Improving the alkali metal electrode/inorganic solid electrolyte contact via room-temperature ultrasound solid welding

The combination of alkali metal electrodes and solid-state electrolytes is considered a promising strategy to develop high-energy rechargeable batteries. However, the practical applications of these two components are hindered by the large interfacial resistance and growth of detrimental alkali metal depositions (e.g., dendrites) during cycling originated by the unsatisfactory electrode/solid electrolyte contact. To tackle these issues, we propose a room temperature ultrasound solid welding strategy to improve the contact between Na metal and Na3Zr2Si2PO12 (NZSP) inorganic solid electrolyte. Symmetrical Na|NZSP | Na cells assembled via ultrasonic welding show stable Na plating/stripping behavior at a current density of 0.2 mA cm−2 and a higher critical current density (i.e., 0.6 mA cm−2) and lower interfacial impedance than the symmetric cells assembled without the ultrasonic welding strategy. The beneficial effect of the ultrasound welding is also demonstrated in Na|NZSP | Na3V2(PO4)3 full coin cell configuration where 900 cycles at 0.1 mA cm−2 with a capacity retention of almost 90% can be achieved at room temperature.

C ommercial lithium ions batteries (LIBs) have promoted the rapid development of electrochemical energy storage technology in the past decades. However, the energy density of LIBs reached a bottleneck and the safety issues of liquid organic electrolyte used in LIBs were denounced 1,2 . Solid-state alkali metal batteries employing solid-state electrolytes are recognized as the next-generation energy storage technology due to their merits in high-energy density and safety performances as well as long-life time 3,4 . Nevertheless, the practical application of solid-state batteries is hindered seriously by the poor contact between electrodes and solid-state electrolyte as well as the resultant interfacial issues 5 . Such as, the poor contact of interface leads to a huge interface resistance 6,7 , preventing the effective transport of ions; the uneven deposition/stripping of metal anode at the discontinuous contact interface results in the growth of metal dendrites during the electrochemical cycling process, eventually causing the failure of batteries 8,9 . Among various types of solid-state electrolytes, oxide ceramic electrolytes have been closely concerned in view of their high ion conductivity (10 −4~1 0 −3 S cm −1 ) and wide voltage window 10,11 . Especially, the good mechanical strength of oxide ceramic electrolytes can effectively alleviate and inhibit the dendritic growth of alkali metal anode 12,13 . However, the problem of poor contact between metal anode and rigid oxide ceramic electrolytes is most prominent among the solid-state batteries involving in various types of solid-state electrolytes.
A series of strategies have been approached to improve the interface contact between metal electrodes and oxide ceramic electrolytes 14,15 . Static pressing was mostly employed to improve the physical contact between the metal anode and solid-state electrolyte to a certain extent, but the improvement effect of interface contact with acceptable interface impedance is limited 16 . Fusion welding with the assistance of heating alkali metals beyond their melting points (sodium: 98 o C, lithium: 180 o C) was widely used to improve the interface contact of metals anode with solid electrolyte 17 . However, the wettability of molten metal to oxide ceramic was hindered seriously because of the formation of undesired layer on the surface of ceramic electrolyte [18][19][20] . Very recently, ultrasonication was employed to aggressively promote the joining of molten Li with Li 7 La 3 Zr 2 O 12 (LLZO) and this technology was referred as ultrasonic-assisted fusion welding method 21 . No matter the traditional fusion welding or the newly exploited ultrasonicassisted fusion welding, heating operation for the formation of molten metals is required. Apparently, the heating operation will complicate the assemble of batteries and is unsuitable for large-scale application. The introduction of an interlayer with lithiophilic /sodiophilic character, exampling with ZnO 22 , Al 2 O 3 5 , Li 3 PO 4 23 , TiO 2 24 , Li 3 N 25 , and others, between alkali metal anode and ceramic electrolyte was recognized as an effective route to promote the interface contact, resulting in the obvious decrease of the interface resistance and improved uniform deposition of the alkali metal. For example, Lu et al. fabricated a 3D porous structure Na 3 Zr 2 Si 2 PO 12 (NZSP) impregnated with SnO 2 on its surface and then filled with molten sodium, and reported the application performances in solidstate sodium metal battery 26 . Miao et al. reported that the AlF 3 interlayer between the sodium anode and NZSP electrolyte could induce the uniform deposition and stripping of Na 27 . Nevertheless, the interlayer with poor adhesion might partially peel off and lead to the interface contact becomes even worse, accompanying with the volume change of electrode and the complex electrochemical process during cycling. Thus, it is urgent to develop promising interface joining technology to fabricate intimate contact interface with atom-level bonding between metal anodes and ceramic electrolytes.
