High-Entropy Doped NASICON Cathode

• The paper introduces a high-entropy doping strategy in NASICON cathodes, where trace multication substitution stabilizes the structure and widens Na⁺ diffusion channels.
• Electrochemical analyses reveal improved kinetics with reduced polarization and enhanced capacity (up to 119 mAh g⁻¹) compared to pristine NVP.
• The integration of precise synthesis, structural characterization, and Bayesian optimization offers a scalable pathway for optimizing sodium-ion battery cathodes.

A high-entropy doped NASICON cathode refers to a sodium superionic conductor (NASICON) with multicomponent cationic doping at a transition-metal site, designed to enhance ionic and structural performance for sodium-ion batteries (SIBs). The prototypical system, Na₃V₂(PO₄)₃ (NVP), is modified by partial substitution of the vanadium site with a configurationally diverse set of cations—at trace levels—yielding a single-phase compound stabilized by high configurational entropy. This approach aims to leverage entropy stabilization, lattice tuning, and redox activation to overcome traditional performance bottlenecks while maintaining the robust polyanion scaffold of the NASICON framework (Singh et al., 4 Feb 2026).

1. Synthesis, Stoichiometry, and Structural Effects

High-entropy doping in NASICON cathodes is exemplified by Na₃V₁.₉(Cr₀.₀₂,Mo₀.₀₂,Al₀.₀₂,Zr₀.₀₂,Ni₀.₀₂)(PO₄)₃ (abbreviated as "NVP-HE"). The vanadium site is shared by V³⁺ (1.90 f.u.) and five dopant cations (0.02 f.u. each, total 0.1 f.u.), giving six distinct cationic species per site. Synthesis proceeds via a sol–gel route: stoichiometric precursors (NH₄VO₃, NH₄H₂PO₄, NaNO₃, metal nitrates), citric acid (80 wt % of precursors), and carbon nanotubes (5 wt %) are gelled at 70–80 °C, dried under vacuum at 120 °C, and calcined at 350 °C (4 h) then 800 °C (8 h) under Ar/5% H₂ atmosphere. Reference NVP is identically prepared excluding dopants (Singh et al., 4 Feb 2026).

Configurational entropy, $$S_{\mathrm{conf}} = -R[x_\mathrm{V} \ln x_\mathrm{V} + 5x_i \ln x_i]$$, is calculated as ≃2.3 J mol⁻¹ K⁻¹ per mole of cation-site mixing, with site fractions $x_\mathrm{V}=0.95$, $x_i=0.01$. Entropy stabilization is manifested structurally: the lattice parameters expand slightly (NVP: $a \approx 8.7343$ Å, $c \approx 21.8210$ Å; NVP-HE: $a=8.7369$ Å, $c=21.8378$ Å), with the unit cell volume increasing by ≃0.2%. X-ray diffraction and Rietveld refinement confirm phase purity (rhombohedral, $R\bar{3}c$, $R_\mathrm{wp} \approx 1.7\%$) and a shift of the (012) reflection consistent with lattice expansion. Refinement reveals elongation of the V–O bond (2.040 Å vs 2.037 Å in NVP) and stiffening of the P–O bond (1.509 Å vs 1.518 Å), suggesting enhanced Na⁺ bottlenecking and rigidity of the PO₄ network (Singh et al., 4 Feb 2026).

2. Electrochemical Performance and Full Cell Metrics

NVP-HE exhibits distinct electrochemical advantages over pristine NVP. Cyclic voltammetry (CV; 0.1 mV s⁻¹, 2.0–4.3 V vs Na/Na⁺) shows the canonical V³⁺/V⁴⁺ redox at ≈3.57/3.21 V and, uniquely to HE doping, a V⁴⁺/V⁵⁺ couple at ≈3.96/3.93 V. The CV peak separation is reduced to ≈50 mV (vs 90 mV), indicating accelerated kinetics. Galvanostatic charge/discharge (GCD; 0.1 C–10 C) yields a specific capacity of 119 mAh g⁻¹ at 0.1 C (vs 105 mAh g⁻¹ for NVP), with two voltage plateaus (3.41/3.34 V and 3.95 V). NVP-HE retains 72% of low-rate capacity at 5 C and recovers 95% when returned to 0.1 C. Long-term cycling retains 93% at 2 C after 100 cycles and 68% at 10 C after 1000 cycles. In full cells (hard-carbon anode, ≈1.2:1 cathode:anode mass ratio), initial discharge (2.0–4.3 V) delivers 106 mAh g⁻¹ (cathode basis) at ≈3.2 V and ≈326 Wh kg⁻¹ (normalized to cathode); 79% capacity is retained after 100 cycles at 2 C (Singh et al., 4 Feb 2026).

