Break it down to speed it up: Na₂O–NaTaCl₆

Abstract

Fast sodium-ion conductors hold strong potential for enabling all-solid-state sodium batteries, offering inherent advantages such as enhanced safety and cost-effectiveness. However, challenges remain in achieving fast ion transport for realizing high-power density. This work reports the synthesis of a novel sodium solid electrolyte, Na₂O–NaTaCl₆ (Na₂O-NTC), via an energy-efficient approach. We achieved a high ionic conductivity of 4.41 mS cm⁻¹ and activation energy of 0.32 eV with only 4 hours of mechanochemical milling. The conductivity of Na₂O-NTC surpassed that of crystalline NaTaCl₆ (NTC) synthesized under similar conditions by more than one order of magnitude. In addition, Na₂O-NTC exhibited a relatively low electronic conductivity of 6.72 × 10⁻¹⁰ S cm⁻¹. Using XRD, Raman, and high-resolution NMR characterizations, the presence of Na₂O as a glass modifier was found to effectively amorphize the crystalline structure of NaTaCl₆, resulting in a glassy oxyhalide material with fast Na⁺ dynamics. This work demonstrates that leveraging inexpensive glass modifiers can effectively break down low-conductivity crystalline materials and tune the local structures to obtain highly conductive glassy solid electrolytes. Cost-effective and energy-efficient synthesis of glassy superionic conductors can aid the development and widespread adoption of high-performance rechargeable solid-state Na batteries.

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Introduction

The increasing demand for energy necessitates the use of renewable energy sources, accompanied by advanced energy storage technologies. All-solid-state batteries (ASSBs) are considered the next-generation energy storage technology, as they offer a potential solution to the safety concerns and energy density limits associated with the current liquid-electrolyte-based batteries.^1–8 Although Li-ion batteries have dominated the electrochemical energy storage research space, Na-ion batteries are also gaining attention as a viable alternative to lithium-ion batteries, primarily because of the high natural abundance and low cost of Na, thus making Na-ion batteries a more sustainable option for large-scale energy storage. However, developing sodium solid electrolytes (Na-SE) with high ionic conductivity and good stability to enable high-power-density batteries remains challenging.^3,7,9–11

Considerable efforts have been made to develop Na-SE, focusing on three main categories of inorganic solid electrolytes: oxides, sulfides, and halides. Oxides such as the NASICON structures are well-known for their high chemical and electrochemical stability; however, achieving high ionic conductivity for oxides often requires high-temperature sintering.^12–15 Sulfides and their derivatives (Na₃PS₄, Na_3.0PS_3.8Cl_0.2, Na₃PS_4−xO_x) are soft and exhibit high ionic conductivity, but their poor chemical and electrochemical stability hinders their practical application in ASSBs.^16–20 Halide-based Na-SEs, including Na₂ZrCl₆ (NZC), Na₂YCl₆ (NYC), Na_3−xY_1−xZr_xCl₆ (NYZC), and Na₂TaCl₆ (NTC), are attractive owing to their promising ionic conductivity and electrochemical stability at high voltages.^21–28

Structurally, these inorganic solid electrolytes can exist in either crystalline or amorphous forms. Crystalline SEs have well-defined, long-range structures, which provide predictable pathways for ion transport. However, ion transport is often slowed down due to the presence of grain boundaries and bottlenecks along the diffusion channels.^7,29–31 Additionally, grain boundaries cause dendrite formation and short circuiting of batteries.³² In contrast, the lack of grain boundaries in glassy materials minimizes dendrite formation and potentially elongates battery lifetime. Glassy SEs lack structural periodicity, and thus, they do not have well-defined ion transport pathways like in crystalline SEs. Therefore, glassy materials often exhibit poorer ionic conductivities. Since ion transport in glassy SEs highly depends on local structures and lattice dynamics, the creation of a more flexible local network can be conducive to fast ion movement, potentially leading to higher ionic conductivity.^8,33–37 Glass modifiers such as Na₂O, Li₂O, and Ag₂O play crucial roles in effectively modifying the anion network and creating bridging and non-bridging oxygen atoms within the glass network, thereby facilitating the formation of favorable conduction pathways for ion movement.^14,38–40 Consequently, glassy Na-SEs with enhanced performance can be developed by introducing glass modifiers into their crystalline structures, creating structural disorder and diffusion pathways for fast Na-ion conduction.

