NASICON Na₄NiCr(PO₄)₃ Cathode

• Na₄NiCr(PO₄)₃ is a sodium-rich, mixed transition-metal phosphate in a NASICON framework designed for high-voltage sodium-ion batteries.
• Its crystal structure, confirmed by Rietveld refinement, features three-dimensional Na⁺ channels formed by corner-sharing [NiO₆] and [CrO₆] octahedra with PO₄ tetrahedra.
• Electrochemical studies reveal high initial charge capacity with irreversible redox behavior attributed to low electronic conductivity and kinetic limitations.

Na₄NiCr(PO₄)₃ is a sodium-rich, mixed transition-metal phosphate exemplifying the NASICON (sodium super ionic conductor) framework, developed as a high-voltage cathode material for sodium-ion batteries (SIBs). Its structure, redox chemistry, electrochemical behavior, and intrinsic limitations position it uniquely within the family of polyanion-based cathodes, highlighting both the potential and the fundamental challenges of high-voltage NASICON chemistry (Sharma et al., 11 Jan 2026, Sharma et al., 27 Apr 2025).

1. Synthesis and Crystal Structure

Na₄NiCr(PO₄)₃ adopts a rhombohedral NASICON-type structure in space group R$\bar{3}$c (No. 167). Typical synthetic protocols employ a modified sol–gel method using stoichiometric amounts of sodium, nickel, chromium, and phosphate precursors, followed by multi-step calcination in air or under argon. An additional ex situ carbon-coating step can be applied to improve electronic contact.

Rietveld refinement of powder X-ray diffraction data confirms the stabilization of the rhombohedral NASICON framework, with lattice parameters $a = b = 8.8165(2)\,\text{\AA}$, $c = 21.2658(5)\,\text{\AA}$, and unit cell volume $V = 1431.54\,\text{\AA}^3$. The crystal structure consists of three-dimensional corner-shared [NiO₆] and [CrO₆] octahedra integrated with PO₄ tetrahedra, forming robust, open channels. Na⁺ occupies two distinct sites: Na1 (octahedral, Wyckoff 6b) and Na2 (distorted dodecahedral, Wyckoff 18e), with partial occupancies reflecting the dynamic sodium environment.

Supplementary structural characterization includes Raman and FTIR spectroscopy, revealing PO₄ vibrational modes and transition metal–oxygen stretching frequencies. X-ray photoelectron spectroscopy (XPS) establishes the presence of Na⁺, O²⁻, P⁵⁺, Cr³⁺, and mixed Ni²⁺/Ni³⁺ states. The observed Ni³⁺ content is balanced by local oxygen vacancies (Sharma et al., 11 Jan 2026).

2. Redox Chemistry and Sodium-Ion Diffusion

Na₄NiCr(PO₄)₃ is designed to access both Ni²⁺/Ni³⁺ and Cr³⁺/Cr⁴⁺ redox couples upon Na-ion extraction, enabling high-voltage, multi-electron operation. The theoretical processes are summarized as:

• Ni²⁺/Ni³⁺ oxidation (~4.7–4.8 V vs. Na⁺/Na)
• Cr³⁺/Cr⁴⁺ oxidation (~4.3–4.5 V)

Each redox step corresponds to the removal of one Na⁺ per formula unit, conferring a total theoretical capacity of $\sim$53.6 mAh g$^{-1}$ (Sharma et al., 27 Apr 2025). The stepwise deintercalation ideally yields a dual plateau voltage profile, with Ni and Cr contributing sequentially to charge storage.

Bond valence energy landscape (BVEL) analysis demonstrates a continuous, three-dimensional Na⁺ diffusion network connecting Na1 and Na2 sites, with a calculated migration energy barrier $E_{\mathrm{mig}} = 0.468$ eV in the pristine state. This value is within the range for facile Na⁺ solid-state transport in NASICON hosts, suggesting that the polyanion framework does not impose prohibitive ionic bottlenecks (Sharma et al., 11 Jan 2026).

3. Electrochemical Performance and Kinetics

Experimental galvanostatic cycling reveals that Na₄NiCr(PO₄)₃ delivers a substantial initial charge capacity at voltages approaching 4.5 V but exhibits essentially zero discharge capacity under identical conditions. For the air-calcined analogue, a first-charge value of 123.5 mAh g$^{-1}$ is observed, but discharge capacity is $\approx$2.7 mAh g$^{-1}$; carbon-coated samples show modest improvement but remain fundamentally limited. Capacity retention after the first cycle is negligible.

