercalation voltage (<3 V Na⁺/Na⁰) and limited cycle life. To tackle these issues, we utilize Li/Cu-cosubstitution into the O3-Na_0.80(Fe_0.50Mn_0.50)O₂ cathode to tune the ionocovalency of Fe/Mn-O bonds, which in turn can modulate the electrochemical properties. With the cosubstitution, the intercalation voltage of Na_0.80(Li_0.10Cu_0.10Fe_0.30Mn_0.50)O₂ is raised to >3.2 V, thanks to the introduction of Cu³⁺/Cu²⁺ redox and enhanced Fe-O bond ionicity. Na_0.80(Li_0.10Cu_0.10Fe_0.30Mn_0.50)O₂ displays extraordinary cycling stability (98 and 60% at 1C after 500 cycles in the window of 4.0-2.0 and 4.0-1.5 V, respectively) compared to the unsubstituted cathode (58 and 24% at 1C after 500 cycles in the window of 4.0-2.0 and 4.0-1.5 V, respectively). The enhanced stability is attributed to the retention of the O3-type structure and suppressed Jahn-Teller MnO₆ distortion during deep sodiation, as revealed by X-ray diffraction and X-ray absorption spectroscopy measurements and DFT calculations. This study highlights the importance of chemical substitution strategies in the development of advanced layered oxide cathodes with higher energy densities and cycling stabilities.