The relentless march towards global digitalization has precipitated an explosive demand for electrochemical energy storage devices. Concurrently, the finite nature of fossil fuel resources and their attendant environmental degradation present a persistent global challenge. The development and utilization of renewable energy sources offer a dual-pronged solution, mitigating both energy scarcity and pollution. In this context, the pursuit of novel and superior energy storage materials is paramount. Among the plethora of storage technologies, rechargeable batteries have garnered significant market favor due to their compact size, high performance, and environmental benignity. Leveraging the abundant reserves and low cost of sodium resources, sodium-ion battery technology exhibits tremendous application potential in the energy storage sector.
Sodium, being far more abundant and widely distributed than lithium, emerges as a more economical alternative for battery chemistry. The inherently lower cost of sodium-ion battery systems makes them exceptionally attractive for large-scale grid storage applications. Sharing similar physicochemical characteristics as alkali metals, sodium-based batteries are widely regarded as the most viable candidates to supplant lithium-ion systems. However, to bolster the competitiveness of sodium-ion battery technology, the development of high-energy-density cathode materials is crucial. Consequently, intensive research efforts are directed towards exploring cathode materials, with sodium layered oxides and polyanionic compounds at the forefront. The key to advancing the commercialization of sodium-ion battery lies in developing cathode materials that excel in safety, energy/power density, cost-effectiveness, and longevity.
Among the most researched polyanionic cathodes are vanadium-based phosphates, notably Na3V2(PO4)3 (NVP), which adopts a Na Superionic Conductor (NASICON) structure. NASICON-type materials are characterized by a robust three-dimensional framework built from corner-sharing [MO6] octahedra and [PO4] tetrahedra. This architecture provides open, three-dimensional channels conducive to rapid Na+ diffusion, while the strong covalent bonding ensures exceptional structural and thermal stability, translating to long cycle life. The general formula for such materials can be represented as NaxMM'(PO4)3, where M and M’ are transition metals. Despite its promise, pristine NVP often suffers from lower-than-theoretical practical capacity and suboptimal cycling stability. Strategies to ameliorate these issues include nanostructuring, carbon coating, and elemental doping. Doping, in particular, is a potent method to tune the composition, enhance electronic conductivity, and improve Na+ diffusion kinetics.
While experimental synthesis and testing are indispensable, they can be time-consuming and costly for exploring vast compositional spaces. The precise measurement of atomic-scale properties, such as migration barriers and electronic structure evolution, also poses significant challenges in the lab. Herein lies the power of computational materials science. With the exponential growth in computational power, the paradigm of “theory-guided material design” has gained prominence. First-principles calculations based on Density Functional Theory (DFT) provide an unparalleled atomic-scale lens to probe the sodiation/desodiation mechanisms, predict electrochemical properties, assess structural stability, and understand the fundamental origins of electrochemical activity in sodium-ion battery cathodes. This work employs systematic DFT calculations to screen and evaluate transition metal-doped NASICON-type materials, aiming to identify high-performance candidates and elucidate the underlying principles governing their performance.
Computational Methodology
All spin-polarized first-principles calculations were performed within the framework of Density Functional Theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP). The ion-electron interactions were described using the Projector Augmented Wave (PAW) method. For the exchange-correlation functional, we employed the generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerhof (PBE). A plane-wave basis set with a kinetic energy cutoff of 400 eV was used. For structural relaxation, the Brillouin zone was sampled using a 3×3×1 Monkhorst-Pack k-point mesh, which was increased to 5×5×1 for electronic structure calculations (e.g., Density of States). All atomic positions and lattice parameters were fully relaxed until the Hellmann-Feynman forces on each atom were less than 0.02 eV/Å, and the self-consistent field energy convergence was set to 1×10−6 eV.
To investigate the Na+ diffusion kinetics, the climbing-image nudged elastic band (CI-NEB) method was employed to locate the minimum energy path (MEP) and the corresponding activation energy barrier (Ea). Five intermediate images were interpolated between the initial and final states along the presumed diffusion pathway.
