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Article

Fabrication and Electrochemical Performance of Br-Doped Na3PS4 Solid-State Electrolyte for Sodium–Sulfur Batteries via Melt-Quenching and Hot-Pressing

by
Ao Ma
1,
Shuhui Liu
1,*,
Degui Li
2,
Bin Gu
2,
Sheng Li
1 and
Jing Wang
2,*
1
School of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China
2
Guangxi Key Laboratory of Green Manufacturing for Ecological Aluminum Industry, Baise University, Baise 533000, China
*
Authors to whom correspondence should be addressed.
Inorganics 2025, 13(3), 73; https://doi.org/10.3390/inorganics13030073
Submission received: 21 January 2025 / Revised: 26 February 2025 / Accepted: 26 February 2025 / Published: 28 February 2025
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Abstract

Room-temperature all-solid-state sodium–sulfur (Na-S) batteries are being regarded as a promising technology for large-scale energy storage. However, the low ionic conductivity of existing sulfide solid electrolytes has been hindering the potential and commercialization of Na-S batteries. Na3PS4 has garnered extensive attention among sulfide solid electrolytes due to its potential ionic conductivity (primarily predominated by vacancies) and ease of fabrication. Herein, we demonstrated a combined melt-quenching with Br doping technique to pre-generate abundant defects (vacancies) in the Na3PS4, which expanded ion transport channels and facilitated Na+ migration. The quenched Na2.9PS3.9Br0.1 holds an ionic conductivity of 8.28 × 10−4 S/cm at room temperature. Followed by the hot-pressed fabrication at 450 °C was conducted on the quenched Na2.9PS3.9Br0.1 to reduce interface resistance, the resultant Na2.9PS3.9Br0.1 pellet shows an ionic conductivity up to 1.15 × 10−3 S/cm with a wide electrochemical window and chemical stability towards Na alloy anodes. The assembled all-solid-state Na2S/Na2.9PS3.9Br0.1/Na15Sn4 cell delivers an initial reversible capacity of 550 mAh/g at a current density of 0.1 mA/cm2. After 50 cycles, it still maintains 420 mAh/g with a capacity retention of 76.4%. The integration of melt-quenching, doping, and hot-pressing provides a new strategy to enable sulfide electrolytes with high ionic conductivity and all-solid-state Na-S batteries with high performance.

