Cold sintering of Na3Zr2Si2PO12 solid electrolyte: Effect of mechanical pressing and post-annealing

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Adetona, Ayorinde O. Nejo, Moses Yibowei This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6479472/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract We investigated the effect of mechanical pressing and post-annealing on aqueous cold-sintered Na₃Zr₂Si₂PO₁₂ (NZSP) solid electrolyte, comparing the results with conventionally sintered NZSP. Various physical, microscopic and spectroscopic techniques, including Archimedes, Impedance spectroscopy, XRD and SEM, were used to investigate the properties of the cold-sintered NZSP solid electrolyte. X-ray diffraction confirmed the retention of the NaSICON phase in cold-sintered NZSP, with the secondary m -ZrO₂ peaks becoming prominent. Scanning electron microscopy revealed that post-annealing improved the ceramic's morphology, showing grain formation and inter-granular porosity remained. With an ionic conductivity of 2.45 × 10 ⁻ ⁵ S/cm at 25 °C, NZSP ceramics post-annealed at 400 °C showed no susceptibility to mechanical pressing. Conductivity increased tenfold after post-annealing at 800 °C, reaching 1.43 × 10 - ⁴ S/cm. Cold sintering Na3Zr2Si2PO12 solid electrolyte X-ray diffraction Electrochemical impedance spectroscopy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Cold sintering is a technique that reduces the high energy consumption and processing temperatures required for ceramic materials, unlike conventional sintering methods, which require temperatures exceeding 1000°C [ 1 – 7 ]. Cold sintering involves densifying ceramics at much lower temperatures, often below 300°C, by applying mechanical pressure and a transient solvent at low temperatures. The process involved in cold sintering includes material dissolution, particle rearrangement, crystal growth, and grain growth [ 5 – 8 ]. This method has gained significant interest due to its potential to lower manufacturing costs [ 3 – 6 ], reduce carbon footprints [ 1 , 7 ], and enable the integration of ceramics with temperature-sensitive materials [ 2 ]. One material of particular interest for cold sintering is Na₃Zr₂Si₂PO₁₂ (NZSP or NaSICON). NaSICON stands for Sodium Super Ionic Conductors [ 9 – 13 ]. NZSP ceramics have been thoroughly studied and used as solid electrolytes in sodium-ion solid-state batteries and sodium-ion conductors because of their high ionic conductivity, high ionic transference number and thermal stability [ 8 – 10 ]. NaSICON has been thoroughly studied as a promising solid electrolyte for all-solid-state batteries (SSSBs) and other solid-state ionic devices. Its typical formula is Na 1 + x Zr 2 Si x P 3−x O 12 (0.0 ≤ x ≤ 3.0) [ 14 – 17 ]. Na-ion concentration, diffusion, Na + mobility and crystal symmetry are some of the variables that affect its ionic conductivity [ 14 ]. Na + ions move via channels made of SiO 4 /PO 4 tetrahedral and ZrO 6 octahedral in the NaSICON structure. Na 1 + x Zr 2 Si x P 3−x O 12 (0.0 ≤ x ≤ 3.0) has two known crystal symmetries: rhombohedral ( R-3c ) and monoclinic ( C12/c ). At room temperature, the monoclinic phase, which occurs between 1.8 ≤ x ≤ 2.2, is thermally stable and has a conductivity of 10^ −3 and 10^ −4 S/cm [ 9 , 11 , 14 ]. Conventional sintering of phosphate-based ceramics solid electrolytes requires high temperatures, typically around 1000°C [ 18 – 20 ], which could lead to undesirable grain growth, phase instability, and increased energy consumption. Cold sintering offers a potential solution by allowing densification at much lower temperatures. However, the effectiveness of cold sintering on NZSP ceramics solid electrolytes still needs to be fully understood [ 21 – 23 ], especially regarding how mechanical pressing and post-annealing treatments influence the material's structure and ionic conductivity. There are a few studies on the cold sintering of NZSP ceramics solid electrolytes in the literature, summarised in Table 1 . While the NZSP structure is retained [ 21 ], achieving high relative density and ionic conductivity comparable to conventionally sintered NZSP solid electrolytes remains challenging. This study investigates the aqueous cold sintering of NZSP solid electrolytes and the effect of mechanical pressure and post-annealing on the ceramics, focusing on the NZSP phase stability, microstructure, and conductivity measured against reported literature. Table 1 NaSICON Compositions, sintering temperatures, times, transient solvents, relative density after post-annealing and total ionic conductivity of NaSICON compositions densified by cold sintering method. NASICON Sintering Temp. (°C) Time (minutes) Transient Solvent ρ r (%) Ionic conductivity (S/cm) Ref. Na 3 Zr 2 Si 2 PO 12 375 180 NaOH > 90 2.0 × 10 − 4 21 Na 3.256 Mg 0.128 Zr 1.872 Si 2 PO 12 140 120 H 2 O 82.4 4.0 × 10 − 5 22 Na 3.256 Mg 0.128 Zr 1.872 Si 2 PO 12 140 120 Bi 2 O 3 86.1 1.1 × 10 − 4 22 Na 3.4 Sc 0.4 Zr 1.6 Si 2 PO 12 250 15 KOH 87 1.2 × 10 − 6 23 2. Experimental methods 2.1 NZSP ceramic fabrication : The NZSP powders employed in this investigation were synthesised through the solid-state reaction method described in [ 9 ]. A pestle, mortar, and 5 µm sieve were used to crush and grind the ceramic pellets into powders. The NZSP powder was mixed in a pestle and mortar with 10 wt.% distilled H 2 O for approximately 4 minutes. The water evaporated during mixing, and the NZSP powder became hygroscopic. After that, the moisture powder was compressed in a 10 mm die platen for an hour at 120°C using a Specac uniaxial hot press managed by Atlas Series Platen Controller. The pressures ranged from 125 to 437 MPa. The pressing time was recorded once the maximum parameters (125 MPa and 120°C) were achieved. A heating rate of 1°C per minute was achieved by applying mechanical pressure. To guarantee reproducibility, two pellets were pressed and inspected at every temperature, pressure level, and time. The Archimedes' method was used to calculate NZSP ceramics density. 2.2 Pellet Surface Preparation : Conventional mechanical polishing with sandpaper was not feasible due to the inherent fragility of the cold-sintered NZSP pellets (prior to and after post-annealing), often leading to cracking or edge damage. The pellet surfaces were gently polished by lightly rubbing against fine-grade (1200 grit) SiC paper on a flat surface, using minimal pressure and no water. 2.3 Microstructural and structural characterisation of NZSP ceramics : The diffraction data of the cold-sintered NZSP solid electrolyte pellets were examined using a PANalytical Aeris X-ray diffractometer (XRD) with Cu-Kα radiation (λ = 0.154 nm) over a 2θ range of 10–60° with a 0.02° step size. Topas 5 software was used to examine the phase refinement of the diffraction data. Using an FEI Inspect F50 scanning electron microscope (SEM), the microstructural analysis of the cold-sintered NZSP ceramic's lightly polished surface was conducted. 2.4 Impedance spectroscopy of NZSP ceramic : Impedance measurements were conducted on the Au paste-coated cold-sintered NZSP pellets. Electrochemical impedance spectroscopy was conducted at 50°C intervals between room temperature (RT) and 300°C, using an Agilent 4294A. Stainless steel | NZSP | stainless steel cell configuration was used for the EIS testing setup. The measurements were conducted in a custom-built cell with spring-loaded electrodes to ensure consistent contact. An applied pressure of approximately 5 MPa was maintained during impedance measurements to improve interface stability and minimise contact resistance. The Agilent 4294A was calibrated to correct measurement errors using blank, open, and closed circuits. A geometric correction factor that considered the sample-electrode surface area and pellet thickness was applied following impedance (Z*) measurements. Data analysis and circuit fitting were done using Scribner Associates' ZVIEW Impedance Software version 2.4. The ionic conductivity was calculated from Eq. 1 below. \(\:\sigma\:=\frac{\text{L}}{\text{R}\text{*}\text{A}}\) …………………………………………………….1. Where, σ – is the ionic conductivity (S/cm) L – is the thickness of the pellets. R – is the bulk resistance obtained from the Nyquist plot (ꭥ) A – is the cross-sectional area of the pellet (cm²). 