Currently, most Na-based glassy SEs that have been developed either require a long synthesis time >40 hours or elevated synthesis temperature; however, the ionic conductivity is still low (<1 mS cm⁻¹) for practical use.^{13,31,41–44} Developing high-performance solid electrolytes through energy- and time-efficient methods is paramount for the commercialization of ASSBs. Efficient synthesis methods reduce production costs and time, making the technology more viable for widespread adoption to meet the ever-increasing energy demand. Therefore, developing materials with high performance using energy-efficient methods is crucial for the future of energy storage technologies.

Leveraging glass modifiers, we have synthesized a highly conductive, glassy Na-SE, Na₂O–NaTaCl₆ (Na₂O-NTC) via efficient room-temperature mechanochemical synthesis. By mechanochemical ball-milling of the starting materials for just 1, 2, 4, and 6 hours, we achieved ionic conductivity exceeding 1 mS cm⁻¹. The most optimal condition was at 4 hours, producing a remarkable conductivity of 4.41 mS cm⁻¹— over one order of magnitude higher than NTC ball-milled for the same duration. In the glassy Na₂O-NTC, the presence of the glass modifier, Na₂O, disrupted the NTC crystal planes, promoting local ion dynamics and efficient Na-ion conduction. Powder-XRD analysis confirmed the progressive amorphization of the SE with extended milling time. The local structure of the glassy Na₂O-NTC was further elucidated via²³Na magic-angle-spinning (MAS) NMR and Raman spectroscopy, and Na⁺-ion dynamics probed by NMR relaxometry. This work highlights the potential of inexpensive glass modifiers to promote energy-efficient synthesis of highly conductive Na-based SE.

Experimental methods

Synthesis of Na₂O-NTC

The mechanochemical method was utilized to synthesize Na₂O-NTC. First, Na₂O (Sigma Aldrich, 99.8%) and NaCl (VWR Chemicals BDH) were pre-dried under vacuum at 200 °C for 12 hours, while other materials were used as purchased. In an Argon-filled glovebox (VTI), all the starting materials, Na₂O (Sigma Aldrich, 99.8%), NaCl (VWR Chemicals BDH), and TaCl₅ (Sigma Aldrich, 99.8%), were ground in an agate mortar and pestle for 5 minutes using a 1 : 1 molar ratio of Na₂O to NaTaCl₆, to form a homogenized mixture. The resulting powder was then transferred into a 25 mL Zirconia jar containing two 10 mm zirconia balls. The jar was vacuum-sealed to retain the powder in an inert atmosphere and mechanochemically ball-milled for 1, 2, 4, and 6 hours using a high-energy ball mill (SPEX8000M). The synthesized samples were then collected for characterization and stored in an Argon-filled glovebox.

Electrochemical impedance spectroscopy (EIS)

The ionic conductivities of all Na₂O-NTC samples were evaluated using Gamry 600+ over a frequency range of 0.1 Hz to 5 MHz. The powder was pressed at 360 MPa between two steels in house-made 8 mm cylindrical split cells with indium foil as the blocking electrode. To determine the activation energy for ion conduction, variable-temperature EIS (VT-EIS) was performed in a Cincinnati Sub-Zero (CZS) Microclimate Chamber under dry air, within a temperature range of −15 and 55 °C at 10 °C intervals.

DC polarization

DC polarization technique was utilized to determine the electronic conductivities of the synthesized electrolytes. Approximately 130 mg of the solid electrolyte was weighed and pressed between two stainless steels of a 8 mm cylindrical split cell made in-house. Electronic conductivity was measured using Gamry 600+ at 0.1 V. The stainless steel served as the current collector and ion-blocking electrodes.

