Exploring Multi-Transition-Metal NASICON Frameworks as High-Performance Cathodes for Sodium-Ion Batteries

Multi-Transition-Metal NASICON Cathodes for Sodium-Ion Batteries: Computational Insights

Introduction

This work delivers a systematic DFT-driven investigation of NASICON-type phosphate frameworks as cathodes for sodium-ion batteries (SIBs), focusing on the role of compositional complexity through multi-transition-metal (TM) substitution. The study explores compositions built from Mn, Cr, and Fe, examining unary, binary, and ternary mixtures, with the overarching objective of identifying compositions that balance phase stability, energy density (via voltage), redox accessibility, and Na⁺ transport. The property mapping addresses core bottlenecks in state-of-the-art SIB cathodes, including limited operational voltage windows, sluggish ionic transport, and structural disruption upon cycling.

Methods and Computational Framework

The authors leverage spin-polarized DFT calculations (SCAN+U, VASP) to determine structural, electronic, and thermodynamic properties across a nine-membered chemical space: three unaries (NaxMn2(PO4)3—NMP, NaxCr2(PO4)3—NCP, NaxFe2(PO4)3—NFP), three binaries (NMCP, NCFP, NFMP), and three ternaries (NMCFP, NCFMP, NFMCP). Redox energetics (Na intercalation voltages) are deduced via DFT-computed total energies; the phase stability is assessed through 0 K formation energies and convex hull analyses in the full Na–Mn–Cr–Fe–P–O space. The nudged elastic band method, using a hybrid machine-learned/DFT approach, quantifies Na⁺ migration barriers (Em). All calculations are referenced consistently, and Na-vacancy orderings are systematically enumerated to avoid configurational bias in bulk properties.

Structural Trends and Phase Stability

All systems largely preserve the canonical rhombohedral NASICON framework across compositions, with only minor lattice distortions. The analysis of formation energies indicates distinct stabilization patterns:

• Unaries: Strong stabilization at $x=3$ (NaxTM2(PO4)3), reflecting the thermodynamic preference of the TM³⁺ oxidation state (Cr^3+, Fe^3+), and associated Na-vacancy orderings.
• Mn-rich Unaries: Exhibit shallow convex hulls due to Jahn-Teller-active Mn^3+, leading to modest intermediate phase stabilization but potential structural instabilities.
• Binaries: Intermediate profiles, with TM-dependent variations—mixed redox activity and the role of local configurational interactions become pronounced.
• Ternaries: Show more complex pseudo-binary hulls and, notably, a shift of thermodynamic minima from $x=3$ to $x=2$ in some systems (e.g., NCFMP), indicating a redistribution of redox contributions and a change in Na-vacancy stabilization mechanism.

Critically, all systems lack sharp phase separation across $1 \le x \le 4$, with at least one stable or metastable intermediate composition, which is favorable for mitigating voltage hysteresis and supporting solid-solution–like electrochemical profiles.

Redox Activity and Intercalation Voltage Mapping

The computed intercalation voltages reveal:

• Fe^4+/Fe3+: Dominant contributor to high average voltage (~4.0 V), most notable in Fe-rich unaries and binaries (NFP, NFMP).
• Mn and Cr Redox: Mn participates at intermediate voltages (3.30–3.34 V; 3.88–3.95 V), while Cr acts at lower (Cr^3+/Cr2+) and higher (Cr^4+/Cr3+, ~4.1 V) voltages depending on local environment.
• Multi-TM Systems: The distribution of redox activity allows voltage tuning, but high-Fe systems can produce plateaus above the oxidative stability window of conventional Na electrolytes, limiting practical extractable capacity.

The ternary NaxMnFe0.5Cr0.5(PO4)3 (NMCFP) emerges as particularly promising: average voltage of 3.79 V, where all voltage plateaus lie within standard electrolyte stability limits. The redox transitions remain sharply defined across the chemical space, and pronounced anion involvement or hybridization is observed in some Fe-rich systems.