1. Introduction

All-solid-state sodium–sulfur (Na-S) batteries, with such potential advantages as enhanced safety, superior energy density, high-temperature resistance, and extended lifespan, are expected to overcome especial the flammability and explosiveness of traditional organic liquid batteries [1]. These attributes position it as a revolutionary technology with the potential to significantly benefit humanity. All-solid-state Na-S batteries can offer unique advantages for large-scale energy storage, especially in the renewable energy grids field, because of large current and high power charge/discharge capabilities, excellent specific energy, and abundant sodium and sulfur resources [2,3,4]. Inorganic solid-state electrolyte is a crucial component for all-solid-state batteries, and its room-temperature ionic conductivity is the core parameter which directly impacts battery performance. Therefore, pursuing the high room-temperature ionic conductivity of solid-state electrolyte has always been the primary goal for all-solid-state Na-S batteries. Inorganic solid-state electrolytes are primarily classified into oxides and sulfides [5].Usually, sulfides present higher room-temperature ionic conductivity than oxides due to the larger radius of the S2− ion than O2−, which results in wider ion transport channels and weakens the attraction between the framework and the ions [6]. Sulfide solid electrolytes also exhibit better mechanical ductility and low elastic modulus, leading to the ease of fabrication and good interface contact of electrolytes with electrodes [7,8]. Among them, Na3PS4 has attracted extensive attention due to its potential high ionic conductivity, low temperature and facile synthesis [9,10,11,12,13].
In recent years, vast efforts have been dedicated to the research of Na3PS4 electrolyte. In 2012, Hayashi et al. [14] first reported the synthesis of cubic Na3PS4 (c-Na3PS4) glass-ceramic electrolyte via ball milling, achieving an ionic conductivity of 2 × 10−4 S/cm at room temperature. In 2014, they again synthesized Na3PS4 using higher-purity Na2S (99.1%), and the ionic conductivity doubled compared to their previous work [15]. Zeier et al. [16] first obtained c-Na3PS4 via ball milling, and the c-Na3PS4 further was transformed into tetragonal Na3PS4 (t-Na3PS4) after annealing at 500 °C. They found that the two electrolytes with different crystal structures yet presented nearly identical ionic conductivities, and the ionic conductivity values were all higher than that of t-Na3PS4 synthesized from high-temperature solid phase sintering method. Then, it was concluded that the high ionic conductivity could be attributed to the plentiful defects generated by ball milling. Wagemaker et al. [17] also revealed through simulations that just two percent sodium vacancy concentration can enhance the ionic conductivity up to 0.2 S/cm. Their experiment results of synthesized c-Na3PS4 and t-Na3PS4 from ball milling further verified that the vacancy concentration is an essential factor dominating the ionic conductivity of sulfide electrolytes [18].
For increasing the defects of Na3PS4 electrolyte to achieve high ionic conductivity, in addition to selecting synthesis process, the substitutional doping at the P or S sites is also an effective strategy for producing defects [19,20]. Molecular dynamics and first-principles simulations both predicted that the doping with aliovalent ions (such as Sn4+, Ge4+, Si4+) to replace P5+ sites or halide ions (such as F−, Cl−, Br−, I−) to replace S2− sites can effectively increase defects, thereby inducing more Na+ vacancies [17,21]. Based on these theoretical studies, Tanibata et al. [22] experimentally synthesized Si-doped c-94Na3PS4·6Na4SiS4, and Chu et al. [23] also prepared Cl-doped t-Na2.9375PS3.9375Cl0.0625. However, investigations of the Br-doped sulfide electrolytes are seldom reported.
In a word, although a lot of progress for Na3PS4 has been made, Na3PS4 electrolyte still faces challenges such as low ionic conductivity, high interface resistance, and poor stability against anodes [24]. Moreover, especially it needs to be pointed out that the currently cold-pressed electrolytes fail to form a close contact and contain plenty of voids, and the uneven deposition of sodium (especially under high current densities) tends to occur here, which will accelerate sodium dendrites growth along the voids and grain boundaries throughout the electrolyte to cathode and render the failure of cells.
To address the aforementioned issues, this work proposes a synergistic strategy integrating melt-quenching, Br doping, and hot-pressing to improve both ionic conductivity and interfacial stability in Br-doped Na3PS4 (Na2.9PS3.9Br0.1) solid electrolyte. The quenching process induces a large number of defects within the electrolyte, increasing the sodium ion conduction pathways. Meanwhile, Br doping further enhances the defect concentration and widens the ion transport channels, improving the ionic conductivity of the electrolyte. The subsequent hot-pressing process is used to relieve the stress generated by quenching, improve interface contact and lower the interface resistance, thereby improving the ionic conductivity of the electrolyte as well as interfacial stability during cycling. The resultant Na2.9PS3.9Br0.1 electrolyte pellet demonstrates good room-temperature ionic conductivity of 1.15 × 10−3 S/cm and maintains stable interfaces with both Na2S cathode and Na15Sn4 anode. Therefore, the assembled Na2S/Na2.9PS3.9Br0.1/Na15Sn4 full cell exhibits superior electrochemical performance. This study proposes a synergetic strategy that contributes to achieving high performance Na-S batteries.