Materials characterization

Powder X-ray diffraction (P-XRD). The powder X-ray diffraction of NTC and Na₂O-NTC composites was carried out on a Rigaku D8 powder diffractometer. Diffraction patterns were collected using a voltage of 45 kV and current of 40 mA with Cu-Kα radiation over the 2θ range of 10–80° with a step size of 0.03^° and at a scan rate of 2.5^° per min. To prevent the exposure of Na₂O-NTC samples to humid air, all samples were prepared in an Ar-filled glovebox and protected with a Kapton film. Rietveld refinement was conducted using the GSAS software suite.⁴⁵

Solid-state nuclear magnetic resonance (SS-NMR). Using a Bruker 2.5 mm Magic Angle Spinning (MAS) NMR probe, ²³Na MAS NMR experiments were performed on a Bruker NMR spectrometer (500 MHz 11.7 T) at a Larmor frequency of 132.294 MHz. To prepare the sample for the experiment, a 2.5 mm zirconia rotor was packed with approximately 30 mg of sample. ²³Na NMR spectra were acquired using a single pulse with a p/2 pulse length of 3.85 μs and a recycle delay of 10 s at a spinning rate of 25 kHz. In addition, ²³Na spin-lattice relaxation time (T₁) measurements were measured using an inversion recovery pulse sequence.

Variable-temperature ²³Na spin-lattice relaxation time (T₁) measurements were carried out using an inversion recovery pulse sequence on a Bruker Avance-I NMR spectrometer (300 MHz, 7.05 T) at a Larmor frequency of 73.39 MHz, and a spinning rate of 10 kHz. Samples were packed into a 4 mm zirconia rotor. Measurements were performed from 25 to 85 °C with an increment of 5 °C. ²³Na shift was calibrated using 0.1 M NaCl solution at 0 ppm.

Scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDS). Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were used to examine the morphology and elemental composition of Na₂O-NTC. Approximately 130 mg of the synthesized Na₂O-NTC powder was pressed into a pellet at 360 MPa, and the analysis was performed using a JEOL JSM-IT800 SEM-EDS system. Images were collected at an accelerating voltage of 15.0 kV with a dwell time of 5 μs.

Raman spectroscopy. Raman spectroscopy was carried out using HORIBA JY LabRam Evolution Raman Spectrograph with a 532 nm excitation laser and a grating size of 600 g mm⁻¹. The powdered samples were placed on glass slides and covered with glass for the analysis.

Results and discussion

Ionic and electronic conductivities

AC electrochemical impedance spectroscopy (EIS) was utilized to determine the conductivities of Na₂O-NTC synthesized via mechanochemical milling, as described in the Method section. Fig. 1a shows the room-temperature Nyquist plot of NTC and Na₂O-NTC mechanochemically ball-milled for 4 hours, denoted as NTC-4 and Na₂O-NTC-4, respectively. The equivalent circuit model for analyzing the Nyquist plots of Na₂O-NTC acquired at −25 °C and 25 °C is shown in Fig. S1. From the Nyquist plot analysis, an overall conductivity of 4.41 mS cm⁻¹ was obtained for Na₂O-NTC-4 , which surpasses that of NTC-4 (0.11 mS cm⁻¹) by over forty times. Only one semicircle in the high-frequency region of the Nyquist plot acquired at −25 °C for Na₂O-NTC-4 (Fig. 1b) suggests that bulk and grain boundary contributions to ion transport cannot be resolved, or the grain boundary contribution is negligible in this material.^4,46–48 Variable-temperature electrochemical impedance spectroscopy (VT-EIS) was used to determine the activation energy for Na-ion transport. Fig. 1c shows the Arrhenius-type plot of Na₂O-NTC composites synthesized at different ball-milling durations (1, 2, 4, and 6 hours). All Na₂O-NTC samples exhibit high ionic conductivities exceeding 1 mS cm⁻¹ (Fig. S2, S3a, and Table S1). The activation energy for Na-ion transport ranges from 0.32 eV (Na₂O-NTC-4 hours) to 0.36 eV (Na₂O-NTC-1 hour). Overall, the activation energy is relatively low compared to other Na-ion conductors.^24–26 The electronic conductivity of Na₂O-NTC-4 hours was calculated to be 6.72 × 10⁻¹⁰ S cm⁻¹ (Fig. 1d), significantly lower than that of NTC-4 hours, 2.54 × 10⁻⁹ S cm⁻¹. This low electronic conductivity also suggests that the measured overall conductivity using EIS is primarily due to Na-ion conduction with negligible electronic contribution.^49,50 Other Na₂O-NTC samples, regardless of different reaction times, also show low electronic conductivities on the order of 10⁻⁹ S cm⁻¹–10⁻¹⁰ S cm⁻¹ (Fig. S3b and Table S1). Notably, Table S2 demonstrates that our Na₂O-NTC sample achieves one of the highest ionic conductivities (4.41 mS cm⁻¹) alongside a comparably low activation energy (0.32 eV), obtained via a short milling time of 4 hours without any thermal treatment. This performance not only matches or surpasses that of many reported sodium solid electrolytes, some of which require prolonged milling durations or additional heat treatment, but also highlights the efficiency and practicality of our synthesis approach for high-performance Na-based solid electrolytes.

