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. 2021 Jan 18;7(2):335–344. doi: 10.1021/acscentsci.0c01560

Prevention of Na Corrosion and Dendrite Growth for Long-Life Flexible Na–Air Batteries

Xizheng Liu †, Xiaofeng Lei †, Yong-Gang Wang ‡,*, Yi Ding †,*
PMCID: PMC7908042  PMID: 33655071

Abstract

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Rechargeable Na–air batteries (NABs) based on abundant Na resources are generating great interest due to their high energy density and low cost. However, Na anode corrosion in ambient air and the growth of abnormal dendrites lead to insufficient cycle performance and safety hazards. Effectively protecting the Na anode from corrosion and inducing the uniform Na plating and stripping are therefore of vital importance for practical application. We herein report a NAB with in situ formed gel electrolyte and Na anode with trace residual Li. The gel electrolyte is obtained within cells through cross-linking Li ethylenediamine at the anode surface with tetraethylene glycol dimethyl ether (G4) from the liquid electrolyte. The gel can effectively prevent H2O and O2 crossover, thus delaying Na anode corrosion and electrolyte decomposition. Na dendrite growth was suppressed by the electrostatic shield effect of Li+ from the modified Li layer. Benefiting from these improvements, the NAB achieves a robust cycle performance over 2000 h in opened ambient air, which is superior to previous results. Gelation of the electrolyte prevents liquid leakage during battery bending, facilitating greater cell flexibility, which could lead to the development of NABs suitable for wearable electronic devices in ambient air.

Short abstract

The Na−air batteries have been developed with in situ formed gel electrolyte on a Li modified Na anode. They display ultrastable cycle performance up to 2000 h in ambient air.

Introduction

Current metal–O2 battery technologies with ultrahigh theoretical energy densities have difficulty satisfying the practical application demands for a long cycle life and working in ambient conditions.1−6 This is despite the significant progress in the development of highly efficient cathode catalysts, oxidation-resistant electrolytes, and stable alkali metal anodes.7−12 The Na–air batteries (NABs) are receiving immense attention owing to their inherent cost benefit and extremely low charge overpotential when compared with that of Li–air batteries.3,13−15 In the typical discharge process of NABs, oxygen is reduced at the cathode and is combined with Na+ which comes from the anode to form Na2O2/NaO2, which is a reversible process occurring during the following charging process.3,16−19 The reported discharge/charge process finishes in a pure oxygen or a gaseous environment without moisture or CO2 contaminants. For practical applications, the NABs must be operated in ambient air which is still a big challenge, and numerous problems have arisen. However, little attention was paid to these obstacles regarding NAB systems.

The Na anode corrosion and the decomposition of the electrolyte are the first and foremost issues with NABs.20−22 In ambient air, the crossover of H2O and CO2 from the cathode to the Na anode inevitably leads to Na anode corrosion and the formation of a NaOH/Na2CO3 passivation layer on the anode, which causes severe electrode polarization and premature cell death. In addition, O2– in the reduced state can also transport to the Na anode, resulting in anode corrosion and lower Coulombic efficiency.13,23,24 Moreover, electrolyte volatilization and decomposition by the metallic Na anode also deteriorates the cycle performance.13,25−27 Functional separators and protective layers have been adopted to suppress these gas contaminant crossover behaviors. La and co-workers introduced a mechanically reinforced membrane to stop gas species crossover to the Na anode, enhancing its stability.28 Zhou et al. tailored the Na anode by in situ preparation of a NaF-rich protective film to prevent electrolyte degradation, thus improving the NAB performance.29 The employment of an ion-selective Na-Nafion separator has also proven to be effective in suppressing the side reactions in NABs.30 In addition, Chen and Cui et al. developed polymer electrolytes and observed a robust cycle ability of the Na-based batteries.31,32 The formation of Na dendrites during repeated cycles is another critical issue. Near the rough surface of the Na anode, the inhomogeneous distributions of local current and Na+ concentration unavoidably lead to deposition hot spots and formation of Na dendrites.15,33,34 The large dendrites penetrate the separator and result in short circuit as well as battery failure. Zhang et al. report the utilization of a bimetallic Li-Na alloy anode and a flexible passivation film for a dendrite-free Li-Na alloy–O2 battery. The successive supplying of Li+ from the Li-Na alloy ensures an electrostatic shield effect, thus inducing uniform Na plating.13 Peng and co-workers constructed a sodiophilic interphase using an oxygen functionalized carbon nanotube network that facilitated homogeneous Na nucleation with no dendrite formation. The fabricated Na–air battery displayed a cycle life of more than 100 h.35 An inorganic solid-state electrolyte could sustain the dendrite penetration, but its low ionic conductivity and poor interfacial contact seriously prohibit its practical applications in NABs.36 All of this aforementioned progress provides useful information for completely understanding the Na anode degradation mechanism and finding possible solutions for further improving the performance of NABs.

