NASICON Interface

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Insights into Sodium-ion Transfer at Na/NASICON Interface Improved by Uniaxial Compression Yasuhiro Uchida, George Hasegawa, Kazunari Shima, Miki Inada, Naoya Enomoto, Hirofumi Akamatsu, and Katsuro Hayashi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00250 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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ACS Applied Energy Materials

Insights into Sodium-ion Transfer at Na/NASICON Interface Improved by Uniaxial Compression Yasuhiro Uchida,† George Hasegawa,†,* Kazunari Shima,† Miki Inada, ‡ Naoya Enomoto, § Hirofumi Akamatsu,† and Katsuro Hayashi†,*

† Department

of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan

‡ Center

of Advanced Instrumental Analysis, Kyushu University, Kasuga, Fukuoka 8168580, Japan

§ National

Institute of Technology, Ariake College, Omuta, Fukuoka 836-8585, Japan

ACS Paragon Plus Environment

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KEYWORDS: Na-ion battery; activation energy; charge transfer; solid electrolyte; allsolid-state battery


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ABSTRACT

A robust ceramic solid electrolyte with high ionic conductivity is a key component for all-solid-state batteries (ASSBs). In terms of the demand for high-energy-density storage, researchers have been tackling various challenges to use metal anodes, where a fundamental understanding on the metal/solid electrolyte interface is of particular importance. The Na+ super ionic conductor, so-called NASICON, has high potential for applying to ASSBs with a Na anode due to its high Na+ ion conductivity at room temperature, which has been, however, faced a daunting issue of the significantly large interfacial resistance between Na and NASICON. In this work, we have successfully reduced the interfacial resistance as low as 14 Ω cm2 at room temperature by a simple mechanical compression of a Na/NASICON assembly.

We also demonstrate a

fundamental study on the Na/NASICON interface in comparison with the Na/β”-alumina counterpart by means of the electrochemical impedance technique, which elucidates a stark difference between the activation energies for interfacial charge transfer: ~0.6 eV for Na/NASICON and ~0.3 eV for Na/β”-alumina. This result suggests the formation of


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a Na+-conductive interphase layer in pressing Na metal on NASICON surface at room temperature.

1. Introduction 　Recent upsurge of research interest in all-solid-state batteries (ASSBs) as a nextgeneration energy storage device with high energy density and safety puts a worldwide spotlight on ceramic ionic conductors.1,2 As compared with the Li-ion secondary battery with a liquid electrolyte, the major drawbacks lie in the relatively low ionic conductivity of solid electrolytes and the significantly high interfacial resistance between solid electrolytes and electrode materials (solid/solid interface).3 With respect to the former point, extensive efforts to explore fast ionic conductors have developed a variety of solid electrolyte candidates to date.

As for the Li+ ion conductors, for example, their

conductivities at room temperature reach the order of 10–3 S cm–1 for oxide-based compounds4-6 while some sulfide ceramics exhibit beyond 10–2 S cm–1, which is comparable to the liquid electrolytes.7,8 On the other hand, the latter point still remains


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a formidable challenge especially for the ASSBs using a Li metal anode due to the poor stability of those electrolytes in contact with Li, forming a highly resistive solid electrolyte interphase (SEI).2,9,10 Although some strategies such as doping,11-14 coating15,16 and microstructural designing17-19 have been proposed for obtaining a solid electrolyte with improved stability in relation to the garnet-type Li7La3Zr2O12 (LLZO), another critical issue of the short-circuit plagued by dendrite formation is yet to be solved.20-25 In view of reducing battery cost, Na+-based ASSBs are fascinating due to the abundant Na resource, which boosts the demand for new Na+ ion conductors with superior conductivity.26-29 However, the emerging fast Na+ ion conductors again suffer from the interfacial problem in contact with Na metal, as is the case with the Li+ ion conductors.30 Meanwhile, the research on fast Na+ ion conductors has a longer history than the Li+ counterpart since the advent of β-alumina in 1960s followed by Na+ super ionic conductor (NASICON).31 A great deal of efforts were dedicated in the subsequent decades, which elucidated their stability against Na metal, ionic conduction pathways, effects of doping and so on.31-33 In this context, researchers have revisited the use of those well-investigated ceramics, in particular NASICON, which shows a higher


