Na3PS4

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Oxide-based Composite Electrolytes Using Na3Zr2Si2PO12/Na3PS4 Interfacial Ion Transfer Kousuke Noi, Yuka Nagata, Takashi Hakari, Kenji Suzuki, So Yubuchi, Yusuke Ito, Atsushi Sakuda, Akitoshi Hayashi, and Masahiro Tatsumisago ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02427 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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ACS Applied Materials & Interfaces

Oxide-based Composite Electrolytes Using Na3Zr2Si2PO12/Na3PS4 Interfacial Ion Transfer Kousuke Noi, Yuka Nagata, Takashi Hakari, Kenji Suzuki, So Yubuchi, Yusuke Ito, Atsushi Sakuda, Akitoshi Hayashi*, and Masahiro Tatsumisago Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuencho, Naka-ku, Sakai, Osaka 599-8531, Japan

KEYWORDS. Sodium ion conductivity; Interfacial ion transfer; NASICON; Sulfide; Composite; Solid-state battery

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT. All-solid-state sodium batteries using Na3Zr2Si2PO12 (NASICON) solid electrolytes are promising candidates for safe and low-cost advanced rechargeable battery systems. Although NASICON electrolytes have intrinsically high sodium ion conductivities, their high sintering temperatures interfere with the immediate development of high-performance batteries. In this work, sinteringfree NASICON-based composites with Na3PS4 (NPS) glass-ceramics were prepared to combine the high grain-bulk conductivity of NASICON and the interfacial formation ability of NPS. Before the composite preparation, the NASICON/NPS interfacial resistance was investigated by modeling the interface between the NASICON sintered ceramic and the NPS glass thin film. The interfacial ion transfer resistance was very small above room temperature; the area specific resistances at 25°C and 100°C were 15.8 Ω cm2 and 0.40 Ω cm2, respectively. Based on this smooth ion transfer, NASICON-rich (70–90 wt%) NASICON–NPS composite powders were prepared by ball-milling fine powders of each component. The composite powders were well densified by pressing at room temperature. Scanning electron microscopy observation showed highly dispersed sub-micrometer NASICON grains in a dense NPS matrix to form closed interfaces between the oxide and sulfide solid electrolytes. The composite green (unfired) compacts with 70 and 80 wt% NASICON exhibited high total conductivities at 100°C of 1.1 × 10−3 S cm−1 and 6.8 × 10−4 S cm−1, respectively. An all-solid-state Na15Sn4/TiS2 cell was constructed using the 70-wt% NASICON composite electrolyte by the uniaxial pressing of the powder materials and its discharge properties were evaluated at 100°C. The cell showed the reversible capacities of about 120 mAh g−1 under the current density of 640 µA cm−2. The prepared oxide-based composite electrolytes were thus successfully applied in all-solid-state sodium rechargeable batteries without sintering. 1. INTRODUCTION


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Sodium batteries utilizing abundant and low-cost sodium resources have attracted significant attention as potential large-scale rechargeable battery systems for use in electric vehicles, backup energy storage, and solar and wind farms.1–4 Cathode and anode materials for such batteries have been studied extensively to develop advanced batteries with high energy and power densities and long cycle lifetimes.5–8 Most such studies have focused on cell configurations similar to those of commercial lithium-ion batteries using organic liquid electrolytes. However, batteries using flammable organic solvents as electrolytes have intrinsic safety issues, which become more severe as the cell capacity increases for large-scale energy storage. To improve the safety and reliability of batteries, all-solid-state sodium batteries using inorganic solid electrolytes instead of liquid electrolytes have been studied as next-generation systems.9–14 Solid electrolytes with high sodium ion conductivities are critical materials for developing all-solid-state sodium batteries. Several oxide and sulfide materials have been proposed as candidates for inorganic solid electrolytes.15,16 The NASICON (Na Super Ionic Conductor) family of ceramics is a well-known group of sodium-ion-conducting oxides with the general formula Na1+xZr2SixP3-xO12 (0 ≤ x ≤ 3).17,18 Compositional studies showed that Na1+xZr2SixP3-xO12 (1.8 ≤ x ≤ 2.2) ceramic electrolytes had especially high conductivities, exceeding 10−3 S cm−1 at room temperature.19–22 Moreover, the manufacturing process for NASICON ceramics is economically favorable, with synthesis performed in air from abundant elemental sources. However, the application of NASICON in allsolid-state batteries is hindered by difficulties in densification. At present, NASICON ceramics must be densified by high-temperature sintering to induce high sodium ion conductivities.17–22 Conventional NASICON sintering processes require heating at ~1200°C. The high-temperature sintering of NASICON for reducing grain-boundary resistance entails several major difficulties. Firstly, high-temperature firing often promotes the volatilization of