The average voltage (V) for a sodium extraction/insertion process between two phases, NaxMP and NayMP (where MP represents the host framework and y > x), can be calculated from the total energies according to the following formula:
$$ V = -\frac{[E(\text{Na}_y\text{MP}) – E(\text{Na}_x\text{MP}) – (y-x)E(\text{Na})]}{(y-x)e} $$
where \( E(\text{Na}_y\text{MP}) \) and \( E(\text{Na}_x\text{MP}) \) are the total energies of the discharged and charged cathode materials, respectively, \( E(\text{Na}) \) is the energy per atom of bulk metallic sodium, and \( e \) is the elementary charge. The voltage is typically reported in volts (V).
Results and Discussion
1. Structural Models and Candidate Screening
The parent compound for this study is Na3V2(PO4)3 with a rhombohedral NASICON structure (space group R-3c). The framework consists of VO6 octahedra sharing corners with PO4 tetrahedra, creating a three-dimensional network. Sodium ions occupy two distinct crystallographic sites: the Na(1) site (6b, octahedral coordination) and the Na(2) site (18e, distorted octahedral coordination). Doping was modeled by substituting one or both vanadium (V) atoms in the unit cell with other transition metals (Ti, Cr, Mn, Fe). We considered various doping configurations, including para-, neighbor-, and far-neighbor substitutions, and fully relaxed all structures to identify the most stable configuration for each composition. A series of doped compounds were systematically investigated: Na3TiV(PO4)3, Na3CrV(PO4)3, Na3MnV(PO4)3, Na3FeV(PO4)3, and the binary-doped systems Na3TiMn(PO4)3, Na3TiFe(PO4)3, Na3CrMn(PO4)3, Na3CrFe(PO4)3, and Na3MnFe(PO4)3.
2. Electronic Structure and Conductivity
The electronic conductivity of a cathode material is a critical factor influencing rate capability and polarization. We analyzed the density of states (DOS) for the pristine and doped compounds. A key finding is that the electronic structures of Na3TiMn(PO4)3 and Na3TiFe(PO4)3 exhibit significantly higher electronic state density near the Fermi level compared to pristine Na3V2(PO4)3.
| Material | Key Electronic Structure Feature | Inferred Conductivity |
|---|---|---|
| Na3V2(PO4)3 (NVP) | Modest state density at Fermi level. | Moderate, typical semiconductor. |
| Na3TiMn(PO4)3 | Enhanced state density near Fermi level. | Improved electronic conductivity. |
| Na3TiFe(PO4)3 | Enhanced state density near Fermi level. | Improved electronic conductivity. |
The enhanced electronic state density near the Fermi level in these Ti-containing double-doped systems suggests a reduced band gap or the presence of more charge carriers, implying superior intrinsic electronic conductivity. This is a crucial advantage for a sodium-ion battery cathode, as it facilitates faster electron transfer during the charge/discharge process, thereby improving rate performance and reducing polarization.
3. Sodium-Ion Diffusion Kinetics
The NASICON structure is renowned for its fast ionic conduction. We mapped out three primary probable diffusion pathways for Na+ migration within the three-dimensional network, labeled as Path I, Path II, and Path III, connecting different Na(1) and Na(2) sites. The CI-NEB method was then used to calculate the energy profiles and activation barriers (Ea) for these paths in Na3V2(PO4)3, Na3TiMn(PO4)3, and Na3TiFe(PO4)3.
For pristine NVP, the calculated barriers are approximately 0.12 eV for Path I and Path III, and 0.11 eV for Path II, consistent with literature values and confirming favorable Na+ mobility. For the doped compounds Na3TiMn(PO4)3 and Na3TiFe(PO4)3, while Path I shows an extremely low barrier (~0.04 eV), it corresponds to a large energy difference (ΔE ~ 0.60 eV) between the initial and final states. This large ΔE indicates the process is thermodynamically unfavorable for reversible cycling. In contrast, Path II and Path III in these materials exhibit near-zero ΔE between endpoints, with activation barriers calculated in the range of 0.12 eV to 0.18 eV. These values are comparable to or slightly higher than those in pristine NVP but remain within a very favorable range for fast ionic transport.
