1. Why Do We Care About New Anode Materials?

Modern technology relies heavily on rechargeable lithium-ion batteries: smartphones, laptops, electric cars, stationary storage and more. As devices get smaller and more powerful, the expectations for batteries keep increasing:

• Higher energy density (more energy per weight),

• Faster charging,

• Good cycle life (many charge/discharge cycles without losing capacity),

• Safe and stable operation.

One way to improve batteries is to develop better electrode materials, especially for the anode. Traditionally, graphite is used as the anode in commercial Li-ion batteries, but researchers are constantly searching for materials that can store more lithium, charge faster and stay stable.

In this context, two-dimensional (2D) materials have attracted strong interest. Because they are extremely thin and have a large surface area, they can:

• Offer many active sites for lithium storage,

• Provide short diffusion paths for Li ions,

• Allow fast charge and discharge.

Among these 2D materials, a relatively new family has become very promising: MXenes.

2. What Are MXenes?

MXenes are 2D materials made from transition metal carbides, nitrides, or carbonitrides.

Their general formula is:

Mₙ₊₁XₙTᵧ

where:

• M = an early transition metal (like Ti, Nb, V, etc.),

• X = carbon (C) and/or nitrogen (N),

• Tᵧ = surface terminations (O, F, OH, Cl, sometimes H).

These surface terminations are very important. They:

• Make MXenes hydrophilic (they like water),

• Modify their electronic properties,

• Affect how ions like Li⁺ interact with the surface.

MXenes are being studied for many applications:

• Energy storage (batteries, supercapacitors),

• Electromagnetic interference shielding,

• Composite reinforcement,

• Water purification, gas and biosensors,

• Lubrication, catalysis (photo-, electro- and chemical).

The first MXene, Ti₃C₂, was reported in 2011. Since then, dozens of MXenes have been synthesized or predicted.

3. Why Study Nb₂C and Nb₂CO₂?

The focus of this paper is on niobium-based MXenes, especially:

• Nb₂C (bare surface),

• Nb₂CO₂ (fully oxygen-terminated surface).

These two are closely related: Nb₂CO₂ is basically Nb₂C whose surfaces are covered by oxygen atoms. That oxygen termination changes the surface charge, bonding, and behavior with lithium ions.

Nb₂C is interesting because:

• It has excellent conductivity,

• It is even reported to be superconducting with a critical temperature of about 12.5 K, which is high for a 2D MXene,

• Previous work suggested it could be a good material for high-rate Li-ion batteries (can handle fast charge/discharge).

Some experiments also showed that oxygen-terminated Nb₂C (Nb₂CTₓ) can store a good amount of lithium. This motivates a more detailed theoretical comparison between bare Nb₂C and oxidized Nb₂CO₂ as anode materials.

4. How Did the Authors Study These Materials?

This is a theoretical (computational) study. No physical electrode was built in the lab in this work; instead, the authors used first-principles calculations based on:

• Density Functional Theory (DFT), using the Quantum Espresso code,

• Generalized Gradient Approximation (GGA) with PBE functional for the exchange–correlation energy,

• Ultrasoft pseudopotentials,

• A supercell with enough vacuum (over 20 Å) so that layers do not interact with their periodic images,

• A 3 × 3 supercell of the MXene to simulate Li coverage on the surface,

• Geometry optimization until forces and energies are well converged,

• CI-NEB (Climbing Image Nudged Elastic Band) method to find the minimum energy path (MEP) and energy barriers for Li diffusion.

In simpler terms:

They built realistic atomic models of Nb₂C and Nb₂CO₂, placed lithium atoms on their surfaces in different positions, and used quantum mechanical calculations to learn:

• Where Li prefers to sit,

• How strongly it binds,

• How easily it can move,

• How much charge is transferred,

• How the lattice changes when more and more Li is added,

• What theoretical capacity and voltage these materials would offer as anodes.

5. Basic Structure of Nb₂C and Nb₂CO₂

The study first checks the geometry and bonding in these two MXenes.

