Abstract:Owning to its natural advantages of safety and element abundance, all-solid-state sodium-ion batteries have great potential for large-scale energy storage applications. Compared with traditional organic electrolyte sodium-ion batteries, solid-state sodium-ion batteries have the advantages of non-flammability and high thermal stability, and solid electrolytes exhibit good mechanical strength, which is conducive to the direct stacking of unit components, and is more in line with the safety and structural requirements of large-scale energy storage equipment. The core component of the all-solid-state battery is the solid electrolyte. The NASICON-type Na3Zr2Si2PO12 (NZSP) electrolyte has been widely studied due to its high ionic conductivity, thermal stability, and excellent mechanical properties. Air plasma spraying (APS) has been widely used in the field of ceramic coating deposition as a cost-effective, large-scale coating preparation process. Our previous investigations showed that for ceramic materials with melting points below about 1500 ℃ , dense coatings with fully bonded splats can be prepared by APS at room temperature. In this study, both single splats and NZSP coatings were deposited by APS using Ar-H2 plasma, and the influence of powder particle size distribution and spray parameters on the elemental preferential evaporation during spraying and microstructure of NZSP coatings were systematically investigated. By depositing individual isolated splats, the influence of plasma spray parameters on the NZSP particle melting degree and the preferential evaporation loss of Na and P elements were examined. The splats were deposited on a preheated substrate and thus the splats deposited from fully molten particles present a regular disk shape while splats from semi-molten droplets present a shape with splashed arms. The morphology of splats was examined by both scaning electron microscopy (SEM) and three dimensional conforcal laser microscopy. Accordingly, the fraction of molten droplets can be estimated based on test of the fraction of disk-shaped splats. The element contents of regular disk splats were estimated by energy dispersive spectroscopy (EDS). The microstructure of the NZSP deposits was examined by SEM and the phase structure of the deposits was characterized by X-ray diffraction (XRD). The results showed that the plasma arc power and hydrogen gas flow significantly affect the evaporation loss of Na and P, and there is an obvious particle size effect on the preferential evaporation loss of elements. With powders of particle size less than about 35 μm, the Na and P evaporization losses are significantly increased with the decrease of the particle size with respect to Zr. Thus, with a small NZSP particle of 20μm, Na and P present about 30% and 60% evaporation loss, respectively, with respect to Zr. When particle size is increased over to 35 μm, the Na and P preferential evaporation losses can be significantly suppressed. The results revealed plasma spraying deposition results in evolution of ZNSP in rhombic phase structure. By controlling the particle size range, the loss of Na elements during plasma spraying could be controlled below 10%, and the loss of P elements could be controlled below 20%. By using NZSP powder with a particle size range of 30~50 μm and controlling the APS parameters, a dense NZSP electrolyte with excellent inter-splat bonding and no obvious layered structure could be achieved, demonstrating the broad application prospects of APS for the preparation of solid electrolytes for all-solid-state sodium-ion batteries.