3. Results 3.1 Relative density The density of the NZSP ceramic was measured using Archimedes' method, and the results were contrasted with the theoretical density of NZSP (3.24 g/cm 3 ) derived from XRD refinement investigations. The cold-sintered Na 3 Zr 2 Si 2 PO 12 ceramics were fragile; therefore, some NZSP ceramics were post-annealed at 400°C and others at 800°C prior to measurement. As shown in Table 2 , cold-sintered NZSP pellets annealed at 400°C achieved a relative density (ρ r ) of ~ 80%, irrespective of the applied mechanical pressure, consistent with literature reports [ 21 – 23 ]. The only exception was the sample pressed at 125 MPa, which exhibited a ρ r of ~ 75%. Since the mechanical pressure had minimal influence on the cold-sintered NZSP ceramics, three NZSP ceramics samples pressed between 312 and 437 MPa were post-annealed at 800°C, resulting in a ρ r increase of over 40%, achieving ρ r > 90%, consistent with the literature [ 21 , 23 ]. The error margin of ± 0.5% reflects the estimated experimental uncertainty associated with density measurements using Archimedes' method. This includes potential variations due to surface porosity, trapped air bubbles, and minor mass and volume measurement fluctuations during immersion. For each processing condition, two pellets were independently prepared and measured. The reported density values in Table 2 represent the average of these measurements. Table 2 Cold-sintered NZSP relative density Relative density of cold-sintered Na 3 Zr 2 Si 2 PO 12 Pressure (MPa) ρ r (%) + 400°C ρ r (%) + 800°C 125 75.3 ± 0.3 - 250 80.0 ± 0.4 - 312 80.2 ± 0.5 89.5 ± 0.5 375 80.6 ± 0.5 91.0 ± 0.5 437 79.6 ± 0.3 88.9 ± 0.5 3.2 X-ray Diffraction NZSP ceramic solid electrolyte diffraction pattern confirms the formation of NaSICON material, with peaks matching monoclinic Na₃Zr₂Si₂PO₁₂ (PDF No: 00-035-0412 and space group C12/c ), as shown in Fig. 1 a. XRD patterns of the cold-sintered NZSP at various mechanical pressures (125–437 MPa) and post-annealed at 400 and 800°C are shown in Fig. 1 (b–f). This revealed that the NZSP framework and C12/c space group were preserved, regardless of the mechanical pressing and post-annealing. The only notable difference in the peak is the formation of m -ZrO 2 secondary peaks, which occur at 2θ° ≈ 28.30 and 31.52, consistent with the literature [ 9 – 13 , 21 ]. However, the secondary m -ZrO₂ peaks were more prominent in samples mechanically pressed at 250–437 MPa, Fig. 1 (c–f). Full-pattern Topas 5 software was used to understand the crystallographic data and the NZSP compositions' phase formation, and the cold-sintered NZSP's diffraction pattern was examined using the Rietveld refinement method. The two phases, C12/c NZSP and m -ZrO 2 , were validated by the refinement results. Figure 2 a revealed the refinement pattern of NZSP conventionally sintered, which was used as a reference [ 9 ] and Figs. 2 (b-f) show the refined pattern of the cold-sintered NZSP. Table 3 shows the lattice parameters, theoretical density, unit cell volume, goodness of fits (GoF), and phase composition percentage. All the cold-sintered NZSP ceramics have approximately 96% monoclinic ( C12/c ) NZSP symmetry and 4% m -ZrO 2 impurity phase. The increase in ZrO₂ content with higher mechanical pressure may be attributed to localized decomposition or partial destabilization of the NZSP phase due to increased stress at particle contacts during pressing. Such conditions can potentially promote the segregation or formation of secondary phases like ZrO₂, especially without sufficient thermal energy to fully densify the NZSP structure. Also, we examined this trend in samples without post-annealing. The PXRD data suggest that a similar, though less pronounced, increase in ZrO₂ content can be observed with increasing pressure, even in the as-pressed samples. Table 3 Refined parameters, the goodness of fit (GoF), phase fractions and theoretical density of NZSP ceramic NZSP Phase fraction (%) Lattice parameters (Å) β (°) GoF Unit cell volume Theoretical density C12/c m -ZrO 2 a b c 125 97.6 2.4 15.642 9.048 9.226 123.71 2.08 1086.2 3.246 250 96.4 3.6 15.642 9.049 9.221 123.69 2.14 1085.9 3.247 312 96.5 3.5 15.645 9.051 9.223 123.7 2.29 1086.5 3.245 375 96.5 3.5 15.646 9.051 9.223 123.7 2.2 1086.6 3.245 437 96.7 3.3 15.642 9.051 9.225 123.7 2.11 1086.5 3.245 3.3 Scanning electron microscopy Figures 3 (a) and (b) display the micrographs of the cold-sintered Na₃Zr₂Si₂PO₁₂ solid electrolytes after they were post-annealed at 400 and 800°C, respectively. Since the mechanical pressing susceptibility had minimal impact on the NZSP ceramic. The micrographs reveal a porous, loosely compacted ceramic with minimal particle agglomeration, which agrees with a ceramic with a ρ r of ~ 80%. In contrast, the micrographs of the ceramic mechanically pressed at 375 MPa and post-annealed at 800°C, Fig. 3 b, show a more compacted structure and well-densified grains, although inter-granular porosity remains. This micrograph supports a ceramic with a ρ r of ≈ 90%, consistent with NZSP SEM reported in the literature [ 21 – 22 ]. According to Table 4 , grain size analysis using the line-intercept method reveals that the mean grain size was ~ 1.24 µm following post-annealing at 400°C and subsequently increased to ~ 1.38 µm following post-annealing at 800°C, showing a 10% grain growth. Compared to NZSP (~ 2.0 µm) conventionally sintered, the mean grain sizes of the cold-sintered NZSP ceramic are smaller [ 9 – 12 ]. Table 4 Cold-sintered NZSP ceramic grain sizes after post-annealing at 400 and 800°C using the line intercept method. Post-annealed at 400°C Post annealed at 800°C. S/n Length Grain intercept Grain size Length Grain intercept Grain size 1 10.58 11 0.96 8.81 7 1.26 2 7.22 7 1.03 11.1 8 1.39 3 11.57 7 1.65 9.42 8 1.18 4 5.96 5 1.19 5.57 4 1.39 5 9.09 8 1.14 11.37 7 1.62 6 10.1 6 1.68 12.04 8 1.51 7 6.01 6 1 10.63 8 1.33 Average grain size 1.24 1.38 3.4 Impedance spectroscopy Cold-sintered Na₃Zr₂Si₂PO₁₂ solid electrolytes, densified at varying mechanical pressures (125–437 MPa) and post-annealed at 400 and 800°C, are depicted in Figs. 4 and 5 as complex impedance (Z*) graphs. The impedance spectra indicate ionic conduction at 25°C, which shows a low-frequency spike and an arc with a non-zero intercept. The total resistivity was determined by intercepting the spike with the Z' axis. The (Z*) plots of NZSP ceramic solid electrolytes that were mechanically pressed at 125 and 250 MPa and then post-annealed at 400°C are shown in Fig. 4 . Within a margin of error, the total ionic conductivities at 25°C were 2.73 × 10⁻⁵ S/cm and 2.45 × 10⁻⁵ S/cm, respectively, showing that pressure had minimal effect on the cold-sintered ceramic conductivity. Similarly, Fig. 5 compares NZSP ceramics mechanically pressed at 312, 375, and 437 MPa and post-annealed at 800°C. At 25°C, the total ionic conductivities were 1.25 × 10⁻⁴, 1.43 × 10⁻⁴, and 1.10 × 10⁻⁴ S/cm, respectively, consistent with the literature [ 21 – 22 ]. The higher conductivity at 375 MPa compared to 437 MPa correlates with the higher density observed at 375 MPa. Overall, post-annealing at 800°C had a ten-fold increment in conductivity compared to 400°C. These results were consistent with literature reports on cold-sintered NZSP solid electrolytes, though the conductivity values for cold-sintered NZSP ceramics remain lower compared to NZSP ceramics sintered conventionally [ 8 – 9 ]. Figure 6 displays the Arrhenius plots of the total conductivity (σ t = 1/Z t ) for cold-sintered NZSP solid electrolytes post-annealed at 400°C and 800°C and