For Nb₂C (bare):

• Lattice parameter (a): about 3.11 Å,

• Nb–C bond length: about 2.15 Å,

• The layer thickness and interlayer distances show a compact 2D structure.

For Nb₂CO₂ (oxidized):

• Lattice parameter slightly larger: about 3.13 Å,

• Nb–O bond length: about 2.10 Å,

• Nb–C bond slightly longer: about 2.20 Å,

• The overall thickness of the MXene layer is larger due to the extra oxygen layers on both sides.

These values agree well with previously reported experimental values for Nb₂CTₓ, which suggests the computational model is realistic.

The authors also mention that phonon calculations (from previous studies) show no negative frequencies, meaning both Nb₂C and Nb₂CO₂ are dynamically stable.

6. Electrostatic Potential: Where Does Li Want to Sit?

To understand where lithium will preferentially adsorb, the authors calculated electrostatic potential isosurfaces.

In simple terms:

• Regions with negative potential (electron-rich) attract positively charged Li⁺ ions,

• Regions with positive potential repel them.

For Nb₂C:

• Nb atoms are surrounded by regions of positive potential,

• C atoms show negative potential,

• So Li⁺ is more attracted to regions above carbon.

For Nb₂CO₂:

• Oxygen atoms are highly electronegative and carry strong negative potential,

• Carbon atoms become more neutral due to the presence of O,

• Therefore, regions near oxygen atoms become very attractive for Li⁺.

This gives a first intuitive picture: Li is expected to bind more strongly to Nb₂CO₂ than to Nb₂C, because the O-terminated surface is more negatively charged.

7. Electronic Properties: Are These Good Conductors?

The authors computed the band structure and projected density of states (PDOS) for both MXenes.

Main conclusions:

• Both Nb₂C and Nb₂CO₂ are metallic.
  This is important because the anode material should conduct electrons easily during charge/discharge.

• For Nb₂C, states near the Fermi level are mainly from Nb orbitals (p and d).

• For Nb₂CO₂, Nb-d and Nb-p orbitals still dominate near the Fermi level, but oxygen states significantly contribute at energies below the Fermi level.

In short, both materials have good electronic conductivity, which is a basic requirement for a good anode.

8. Where Exactly Does Li Adsorb on the Surface?

To be precise, the authors tested several high-symmetry adsorption sites on the surface:

• Top: directly above a surface atom,

• Bridge: between two neighboring atoms,

• T4: above an atom in the second layer,

• H3: above a third-layer atom, making a threefold hollow site.

They calculated the adsorption energy of a single Li atom at each site. A more negative adsorption energy means more stable binding.

For Nb₂C

Ordering of stability (from most to least stable):

T4 < H3 < Bridge < Top

(“<” here means “more negative than”, i.e. more stable.)

This means that Li prefers to sit above carbon-related positions (T4 and H3), consistent with the electrostatic potential picture.

For Nb₂CO₂

The most stable site is H3, where Li is surrounded by three oxygen atoms and forms a threefold coordination.
The interaction is stronger because the O atoms carry strong negative potential.

In general, Li binds more strongly to Nb₂CO₂ than to Nb₂C, again confirming that the oxidized surface interacts strongly with Li.

9. How Easily Can Li Move? (Diffusion Barriers)

A good anode not only needs to bind Li, but also allow Li to move quickly during charging and discharging. This is controlled by the diffusion energy barrier.

Using the CI-NEB method, the authors computed the minimum energy path for Li diffusion across the surface.

On Nb₂C

• Li moves from one T4 site to another equivalent T4 site,

• The bridge site is the transition state (energy barrier),

• The calculated diffusion barrier is 35 meV, which is very low.

A low diffusion barrier means Li can move easily on the surface, which is good for fast charging/discharging.

On Nb₂CO₂

• The diffusion path goes from one H3 site to another,

• The transition state corresponds roughly to a T4 position near O atoms,

• The diffusion barrier is about 250 meV.