2. Results and Discussion

2.1. Crystal Structure Analysis

X-ray diffraction (XRD) was used to analyze the phase structure of the electrolytes synthesized by different methods (Figure 1). The as-quenched Na3PS4 at 650 °C presents a cubic phase structure (Figure 1a). After annealing at 270 °C, the diffraction peaks started to show a tendency of peak splitting, the enlargement of the peaks at around 31° (inset in Figure 1a) had more typical split characteristics. Then, after annealing at a higher temperature of 450 °C, the c-Na3PS4 phase has been completely transformed into the t-Na3PS4 (JCPDS #081-1472). Moreover, unlike the ball-milled Na3PS4, which are both t-Na3PS4 annealed at 270 °C or 450 °C, the t-Na3PS4 that was quenched and then annealed at 450 °C showed weaker and wider peaks with respect to the ball-milled t-Na3PS4 annealed at 450 °C. This is due to the fact that the as-quenched cubic phase is thermodynamically less stable owe to the vast defects and the lattice distortion energy induced by quenching [21]. Subsequent annealing can incite the lattice distortion energy to relieve and provide sufficient energy to surmount the phase transformation energy barrier, driving the transformation of c-Na3PS4 phase towards t-Na3PS4. Figure 1b demonstrates the XRD patterns of Na3−xPS4−xBrx (x = 0, 0.0625, 0.1, 0.125, 0.15) prepared from the quenching at 650 °C followed by annealing at 450 °C. It can be observed that all the diffraction peaks of Br-doped Na3−xPS4−xBrx gradually and slightly shift to the left (x < 0.1), suggesting a decrease in the glancing angles θ, and reversely to the right once the Br doping level x exceeds 0.1, meaning larger θ, which is particularly evident in the enlargement of the peaks at around 31° (inset in Figure 1b). According to Bragg’s equation, this shift can be attributed to the partial replacement of S2− by the larger Br− ions, the proper Br doping (such as x < 0.1) can increase the lattice constants to a certain extent. However, excessive Br (x > 0.1) has failed to be incorporated into the Na3PS4 lattice (because NaBr was verified) and has potentially even destructed the periodic regularity of the Na3PS4 crystal structure, presenting relatively poor crystallinity, which was reflected by the gradually broadened and weakened peaks (Figure 1b). Figure 1c compares the phase structure of the hot-pressed and cold-pressed Na2.9PS3.9Br0.1 electrolyte pellets. The subjected hot-pressed Na2.9PS3.9Br0.1 pellet still retains its tetragonal phase structure without additional peaks. This indicates that the hot-pressing has not affected the phase composition of the electrolyte.

2.2. Morphological and Elemental Analysis

To examine the effect of the hot-pressing and Br doping on the morphologies and microstructure of the electrolytes, we performed scanning electron microscope (SEM) observations on the Na2.9PS3.9Br0.1 and Na3PS4 synthesized via quenching then annealing at 450 °C and the corresponding elemental mappings of Na, P, S and Br (Figure 2a,b). The high-magnification SEM images reveals that the both samples are composed of well-developed particles, which are fused together to form a lava-like structure. The particle diameter is less than 1 μm, while the fused lava-like structures are about 1–5 μm, and no significant differences are observed between the two samples (Figure 2c,d). This demonstrates that the proper Br doping (x = 0.1) does not alter the microstructure of the electrolytes. The elemental mapping analysis displays the presence and uniform distribution of Na, P, S, and Br elements because of no noticeable segregation or localized enrichment of the elements observed. Figure 2e,f show the cross-sectional SEM images of the hot-pressed and cold-pressed Na2.9PS3.9Br0.1 pellets. The hot-pressed pellet (Figure 2e) behaves as if it has a compact cross-section with a molten lava-like feature. Instead, the cold-pressed one manifests loose and many such voids (Figure 2f). In spite of the low elastic modulus E of sulfides and good mechanical ductility, the cold-pressed fabrication still fails to address the knots of solid electrolytes lack of compactness. It is these many voids that will promote dendrites to grow to penetrate through the electrolytes all the way to cathode. Thus, the cells shortcut and fail. On the contrary, the hot-pressed electrolyte pellets shake off the voids and enhance the density of electrolyte accordingly, effectively alleviating such issues as the cold-pressed electrolyte suffered from.

