Advancing Solid-State Sodium Batteries with Na-Ion Conducting Oxychloride Nanocomposites

By Nidhi DhullReviewed by Bethan DaviesOct 8 2024

A recent article published in Chemistry of Materials reported on oxychlorides embedded in a ternary system of NaCl-TaCl₅-Ta₂O₅, demonstrating high conductivities, formabilities, and oxidation-reduction stabilities. The mechanochemically synthesized samples consisted of NaCl and Ta₂O₅ nanoparticles within a Na-Ta-Cl-O amorphous matrix.

Background

All-solid-state sodium batteries are gaining popularity due to their abundant raw materials, high energy density, robust safety, and wide operating temperature range. A key factor in the commercialization of these batteries is the solid electrolyte, which must strike a balance between formability, conductivity, and electrochemical stability.

Common inorganic solid electrolytes include oxides, hydrides, sulfides, and halides. Sulfides, while offering high ionic conductivity, good formability, and sufficient electrochemical stability to create a stable interface with negative electrodes, often lack oxidation stability. On the other hand, chlorides exhibit high ionic conductivity, exceptional compatibility with positive electrodes, and good processability at room temperature but suffer from poor reduction stability.

A promising approach for all-solid-state sodium batteries is to use halides on the positive electrode side and sulfides on the negative side. Chlorides, in particular, can enhance the energy density of these batteries. As a result, this study proposes Na-ion conducting amorphous oxychloride electrolytes, which incorporate nanocrystals to achieve an optimal balance of properties.

Methods

Anhydrous NaCl, TaCl₅, and Ta₂O₅ were mixed in different ratios to form Na_1+xTaCl_6–5xO_5x (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3) and sealed in a ZrO₂ pot with ZrO₂ balls under a dry Ar atmosphere. Notably, the NaCl reagent was pre-dried at 200 °C for 12 hours under vacuum conditions. The mixtures were then mechanochemically treated at 510 rpm for 30 hours using a planetary ball mill.

The synthesized powders were characterized using X-ray diffraction (XRD) and Raman spectroscopy. Additionally, their X-ray absorption fine structure (XAFS) spectra were obtained by diluting the powders in boron nitride at suitable concentrations. Solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) experiments were performed on the powders using the ²³Na isotope.

The densities of the compacted samples were calculated based on their volume and weight, while the densities of the powders were measured using a gas pycnometer. The powder samples' morphologies were observed with a field-emission scanning electron microscope (FESEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) system.

Further analysis of the microstructures was conducted using a field-emission transmission electron microscope (TEM), and elemental distribution was investigated via scanning transmission electron microscopy (STEM) coupled with EDS. The samples were also pelletized at 360 MPa using uniaxial pressing for electrochemical characterization. Electrochemical impedance spectroscopy (EIS) measurements were carried out, and the oxidation stability of the samples was assessed using linear sweep voltammetry (LSV).

Results and Discussion

XRD patterns of the prepared Na_1+xTaCl_6–5xO_5x sample differed with varying x values. The sample with an x=0 exhibited broad peaks similar to that of NaTaCl₆. Alternatively, the XRD patterns of the x=0.05 to 0.15 samples were attributed to NaTaCl₆, Ta₂O₅, and NaCl. Notably, the NaCl and Ta₂O₅ peaks increased with increasing amounts of the Ta₂O₅ reagent, while the NaTaCl₆ peak gradually decreased, disappearing for x=0.2 to 0.3. 

The amorphous fraction of all synthesized samples was higher compared to the oxide-free (x = 0) sample. The addition of Ta₂O₅, acting as a glass-forming intermediate oxide, enhanced the amorphous composition. SEM images showed that the sample with x = 0.05 contained particles ranging from a few micrometers to several tens of micrometers in size. Similarly, the x = 0.3 sample had particles several micrometers in size, whereas the x = 0.01 to 0.25 samples exhibited particles measuring several tens of micrometers.

Electron diffraction (ED) patterns from TEM of the x = 0.25 sample revealed several Debye-Scherrer rings with spots and a halo pattern, indicating an amorphous structure. The diffraction peaks were consistent with the XRD pattern of NaCl. Additionally, EDS maps of this sample showed segregated Ta and O species. Therefore, the microstructure of the prepared samples was characterized by NaCl and Ta₂O₅ nanoparticles randomly dispersed within an amorphous, oxygen-containing matrix.

The Na_2.25TaCl_4.75Cl_1.25 (x=0.25) exhibited the maximum conductivity (2.5×10^-3 S/cm at 25 °C) among the prepared samples. Though this conductivity was not comparable to Na_2.88Sb_0.88W_0.12S₄ (3.2×10^-2 S/cm), it was higher than the representative sulfides, such as Na₃PS₄ glass-ceramics and Na₃SbS₄. This was attributed to the space charge layer effect between the ionic conductor and oxide despite the insulating nature of NaCl and Ta₂O₅.

Conclusion

Overall, the researchers successfully developed a novel Na-ion-conducting oxychloride (Na-Ta-Cl-O) system, consisting of an amorphous oxychloride matrix embedded with NaCl and and Ta₂O₅ nanoparticles. The synthesized composite oxychlorides demonstrated high ionic conductivities, ranging from 0.8×10^-3  to 2.5×10^-3 S/cm at 25 °C.

The electrochemical potential window of the oxychlorides was measured to be 0.4–4.1 V versus Na⁺/Na, attributed to NaCl and Ta₂O₅ nanoparticles embedded in the Na-Ta-Cl-O amorphous matrix. Additionally, the samples exhibited superior formability and greater mechanical strength compared to NaTaCl₆.

In conclusion, the incorporation of nanocrystals into amorphous electrolytes effectively enhanced their ionic conductivity, electrochemical stability, and mechanical properties, offering a promising route for the development of high-performance solid electrolytes in all-solid-state batteries.

Journal Reference

Motohashi, K., Tsukasaki, H., Mori, S., Sakuda, A., & Hayashi, A. (2024). Fast Sodium-Ion Conducting Amorphous Oxychloride Embedding Nanoparticles. Chemistry of Materials. DOI: 10.1021/acs.chemmater.4c02104,‌ https://pubs.acs.org/doi/10.1021/acs.chemmater.4c02104

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