Interplay between Electrochemistry and Phase Evolution of the P2-type Nax(Fe1/2Mn1/2)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 Na2/3(Fe1/2Mn1/2)O2 with a hexagonal structure and P63/mmc 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 operando synchrotron X-ray powder diffraction to understand the origins of the capacity fade of the Na2/3(Fe1/2Mn1/2)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 P63/mmc (P2-type at the open-circuit voltage) to P63 (OP4-type when fully charged) to P63/mmc (P2-type at 3.4–2.0 V) to Cmcm (P2-type at 2.0–1.5 V) symmetry structures during the desodi...