Previously, we reported the in situ preparation of a gel electrolyte through a cross-linking reaction between Li-EDA and an ether-based solvent.37 However, the corresponding reactions cannot occur based on Na-EDA. Herein, we proposed a new Li modified Na anode instead of a pure Na anode to obtain a Na-contained gel electrolyte within the fabricated cells. The Li at the surface serves as a sacrificial layer to form Li-EDA cross-linked with the ether-based solvent. The residual trace amount of Li alloyed with Na could regulate the Na deposition and ensure dendrite-free performance. The crossover of H2O and O2 was greatly suppressed and the stability of the metallic Na anode was clearly improved, facilitating the operation of NABs in ambient air with superior cycling performance.

Results and Discussion

Gel Characterization and Mechanism

The electronegativity of metallic Na is lower than that of metallic Li which suggestions its higher reactivity in ambient air. The Na anodes in metal–air batteries can be easily corroded and thus exhibit poor electrochemical performance.38 As shown in Figure S1, the metallic luster of Na disappeared rapidly when exposed to ambient air, and it quickly converted to NaOH and Na2CO3 within 1 h. For metallic Li, a black Li3N layer appeared after 5 min and became dark with prolonged exposure time. The corrosion of Na in ambient air is more severe than that of Li, and it is therefore more urgent to protect the Na anode of the metal–air battery. The cross-linking of EDA with the ether solvent was achieved at the Li anode surface (Figure S2) for ambient Li–air batteries; unfortunately, it failed with a Na anode (Figure S3). We herein proposed a new strategy by using a Li modified Na (Na@Li) composite anode (Figure S4) to obtain a gel electrolyte within the assembled cells. Figure 1a and Scheme S1 schematically illustrate the G4 gelation process. The surface Li reacted with EDA to form a LiEDA layer spontaneously, and then cross-linked with G4 to form a gel electrolyte at the surface of the Na anode. Almost all of the surface Li converted into LiEDA with only a trace amount remaining on the Na surface. With a fresh Na anode, the similar reaction has not been observed. Figure 1b shows the feasibility of the preparation of gel electrolytes with a Na@Li anode. A more detailed gelation process is displayed in Figure S5. During the gelation process, there is an exponential growth of viscosity and a fluctuation of ionic conductivity. As disclosed in Figure 1c and Figure S6, the rapid increase of viscosity after 5 days indicates the formation of a gel. There is a slight increase in ionic conductivity at the third and fourth day, which is associated with dissolution of LiEDA and the increase of ionic concentration (Figure 1c and Figure S7). It is observed that the ionic conductivity decreases with the rising of viscosity, accompanied by the cross-linking reactions (Figure S8). The 1H NMR and 13C NMR analyses (Figure 1d,e and Figure S9) confirm that no obvious difference in chemical shift before and after gelation is seen. The slight shift and newly appeared peaks are associated with the cross-linking reactions.39 Fourier transform infrared spectra (FTIR, Figure 1f) were obtained to further reveal a rather similar chemical structure of the G4 gel and its liquid counterpart, especially in the fingerprint region of 1300–400 cm–1. Only a new shoulder peak appears at about 1600 cm–1, which is associated with the shear vibration of −NH2 or the bending vibration of N-H.40,41 Markedly, the FTIR results show that the G4 gel contained both G4 and EDA components.

Figure 1.

