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Influence of stacking, coordination, and surface chemistry on Al intercalation in V$_2$CT$_2$ and Ti$_3$C$_2$T$_2$ MXenes for Al-ion batteries

Amal Raj Veluthedath Nair, Nuala M. Caffrey

Abstract

As the energy storage ecosystem evolves beyond lithium, MXenes, a versatile family of 2D materials derived from MAX phases, have emerged as promising candidates for next-generation energy storage electrodes due to their tunable surface chemistry, large interlayer spacing, and excellent electronic conductivity. In this work, we use density functional theory to investigate Ti$_3$C$_2$ and V$_2$C MXenes as cathodes in Al-ion batteries. Four stacking configurations of the two-dimensional sheets and two different ion coordination sites are evaluated to understand their influence on ion intercalation and mobility. We find that the stacking configuration and surface chemistry critically impact interlayer spacing and electrochemical performance. O-terminated layers in an octahedral stacking exhibit remarkable structural stability with minimal interlayer expansion upon ion intercalation, particularly for Al intercalation in V$_2$C which exhibits an interlayer expansion of 0.1 angstrom, consistent with experimental findings. While octahedral stacking is observed to be energetically more favourable, it reduces ion mobility compared to prismatic stacking. Furthermore, O-terminated MXenes exhibit high theoretical specific capacities, reaching more than 270 mAh/g. F-terminated MXenes are considerably more unstable after intercalation and as a result exhibit much lower Al capacities. These findings highlight the importance of stacking configurations, termination and intercalant chemistry in MXenes for battery applications.

Influence of stacking, coordination, and surface chemistry on Al intercalation in V$_2$CT$_2$ and Ti$_3$C$_2$T$_2$ MXenes for Al-ion batteries

Abstract

As the energy storage ecosystem evolves beyond lithium, MXenes, a versatile family of 2D materials derived from MAX phases, have emerged as promising candidates for next-generation energy storage electrodes due to their tunable surface chemistry, large interlayer spacing, and excellent electronic conductivity. In this work, we use density functional theory to investigate TiC and VC MXenes as cathodes in Al-ion batteries. Four stacking configurations of the two-dimensional sheets and two different ion coordination sites are evaluated to understand their influence on ion intercalation and mobility. We find that the stacking configuration and surface chemistry critically impact interlayer spacing and electrochemical performance. O-terminated layers in an octahedral stacking exhibit remarkable structural stability with minimal interlayer expansion upon ion intercalation, particularly for Al intercalation in VC which exhibits an interlayer expansion of 0.1 angstrom, consistent with experimental findings. While octahedral stacking is observed to be energetically more favourable, it reduces ion mobility compared to prismatic stacking. Furthermore, O-terminated MXenes exhibit high theoretical specific capacities, reaching more than 270 mAh/g. F-terminated MXenes are considerably more unstable after intercalation and as a result exhibit much lower Al capacities. These findings highlight the importance of stacking configurations, termination and intercalant chemistry in MXenes for battery applications.
Paper Structure (8 sections, 4 equations, 7 figures, 2 tables)

This paper contains 8 sections, 4 equations, 7 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Crystal Structure
  3. Theoretical Methods
  4. Results and discussion
  5. Unintercalated MXenes
  6. Intercalated MXenes - Dilute Limit
  7. Stacking Dependence of Al-ion Diffusion Barriers
  8. Intercalated MXenes - Concentrated Limit

Figures (7)

  • Figure 1: (a) WS--oct Ti$_3$C$_2$T$_2$, (b) WS--pris Ti$_3$C$_2$T$_2$, (c) ZZ--oct Ti$_3$C$_2$T$_2$, and (d) WS--pris Ti$_3$C$_2$T$_2$. (e), (f), (g), and (h) present the corresponding V$_2$CT$_2$ structures.
  • Figure 2: (a) Energy difference between ZZ--oct and ZZ--pris stacking ($\Delta E_\mathrm{oct-pris}$) for Ti$_3$C$_2$T$_2$ (b) Energy difference between ZZ--oct and ZZ--pris stacking for V$_2$CT$_2$, where T = --O and T = --F terminations are denoted as solid blue and dashed orange lines. Units are meV per formula unit (meV/f.u.)
  • Figure 3: Change in interlayer distances ($\Delta d$) after intercalation of Na, Mg, Al for ZZ--oct and ZZ--pris stacking, denoted as open squares and solid squares, for (a) Ti$_3$C$_2$T$_2$ and (b) V$_2$CT$_2$. O--terminations as long dashed blue lines and F--terminations as short dashed orange lines.
  • Figure 4: Calculated Al migration energy barriers in (a) ZZ--pris stacked Ti$_3$C$_2$T$_2$ (0.59eV), (b) ZZ--pris stacked V$_2$CT$_2$ (0.50eV), (c) ZZ--oct stacked Ti$_3$C$_2$T$_2$ (1.32eV), and (d) ZZ--oct stacked V$_2$CT$_2$ (1.44eV).
  • Figure 5: Change in interlayer spacing ($\Delta d$) with increasing Al concentration intercalated in ZZ--oct stacking. Solid circles represent Ti$_3$C$_2$O$_2$ and open squares represent V$_2$CO$_2$.
  • ...and 2 more figures