2.3. Electrochemical Performance Analysis

Electrochemical impedance spectroscopy (EIS) was conducted on the electrolytes to calculate and assess their ionic conductivity. The Nyquist plots of electrolytes all present twisted semicircles and straight lines, indicating their typical characteristics as ionic conductors (Figure 3). The intercept of the high-frequency semicircle with the X-axis and the diameter of the semicircle correspond to the bulk resistance and the interface resistance of the electrolytes, respectively. The calculation is determined based on the total resistance, which typically consists of bulk resistance and interface resistance [25]. Among them, the as-quenched Na3PS4 electrolyte shows a relatively large semicircle but gradually becomes small as an increasing annealed temperature from 270 °C to 450 °C (Figure 3a), meaning a decreasing resistance. Compared to the Na3PS4 electrolyte ball-milled and annealed at 450 °C (Figure 3b), the resistance of the quenched then annealed at 450 °C Na3PS4 electrolyte is much lower. Figure 3c demonstrates the Nyquist plots of Na3−xPS4−xBrx (x = 0, 0.0625, 0.1, 0.125, 0.15) prepared from the quenching at 650 °C followed by annealing at 450 °C. As the Br doping level increases (x from 0 to 0.1), the semicircle gradually becomes smaller, and yet larger after x exceeds 0.1 (Figure 3c), which implies the lowest resistance upon the Br-doping amount x = 0.1. This result is aligned with that of the XRD. The Nyquist plot of the hot-pressed Na2.9PS3.9Br0.1 pellet shows much lower resistance than the cold-pressed one (Figure 3d). The relative calculation data listed in Table 1 clearly demonstrate the differences in resistance and ionic conductivity of the electrolytes synthesized by different methods. The ionic conductivities of the Na3PS4 quenched followed by annealing at 270 °C or 450 °C are 1.35 × 10−4 S/cm and 4.41 × 10−4 S/cm, respectively. Which are far higher than the conductivity of 1.72 × 10−5 S/cm and 2.50 × 10−5 S/cm of ball-milled ones. This is because the rapid quenching refines grains and enhances defect concentrations. The ionic conductivity of the Na3PS4 quenched and then annealed at 450 °C is as nearly three times as high as that of the annealing at 270 °C, which is attributed to a high heat treatment temperature being more favorable for relieving the stress produced during synthesis and grain development [26]. The Br doping (x = 0.1) further broadens the ionic channels and doubles the conductivity of Na2.9PS3.9Br0.1 electrolyte to 8.28 × 10−4 S/cm. The subsequent application of hot-pressing further enhances grain contact and decreases the resistance of electrolyte, leading to the ionic conductivity as high as 1.15 × 10−3 S/cm at room temperature.
Moreover, we also calculated the activation energies of 450 °C annealed Na3PS4 and Na2.9PS3.9Br0.1 as well as hot-pressed Na2.9PS3.9Br0.1 according to the Arrhenius equation within a temperature range from 25 °C to 85 °C. Three electrolytes’ computations were 0.27 eV, 0.22 eV, and 0.21 eV, respectively (Figure 3e). When Br doping x = 0.1, the activation energy of Na2.9PS3.9Br0.1 greatly lowers compared to Na3PS4 (from 0.27 eV to 0.22 eV), which aligns well with the theoretically predicted results [27]. The reason for this is that the introduction of Br− with proper ionic radius not only broadens the ion transport pathways but also can weaken the interaction between the electrolyte framework and Na+. The subsequent hot-pressing guaranteed interfaces to fully contact and reduce interface resistance, which further decreased activation energy [28]. These findings highlight the efficacy of the synergistic optimization strategy combining melt-quenching, doping, and hot-pressing, providing valuable technical insights for the design and development of high-performance solid electrolytes.
