Ultra-High-Energy Density in Layered Sodium-Ion Battery Cathodes through Balancing Lattice-Oxygen Activity and Reversibility

Lattice-oxygen redox in layered metal oxide cathodes offers a promising way to exploit high-energy density sodium-ion batteries. However, oxidation and reduction of lattice-oxygen are always asymmetric, showing poor reversibility upon charging and discharging due to the activated oxygen loss and subsequent structural rearrangement. Here, a layered Na0.7[Li0.2Mn0.7Co0.1]O2 (NLMCO) is developed by balancing lattice-oxygen activity and reversibility, which can deliver a record energy density of 729.7 Wh kg−1, further exceeding the state-of-the-art Na0.75[Li0.25Mn0.75]O2 (NLMO, 638.4 Wh kg−1). In light of electron paramagnetic resonance spectroscopy, in situ differential electrochemical mass spectroscopy, and electrochemical testing results, the highly activated lattice-oxygen is effectively stabilized in NLMCO without oxygen molecule release while obvious oxygen release is detected in the highly activated NLMO. Benefiting from the enhanced transition metal-oxygen covalency and reduced band energy gap, the NLMCO electrode demonstrates simultaneously high lattice-oxygen activity and reversibility, thus resulting in excellent rate and cycling performance, as well as ultra-high energy density. The findings highlight the critical association of energy density and lattice-oxygen redox reversibility, which will inspire more interest in anionic redox-based high-energy batteries.