Designing High Energy Sodium‐Ion Battery Cathodes by Utilizing P2/O3 Biphasic Structure and Lithium Honeycomb Ordering

Layered transition metal oxides, in particular P2-type ones, are considered as promising cathode materials for sodium-ion batteries on account of their high specific capacity and rate capability. Nevertheless, conventional layered compounds involve detrimental phase transformation throughout repeated cycles, which results in electrochemical performance degradation. Therefore, finding structurally stable layered compounds, featuring minimal phase transition has been a key theme of the sodium-ion battery research. Here lithium substituted Fe/Mn-based P2/O3 layered oxide-Na_0.67 Li_0.2 Fe_0.2 Mn_0.6 O₂ -that overcomes the inherited structural instability, is reported. In situ synchrotron-based diffraction measurements and DFT calculations are utilized, in order to identify the association between P2/O3 biphasic structure and electrochemical performances. The lithium honeycomb ordering within the P2/O3 biphasic layered compound effectively constrains the undesirable phase transitions; more specifically, both P2-Z phase transition and Jahn-Teller distortion are suppressed throughout wide potential range of 1.5-4.5 V. The DFT calculation further discovers that the presence of honeycomb ordering is crucial for achieving the structural stability by forming Na-vac-Li and Na-Li-Na pairing at highly charged state. The results highlight that the synergetic effect of P2/O3 biphasic structure and lithium substitution can provide an effective strategy toward achieving electrochemically stable layered cathode material for sodium-ion batteries.