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sodium and phosphorus, causing NASICON decomposition.23–25 Next, severe and undesirable side reactions with the electrode active materials can occur at high temperatures during the co-sintering fabrication of batteries to ensure the formation of close solid–solid interfaces.10 Therefore, NASICON sintering temperatures must be decreased for applications in all-solid-state batteries. In several studies, the NASICON sintering temperature has been successfully lowered to less than 1000°C using the pressure- and/or electric field-assisted sintering techniques of hot-pressing10 and spark plasma sintering.11 Noguchi et al.10 prepared an all-solid-state sodium-ion cell by hotpressing at 700°C using Na3V2(PO4)3 as both the positive and negative electrode materials. Although the obtained solid-state symmetrical cell Na3V2(PO4)3/NASICON/Na3V2(PO4)3 worked as a secondary battery at room temperature, the cell operated with the low current density of 10 µA cm−2. Very recently, we demonstrated the effectiveness of liquid-phase sintering with Na3BO3 to lower the NASICON sintering temperature.26,27 A NASICON ceramic was sintered with 9 wt% Na3BO3 at 700°C and showed the room-temperature conductivity of 1 × 10−4 S cm−1. Moreover, this liquid-phase sintering at 700°C was free of undesirable reactions with a NaCrO2 positive electrode material. However, the technique still required high-temperature heating; thus, the development of high-performance batteries requires the optimization of co-sintering conditions with sophisticated ceramics engineering. Unlike oxide electrolytes, sulfide electrolytes show good interface formation abilities for the construction of all-solid-state batteries; they densify well under external pressure at room temperature (by cold-pressing).28–30 We previously reported on a sodium ion-conducting Na3PS4 (NPS) glass-ceramic with a cubic Na3PS4 phase.9,31,32 Green (unfired) compacts of NPS glassceramic powder showed conductivities exceeding 10−4 S cm−1 at 25°C. Close interfaces with large contact areas between NPS particles were formed within these green compacts.9,32 All-solid-state


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sodium batteries using the NPS electrolytes were fabricated by cold-pressing powders of the NPS and several electrode materials.9,31,33,34 The cells operated successfully as rechargeable batteries at room temperature. Therefore, the interface formation ability of the NPS is a great advantage in application to solid-state batteries because NPS-based cells can be constructed without sintering. However, it is necessary to manufacture and use the NPS electrolytes under a strict control of an inert gas atmosphere because they react with moist air to generate hydrogen sulfide gas. If the NPS usage ratio in batteries decrease, it will contribute to reducing the amount of the inert gas used in the whole processes of battery manufacture and decreasing the maximum amount of hydrogen sulfide produced in case of unintended atmospheric exposure. Herein, we propose a sintering-free approach for using NASICON electrolytes, in which NASICON–NPS composites are created with NASICON and NPS as dominant and secondary components, respectively. These NASICON-rich composite electrolytes are attractive from the standpoint of safety and productivity improvement in synthesizing materials and fabricating batteries. The expected ion-transport pathways of our proposed composite electrolytes are illustrated in Figure S1. Our approach exploits the synergy of the high grain-bulk conductivity of NASICON and the interface formation ability of NPS. NPS behaves as an ion-conducting adhesive material among the NASICON grains. The prepared oxide-based composite powders can be densified simply by cold-pressing, as with the pure NPS material. In the composite green compacts, solid–solid interfaces form between the oxide electrolyte NASICON and the sulfide electrolyte NPS. If sodium ion transfer occurs smoothly across these electrolyte interfaces, the NASICON–NPS composites will be good high-conductivity solid electrolytes. We quantify the interfacial ion transfer resistances between NASICON and NPS, as these have not yet been measured. The accurate quantification of the interfacial resistances permits the informed design of


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the composite component ratios and sizes and provides an understanding of the properties of the composite green compacts. This report describes the preparation and characterization of novel NASICON–NPS composite electrolytes with NASICON-rich compositions (70–90 wt%). Before composite preparation, the NASICON/NPS interfacial resistance is quantified for a model of a NASICON sintered body coated with a glassy NPS thin film35 prepared by pulsed laser deposition (PLD). Then, using the obtained information regarding interfaces suitable for sodium ion transfer, oxide-based NASICON–NPS composite powders with many oxide–sulfide interfaces are prepared. NASICON fine grains are mixed with a small amount of the NPS glass-ceramic by ball milling. These powders are pelletized by cold-pressing and the obtained composite green compacts are characterized. The ion transport pathways in the NASICON–NPS compacts are discussed based on measured results of various ion transport properties and the composite microstructures. We suggest that NASICON– NPS ion transfer plays an important role for the ion transport of the composites. Moreover, the obtained composite electrolytes showing high conductivities of ~10−3 S cm−1 at 100°C are applied to all-solid-state cells and their discharge-charge properties are evaluated.