This is significantly higher than for bare Nb₂C, but still reasonable and actually lower than for some Ti-based functionalized MXenes. The higher barrier comes from the stronger Li–O interaction, which makes Li more “anchored” to the surface.

So there is a trade-off:

• Nb₂C: weaker binding, but very fast Li diffusion.

• Nb₂CO₂: stronger binding (more stable lithiation), but slower diffusion.

10. Lithiation Mechanism: How Does the Surface Fill with Li?

Next, the authors studied what happens when you add more and more Li atoms to the surface, not just one.

They placed Li atoms step by step, up to forming a full Li monolayer (ML), and then even a second Li monolayer above the first. They focused the description mostly on Nb₂CO₂, but Nb₂C behaves in a similar ordered way.

Some key observations:

• At low Li coverage, Li atoms occupy the most favorable adsorption sites (T4 on Nb₂C, H3 on Nb₂CO₂).

• As more Li atoms are added, they tend to form regular patterns, such as lines and then hexagonal arrangements.

• For Nb₂CO₂, at around seven Li atoms (in the chosen supercell), hexagonal patterns appear with a hollow center that finally fills when a full monolayer is formed.

• The overall process is ordered and systematic, not random.

This ordered lithiation is important: it affects the structural stability, volume changes, and electrochemical behavior during cycling.

11. Lattice Expansion vs Contraction Under Lithiation

A very practical question for anodes is: Does the material swell a lot when it stores Li? Large volume changes can cause mechanical damage and capacity fading.

The authors tracked how the lattice parameter changes as Li concentration increases.

For Nb₂C

• The lattice parameter increases with Li content:

  • From about 9.33 Å (3×3 supercell) with no Li,

  • Up to about 9.37 Å at full monolayer coverage,

  • Then slightly reduces to about 9.35 Å when the second Li monolayer is added.

• This expansion is mainly due to:

  • Li–Li electrostatic repulsion,

  • Relatively weak Nb–Li bonding (more ionic, less structural “tightening”).

For Nb₂CO₂

• The behavior is different: the lattice parameter decreases as Li is added:

  • From about 9.39 Å (no Li),

  • Down to 9.35 Å with one Li atom,

  • And to about 9.32 Å at full Li monolayer.

• When two Li monolayers are present, the lattice parameter stays almost constant.

• This contraction is linked to:

  • Stronger Li–O interactions,

  • Charge redistribution that reduces Li–Li repulsion and pulls the structure slightly together.

So, Nb₂C tends to expand, while Nb₂CO₂ slightly contracts under lithiation. Both remain structurally stable, but the oxidized phase shows a more “tight” bonding with Li.

12. Is the Lithiation Thermodynamically Stable?

To check whether the lithiated systems are thermodynamically favorable, the authors calculated the formation energy per Li atom for each lithiation level.

• Negative formation energy means the structure is stable with respect to separate Li metal and bare MXene.

For Nb₂C

• The first Li atom has a formation energy of about −0.72 eV/atom,

• As more Li is added, the formation energy stays negative,

• Even the second Li monolayer remains stable, with formation energies around −0.28 eV/atom.

For Nb₂CO₂

• The first Li atom is much more stable, with formation energy about −1.87 eV/atom,

• Values remain more negative than for Nb₂C across the whole lithiation range,

• Two full Li monolayers still have a negative formation energy (around −0.47 eV/atom).

Conclusion:
Lithiation is thermodynamically favorable for both Nb₂C and Nb₂CO₂, but clearly more stable in the oxidized Nb₂CO₂ phase.

13. Charge Transfer and Bonding Nature

The authors also looked at how charge is distributed using:

• Bader charge analysis,

• Electron Localization Function (ELF) line profiles.

Key points:

• In both MXenes, carbon gains electron density from niobium, indicating strong Nb–C bonds with an ionic component.

• In Nb₂CO₂, oxygen atoms draw even more electron density from Nb, as expected due to their high electronegativity.