The application of ultrasonic welding has been successfully demonstrated in joining metal and ceramic in the past years [28][29][30][31] .
High-frequency ultrasonic vibration energy could be converted into the deformation energy, frictional work and limited temperature rise, leading to the quick diffusion of atoms at the interface and the formation of intimate bonding interface 29 . Especially for the joining of metal and ceramic, ultrasonic friction could quickly break oxide film on metal surface and gas film on ceramic surface, promoting the wettability spreading of metal to ceramic [32][33][34] .
In this work, room temperature ultrasonic solid welding without heating operation is demonstrated to fabricate an intimate contact interface between sodium metal anode and Na 3 Zr 2 Si 2 PO 12 ceramic electrolyte. The resultant interface properties and the boosting application performances in solid-state sodium metal batteries are investigated in detail.

Results
Characterizations of Na 3 Zr 2 Si 2 PO 12 electrolyte. Figure 1a depicted the XRD pattern of the prepared Na 3 Zr 2 Si 2 PO 12 ceramic pellets, which could be well indexed as the NASICON-type Na 3 Zr 2 Si 2 PO 12 targeted phase (PDF #84-1200). Two weak peaks positioned at~28.28°and~31.51°were corresponded to the impurity phase of ZrO 2 (PDF #86-1450). The coexistence of small amount of ZrO 2 impurity was a common phenomenon caused by the volatility of Na and P during high temperature calcination [35][36][37][38] . The Nyquist plots of impedance for the prepared Na 3 Zr 2 Si 2 PO 12 sheet at different temperature were shown in Fig. 1b. The impedance spectra of electrolyte consisted of a semicircle at high frequency and a diagonal line at low frequency. The size of semicircle represented the grain boundary resistance, and the intercept of semicircle with X-axis was the total resistance of bulk and grain boundary. The ionic conductivity of the obtained electrolyte at room temperature was calculated to be 4.3 × 10 −4 S cm −1 . With the increasing of test temperature, the semicircle representing the grain boundary resistance decreased gradually, indicating that the grain boundary resistance occupied a smaller proportion of the total resistance at high temperature. The activation energy (E a ) was determined to be 0.291 eV from the slope of the Arrhenius plot in the form of ln (σT) versus 1000/T. The top SEM view of the prepared electrolyte was shown in Fig. 1c. It was shown that the crystalline grains of Na 3 Zr 2 Si 2 PO 12 show a cube-like morphology and some gaps existed between grains. In addition, the cross-section SEM image ( Fig. 1d) of Na 3 Zr 2 Si 2 PO 12 pellet showed that there were voids in the ceramic even though the electrolyte has the acceptable ionic conductivity (4.3 × 10 −4 S cm −1 at room temperature) and high density (the density of the prepared electrolyte pellets was 3.10~3.13 g cm −3 with a relative density of~95%).
Fabrication of intimate contact interface by ultrasound welding. Room temperature ultrasound solid welding was employed to fabricate the intimate contact interface between Na metal anodes and Na 3 Zr 2 Si 2 PO 12 (NZSP) electrolytes, as schematically illustrated in Fig. 2. The Na metal foil was placed on the top of Na 3 Zr 2 Si 2 PO 12 pellet, then the ultrasonic horn of a household ultrasonic cleaner (photographic picture, power and frequency of the household ultrasonic cleaner was shown in Supplementary  Fig. 1) was loaded on the Na metal foil with ultrasonic vibration for~25 s at room temperature (25-27°C) (Supplementary Movie). The fabricated intimate interface between Na anode and Na 3 Zr 2 Si 2 PO 12 electrolyte by ultrasound welding was indexed as UW-Na/Na 3 Zr 2 Si 2 PO 12 . As a comparison, Na/Na 3 Zr 2 Si 2 PO 12 interface without ultrasonic vibration was fabricated by static pressing method (Kejing MSK-160E, China) with a pressure intensity of~300 kg (~26 MPa). Supplementary Fig. 2a displayed the digital photographs of the Na/Na 3 Zr 2 Si 2 PO 12 , in which Na metal anode fabricated by static pressing could be scraped off ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-27473-4 easily from the surface of Na 3 Zr 2 Si 2 PO 12 pellet, revealing the contact between Na and electrolyte pellet is poor. On the contrary, Na electrode still had a large area residue on the surface of Na 3 Zr 2 Si 2 PO 12 pellet for the UW-Na/Na 3 Zr 2 Si 2 PO 12 even after scraping with a knife, indicating the intimate contact between Na foil and Na 3 Zr 2 Si 2 PO 12 pellet with the aid of ultrasonic welding (Supplementary Movie and Supplementary Fig. 2b).