Fig. 1 (a) Nyquist plots of Na₂O–NaTaCl₆ and NaTaCl₆ prepared via mechanochemical ball-milling for 4 hours, measured at 25 °C. (b) Nyquist plots for Na₂O–NaTaCl₆ measured at −25 °C with equivalent circuit fitting (inset). (c) Arrhenius plot of Na₂O–NaTaCl₆ synthesized via mechanochemical ball-milling for 1, 2, 4, and 6 hours. (d) DC polarization curve of Na₂O–NaTaCl₆ ball-milled for 4 hours and its corresponding electronic conductivity of 6.72 × 10⁻¹⁰ S cm⁻¹.

Structure

The Raman spectra in Fig. 2a reveals the bonding characteristics in NTC and Na₂O–NTC. The bands at 100–200 cm⁻¹ and 300–420 cm⁻¹ correspond to the T₂g bending vibration and the A₁g stretching vibration of Ta–Cl, respectively.^21,35,51 In particular, the bands between 100–200 cm⁻¹ originate from Ta–Cl bending modes; specifically, those below 150 cm⁻¹ arise from out-of-plane bending due to Cl wagging or twisting, and those between 150–200 cm⁻¹ from in-plane Cl–Ta–Cl bending. The peaks between 200–400 cm⁻¹ are attributed to Ta–Cl stretching, including the intense sharp peak at ∼405 cm⁻¹ from axial Ta–Cl symmetric stretch and the weaker peaks at slightly lower wavenumbers from equatorial Ta–Cl symmetric stretch. The broadness of these bands in NTC and Na₂O-NTC is typical of extended 3D lattices with long-range interactions, lattice vibrations, and possible site disorder. The peak intensity decrease at ∼405 cm⁻¹ from axial Ta–Cl symmetric stretch suggests that O substitution of Cl mainly occurs at axial positions. Additionally, the Na₂O-NTC spectra display extra peaks between 700–900 cm⁻¹, which correspond to Ta–O–Ta stretching vibrations.^35,52 This confirms that Cl is being partially substituted by O in some of the [TaCl₆]⁻ octahedra, with possible structural geometries illustrated in Fig. 2b.

Fig. 2 (a) Raman spectra of Na₂O–NaTaCl₆, NaTaCl₆, TaCl_5, and Ta₂O_5. (b) Possible Ta coordinations and local geometries in Na₂O–NaTaCl₆.

NaTaCl₆ (NTC) crystallizes in the monoclinic crystal system with the space group P2₁/n (ICSD #36519).⁵³ In its structure (Fig. 3a and b), Na, Cl, and Ta occupy the 4e Wyckoff positions. Specifically, Na⁺ occupies the prismatic sites, while Ta occupies the octahedral sites; both are coordinated with chlorine ions (Fig. 3a and b). The powder XRD patterns (Fig. 3c) of Na₂O-NTC reveal a gradual disappearance of the NTC diffraction peaks with increasing ball-milling time. With 4 hours of ball-milling, Na₂O-NTC became glassy, with no significant NTC peaks remaining, leaving only a trace of NaCl impurities, as confirmed with the Rietveld refinement (Fig. 3d). This transition from a crystalline to an amorphous phase is attributed to significant structural modification of the NTC framework induced by Na₂O.

