Progressive Assessment on the Decomposition Reaction of Na

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Progressive Assessment on the Decomposition Reaction of Na Superionic Conducting (NASICON) Ceramics Jae-Il Jung, Daekyeom Kim, Hyojin Kim, Yong Nam Jo, Jung Sik Park, and Youngsik Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09316 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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ACS Applied Materials & Interfaces

Progressive Assessment on the Decomposition Reaction of Na Superionic Conducting (NASICON) Ceramics

Jae-Il Jung†‡, Daekyeom Kim†‡, Hyojin Kim†, Yong Nam Jo§, Jung Sik Park⊥ and Youngsik Kim*† †

School of Energy and Chemical Engineering, Ulsan National Institute of Science & Technology (UNIST), Ulsan 689–798, Korea § Advanced Batteries Research Center, Korea Electronics Technology Institute (KETI), Gyeonggi-do 463−816, Korea ⊥Analysis

Support TEAM, JEOL Korea Ltd., Seoul 134−814, Korea

Abstract The successful analysis on the microstructure of Hong-type Na superionic conducting (NASICON) ceramics revealed that it consists of several heterogeneous phases: NASICON grains with rectangular shapes, monoclinic round ZrO2 particles, grain boundaries, a SiO2rich vitrified phase, Na-rich amorphous particles, and pores. A dramatic microstructural evolution of NASICON ceramics was demonstrated via an in-situ analysis, which showed that NASICON grains sequentially lost their original morphology and were transformed into comminuted particles (as indicated by the immersion of bulk NASICON samples into seawater at a temperature of 80 °C). The consecutive X-ray diffraction analysis represented

that the significant shear stress inside NASICON ceramics caused their structural decomposition, during which H3O+ ions occupied ceramic Na+ sites (predominantly along the (111) and (133) planes), while the original Na+ cations

came out in the (020) plane of the

NASICON ceramic crystalline structure. The results of time-of-flight secondary-ion mass spectrometry analysis confirmed that large concentrations of Cl- and Na+ ions were

*

Corresponding author : Youngsik Kim E-mail: [email protected] 1

ACS Paragon Plus Environment


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distributed across the surface of NASICON ceramics, leading to local densification of a 20 µm thick surface layer after treatment within seawater solution at a temperature of 80 °C. Keywords: Seawater battery, Solid electrolyte, Hong-type NASICON ceramics, Microstructure corrosion, Structural decomposition/degradation

1.

Introduction NASICON or sodium (Na) super (S) ionic (I) conductor (CON) compounds have been

considered strong candidates for solid electrolyte membranes, which can be used in Na/S secondary batteries1, 2 and various gas sensors (including CO2 ones). 3-6 Owing to the threedimensional pathway of ionic migration, NASICON demonstrates a relatively high ionic conductivity of around 10-3 S/cm, which is equivalent to that of β-Al2O3 at room temperature (25 °C), with the corresponding activation energies of 0.2 eV and 0.15 eV, respectively. 7-11 In general, NASICON is categorized greatly into Hong type (Na1+xZr2SixP3-xO12) and von Alpen type (Na1+xZr2-x/3SixP3-xO12-x/3) NASICONs, which have different composition ratios. Hong type NASICON has the advantage of being manufactured at a low temperature of 1250 °C (in contrast to β-Al2O3 and von Alpen type NASICON which have to be sintered at ≥ 1600, ≥ 1300 °C, respectively, in order to reach a sufficient degree of densification).12, 13 Many efforts have focused on enhancing the properties of NASICON ceramics and thus converting them into superior superionic conductors by modifying the utilized synthesis methods via additional heat treatment, compositional ratio variations, and dopant usage. 12-17 The Hong type NASICON ceramic has complex crystalline structure with a rigid three2


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dimensional network of (PO4)3- or (SiO4)4- tetrahedron sharing corners with (ZrO6)8octahedron. Each (ZrO6)8- octahedron is connected to four (SiO4)4- and tow (PO4)3-, while each tetrahedron is connected to four (ZrO6)8- octahedron. Na+ cation occupies the interstitial site of network structure. In addition, Hong type NASICON varies between the rhombohedral (≥ 200 °C) and monoclinic (≤ 200 °C) ones within the compositional range of 1.8 ≤ x ≤ 2.2 in Na1+xZr2SixP3-xO12. 1, 2, 8, 16-20 The successful synthesis of homogenous and intact NASICON species is a significant technical challenge because Na and P elements tend to evaporate from the ceramic surface during sintering at ≥ 1200 °C, resulting in the precipitation of monoclinic ZrO2 particles. 13-17, 21, 22