densified at different mechanical pressures. The associated activation energies (E a ) and uncertainties were derived from data shown in Table 5 , and the slope of the temperature-dependent data was calculated using the least-squares method. After being mechanically pressed at 125 and 250 MPa and post-annealed at 400°C, the E a for cold-sintered NZSP solid electrolytes was ~ 0.41 ± 0.11 eV. Conversely, samples that were post-annealed at 800°C and mechanically pressed between 312–437 MPa showed lower E a values of ~ 0.34 ± 0.05 eV. However, these E a values are consistent with the literature and NZSP conventionally sintered [ 9 – 14 , 21 – 23 ]. Table 5 Temperature-dependent conductivity table of cold-sintered NZSP and the pressing pressure Post annealed 400°C/ 12hr Post annealed 800°C/12hr 125MPa 250MPa 312MPa 375MPa 437MPa Temperature (°C) 1000/T (K) Log σ t Log σ t Temperature (°C) 1000/T (K) Log σ t Log σ t Log σ t 80 2.83 -3.27 -3.27 40 3.19 -3.72 -3.65 -3.78 120 2.54 -2.6 -2.57 80 2.83 -3.05 -3.02 -3.12 160 2.31 -2.15 -2.13 120 2.54 -2.57 -2.54 -2.45 200 2.11 -1.8 -1.78 160 2.31 -2.12 -2.04 -2.18 200 2.11 -1.88 -1.74 -1.97 4. Conclusions We investigated the effects of mechanical pressing and post-annealing on NZSP ceramic solid electrolyte cold-sintered by XRD, Archimedes, SEM, and impedance spectroscopy and compared the results to conventionally sintered NZSP and cold-sintered NZSP in the literature. XRD confirmed that the phase assemblage remained unchanged, though m -ZrO₂ secondary peaks were more prominent. SEM revealed the influence of post-annealing on morphology with persistent intergranular porosity. Ionic conductivities at 25°C showed minimal pressure impact at 250 MPa, with 2.45 × 10⁻⁵ S/cm and E a ≈ 0.41 eV. Post-annealing at 800°C improved conductivity, achieving 1.43 × 10⁻⁴ S/cm and E a ≈ 0.34 eV. In its present form, the cold-sintered NZSP process is energy-intensive and not viable without improvements. Declarations Author Contribution Ademola Adetona, Ph.D.Conceptualization: The author conceived the primary ideas and research question.Methodology: The author developed the research design, collected and analysed the data.Investigation: The author conducted experiments and gathered relevant data.Writing - Original Draft: The author wrote the initial draft of the manuscript.Writing - Review & Editing: The author contributed to revising and editing.Visualisation: The author created the figures and visual elements used in the manuscript.Validation: The author verified the accuracy and integrity of the research findings.Funding Acquisition: The author secured financial support for the project.Project Administration: The author managed the project and ensured its smooth execution.Olufunke Nejo, Ph.D.Writing - Original Draft: The author contributed to the initial draft of the manuscript.Writing - Review & Editing: The author provided critical feedback and contributed to improving the manuscript.Validation: The author verified the accuracy and integrity of the research findings.Mr Moses YiboweiWriting - Original Draft: The author contributed to the initial draft of the manuscript.Writing - Review & Editing: The author provided critical feedback and contributed to improving the manuscript.Validation: The author verified the accuracy and integrity of the research findings. Acknowledgement The authors acknowledge the financial support of the Tertiary Education Trust Fund of Nigeria (TETFUND), the Department of Chemistry at the University of Lagos, Nigeria, the Functional Materials and Devices Group in the Department of Materials Science and Engineering at The University of Sheffield, United Kingdom, and Ge Wang (Dame Kathleen Ollerenshaw Fellow) from the Materials Department of the University of Manchester. Data Availability Upon a reasonable request, the data for this article is available. Declaration and Conflict of Interest The authors declare no known financial conflicts that could have affected the work presented in this article. References L. Li, J. Andrews, R. Mitchell, D. Button, D.C. Sinclair, I.M. Reaney, Aqueous Cold Sintering of Li-Based Compounds, ACS Applied Materials & Interfaces. 15 (2023) 20228–20239. https://doi.org/10.1021/acsami.3c00392. J. Andrews, D. Button, I.M. 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H. Leng, J. Huang, J. Nie, J. Luo, Cold sintering and ionic conductivities of Na 3.256 Mg 0.128 Zr 1.872 Si 2 PO 12 solid electrolytes, Journal of Power Sources. 391 (2018) 170–179. https://doi.org/10.1016/j.jpowsour.2018.04.067. J. G. Pereira da Silva, M. Bram, A.M. Laptev, J. Gonzalez-Julian, Q. Ma, F. Tietz, O. Guillon, Sintering of a sodium-based NASICON electrolyte: A comparative study between cold, field-assisted and conventional sintering methods, Journal of the European Ceramic Society. 39 (2019) 2697–2702. https://doi.org/10.1016/j.jeurceramsoc.2019.03.023. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6479472","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":458088927,"identity":"0fda727a-72eb-4822-919f-e5021061647d","order_by":0,"name":"Ademola J. Adetona","email":"data:image/png;base64,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","orcid":"","institution":"University of Sheffield","correspondingAuthor":true,"prefix":"","firstName":"Ademola","middleName":"J.","lastName":"Adetona","suffix":""},{"id":458088928,"identity":"221de90e-3a5d-4a1a-bb6f-d6af4dcc71a8","order_by":1,"name":"Ayorinde O. Nejo","email":"","orcid":"","institution":"University of Lagos, Lagos State","correspondingAuthor":false,"prefix":"","firstName":"Ayorinde","middleName":"O.","lastName":"Nejo","suffix":""},{"id":458088929,"identity":"90503fad-8ec5-4920-a26b-ee2005b45044","order_by":2,"name":"Moses Yibowei","email":"","orcid":"","institution":"University of Sheffield","correspondingAuthor":false,"prefix":"","firstName":"Moses","middleName":"","lastName":"Yibowei","suffix":""}],"badges":[],"createdAt":"2025-04-18 13:53:04","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6479472/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6479472/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":83103053,"identity":"7b974f3d-0e58-4b50-b6a5-e89c12cc745f","added_by":"auto","created_at":"2025-05-20 05:33:13","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":37561,"visible":true,"origin":"","legend":"\u003cp\u003eX-ray diffraction patterns of cold-sintered NZSP (*) show the \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e impurity phases.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6479472/v1/e38aca55683166eb5dabd28e.png"},{"id":83103878,"identity":"819921e7-388a-4e43-b18e-bd871f0746a4","added_by":"auto","created_at":"2025-05-20 05:41:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":217870,"visible":true,"origin":"","legend":"\u003cp\u003eThe full pattern Rietveld refinement of cold-sintered Na\u003csub\u003e3\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e12\u003c/sub\u003e at different mechanical pressing and post-annealing (a) conventional NZSP (b) 125MPa/400 °C (c) 250MPa/400 °C (d) 312MPa/800 °C (e) 375MPa/800 °C (f) 412MPa/800 °C.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6479472/v1/e4dc6cd68b653c4ce2e8f137.png"},{"id":83105062,"identity":"bb6cbb87-5f79-44cb-8ae5-18eea78584f6","added_by":"auto","created_at":"2025-05-20 06:05:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":455118,"visible":true,"origin":"","legend":"\u003cp\u003eThe micrographs of the polished surface of cold-sintered NZSP ceramic (a) after post-annealed at 400 °C/12hr (b) after post-annealed at 800 °C/12hr.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6479472/v1/a7d792691170ec849a53559a.png"},{"id":83103055,"identity":"d7981663-63d1-4462-a2fc-93ce3c60b437","added_by":"auto","created_at":"2025-05-20 05:33:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":24108,"visible":true,"origin":"","legend":"\u003cp\u003e25 °C impedance plots of cold sintered Na\u003csub\u003e3\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e12\u003c/sub\u003e at pressing pressures (125 and 250MPa) and post-annealed at 400 °C.