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conductivity than β-alumina at room temperature. Nevertheless, only a few have been reported on the solid/solid interface between Na and NASICON because Na+ ion conductors were first developed as a solid electrolyte aiming at a rechargeable sodiumsulfur (NAS) battery operated at 300–350 °C, where both Na and S electrodes are in liquid form.34,35 Recently, Goodenough and co-workers documented that the NASICON surface is incompatible with Na metal at ambient condition, which is responsible for a large interface resistance between Na and NASICON.36 The poor contact also precludes a uniform Na+ flux across the interface, leading to severe dendrite formation and rapid short-circuit. They concluded that the fabrication of a good interlayer between Na and NASICON is a key to solve the overall issue and put forward the pretreatment of NASICON with Na melt to form a favorable SEI. In this study, we have tackled this crux by a direct approach: a uniaxial compression loaded on a Na/NASICON assembly. It was revealed that the high-pressure pressing can effectively reduce the resistance due to the improved interfacial contact. Here, we also discuss the activation energy of Na+ ion transfer at the solid/solid interface in comparison with Na/β”-alumina.


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2. Experimental 2.1 Synthesis of NASICON mother powder 　Reagent grade ZrO2, SiO2 and Na3PO412H2O (Purity of 98 %, 99.9 %, and 99 %, respectively; purchased from Kojundo Chemicals, Japan) were weighed to obtain a composition of Na3Zr2Si2PO12, and mixed by planetary ball milling with zirconia balls in ethanol using a Pulverisette 6 milling machine (Frichche, Germany) at 150 rpm for 1 h. The mixture was calcined at 1100 °C for 12 h. The calcined powder was pulverized by ball milling at 150 rpm for 18 h with zirconia balls and ethanol. The dried powder was screened with a sieve of 355 µm to obtain the fine NASICON mother powder. 　 2.2 Synthesis of sintered NASICON electrolyte 　The NASICON fine powder was pressed into a pellet followed by the cold isostatic pressing (CIP) at 200 MPa for 5 min.

Then, the pellet was placed in an alumina

crucible, covered with the NASICON mother powder, and calcined at 1270 °C for 12 h in air. Thus obtained NASICON sintered compact was cut into thin discs with a thickness


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of ~1.1 mm. Both surfaces of the NASICON disc were polished into ~1 mm thick with a rotary polisher (MA-150, Musashinodenshi Corp., Japan) using 9 µm and 3 µm diamond pastes (Hyprez®, ENGIS Japan Corp., Japan) in sequence.

The mirror-polished

samples were finally annealed again at 1100 °C for 30 min in the NASICON mother powder.

2.3 Synthesis of sintered β”-alumina electrolyte 　 The β”-alumina electrolyte was prepared by the solid-state reaction similar to the NASICON pellets. The β”-alumina (Na1.67Mg0.67Al10.33O17) fine powder was synthesized by firing the stoichiometric mixture of γ-Al2O3, Na2CO3 and Mg(OH)2 (Kojundo Chemicals, Japan) at 1500 °C for 2 h in air followed by ball-milling at 300 rpm for 10 h with zirconia balls and ethanol. The obtained fine powder was pressed into a pellet and subjected to CIP at 200 MPa for 5 min. Then, the pellet was placed in an alumina crucible filled with the β”-alumina powder and calcined at 1650 °C for 30 min in air.

2.4 Characterization


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　 Microstructure on sintered surface and fractured cross-section of the NASICON pellet was observed by field-emission scanning electron microscopy (FE-SEM) using an S-5200 SEM (Hitachi Ltd., Japan).

Crystal phases of the sintered products were

identified by powder X-ray diffractometry (XRD) using a D8 ADVANCE (Bruker AXS GmbH, Germany).

Bulk density for the sintered pellets was calculated by

[weight]/[volume], which was used for obtaining the relative density.

Compressive

mechanical testing was performed using a uniaxial electromechanical testing device (EZGraph, Shimadzu Corp., Japan) at a crosshead speed of 1.0 mm min–1.