Figure 1

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In situ gel electrolyte preparation and characterizations. (a) Schematic illustrating the in situ preparation of gel electrolyte. (b) Digital photos of gel electrolyte formation with Li covered Na plate. (c) The ionic conductivity–viscosity–time curve during the formation process of the gel electrolyte with 0.1 M NaCF3SO3. (d–f) 1H NMR spectrum (d), 13C NMR spectrum (e), and FTIR spectra (f) of pure solvents and gel G4.

To further understand the gelation mechanism, we calculated the Gibbs free energy of possible chemical reactions during the cross-linking process by using density functional theory (DFT) calculation. As disclosed in Figure 2a, the ΔG for the reaction between metallic Li and EDA is negative while a spontaneous reaction occurred. Moreover, the Li from LiEDA or Li2EDA could cross-link with O from the G4 molecule to reduce the free energy and achieve a stable state. The geometric structures of the LiEDA-2G4 and Li2EDA-2G4 are shown in Figure 2b. It can be found that a strong coordination bond of Li–O is observed. However, the similar reaction between metallic Na and EDA cannot occur spontaneously. The following cross-linking reaction with G4 is thus stopped. Moreover, the H–O coordination bond cannot be obtained, and thus no gel formed when there is only pure EDA and G4. More detailed data can be found in Tables S1 and S2. Furthermore, plenty of H2 can be collected when EDA reacted with Li (Figure S10), while no bubble was observed with EDA and Na.

Figure 2.

Figure 2

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Gibbs free energy and noncovalent interaction maps of possible gel unit. (a) The Gibbs free energy (ΔG) of possible reactions among Na, Li, EDA, G4, LiEDA, and Li2EDA. (b) The geometric and interaction structures of LiEDA-2G4 and Li2EDA-2G4.

Electrochemical Characterization

To illustrate the effects of the G4 gel in suppressing the formation of Na dendrites, Na||SS cells were assembled to investigate the reversibility during Na plating/stripping. As shown in Figure 3a, a lower overpotential with the gel electrolyte is observed, which remains below 20 mV even after 300 h. However, with liquid electrolyte, there is a gradual increase of electrode polarization after only 70 h and a sudden surge in voltage occurs at the 96th hour, which can be ascribed to short circuiting by Na dendrites. With the Na@Li electrode, there is an electrostatic shield effect by Li+ which can be continuously supplied by the residual Li at the Na surface.42 This induces uniform Na deposition and dendrite-free performance. Linear sweep voltammetry was adopted to study the electrochemical stability of the electrolytes. Figure 3b clearly shows that G4 gel is above 5.50 V. This means that the electrochemical stability has been clearly improved. The electrochemical performance of NABs with in situ gel electrolytes has been evaluated in ambient air. Figure 3c discloses the voltage profiles at different discharge/charge current densities with a limited capacity of 500 mAh g–1 (based on the cathode catalyst). It delivers a discharge plateau at about 2.27 V when discharging at current densities of 100 and 250 mA g–1. This is comparable with the previous results of NABs in a pure O2 environment.3 During the discharging process, O2 is reduced at the cathode and combines with Na+ to form Na2O2 or NaO2 in pure O2.17 In the present study, the Na2O2 or NaO2 would react with H2O and CO2 in ambient air, and the final discharge products should be NaOH and Na2CO3. The discharge plateau decreases to 2.19 V at a higher current density of 500 mA g–1. During the following charging process, there are three charging plateaus which correspond to the decomposition of NaO2/Na2O2, NaOH, and Na2CO3, respectively. It is observed that the charging overpotentials of NaOH and Na2CO3 are much higher than that of NaO2/Na2O2, and a higher efficiency cathode catalyst should be further developed. This is similar to the situation with Li–O2 batteries in ambient air.

Figure 3.

Figure 3

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Electrochemical performance with liquid and gel electrolytes. (a) Na plating/stripping curves of Na||SS cells with gel or liquid electrolytes. The current density is 1 mA cm–2, and 1 mAh cm–2 of Na is plated and stripped per cycle. Insets: enlarged profiles at the five consecutive cycle numbers noted. (b) Linear sweep voltammetry (LSV) of in situ G4 gel and liquid electrolytes with 0.5 M NaCF3SO3 at a sweep rate of 1 mV s–1. (c–e) Electrochemical performance of ambient Na–air batteries. The discharge/charge profiles at different current densities of 100, 250, and 500 mA g–1 (c), selected cycles of discharge/charge profiles at a current density of 250 mA g–1 (d), and long-term cycling of ambient Na–air batteries (e) and insets: enlarged profiles at the five consecutive cycle numbers noted. The RH is about 10%, and the temperature is about 25 ± 2 °C.