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6479472/v1/b4b1306e35ca50fdb5e86a2d.png"},{"id":83103880,"identity":"d03ba62e-0581-4535-ad61-e4a766d0d69d","added_by":"auto","created_at":"2025-05-20 05:41:13","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":28315,"visible":true,"origin":"","legend":"\u003cp\u003e25 °C impedance plots of cold sintered Na\u003csub\u003e3\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e12\u003c/sub\u003e ceramic at pressing pressures (312, 375 and 437MPa) and post-annealed at 800 °C.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6479472/v1/2fac0d1e54a548178de28697.png"},{"id":83103060,"identity":"2cf7e398-3759-4800-a7be-e3dad5b8ec21","added_by":"auto","created_at":"2025-05-20 05:33:13","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":34290,"visible":true,"origin":"","legend":"\u003cp\u003eArrhenius plots of the total ionic conductivity of cold-sintered NZSP\u003csub\u003e \u003c/sub\u003eat various pressing pressures and post-annealed.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6479472/v1/b9648d068ff8e782402af42d.png"},{"id":83105068,"identity":"183c8ed1-fe1d-4cfd-a532-14b1b0231f7c","added_by":"auto","created_at":"2025-05-20 06:05:18","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1796799,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6479472/v1/b17e403f-7ad1-41db-bd8f-6146eeca03d9.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Cold sintering of Na3Zr2Si2PO12 solid electrolyte: Effect of mechanical pressing and post-annealing","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eCold sintering is a technique that reduces the high energy consumption and processing temperatures required for ceramic materials, unlike conventional sintering methods, which require temperatures exceeding 1000\u0026deg;C [\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5 CR6\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Cold sintering involves densifying ceramics at much lower temperatures, often below 300\u0026deg;C, by applying mechanical pressure and a transient solvent at low temperatures. The process involved in cold sintering includes material dissolution, particle rearrangement, crystal growth, and grain growth [\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. This method has gained significant interest due to its potential to lower manufacturing costs [\u003cspan additionalcitationids=\"CR4 CR5\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], reduce carbon footprints [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], and enable the integration of ceramics with temperature-sensitive materials [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. One material of particular interest for cold sintering is Na₃Zr₂Si₂PO₁₂ (NZSP or NaSICON). NaSICON stands for Sodium Super Ionic Conductors [\u003cspan additionalcitationids=\"CR10 CR11 CR12\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. NZSP ceramics have been thoroughly studied and used as solid electrolytes in sodium-ion solid-state batteries and sodium-ion conductors because of their high ionic conductivity, high ionic transference number and thermal stability [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. NaSICON has been thoroughly studied as a promising solid electrolyte for all-solid-state batteries (SSSBs) and other solid-state ionic devices. Its typical formula is Na\u003csub\u003e1\u0026thinsp;+\u0026thinsp;x\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003ex\u003c/sub\u003eP\u003csub\u003e3\u0026minus;x\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e (0.0\u0026thinsp;\u0026le;\u0026thinsp;x\u0026thinsp;\u0026le;\u0026thinsp;3.0) [\u003cspan additionalcitationids=\"CR15 CR16\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Na-ion concentration, diffusion, Na\u003csup\u003e+\u003c/sup\u003e mobility and crystal symmetry are some of the variables that affect its ionic conductivity [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Na\u003csup\u003e+\u003c/sup\u003e ions move via channels made of SiO\u003csub\u003e4\u003c/sub\u003e/PO\u003csub\u003e4\u003c/sub\u003e tetrahedral and ZrO\u003csub\u003e6\u003c/sub\u003e octahedral in the NaSICON structure. Na\u003csub\u003e1\u0026thinsp;+\u0026thinsp;x\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003ex\u003c/sub\u003eP\u003csub\u003e3\u0026minus;x\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e (0.0\u0026thinsp;\u0026le;\u0026thinsp;x\u0026thinsp;\u0026le;\u0026thinsp;3.0) has two known crystal symmetries: rhombohedral (\u003cem\u003eR-3c\u003c/em\u003e) and monoclinic (\u003cem\u003eC12/c\u003c/em\u003e). At room temperature, the monoclinic phase, which occurs between 1.8\u0026thinsp;\u0026le;\u0026thinsp;x\u0026thinsp;\u0026le;\u0026thinsp;2.2, is thermally stable and has a conductivity of 10^\u003csup\u003e\u0026minus;3\u003c/sup\u003e and 10^\u003csup\u003e\u0026minus;4\u003c/sup\u003e S/cm [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Conventional sintering of phosphate-based ceramics solid electrolytes requires high temperatures, typically around 1000\u0026deg;C [\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], which could lead to undesirable grain growth, phase instability, and increased energy consumption. Cold sintering offers a potential solution by allowing densification at much lower temperatures. However, the effectiveness of cold sintering on NZSP ceramics solid electrolytes still needs to be fully understood [\u003cspan additionalcitationids=\"CR22\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], especially regarding how mechanical pressing and post-annealing treatments influence the material's structure and ionic conductivity. There are a few studies on the cold sintering of NZSP ceramics solid electrolytes in the literature, summarised in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. While the NZSP structure is retained [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], achieving high relative density and ionic conductivity comparable to conventionally sintered NZSP solid electrolytes remains challenging. This study investigates the aqueous cold sintering of NZSP solid electrolytes and the effect of mechanical pressure and post-annealing on the ceramics, focusing on the NZSP phase stability, microstructure, and conductivity measured against reported literature.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eNaSICON Compositions, sintering temperatures, times, transient solvents, relative density after post-annealing and total ionic conductivity of NaSICON compositions densified by cold sintering method.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026times;\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNASICON\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSintering Temp. (\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTime (minutes)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTransient Solvent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eρ\u003csub\u003er\u003c/sub\u003e (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eIonic conductivity (S/cm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eRef.\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa\u003csub\u003e3\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e12\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e375\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e180\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eNaOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026gt;\u0026thinsp;90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c6\"\u003e \u003cp\u003e2.0 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e21\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa\u003csub\u003e3.256\u003c/sub\u003eMg\u003csub\u003e0.128\u003c/sub\u003eZr\u003csub\u003e1.872\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e12\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e140\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e82.