The results indicate that Na+ diffusion in these promising doped cathodes is both thermodynamically reversible (small ΔE for dominant paths) and kinetically facile (low Ea), which is essential for achieving high power density in a sodium-ion battery.
| Material | Primary Diffusion Path | ΔE (eV) | Ea (eV) | Kinetic Assessment |
|---|---|---|---|---|
| Na3V2(PO4)3 | Path II | ~0 | ~0.11 | Excellent |
| Na3TiMn(PO4)3 | Path I | ~0.60 | ~0.04 | Thermodynamically unfavorable |
| Paths II & III | ~0 | 0.12 – 0.18 | Favorable | |
| Na3TiFe(PO4)3 | Path I | ~0.60 | ~0.04 | Thermodynamically unfavorable |
| Paths II & III | ~0 | 0.12 – 0.18 | Favorable |
4. Voltage Profile and Electrochemical Stability
The operating voltage is a key parameter determining the energy density of a sodium-ion battery. Using the formula above, we computed the average voltages for sequential Na+ extraction from the screened materials. In NASICON structures, Na+ ions at the Na(2) sites are generally electrochemically active and extract first, which aligns with our computational models.
A notable finding is the evolution of the voltage profile. Pristine Na3V2(PO4)3 shows significant voltage steps during desodiation, which can be linked to phase transformations and different local structural rearrangements as Na+ is removed. In contrast, the calculated voltage profiles for Na3TiMn(PO4)3 and Na3TiFe(PO4)3 are characterized by more stable voltage plateaus. A flatter voltage profile is highly desirable for battery applications as it indicates smoother phase transitions, less structural strain, and more predictable state-of-charge management in a sodium-ion battery system.
The combination of a stable voltage profile, good electronic conductivity, and favorable Na+ diffusion kinetics positions these double-doped materials as highly promising candidates. The multi-electron redox activity of the Mn3+/Mn4+ and Fe3+/Fe4+ couples, in combination with Ti4+ acting as a stabilizing spectator ion, can potentially offer high capacity while maintaining structural integrity.
Conclusion
This comprehensive first-principles computational study systematically investigated the geometric structure, electronic properties, Na+ diffusion kinetics, and voltage characteristics of NASICON-type polyanionic cathode materials for sodium-ion battery applications, focusing on transition metal doping of the benchmark Na3V2(PO4)3 compound. The key conclusions are summarized as follows:
- Electronic Structure Screening: Analysis of the density of states revealed that the double-doped systems Na3TiMn(PO4)3 and Na3TiFe(PO4)3 possess a richer electronic state density near the Fermi level compared to the parent material. This characteristic predicts enhanced intrinsic electronic conductivity, which is crucial for achieving high-rate capability in a sodium-ion battery.
- Ion Transport Properties: CI-NEB calculations mapped the Na+ migration pathways and energy barriers. For the promising candidates Na3TiMn(PO4)3 and Na3TiFe(PO4)3, the dominant diffusion paths exhibit near-zero thermodynamic energy differences and favorable activation barriers in the range of 0.12–0.18 eV. This confirms that these materials support both thermodynamically reversible and kinetically facile sodium-ion transport.
- Electrochemical Performance Prediction: Voltage profile calculations indicated that Na3TiMn(PO4)3 and Na3TiFe(PO4)3 offer more stable voltage plateaus during charge/discharge compared to the stepped profile of pristine NVP. A flat voltage plateau is a key indicator of smooth electrochemical reactions and good structural stability.
By integrating the assessments of electronic conductivity, ionic diffusivity, and electrochemical stability, this theoretical work successfully identifies Na3TiMn(PO4)3 and Na3TiFe(PO4)3 as high-performance NASICON-type cathode candidates for advanced sodium-ion battery technology. These findings provide valuable atomic-scale insights and a clear theoretical guideline for the experimental synthesis and development of next-generation polyanionic cathode materials, accelerating the rational design of efficient and cost-effective energy storage systems.