• Each Li atom donates most of its valence electron (~0.8e) to the MXene surface, behaving essentially as Li⁺.

From ELF line profiles:

• Nb–C and Nb–O bonds show ionic character,

• Nb–Li bonds in Nb₂C are relatively weak, which explains the low diffusion barrier,

• O–Li bonding in Nb₂CO₂ is stronger, explaining the higher diffusion barrier and stronger lithiation.

This helps connect microscopic bonding with macroscopic behavior (diffusion, expansion, stability).

14. Electrochemical Performance: Voltage and Capacity

Finally, the authors evaluated how these materials would perform as anode materials by calculating:

• Average electrode potential (V) as a function of lithiation level,

• Theoretical gravimetric capacity (Q, in mA h/g).

They considered Li intercalation reactions of the form:

• Nb₂C + yLi → Nb₂CLiᵧ

• Nb₂CO₂ + yLi → Nb₂CO₂Liᵧ

and used standard formulas to compute V and Q.

Maximum Theoretical Capacity

They considered Li intercalation on one side and on both sides of the MXene sheet:

So in theory:

• Nb₂C can reach up to 275 mA h/g,

• Nb₂CO₂ can reach around 233 mA h/g.

These values are comparable to or higher than many commonly used anode materials.

The authors also compare with experimental data:

• Experiments on Nb₂CTₓ gave about 170 mA h/g,

• This is closer to the oxidized Nb₂CO₂ theoretical value, which makes sense: real surfaces are not purely bare; they carry mixed terminations (O, OH, F).

Voltage Profile

• Nb₂CO₂ shows a higher voltage at low capacity (up to ~1.87 V),

• Nb₂C starts lower (~0.72 V),

• At higher capacities, both materials show low voltages (~0.1 V), which is typical of effective anode materials.

Both MXenes show stable voltage–capacity behavior, which is desirable for practical cycling.

15. How Do These MXenes Compare to Other 2D Anode Candidates?

The paper also compares Nb₂C and Nb₂CO₂ to other 2D materials and MXenes studied theoretically.

Some points from the comparison:

• Diffusion barriers:

  • Nb₂C (35 meV) is similar to fast-diffusion MXenes like Ti₄C₃, Ti₂Ta₂C₃, Ti₂C,

  • Nb₂CO₂ (250 meV) has higher barriers, but still lower than some Ti-based functionalized MXenes, and much lower than some non-MXene 2D materials like g-CN or BC₃N₃.

• Capacities:

  • Nb-based MXenes have capacities comparable to many Ti-based MXenes,

  • Some V-based MXenes can achieve even higher capacities,

  • Some other 2D systems may have high capacity but very large diffusion barriers, making them less practical.

Overall, Nb₂C and Nb₂CO₂ balance capacity, conductivity, diffusion, and stability quite well, making them promising anode candidates.

16. Final Takeaways

Let’s summarize the main conclusions of this study in plain language:

• Both Nb₂C and Nb₂CO₂ are metallic, which is great for anode conductivity.

• Lithium binds more strongly to Nb₂CO₂ than to Nb₂C, thanks to the negatively charged oxygen surface.

• Li diffusion is very fast on Nb₂C (very low barrier ~35 meV), but slower on Nb₂CO₂ (barrier ~250 meV) due to stronger Li–O interactions.

• Under lithiation:

  • Nb₂C expands,

  • Nb₂CO₂ slightly contracts,
    and both remain structurally stable even with two Li monolayers.

• Thermodynamically, lithiation is stable for both materials, with the oxidized Nb₂CO₂ phase being more stable overall.

• The maximum theoretical capacities are:

  • ~275 mA h/g for Nb₂C (two-sided),

  • ~233 mA h/g for Nb₂CO₂ (two-sided),
    which are competitive with many state-of-the-art anode materials.

• The theoretical values are somewhat higher than experimental ones, as expected, because real materials have mixed functional groups and defects.