NASICON Nanofilms in Sodium ASSBs

• NASICON nanofilms are nanoscale thin films derived from sodium superionic conductor materials, serving as solid electrolytes in sodium-based all-solid-state batteries.
• Common synthesis methods include ion-beam sputter deposition and sol–gel spray deposition with rapid NIR sintering, enabling energy-efficient film production.
• Ion implantation and defect engineering tailor ionic conductivity and microstructure, reducing device resistance and enhancing performance in energy storage systems.

NASICON nanofilms are nanoscale thin films based on sodium superionic conductor (NaSICON) materials, a family of inorganic compounds with the general formula Na₃Zr₂Si₂PO₁₂ and related stoichiometries. These films are distinguished by their high ionic conductivity for Na⁺ and have critical relevance as solid electrolytes in sodium-based all-solid-state batteries (ASSBs) and other electrochemical energy storage systems. NASICON nanofilms offer prospects for reduced device resistance, tunable defect landscapes, and compatibility with micro-fabrication, enabling innovative approaches to miniaturized and high-performance sodium-ion devices.

1. Synthesis Approaches for NASICON Nanofilms

Two principal methods for NASICON nanofilm fabrication are documented: ion-beam sputter deposition and sol–gel based spray deposition with near-infrared (NIR) sintering.

Ion-Beam Sputter Deposition

The mixed-oxide solid-state reaction is used to synthesize NASICON targets. Precursor reagents include NH₄H₂PO₄, ZrO₂, Na₂CO₃, and fumed SiO₂, combined in stoichiometric ratios and processed via planetary ball milling and multiple high-temperature calcinations (first at 1150 °C, then at 1250 °C). The dense pellets, either made in-house (with or without intermediate regrinding) or from commercial NZSP powders, serve as targets for sputtering (Ceccio et al., 22 Jan 2026).

A home-built Low Energy Ion Facility delivers Ar⁺ ions at 20 keV onto the target, with the material sputtered onto Si <100> substrates mounted at an angle. Film growth is conducted at ambient temperature (~25 °C). The resulting nanofilms, denoted according to their parent pellet batch (“NASICON 2,” “NASICON 4,” “NASICON 5”), are continuous and uniform, although growth rate, chamber pressure, and final thickness are not specified (Ceccio et al., 22 Jan 2026).

Sol–Gel Spray Deposition and NIR Sintering

A complementary methodology utilizes a sol–gel route, where aqueous solutions of ammonium dihydrogen phosphate, sodium metasilicate, tartaric acid, and zirconyl chloride are combined in an Na:Zr:Si:P molar ratio of 4:2:2:1, with tartaric acid acting as a chelating agent (Valls et al., 2024). Spray deposition onto O₂ plasma-treated fused silica at 120 °C builds up microns-thick precursor films by serial layering.

These “green” films undergo ultrafast NIR sintering using a 36 kW adphosNIR® system. Achieving ≥1000 °C in 60 s is facilitated by a prior 1 h 750 °C step that forms a carbon-rich intermediate with heightened optical absorbance in the NIR spectrum, which combusts during the NIR pulse, driving rapid densification and crystallization (Valls et al., 2024). This process contrasts with traditional oven sintering (>3 h at 1000 °C), offering ~180× time and energy reductions while preserving phase composition and eliminating micron-scale heterogeneity.

2. Morphology and Structural Characteristics

Sputter-Deposited Nanofilms

Scanning electron microscopy reveals that as-deposited NASICON nanofilms are smooth and uniform, with minimal particulates. Post-nickel implantation, surface nanostructures emerge depending on fluence and target origin: low-fluence (1×10¹⁴ Ni/cm²) introduces incipient roughening, while higher fluences promote nanocluster formation, especially in films derived from commercial NZSP (Ceccio et al., 22 Jan 2026).

X-ray diffraction (XRD) demonstrates that these films are predominantly amorphous, presenting only a broad hump between 18°–39° 2θ (Cu Kα); sharp crystalline NASICON peaks are absent. The Si substrate diffraction peaks remain distinct (Ceccio et al., 22 Jan 2026).

Spray/Sintered Microstructure

Cross-sectional SEM of NIR- and conventionally sintered films shows crack-free, continuous films with thicknesses of ~7–17 µm (depending on number of layers). Top-view imaging displays that furnace-sintered material has micron-scale + sub-micron phase inhomogeneity (SiO₂ islands, NASICON-rich regions), while NIR-sintered films are microstructurally homogeneous with grain sizes <200 nm and no significant SiO₂ segregation (Valls et al., 2024). Porosity data is not reported but films appear dense at SEM scale.

XRD confirms dominant Na₃Zr₂(SiO₄)₂PO₄ phase for successfully processed films, with secondary silica and zirconia phases detected when sintering is incomplete or the protocol is suboptimal. GI-XRD identifies substrate-near silica phases confined to the interface (Valls et al., 2024).

3. Ion-Beam Modification and Defect Engineering

Sputtered nanofilms undergo post-deposition ion implantation with Ni²⁺ at 1.1 MeV across fluences of 1×10¹⁴, 5×10¹⁴, 1×10¹⁵ Ni/cm² using a tandem accelerator. At these energies, the projected Ni range is several µm — far exceeding film thickness — so ions traverse the films, with energy loss primarily via electronic stopping (ionization, bond breaking) rather than nuclear collisions (Ceccio et al., 22 Jan 2026).

