Characterization of hot-pressed von Alpen type NASICON ceramic electrolytes
Introduction
Li-ion conducting solid state electrolytes (SSE) have garnered much interest as a possible solution to limits faced by liquid electrolytes traditionally used in Li ion batteries. Much of this interest for Li based SSE stems from the improved safety solid electrolytes offer compared to non-aqueous liquid electrolytes as well as their ability to be used in higher energy density cells when paired with Li metal anodes [[1], [2], [3]]. While there have been a number of Li based SSE systems which have shown promise in enabling the use of Li metal anodes, serious concerns remain around the quantity of available Li and conflict associated with accessing remaining sources of available Li [[4], [5], [6], [7]].
Na based SSEs offer the same form of improvements to safety and energy density that Li based SSEs do compared to Li ion batteries, while having the added benefit of allowing for pairing with Na metal anodes [8]. Na metal has a higher propensity to creep than Li, which can result in higher charge rate capabilities when compared to Li [[9], [10], [11]]. While Li-based battery technology has significantly advanced in recent decades, the cost and availability of Li could be potential issues for application such as grid storage [12]. Sodium has a higher abundance than Li, is more affordable, and presents a possibility for fewer conflicts in sourcing [13].
Alongside the need for higher performance batteries for electric vehicles comes a need for improved grid energy storage. Flow cells are one format of electrochemical cells which show promise as grid energy storage systems, however a need remains for effective, low-cost single-ion conducting membranes to prevent cross contamination [14]. SSEs also show promise as membranes for flow cells, as there is much overlap between the engineering requirements for SSEs and flow cell membranes. One of the most promising Na based SSEs have been the Na super-ion conductor (NASICON) materials of the form NaxM2(XO4)3. NASICON materials, particularly variations of Na3Zr2Si2PO12 (NSZP), have long been investigated as solid electrolytes due to their high ionic conductivity [[15], [16], [17]]. These NASICON materials have been used commercially in molten sodium batteries, such as sodium sulfur and ZEBRA cells operating near 300 °C [18]. Hong originally described NASICON formulations based on Na1+xZr2SixP3-xO12 consisting of a solid-solution series between NaZr2P3O12 and Na4Zr2Si3O12 in 1976 [15]. Due to the presence of zirconia formation upon sintering, von Alpen suggested a zirconia-deficient form of NASICON with the general formula Na1+xZr2−x/3SixP3−xO12−2x/3 with end members of NaZr2P3O12 and Na4ZrSi3O10 [16]. Kuriakose et al. and Ahmad et al. analyzed the effects of sintering both Hong and von Alpen type NASICON samples, observing that von Alpen samples typically did not form the secondary phase zirconia upon sintering, while Hong samples did contain zirconia [19,20].
NASICON materials also have demonstrated effectiveness at operating as membranes in aqueous flow cells [21]. However, the ability to densify and manufacture thin films of NASICON remains a challenge. The processing methods used to create NASICON have been shown to have drastic impacts on the microstructure, mechanical, and electrochemical properties of NASICON [[22], [23], [24]]. While much work has been done to investigate the effects of various NASICON synthesis methods [19,[23], [24], [25], [26], [27]] and various sintering conditions [20,24,[28], [29], [30]], comparatively few studies have been performed on the effects of hot pressing on NASICON NZSP samples. Perthuis observed that hot pressing Hong-type NASICON at 1250C resulted in highly dense (98% theoretical density) and conductive (1mS/cm at room temperature) NASICON samples, however the effects of hot pressing on phase purity, microstructural evolution, and mechanical properties were not described [23]. Yde-Andersen hot pressed varied NASICON compositions prepared from gel-derived raw materials and obtained high densities (~95–99% theoretical density) and varied conductivities (~0.30–2.94 mS/cm at room temperature), however the effects on mechanical properties and phase purity, including the presence of a glassy phase, were only minimally described and effects on microstructural evolution were not described [31]. Moreover, the presence of the glassy impurity phase has been believed to affect the NASICON bulk phase composition and therefore the bulk properties [20,25]. In addition, the glassy phase has been observed to be present at grain boundaries, which likely affects ionic transport, stability, and perhaps mechanical properties. Comparatively few studies have also investigated mechanical properties of NASICON samples. Nonemacher investigated the elastic modulus (72–88 GPa), hardness (5.6–7.6 GPa), and fracture toughness (1.30–1.58 MPa m0.5) of Al and Y substituted NASICON solid electrolytes [32]. Fracture strengths ranging from 55 to 212 MPa have been found using three point bend, four point bend, and diametral strength tests [24,28,31,33].
In this study, we hypothesize that a combination of optimized hot-pressing conditions and powder processing could enable a processing route to produce a dense NASICON with a reduced glass content compared to conventional sintering. Furthermore, there is very limited information on the mechanical properties of VA NASICON. Hot-pressing has previously been shown to produce ceramic samples enabling reliable testing of mechanical properties such as elastic modulus, hardness, and fracture toughness [[34], [35], [36], [37]]. Thus, we hypothesize that hot-pressing can be used to prepare dense NASICON samples that allow for increased understanding of the mechanical properties of VA NASICON. Powder x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), impulse excitation, Vickers hardness, indentation fracture toughness, and electrochemical impedance spectroscopy (EIS) were used to investigate the effects of hot-pressing von Alpen type NASICON powders. Von-Alpen type NASICON was selected due to its lower propensity to form zirconia as compared to Hong type NASICON, which leads to higher conductivity and fracture strength [38]. Von Alpen NASICON powder synthesis was varied to alter particle morphologies to see how this effects the microstructure and properties of the hot-pressed samples.
Section snippets
Materials synthesis and processing
Von Alpen (VA) type NASICON (Na3.1Zr1.55Si2.3P0.7O11) powders were prepared through the following process. Reagent grade Na3PO4·12H2O, Na2CO3, ZrO2 and SiO2 were used as starting materials. The starting materials were mixed into ethanol. The resulting mixture was ball milled (Pulverisette 5, Fritsch) at 400 rpm for 0.5 h. After ball milling it mixture was dried at 80 °C in an oven for 24 h. After drying, the resulting powder was calcined at 1100 °C for 10 h. The calcined powder was ball milled
Materials characterization
SEM images of NASICON powders were taken for the three different powders used prior to hot pressing. Fig. 1 shows the particle morphology for NS, S, and S900C powders. A significant difference in particle morphology can be observed between NS and S powders prior to hot pressing. The NS particles consisted of irregular and larger particles while the S powders had a more consistent spherical morphology, in line with [39]. After undergoing heat treatment, S900C samples did not undergo any
Conclusions
A RIHP technique was used to synthesize dense von Alpen NASICON-type materials using three different precursor powders. Changes in particle morphology resulted in differences in phase purity, microstructure, mechanical properties, and electrochemical properties. The formation of a secondary, glassy phase impurity along the grain boundaries was more prevalent in NS precursor powders with larger, more irregular particles. The heightened presence of the glassy phase served to decrease the elastic
Author statement
Jeff Sakamoto and Joey Valle: Conceptualization the study. Joey Valle conducted materials densification, characterization, and EIS measurements. Jeff Wolfenstine interpreted data and provided experience and expertise in writing the manuscript. Wooseok Go synthesized the NaSICON powder. Youngsik Kim supervised Wooseok Go's synthetic work. Claire Huang conducted the mechanical property measurement. Dhruv Tatke assisted in SEM characterization.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.
Acknowledgements
This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.This work was supported by the 2021 Research Fund (1.210041.01) of UNIST(Ulsan National Institute of Science & Technology)
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