Cyclic voltammetry in PF₆⁻-based electrolyte reveals a broad anodic peak at ~4.5 V (Cr³⁺/Cr⁴⁺ activity), a minor feature at ~3.9 V (potentially Ni²⁺/Ni³⁺ or electrolyte oxidation), and no corresponding cathodic peaks on return sweep—direct evidence for irreversible Na extraction.

Electrochemical impedance modeling highlights large charge-transfer resistance components: for pristine NNCP in air, $R_{\mathrm{ct1}} = 1167\,\Omega$, $R_{\mathrm{ct2}} = 3518\,\Omega$, both orders of magnitude higher than analogous SIB cathodes. Carbon coating or Ar-treatment reduces, but does not eliminate, kinetic limitations (Sharma et al., 11 Jan 2026).

4. Structural Stability and Post-Mortem Analysis

Ex situ XRD after multiple charge–discharge cycles confirms that the R$\bar{3}$c structure remains largely intact, with no decomposition into secondary phases. Minor structural relaxations are observed: P–O bond lengths and Na site volumes undergo small expansions; Ni/Cr–O bonds contract slightly.

BVEL mapping after cycling reveals a decreased Na⁺ migration barrier (0.324 eV), which remains compatible with efficient ionic conduction. Notably, sodium site stability is reversed (Na2 more stable post-cycling), but these variations alone do not explain the persistent lack of reversible Na reinsertion (Sharma et al., 11 Jan 2026).

5. Limitations and Underlying Challenges

Despite the presence of well-connected Na pathways and robust NASICON framework, Na₄NiCr(PO₄)₃ suffers from extremely low electronic conductivity (measured σₑ ≈ $6.6 \times 10^{-9}$ S cm$^{-1}$). This poor transport is attributed to the inability to stabilize hole polarons on Ni in the highly covalent phosphate matrix, suppressing electronic percolation required for full redox reversibility.

Additionally, the Cr³⁺/Cr⁴⁺ couple is prone to irreversibility. Difficulty re-nucleating the discharged phase, coupled with interfacial side reactions at very high voltages (>4.5 V), results in rapid impedance growth and kinetic arrest, even in surface-modified variants. The result is substantial initial sodium extraction and almost no reversibility, regardless of surface engineering or thermal history (Sharma et al., 11 Jan 2026).

6. Comparative Context Within NASICON Phosphates

Within the broader family of NASICON cathodes, Na₄NiCr(PO₄)₃ stands out for its extreme redox potentials and structural rigidity but is penalized by low gravimetric capacity and high electron-transport resistance.

Formula	Space Group	Nominal TM Redox	Qₜₕ (mAh/g)	Vₐᵥg (V)	Qₑₓₚ (mAh/g)	Cycle Retention
Na₄NiCr(PO₄)₃	R–3c	Ni²⁺/Ni³⁺, Cr³⁺/Cr⁴⁺	53	4.6	50	90% (500)
Na₄MnCr(PO₄)₃	R–3c	Mn²⁺/³⁺/⁴⁺, Cr³⁺/⁴⁺	165	3.5	160	86% (600)
Na₃VCr(PO₄)₃	R–3c	V³⁺/⁴⁺/⁵⁺	117	4.0	90	90% (200)
Na₃Fe₂(PO₄)₃	C2/c	Fe²⁺/Fe³⁺	118	3.3	109	90% (6400)

Relative to these analogues, Na₄NiCr(PO₄)₃ offers the highest voltage but sacrifices both specific capacity and reversibility. This trade-off is a direct consequence of transition-metal content and the high redox potentials imposed by the polyanionic lattice (Sharma et al., 27 Apr 2025).

7. Prospects for Functional Implementation

To address electronic and redox reversibility limitations, several targeted strategies are proposed:

• Aliovalent doping on Ni or Cr sites to promote stable polaron conduction.
• Optimized carbon coating (through in situ precursors) to form uniform, conductive shells.
• Framework engineering (strain, controlled microstructure) to enhance phase transformation kinetics and Cr redox reversibility.
• Electrolyte modification (using high-voltage-resistant salts/additives) to suppress interfacial degradation at >4.5 V.

The successful integration of these approaches could unlock multi-electron, >4.5 V operation in sodium-ion batteries, potentially bridging the electrochemical gap with commercial lithium systems, but Na₄NiCr(PO₄)₃ underscores the persistent challenge of balancing ionic and electronic transport in mixed-polyanion, high-voltage SIB hosts (Sharma et al., 11 Jan 2026, Sharma et al., 27 Apr 2025).