Constructing MgMn6 Superlattice to Enable Highly Reversible Anionic Redox in Layered Oxide Cathodes for High-Capacity Sodium-Ion Batteries

Abstract

In layered oxides of sodium-ion batteries, the anion redox reaction (ARR) that activates lattice oxygen has been regarded as one of the effective ways to break through the traditional transition metal cationic redox capacity limitation. Nevertheless, under high-voltage conditions, problems of transition metal migration, irreversible oxygen release, and phase transformation often lead to rapid capacity decay and aggravated voltage hysteresis. To address these challenges, this study successfully constructed a P2-type Na0.72Li0.24Mg0.05Mn0.71O2 (NLMMO), including LiMn6 and MgMn6 dual superlattice structural units. This exhibits significant lattice oxygen activity, combining excellent specific capacity, enhanced cycling stability, and good structural integrity. Experimental results indicate that the introduction of Mg into the transition metal layer generates Na−O−Mg configuration, which contributes to enhancing oxygen redox activity, thus reaching an ultrahigh specific capacity of 266.9 mAh g−1 and outstanding rate performance (>120 mAh g−1 at 5 C). Meanwhile, the MgMn6 superlattice structure strengthens the TM−O bond, effectively suppressing Mn in-plane migration under deep desodiation, which in turn prevents generating Mn vacancy clusters and O2 release, and preserves local structural stability, consequently achieving highly reversible anionic redox. In situ XRD further verifies that NLMMO achieves a highly reversible “near-zero-strain” solid-solution reaction. This work offers novel insight into designing ARR-type layered oxides that combine high energy density with structural stability.