Moreover, Fuents et al. reported the implausibility of producing a uniform crystalline

structure of NASICON ceramics due to thermodynamic instability; hence, it could only exist as a metastable state between the crystalline and amorphous phases, making the detailed analysis of the NASICON ceramic microstructure very difficult. 16, 19, 23-25 The limited analytical approach to the NASICON microstructure impedes its progressive reinforcement against structural decomposition under corrosive solution conditions. 14, 26 A series of intensive studies on the corrosion mechanism of NASICON ceramics have recently been conducted. Auborn et al. suggested that the replacement of Na+ ions with H3O+ plays a critical role in the NASICON structural degradation in acidic solutions. 12-17, 23, 27 In particular, when Hong type NASICONs were utilized as solid-state electrolytes for new aqueous battery system operated at a current density of 0.025 mA⋅cm-2, they demonstrated superior durability as compared to that of β–Al2O3, corresponding to a high efficiency of up to 91% after 20 cycles (a significant increase in impedance was observed for β–Al2O3 only after 10 cycles) 3



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(Figure S1).28 The system lasted for a long period of exceeding 600 h of operation time at room temperature (25 °C) without breakout (Figure S2). Thus, in order to develop a NASICON-type solid electrolyte with enhanced properties, the stability of NASICON ceramics in natural and modified seawater solution must be investigated. Therefore, a systematic investigation of the NASICON microstructure would allow a realistic understanding of its corrosion process (which occurs due to the chemical reaction of H3O+ or Cl- ions with Na+ NASICON species) and the related microstructural evolution observed during seawater treatment.14, 22, 29 In this work, we successfully executed a detailed transmission electron microscopy (TEM) analysis on the typical Hong-type NASICON ceramic structure with the composition of Na1+xZr2SixP3-xO12 (x = 2) and then observed the unprecedented phenomena during the dynamic corrosion trend of NASICON ceramics under different seawater conditions (corresponding to different kinds of bases). During immersion testing in seawater at 80 °C, a noticeable amount of NaCl(s) precipitate was formed, indicating severe permeation of Clanions and their possible reaction with Na+ cations inside the NASICON ceramic lattice while H3O+ cations occupy empty Na+ sites. As a result, the following planes of stress loading were obtained for NASICON ceramics: expansion along the (111) and (133) planes for the hydronium NASICON phase (H3O+− NASICON) and contraction in the (020) plane for the main NASICON phase (Na+− NASICON), which play a significant role in the degradation of NASICON ceramics due to the large amount of loaded shear stress.

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2.

Experimental Section

Synthesis Hong-type NASICON ceramic powder with the composition of Na3Zr2Si2PO12 was prepared by solid-state mixing. The starting materials consisted of ZrO2 (Daejung, 99%, 5 µm), SiO2 (Junsei, 99%, 0.5−10 µm), and Na3PO412H2O (Daejung, 99%). In order to obtain the stoichiometry of Na3Zr2Si2PO11, 0.2 mol of ZrO2, 0.2 mol of SiO2 and 0.1 mol of Na3PO412H2O were mixed for 30 min using a pestle and a mortar, and the resulting mixture was dried at 120 °C for 12 h in a drying oven. After that, it was calcined in air in two steps: first at 400 °C for 5 h and then at 1100 °C for 12 h followed by grinding. The calcined mixture was ball-milled in ethanol at a rotation speed of 250 rpm for 30 min inside a planetary mixer and then subsequently dried at 80 °C for 3 h in the drying oven, resulting in Hong-type NASICON ceramics powder (the detailed synthesis procedure for this material has been reported elsewhere).28 1.25 g of the synthesized powder was weighed and pressed into disk shapes with diameters of 20 mm and thicknesses of 1.5 mm inside steel mold at 2 tons of uniaxial pressure. The obtained samples were sintered inside an electrical furnace at 1280 °C for 10 h in air at a heating rate of 2 °C/min (in order to prevent the reaction between the samples and an Al2O3 plate, the former were placed on the top of a 0.01 inch thick Pt sheet). Finally, the sintered samples were sequentially polished with 120, 320, 600, 1000, and 2000 grit SiC abrasives to produce 0.7 mm thick pieces and then finalized in the aqueous solution containing Al2O3 particles with diameters of 0.5 µm. The relative density of each sample was measured by utilizing Permeameter-Porosimeter (AccuPyc Ι Ι 1340).