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c6\"\u003e \u003cp\u003e4.0 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa\u003csub\u003e3.256\u003c/sub\u003eMg\u003csub\u003e0.128\u003c/sub\u003eZr\u003csub\u003e1.872\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e12\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e140\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBi\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e86.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c6\"\u003e \u003cp\u003e1.1 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa\u003csub\u003e3.4\u003c/sub\u003eSc\u003csub\u003e0.4\u003c/sub\u003eZr\u003csub\u003e1.6\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e12\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e250\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eKOH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026times;\" colname=\"c6\"\u003e \u003cp\u003e1.2 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e23\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"2. Experimental methods","content":"\u003cp\u003e\u003cspan\u003e\u003c/span\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.1 NZSP ceramic fabrication\u003c/strong\u003e: The NZSP powders employed in this investigation were synthesised through the solid-state reaction method described in [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e]. A pestle, mortar, and 5 \u0026micro;m sieve were used to crush and grind the ceramic pellets into powders. The NZSP powder was mixed in a pestle and mortar with 10 wt.% distilled H\u003csub\u003e2\u003c/sub\u003eO for approximately 4 minutes. The water evaporated during mixing, and the NZSP powder became hygroscopic. After that, the moisture powder was compressed in a 10 mm die platen for an hour at 120\u0026deg;C using a Specac uniaxial hot press managed by Atlas Series Platen Controller. The pressures ranged from 125 to 437 MPa. The pressing time was recorded once the maximum parameters (125 MPa and 120\u0026deg;C) were achieved. A heating rate of 1\u0026deg;C per minute was achieved by applying mechanical pressure. To guarantee reproducibility, two pellets were pressed and inspected at every temperature, pressure level, and time. The Archimedes\u0026apos; method was used to calculate NZSP ceramics density.\u003c/p\u003e\u003cspan\u003e\n \u003cp\u003e\u003cstrong\u003e2.2 Pellet Surface Preparation\u003c/strong\u003e: Conventional mechanical polishing with sandpaper was not feasible due to the inherent fragility of the cold-sintered NZSP pellets (prior to and after post-annealing), often leading to cracking or edge damage. The pellet surfaces were gently polished by lightly rubbing against fine-grade (1200 grit) SiC paper on a flat surface, using minimal pressure and no water.\u003c/p\u003e\n\u003c/span\u003e\u003cspan\u003e\n \u003cp\u003e\u003cstrong\u003e2.3 Microstructural and structural characterisation of NZSP ceramics\u003c/strong\u003e: The diffraction data of the cold-sintered NZSP solid electrolyte pellets were examined using a PANalytical Aeris X-ray diffractometer (XRD) with Cu-K\u0026alpha; radiation (\u0026lambda;\u0026thinsp;=\u0026thinsp;0.154 nm) over a 2\u0026theta; range of 10\u0026ndash;60\u0026deg; with a 0.02\u0026deg; step size. Topas 5 software was used to examine the phase refinement of the diffraction data. Using an FEI Inspect F50 scanning electron microscope (SEM), the microstructural analysis of the cold-sintered NZSP ceramic\u0026apos;s lightly polished surface was conducted.\u003c/p\u003e\n\u003c/span\u003e\u003cspan\u003e\n \u003cp\u003e\u003cstrong\u003e2.4 Impedance spectroscopy of NZSP ceramic\u003c/strong\u003e: Impedance measurements were conducted on the Au paste-coated cold-sintered NZSP pellets. Electrochemical impedance spectroscopy was conducted at 50\u0026deg;C intervals between room temperature (RT) and 300\u0026deg;C, using an Agilent 4294A. Stainless steel | NZSP | stainless steel cell configuration was used for the EIS testing setup. The measurements were conducted in a custom-built cell with spring-loaded electrodes to ensure consistent contact. An applied pressure of approximately 5 MPa was maintained during impedance measurements to improve interface stability and minimise contact resistance. The Agilent 4294A was calibrated to correct measurement errors using blank, open, and closed circuits. A geometric correction factor that considered the sample-electrode surface area and pellet thickness was applied following impedance (Z*) measurements. Data analysis and circuit fitting were done using Scribner Associates\u0026apos; ZVIEW Impedance Software version 2.4. The ionic conductivity was calculated from Eq. 1 below.\u003c/p\u003e\n\u003c/span\u003e\n\u003cp\u003e\u003c/p\u003e\n\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u0026nbsp;\u003cspan class=\"mathinline\"\u003e\\(\\:\\sigma\\:=\\frac{\\text{L}}{\\text{R}\\text{*}\\text{A}}\\)\u003c/span\u003e\u0026nbsp;\u003c/span\u003e\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;\u0026hellip;.1.\u003c/p\u003e\n\u003cp\u003eWhere,\u003c/p\u003e\n\u003cp\u003e\u0026sigma; \u0026ndash; is the ionic conductivity (S/cm)\u003c/p\u003e\n\u003cp\u003eL \u0026ndash; is the thickness of the pellets.\u003c/p\u003e\n\u003cp\u003eR \u0026ndash; is the bulk resistance obtained from the Nyquist plot (ꭥ)\u003c/p\u003e\n\u003cp\u003eA \u0026ndash; is the cross-sectional area of the pellet (cm\u0026sup2;).\u003c/p\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Relative density\u003c/h2\u003e\n \u003cp\u003eThe density of the NZSP ceramic was measured using Archimedes\u0026apos; method, and the results were contrasted with the theoretical density of NZSP (3.24 g/cm\u003csup\u003e3\u003c/sup\u003e) derived from XRD refinement investigations. The cold-sintered Na\u003csub\u003e3\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e12\u003c/sub\u003e ceramics were fragile; therefore, some NZSP ceramics were post-annealed at 400\u0026deg;C and others at 800\u0026deg;C prior to measurement. As shown in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, cold-sintered NZSP pellets annealed at 400\u0026deg;C achieved a relative density (\u0026rho;\u003csub\u003er\u003c/sub\u003e) of ~\u0026thinsp;80%, irrespective of the applied mechanical pressure, consistent with literature reports [\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]. The only exception was the sample pressed at 125 MPa, which exhibited a \u0026rho;\u003csub\u003er\u003c/sub\u003e of ~\u0026thinsp;75%. Since the mechanical pressure had minimal influence on the cold-sintered NZSP ceramics, three NZSP ceramics samples pressed between 312 and 437 MPa were post-annealed at 800\u0026deg;C, resulting in a \u0026rho;\u003csub\u003er\u003c/sub\u003e increase of over 40%, achieving \u0026rho;\u003csub\u003er\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;90%, consistent with the literature [\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]. The error margin of \u0026plusmn;\u0026thinsp;0.5% reflects the estimated experimental uncertainty associated with density measurements using Archimedes\u0026apos; method. This includes potential variations due to surface porosity, trapped air bubbles, and minor mass and volume measurement fluctuations during immersion. For each processing condition, two pellets were independently prepared and measured. The reported density values in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e represent the average of these measurements.