A stable solid-state electrolyte (SE) is key for high-performance all-solid-state batteries. To inspect the electrochemical window of Na3PS4 and Na2.9PS3.9Br0.1 electrolytes, Na/Na3PS4/SS (stainless steel), Na/Na2.9PS3.9Br0.1/SS and Na15Sn4/Na2.9PS3.9Br0.1/SS cells were assembled using hot-pressed Na3PS4 and Na2.9PS3.9Br0.1 pellets, with Na or Na15Sn4 and stainless steel as electrodes. Cyclic voltammetry (CV) curves of the three cells were recorded over a voltage range of −0.5 to 5 V (vs. Na/Na+) at a scan rate of 0.1 mV/s (Figure 4a). It can be seen that the three curves show a pair of redox peaks at around 0 V, corresponding to the deposition and stripping of Na. The Na3PS4 against Na CV curve presents a distinct oxidation current peak during the forward scan to 3.25 V, which indicates an irreversible decomposition reaction of the electrolyte with Na metal. In contrast, the Br-doped Na2.9PS3.9Br0.1 against Na CV displays a much weaker and wider oxidation current peak, with the peak potential even delaying to 3.55 V, which suggests an availability of Br doping for stable electrolytes. The phenomenon is probably attributed to the Br doping, which facilitates a more stable solid electrolyte interphase (SEI) film (composed of organic and NaBr inorganic species) at the electrolyte–electrode interface, effectively suppressing the side reactions. Instead, the Na2.9PS3.9Br0.1 against Na15Sn4 CV always keeps flat apart from the deposition and stripping peaks of Na near 0 V and demonstrates the excellent compatibility of Na2.9PS3.9Br0.1 with Na15Sn4 anode.
To further inspect the stability of the electrolytes, the Na/Na2.9PS3.9Br0.1/Na and Na15Sn4/Na2.9PS3.9Br0.1/Na15Sn4 symmetric cells were assembled using the quenched and hot-pressed Na2.9PS3.9Br0.1 pellets, and performed galvanostatic cycling measurement at a current density of 0.1 mA/cm2 (Figure 4b). It can be found that the Na symmetric cell shows a high polarization potential that increases rapidly within a short period of approximately 200 h and fails, while the Na15Sn4 symmetric cell always maintains a very low polarization potential (approximately 10 mV) over 400 h and exhibits excellent stability. The EIS of the Na15Sn4/Na2.9PS3.9Br0.1/Na15Sn4 symmetric cell also verify its low resistance either beginning or during cycles (Figure 4c). The above results suggest that the prepared electrolytes possess excellent compatibility with Na alloys yet are still relatively unstable against pure Na. Although Br doping can, to a certain extent, relieve the side reactions, the stability of electrolytes against Na needs further improvement.
For further verifying the practicability of the solid electrolytes, the all-solid-state cells were assembled based on the hot-pressed and cold-pressed Na2.9PS3.9Br0.1 electrolytes, with Na2S composite as cathode [29] and Na15Sn4 anode [22], and cycle performances were tested within a voltage range of 0.5–3V at a constant current density of 0.1mA/cm2 (Figure 4d,f). Figure 4d compares the sodiation/desodiation profiles for the 1st and 50th cycles of these two cells based on the different solid electrolytes, and Figure 4f shows their cycle performances. The hot-pressed Na2.9PS3.9Br0.1 cell presents a lower charge platform and a higher discharge platform compared to the cold-pressed one, implying a smaller polarization and internal resistance, Figure 4e also confirms this, and exhibits an initial charge capacity of 645 mAh/g and a discharge capacity of 550 mAh/g with an initial Coulombic efficiency of 85.3%, significantly outperforming the cold-pressed one, which has a charge capacity of 524 mAh/g, a discharge capacity of 355 mAh/g, and an initial Coulombic efficiency of 67.8%. After 50 cycles, the hot-pressed Na2.9PS3.9Br0.1 cell still maintained a high reversible capacity of 420mAh/g, with a capacity retention of 76.4%. The sodiation/desodiation profile for the 50th cycle is basically the same as that of the first cycle, while the capacity of the cold-pressed cell is less than 100 mAh/g after the 50th cycle, and the voltage increases/decreases sharply when sodiation/desodiation. The good performance of the hot-pressed Na2.9PS3.9Br0.1 cell ascribed to the high ionic conductivity of the electrolytes induced by quenching and doping and low resistance by hot-pressing.