X-ray

photoelectron spectroscopy (XPS; AXIS-ULTRA; Shimadzu Corp.) was performed for the surface elemental analyses of NASICON samples. The Na metal adhered on the NASICON electrolytes was dissolved in ethanol (EtOH) followed by drying under vacuum. The monochromatized X-ray Al K radiation (1486.6 eV) was used. The core levels were calibrated by reference to the first component of the C ls core level peak (unfunctionalized hydrocarbons) set at 284.5 eV. In the case of EtOH-washed samples, the surface was slightly etched by Ar-sputtering to remove impurities like organic and carbonate species.


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2.5 Electrochemical test conditions The NASICON solid electrolytes with a thickness of 1 mm were employed for the electrochemical tests. To obtain an Au | NASICON | Au cell, Au electrodes were put on to the both sides of the NASICON pellet with Au paste (No.8556, Tokuriki Honten Co., Ltd., Japan), which was heated at 200 °C to remove organic moieties. A Na | NASICON | Na cell was also prepared by sandwiching a NASICON plate between two Na foils, which was subjected to a uniaxial compression at 0–30 MPa using an oil hydraulic press (MS05-100, Riken Kiki Co., Ltd., Japan) in an Ar-filled glove box. The typical holding time for pressing was 5 sec.

Electrochemical impedance spectroscopy (EIS) was

recorded using a 1260A impedance/gain-phase analyzer (Solartron Analytical, UK) with a bias voltage of 100 mV in a frequency range from 1 MHz to 10 Hz at different temperatures. The impedance data were analyzed using a fitting program (ZView 2; Solartron Analytical, UK).

Galvanostatic cycling tests were performed on the Na|

NASICON | Na cell subjected to 30 MPa compression prior to assembling the cell using a potentiostat/galvanostat apparatus (S1287; Solartron Analytical, UK). The charge and


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discharge tests were conducted at varied current densities with 30 min-duration per each step. The activation energy for the charge transfer at Na/solid electrolyte interface was calculated according to the Arrhenius equation (1): 1 𝑅

𝐴

𝐸𝑎

= 𝑇 exp ( ― 𝑘𝐵𝑇)

(1)

where R is the interface resistance, A is the pre-exponential factor, T is the temperature,

Ea is the activation energy, and kB is the Boltzmann constant.

3. Results and Discussion 3.1 Characterization of sintered NASICON pellet The NASICON with a composition of x = 2 in Na1+xZr2SixP3–xO12 (0 < x < 3), where the electrical resistivity is reported to be minimum,37 was employed in this study. The NASICON mother powder with the corresponding composition (Na3Zr2Si2PO12) was first prepared by the conventional solid state reaction, which was pressed into a pellet followed by sintering to obtain a well-densified NASICON ceramic body. As the surface


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properties such as roughness and impurity of a solid electrolyte influence on the charge transfer at the electrolyte and electrode interface,17-19 the surface of the NASICON electrolyte was mirror-polished followed by annealing to cure the surface damage during polishing in order to minimize the individual difference among the specimens. Figure 1 (a) shows the surface microstructure of the representative NASICON pellet, signifying the smooth surface composed of densely packed grains with a size of submicrons to a few microns containing only a few voids. The cross-sectional SEM image in Figure 1 (b) verifies the whole pellet was well-densified. The typical relative density of the NASICON specimens was assessed as 97–98% based on the theoretical density of 3.27 g cm–3 for Na3Zr2Si2PO12. The XRD pattern for the NASICON pellet in Figure 1 (c) can be assigned to Na3Zr2Si2PO12 with a trace of monoclinic ZrO2 phase. The quality of the NASICON pellet was also examined in terms of the electrical conductivity.


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Figure 1. (a,b) SEM images of the NASICON pellet after mirror-polishing: (a) pellet surface and (b) cross-section. (c) XRD pattern and appearance (inset) of the NASICON pellet. (d) Nyquist plot of the NASICON pellet with the blocking electrodes measured at 25 °C.

The Nyquist diagram of the Au | NASICON | Au cell is presented in Figure 1 (d). The profile exhibits a depressed semicircle in the frequency range from 1 MHz to 100 kHz and a near-vertical line at the lower frequencies. The former is attributed to the grain


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boundary resistance in the NASICON electrolyte, while the latter is predominantly derived from a capacitive component corresponding to the blocking nature of the Au electrodes.