Fig. 3 Crystal structure of NaTaCl₆ (ICSD #36519) showing Na in the prismatic site and Ta in the octahedral site, symmetry along (a) 010 and (b) 100. (c) Lab powder XRD pattern of Na₂O-NaTaCl₆ mechanochemically ball-milled for 1, 2, 4, and 6 hours, and NaTaCl₆ for 4 hours. The Broad peak at ∼2θ = 20^° originates from the Kapton film used to seal the sample container. (d) Rietveld refinement of Na₂O–NaTaCl₆ ball-milled for 4 hours, revealing the only crystalline phase as NaCl.

The SEM of Na₂O-NTC reveals smooth surfaces with no observable granular structures, typical of glassy materials.^35,41 This corresponds well with EIS measurements in which no grain boundary contribution to ion conduction was observed, even at low temperatures. Furthermore, the EDS mapping indicated that all the elements – Na, Ta, O, and Cl—are evenly distributed (Fig. S4). This glassy surface morphology, coupled with nearly even elemental distribution, aligns with the findings from the P-XRD analysis, reinforcing the characterization of the Na₂O-NTC as amorphous.

To investigate the impact of post-annealing, the most conductive Na₂O-NTC-4 hours powder was subjected to heat treatment at 150 °C for 2 hours. The corresponding EIS Nyquist plot is represented in Fig. S5. This resulted in a significant reduction in ionic conductivity, nearly two orders of magnitude lower than the as-prepared ball-milled Na₂O-NTC-4 hours (Table S3). A visible color change from creamy yellow to white was also observed after annealing, and XRD analysis revealed the gradual appearance of NTC crystalline peaks (Fig. S6). The yellow color is often attributed to a mixed-ligand Ta–O–Cl environment with an amorphous structure. The color change may be induced by crystallization and possible O loss. To further investigate the thermal stability of the as-prepared Na₂O–NaTaCl₆ solid electrolyte, ionic conductivity measurements were performed at progressively elevated temperatures: 25 °C, 50 °C, 75 °C, 100 °C, 150 °C, and 200 °C. As shown in Fig. S7, the conductivity decreased significantly with increasing temperature, dropping from ∼4.41 mS cm⁻¹ at 25 °C to 0.023 mS cm⁻¹ at 200 °C, over two orders of magnitude lower than that of the as-prepared ball-milled Na₂O-NTC-4 hours. Corresponding EIS Nyquist plots are provided in Fig. S7b. This significant decrease in conductivity upon heating supports the conclusion that the as-prepared material is only highly conductive in the amorphous phase. Exposure to elevated temperatures likely induces structural rearrangement or partial crystallization, which impedes Na⁺-ion transport. These findings confirm that post-synthesis annealing is detrimental to ionic conductivity.

To evaluate the chemical stability of the solid electrolyte, an ionic-conductivity-retention test was conducted over a period of 10 days. The initial conductivity was 4.35 mS cm⁻¹ and gradually decreased to 2.48 mS cm⁻¹ by Day 10. Although a reduction in conductivity was observed, the electrolyte retained approximately 57% of its initial conductivity, remaining above 2 mS cm⁻¹ throughout the test period (Fig. S8).