3. Materials and Methods

3.1. Synthesis

The synthesis of pure Na3PS4 via melt-quenching involves vacuum-sealing the uniformly mixing Na2S (95.0 wt%) and P2S5 (99.5 wt%), heating to 650 °C at a rate of 2 °C/min, holding for 1 h, and then quenching. To promote further crystallization, the sample was vacuum-sealed again and annealed at 270 °C and 450 °C for 2 h, respectively. To introduce Br doping, the precursors were weighed according to the reaction equation (1.5 − x)Na2S + 0.5P2S5 + xNaBr → Na3−xPS4−xBrx (x = 0, 0.0625, 0.1, 0.125, 0.15). After vacuum sealing, the mixture was subjected to melt-quenching at 650 °C, followed by annealing at 450 °C to enhance crystallinity. For comparison, Na3PS4 was also synthesized via ball milling at 510 rpm for 48 h. Raw materials and grinding media were loaded into the milling jar under argon atmosphere, which was then sealed. After ball milling, the samples were annealed at 270 °C and 450 °C, respectively. Hot-pressed electrolyte pellets were prepared by placing the powder into a graphite mold, heating to 450 °C under 80 MPa for 30 min and then cooled to room temperature before removal. The preparation of hot-pressed Na2.9PS3.9Br0.1 electrolyte pellet is shown in Figure 5. All processes were carried out without contact with air.

3.2. Characterization of Solid-State Electrolytes

The structural characterization was performed using an X-ray diffractometer (XRD; Rigaku Smartlab X-ray Diffractometer, Rigaku, Tokyo, Japan) in the 2θ range of 30° to 60°, with a Cu-Kα source (40 kV, 150 mA). Samples were sealed with Kapton tape to prevent air exposure during testing. The electrolyte powder and cross-sections of electrolyte pellets were obtained using a scanning electron microscope (SEM; Schottky Field Emission Scanning Electron Microscope SU5000, Hitachi, Tokyo, Japan), with elemental mapping performed via Energy Dispersive Spectroscopy (EDS).

3.3. Electrochemical Characterization

CV and EIS measurements were performed using an electrochemical workstation (SP-300 Potentiostat, Bio-Logic, Seyssinet-Pariset, France). EIS measurements were conducted across a frequency range of 7 MHz to 0.1 Hz in a potentiostatic mode with an amplitude of 50 mV. Temperature dependence of conductivity was obtained by heating the cell from 25 °C to 85 °C in 15 °C increments, with 1 h thermal equilibrium before each measurement. Sodium ion migration activation energy was calculated from the Arrhenius plot slope. For the CV measurements, a voltage range of −0.5 to 5V was applied at scan rate of 0.1 mV/s.
Furthermore, the performance of symmetric cells and full cells was evaluated using a Lanhe Battery Testing System (CT2001A, Wuhan Land, Wuhan, China).
All assembly and testing procedures were conducted in an environment isolated from air.

4. Conclusions

In this work, the Br-doped t-Na2.9PS3.9Br0.1 electrolyte was successfully synthesized through melt-quenching and hot-pressing, significantly reducing the synthesis cycle. The resultant Br-doped t-Na2.9PS3.9Br0.1 electrolyte delivered a high ionic conductivity of 1.15 × 10−3 S/cm at room temperature, with activation energy as low as 0.21 eV. Which is due to the fact that melt-quenching and Br-doping greatly generated defect concentration within the electrolyte, widened the Na+ transmission channels, and weakened the adsorption of scaffolds to Na+, followed by hot-pressing at 450 °C to further diminish the interface resistance. The assembled Na2S/Na2.9PS3.9Br0.1/Na15Sn4 cell exhibited a highly reversible capacity of 550 mAh/g at room temperature, and it still maintains a capacity of 420 mAh/g with a capacity retention of 76.4% over 50 cycles. The combined strategy of melt-quenching, Br doping and hot-pressing leverages ideal liquid-phase reaction kinetics, offering an efficient alternative to existing methods and providing a promising new pathway for synthesizing high-performance sulfide electrolytes for all-solid-state Na-S batteries.