2. EXPERIMENTAL SECTION 2.1 Evaluation of interfacial resistance The interfacial resistances between NPS and the NASICON ceramic Na3Zr2Si2PO12 were evaluated using alternating-current (AC) impedance measurements for a symmetrical cell of gold/NPS/NASICON/NPS/gold containing a disk-shaped NASICON sintered body. Before the cell preparation, the NASICON sintered ceramic was polished using an oil-based diamond slurry


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(average abrasive grain size: 1 µm) and rinsed with dehydrated hexane. After rinsing, the ceramic specimen was moved quickly into an argon-filled glove box. All subsequent operations were performed without air exposure. NPS glass thin films were formed on the polished NASICON ceramic by PLD, conducted based on a previous report.35 An apparatus with a KrF excimer laser (λ = 248 nm, Lambda Physik, LPXPro) was used. The pulsed laser energy was 200 mJ/pulse and the repeating frequency was 10 Hz. Pellets of the Na3PS4 powders, obtained by mechanochemical synthesis,31 were used as targets. The NASICON sintered body was placed 7 cm from the target. NPS thin films were grown on both surfaces of the NASICON ceramic at room temperature under the Ar gas pressure of 5 Pa. The structural components of the formed films were confirmed by Raman spectroscopy using a spectrometer (Horiba, LabRam HR-800). The main structural unit was PS43−; some P2S64− was also detected, with a peak profile almost the same as that reported for an NPS film on a Si substrate.35,36 The desired NPS film was thus formed on the surfaces of the NASICON sintered ceramic. Then, gold thin films as ion-blocking electrodes were formed on the NPS

films

by

direct-current

(DC)

sputtering

to

obtain

the

symmetrical

gold/NPS/NASICON/NPS/gold cell. AC impedance measurements were performed using an impedance analyzer (Solartron, SI1260). The impedances were measured in the frequency range of 1 Hz to 2 MHz at temperatures of 0 to 52°C. After the measurements of the symmetrical cell, the impedances of NASICON itself were evaluated in the same manner by sputtering the sintered body directly with gold electrodes (Au/NASICON/Au). The morphologies and thicknesses of the deposited thin films were observed by scanning electron microscopy (SEM, Hitachi, SU8220).

2.2 Preparation of composite powders and green compacts


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NASICON–NPS composite powders and green compacts thereof were prepared using the raw materials of NASICON and NPS glass-ceramic powders. The fine NASICON powder was obtained by the ball-mill pulverization of the NASICON ceramic used for the interfacial resistance evaluation. The NPS glass-ceramic powder was obtained by mechanochemical reaction followed by heat treatment.31,32 The raw materials were mixed in specific weight ratios where the NASICON nominal weight fractions were 70, 80, and 90 wt%. The mixtures were placed in a 45-mL ZrO2 vessel with 4-mm-diameter ZrO2 balls. The weights of the total raw materials and the balls in each batch were 1 g and 60 g, respectively. Ball milling was performed using a planetary ball-mill apparatus (Fritsch, Pulverisette 7) at the base disk rotation speed of 230 rpm for 1 h. After milling, the product powders were screened through a 270 mesh sieve to obtain the NASICON/NPS composite powders. The powders were then press-molded by applying a uniaxial pressure of 720 MPa to prepare disk-shaped NASICON–NPS composite green compacts. Green compacts of each raw material were prepared in the same manner.

2.3 Characterization of composite electrolytes All the characterizations described below were conducted without air exposure using argonfilled and airtight sample holders. X-ray diffraction (XRD) measurements were conducted to identify the crystalline phases of the individual and composite powder materials of NASICON, NPS, and the NASICON–NPS composites. A laboratory X-ray diffractometer (Rigaku, SmartLab) with Cu Kα radiation was used. The morphologies of the obtained powders and the microstructures of the green compacts were observed by SEM. To observe the green compact cross-sections, argon ion milling was performed using an ion milling system (Hitachi, IM4000) to


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obtain flat sample surfaces. Topological and compositional contrasts were obtained by observing the flat surfaces with secondary electron (SE) images and backscattered electron (BSE) images, respectively. Elemental mapping analyses for the composite powders were obtained using energydispersive X-ray spectroscopy (EDS, Horiba, EMAXEvoution X-Max). The bulk densities (ρbulk) of all green compacts were determined from their weights, areas, and thicknesses. The true densities (ρtrue) of the NPS, NASICON, and 80 wt.% NASICON composite powders were determined using an argon pycnometer (Micromeritics, AccuPyc II 1340).30 The nominal volume fractions for the composites were estimated from the ρtrue of the raw materials and the nominal weight fractions. The measured ρtrue of the composite was almost equal to the calculated density (ρcalc.) based on the ρtrue of the raw materials and the nominal volume fraction. The values of ρtrue and ρcalc. for the 80-wt% NASICON composite were 3.00 and 2.96 g cm−3, respectively. Therefore, the relative densities of the green compacts were defined as (ρbulk/ρtrue) and (ρbulk/ρcalc.) for the raw materials and composites, respectively. The ionic conductivities of the green compacts were determined by AC impedance measurements. Gold electrodes were sputter-coated on both surfaces of the as-molded compacts. Impedances were measured using the SI1260 analyzer in the frequency range of 1 Hz to 2 MHz for the temperature range from 11°C to 108°C. Here, the samples were 0.05–0.08 cm in thickness (L) and the surface area (S) of the Au electrodes was 0.159 or 0.785 cm2. L and S were varied to adjust the magnitude of resistance within the range measurable by the impedance analyzer. The L/S values of each specimen were noted with Nyquist plots. An all-solid-state test cell (Na15Sn4/NASICON–NPS/TiS2) was constructed to investigate the applicability of the NASICON–NPS composite powders as solid electrolytes. In this cell, Na15Sn4, NASICON–NPS, and TiS2 were respectively used as the counter electrode active material, a solid