3. Na⁺ Diffusion Mechanisms and Kinetics

Multiple techniques quantify Na⁺ transport kinetics in NVP-HE. CV analysis via the Randles–Sevčík equation yields diffusion coefficients $D_{\mathrm{Na}^+}$ of $6.0\times10^{-12}$ cm² s⁻¹ (anodic) and $3.7\times10^{-11}$ cm² s⁻¹ (cathodic). Galvanostatic intermittent titration (GITT) measurements fit Fick’s law, providing $D_\mathrm{charge}\approx7.5\times10^{-11}$ cm² s⁻¹, $D_\mathrm{discharge}\approx3.1\times10^{-11}$ cm² s⁻¹. Electrochemical impedance spectroscopy (EIS), analyzed via the Warburg regime, yields $D_{\mathrm{Na}^+}\approx10^{-11}$–$10^{-12}$ cm² s⁻¹, consistent across methods. The diffusion coefficients fall within $10^{-11}$–$10^{-13}$ cm² s⁻¹, with both interfacial transfer and bulk migration accelerating at elevated temperatures. Arrhenius fits for charge-transfer and SEI passage yield activation energies $E_\mathrm{a}\approx0.68$ eV and $0.25$ eV, respectively (Singh et al., 4 Feb 2026).

4. Interfacial Kinetics and Distribution of Relaxation Times

Detailed interfacial processes are resolved through EIS-DRT (distribution of relaxation times) analysis. The circuit model includes $R_0$ (ohmic), $R_1$/$Q_1$ (SEI resistance/CPE), $R_2$/$Q_2$ (charge transfer), and Warburg ($Q_3$) elements. DRT recovers the polarization resistance distribution $g(\tau)$, identifying distinct processes without circuit presumption. Peaks are assigned to: P1 (≈12 μs, CNT contact), P2 (≈0.1 ms, CNT network), P3 (≈1.5 ms, Na/electrolyte charge transfer), P4 (≈22 ms, Na⁺/SEI migration), P5 (≈0.3 s, cathode/electrolyte charge transport), and P6 (≈6.6 s, solid-state Na⁺ diffusion). R₁ and R₂ decrease significantly during charge (307→30 Ω, 1680→184 Ω, 2.4→4.3 V), indicating rapid Na⁺ extraction and multivalent redox activation. At low voltages, resistances rise due to site re-occupation. Elevated temperatures lower all R values (e.g., R₀: 5.7→2.9 Ω at 28→55 °C); a new relaxation (17 ms) emerges above 35 °C, attributed to secondary interphase formation (Singh et al., 4 Feb 2026).

5. Design Principles and Guidelines for High-Entropy Doped NASICON

High-entropy doping at the vanadium site stabilizes the NASICON lattice via $S_{\mathrm{conf}}$ increase, modulates local structure (V–O elongation, P–O contraction), and widens Na⁺ diffusion bottlenecks while activating the high-voltage V⁴⁺/V⁵⁺ couple. Electrochemical metrics—capacity, polarization, rate performance, cycling retention—are consistently improved vs. undoped NVP. Interface engineering via in-situ EIS+DRT resolves and optimizes charge transfer and diffusion bottlenecks. For future cathode design: trace multi-cation doping (≤0.1 f.u.) preserves phase purity and entropy benefits; dopant selection criteria include mixed valence and variable ionic radii to optimize both electronic structure and Na⁺ mobility. Carbon/CNT integration remains essential for electron percolation (Singh et al., 4 Feb 2026).

6. Computational Design and Bayesian Optimization

Element mapping Bayesian optimization (BO) provides a systematic framework for dopant selection in NASICON-type cathodes (Park et al., 2024). The discrete elemental space is embedded in a continuous, chemically meaningful descriptor space (e.g., unary score reflecting redox voltage compatibility). Gaussian Process surrogates (with Matérn kernel) operate in this mapped space and drive iterative composition selection. The methodology identifies optimal binary doped Na₃(Eᵢ₍²⁻ʸ₎Eⱼʸ)(PO₄)₂F₃ compositions targeting all plateau voltages within a desired window. The approach is extendable to true high-entropy systems ($r\geq4$ distinct dopants, $x_m\approx2/r$), with performance validated through DFT and experiment. A plausible implication is that such data-driven optimization, when combined with high-entropy design, can expedite the discovery of multi-cation frameworks with targeted redox and transport properties (Park et al., 2024).

7. Context and Generalization

NVP-HE demonstrates how trace-level high-entropy doping triggers synergistic improvements in structural robustness, Na⁺ mobility, and redox activity, providing broad guidelines for future cathode materials. Element-mapping-driven Bayesian optimization broadens the compositional search space, especially for multi-dopant (high-entropy) phases. However, while current optimization frameworks have predominantly produced binary-doped compounds, extension to $r\geq4$ (true high entropy) remains an active research avenue. The combined synthesis, electrochemical, and computational methodologies present a scalable paradigm for rational multi-cation optimization in SIB cathodes (Singh et al., 4 Feb 2026, Park et al., 2024).