Author Contributions

Conceptualization, J.W.; methodology, J.W.; formal analysis, A.M. and B.G.; investigation, A.M.; validation, J.W.; visualization, A.M.; writing—original draft, A.M.; writing—review and editing, J.W. and S.L. (Shuhui Liu); supervision, D.L. and S.L. (Sheng Li) All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by National Natural Science Foundation of China (Grant No. 52161032), Scientific Research and Technological Development of Baise City (Grant No. 20230556).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. XRD patterns of the Na3PS4 (Na3−xPS4−xBrx) through different synthesis methods: (a) ball milled and melt-quenched Na3PS4 followed by different annealing conditions; (b) melt-quenched Na3−xPS4−xBrx (x = 0, 0.0625, 0.1, 0.125, 0.15) followed by annealing at 450 °C; (c) Na2.9PS3.9Br0.1 pellets hot-pressed at 450 °C and cold-pressed, respectively.
Figure 1. XRD patterns of the Na3PS4 (Na3−xPS4−xBrx) through different synthesis methods: (a) ball milled and melt-quenched Na3PS4 followed by different annealing conditions; (b) melt-quenched Na3−xPS4−xBrx (x = 0, 0.0625, 0.1, 0.125, 0.15) followed by annealing at 450 °C; (c) Na2.9PS3.9Br0.1 pellets hot-pressed at 450 °C and cold-pressed, respectively.
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Figure 2. SEM images of (a) Na2.9PS3.9Br0.1 and (b) Na3PS4 via melt-quenching followed by annealing at 450 °C and the corresponding elemental mapping Na, P, S, and Br; SEM images of (c) Na2.9PS3.9Br0.1 and (d) Na3PS4 at higher magnification; the cross-sectional images of (e) the hot-pressed and (f) cold-pressed Na2.9PS3.9Br0.1 pellets.
Figure 2. SEM images of (a) Na2.9PS3.9Br0.1 and (b) Na3PS4 via melt-quenching followed by annealing at 450 °C and the corresponding elemental mapping Na, P, S, and Br; SEM images of (c) Na2.9PS3.9Br0.1 and (d) Na3PS4 at higher magnification; the cross-sectional images of (e) the hot-pressed and (f) cold-pressed Na2.9PS3.9Br0.1 pellets.
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Figure 3. Nyquist plots of the Na3PS4 (Na3−xPS4−xBrx) through different synthesis methods: (a) melt-quenched and (b) ball-milled Na3PS4 fallowed by different annealing conditions; (c) Na3−xPS4−xBrx (x = 0, 0.0625, 0.1, 0.125, 0.15) quenched then annealed at 450 °C; (d) cold-pressed and hot-pressed Na2.9PS3.9Br0.1 pellets; (e) activation energies of electrolytes synthesized via different methods.
Figure 3. Nyquist plots of the Na3PS4 (Na3−xPS4−xBrx) through different synthesis methods: (a) melt-quenched and (b) ball-milled Na3PS4 fallowed by different annealing conditions; (c) Na3−xPS4−xBrx (x = 0, 0.0625, 0.1, 0.125, 0.15) quenched then annealed at 450 °C; (d) cold-pressed and hot-pressed Na2.9PS3.9Br0.1 pellets; (e) activation energies of electrolytes synthesized via different methods.
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Figure 4. (a) CV curves of different cells measured at a scan rate of 0.1 mV/s; (b) voltage profiles of symmetric cells with Na15Sn4 and Na metal electrodes at 0.1 mA/cm2; (c) Nyquist plots acquired from the symmetric cell with Na15Sn4 electrode; (d–f) electrochemical performance of all-solid-state full cells assembled with hot-pressed and cold-pressed Na2.9PS3.9Br0.1.