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electrolyte separator, and a working electrode active material. The combination of model electrode active materials Na15Sn4 and TiS2 for an all-solid-state cell was proposed previously.9 The test cell was fabricated as a trilayer pellet by cold-pressing without firing. The powder materials for each pellet are as follows. The Na15Sn4 counter electrode powder was prepared as a composite with an electron-conducting carbon additive (Lion, Ketjenblack, KB) by mechanical milling.37 The weight ratio of Na15Sn4:KB was 19:2. The TiS2 working electrode powder was prepared as a mixture with the NPS glass-ceramic to form ion conduction paths in the electrode layer.9 The weight ratio of TiS2:NPS was 2:3. The solid electrolyte powder for the separator was the NASICON–NPS composite with 70 wt% NASICON. The working electrode (7 mg), solid electrolyte (100 mg), and counter electrode (20 mg) powders were placed in a 10-mm-diameter polycarbonate tube and pressed together by applying a uniaxial pressure of 360 MPa. This trilayer pellet was sandwiched between two stainless-steel rods as current collectors to obtain the all-solid-state cell. A comparative cell using only the pure NPS glass-ceramic instead of the composite electrolyte was also constructed. Constant-current discharge-charge tests were conducted at 100 oC in a voltage range from 1.17 to 2.40 V using a charge–discharge measurement device (Nagano, BTS-2004).

3. RESULTS AND DISCUSSION 3.1 Evaluation of interfacial resistance between NASICON and NPS Figure S2 shows the ion-conducting properties of the NASICON sintered body used for interfacial resistance evaluation and those of the composite powders. The NASICON ceramic has a very high conductivity of 4.7 × 10−3 S cm−1 at 25°C and a low activation energy of 26 kJ mol−1. These characteristic values were estimated using the total resistance of the NASICON ceramic.


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The Nyquist plot (inset in Figure S2) shows that the bulk and grain-boundary resistances of the ceramic are inseparable. NPS glass thin films were formed on the NASICON ceramic by PLD. Figure S3 shows SEM images of the surfaces of the NASICON ceramic before and after PLD. The polished surface of the ceramic is very smooth without visible polishing scratches (Figure S3a). After PLD (Figure S3b), a smooth film without cracks is formed. Although some small pores (~1 µm), originally present on the as-polished ceramic surfaces, are observed, they occupy only a small fraction of the total surface area and the variation they induce in the contact area between the sintered body and the NPS film is negligible. A symmetrical gold/NPS/NASICON/NPS/gold cell was then prepared by DC sputtering of gold films onto the NPS films. SEM images of the


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fractured cross-section of the symmetrical cell are shown in Figure 1. A dense NPS glass thin film with uniform ~1.0-µm thickness is formed on the NASICON sintered body. A dense gold layer of ~50 nm in thickness is also observed. No cracks or boundaries in the NPS film and no voids at the NASICON/NPS interface are observed. This uniform thin film morphology is expected to allow accurate resistance measurements without contact losses and extra interfacial resistances. The opposite side of the cell was also observed to show a similar thin film with the NPS thickness of ~1.8 µm. The total thickness of the NPS glass films is ~2.8 µm for the symmetrical cell.

Figure 1. SEM images of the fractured cross-section of the symmetrical gold/NPS/NASICON/NPS/gold cell; (a) lower-magnification and (b) highermagnification images.


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AC impedance measurements of the gold/NPS/NASICON/NPS/gold symmetrical cell were performed. This measurement evaluated the ionic resistances of the cell with series connection of two kinds of sodium ion-conducting materials. Total resistances of the cell were in a measurable range of an impedance analyzer. Therefore, it proved that interfacial sodium ion transfer occurred between these materials. Figure 2 shows the Nyquist impedance plot of the symmetrical cell at room temperature. An inseparable arc at high frequencies (from 106 Hz to the middle of 104 Hz) and a tail at low frequencies (below the middle of 104 Hz) are observed. This low-frequency

Figure 2. Nyquist impedance plot of the symmetrical gold/NPS/NASICON/NPS/gold cell at room temperature. Inset: Magnification of high-frequency region.

response is typical of the interface between an ionic conductor such as NPS and an ion-blocking electrode such as gold.31,38 The total ionic resistance of the cell (Rtotal), consisting of the series connection of the NASICON ceramic resistance (RNASICON), NPS glass films resistances (RNPS), and the NASICON/NPS interfacial resistances (Rint, two interfaces), is estimated from the highfrequency arc shown in the figure. Although these resistance components should be respectively


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accompanied by capacitor components, the relaxation times of the components are too similar to distinguish on the Nyquist plot. Therefore, Rint is indirectly calculated from the following equation (1): Rint = ½｛Rtotal―(RNASICON + RNPS)}

