Effect of sintering and annealing on electrochemical and mechanical characteristics of Na₃Zr₂Si₂PO₁₂ solid electrolyte

Abstract

NASICON (Sodium superionic conductor) type Na3Zr2Si2PO12 (NZSP) has received a lot of interest as the solid electrolyte for all‐solid‐state sodium‐ion batteries (ASSIBs). The electrolyte has superior interfacial characteristics, high thermal stability, and good ionic conductivity. Because of their higher energy density, improved mechanical stability, no liquid leakage problem, and higher operating voltages, All solid‐state batteries are expected to replace liquid electrolyte‐based batteries in many applications. The solid electrolyte also acts as a separator, and hence additional separator is not required for cell operations. Because of its 3D open architecture and continuous diffusion channels, NZSP is considered a better solid electrolyte. The NZSP solid electrolyte has been synthesized by spark plasma sintering (SPS) followed by annealing the sintered materials. The SPS method leads the material to have higher density and ionic conductivity. Conventional sintering of the materials requires a temperature as high as 1225°C; however, the temperature required for the SPS is as low as 1050°C. Moreover, conventional sintering yields samples of relative density up to 91%, while SPSed samples have achieved a maximum density of around 98%. The ionic conductivity of solid electrolyte SPSed at 1050°C for 10 min is found to be 3.5 × 10−4 S/cm with an activation energy of 0.27 eV. The annealing of the SPSed samples improves the ionic conductivity of the SPS1050‐20mins sample to roughly double the value obtained from the as‐prepared SPS sample because there are fewer secondary phases and a structural change from a rhombohedral to a monoclinic system. To ascertain the samples' crystal structure, particle shape, and ionic conductivity, materials were characterized using X‐ray diffraction, scanning electron microscopy, and electrochemical impedance spectroscopy. The samples' mechanical characteristics, for example, the hardness and fracture toughness of the samples, were also determined.

Correlation between mechanical properties and ionic conductivity of sodium superionic conductors: a relative density-dominant relationship

“…Recent studies have increasingly focused on the mechanical properties of sintered polycrystalline NZSP, with specific attention to hardness [10,[24][25][26][27][28]44], elastic modulus [24,25,27,28], bending strength [10], and fracture toughness [24,28]. Notably, the latest measurements for hardness and elastic modulus exceed the ranges previously summarized by Wolfenstine et al [47] in 2022, which were 4.4-4.9 GPa for hardness and 56-97 GPa for elastic modulus.…”

“…The conventional preparation method, i.e., high-temperature and pressure-less sintering, is straightforward and scalable. However, they often result in NZSP electrolytes characterized by a high concentration of defects, such as pores, yielding relatively low relative densities around 90% [28]. High-energy ball-milling enhances the densification of NZSP pellets during sintering, but this approach concurrently introduces disordering and partial amorphization to the particle surface [51].…”

“…Techniques such as sol-gel which facilitates uniform precursor mixing at the molecular level, and spark plasma sintering (SPS), https://doi.org/10.26434/chemrxiv-2024-d0j6v-v3 ORCID: https://orcid.org/0000-0003-0904-802X Content not peer-reviewed by ChemRxiv. License: CC BY-NC 4.0 known for its rapid densification capabilities through micro-discharges between particles, have successfully produced polycrystalline NZSP with notably high relative densities of approximately 98% and above [28,[52][53][54][55]. Moreover, employing a polydisperse particle size distribution in the sintering process can improve the compactness or relative density of sintered polycrystalline NZSP [51].…”

“…Since the 1980s, various studies have reported significant progress in this area through the optimization of processing conditions (e.g., modifying sintering parameters, incorporating sintering aids, employing novel sintering techniques) and composition (e.g., adjusting the x value and ionic doping). Recent advancements have greatly improved the RT ionic conductivity of NZSP to 1.0 mS/cm [10,[24][25][26][27][28]. In 1990, a direct analogue of NZSP, Li1.3Al0.3Ti2(PO4)3 (LATP), was discovered [29].…”

“…Research on the mechanical properties of NASICON electrolytes has emerged as a critical area for advancing energy storage technologies [10,[24][25][26][27][28][44][45][46]. Wolfenstine et al [47] provided a comprehensive summary of the primary mechanical properties of sintered polycrystalline NZSP, which include hardness (4.4-4.9 GPa), elastic modulus (56-97 GPa), fracture toughness (1-1.5 MPa•m 0.5 ), and fracture strength .…”