Figure 3d,e shows the cycle performance of NABs in ambient air at a current density of 250 mA g–1 (room temperature, relative humidity 10%), indicating a remarkable cycle performance up to 500 cycles, which corresponded to 2000 h; it is superior to previous results (Table S3). This superior cycle performance can be attributed to the improved protection of the Na anode by the in situ formed gel electrolyte and the Li+ electrostatic shield effect. Moreover, we further optimized the amount of residual Li on the Na surface by controlling the EDA processing time and thickness of Li. As disclosed in Figure S11, more stable electrochemical performance can be observed with more residual Li on the Na anode surface. The discharge plateau remains at 2.10 V during most repeated cycles, while only slightly increasing the charging voltage from 4.10 V (50th cycle) to 4.27 V (500th cycle). The MnO cathode catalyst was revealed to be highly efficient toward the decomposition of NaOH and Na2CO3. Performance fading became apparent after 480 cycles, and the discharge terminal voltage gradually decreased to less than 2.00 V. To our knowledge, this is one of the best performances of NABs in ambient air.20,38,43 The accumulation of undecomposed discharge products and some products from the side reactions on the cathode lead to the performance deterioration. We therefore reassembled the cells with a cycled Na anode (for more than 2000 h) and a fresh cathode. As shown in Figure S12, it displayed stable performance for more than 50 cycles. These results illustrate that the Na anode had been well protected and is suitable for long-term cycling in ambient air. The improved cycle performance is mainly associated with the anticorrosion performance of the Na anode endowed by the in situ formed gel electrolyte.

Gas Permeability

The gas permeability experiments were carried out on the G4 gel and liquid electrolyte to further investigate the anode protection mechanism. As illustrated in Figure 4a, gas diffusion in the gel electrolyte was greatly suppressed and thus anode corrosion was inhibited. The measurement was carried out by homemade equipment as shown in Figures S13 and 14. The desiccant-silica gel (DSG) was adopted as an indicator of the moisture diffusion, while the O2 and CO2 were detected by gas chromatography. There are three DSG particles locked in the thin glass tube that allows vapor to pass through the Celgard separator soaked with the G4 gel or liquid G4 electrolyte. As observed in Figure 4b, the first particle turned pink after about 1 h for G4 liquid, while it took 4 h for the G4 gel. It took 3 (4) hours for the second (third) particle to change into pink, respectively. Meanwhile, the corresponding time was 8 (10) hours for the G4 gel. Thus, H2O vapor diffusion with the gel electrolyte was suppressed by a factor of approximately 2.5 compared to that of the corresponding liquid electrolyte. The amount of O2 crossover is only 40% for the gel (Figure 4c). There is little difference for the case of CO2 diffusion in the gel electrolyte. No CO2 can be detected during the initial 20 min, while the diffused CO2 increases rapidly afterward. The amount of CO2 that crossed the liquid electrolyte is about 55% for the G4 gel. This may be associated with the promoted absorption of CO2 by EDA contained in the gel. The suppression of H2O and O2 crossover remarkably enhanced the Na anode stability during cycling in ambient air.

Figure 4.

Figure 4

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Improved anti-gas permeability studies. (a) Schematic illustration of the anti-water vapor/oxygen/permeability test after electrolyte gelation. (b) Digital photos of the homemade anti-water vapor permeability test and recorded time-dependent water vapor diffusion contents. Inset: data of the tests. (c, d) The relative contents of permeated O2 (c) and CO2 (d) by GC.