(1)

Here, RNASICON and RNPS are estimated using the conductivities shown in Figure S2 and the conductivities reported by Ito,35 respectively. The surface area of the gold electrode (0.159 cm2) and the NASICON and NPS sample thicknesses of 7.42 × 10−2 cm and 2.8 × 10−4 cm, respectively, are also used for the calculation. For example, each resistance at 22°C is as follows: Rtotal = 818 Ω, RNASICON = 112 Ω, RNPS = 469 Ω, and Rint = 118.5 Ω (area specific resistance, ASR in RintS: 18.8 Ω cm2). The same NASICON specimen is used for the impedance measurements shown in Figure S2 and Figure 2. Although the conductivities of sintered bodies often have individual differences, these differences are negligible in this experiment. The temperature dependence of the ASR is depicted in Figure 3. The calculated ASR for the NASICON/NPS interfacial component is decreased with increasing temperature, obeying an Arrhenius-type equation (2): 1/(RintS) = 1/(Rint,0S) exp(−Eint/RgasT)

(2)

where Rint,0, Eint, T, S and Rgas are the pre-exponential factor, activation energy for interfacial ion transfer, absolute temperature, electrode surface area, and the gas constant, respectively. The extrapolated ASR values at 25°C and 100°C are 15.8 Ω cm2 and 0.40 Ω cm2, respectively. The interfacial resistances between NASICON and NPS are very small above room temperature,


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Figure 3. Temperature dependence of ionic resistance in the symmetrical gold/NPS/NASICON/NPS/gold

cell.

Calculated

NASICON/NPS

interfacial

resistance (Rint, circular symbols), total resistance of the cell (Rtotal, square symbols), estimated NASICON ceramic resistance (RNASICON, dashed line), and estimated NPS thin film resistance35 (RNPS, dotted line). especially at the high temperature of 100°C. The activation energy is calculated as 45 kJ mol−1. To the best of our knowledge, this is the first report of the activation energy for interfacial ion transfer between two types of alkaline-ion-conducting ceramic solid electrolytes (here, oxide and sulfide electrolytes). Previously, Sagane et al. studied interfacial sodium ion transfer between ceramic and polymer solid electrolytes.39,40 They found that the activation energies for interfacial ion transfer between oxide ceramics (NASICON or β″-alumina) and polymers (NaCF3SO3-poly(ethylene oxide)) were approximately 70 kJ mol−1. Therefore, the present Eint value of 45 kJ mol−1 is much lower than that for ceramic/polymer interfacial sodium ion transfer. Moreover, unlike the very small ASR for the present NASICON/NPS interface (0.4 Ω cm2 at 100°C), the NASICON/polymer


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interfacial resistance was much larger at approximately 1900 Ω cm2 at 100°C.39 Sagane et al. suggested that the activation energies for interfacial ion transfer were determined by the chemical potential difference of sodium ions in materials and a transition state.40 The NASICON/NPS interface may favor sodium ion transfer because of a smaller difference in the chemical potential of sodium ions in the two materials, compared to that in the combination of NASICON and polymers. Therefore, highly conductive composite solid electrolytes are expected to be achieved even by mixing fine powders of the NASICON and NPS, despite the formation of many interfaces and the serial connection of each interfacial resistance.

3.2 Characterization of Na3Zr2Si2PO12-Na3PS4 composite electrolytes In section 3.1, the NASICON/NPS interfacial resistance is evaluated using NPS glass thin film. Dense glassy films with no cracks or boundaries permitted accurate quantitation of the NASICON/NPS interfacial resistances. The NPS glass-ceramic (crystallized glass) is used to prepare the powder composite electrolytes instead of NPS glass to decrease the resistances derived from the NPS components. NPS glass-ceramics with cubic Na3PS4 nanocrystals show conductivities several tens of times higher than that of NPS glass.9,31,32 Moreover, these NPS glassceramic powders have good interface formation abilities; the powders can be significantly densified by cold-pressing to obtain highly conductive green compacts. Therefore, composite powders of highly conducting NASICON and the NPS glass-ceramic were prepared and characterized. Figure 4 presents the XRD pattern of the NASICON–NPS composite powder with 80 wt% NASICON obtained by ball-milling of the NASICON ceramic and NPS glass-ceramic powders.


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Figure 4. XRD patterns of the 80 wt% NASICON–NPS composite powder and of raw NASICON and NPS.

The dominant phase of the NASICON raw material is monoclinic Na3Zr2Si2PO12, which is a wellknown superionic phase.17 ZrO2 is detected as an impurity phase, which is common in the firing process for synthesizing NASICON.23–25 The NPS raw material contains a highly conductive cubic Na3PS4 phase.9 The broad peaks of cubic Na3PS4 indicate the precipitation of nanocrystals thereof in a glassy matrix, although nanocrystals are not observed directly by SEM. After ball milling, only the peaks from the raw materials are observed, indicating no unfavorable reactions during milling. Figure S4 shows SEM images of the raw powder materials directly used for the composite electrolytes. The particle sizes of the NASICON and NPS powders are approximately 0.2–5 µm and 0.3–10 µm, respectively. These particles were used in the composite powders. A SEM image and EDS elemental mapping of the composite powder with 80 wt% NASICON are portrayed in Figure 5. The signals of O Kα and S Kα respectively represent the distribution of the oxide NASICON and the sulfide NPS. Although this composite powder contains only 20 wt% (28 vol%)


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Figure 5. (a) SEM image and EDS elemental mappings of (b) O and (c) S in 80 wt% NASICON–NPS composite powder.