Due to the largely amorphous nature of Na₂O-NTC, Na⁺-ion conduction is likely governed by local structural order; therefore, it is critical to acquire insights into the local structural arrangements. ²³Na MAS NMR experiments were utilized to probe the local structures and dynamics of Na⁺ ions. Fig. 4 shows the ²³Na MAS NMR spectra of ball-milled Na₂O-NTC (1 hour and 4 hours), NTC-4 hours, and bulk Na₂O and NaCl references. In the ²³Na MAS NMR spectra, crystalline NaTaCl₆ shows a sharp resonance at −10.6 ppm, assigned to Na⁺ in a TaCl₆ octahedral environment, consistent with prior studies.²¹ In contrast, the 1-h and 4-h ball-milled Na₂O–NaTaCl₆ samples exhibit similar peaks shifted upfield to −11.16 and −11.18 ppm, respectively. This shift is attributed to the formation of a disordered Na–Ta–O–Cl (NTOC) environment, supported by the Raman characterization (Fig. 2). The broader resonance and higher oxygen content in the 4-h ball-milled sample support an amorphous, oxygen-incorporated phase associated with enhanced ionic conductivity. The resonance at ∼7 ppm in all the spectra is assigned to NaCl, while the peak at ∼55 ppm in Na₂O-NTC samples is from Na₂O.^17,54 Compared with the reference spectra acquired on bulk NaCl and Na₂O, the NaCl and Na₂O resonances in Na₂O-NTC are significantly broader with a Lorentzian line shape; the former is indicative of structural disorder in nano-sized particles, and the latter enhanced Na⁺-ion dynamics. In the Na₂O-NTC samples, the peak near ∼11 ppm broadens with increasing ball-milling time, with the linewidth increased from 274.99 Hz in NTC to 310.34 Hz (1 hour) and 380.15 Hz (4 hours) for Na₂O-NTC (Table S4), indicative of increased structural disorder induced by the incorporation of O, which often promotes the formation of an amorphous or glassy phase.⁴ Meanwhile, the ²³Na MAS NMR resonance transitions from a Gaussian-dominated line shape in the NTC-4 to a Lorentzian line shape in the Na₂O-NTC-4 sample, suggesting ion motion-induced fast spin relaxation in the latter.

Fig. 4 ²³Na MAS NMR spectra of the Na₂O–NaTaCl₆ samples ball-milled for 4 hours and 1 hour, NaTaCl₆, and bulk Na₂O and NaCl.

Quantitative analysis reveals a correlation between higher O-content and faster Na⁺ dynamics in NTOC samples (Fig. S9 and Table S5). The analysis confirms that oxygen substitution within the [TaCl₆]⁻ framework plays a critical role in enhancing ionic conductivity, consistent with recent MD simulations suggesting that O-for-Cl substitution in halide frameworks (NaAlCl_2.5O_0.75 and LiAlCl_2.5O_0.75) can significantly enhance Na⁺ and Li⁺ ionic conductivity by widening diffusion pathways and reducing migration barriers.⁵⁵

Furthermore, the Na₂O-NTC-4 sample shows a higher proportion, a 30.46% increase, of the fast-conducting NaTaO_xCl_6−2x (NTOC) phases compared to Na₂O-NTC-1 based on the relative integral. In the NaTaCl₆ sample, NaCl accounts for 5.07%, a residual from incomplete reaction of NaCl with TaCl₅. While in Na₂O-NTC samples, NaCl is a byproduct from O-incorporation into NaTaCl₆ (Reaction 1). NaCl amount increase from 2.29% (1 h milling) to 4.80% (4 h milling), suggesting more oxygen incorporation in the latter, which correlates with enhanced Na⁺ mobility. The chemical compositions of NTOC in the 1-hour and 4-hour Na₂O-NTC NMR spectra were determined to be NaTaO_0.04Cl_5.92 and NaTaO_0.06Cl_5.88, respectively, using Chemical Reaction (1) and eqn (1) below.

xNa₂O + NaTaCl₆ → NaTaO_xCl_6−2x + 2xNaCl (Reaction 1)

To calculate the mole ratio of Na in the highest conductive Na₂O-NTC, eqn (2) was adopted:

A and B represent NaCl and NTOC, respectively, and N is the number of Na per formular. Area is the areal integral of the corresponding NMR peak. The mole ratio of NTOC to NaCl was calculated to be 8 : 1.