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Powder X-ray diffraction measurements Crystal structures of the synthesized materials were analyzed by X-ray diffraction (XRD; D/Max−2200, Rigaku) using Cu-Ka radiation in the scan range of 10°–40° at a step size of 0.02° and counting time of 5 s. Lattice parameters were determined using the least-squares method.

Structural analysis The material morphology and structure were analyzed using a scanning electron microscopy (SEM) apparatus (Helios, FEI) (operated at an applied voltage of 10 kV) and a TEM instrument (JEM−ARM200F, JEOL) operated at a voltage of 80 kV. Cross-sectional TEM particle samples were prepared by a focused ion beam (FIB) technique (Verios 460, FEI). Before the FIB cutting procedure, the entire powder surface was preliminarily coated with sputtered C species to avoid surface contamination and prevent it from the damage caused by a Ga ion beam (utilized in the FIB process).

Cell fabrication and test An electrochemical cell was fabricated using a positive electrode, a negative electrode, and a hybrid electrolyte (Figure S1). The positive and negative electrode corresponded to seawater and Na metal (Sigma−Aldrich, USA), respectively.30 while the hybrid multilayer electrolyte contained 1.0 M solution of NaCF3SO4 (Sigma−Aldrich, USA; organic liquid electrolyte) dissolved within the solvent of triethylene glycol dimethyl ether (TEGDME) (Sigma−Aldrich, USA) in and Hong-type NASICON ceramic (Na3Zr2Si2PO12; solid electrolyte). C felt (Fuel Cell Store Inc.) attached to the NASICON ceramic surface was used 6


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as current collector. Each cell part was fabricated inside an Ar-filled glove box; the resulting cell assembly was subsequently connected to a testing station during seawater exposure. A WBCS3000 battery testing apparatus was used to characterize charge and discharge cycles for the produced cell. Charge and discharge cycles test was performed at 0.025 mA⋅cm-2 current density for 10h, respectively.

Seawater immersion testing After polishing the surfaces of the obtained NASICON ceramic pellets with SiC abrasives, corrosion tests were conducted by immersing NASICON samples into different seawater solutions at a temperature of 80 °C for periods ranging from 1 d to 15 d followed by treatment with a leather cloth soaked in the aqueous suspension of 0.5 µm Al2O3 particles. The modified seawater solutions were prepared by adding 10 vol.% of the following reagents into the natural seawater (seawater) with a pH of 8.17: HCl (l) (pH = 0.13), CH3COOH (l) (pH = 1.92), and NaOH (l) (pH = 9.69). Time-of-flight secondary ion mass spectrometry studies Depth profiling was performed for each constituent element of the NASICON ceramic surface by using a time-of-flight secondary ion mass spectrometry (TOF−SIMS 5, ION TOF) instrument equipped with a pulsed Bi cluster primary ion source. The primary ion gun was operated at an energy of 25 keV and target current of 1.1 pA (the analysis area of 50 × 50 µm2 was utilized in all experiments). O2+ and Cs+ ion guns were operated at an energy of 2 keV and a target current of 620 nA was used on the sputter area of 200 × 200 µm2 to obtain positive and negative depth profiles, respectively. 7



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3.

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Results and Discussion The Hong-type NASICON ceramic discs prepared according to the procedure described

in the previous reports28, 30 were characterized by the relative density of 94.2% and ionic conductivity of 7.5 × 10-4 S/cm. The microstructure of the sintered Hong-type NASICON ceramics was examined by the SEM and TEM methods.28-30 Figures 1a and b display the overall microstructures of the Hong-type NASICON ceramics, which were analyzed by studying the fractured surfaces depicted in the SEM and cross-sectional TEM images. The SEM image shown in Figure 1a indicates that the morphology of the main Hong-type NASICON ceramic grains contains faceted cubes and rectangles. It is composed of multiple heterogeneous phases represented by NASICON grains, grain boundaries, monoclinic ZrO2 particles (ZrO2,m), a SiO2-rich vitrified phase, Na-rich amorphous particles, and pores. According to the TEM image presented in Figure 1b, the grain sizes of NASICON ceramics and ZrO2,m are equal to 3.8 ± 2.4 µm and 0.3 ± 0.2 µm, respectively (the ZrO2,m grains are randomly distributed among the NASICON grains). The previous results of the XRD analysis confirm the existence of the ZrO2,m crystalline phase in the sintered NASCION ceramics.13, 15, 17, 21