As the total resistance (bulk and grain boundary resistances) of the

NASICON pellet can be obtained from the intercept of the semicircle with the real axis at low frequency, the total conductivity was determined as 1.1 × 103 S cm1 at 25 °C, which is comparable to the value reported previously.37

3.2 Effect of uniaxial compression on Na/NASICON interface resistance The electrochemical property of the Na/NASICON interface was probed by using a Na | NASICON | Na symmetric cell. The AC impedance profile of the cell without any intentional compression is displayed in Figure 2 (a). The cell configuration used for this study involved a spring, which applied ca. 20 N corresponding to the uniaxial stress of approximately 0.25 MPa.

The impedance spectrum comprises a high-frequency

semicircular component similar to the cell with Au blocking electrodes (Figure 1 (d)) and another large semicircle in the low frequency range. When the impedance data was fitted with the equivalent circuit illustrated in the inset of Figure 2 (a), the low-frequency


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semicircle resistance and capacity resulted in 1.3 × 103 Ω cm2 and 1.2 × 107 F, respectively. As the capacitance values of 107–105 F is generally interpreted as an interface resistance between electrode and electrolyte,38 this resistance can be ascribed to the two Na/NASICON interfaces in the symmetric cell.

It follows that the

Na/NASICON interface resistance was determined as 6.6 × 102 Ω cm2, which is 6 times higher than the total resistance of the 1 mm-thick NASICON electrolyte (110 Ω cm2). A mechanical pressing of a metal electrode/solid electrolyte assembly is simple yet effective to improve the interfacial contact.11,13

In this research, the uniaxial

compression was applied to the NASICON pellet sandwiched between a pair of Na foils, as schematically shown in Figure 2 (b). Note that the pressed pellet was placed in the same cell described above, which means that the pressure that the Na/NASICON/Na assemblies experienced during the electrochemical test came solely from the spring (~0.25 MPa). We have also confirmed by the mechanical test that the NASICON pellet shows a linear elastic behavior without any eternal strain after deloading within the pressure range in this study.


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Figure 2. (a) Nyquist plot of the Na | NASICON | Na cell without compression prior to the cell assembly. The equivalent circuit for fitting is displayed in the inset. (b) Schematic illustration of the uniaxial compression of the Na/NASICON assembly.

(c) Nyquist plots for the Na |

NASICON | Na cells with pressing at varied pressures. (d) Variation of the areal resistances for the NASICON total resistance (RNASICON) and the Na/NASICON interface resistance (Rinterface) as a function of the uniaxial compression pressure.

Figure 2 (c) shows the Nyquist plots for the samples pressed with different uniaxial pressures. All the spectra consist of a small semicircular portion and a semicircle. The


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curve fitting was performed using the equivalent circuit as is the case with the spectrum in Figure 2 (a), and the capacitance values of 0.5–1.6 × 105 F were obtained for the low-frequency semicircle in each spectrum. Hence, the low-frequency semicircles are attributable to the Na/NASICON interface resistance. The fitting results are summarized in Figure 2 (d), where the NASICON total resistance (bulk and grain boundary) and the interface resistance are separately plotted as a function of the compressing pressure. The similar NASICON total resistivities testify that the electrical properties of the NASICON pellets for these measurements were almost identical. It is noteworthy that the interface resistance dramatically decreased by pressing the Na/NASICON/Na assembly. As a result, the interface resistance decreased to 14 Ω cm2 by pressing at 30 MPa, which is an order of magnitude smaller than the total resistance of the 1 mmthick NASICON electrolyte and thereby equivalent to that for a 100 µm-thick electrolyte. It should be also pointed out that the typical charge transfer resistance of Na anode in a carbonate-based organic electrolyte ranges from 700–1000 Ω cm2 due to the highly resistive SEI layer.39


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Figure 3 (a) presents the impedance spectra recorded at different temperatures and the logarithmic interface resistances calculated from these impedance data are plotted as a function of temperature in Figure 3 (b). The activation energy for Na+ ion transfer at the Na/NASICON interface was evaluated from the corresponding Arrhenius plot (Figure 3 (c)), providing 0.60 eV, 0.67 eV, 0.53 eV and 0.63 eV for the Na/NASICON/Na assemblies pressed at 10 MPa, 14 MPa, 20 MPa and 30 MPa, respectively.