To elucidate the role of Na₂O in Na₂O–NaTaCl₆ and the reaction pathways, we synthesized a comparative binary composition comprising only Na₂O and TaCl₅ in a 1 : 1 molar ratio. The reaction was carried out under similar high-energy ball milling conditions (4 hours) as used for Na₂O–NaCl–TaCl₅ (1 : 1 : 1). The Na₂O–TaCl₅ sample exhibited an ionic conductivity of 1.54 mS cm⁻¹ at room temperature, whereas Na₂O–NaTaCl₆ showed a substantially higher conductivity of 4.41 mS cm⁻¹ (Fig. S10a). To investigate the origin of this disparity, XRD and NMR analyses were performed on both samples. The Na₂O–TaCl₅ sample exhibits peaks corresponding to the crystalline phase of NaTaCl₆. In contrast, the XRD pattern of Na₂O–NaTaCl₆ exhibits no observable NaTaCl₆ peaks, confirming the formation of a fully amorphous phase (Fig. S10b). These results suggest crystalline NaTaCl₆ is a reaction intermediate. NMR data reveal a smaller fraction of the conductive NTOC phase with a lower O-content in the Na₂O–TaCl₅ sample, compared with Na₂O–NaCl–TaCl₅ (Fig. S9, S11 and Table S6), echoing the observed relatively lower ionic conductivity.

Na⁺-ion dynamics

To explore the Na-ion dynamics in Na₂O-NTC, we conducted ²³Na T₁ NMR relaxation time measurements. The T₁ values for the NTOC, Na₂O, and NaCl resonances, measured at room temperature, were 0.0012, 2.06, and 0.24 seconds, respectively (Table S5). Notably, NTOC exhibited the shortest ²³Na T₁ NMR relaxation time. Significant changes in the ²³Na T₁ NMR relaxation times of Na₂O and NaCl before and after ball-milling are observed. The bulk forms of Na₂O and NaCl have T₁ values of 5.69 and 11.43 seconds, respectively. After ball-milling, the T₁ relaxation times decreased to 0.58 and 6.49 s for nano-sized Na₂O and NaCl. This significant drop in T₁ suggests fast spin relaxation, likely due to structural defects and distortion-enhanced quadrupolar interactions coupled with increased ion dynamics. This observation echoes with the peak broadening and Lorentzian line shape of both compounds before and after ball-milling (Fig. S12), indicating reduced crystallinity, increased structural distortions, and enhanced ion dynamics within the materials.

To further understand the relationship between ²³Na T₁ relaxation time and ion mobility, variable-temperature NMR (VT-NMR) was employed. According to the Bloembergen, Purcell, and Pound (BPP) theory (eqn (3)), the T₁ relaxation time relates to the motional correlation time (τ_c).^56,57 It is important to note that, in ²³Na (spin-3/2) NMR, quadrupolar coupling is often the dominant interaction in asymmetric local environments, which, when coupled with ion dynamics at a rate in the vicinity of the ²³Na Larmor frequency, drives fast spin relaxation (eqn (3)). Fast spin relaxation often yields a typical Lorentzian line shape, as we observed in Fig. 4.

where T₁ is the spin-lattice relaxation time, I is the nuclei spin number (3/2 for ²³Na), Q_c is the nuclear quadrupolar coupling constant, η is the asymmetry parameter, ω₀ is the Larmor frequency, and τ_c is the motional correlation time constant. Fig. 5 demonstrates that as the temperature increases, the ²³Na T₁ values decrease for all resonances, indicating that all resonances are in the slow-motion regime (w₀τ_c >>1) and that shorter T₁ values correlate to faster Na-ion mobility. Therefore, NTOC exhibits fastest Na⁺ dynamics in Na₂O-NTC, because of the shortest T₁ relaxation time of 1.2 ms among the Na-containing species in Na₂O-NTC. In comparison, the ²³Na T₁ for Na in NTC is 7.3 ms, longer than that for NTOC, suggesting slower Na⁺ dynamics. The other components, Na₂O and NaCl, show much longer ²³Na T₁ values and limited ion exchange with NTC or NTOC, likely no significant contribution to overall ionic conductivity. Additionally, the electrochemical impedance measurements of ball-milled NaCl and Na₂O exhibited very large impedance. The Nyquist plots appear scattered and do not form a semicircle because Na₂O and NaCl are poor ion conductors with very high resistance. Consequently, the impedance response is dominated by noise and measurement limitations, preventing the formation of a clear semicircle typically seen in ionically conductive materials (Fig. S13).

