Synergetic Enhancement on Bulk and Grain Boundary Ionic Conduction of Mg Doped High-Entropy NASICON-Type Solid Electrolyte for Solid-State Na+ Batteries by Spray Flame Synthesis

TL;DR

This work targets improving Na+ conduction in NASICON-type NZSP solid electrolytes for all-solid-state Na batteries by using Mg doping and scalable gas-phase synthesis. A swirling spray flame system produces Mg-doped NZSP nanoparticles with nano-scale high-entropy mixing, enabling reactive sintering to form dense NASICON pellets at reduced temperatures, and a two-step sintering route further enhances densification. The Mg0.25NZSP composition achieves a room-temperature conductivity of $1.91$ mS/cm with an activation energy of $E_a$=$0.200$ eV, driven by simultaneous improvements in bulk and grain boundary transport due to the formation of a Na3-2yMg_yPO4 secondary phase that improves intergranular contact. The approach is scalable (>$1$ kg/h) and cost-efficient, offering a practical pathway for high-performance, solid-state Na batteries and suggesting potential for multi-element high-entropy doped NASICON systems in the future.

Abstract

All-solid-state sodium batteries represent a promising next-generation energy storage technology, owing to cost-effectiveness and enhanced safety. Among solid electrolytes for solid-state sodium batteries, NASICON-structured Na3Zr2Si2PO12 has emerged as a predominant candidate. However, its widespread implementation remains limited by suboptimal ionic conductivity in both bulk and grain boundary regions. In this study, we demonstrate a novel approach utilizing swirling spray flame synthesis to produce Mg-doped NASICON solid electrolyte nanoparticles. This method facilitates efficient doping and homogeneous mixing for scalable production, resulting in core-shell non-NASICON structures with nano-scale high-entropy mixing. Notably, the atomic migration distances achieved by flame synthesis are significantly reduced compared to conventional solid-state reactions, thereby enabling reactive sintering to preserve high sinterability of nanoparticles during post-treatment processes. High-temperature sintering yields dense NASICON-structured solid electrolytes. Among those, Mg0.25NZSP exhibits an optimal ionic conductivity of 1.91 mS/cm at room temperature and an activation energy of 0.200 eV. The enhancement mechanism can be attributed to incorporation into the NASICON phase and formation of a secondary phase. The low-melting-point secondary phase significantly improves grain boundary contact to enhance grain boundary conductivity. The process achieves simultaneous enhancement of both bulk and grain boundary conduction through a single-step procedure. Comparative analysis of sintering temperatures and ionic conductivities among NASICON solid electrolytes synthesized via different methods demonstrates flame-synthesized nanoparticles offer superior performance and reduced post-treatment costs, owing to their exceptional nano-scale sinterability and uniform elemental distribution.

Figures (4)

• Figure 1: Synthesis and characterization of Mg-doped NZSP nanoparticles. a) Schematic illustration of the swirling spray flame synthesis system for Mg-doped NZSP nanoparticle production. b) TEM images and corresponding SAED patterns of as-synthesized Mg$_{0.00}$NZSP, Mg$_{0.25}$NZSP, and Mg$_{0.50}$NZSP nanoparticles. c) XRD patterns of as-synthesized Mg$_x$NZSP nanoparticles ($x$=0-0.5). d) TEM micrograph and corresponding EDS elemental mapping of as-synthesized Mg$_{0.25}$NZSP nanoparticles.
• Figure 2: Phase formation mechanisms and microstructural characterization of sintered Mg-doped NZSP electrolytes. a) Schematic illustration depicting elemental mixing scales and corresponding NASICON phase formation mechanisms in Mg$_{x}$NZSP particles. b) XRD patterns of two-step sintered (1200/1100$\celsius$ for 12h) Mg$_x$NZSP electrolyte pellets ($x$=0-0.5). c) SEM cross-sectional micrographs of Mg$_{0.25}$NZSP electrolyte pellets processed under various sintering conditions. d) SEM micrograph and corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping of Mg$_{0.25}$NZSP electrolyte sintered at 1200/1100$\celsius$ for 12h.
• Figure 3: Electrochemical characterization of Mg-doped NZSP electrolytes. Normalized Nyquist plots at 25$\celsius$ for a) Mg$_{x}$NZSP, $x$=0-0.3, and b) Mg$_{0.5}$NZSP electrolyte symmetrical cells processed by two-step sintering (1200/1100$\celsius$-12h). c) Normalized impedance spectra at 25$\celsius$ for Mg$_{0.25}$NZSP electrolytes prepared using various sintering protocols. d) Temperature-dependent normalized impedance spectra (25-95$\celsius$) for Mg$_{0.25}$NZSP electrolyte sintered at 1200/1100$\celsius$-12h. (e) Arrhenius plots comparing ionic conductivity of Mg$_{0.25}$NZSP electrolytes processed under different sintering conditions with undoped Mg$_{0}$NZSP (1200/1100$\celsius$-12h). Inset: Comparative analysis of bulk, grain boundary, and total activation energies for Mg$_{0.25}$NZSP samples across different sintering protocols.
• Figure 4: Conductivity analysis and mechanistic insights of Mg-doped NASICON electrolytes. (a) Room temperature (25$\celsius$) bulk ($\sigma_{\rm{b}}$), grain boundary ($\sigma_{\rm{gb}}$), and total ($\sigma$) ionic conductivities as functions of Mg content for samples sintered at 1150$\celsius$-12h. b) Comparative analysis of bulk, grain boundary, and total conductivities for Mg$_{0.25}$NZSP processed under various sintering conditions. c) Schematic illustration of microstructural evolution and Na$^+$ transport enhancement mechanism in Mg-doped NASICON electrolytes, highlighting secondary phase formation at grain boundaries. d) Performance comparison of Na$_3$Zr$_2$Si$_2$PO$_{12}$-based NASICON electrolytes: sintering temperature versus electrical conductivity for pristine and doped compositions synthesized via different processing routes.