Investigating the Electronic and Magnetic Properties of Na$_x$Fe$_{1/2}$Mn$_{1/2}$O$_2$ Cathode Materials with X-ray Compton Scattering

TL;DR

This study addresses the redox mechanisms and conductivity in Na$_x$Fe$_{1/2}$Mn$_{1/2}$O$_2$ cathodes for sodium-ion batteries by combining high-energy x-ray Compton scattering, SQUID magnetometry, and DFT with the r$^2$SCAN functional. It demonstrates that oxygen $2p$ orbitals dominate redox during desodiation, while transition-metal $3d$ electrons delocalize to yield a metallic state at $x=\tfrac{2}{3}$, a behavior quantified by a negative excursion in the difference Compton profile $\Delta J(p)$ and a calculated electron-hole transfer $n_e\approx 0.17$ e$^{-}$ per Na. Magnetic Compton measurements reveal a net spin moment at $x=\tfrac{2}{3}$ with a measurable O-$2p$ contribution to magnetization, consistent with DFT magnetization density that shows appreciable O magnetization. The work provides a conductivity-descriptor based on momentum-space profiles and highlights oxygen’s active role, offering a robust Hubbard-parameter-free theoretical framework and advocating operando Compton studies to optimize Na-ion cathodes.

Abstract

We discuss electronic and magnetic properties of Na$_x$Fe$_{1/2}$Mn$_{1/2}$O$_2$, a promising Na-ion battery cathode material. Using x-ray Compton scattering, SQUID magnetometry, and density-functional-theory based modeling, we probe how electrons and spins evolve during sodiation. By comparing Compton profiles of sodiated and desodiated samples, we show that oxygen 2$p$ orbitals drive the redox process, while transition-metal 3$d$ electrons become more delocalized, explaining the metallic phase at $x=2/3$. These profile differences define a quantitative descriptor for the sodiation range associated with improved conductivity. Electron holes on oxygen, reflected in oxygen magnetization, confirm the important role of oxygen in the electrochemical activity of the cathode.

Figures (8)

• Figure 1: Structures of (a) P2-Na$_{2/3}$Fe$_{1/2}$Mn$_{1/2}$O$_2$ and (b) O2-Na$_{2/9}$Fe$_{1/2}$Mn$_{1/2}$O$_2$ used in our DFT calculations.
• Figure 2: A schematic of the experimental magnetic Compton scattering setup at BL08W of SPring-8. Scattering geometry and orientation of the magnetic field are shown. The scattering vector lies along the z-direction of the Compton profile, which is nearly parallel to the incident photon direction, corresponding to a scattering angle of 178$^{\circ}$. Momentum densities are spherically averaged in polycrystalline samples, while the spin remains aligned with the magnetic field.
• Figure 3: Spin and orbital resolved total and partial densities of states on various atomic sites (see legend) in (a) P2-Na$_{2/3}$Fe$_{1/2}$Mn$_{1/2}$O$_{2}$ and (b) O2-Na$_{2/9}$Fe$_{1/2}$Mn$_{1/2}$O$_{2}$.
• Figure 4: Spherically averaged theoretical and experimental valence Compton profiles of P$_2$-Na$_{2/3}$Fe$_{1/2}$Mn$_{1/2}$O$_{2}$. Results for three different theoretical models are given (see text for details). Theoretical profiles are convoluted with a Gaussian of 0.5 a.u. full-width-at-half-maximum.
• Figure 5: Experimental valence Compton profile difference $\Delta J(p)$ between P$_2$-Na$_{2/3}$Fe$_{1/2}$Mn$_{1/2}$O$_{2}$ and O$_2$-Na$_{1/3}$Fe$_{1/2}$Mn$_{1/2}$O$_{2}$, along with the corresponding curve-fitting results (see text for details), including contributions from O-$2p$ and Fe/Mn $3d$ electrons. Inset shows the Compton profiles of transition metal $3d$ orbitals for the indicated values of the $Z_2$ and $Z_3$ parameters. The profile $D(p)$ is obtained by taking the difference between the profiles for $Z_2 = 1.5$ and $Z_3 = 3$. The area under the negative excursion of the $D(p)$ profile quantifies the number of $3d$ electrons displaced during the sodiation process.