Anode and Cathode Characterization

The morphology and spectral characterizations of the cycled Na anode are shown in Figure 5. The surface morphology of Na@Li is smooth and flat as shown in Figure 5a. After the formation of the LiEDA layer, its surface becomes rough (Figure 5b). This is just an intermediate state before the gelation. However, a relatively smooth surface of the cycled Na anode was observed (Figure 5c) with in situ formed G4 gel, which indicates the prevention of Na corrosion and dendrite growth. The XRD patterns in Figure 5d reveal that NaOH and NaOH·H2O were generated after being cycled for 500 times, while metallic Na was still present in this cycled anode. Figure 5e shows the FTIR spectra that further confirmed the G4 gel and NaOH on the anode surface. In addition, the typical wide peak within 1350–1550 cm–1 corresponds to Na2CO3 which reacted with CO2 form the gel electrolyte. And this Na2CO3 film contributes to the stability of the metallic Na anode.44 The gel electrolyte was also collected from the surface of the cycled Na anode and investigated by NMR. As shown in Figure S15, there are no obvious chemical shifts for both of 1H NMR and 13C NMR compared with the fresh G4 liquid, suggesting the superior stability of the as-prepared gel electrolyte and robust surface layer on the surface of the Na anode. These results imply that the suppression of H2O and O2 crossover of electrolyte can distinctly protect the metallic Na anode from corrosion by containment gases, and thus facilitates the long-term operation of NABs in ambient air.

Figure 5.

Figure 5

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Characterizations of the cycled Na anodes. (a–c) SEM images of pristine Li modified Na anode (a), LiEDA modified Na anode (b), and cycled Na anode (c) for an ambient Na–air battery that utilized a gel electrolyte. (d, e) XRD patterns (d) and FTIR spectra (e) of cycled Na anode.

The discharge products on the cathode were analyzed by multiple techniques to further investigate the battery reaction mechanism. The electrodes were first subjected to galvanostatic discharge/charge for a certain time (Figure 6a), and the cathodes were retrieved from the disassembled cells. The clear and clean cathode surface (Figure 6b) was covered by a dense deposit layer (Figure 6c) after discharging. This deposit layer disappeared, and the surface recovered (Figure 6d) after recharging. The process of gradual generation/decomposition of discharge products can be more clearly observed in Figure S16. The amorphous appeared first, and then large particles accumulated together, and finally a sheetlike products layer formed. The main phase of discharge products was confirmed as Na2CO3 by XRD (Figure 6e and Figure S17). The NaO2/Na2O2 and NaOH have not been detected due to their low crystallinity and fast reactions with CO2 in ambient air. Besides, the peaks at around 1454 and 879 cm–1 of the discharged cathode originated from the carbonate.44 These peaks disappeared in the charged cathode, which suggests decomposition occurred. These results demonstrate that the main discharge product in ambient air is Na2CO3 and illustrate the high reversibility accompanying the repeated discharge/charge cycles. Therefore, the three charging voltage plateaus can be ascribed to the decomposition of NaO2/Na2O2 and NaOH with low crystallinity, amorphous Na2CO3, and crystalline Na2CO3, respectively.45−47

Figure 6.

Figure 6

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Examinations of the discharge products on the cathodes of Na–air batteries with gel electrolyte. (a) The corresponding voltage profiles with a limited capacity of 2500 mAh g–1 at a current density of 250 mA g–1. (b–d) SEM images of pristine (b), discharged (c), and charged (d) cathodes. (e, f) The corresponding XRD patterns (e) and FTIR spectra (f) of cathodes at different states.

Laminated cells with the in situ formed gel electrolyte were prepared to investigate their potential application as batteries for flexible devices. The light-emitting diode (LED) display can be illuminated by one NAB pouch cell whether in flat, bended, folded, or twisted conditions (Figure S18a). Illumination of the LED display can be well-maintained, even with the puncturing or cutting of the pouch cells (Figure S18b). The superior electrochemical performance, excellent flexibility, and safety in ambient air can be attributed to the in situ formed G4 gel on the Na anode assisted by the Li sacrificial layer. (1) The in situ fabrication strategy endows good interfacial contact between the electrode and solid electrolyte, (2) the as-formed gel electrolyte served as a protection layer that enhanced resistance to corrosion and guaranteed battery utilization in ambient air, (3) the as-introduced Li improved the ionic conductivity and directed the Na deposition, thus facilitating nondendrite performance, and (4) the complete protection of the metallic Na anode prohibited cells from sudden death or degradation caused by anode corrosion, puncture and deformation, therefore significantly enhancing the cycle life and safety of flexible NABs in ambient air.