The

activation energy seems to be irrespective of the compression stress and amounts to approximately 0.6 eV.


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Figure 3. (a,b) Temperature dependences of (a) the Nyquist plots and (b) the Na/NASICON interface resistances obtained by fitting for the Na | NASICON | Na cells with pressing at different pressures. (c) Arrhenius plots of the interface resistances for the Na | NASICON | Na cells with pressing at different pressures.

3.3 Insight into pressed Na metal on NASICON In the Li/LLZO system, it was reported that the heat-treatment at slightly below the melting point of Li (175 °C) can effectively reduce the interface resistance.19,21 However, it was found that the interface resistance of the Na | NASICON | Na cell at 25


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°C after heating to 90 °C, which is slightly lower than the melting point of Na (98 °C), remained similar to the initial value as shown in Figure S1. This result indicates that the compression at > 10 MPa provides a better Na/NASICON interface in comparison with the pressing by the spring in the cell (~0.25 MPa) at 90 °C. In order to check the mechanical feature of a Na foil on pressing, the uniaxial compression test was performed on a Na plate at room temperature. A Na plate with a dimension of ca. (L) 12 mm × (W) 15 mm × (H) 4 mm was cut out from a Na block, which was sealedin a plastic bag in an Ar-filled glove box. The Na plate was taken out from the glove box and subjected to the compression test immediately. The same mechanical test was carried out on a Li plate as well for comparison. The metal surface maintained shiny after the mechanical tests. Figure S2 represents the load-strain curve for the representative Li and Na metal plates. Since the cross sectional area was substantially changed during pressing, the mechanical stress values based on the original plate dimension are presented in the graphs. Note that the loading curves might partially reflect the friction between the metal plate and a plastic bag. Both load-strain curves exhibit the similar mechanical


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behavior consisting of the initial elastic region followed by yielding and densification. As is well known, the Na plate was apparently softer than the Li plate in terms of elasticity and yield stress. Besides, the onset of the steep increase in loading force shifts to the larger strain for the Na metal, indicating that a Na electrode can deform to a larger extent in accordance with the surface microstructure of a solid electrolyte under the same compression pressure as compared with a Li electrode.

3.4 Insight into Na+ ion transfer at Na/NASICON interface Ion transfer at interfaces between two different electrolytes (solid/liquid or solid/polymer) were intensively investigated by Abe et al.40-43 They disclosed that the dominant factor changes depending not only on the type of electrolytes but also on the ion species (Li+ or Na+). In the case of Li+ ion transfer, the thorough desolvation of Li+ in an organic electrolyte is the rate-determining step for the both systems.40-42

By

contrast, in the case of Na+, the weaker Lewis acidity of Na+ than Li+ renders the activation barrier for desolvation smaller, and the extraction of Na+ ion from a solid electrolyte dominates the charge transfer at solid/liquid electrolyte interfaces.43


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Nevertheless, regarding the Na+ ion transfer at a solid/poly(ethylene oxide)-based electrolyte interface, the desolvation process still remains rate-determining because of the strong interaction between Na+ and polymer. The activation energies associated with Na+ ion transfer are compiled in Table 1. In the present research, we have approximated the activation energy at Na/NASICON interface as about 0.6 eV, which lies between the activation energies of the NASICON/organic electrolyte and the NASICON/polymer electrolyte. In order to foster better understanding, we have also examined a different type of Na+-conducting ceramics, Na-β-alumina, which is renowned as a sustainable Na+ ion conductor33 and practically applied to the NAS battery developed by NGK Insulators Ltd. (Japan) and Tokyo Electric Power Co. (Japan) as well as the ZEBRA battery by FZ Sonick SA (Switzerland).34,35

Table 1. Activation energy for interfacial Na+ ion transfer.