The TEM area measurements reveal that the sintered NASICON ceramics consist of 84.7vol.% of NASICON grains, 7.7 vol.% of ZrO2,m grains, 6.1vol.% of SiO2, and 1.5vol.% of pores (the siliceous phase and pores are located in the cornered and faceted areas between the interlocked NASICON grains). The TEM image of the inner part of a NASICON grain (depicted in Figure 1c) demonstrates that the crystalline micro-domain phases with sizes ranging from a few nm to 10 nm are randomly distributed among the main amorphous phase, which is consistent with the previous results stating that NASICON ceramics are 8


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characterized by the thermodynamically metastable equilibrium between the crystalline and amorphous phases.18, 25 The round-shaped ZrO2,m grains contain around 1 at.% of P and exhibit stacking faults, as shown in Figure 1d and Figure S3. According to the fast Fourier transform analysis of the image depicted in Figure 1d, it contains the (100)- and {100}-type twin crystals of ZrO2,m, embedded inside the NASICON ceramics.31, 32 Other precipitated species (corresponding to the Na-rich amorphous particles with diameters of 50 nm) are distributed randomly between the NASICON ceramic grains (see Figure 1e and Figure S4). In general, in order to eliminate such residual elements or dissolve each component inside the NASICON ceramic structure, either the heating temperature or heating time must be increased. As a result, Na and P elements easily evaporate, while the ZrO2,m particle grains precipitate, leading to undesirable compositions of NASICON ceramics (Figure S5). On the other hand, a higher fraction of Si would result in more homogeneous SiO2-rich von Alpen NASICON ceramics.24, 33 The successful removal of the Na-rich amorphous particles from the NASICON microstructure is another critical issue, which must be resolved while developing advanced Hong-type NASICON ceramics. The image of the grain boundary phase depicted in Figure 1f shows that the average width of a grain boundary is around 3.4 nm, which is 10 times bigger than the thickness of the ideal bilayer grain boundary (0.35 nm). 34, 35

The energy dispersive spectroscopy (EDS) analysis of the areas near the grain

boundaries reveal that Na is likely to be segregated as a main element (see Figure S6; the detailed understanding of the grain boundary structure requires additional studies). However, a proper analysis of the microstructure of Hong-type NASICON ceramics cannot be accomplished without differentiated analysis skills and operational conditions (in addition to the top-notch level of TEM functionality), and the obtained results are comparable with the 9



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previously reported data.13, 15, 17, 21, 24, 28-30, 33 Thus, the robust understanding of the NASICON ceramic microstructure is important for the systematic investigation of the corrosion mechanism of NASICON ceramics. When a Hong-type NASICON ceramic disc is used as a solid electrolyte during seawater battery cell testing (Figure S1), its grain particles become comminuted after repeated cycles of operation, leading to structural disintegration (Figure S2). While both the electrical potential and chemical processes simultaneously affect the structural durability of NASICON ceramics in seawater, a detailed investigation of the chemical reaction between seawater and NASICON ceramics must be conducted; in particular, the microstructural evolution of the entire NASICON ceramic discs has to be examined since they were immersed in various seawater solutions with different temperatures and pH values. When the experiment was performed at 25 °C, no noticeable structural changes were detected (Figure S7). Thus, additional experiments had to be conducted at 80 °C in order to increase the reaction rate between NASICON ceramics and aqueous solutions (Table S2).36 The durability of Hong-type NASICON ceramics was evaluated by immersing the NASICON ceramic disks in seawater at 80 °C for different periods to simulate the realistic experimental conditions (instead of treating powder ceramics with water or diluted acid solutions).26, 27, 37 Figures 2a, b, and c describe the sequential morphology changes observed for the identical surface areas of NASICON ceramics after 0, 5, and 10 d of treatment, respectively. After 5 d, the ceramic grains begin to lose their original morphology and become distorted; after 10 d, the NASICON phase is ultimately decomposed into small granular particles with sizes below 0.3 µm. The observed gradual microstructural degradation 10


