Characterizing High-Capacity Janus Aminobenzene-Graphene Anode for Sodium-Ion Batteries with Machine Learning

Abstract

Sodium-ion batteries require anodes that combine high capacity, low operating voltage, fast Na-ion transport, and mechanical stability, which conventional anodes struggle to deliver. Here, we use the SpookyNet machine-learning force field (MLFF) together with all-electron density-functional theory calculations to characterize Na storage in aminobenzene-functionalized Janus graphene (Na$_x$AB) at room-temperature. Simulations across state of charge reveal a three-stage storage mechanism-site-specific adsorption at aminobenzene groups and Na$_n$@AB$_m$ structure formation, followed by interlayer gallery filling-contrasting the multi-stage pore-, graphite-interlayer-, and defect-controlled behavior in hard carbon. This leads to an OCV profile with an extended low-voltage plateau of 0.15 V vs. Na/Na$^{+}$, an estimated gravimetric capacity of $\sim$400 mAh g$^{-1}$, negligible volume change, and Na diffusivities of $\sim10^{-6}$ cm$^{2}$ s$^{-1}$, two to three orders of magnitude higher than in hard carbon. Our results establish Janus aminobenzene-graphene as a promising, structurally defined high-capacity Na-ion anode and illustrate the power of MLFF-based simulations for characterizing electrode materials.

Figures (5)

• Figure 1: Anode characterization as a function of state of charge.A) Open-circuit voltage profile extracted from Machine-learning Molecular Dynamics at 300 K. B) Average interlayer distance between graphene layers as Na content increases. The individual dots display the specific intra-plane distances. C) Sodium-ion diffusion coefficient computed from the mean-squared displacement. The grey-boxes highlight the plateaus for the voltage, volume and ion diffusivity reached over the high-loading stable domain: The working state-of-charge regime.
• Figure 2: Radial Distribution for Na--N and Na--Na interactions in Na$_{x}$AB.A) Radial distribution function $g(r)$ for Na–N pairs at $x = 2.0$. The first peak at approximately 2.5 Å (red dashed line) corresponds to Na directly adsorbed on the --NH$_{2}$ groups, while the second peak near 4.0 Å (yellow dashed line) arises from Na atoms positioned above the aromatic rings. A broader peak at 7.0 Å (green dashed line) reflects more weakly correlated Na ions located in adjacent interlayer environments. B) Na--Na radial distribution function at $x=4.0$. The first peak at 3.0 Å (red dashed line) indicates short-range Na--Na correlations characteristic of Na-cluster formation. A broader second peak at 6.5 Å (green dashed line) corresponds to Na atoms in higher coordination environments, including those situated on the graphene layers. Dashed lines in the schematic illustrate the spatial arrangement associated with these coordination distances.
• Figure 3: Average Hirshfeld charges for each atomic species as a function of sodium content ${x}$. C$_\mathrm{AB}$, H$_\mathrm{AB}$, and N$_\mathrm{AB}$ denote atoms in the aminobenzene groups. Values represent averages over ten MD configurations per composition; error bars indicate the standard deviation. Snapshots embedded in the plot show representative configurations, with Na atoms colored according to their Hirshfeld charge (color scale: black=0, white = 0.6).
• Figure 4: Projected Density of States (PDOS) by atomic species in Na$_{x}$AB. The PDOS is averaged over ten single-point calculations for each sodium concentration $x$. C$_\mathrm{AB}$, H$_\mathrm{AB}$, and N$_\mathrm{AB}$ denote atoms belonging to the aminobenzene groups. The Fermi level is set to 0 eV. Shaded regions indicate the standard deviation across sampled configurations.
• Figure 5: Orbital-resolved electronic structure and charge-density difference for $x =0.5$.A) Projected density of states (PDOS) averaged over ten single-point calculations. Shaded regions indicate the standard deviation across sampled configurations. B) and C) Charge-density difference ($\Delta{\rho{}}$) isosurfaces for a representative configuration at $x =0.5$, plotted at ±0.004 e Å$^{-3}$. Red denotes electron accumulation and blue denotes electron depletion. The $\Delta{\rho{}}$ reveals depletion around the Na atom and extended accumulation over the benzene ring, characteristic of a cation–$\pi{}$ interaction.