## 2 Materials and Methods
### Instrumentation and Characterization Methods  
Ball-milling was performed on planetary ball-mills (PM 200, \(<\)10 mm feed size, \(<\)1 \(\upmu\)m (normal grinding based on feed material),and \(<\)0.1 \(\upmu\)m (colloidal) grind size, Retsch, Haan, Germany), using zirconia holders of corresponding volumetric capacities and zirconia balls of 10-, 5-, and 3-mm diameters; ball charging was selected for maximum energy output based on sample mass and applied technique (wet- vs. dry-grinding). Particle size distributions for NASICON powders, before and after processing, were measured using a Nano DS Dual Light Scattering Particle Size Analyzer (CILAS, Orleans, France), operated in static light scattering (SLS) mode to comply with the estimated micronic particle size of the NASICON phases, and in dynamic (DLS) mode for the detection of submicronic particles. Surface area and microporosity analyses were performed on an Autosorb iQ analyzer (Quantachrome, Boynton Beach, FL, USA). Samples were degassed prior to analysis at 115 \({}^{\circ}\)C for 4 to 6 h; nitrogen adsorption-desorption isotherms were obtained at the temperature of 77 K. Specific surface areas were calculated according to the Brunauer-Emmett-Teller (BET) equation using the linearity criterion over the range of 0.1 to 0.3(\(P/P_{0}\)); the Barrett-Joyner-Halenda (BJH) equation was used to characterize pore size distributions based on the desorption isotherms. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted on a STA 449 F3 Jupiter thermal analyzer (NETZSCH, Germany) equipped with a water-cooled graphite furnace. Scans were performed over the temperature range of 20 \({}^{\circ}\)C to 800 \({}^{\circ}\)C in an inert argon atmosphere using an open 85-\(\upmu\)L (137.72 mg) alumina crucible, at the scanning step of 5 K min\({}^{-1}\) to emulate the heating rate during the thermal processing of NASICON. Prior to DSC measurements, the equipment was calibrated at the same conditions by loading only the empty alumina crucible. Sample masses were 43.13 mg of untreated NASICON powder and 42.40 mg of the calculated material, the latter produced by pulverizing an in-lab manufactured pellet. Optical microscopies were collected using a portable electronic microscope at \(\times\)100 magnification; pellets were manually broken along their diameters so that cross-sectional profiles could be imaged; micrography images were processed using the ImageJ software package (FIJI version) for scaling, without applying any form of image manipulation. Crystal lattice models were portrayed using the Jmol software for molecular modeling. X-ray diffraction (XRD) measurements were conducted on a MiniFlex600 X-ray diffractometer (Rigaku, Japan) equipped with a CuK\({}_{a}\) X-ray source (\(\lambda\) of 1.541838 A). Scans were performed over the 10\({}^{\circ}\) to 60\({}^{\circ}\) range at high resolution (0.01\({}^{\circ}\) step, 1.0\({}^{\circ}\) min\({}^{-1}\) scanning speed) using a monochromator to suppress background fluorescence. NASICON samples were scanned in powder form as received, or were pulverized in a mortar when in pellet form; persistent zirconia aggregates were retained to consistently portray the corresponding phase of the composite material. Crystallographic lattice parameters were indexed according to COD cards 1529809 and 1530657 for NASICON (PDF 01-084-1190 and 00-035-0412) as well as COD 5000038 for zirconia (PDF 01-070-2491) [13; 57; 58]. Broadband dielectric and conductivity measurements were performed on an Alpha-A Modular Measurement System (NOVOCONTROL, Montabaur, Germany) for dielectric, conductivity, and electrochemical impedance spectroscopy, equipped with a Quattro Cryosystem temperature controller.