\u003c/p\u003e\n \u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eCold-sintered NZSP relative density\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colspan=\"3\"\u003e\n \u003cp\u003eRelative density of cold-sintered Na\u003csub\u003e3\u003c/sub\u003eZr\u003csub\u003e2\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e12\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePressure (MPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026rho;\u003csub\u003er\u003c/sub\u003e (%)\u0026thinsp;+\u0026thinsp;400\u0026deg;C\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026rho;\u003csub\u003er\u003c/sub\u003e (%)\u0026thinsp;+\u0026thinsp;800\u0026deg;C\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e125\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e75.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e250\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e80.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e312\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e80.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e89.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e375\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e80.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e91.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e437\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e79.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e88.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 X-ray Diffraction\u003c/h2\u003e\n \u003cp\u003eNZSP ceramic solid electrolyte diffraction pattern confirms the formation of NaSICON material, with peaks matching monoclinic Na₃Zr₂Si₂PO₁₂ (PDF No: 00-035-0412 and space group \u003cem\u003eC12/c\u003c/em\u003e), as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea. XRD patterns of the cold-sintered NZSP at various mechanical pressures (125\u0026ndash;437 MPa) and post-annealed at 400 and 800\u0026deg;C are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e(b\u0026ndash;f). This revealed that the NZSP framework and \u003cem\u003eC12/c\u003c/em\u003e space group were preserved, regardless of the mechanical pressing and post-annealing. The only notable difference in the peak is the formation of \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e secondary peaks, which occur at 2\u0026theta;\u0026deg; \u0026asymp; 28.30 and 31.52, consistent with the literature [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e]. However, the secondary \u003cem\u003em\u003c/em\u003e-ZrO₂ peaks were more prominent in samples mechanically pressed at 250\u0026ndash;437 MPa, Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e(c\u0026ndash;f).\u003c/p\u003e\n \u003cp\u003eFull-pattern Topas 5 software was used to understand the crystallographic data and the NZSP compositions\u0026apos; phase formation, and the cold-sintered NZSP\u0026apos;s diffraction pattern was examined using the Rietveld refinement method. The two phases, \u003cem\u003eC12/c\u003c/em\u003e NZSP and \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e, were validated by the refinement results. Figure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea revealed the refinement pattern of NZSP conventionally sintered, which was used as a reference [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e] and Figs. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e(b-f) show the refined pattern of the cold-sintered NZSP. Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e shows the lattice parameters, theoretical density, unit cell volume, goodness of fits (GoF), and phase composition percentage. All the cold-sintered NZSP ceramics have approximately 96% monoclinic (\u003cem\u003eC12/c\u003c/em\u003e) NZSP symmetry and 4% \u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e impurity phase. The increase in ZrO₂ content with higher mechanical pressure may be attributed to localized decomposition or partial destabilization of the NZSP phase due to increased stress at particle contacts during pressing. Such conditions can potentially promote the segregation or formation of secondary phases like ZrO₂, especially without sufficient thermal energy to fully densify the NZSP structure. Also, we examined this trend in samples without post-annealing. The PXRD data suggest that a similar, though less pronounced, increase in ZrO₂ content can be observed with increasing pressure, even in the as-pressed samples.\u003c/p\u003e\n \u003ctable id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eRefined parameters, the goodness of fit (GoF), phase fractions and theoretical density of NZSP ceramic\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNZSP\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003ePhase fraction (%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"3\"\u003e\n \u003cp\u003eLattice parameters (\u0026Aring;)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u0026beta; (\u0026deg;)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eGoF\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eUnit cell volume\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTheoretical density\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eC12/c\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003em\u003c/em\u003e-ZrO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ea\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eb\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ec\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e125\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e97.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15.642\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.048\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.226\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e123.71\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1086.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.246\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e250\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e96.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15.642\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.049\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.221\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e123.69\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1085.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.247\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e312\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e96.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15.645\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.051\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.223\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e123.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.29\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1086.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.245\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e375\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e96.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15.646\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.051\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.223\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e123.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1086.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.245\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e437\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e96.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e15.642\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.051\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.225\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e123.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1086.