Fig. 5 (a) Variable-temperature ²³Na T₁ NMR relaxation times of Na₂O–NaTaCl₆ as a function of temperature. (b) Comparison of ²³Na NMR T₁ relaxation times of different components in NaTaCl₆ and Na₂O–NaTaCl₆.

Conclusion

In this study, we report a glassy sodium solid electrolyte Na₂O–NaTaCl₆. Using a one-step room-temperature mechanochemical synthesis over 4 hours, we achieved a high ionic conductivity of 4.41 mS cm⁻¹ and an activation energy of 0.32 eV. P-XRD on Na₂O–NaTaCl₆ revealed a highly disordered structure. The SEM-EDS analysis showed no significant grain boundaries and a homogenous distribution of Na, Ta, O, and Cl. The Raman analysis confirmed the substitution of Cl with O to form NTOC. High-resolution ²³Na NMR spectroscopy quantifies the amount of O incorporated into NTOC and NMR relaxometry revealed fast Na⁺-ion dynamics in NTOC, which is responsible for the observed fast Na-ion conduction. Na₂O acted as a glass modifier that facilitated the formation of the highly conductive glassy phase within a short time of mechanochemical synthesis, compared with typical duration of tens of hours. This study underscores the role of glass modifiers in creating superionic conductors with strong potential for cost-effective, commercially viable solid electrolytes.

Author contributions

Islamiyat A. Ojelade: data curation: lead; investigation: supporting; validation: supporting; vsualization: lead; writing – original draft: lead. Erica Truong: data curation: lead; investigation: supporting; validation: supporting; visualization: lead; writing – original draft: lead. Ifeoluwa P. Oyekunle: data curation: supporting; formal analysis: supporting; investigation: supporting; methodology: supporting; validation: supporting; visualization: supporting; writing – original draft: supporting. Tej P. Poudel: data curation: supporting; formal analysis: supporting; investigation: supporting; methodology: supporting; validation: supporting; visualization: supporting; writing – original draft: supporting. Yudan Chen: data curation: supporting; formal analysis: supporting; investigation: supporting; methodology: supporting; software: supporting; writing – origwnal draft: supporting. Michael J. Deck: data curation: supporting investigation: supporting; validation: supporting; visualization: supporting; writing – original draft: supporting. Yongkang Jin: data curation: supporting investigation: supporting; validation: supporting; writing – original draft: supporting. Bright Ogbolu: investigation: supporting; validation: supporting; visualization: supporting; writing – review & editing: supporting. Md. Mahinur Islam: data curation: supporting; validation: supporting; writing – review & editing: supporting. Pawan K. Ojha: data curation: supporting; validation: supporting; writing – review & editing: supporting. Thilina N. D. D. Gamaralalage: validation: supporting; visualization: supporting; writing – review & editing: supporting. J. S. Raaj Vellore Winfred: data curation: supporting; formal analysis: supporting. Yan-Yan Hu, Ph.D.: conceptualization: lead; formal analysis: lead; funding acquisition: lead; methodology: lead; project administration: lead; resources: lead; supervision: lead; validation: lead; visualization: equal; writing – original draft: equal; writing – review & editing: lead.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data supporting this article have been included as part of the SI. Supplementary information is available. See DOI: https://doi.org/10.1039/d5sc05307b.

Acknowledgements

The authors acknowledge the support from the National Science Foundation under grant no. DMR-1847038. All solid-state NMR experiments were performed at the National High Magnetic Field Laboratory by the National Science Foundation Cooperative Agreement No. DMR-1644779 and DMR-2128556, and the State of Florida.

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Footnote

† These authors contributed equally.

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