Na

PEO

PC

DMSO


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NASICON

0.53–0.67 eV a

0.75 eV b,c

0.35 eV b,c

0.31 eV c,d

β”-Alumina

0.28 eV a

0.73 eV d

0.54 eV d

0.47 eV d

a

This study.

b

ref. 41.

c

NASICON with a composition of Na3Zr1.88Y0.12Si2PO12.

d

ref.

43.

A sintered pellet of the Mg2+-stabilized β”-alumina with a composition of Na1.67Mg0.67Al10.33O17 (1 mm thick) was employed for the electrochemical study as the aforementioned NASICON system. The XRD pattern in Figure 4 (a) evidences very few impurities in the sintered pellet. The relative density of the pellet was estimated as ca. 86%. As β”-alumina has good compatibility with Na metal,30 the pressure applied to the Na/β”-alumina assembly prior to the impedance tests was 2 MPa. Figure 4 (b) shows the Nyquist plots for the Na | β”-alumina | Na cells at different temperatures. Each spectrum is composed of two semicircles and the low-frequency portion with a capacitance of 2.5–5.1 × 105 F can be attributed to the interface resistance according to the fitting analysis similar to the NASICON system. Based on the semicircle at high frequencies, the total resistance (bulk and grain boundary) for β”-alumina was evaluated as 128 Ω cm2 at room temperature leading to the conductivity of 1.5 × 103 S cm–1. The


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good agreement of the conductivity with the values reported previously30-34 assures the acceptable quality of the β”-alumina electrolyte. The small interface resistance of 21 Ω cm2 (25 °C) was observed, which is comparable to that at Na/NASICON with pressing at > 20 MPa.

Figure 4. (a) XRD pattern of the β”-alumina electrolyte. (b) Nyquist plots for the Na | β”alumina | Na cell measured at different temperatures.

(c,d) Arrhenius plots of the Na |


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NASICON | Na cells: (c) total conductivity of β”-alumina and (d) interface resistance at Na/β”alumina.

The Arrhenius plots for the Na | β”-alumina | Na are depicted in Figure 5 (c) and (d). The activation energy for the total conductivity of β”-alumina was calculated as 0.21 eV, which is in line with the reported value,33 while the value of 0.28 eV was obtained for the interfacial Na+ ion transfer at Na/β”-alumina, which is smaller than a half of that at Na/NASICON and similar to the interfacial Li+ ion transfer at Li/Nb-doped LLZO (0.31 eV).12 According to the previous report,43 the activation energies for Na+ ion transfer between solid and liquid electrolytes were reported to be higher for the β”-alumina solid electrolyte than for the NASICON (see Table 1).

This tendency is opposite to the

Na/solid electrolyte systems in this study. In addition, the activation energies at the NASICON/liquid electrolyte interfaces also lie in 0.31–0.35 eV.43 These systems with low activation energy were explained by the interfacial charge transfer free from the


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desolvation process, which generally associates with high activation barrier (ca. 0.6 eV for the Li+ in typical organic electrolytes).40 One possible interpretation for the high activation energy at Na/NASICON is the constriction effect originated from the imperfect contacts between Na metal and NASICON.44,45 In this study, the mirror-polished NASICON plates were employed and high pressure was applied to make a good Na/NASICON contact.

However, it is

possible that the contact area gradually decreased during the repeated AC impedance tests.46 Fleig and Maier put forward that the similar activation energy values for the two semicircular components in the complex impedance plane signify the existence of geometrically imperfect contacts.45 In the present case, the activation energy for Na+ ion conduction in bulk and grain boundary of NASICON were evaluated as 0.22 eV and 0.75 eV, respectively (see Figure S3). These values are apparently different from the activation energy derived from the interfacial resistance at Na/NASICON, which indicates the small contribution of the ohmic constriction effect. As a different interpretation for the high activation energy at Na/NASICON interface obtained in this study, the presence of another barrier between Na and NASICON is