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.245\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 Scanning electron microscopy\u003c/h2\u003e\n \u003cp\u003eFigures \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(a) and (b) display the micrographs of the cold-sintered Na₃Zr₂Si₂PO₁₂ solid electrolytes after they were post-annealed at 400 and 800\u0026deg;C, respectively. Since the mechanical pressing susceptibility had minimal impact on the NZSP ceramic. The micrographs reveal a porous, loosely compacted ceramic with minimal particle agglomeration, which agrees with a ceramic with a \u0026rho;\u003csub\u003er\u003c/sub\u003e of ~\u0026thinsp;80%. In contrast, the micrographs of the ceramic mechanically pressed at 375 MPa and post-annealed at 800\u0026deg;C, Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb, show a more compacted structure and well-densified grains, although inter-granular porosity remains. This micrograph supports a ceramic with a \u0026rho;\u003csub\u003er\u003c/sub\u003e of \u0026asymp;\u0026thinsp;90%, consistent with NZSP SEM reported in the literature [\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eAccording to Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, grain size analysis using the line-intercept method reveals that the mean grain size was ~\u0026thinsp;1.24 \u0026micro;m following post-annealing at 400\u0026deg;C and subsequently increased to ~\u0026thinsp;1.38 \u0026micro;m following post-annealing at 800\u0026deg;C, showing a 10% grain growth. Compared to NZSP (~\u0026thinsp;2.0 \u0026micro;m) conventionally sintered, the mean grain sizes of the cold-sintered NZSP ceramic are smaller [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e\n \u003ctable id=\"Tab4\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eCold-sintered NZSP ceramic grain sizes after post-annealing at 400 and 800\u0026deg;C using the line intercept method.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003ePost-annealed at 400\u0026deg;C\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\" colspan=\"3\"\u003e\n \u003cp\u003ePost annealed at 800\u0026deg;C.\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS/n\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLength\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGrain intercept\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGrain size\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLength\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGrain intercept\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGrain size\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10.58\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.96\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8.81\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.26\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7.22\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.39\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11.57\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.18\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.96\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.57\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.39\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e9.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11.37\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.62\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.68\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e12.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.51\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10.63\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.33\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAverage grain size\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.24\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.38\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4 Impedance spectroscopy\u003c/h2\u003e\n \u003cp\u003eCold-sintered Na₃Zr₂Si₂PO₁₂ solid electrolytes, densified at varying mechanical pressures (125\u0026ndash;437 MPa) and post-annealed at 400 and 800\u0026deg;C, are depicted in Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e as complex impedance (Z*) graphs. The impedance spectra indicate ionic conduction at 25\u0026deg;C, which shows a low-frequency spike and an arc with a non-zero intercept. The total resistivity was determined by intercepting the spike with the Z\u0026apos; axis. The (Z*) plots of NZSP ceramic solid electrolytes that were mechanically pressed at 125 and 250 MPa and then post-annealed at 400\u0026deg;C are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. Within a margin of error, the total ionic conductivities at 25\u0026deg;C were 2.73 \u0026times; 10⁻⁵ S/cm and 2.45 \u0026times; 10⁻⁵ S/cm, respectively, showing that pressure had minimal effect on the cold-sintered ceramic conductivity. Similarly, Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e compares NZSP ceramics mechanically pressed at 312, 375, and 437 MPa and post-annealed at 800\u0026deg;C. At 25\u0026deg;C, the total ionic conductivities were 1.25 \u0026times; 10⁻⁴, 1.43 \u0026times; 10⁻⁴, and 1.10 \u0026times; 10⁻⁴ S/cm, respectively, consistent with the literature [\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]. The higher conductivity at 375 MPa compared to 437 MPa correlates with the higher density observed at 375 MPa. Overall, post-annealing at 800\u0026deg;C had a ten-fold increment in conductivity compared to 400\u0026deg;C. These results were consistent with literature reports on cold-sintered NZSP solid electrolytes, though the conductivity values for cold-sintered NZSP ceramics remain lower compared to NZSP ceramics sintered conventionally [\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e displays the Arrhenius plots of the total conductivity (\u0026sigma;\u003csub\u003et\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1/Z\u003csub\u003et\u003c/sub\u003e) for cold-sintered NZSP solid electrolytes post-annealed at 400\u0026deg;C and 800\u0026deg;C and densified at different mechanical pressures. The associated activation energies (E\u003csub\u003ea\u003c/sub\u003e) and uncertainties were derived from data shown in Table \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e, and the slope of the temperature-dependent data was calculated using the least-squares method. After being mechanically pressed at 125 and 250 MPa and post-annealed at 400\u0026deg;C, the E\u003csub\u003ea\u003c/sub\u003e for cold-sintered NZSP solid electrolytes was ~\u0026thinsp;0.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.11 eV. Conversely, samples that were post-annealed at 800\u0026deg;C and mechanically pressed between 312\u0026ndash;437 MPa showed lower E\u003csub\u003ea\u003c/sub\u003e values of ~\u0026thinsp;0.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05 eV. However, these E\u003csub\u003ea\u003c/sub\u003e values are consistent with the literature and NZSP conventionally sintered [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e\n \u003ctable id=\"Tab5\" border=\"1\" class=\"fr-table-selection-hover\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eTemperature-dependent conductivity table of cold-sintered NZSP and the pressing pressure\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003ePost annealed 400\u0026deg;C/ 12hr\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\" colspan=\"3\"\u003e\n \u003cp\u003ePost annealed 800\u0026deg;C/12hr\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e125MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e250MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e312MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e375MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e437MPa\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTemperature (\u0026deg;C)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1000/T (K)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLog \u0026sigma;\u003csub\u003et\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLog \u0026sigma;\u003csub\u003et\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTemperature (\u0026deg;C)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1000/T (K)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLog \u0026sigma;\u003csub\u003et\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLog \u0026sigma;\u003csub\u003et\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLog \u0026sigma;\u003csub\u003et\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.83\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-3.27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-3.27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-3.72\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-3.65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-3.78\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e120\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.54\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.57\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.83\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-3.