Characterizations of interface properties. The contact property of the interface between Na metal and Na 3 Zr 2 Si 2 PO 12 pellet was evaluated by the cross-sectional scanning electron microscopy (SEM) image, as shown in Fig. 3a. The formed interface between Na metal and Na 3 Zr 2 Si 2 PO 12 pellet for UW-Na/Na 3 Zr 2 Si 2 PO 12 was intimate contact without microscopic void defects. In contrast, the Na/Na 3 Zr 2 Si 2 PO 12 interface (Fig. 3b)  pressing without ultrasound welding exhibited obviously gap between Na anode and Na 3 Zr 2 Si 2 PO 12 electrolyte, resulting in large interfacial resistance because of the poor ions transport at the interface 39 . This result verified that ultrasound solid welding was an effective method to construct intimate contact interface between Na anode and Na 3 Zr 2 Si 2 PO 12 electrolyte. The EDX elemental mapping of the selected cross-sectional area for UW-Na/Na 3 Zr 2 -Si 2 PO 12 interface was reported in Fig. 3c. It was found that the elements of Zr, Si and P pertaining to Na 3 Zr 2 Si 2 PO 12 pellet showed a similar distribution whereas the Na element has a significant diffusion from the Na anode into the surface thin layer of Na 3 Zr 2 Si 2 PO 12 pellet. This revealed that the Na metal could diffuse into Na 3 Zr 2 Si 2 PO 12 pellet along the grain boundaries or the pores of ceramics in view of its rapid plastic deformation under ultrasonic vibration 40,41 . Different from the sodium dendrites that grow along the grain boundary resulted from the poor contact between anode and solid electrolyte, intimate bonding interface with a stable interfacial layer was constructed through interfacial reaction between the diffused Na metal and Na 3 Zr 2 Si 2 PO 12 with the aid of ultrasound. The interfacial reaction mechanism between the Na and Na 3 Zr 2 Si 2 PO 12 induced by ultrasonication was revealed by the XRD data, as disclosed in Supplementary Fig. 3. Comparing with the pristine Na 3 Zr 2 Si 2 PO 12 powder, the ultrasound welding sample of Na 3 Zr 2 Si 2 PO 12 powder with sodium metal exhibits some new diffraction peaks: some are originated from the metal sodium but disappears once exposed in air, and others are indexed to be a newly-formed phase of Na 2 SiO 3 , revealing the formation of Na 2 SiO 3 due to the interfacial reaction. Zhang et al. investigated that Na 3 Zr 2 Si 2 PO 12 could be reduced by metal Na to produce Na 4 SiO 4 and other reduction products at 0 V, and will further to produce Na 2 SiO 3 at a higher voltages 42 . In the present work, the newly-formed Na 2 SiO 3 product was detected during the interfacial reaction under ultrasonication. It was reported that Na 2 SiO 3 was an ion conductor 43  the cross-section and the corresponding EDX elemental mappings for Li/Na 3 Zr 2 Si 2 PO 12 ( Supplementary Fig. 4), Na/Li 7 La 3 Zr 2 O 12 ( Supplementary Fig. 5), Li/Li 7 La 3 Zr 2 O 12 ( Supplementary Fig. 6), Na/Na-β''-Al 2 O 3 ( Supplementary Fig. 7) and Li/Na-β''-Al 2 O 3 ( Supplementary Fig. 8) interfaces revealed that these interfaces constructed by ultrasonication are intimate without microscopic void defects. Above results verified that ultrasound solid welding is an effective route to construct intimate contact interface between alkali metals anode and ceramic electrolytes, beneficiating to settle the interfacial issues denounced in solid-state alkali metals batteries.