Discussion

In summary, a Na-based gel electrolyte was achieved by an in situ method within assembled NABs toward their operation in ambient air. Surface Li served as a sacrificial layer that reacted with EDA to form a LiEDA layer, and then cross-linked with G4 molecules to complete the electrolyte gelation. Both H2O and O2 crossover was restrained, and thus Na anode corrosion and electrolyte decomposition were effectively avoided, ensuring the operation of the corresponding NABs in ambient conditions. The possible gel formation mechanism has been disclosed by DFT calculation. Furthermore, highly reversible Na stripping/plating performance was observed in a Na||SS cell. There was no obvious increase in electrode polarization over 300 h, compared with a rapid increase for bare Na electrodes with a liquid electrolyte. This may be associated with residual trace amounts of Li that could induce uniform Na deposition without Na dendrite formation through a Li+-induced electrostatic shield effect. Benefiting from these enhancements, the NABs display a robust cycle performance over more than 2000 h in ambient air. Furthermore, laminated NABs with in situ fabricated gel electrolytes demonstrated robust flexibility even under harsh conditions, showing particular promise for use in wearable devices. The present results may inspire more studies on high energy density alkali metal–O2 batteries in practical ambient air and corresponding fabrication of flexible devices.

Experimental Methods

Preparation of Na Anode

The anode of the Na–air battery was fabricated by a sample aminating treatment on the surface of Li modified Na (Na@Li). First, Na@Li was prepared by a cold-rolling bonding process of sodium metal (99%, Aladdin) and ultrathin lithium metal foil (thickness 46 μm, 99%, China Energy Lithium). Subsequently, the Na@Li was processed by 2% ethylenediamine (EDA, 99.5%, Aladdin)/tetraethylene glycol dimethyl ether (TEGDME, G4, 99%, Aladdin) solution for 1 day, and the LiEDA@Na anode is obtained. All of the aforementioned operations were carried out in an Ar-filled glovebox.

Preparation of MnO

Porous MnO was prepared according to our previous work.37,48 The typical synthesis procedure was 1 g of Mn(CH3COO)2·4H2O and 1.5 g of polyvinylpyrrolidone dissolved in 100 mL of absolute ethanol to form a transparent solution. Then, the mixture was maintained at 85 °C for 4 h under magnetic stirring. The resulting white precipitate was washed with absolute ethanol for several times, and then dried at 50 °C overnight. The porous MnO was obtained by calcination at 300 °C for 2 h under an Ar-flow atmosphere.

Preparation of the G4 Gel Electrolyte

First, the Na-Li composite metal foils were immersed in G4 electrolyte with 4% EDA (volume ratio) for 2 days. The electrolyte was 0.1 M sodium trifluoromethanesulfonate (NaCF3SO3) in G4. Second, the metal foils were separated and stored in the residual solution at room temperature. The G4 gelation gradually occurred within 3 days. All of the experiments were completed in an Ar-filled glovebox.

Na–Air Battery Assembly

The air cathode was prepared by mixing Kejten black (KB), MnO, and polytetrafluoroethylene (PTFE) at a weight ratio of 4.5:4.5:1, rolling into a film and coated on a carbon paper current collector. The total mass loading of composite of MnO and KB is about 1 mg cm–2. The air cathode and LiEDA@Na anode were separated by a Whatman glass fiber (GF/A) separator which was immersed by electrolyte (0.5 M NaCF3SO3/G4) and sealed into coin-type CR 2032 with air holes. The G4 electrolyte gelation gradually occurred after battery assembly. To ensure the complete G4 liquid gelation, the as-fabricated cells were left to stand for more than 4 days. All of the experiments were carried out in an Ar-filled glovebox. No unexpected or unusually high safety hazards were encountered.

Assembling of the Flexible Pouch-Cell Na–Air Battery

The pouch-cell Na–air battery was assembled with a Na@Li anode, a glass fiber separator, an air cathode, and a 0.5 M NaCF3SO3/G4 with 2% EDA electrolyte for easily gelation. The packaged cell was rested for more than 5 days. All of the operations were completed in an Ar-filled glovebox.