Irradiation at low fluence initially raises film impedance due to disruption of Na⁺ migration pathways. At medium fluence (5×10¹⁴ Ni/cm²), impedance decreases markedly, attributable to the formation of mobile-vacancy defects and hopping sites in the amorphous matrix. At highest fluence, trends diverge: some films exhibit impedance recovery (over-damage and recombination), others retain low resistance if the defect density optimally balances percolation and structure (Ceccio et al., 22 Jan 2026). These results emphasize that defect engineering by precise ion dosing can optimize ionic conductivity.

4. Functional Properties: Ionic Conductivity and Electrochemical Behavior

Electrochemical impedance spectroscopy (EIS) characterizes the films’ transport properties at 25 °C, using Nyquist plots and equivalent circuits comprised of a bulk resistance $R_b$ in series with a constant phase element (CPE). The ionic conductivity, $\sigma$, is calculated as:

$\sigma = \frac{L}{R_b \, A}$

where $L$ is film thickness and $A$ electrode area (Ceccio et al., 22 Jan 2026).

While absolute conductivity values are not reported, the qualitative trends are:

• Lowest $\sigma_{RT}$ (highest $R_b$) at low implantation fluence.
• Maximum $\sigma_{RT}$ (lowest $R_b$) at medium fluence for most films.
• Either a partial recovery (increased $R_b$) or maintenance of low $R_b$ at highest fluence, depending on preparation and defect balance (Ceccio et al., 22 Jan 2026).

For amorphous NASICON, the activation energy $E_a$ for transport is typically higher ($\sim$0.2–0.3 eV) than for bulk crystalline NASICON, but $E_a$ itself is unreported here. Dielectric properties and temperature dependence for these nanofilms have not been explicitly measured (Ceccio et al., 22 Jan 2026, Valls et al., 2024).

Functional properties (ionic conductivity, adhesion, thermal stability) for NIR-sintered spray-cast films have yet to be evaluated. Qualitative adhesion is robust enough for post-processing, but no mechanical or electrochemical data are provided (Valls et al., 2024).

5. Comparison of Processing Techniques and Performance Implications

Method	Amorphous/Crystalline	Thickness	Microstructure	Processing Time	Conductivity Data
Sputter + Ni irradiation	Predominantly amorphous	Not specified (nano)	Smooth, then nanostructured after implantation	Minutes+	Enhanced at optimal ion fluence
Spray + NIR sintering	Crystalline	1.4–17 µm	Dense, uniform, <200 nm grains	1 min (NIR), 3 h (furnace)	Not reported

The sputter-deposited films permit amorphous NASICON formation, with defect engineering tuning ionic transport. Their ultrathin geometry (inferred as nanometers) is intrinsically advantageous for minimizing solid electrolyte resistance in devices. Ion-beam modifications can further optimize performance via direct control of defect populations (Ceccio et al., 22 Jan 2026).

Spray + NIR techniques yield crystalline µm-scale films suitable for scalable, rapid deposition over large areas. NIR processing dramatically reduces energy and time, improved homogeneity, and maintains phase purity compared to conventional oven sintering, with potential significance for industrial-scale fabrication (Valls et al., 2024). Ionic transport properties remain to be systematically quantified in NIR-processed films.

6. Implications for Sodium All-Solid-State Batteries and Related Technologies

Reduction of NASICON electrolytes to nanometer thicknesses lowers the geometric resistance, offering an essential lever for realizing high-rate, low-impedance sodium ASSB architectures. Amorphous nanofilms, especially post-irradiation, present a dense, tunable hopping network for Na⁺, where balance between structural disorder and percolation can be fine-tuned through ion-beam parameters (Ceccio et al., 22 Jan 2026).

The ability to rapidly synthesize dense, crack-free, and microstructurally homogeneous NASICON films via spray/NIR approaches further broadens the technological basis for large-area, patterned, or flexible sodium-ion devices (if comparable conductivity and stability are attained) (Valls et al., 2024). These developments position NASICON nanofilms as key candidates for next-generation solid-state sodium batteries, planar microdevices, and potentially as platforms for fundamental Na-ion transport studies.

7. Outlook, Open Issues, and Research Directions

The primary open questions include:

• Direct measurement and systematic comparison of absolute Na⁺ conductivities and activation energies for all classes of NASICON nanofilms.
• Thickness dependence and scaling of ionic transport, particularly for films below 1 µm.
• In-depth examination of dielectric, mechanical, and interfacial properties, especially for integration in practical electrochemical cells.
• Quantitative mechanical adhesion and thermal cycling behavior of both amorphous and crystalline films.
• The extension of rapid NIR sintering methods to sub-micron thicknesses and alternative substrates.

A plausible implication is that ongoing optimization of synthesis, nanostructure, and defect profiles in NASICON nanofilms will be key to leveraging their fundamental advantages for sodium ASSB and microelectronics applications (Ceccio et al., 22 Jan 2026, Valls et al., 2024).