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 Na-2/3(Fe1/2Mn1/2)O-2 With a hexagonal structure and P6(3)/mmc space group is considered a candidate sodium-ibn battery cathode Material due to its high capacity (similar to 190 mAh.g(-1)) and energy density (similar to 520 mWh-g(-1)); which are comparable to those of the commercial LiFePO4 and LiMn2O4 lithium-ion battery tathodes, 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 Na-2/3(Fe1/2Mn1/2)O-2 material during cycling over the relatively wide 1.5-4.2 V (vs Na) window. We found a compleir 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 cmon (P2-type at 201.5 V) symmetry structures during the desodiation and sodiation of the Na-2/3(Fe1/2Mn1/2)O-2 cathode. The associated large cellvolume 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(Fe1/2Mn1/2)O-2 cathode is able to deliver similar to 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.