Interplay between Electrochemistry and Phase Evolution of the P2-type Na_<i>x</i>(Fe_1/2Mn_1/2)O₂ Cathode for Use in Sodium-Ion Batteries

Sodium-ion batteries are the next-generation in battery technology; however, their commercial development is hampered by electrode performance. The P2-type Na_2/3(Fe_1/2Mn_1/2)­O₂ with a hexagonal structure and <i>P</i>6₃/<i>mmc</i> space group is considered a candidate sodium-ion battery cathode material due to its high capacity (∼190 mAh·g^–1) and energy density (∼520 mWh·g^–1), which are comparable to those of the commercial LiFePO₄ and LiMn₂O₄ lithium-ion battery cathodes, with previously unexplained poor cycling performance being the major barrier to its commercial application. We use <i>operando</i> synchrotron X-ray powder diffraction to understand the origins of the capacity fade of the Na_2/3(Fe_1/2Mn_1/2)­O₂ material during cycling over the relatively wide 1.5–4.2 V (vs Na) window. We found a complex phase-evolution, involving transitions from <i>P</i>6₃/<i>mmc</i> (P2-type at the open-circuit voltage) to <i>P</i>6₃ (OP4-type when fully charged) to <i>P</i>6₃/<i>mmc</i> (P2-type at 3.4–2.0 V) to <i>Cmcm</i> (P2-type at 2.0–1.5 V) symmetry structures during the desodiation and sodiation of the Na_2/3(Fe_1/2Mn_1/2)­O₂ cathode. The associated large cell-volume changes with the multiple two-phase reactions are likely to be responsible for the poor cycling performance, clearly suggesting a 2.0–4.0 V window of operation as a strategy to improve cycling performance. We demonstrated here that the P2-type Na_2/3(Fe_1/2Mn_1/2)­O₂ cathode is able to deliver ∼25% better cycling performance with the strategic operation window. This significant improvement in cycling performance implies that by characterizing the phase evolution and reaction mechanisms during battery function we are able to propose these modifications to the conditions of battery use that improve performance, highlighting the importance of the interplay between structure and electrochemistry