NPS, S Kα signals are detected from most particles showing O Kα signals. This implies that NASICON particle cores are coated with NPS shells. Because minor regions are identified where only S Kα signals are detected, small amounts of NPS are not incorporated in these coatings. NASICON–NPS composite green compacts were prepared by the uniaxial pressing of the ballmilled powders. Pure NASICON and NPS green compacts were prepared under the same conditions. Figure 6 shows the relative densities of the green compacts with various NASICON fractions. The insets show photographs of specimens with 70, 90, and 100 wt% NASICON. The NPS powder (0 wt% NASICON) shows the unique behavior of densifying under cold-pressing alone; the relative density of the green compact reaches ~90%. In contrast, the green body of 100


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Figure 6. Relative densities of the NASICON–NPS composite green compacts. Insets: Photographs of specimens with 70, 90, and 100 wt% NASICON.

Figure 7. SEM observations of the ion-milled surfaces of the 80 wt% NASICON–NPS composite green compact; (a, b) secondary electron images and (c, d) backscattered electron images.


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wt% NASICON has a much lower relative density of 77%. The pellet is brittle and cannot be removed from the die as a complete circular disk (see the inset photograph). The addition of a small amount of NPS to NASICON dramatically improves powder compaction (solidification). The composite green pellet with 90 wt% (86 vol%) NASICON has shiny surfaces and sufficient mechanical strength to withstand various evaluations, although its relative density (79%) is not much higher than that of the pure NASICON pellet. The relative density of the composite monotonically increases as the NASICON content is decreased. Composite green compacts with relative densities exceeding 80% are obtained for up to 20 wt% NPS. Figure 7 portrays SEM images in SE and BSE modes for the ion-milled surfaces of the composite green compact with 80 wt% NASICON. The SE images reveal that the majority of the green body is densified, although interparticle voids appear, corresponding to the relative density of 83%. In the BSE images, compositional contrast appears; the majority bright-contrast area is designated as NASICON and the minor darker region is NPS. These BSE images show the dispersion of fine NASICON grains of 0.2–4 µm in size within an NPS matrix. In other words, the NASICON grains are separated from each other by the NPS component. Higher-magnification SE and BSE images show close NASICON/NPS interfacial contact, supporting the expected smooth ion transfer across the interfaces. It has been reported that thiophosphate-based glass phases remain in obtained glassceramics by about 20-30%.41,42 In the composite green compacts, the NPS glass phases remaining in the glass-ceramics will be deformed by an external uniaxial pressure to form closed interface with NASICON particles (see Fig. 7). Therefore, we guess that interfacial transfer of sodium ions in the symmetrical cell discussed in chapter 3.1 similarly occurs in the composite green compacts. The ionic conductivities of the NASICON–NPS composite green compacts were evaluated by AC impedance measurement using ion-blocking cells. All measurements at various temperatures gave


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impedance plots that could not be separated into the individual components of NASICON, NPS, and NASICON–NPS interfaces. Typical Nyquist plots of the composite green compact with 80 wt% NASICON are presented in Figure 8 at 25°C and 100°C. An inseparable arc or a partial arc is observed in the high-frequency region for each temperature. Blocking behaviors are apparent in

Figure 8. Nyquist impedance plots of the 80 wt% NASICON–NPS composite green compact at (a) 25°C and (b) 100°C. The specimen L/S value is 0.314 cm−1.


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the low-frequency region. Similar impedance profiles are observed for other compositions (see Figure S5 for 70 and 90 wt% composites). Therefore, the total resistances of the composite green compacts (Rcomp), considered the DC resistances of the green compacts, can be determined as shown in the figures. The total conductivities of the green compacts are estimated using these Rcomp. Figure 9 shows the temperature dependences of the conductivities of the NASICON–NPS composite green compacts. Conductivities for pure NPS, as reported previously,32 are shown for comparison. These dependences obey the Arrhenius equation. The conductivity of the composite with 90 wt% NASICON shows a slight upward deviation from the Arrhenius line at temperatures above 60°C. The relative density and XRD pattern of this pellet did

Figure 9. Temperature dependence of conductivities for the NASICON–NPS composite green compacts. Inverted-triangle, square, and triangle symbols represent the 70, 80, and 90 wt% NASICON specimens, respectively. Conductivities of pure NPS32 (open symbols) and NASICON (closed symbols) green compacts are also plotted for comparison.