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matches very well the systematic changes of the XRD peak patterns depicted in Figure 2d, which shows that H3O+−NASICON species (detected at 2θ of 30.46° and corresponding to the (133) plane denoted as ★) begin to form after 2 d of seawater treatment (Table S1). In order to achieve a better understanding of the XRD analysis results, the NASICON ceramic disc samples were ultrasonicated with ethanol for 30 min after immersion testing in order to remove any surface by-products. The obtained data suggest that the formation of H3O+−NASICON plays a critical role in the degradation of Hong-type NASICON ceramics. Identical experiments were conducted using the different seawater solutions previously described in the Experimental section (Figure 3). The H3O+−NASICON phase (I) was the most distinct one, and its Io(020) peak corresponding to the monoclinic NASICON structure shifted considerably after 2 d and 5 d of treatment with HCl- and CH3COOH-modified seawater solutions, respectively. The peak height ratios (I/Io) measured after 5 d (the magnitudes of I obtained for peaks ★ at 2θ = 30.46° (133) were equal to 33.1, 18.9, 17.40, and 17.28 for the HCl-modified, CH3COOH-modified, seawater, and NaOH-modified seawater solutions, respectively) are summarized in Table S1. They reveal that the severity of the NASICON ceramic structural degradation decreases in the order of HCl (l)-modified > CH3OOH (l)- modified >> seawater > NaOH (l)-modified seawater solution. The obtained results are consistent with the previous studies, indicating that NASICON ceramics deteriorate severely at low pH since H3O+ ions with high concentrations tend to occupy Na+ sites; on the other hand, the ions originating from the NaOH dissociation (either Na+ or OHones) at low pH would rather inhibit the structural degradation at low pH.23, 27

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The observed structural degradation trend can also be explained by taking into account the time-dependent peak location shifts ∆θ (mostly at low pH), which are summarized in Table S1. As shown in detail in Figure S8 and Figures 3a and b, the H3O+−NASICON peaks at 19.71° (111) and 31.04° (133) shift to the left by ∆θ = −0.36° and ∆θ = −0.58° after 4 d of treatment, respectively, in both HCl- and CH3OOH-modified seawater solutions and level off afterwards (indicating the structural expansion due to the substitutional replacement of Na+ with H3O+ within 5 d of treatment with HCl- and CH3OOH-modified seawater). The main NASICON peaks at 20.21° (020) shift incrementally to the right by ∆θ = +0.69° and +0.70° after 15 d of treatment with the HCl- and CH3OOH-modified seawater solutions. The obtained results suggest that severe shear stress is developed inside NASICON ceramics after the immersion into the seawater solutions with low pH, which corresponds to different stresses, tension in the (111) and (133) planes, and incremental compressive stress in the (020) plane. This phenomenon differs from the overall chemical volume expansion, which occurs when H3O+ ions with a large radius of 0.15 nm are inserted into the NASICON crystal structure.26, 38-40 Another noticeable issue is the formation of NaCl(s) precipitates during seawater treatment, which is shown in Figures. 3a and c. According to the XRD analysis (Figure S9), when NASICON ceramic is immersed into deionized (DI) water (pH = 7), the H3O+−NASICON phase is more intensively formed with a time delay greater than that observed for seawater (pH = 8.17). While it is consistent with the previous argument that lower pH values accelerate corrosion, the microstructure analysis demonstrates little structural degradation of NASICON ceramics in DI water as compared with the seawater treatment (Figure S9), suggesting that different corrosion mechanisms of NASICON 12


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ceramics exist in different solutions. In particular, the corrosion of NASICON ceramics in seawater must be investigated from different perspectives, including the reaction of Clanions with Na+ species of NASICON ceramics (which leads to the formation of NaCl(s) and the reentrance of Na+ ions from seawater into available Na+ NASICON sites). In order to conduct a more detailed investigation, Cl-, Na+, and H3O+ element distributions were analyzed and profiled for each sample via TOF−SIMS after various treatment periods (see Figure 4). As compared with other ions (Figure S10), Cl- ions are found to be the most prevalent ones on the surfaces with depths below 1 µm (followed by Na+ and H3O+). These Cl- ions (either diffused or infiltrated inside NASICON ceramics) can easily react with Na+ ions producing NaCl(s) precipitates. On the other hand, the crosssectional microstructure was analyzed by daily monitoring the microstructural evolution of Hong-type NASICON ceramics immersed in seawater at 80 °C (see Figure 5). It was prepared by cutting the immersed sample in half from the top to the bottom (here the top side was open to the seawater, while the bottom one was in contact with the container). Instead of the significant structural degradation, gradual densification was locally observed on the surface of the samples, the porosity of which decreased from 11.2% in the beginning of the process to 6.5% after 5 d of treatment. This suggests that the precipitated NaCl(s) particles are located in the middle of the NASICON structures, leading to the accumulation of compressive stress on the sample surface accompanied by local densification. These simultaneous phenomena (the severe precipitation of NaCl(s) particles and significant structural degradation) point to the possibility that vacant Na+ sites (produced during the reaction of Na+ ions with Cl- ions described by equations (1) and (2) below) can 13