Electronic Structure

Band structure analysis indicates:

• Simple Unaries: Predictable narrowing of the band gap with increased sodiation, as the TM d states are filled.
• Multinary Systems: Non-monotonic evolution of Eg—combination of local TM arrangements, redox redistribution, and possible orbital hybridization yields nontrivial electronic structure evolution, leading to cases where increased sodiation does not decrease Eg or improve electronic conductivity.
• Valence Composition: The valence band edge is predominantly O 2p, with TM d state admixture; conduction bands derive mainly from unoccupied TM d states.

Thermodynamic Stability

The convex hull (Ehull) analysis demonstrates:

• Unaries (NMP, NCP): Maintained stability or marginal metastability (Ehull ≤ 25 meV/atom) across all $x$, consistent with viable synthesis and cycling.
• NFP: Marked instability at low Na content ($x=1,2$; Ehull > 100 meV/atom), underscoring Fe^4+'s unfavorable nature.
• Binaries: Most exhibit Ehull within the metastable regime (6–28 meV/atom for $x=3,4$). However, Fe-rich binaries become unstable at high oxidation states.
• Ternaries: Typically in the metastable regime ($\sim$20–50 meV/atom across $x$); the most favorable case (NMCFP) avoids strong phase separation and maintains moderate hull energies, qualifying for experimental accessibility through entropic or non-equilibrium stabilization mechanisms.

The lowest Ehull for NMCFP is at $x=4$, but synthetic routes avoiding formation of unary/binary decomposition products must be used to realize this phase.

Na⁺ Transport Properties

Na⁺ migration barriers (Em), calculated for the fully sodiated state ($x=3$0), fall within a favorable range for all compositions:

• NMP, NCP (Unaries): Em ≈ 0.30–0.32 eV; consistent with high Na⁺ mobility.
• NFP: Em = 0.56 eV; demonstrates the poor Na⁺ mobility characteristic of Fe-rich NASICONs.
• Binaries and Ternaries: Em typically in the range 0.30–0.40 eV. Mixed TM environments generally reduce migration barriers compared to pure Fe frameworks, supporting compositional complexity as a lever for improved ionic kinetics.
• Em values in all cases remain below thresholds (~0.65 eV) considered prohibitive for SIB cathode applications.

While calculated only at the fully discharged state, Em trends suggest facile Na⁺ transport is preserved across most (meta)stable multi-TM NASICONs.

Implications and Outlook

This work integrates compositional, electronic, and transport perspectives to qualify multi-TM NASICONs for practical SIB cathode deployment. The computational findings offer several design principles for next-generation NASICONs:

• Fe content elevates voltage but incurs instability and poor Na⁺ mobility in extreme cases.
• Mn and Cr introduce intermediate voltages with lower migration barriers and structural disorder due to Jahn-Teller effects.
• Multinary substitution allows rational decoupling of redox activity and ionic transport, yielding voltage tuning, phase behavior modification, and improved Na⁺ kinetics.
• The ternary NMCFP balances these trade-offs and represents a strong candidate for experimental validation.

Future work should integrate finite-temperature effects (vibrational and configurational entropy), ab initio molecular dynamics to directly access diffusion coefficients, and hybrid DFT functionals or GW for refined electronic properties. The intersection of computational prediction and targeted synthesis is likely to accelerate the realization of sustainable, high-rate, and high-voltage SIBs based on NASICON-type cathodes.

Conclusion

This comprehensive DFT-guided mapping of Mn–Cr–Fe NASICON frameworks delineates the connections between compositional complexity, redox landscape, phase stability, and Na⁺ mobility. The results support the rational design and experimental pursuit of metastable multi-TM NASICONs—particularly NaxMnFe0.5Cr0.5(PO4)3—for SIB cathodes. Key insights include the capacity for multi-TM substitution to enhance average voltage while preserving competitive ionic mobility and (meta)stability, thus positioning NASICON phosphates as robust, sustainable candidates within the SIB cathode domain. Continued integration of computational and experimental methodologies will further refine these design rules and facilitate next-generation sodium-based energy storage deployment.