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not vary before and after the impedance measurements. Although no detailed microstructural observation was performed, we suspect that the interfacial contact was slightly improved by heating. For the pure NASICON green compact, the conductivity was calculated only at 100°C because the magnitude of the resistance exceeded the measurement limits of the impedance analyzer in the lower-temperature range. The conductivities at 25°C (σ25) and 100°C (σ100) and the conduction activation energies (Ecomp) for the green compacts with various NASICON contents are plotted in Figure 10. This figure demonstrates that the addition of a small amount of NPS to NASICON greatly improves the ion-conducting properties of the green compact. The σ100 of the composite with 90 wt% NASICON is 19 times higher than that of pure NASICON. Although σ25

Figure 10. Relationship between NASICON fractions of the green compacts and ion-conducting properties; conductivities at 25°C (σ25, open circles) and 100°C (σ 100, closed circles) and conduction activation energies (Ecomp, open diamonds).

and σ100 of the composite green compacts are both lower than those of pure NPS, these conductivities do not decrease markedly even at the NASICON content of 80 wt% (72 vol%). The


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conductivity decrease as the NASICON content is increased is particularly slight at 100°C. The composites with 70 and 80 wt% NASICON show high σ100 of 1.1 × 10−3 S cm−1 and 6.8 × 10−4 S cm−1, respectively. These values are several times higher than the σ25 (2.1 × 10−4 S cm−1) of pure NPS. NPS electrolytes have been applied to all-solid-state cells operating at room temperature. Therefore, higher rate performances are expected for all-solid-state cells using the present NASICON–NPS composite electrolytes at 100°C. The ion conduction activation energies of the composite green compacts (Ecomp) are larger than the 32 kJ mol−1 shown by pure NPS at 40 kJ mol−1and 38 kJ mol−1 for 70 and 80 wt% NASICON, respectively. The NASICON/NPS interfacial ion-transfer resistance with the relatively large activation barrier Eint of 45 kJ mol−1 should contribute to Ecomp for the composites. This means that ion transfers occur across the NASICON/NPS interfaces in the composites. As discussed previously, the NASICON/NPS interfacial ion transfer resistances are very small, 15.8 Ω cm2 at 25°C and 0.40 Ω cm2 at 100°C. Thus, interfacial ion transfer can connect smoothly transport pathways in the NPS component and within NASICON grains, which were otherwise isolated (see Figure 7). Conversely, if the NASICON intra-grain pathways are not used, the dense NPS matrix provides the only conduction pathway, having the same activation energy as the pure NPS green compact; the only pre-exponential factors would be decreased as the NPS volumetric fractions of the composites decreased. Therefore, smooth NASICON/NPS interfacial ion transfer at high temperatures should activate the ion conduction pathways within the NASICON grains, promoting high total conductivities of the composites. The same idea regarding interfacial ion transfers and NASICON intra-grain conduction pathways in the composites is derived by considering the conduction channel tortuosity and the effective conductivity in the composite electrolytes. For commercial lithium-ion batteries, the


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porous composite electrodes and separators have multi-phase structures including networks of inhomogeneous and interconnected pores or channels impregnated with liquid electrolytes. In general, the electrolyte effective conductivities (σeff) within such electrodes or separators are corrected downward by the geometrical parameter of tortuosity (τ).43–45:

σeff = σbulk ・(ε /τ)

(3)

Here σbulk and ε are the intrinsic conductivity and volumetric fraction of the conducting phase (liquid electrolyte), respectively. For lithium-ion batteries, ε corresponds to the porosity or the void fraction, which is filled with liquid electrolyte. The value τ indicates the length of the tortuous conduction path formed in the multi-phase structures. For conduction pathways composed of straight, uniform channels that are fully parallel to the ion transport direction, τ = 1. Otherwise, in most real cases, τ > 1 for tortuous pathways. Although the dependence of τ on the morphologies and size distributions of electrode and separator materials is complex, τ is generally increased as

ε is decreased. Experimental studies of lithium-ion batteries have permitted the estimation of τ for the ionic conduction of liquid electrolytes in many systems, such as polypropylene separators,44,46– 48

porous carbon electrodes,48–50 and porous LiFePO4 electrodes.45,48,49 In such studies, τ ranges

from 3 to 7 for ε values of 0.4 to 0.2. Herein, unrealistic tortuosity values in the NASICON–NPS composites, i.e., τ ≈ 1, are calculated assuming that only the NPS matrices contribute to the ionic conduction of the composites. As shown in the Supporting Information, the volumetric fractions of NPS (ε) were calculated as 0.34 and 0.23 for the composites with 70 wt% and 80 wt% NASICON, respectively. The bulk conductivity of NPS σbulk was 3.1 × 10−3 S cm−1 (also see Supporting Information). Thus, the unrealistic tortuosity values τ for the composite green compacts are calculated from Equation (3) using the measured conductivities of the composites as


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σeff. The tortuosity values of the 70- and 80-wt% NASICON composite green compacts are respectively calculated as 0.96 and 1.1 at 100°C. These τ values near 1 indicate straight conduction paths; they are unrealistically smaller than those reported for systems with similar ε (0.2−0.4).44– 50