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provide available sites for H3O+ species to occupy. Thus, high concentrations of Cl- indirectly contribute to the structural decomposition of NASICON ceramics. Na+ + Cl- + H3O+ + OH-

Na+−NASICON

Na+ + Cl- + H3O+ + OH-

Cl-

(1)

Na+−NASICON

(2)

On the other hand, it is hard to expect that H3O+ ions will directly occupy all the available Na+ sites (which amount to about one third of all Na+ sites in the NASICON ceramic structure)19 − not only because of the large H3O+ ionic radius of 0.15 nm, but also due to the pre-existent charge balance around the network of Na+ sites. Hence, the formation of H3O+−NASICON most likely results from the substitutional replacement of Na+ ions in Na+−NASICON with H3O+ species from solution9,

26, 27, 41

(the plausible reactions of

NASICON ceramics are summarized in Figure 6). While the observed correlation between the precipitation of NaCl(s) crystals and the ceramic structural degradation requires a more detailed investigation, the severe structural degradation of NASICON ceramics due to the occupation of Na+ sites with H3O+ species after seawater treatment was confirmed in this work (see Figure S10).

4.

Conclusions In this study, typical Hong-type NASICON ceramic structures with the composition

Na1+xZr2SixP3-xO12 (x=2) were investigated via functional TEM analysis. No apparent selective corrosion was observed around fragile areas such as vitrified zones and structural defects, while the formation of the H3O+−NASICON phase was accompanied by structural 14


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degradation. When the immersion experiment was conducted under various seawater conditions, both the obtained XRD peak height ratios (I/Io) and peak shift change values (∆θ) demonstrated that structural degradation decreases in the order of HCl (l)-modified > CH3OOH (l)- modified >> seawater > NaOH (l)-modified seawater solution. During the structural degradation process, different stresses were formed inside the NASICON ceramics, corresponding to constant tension in the (111) and (133) planes and incremental compressive stress in the (020) plane. Local densification (as well as the precipitation of NaCl(s)) was observed for the surface of NASICON ceramics, as indicated by the obtained SEM and TOF−SIMS results. It can be assumed by the reaction of Cl- ions from solution with Na+ species from Na+−NASICON followed by the occupation of vacant Na+ sites with H3O+ ions, which leads to shrinkage and expansion as well as the severe precipitation of NaCl(s) particles inside the NASICON ceramic structure. However, further in-depth studies are required to explain the observed reactivity of Cl- ions with NASICON ceramics and its correlation with possible corrosion mechanisms.

Figure 1. (a) An SEM image of the fractured NASICON ceramic surface. (b) A TEM image of the entire ceramic sample. High-resolution TEM (HRTEM) images of the (c) internal part of a NASICON grain, (d) internal part of a ZrO2,m grain, (e) Na-rich amorphous area and (f) grain boundary area for Hong-type NASICON ceramics. Figure 2. SEM images of the identical Hong-type NASICON ceramic surfaces immersed in seawater at 80 °C for (a) 0 d, (b) 5 d, and (c) 10 d. (d) The XRD patterns corresponding to the images depicted in panels a−c.

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Figure 3. XRD patterns recorded for the Hong-type NASICON ceramic samples, which were immersed in the (a) HCl-modified, (b) CH3COOH-modified (c) intrinsic, and (d) NaOHmodified seawater solutions for 0, 2, 5, 10, and 15 d at 80 °C. Figure 4. TOF−SIMS depth profiles for (a) Cl-, (b) Na+, and (c) H3O+ ions, which were obtained for the Hong-type NASICON ceramic samples immersed in seawater at 80 °C for 0, 5, and 15 d. (d) TOF−SIMS depth profiles obtained for Cl-, Na+, and H3O+ ions after 15 d of seawater treatment. Figure 5. SEM images of the polished surfaces of the Hong-type NASICON ceramic samples immersed in seawater at 80 °C for (a) 0 d, (b) 2 d, (c) 5 d, and (d) 15 d. Figure 6. A schematic diagram summarizing the reactions between NASICON ceramics and seawater solution. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acsami.xxxxxxx. A schematic diagram of the seawater battery cell, SEM, HRTEM, EDS, XRD, TOF-SIMS study Corresponding Author *E-mail: [email protected]. Author Contributions ‡These authors contributed equally to this article. Acknowledgements This work was supported by the research grant (No. 1.160004.01) provided by the Ulsan National Institute of Science and Technology in 2016.

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