Testing of solid-state cells assembled via ultrasonic welding method. The electrochemical impedance spectra of assembled Na | Na 3 Zr 2 Si 2 PO 12 | Na and UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cells were measured to evaluate their interface resistance, as displayed in Fig. 4a. Inset showed the fitted data and corresponding equivalent circuit, where R b , R gb , R int and CPE indicated the bulk resistance, grain boundary resistance, interface resistance and constant phase element, respectively. The left intercept of the arc with x-axis at high frequency represents the bulk resistance of Na 3 Zr 2 Si 2 PO 12 electrolyte whereas the arc represents to the overlap of grain boundary impedance and interface impedance 5 . The sum of R b and R gb could be designated as the total resistance of Na 3 Zr 2 Si 2 PO 12 electrolyte, which was 100 Ω measured in the Fig. 1b. R int was determined for the interface resistance on either side of the Na symmetric cells by subtracting Na 3 Zr 2 Si 2 PO 12 electrolyte resistance from UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW or Na|Na 3 Zr 2 Si 2 PO 12 | Na cell resistance, dividing by two, and then normalizing to the Na electrode surface area. The UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW cell showed a significantly lower total resistance of~140 Ω than the Na | Na 3 Zr 2 Si 2 PO 12 | Na cell (~350 Ω) at room temperature. The interfacial resistance of UW-Na/Na 3 Zr 2 Si 2 PO 12 was evaluated to be~22.6 Ω cm 2 , which was lower than that of Na/Na 3 Zr 2 Si 2 PO 12 (141.3 Ω cm 2 ). This significantly lower interfacial resistance for UW-Na/Na 3 Zr 2 Si 2 PO 12 was dominantly ascribed to the intimate contact and the formed ion conductive Na 2 SiO 3 induced by ultrasonication. The evolution of electrochemical impedance spectra for the assembled UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cell during the storage for 10 days was recorded in Fig. 4b. There was nearly no change for the impedance spectra  Fig. 4 Electrochemical characterizations of symmetrical Na metal coin cells with Na 3 Zr 2 Si 2 PO 12 solid electrolyte. a The electrochemical impedance spectra for the assembled Na | Na 3 Zr 2 Si 2 PO 12 | Na and UW-Na|Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cells. b Electrochemical impedance spectra during the storage for 10 days of the assembled UW-Na|Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cell. c Galvanostatic critical current density test. d Galvanostatic cycling of Na | Na 3 Zr 2 Si 2 PO 12 | Na and UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cells at current density of 0.1 mA cm −2 . e The electrochemical impedance spectra for the initial and cycled UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cell at current density of 0.1 mA cm −2 . f Galvanostatic cycling of UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cell at current density of 0.2 mA cm −2 . with a constant interfacial resistance, indicating the constructed interface by ultrasonication was extremely stable. Galvanostatic critical current density (CCD) was a benchmark to evaluate the electroplating and stripping ability of Na at the interface under gradually increasing current density 45 . The CCD measurement result for the UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cell was presented in Fig. 4c. It was observed that the maximum current density was recorded to be as high as 0.6 mA cm −2 with a capacity of 0.6 mA h cm −2 , which was much higher than that of the most reported values (0.07~0.2 mA cm −2 ) for the Na | Na 3 Zr 2 Si 2 PO 12 | Na symmetric cells without any interfacial modification 27,42,45 . Supplementary Fig. 9 displayed the EIS measurements of the correlated total electrochemical resistance during CCD measurements. When the current density was 0.1~0.35 mA cm −2 , the recorded impedance spectra was changeless, implying the stability of the Na/Na 3 Zr 2 Si 2 PO 12 interface.
With the increase of current density to 0.4~0.6 mA cm −2 , the impedance values of the battery were larger than the initial, implying the effective contact of UW-Na/Na 3 Zr 2 Si 2 PO 12 interface was deteriorated due to Na dissolution and deposition at higher current density. The cycle stability of the assembled cells was evaluated by the Galvanostatic cycling measurements at a current density of 0.1 mA cm −2 , as demonstrated in Fig. 4d. The UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cell exhibited a stable voltage profile with a small overvoltage and without any voltage disturbance during a long cycling time of 1300 h. In contrast, the Na | Na 3 Zr 2 Si 2 PO 12 | Na symmetric cell have a large voltage and short-circuited after only 30 h of cycling, resulting from the poor contact between Na and Na 3 Zr 2 Si 2 PO 12 . The nearly unchanged electrochemical impedance spectra (Fig. 4e) for the initial and cycled UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cell also verified the cycling stability of the ultrasound welding constructed interface. At a higher current density of 0.2 mA cm −2 , it was also found that the assembled UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetrical cell could be cycled firmly with stable voltage and changeless impedance values for a long cycling time ( Fig. 4f and Supplementary Fig. 10). Supplementary Fig. 11a exhibited the cross-section of UW-Na/Na 3 Zr 2 Si 2 PO 12 recovered from cycled Na symmetrical cells at 0.2 mA cm −2 after 100 h cycling. It was observed that the intimate contact interface between Na and Na 3 Zr 2 Si 2 PO 12 could be maintained after cycling. Additionally, the relatively intact cube-like morphology of the electrolyte pellet surface after removing Na foil with ethanol also confirmed the stability of the UW-Na/Na 3 Zr 2 Si 2 PO 12 interface ( Supplementary  Fig. 11b).
