Crystallographic Evolution of P2 Na2/3Fe0.4Mn0.6O2 Electrodes during Electrochemical Cycling

The development of new insertion electrodes requires an in-depth understanding of the structure–function relationships in order to rationally develop better electrodes. Sodium layered oxides such as P2 Na2/3Fe1/2Mn1/2O2 and Na2/3Fe2/3Mn1/3O2 are particularly interesting due to their performance, price, and low toxicity. Using in situ synchrotron X-ray diffraction during electrochemical cycling of P2 Na2/3Fe0.4Mn0.6O2, changes in the phase composition, lattice parameters, and critically sodium content within the crystal structure are determined. Approaching the charged state, there is an increase in the interlayer distance, brought about via a subtle two-phase region that maintains the structure type as P2. Interestingly, this appears to stabilize the P2 structure in the charged state and inhibits the formation of the highly disordered and typically unfavorable "Z" or OP4 phases up to 4.2 V at 20 mA/g. At the discharged state, at least three phases are present, including P′2 and two subtly different P2 phases. This structural evolution and these parameters are critically assessed in light of data from other compositions in the P2 Na2/3Fe1–yMnyO2 system. This study represents a method for performance optimization by tuning the Fe:Mn ratio.