Characterization

X-ray diffraction (XRD) measurements were performed on a MiniFlex600 diffractometer (Rigaku) with Cu K radiation. A scanning electron microscope with FEG (SEM) was conducted using a Verios 460L microscope (FEI). Fourier transform infrared spectroscopy (FT-IR) was recorded using a Frontier Mid-IR FTIR spectrometer (PerkinElmer). The viscosity of the gel electrolyte was measured by a rotating viscometer SNB-1 instrument (Techcomp, 0∼100 Pa·s). A self-made device was used to collect reaction gas of Li and EDA in an Ar-filled glovebox; after that, the gas product was analyzed by a gas chromatograph (GC9790II, FULI INSTRUMENTS). The separator (Celgard 2400) with blank liquid and gel electrolytes were used for gas permeability tests; the gas permeability tests of carbon dioxide and oxygen were performed using a self-designed experimental device (I) and GC. In addition, the gas permeability of water vapor was analyzed by another self-designed experimental device (II) and silica gel (SCR) used as an indicator. 1H NMR and 13C NMR spectra of liquid and gel G4 were collected on a Bruker GPX 400 MHz spectrometer with D2O as a deuterated solvent.

Electrochemical Measurements

The Na||SS cells were cycled at current densities of 1.0 mA cm–2 with a fixed capacity of 1.0 mAh cm–2 by a LAND testing system (LANHE CT2001A). The galvanostatic discharge/charge performances were conducted on an Arbin multichannel electrochemical testing system (BT 2043) at a current density of 250 mA g–1 with a limited capacity of 500 mAh g–1 within the potential window between 1.5 and 4.8 V. All of the results for specific capacities and current densities were calculated based on the total mass of MnO and KB. The linear sweep voltammograms (LSV) of G4 liquid and G4 gel electrolytes were investigated by the linear sweep voltammograms on an electrochemical workstation (CHI 760E) at a scanning rate of 1.0 mV s–1. The electrolyte was sandwiched between lithium anode and stainless steel sheet. The conductivity of G4 gel electrolyte was performed by a conductivity meter (DDSJ-308F, INESA). All electrochemical measurements were carried out in ambient air with a relative humidity (RH) of 10% and at room temperature.

Theoretical Calculations

DFT Calculations

The molecular geometries at ground and transition states were optimized using density functional theory (DFT) at the PBE0/def2-SVP level. A frequency analysis of the optimized molecular was also carried out at the same functional and basis set to obtain the correction term of Gibbs free energy. The single-point calculations at a higher level of m062x/def2-TZVP were then performed to obtain the more precise electronic energy. Finally, the electron energy plus the free energy correction term is the Gibbs free energy. All the calculations above were performed using the Gaussian 16c program suite.49

Acknowledgments

This work was financially supported by the National Key Research and Development Program of China (2018YFE0124500, 2019YFA0705700, and 2018YFE0201702), the National Science Fund for Distinguished Young Scholars (51825102), and the National Natural Science Foundation of China (U1804255). The authors would like to thank Mr. Chao Ma at Tianjin University of Technology and Dr. Songyan Bai at Seoul National University for their support in image design and valuable discussions.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.0c01560.

  • Photos for pure Li metal anodes and their evolution in ambient air and gelation process on different anode, the viscosity and ionic conductivity of G4 electrolyte, the enlarged NMR data and gas analyses in the reaction of Li and EDA, selected cycle performance of Na–air battery with cycled anode and fresh cathode, the scheme and photo of water vapor penetration experiment and oxygen penetration experiment, the XRD and SEM of cycled cathode, flexible and anti-abuse test of pouch-cell Na–air battery, and Gibbs free energy of different components during in situ gelation process (PDF)

Author Contributions

§ X. Liu and X. Lei contributed equally. X. Liu and Y. Ding conceived the project and designed the experiments. X. Lei performed the electrochemical studies and characterizations. X. Liu and X. Lei analyzed the results in which Y. Ding contributed. X. Liu and X. Lei co-wrote the manuscript in which Y.-G. Wang and Y. Ding made the revision. All of the authors discussed the results and commented on the manuscript.

The authors declare no competing financial interest.

Supplementary Material

oc0c01560_si_001.pdf (1.2MB, pdf)

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