Supplementary Table 1 provided a detail comparison about the interface resistance and the cycling stability for Na | Na 3 Zr 2 -Si 2 PO 12 | Na symmetric cells improved with different interface modification routes. It was found that ultrasound solid welding technology facilitates the decrease of interface resistance and improvement of cycling stability comparing with the referring routes, enabling its promising application to settle the interfacial issues of solid-state metal batteries. These results confirmed that the fabricated intimate contact interface induced by ultrasonication not only significantly reduce the interface impedance but also build a stable interface layer to induce the uniform deposition and stripping of Na.
The cycling performance of the assembled UW-Na | Na 3 Zr 2 -Si 2 PO 12 | Na 3 V 2 (PO 4 ) 3 sodium metal battery at 0.1 mA cm −2 was displayed in Fig. 5a. The first discharge specific capacity was recorded to be 110 mAh·g −1 with a coulombic efficiency of 95.27%. After 900 cycles, the discharge capacity was recorded to be 98.8 mAh·g −1 with a capacity retention ratio of 89.81% and high coulombic efficiency of >99.5%, demonstrating the feasibility of full cells based on the highly stable UW-Na/Na 3 Zr 2 Si 2 PO 12 interface. Figure 5b showed electrochemical impedance spectra for the initial and cycled UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na 3 V 2 (PO 4 ) 3 cell. It was found that the total impedance resistance of the cell shows a slight increase from 400 Ω to 530 Ω. This slight increase of total impedance resistance was mainly ascribed to the increasing of cathode interface resistance because the interface resistance of anode was nearly unchangeable. The typical charge/discharge curves of the assembled UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na 3 V 2 (PO 4 ) 3 cell at 0.1 mA cm −2 was displayed in Figure 5c. During 900 times cycling, the overpotential was is stable at~150 mV, which also proved the stability of UW-Na/Na 3 Zr 2 Si 2 PO 12 interface. Fig. 5d  showed rate performance of the assembled UW-Na | Na 3 Zr 2 -Si 2 PO 12 | Na 3 V 2 (PO 4 ) 3 cell when charged/discharged with the current densities ranging from 0.05 mA cm −2 to 0.5 mA cm −2 , which delivered reversible capacity of 104. 4  The corresponding typical charge/discharge curves in Fig. 5e validate the good rate performance of the cell under different current densities between 2.5-4 V. In addition, the polarization increased from 42 mV to 460 mV can be observed with the increasing of current density from 0.05 mA cm −2 to 0.5 mA cm −2 , which was due to the slow diffusion of sodium ions under the subsequent high current density 46 . The stable interface constructed by ultrasonication between Na and Na 3 Zr 2 Si 2 PO 12 enables this excellent rate performance 26 . Supplementary Table 2 provides a detail comparison about the reported electrochemical performances of the solid-state sodium metal batteries basing on Na 3 Zr 2 Si 2 PO 12 electrolyte. UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na 3 V 2 (PO 4 ) 3 cell assembled by the ultrasound solid welding delivers good cycle stability and competitive rate performance.
In summary, room temperature ultrasonic solid welding strategy was demonstrated in this work to construct intimate contact interface between alkali metal anode and oxide solid-state electrolyte. The intimate contact interface with lower interfacial impedance (~22.6 Ω cm 2 ) constructed by ultrasonication enabled the Na symmetric cell could be stably operated at 0.2 mA cm −2 for long cycling time at room temperature. Additionally, the significantly higher critical current density (0.6 mA cm −2 ) illustrated the stable plating/stripping behaviour of Na metal during cycling. As a result, the assembled UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na 3 V 2 (PO 4 ) 3 cell can be well cycled over 900 cycles with a highcapacity retention ratio of 89.81%. This work demonstrated a fast, convenient, low-cost, energy-saving and environmental friendly method to fabricate intimate contact interface between alkali metal anodes and ceramic electrolytes, providing a new gateway to address the interface issues denounced in solid-state metal batteries.