Figure 4. (a) CV curves of different cells measured at a scan rate of 0.1 mV/s; (b) voltage profiles of symmetric cells with Na15Sn4 and Na metal electrodes at 0.1 mA/cm2; (c) Nyquist plots acquired from the symmetric cell with Na15Sn4 electrode; (d–f) electrochemical performance of all-solid-state full cells assembled with hot-pressed and cold-pressed Na2.9PS3.9Br0.1.
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Figure 5. Schematic diagram of the Na2.9PS3.9Br0.1 electrolyte pellet preparation.
Figure 5. Schematic diagram of the Na2.9PS3.9Br0.1 electrolyte pellet preparation.
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Table 1. Ionic-conductivity-related parameters of electrolytes synthesized via different methods.
Table 1. Ionic-conductivity-related parameters of electrolytes synthesized via different methods.
Preparation MethodThickness (mm)Bulk Resistance (Ω)Interface Resistance (Ω)Total Resistance (Ω)Ionic Conductivity (S/cm)
Ball-milled Na3PS4-270 °C0.81421.245585.006006.241.72 × 10−5
Ball-milled Na3PS4-450 °C0.81401.883722.004123.882.50 × 10−5
Na3PS4 without annealing0.59257.71855.001112.716.76 × 10−5
Na3PS4-270 °C0.50180.53292.00472.531.35 × 10−4
Na3PS4-450 °C0.58105.5262.16167.684.41 × 10−4
Na2.9375PS3.9375Br0.0625-450 °C0.5758.6245.81104.436.95 × 10−4
Na2.9PS3.9Br0.1-450 °C0.5662.9323.1986.128.28 × 10−4
Na2.875PS3.875Br0.125-450 °C0.5161.10111.2172.303.77 × 10−4
Na2.85PS3.85Br0.15-450 °C0.5564.34212.6276.942.53 × 10−4
Na2.9PS3.9Br0.1 450 °C hot-pressing0.6344.9325.1070.031.15 × 10−3
Captions: Except where specifically denoted, the rest of the samples were fabricated through quenching, annealing, and cold-pressing.
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Ma, A.; Liu, S.; Li, D.; Gu, B.; Li, S.; Wang, J. Fabrication and Electrochemical Performance of Br-Doped Na3PS4 Solid-State Electrolyte for Sodium–Sulfur Batteries via Melt-Quenching and Hot-Pressing. Inorganics 2025, 13, 73. https://doi.org/10.3390/inorganics13030073

AMA Style

Ma A, Liu S, Li D, Gu B, Li S, Wang J. Fabrication and Electrochemical Performance of Br-Doped Na3PS4 Solid-State Electrolyte for Sodium–Sulfur Batteries via Melt-Quenching and Hot-Pressing. Inorganics. 2025; 13(3):73. https://doi.org/10.3390/inorganics13030073

Chicago/Turabian Style

Ma, Ao, Shuhui Liu, Degui Li, Bin Gu, Sheng Li, and Jing Wang. 2025. "Fabrication and Electrochemical Performance of Br-Doped Na3PS4 Solid-State Electrolyte for Sodium–Sulfur Batteries via Melt-Quenching and Hot-Pressing" Inorganics 13, no. 3: 73. https://doi.org/10.3390/inorganics13030073

APA Style

Ma, A., Liu, S., Li, D., Gu, B., Li, S., & Wang, J. (2025). Fabrication and Electrochemical Performance of Br-Doped Na3PS4 Solid-State Electrolyte for Sodium–Sulfur Batteries via Melt-Quenching and Hot-Pressing. Inorganics, 13(3), 73. https://doi.org/10.3390/inorganics13030073

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