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proposed. As for the Li/solid electrolyte systems, the relatively high activation energy of ~0.63 eV was reported for the charge transfer at Li/lithium phosphorous oxynitride (LiPON).9 Although LiPON had been believed to be stable against Li metal for a long period,47 the formation of a thin passivation layer at the Li/LiPON interface was exemplified by Schwöbel et al.,48 which could be responsible for the high activation energy. We therefore came up with an assumption that NASICON outermost surface reacts with Na metal forming a sort of SEI layer, which might mediate the interface between Na and NASICON yet behaves as a barrier for the interfacial Na+ ion transfer. It is difficult to analyze the interphase because the Na metal stuck to the surface too strongly to be removed without chemical treatment. In this study, the Na metal was dissolved in EtOH followed by drying under vacuum for the XPS analysis. The obtained XPS spectra were displayed in Figure S4. The core-level spectra of the NASICON after Na-pressing are slightly different from those of the pristine NASICON; the peak shift to the lower binding energy is observed in the Na 1s, P 2p and Zr 3p spectra, whereas it is negligible in the Si 2p and O 1s spectra. In the case of Si 2p core-level spectrum, however, the band becomes broader by the Na-pressing. It is deduced that all the


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changes in XPS spectra associates with the formation of reduced components by the reaction between NASICON and Na. Note that the EtOH washing and drying process probably changed the SEI layer, but the XPS results evidence the formation of Na/NASICON interphase. In addition, the similar XPS spectra for the samples before and after the AC impedance measurements prove that the activation energy of ~0.6 eV for the charge transfer at Na/NASICON is highly responsible for this SEI layer. The formation of interlayer species on NASICON surface has been already exemplified by Zhou et al.36

The literature reports that the NASICON surface was

hardly reacted with Na melt at 175 ºC because of the fairly low compatibility and thereby the non-wetting nature, while the reduction to form a black interlayer took place at 380 ºC. Comparing the XPS spectra of the interlayer species to our result in Figure S4, the larger peak shift to the lower energy is observed in the previous results. Hence, the SEI layer formed by Na-pressing at room temperature is less reduced than the interlayer formed by the reaction with Na melt at 380 ºC. We further examined the compatibility of Na and the SEI layer in heating beyond the solid/liquid phase transition of Na. Figure S5 shows the variation of electrical resistivity


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measured at 25 °C for the Na | NASICON | Na cell with a compression at 20 MPa before and after heating the Na/NASICON assembly at 150 °C for 30 min in an Ar atmosphere. It was found that the improved Na/NASICON interface resistance was completely spoiled. It follows that the SEI layer formed by Na-pressing is incompatible with Na melt and different from the interlayer reported previously.36 It is also noteworthy that this behavior is in stark contrast with the Li/Al-stabilized LLZO system demonstrated by Buschmann et al. previously,11 where the fusion of Li onto the solid electrolyte significantly decreased the interface resistance.

3.5 Cycling stability of NASICON electrolyte The improvement of Na/NASICON interface by pressing distinctly reduces the interface resistance between the Na electrode and NASICON solid electrolyte, which is expected to allow stable Na plating/stripping cycles. The DC cycling behavior of the Na | NASICON | Na cell was explored using a NASICON electrolyte with a thickness of 1 mm. We employed the similar method reported by Sharafi et al. to assess the critical current density (CCD), which is a benchmark current density for sustainability of a solid


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electrolyte operated with metallic electrodes.21 It was confirmed that, without pressing, the cell shorting occurred at < 0.1 mA cm–2 due to the high cell voltage suffering from the high interface resistivity (see Figure S6). The higher resistivity enhances the local electric field at the tip of growing dendrites, which accelerates the metal propagation resulting in a more rapid short.36 Recent studies on Li metal electrode on various solid electrolytes validate that a shortcircuit takes place by the dendrite growth of metal both through transgranular pathway and between grain boundaries,22,49,50 which is hardly prevented by the Monroe and Newman strategy, where a solid electrolyte with adequately high mechanical strength can suppress the dendrite propagation.51

Porz et al. reported that Li filament can

penetrate even through a single-crystalline ceramic electrolyte by initial opening cracks against mechanical resistance followed by crack extension.49 It follows that the high mechanical stability and minimized surface roughness of a solid electrolyte are minimal requirements for blocking a short-circuit. This fact also underlines the importance of the metal/solid electrolyte interface; the key to use a metal anode is a stable interface that could minimize interfacial defects.49


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In this context, the nature of Na/NASICON interface formed by the uniaxial compression was examined by the galvanostatic cycling tests.