Methods
Solid-state electrolyte synthesis. Na 3 Zr 2 Si 2 PO 12 (NZSP) solid electrolyte was synthesized by the conventional solid-state sintering method. Stoichiometric amount of Na 2 CO 3 (99.5%, Aladdin), ZrO 2 (99.99%, Aladdin), SiO 2 (99.99%, Aladdin) and NH 4 H 2 PO 4 (99%, Aladdin) raw materials were mixed by ball milling with ZrO 2 balls (Φ10 mm) at 1200 r/min for 1 h in environment atmosphere, and then calcined at 1100°C for 12 h in air atmosphere to obtain precursor. 15 wt.% quantities of excessive Na 2 CO 3 and NH 4 H 2 PO 4 were added to compensate the volatilization of Na and P elements at high temperature. The obtained precursor was ball-milled at 1200 r/min for 1 h and pressed into green pellets at 200 MPa, then sintered at 1250°C for 6 h in air atmosphere. The final as-obtained electrolyte pellets were~800 μm in thickness and~15 mm in diameter.
Materials characterizations, cells assemble and electrochemical measurements. The phase structure of the prepared Na 3 Zr 2 Si 2 PO 12 pellet was analyzed by X-ray diffraction technology (ARL Equinox 3000, France). The microstructure and elemental composition were characterized by field emission scanning electron microscope equipped with energy dispersive spectrometer (Quanta FEG, America). Ionic conductivity of the prepared Na 3 Zr 2 Si 2 PO 12 pellet was evaluated by the electrochemical impedance spectroscopy (EIS) of the Au | Na 3 Zr 2 Si 2 PO 12 | Au symmetrical cell operating at a quasi-stationary potential with two-electrode system on an electrochemical workstation (CHI760E, China) in the frequency range from 1 MHz to 1 Hz at open-circuit voltage, 5 mV amplitude. The recording number of data points was 12 (per decade). UW-Na/Na 3 Zr 2 Si 2 PO 12 /Na-UW symmetrical cell was assembled by the proposed ultrasound solid welding for the measurement of interface resistance and cycling stability of the fabricated interface. To evaluate the evolution of interface contact after cycling, the cell was disassembled manually in the glove box by a nipper plier to harvest the solid electrolyte and electrodes for the ex situ SEM measurement transferred by a sealed cans.
The cycle stability of the assembled cells was evaluated by the Galvanostatic cycling measurements at a current density of 0.1~0.2 mA cm −2 at room temperature (25-27°C). The UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na-UW symmetric cells were tested by rate cycling under the initial current density of 0.1 mA cm −2 with an increasing step of 0.05 mA cm −2 to determine the critical current density (CCD) at room temperature (25-27°C). For the assemble of UW-Na | Na 3 Zr 2 Si 2 PO 12 | Na 3 V 2 (PO 4 ) 3 solid-state sodium metal batteries, Na 3 V 2 (PO 4 ) 3 cathode was prepared by mixing the Na 3 V 2 (PO 4 ) 3 /C (Hubei Energy Technology Co., Ltd, China) with acetylene black carbon and PVDF at a weight ratio of 8:1:1 in the solvent of NMP, then the obtained slurry was casted on Al foil and dried at 120°C in vacuum for 24 h. The load of Na 3 V 2 (PO 4 ) 3 /C in the obtained cathode was~1 mg cm −2 . The thickness of the Na 3 V 2 (PO 4 ) 3 /C-based positive electrode was~0.033 mm with diameter of 12 mm. CR 2032 coin-type cells were assembled in an argon-filled glovebox ([H 2 O] < 0.1 ppm, [O 2 ] < 0.1 ppm) using Na 3 V 2 (PO 4 ) 3 /C as the cathode, Na metal as anode and Na 3 Zr 2 Si 2 PO 12 as electrolyte respectively. The Na anode was ultrasound welding on the one side of the Na 3 Zr 2 Si 2 PO 12 pellet as described in above. The Na foil (99.7%, Aladdin) was prepared with~12 mm in diameter and~120 μm in thickness. 7.5 μL of liquid electrolyte (1 M NaClO 4 in EC/DMC (1:1) + 5% FEC) was used to wet the interface between cathode and Na 3 Zr 2 Si 2 PO 12 sheet. The water content of the electrolyte solution ≤50 ppm. The assembled Na metal batteries were galvanostatically charged and discharged in the voltage range of 2.5-4.0 V vs Na/Na + at room temperature (25-27°C) using an automatic battery tester system (Land CT2001A, China). All the electrochemical measurements were performed on a lab bench without temperature control.