However, SEM observation of the NASICON–NPS composite clearly demonstrated that

NASICON grains and NPS components were dispersed in sub-micrometer-order scale (see Figure 7). This result strongly suggests that the conduction tortuosity of the NPS components is far larger than 1. Therefore, it is unlikely that sodium ions migrate through only the NPS components in the composites. NASICON intra-grain paths must be used in the composites and the smooth NASICON/NPS interfacial ion transfer must induce the high total conductivities observed, especially at high temperatures. Finally, an all-solid-state test cell was fabricated using the NASICON–NPS composite electrolyte and its discharge properties were evaluated at 100°C. The composite with 70 wt% NASICON with the highest conductivity among the composites was used; the composite shows a high conductivity of 1.1 × 10−3 S cm−1 under the cell operation temperature of 100oC. A trilayer pellet cell with a Na–Sn alloy counter electrode, NASICON–NPS composite electrolyte, and TiS2 working electrode was prepared by uniaxial pressing at room temperature. The model electrode materials were selected with reference to our earlier study.9 The Na–Sn alloy electrode was used as a powder mixed with a carbon additive KB because it is advantageous in high-current-density operation.37 Figure 11 shows the discharge-charge profiles of the cell using the composite electrolyte at 100°C. The cell shows the reversible capacities of about 120 mAh g−1 for three cycles at a high current density of 640 µA cm−2. Therefore, we have successfully prepared sintering-free oxide-based NASICON–NPS composite electrolytes applicable to all-solid-state sodium batteries. The discharge-charge test was conducted for 10 cycles. After the third cycle, a reversible cell


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capacity decreased continuously during the cycles. The reversible capacity at the 10th cycle was about 110 mAh g-1. This is probably due to low chemical stability of the Na-Sn negative electrode at 100°C. A similar capacity-degradation was observed when a cell using the pure NPS electrolyte instead of the composite electrolyte operated at 100°C. Although an optimization of cell configuration is necessary to ensure the long cycle lifetime of batteries, this study demonstrates the potential applicability of the proposed composite electrolytes in solid-state rechargeable batteries operating at 100°C. 4. CONCLUSIONS

Figure 11. Discharge-charge curves of the all-solid-state Na–Sn/TiS2 cell using the 70 wt% NASICON–NPS composite electrolyte. The cell operation was carried out at 100°C. The numbers 1, 2 and 3 in the figure represent the first, second, and third cycles, respectively. This report described the preparation and characterization of a new oxide-based composite solid electrolyte of Na3Zr2Si2PO12 (NASICON)–Na3PS4 (NPS). Before preparing the composite, the interfacial resistances of the two materials were investigated using the NASICON sintered ceramic


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and NPS glass thin film. Dense film morphologies and simplified void-free interfaces were obtained by PLD. AC impedance measurements of a symmetrical gold/NPS/NASICON/NPS/gold cell provided the ion transfer properties of the NASICON/NPS interface. The area specific resistances at 25°C and 100°C were 15.8 Ω cm2 and 0.40 Ω cm2, respectively, with the activation energy of 45 kJ mol−1. Therefore, the interfacial ion transfer resistances were very small above room temperature. Based on these results, NASICON–NPS composite powders with NASICON-rich compositions of 70–90 wt% were prepared by ball milling using the NASICON ceramic and NPS glass-ceramic powders. SEM-EDS analysis implied that NASICON core particles were coated with NPS shells in the obtained composite powders. The composite powders were compacted (solidified) well by cold pressing alone, achieving high relative densities exceeding 80% for 70 and 80 wt% NASICON. SEM observation of the NASICON-NPS green compacts revealed that submicrometer NASICON particles were highly dispersed in a dense NPS matrix to form closed interfaces between the oxide and sulfide solid electrolytes. The composite green compacts with 70 and 80 wt% NASICON exhibited the high total conductivities at 100°C of 1.1 × 10−3 S cm−1 and 6.8 × 10−4 S cm−1, respectively. A discussion of the activation energies and ion-conduction tortuosities suggested that highly ion-conductive pathways within the NASICON grains and the smooth NASICON–NPS interfacial ion transfer process contributed to the high composite conductivities, especially at high temperatures. An all-solid-state test cell was constructed using the composite electrolyte with 70 wt% NASICON and its discharge-charge property was evaluated at 100°C. The Na15Sn4/NASICON–NPS/TiS2 cell obtained by the room-temperature uniaxial pressing of powder materials showed the reversible capacities of about 120 mAh g−1 under the


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current density of 640 µA cm−2. Therefore, we successfully prepared a new sintering-free oxidebased composite electrolyte with applicability in all-solid-state sodium batteries.

ASSOCIATED CONTENT Supporting information. Schematic of sodium ion conduction mechanism in the NASICON– NPS composite; conductivity data for raw and composited materials; SEM images of raw and composited materials; AC impedance plots for composite electrolytes; discussion of tortuosity calculation. This information is available free of charge.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected]

Note The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by a Grant-in-Aid for Japan Society for the Promotion of Science (JSPS) Research Fellow (17J09297, KN).


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Na3PS4

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