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-3.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-3.12\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e160\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.31\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e120\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.54\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.57\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.54\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.45\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e200\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-1.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-1.78\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e160\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.31\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2.18\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e200\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-1.88\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-1.74\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-1.97\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eWe investigated the effects of mechanical pressing and post-annealing on NZSP ceramic solid electrolyte cold-sintered by XRD, Archimedes, SEM, and impedance spectroscopy and compared the results to conventionally sintered NZSP and cold-sintered NZSP in the literature. XRD confirmed that the phase assemblage remained unchanged, though \u003cem\u003em\u003c/em\u003e-ZrO₂ secondary peaks were more prominent. SEM revealed the influence of post-annealing on morphology with persistent intergranular porosity. Ionic conductivities at 25\u0026deg;C showed minimal pressure impact at 250 MPa, with 2.45 \u0026times; 10⁻⁵ S/cm and E\u003csub\u003ea\u003c/sub\u003e \u0026asymp; 0.41 eV. Post-annealing at 800\u0026deg;C improved conductivity, achieving 1.43 \u0026times; 10⁻⁴ S/cm and E\u003csub\u003ea\u003c/sub\u003e \u0026asymp; 0.34 eV. In its present form, the cold-sintered NZSP process is energy-intensive and not viable without improvements.\u003c/p\u003e "},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAdemola Adetona, Ph.D.Conceptualization: The author conceived the primary ideas and research question.Methodology: The author developed the research design, collected and analysed the data.Investigation: The author conducted experiments and gathered relevant data.Writing - Original Draft: The author wrote the initial draft of the manuscript.Writing - Review \u0026amp; Editing: The author contributed to revising and editing.Visualisation: The author created the figures and visual elements used in the manuscript.Validation: The author verified the accuracy and integrity of the research findings.Funding Acquisition: The author secured financial support for the project.Project Administration: The author managed the project and ensured its smooth execution.Olufunke Nejo, Ph.D.Writing - Original Draft: The author contributed to the initial draft of the manuscript.Writing - Review \u0026amp; Editing: The author provided critical feedback and contributed to improving the manuscript.Validation: The author verified the accuracy and integrity of the research findings.Mr Moses YiboweiWriting - Original Draft: The author contributed to the initial draft of the manuscript.Writing - Review \u0026amp; Editing: The author provided critical feedback and contributed to improving the manuscript.Validation: The author verified the accuracy and integrity of the research findings.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors acknowledge the financial support of the Tertiary Education Trust Fund of Nigeria (TETFUND), the Department of Chemistry at the University of Lagos, Nigeria, the Functional Materials and Devices Group in the Department of Materials Science and Engineering at The University of Sheffield, United Kingdom, and Ge Wang (Dame Kathleen Ollerenshaw Fellow) from the Materials Department of the University of Manchester.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e \u003cp\u003eUpon a reasonable request, the data for this article is available.\u003c/p\u003e\u003ch2\u003eDeclaration and Conflict of Interest\u003c/h2\u003e\n\u003cp\u003eThe authors declare no known financial conflicts that could have affected the work presented in this article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eL. 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Luo, Cold sintering and ionic conductivities of Na\u003csub\u003e3.256\u003c/sub\u003eMg\u003csub\u003e0.128\u003c/sub\u003eZr\u003csub\u003e1.872\u003c/sub\u003eSi\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e12\u003c/sub\u003e solid electrolytes, Journal of Power Sources. 391 (2018) 170\u0026ndash;179. https://doi.org/10.1016/j.jpowsour.2018.04.067.\u003c/li\u003e\n\u003cli\u003eJ. G. Pereira da Silva, M. Bram, A.M. Laptev, J. Gonzalez-Julian, Q. Ma, F. Tietz, O. Guillon, Sintering of a sodium-based NASICON electrolyte: A comparative study between cold, field-assisted and conventional sintering methods, Journal of the European Ceramic Society. 39 (2019) 2697\u0026ndash;2702. https://doi.org/10.1016/j.jeurceramsoc.2019.03.023.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Cold sintering, Na3Zr2Si2PO12, solid electrolyte, X-ray diffraction, Electrochemical impedance spectroscopy","lastPublishedDoi":"10.21203/rs.3.rs-6479472/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6479472/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWe investigated the effect of mechanical pressing and post-annealing on aqueous cold-sintered Na₃Zr₂Si₂PO₁₂ (NZSP) solid electrolyte, comparing the results with conventionally sintered NZSP. Various physical, microscopic and spectroscopic techniques, including Archimedes, Impedance spectroscopy, XRD and SEM, were used to investigate the properties of the cold-sintered NZSP solid electrolyte. X-ray diffraction confirmed the retention of the NaSICON phase in cold-sintered NZSP, with the secondary \u003cem\u003em\u003c/em\u003e-ZrO₂ peaks becoming prominent. Scanning electron microscopy revealed that post-annealing improved the ceramic's morphology, showing grain formation and inter-granular porosity remained. With an ionic conductivity of 2.45 × 10\u003csup\u003e⁻\u003c/sup\u003e⁵ S/cm at 25 °C, NZSP ceramics post-annealed at 400 °C showed no susceptibility to mechanical pressing. Conductivity increased tenfold after post-annealing at 800 °C, reaching 1.43 × 10\u003csup\u003e-\u003c/sup\u003e⁴ S/cm.\u003c/p\u003e","manuscriptTitle":"Cold sintering of Na3Zr2Si2PO12 solid electrolyte: Effect of mechanical pressing and post-annealing","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-20 05:33:08","doi":"10.21203/rs.3.rs-6479472/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"44dc516a-a24c-4745-ba1a-bb9c3a87350f","owner":[],"postedDate":"May 20th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-05-20T05:33:08+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-20 05:33:08","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6479472","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6479472","identity":"rs-6479472","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}