Figure 5 (a)

demonstrates the charge and discharge cycling results for the cell pressed at 30 MPa prior to the test at 25 °C.

By virtue of the small cell resistance with the reduced

interface resistivity,52 the cell was stably operated at up to 0.8 mA cm–2 (corresponding to the Na-plating/stripping capacity of 0.4 mAh cm–2). However, the gradual increase in voltage was observed at 0.9 mA cm–2, indicating the degradation of the Na | NASICON | Na cell probably due to the influence of Li dendrite propagation, and became out of order at 1.0 mA cm–2.


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Figure 5. Galvanostatic cycling results of the Na | NASICON | Na cell with pressing at 30 MPa measured at (a) 25 °C and (b) 90 °C. The blue circle and arrow indicate the step where the large voltage change was observed.

In the Li | LLZO | Li system reported previously,21 the substantial increment of CCD by the elevation of operating temperature approaching the melting point of Li was demonstrated. This was explained by the improved Li/LLZO interfacial contact and the reduced charge transfer resistance. The galvanostatic cycling behavior at 90 °C in Figure 5 (b) signifies the further enhancement of CCD for the Na | NASICON | Na cell.


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The fairly small cell resistance at the elevated temperature allows the sustainable Na plating/stripping at up to 3 mA cm–2 corresponding to 1.5 mAh cm–2.52 However, these results indicate that the SEI layer formed by pressing Na on NASICON does not perfectly prevent forming Na filaments, and hence, more improvement of Na/NASICON interface is still necessary from a practical point of view.

4. Conclusions The mechanical compression of Na metal onto a NASICON solid electrolyte can effectively reduce the Na/NASICON interface resistance to 14 Ω cm2 by pressing at 30 MPa, which is comparable to the ionic conductivity for a 100 µm-thick NASICON electrolyte at room temperature.

The activation energy for the Na+ ion transfer at

Na/NASICON interface was evaluated as ca. 0.6 eV, which is fairly high as compared with those for Na/β”-alumina and NASICON/organic liquid electrolytes.

This result

suggests the formation of an interstitial layer between Na metal and NASICON by pressing Na metal even at room temperature.


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As the reduced cell resistance results in the better short-circuit prevention, the Na | NASICON (1 mm thick) | Na cell experienced a short at < 1 mA cm–2. The further increase of critical current density is allowed by raising the operation temperature due to the enhancement of conductivity. From a practical viewpoint, the thickness of a solid electrolyte should be thinner than the NASICON plates used in this study (1 mm). It follows that the resistivity related to the bulk and grain boundary decreases, which is advantageous for the sustainability of Na plating/stripping. At the same time, however, the dendrite growth to short more readily occurs in the thinner electrolyte.

Hence,

further effort to block the dendrite formation is still required for high energy density ASSBs.

Supporting Information. The following files are available free of charge. Load-strain curves for the Li and Na metal plates; XPS spectra for the NASICON electrolytes before and after Na-pressing and impedance measurements; variation of


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the Nyquist plots for the Na | NASICON | Na cell during pressing followed by heating

AUTHOR INFORMATION

Corresponding Author * Dr. George Hasegawa E-mail: [email protected] * Prof. Katsuro Hayashi E-mail: k.hayashi @cstf.kyushu-u.ac.jp

ACKNOWLEDGMENT

This work was supported by a Grant-in-Aid for Scientiﬁc Research (KAKENHI No. 26289235 and JP16H6440) from the Japan Society for the Promotion of Science (JSPS), and the Elements Strategy Initiative to Form Core Research Center, Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. In addition, a part of this work was conducted in Kyushu University, supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of MEXT.


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Table of Contents (Graphical Abstract)


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Figure 1 128x105mm (300 x 300 DPI)


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Figure 2 129x120mm (300 x 300 DPI)



Figure 3 176x107mm (300 x 300 DPI)


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Figure 4 129x128mm (300 x 300 DPI)



Figure 5 84x101mm (300 x 300 DPI)


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Table of Contents 82x43mm (300 x 300 DPI)


NASICON Interface

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