Abstract
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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 desodiation and sodiation of the Na2/3(Fe1/2Mn1/2)O2 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 Na2/3(Fe1/2Mn1/2)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.