Influence of Mn/Fe Ratio on Electrochemical and Structural Properties of P2-Na_xMn_1–yFe_yO₂ Phases as Positive Electrode Material for Na-Ion Batteries

Abstract

The comparative structural, Mössbauer and electrochemical studies of the NaxMn2/3Fe1/3O2 and NaxMn1/2Fe1/2O2 systems show that the change in the Mn/Fe ratio has a significant influence on the overlap between the Mn 3+/4+ and Fe 3+/4+ redox couples. The P2-type structure is maintained in the 0.3 < x < 0.8 domain. For the highest intercalation amount, structural distortions occur due to the Jahn-Teller effect of the Mn 3+ ions. The macroscopic distortion results from a competition between the opposite effects of Mn 3+ and Fe 3+ : the isotropic character of Fe 3+ tends to prevent the macroscopic distortion. For the lower sodium amounts, the instability of the interstitial trigonal prismatic space leads to the formation, by slab gliding, of a very disordered structure.Even if this structural transition is reversible a strong capacity fading is observed if the cell is charged above 4V vs Na/Na + .

Decoupling the effect of vacancies and electropositive cations on the anionic redox processes in Na based P2-type layered oxides

“…6b). The calculated interlayer distance (4.82 Å) being close to the d value of O2 type structure [24] and much more shorter than the one of OP4 [13,25] or Z [17,26] phase (5.1 5.2 Å), we can consider that O2 type is formed at the end of charge of the y 0.3 sample. Upon the first discharge, the y 0.3 and y 0.15 samples undergo different structural evolutions.…”

Whole‐Voltage‐Range Oxygen Redox in P2‐Layered Cathode Materials for Sodium‐Ion Batteries

“…It is worth noting that the charge curve of MFM‐0 within the voltage ranges of 3.2–3.7 V (circled by green ellipse in Figure S3, Supporting Information) shows a rapid jump, but increase gradually for MFM‐1 and MFM‐2 samples. As Delmas and co‐workers [ 26 ] have manifested, each TM cation is surrounded by six other TM cations in a triangular lattice. When a large cation (Fe 3+ ) is surrounded by a large number of small cation (such as Mn 4+ ), the crystal field could be intensified, which makes the iron oxidation process easier.…”

Hierarchical Engineering of Porous P2‐Na2/3Ni1/3Mn2/3O2 Nanofibers Assembled by Nanoparticles Enables Superior Sodium‐Ion Storage Cathodes

“…The CV curves and charge/discharge profiles featuring smoother peaks or plateaus than those of the bulk materials, [28,36,41,43,44,46] along with the well-maintained voltage plateau at ≈4.2 V during cycling, suggest that the hierarchical nanofibers could effectively alleviate the Na + /vacancy ordering and P2-O2 phase transition occurring for the pristine materials. [36,[54][55][56] To gain insight into the underlying Na-storage reaction mechanism of the P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 nanofiber electrode, in operando XRD was performed between 1.5 and 4.0 V using Al foil as both the current collector and testing window. Figure 2d displays the rate capability of the P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 nanofiber cathode.…”

“…The slight capacity fading mainly resulted from the Jahn-Teller lattice distortion accompanied by the reduction of Mn(IV) to Mn(III) upon Na + intercalation since the MnO 6 (Mn 3+ ) octahedra would experience an anisotropic change with four lengthened Mn-O bonds and two shortened Mn-O bonds. [36,[54][55][56] To gain insight into the underlying Na-storage reaction mechanism of the P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 nanofiber electrode, in operando XRD was performed between 1.5 and 4.0 V using Al foil as both the current collector and testing window. As shown in Figure 3a,b, during the charging process, the (002) and (004) diffraction peaks gradually shift to lower 2θ angles, while the (100) and (102) peaks shift to higher 2θ angles.…”