Gel Adsorbed Redox Mediators Tempo as Integrated Solid‐State Cathode for Ultra‐Long Life Quasi‐Solid‐State Na–Air Battery

In metal–air batteries, the integrated solid‐state cathode is considered a promising design because it can solve the problem of high interfacial resistance of conventional solid‐state cathodes. However, solid discharge products cannot be efficiently decomposed in an integrated solid‐state cathode, resulting in batteries that are unable to operate for long periods of time. Herein, an integrated solid‐state cathode (Gel‐Tempo cathode) of sodium–air batteries (SABs) capable of promoting efficient decomposition of discharge product Na2O2 is designed. The Gel‐Tempo cathode is synthesized by cationic–π interaction of redox mediator 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (Tempo) and ionic liquid with carbon nanotubes. The Gel‐Tempo cathode serves multiple functions as a redox mediator, flame retardancy, and high stability to air. In quasi‐solid‐state SABs, the Gel‐Tempo cathode reduces overpotential to 1.15 V and improves coulomb efficiency to 84.5% (at a limited discharge capacity of 3000 mAh g−1) compared to gel cathodes. Experiments and density functional theory calculations indicate that Tempo significantly reduces the Gibbs free energy in the decomposition reaction of Na2O2, and high Tempo content is more conducive to enhancing the decomposition kinetics of Na2O2 and hence resulting in an ultra‐long cycle life (1746 h). This work is crucial to promote practical applications of SABs, providing guidelines for functionalization design of integrated solid‐state cathodes for metal–air batteries.


Introduction
With the increasing environmental and energy problems, the development of clean and renewable electrochemical energy storage technologies has been widely researched and developed. [1]odium-air batteries (SABs) have been recognized as a promising energy storage technology because they combine the advantage of high energy density of metal-air batteries with the high abundance feature of sodium. [2]Nonetheless, like other existing alkaline metal-air batteries, SABs use liquid electrolyte in cathode, which faces serious side reactions due to chemical and/or electrochemical instabilities of electrolyte.In addition, there are some potential risks for liquid electrolyte, such as volatilization, leakage problems, and thermal failure issues at high temperatures, which limit the practical application of SABs. [3]ortunately, safer solid electrolytes have been acknowledged as competitive alternative to liquid electrolyte that can be used in metal-air batteries. [4]owever, despite the noteworthy superiorities of solid electrolytes, they inevitably bring large interfacial impedance, which will lead to serious attenuation of battery performance. [5]A promising strategy to solve these problems is to construct integrated solid-state cathode, such as solid electrolyte containing catalyst design, which can largely improve the interface contacting properties. [6]A solid-state cathode of nitrogen-doped CNT in situ grown on zeolite X membrane was reported recently with high safety and air stability. [7]A more recent work deposited carbon nanotubes (CNT) onto a solid electrolyte of Li 7 La 3 Zr 2 O 12 as integrated cathode, effectively reducing the interfacial impedance of battery. [8]Although great progress has been made in recent years, integrated solid cathode designs for metal-air batteries still suffer from some problems.The interface contacts between the insoluble discharge products (i.e., Na 2 O 2 for SABs) and cathode is poor, which leads to an unsatisfactory decomposition reaction of discharge products.In addition, their accumulation on the cathode surface to block O 2 transport channels result in highly limited cycling and even sudden death of the battery.For these reasons, the cycling life of all quasi-solid-state metal-air batteries so far is lower than 1000 h, which is insufficient for practical applications. [9]Therefore, how to design an integrated solid cathode that can achieve efficient decomposition of insoluble discharge products has become the focus and difficulty of current research in quasi-solid-state metal-air battery.
In this work, an integrated solid-state cathode (Gel-Tempo cathode) of quasi-solid-state SAB capable of promoting efficient decomposition of the discharge product Na 2 O 2 has been rationally designed.Gel-Tempo cathode was prepared by a cationic- interaction of redox mediator Tempo and ionic liquid of 1-Ethyl-3methylimidazolium Tetrafluoroborate ([C 2 C 1 im]BF 4 ) with CNT.Experiments and density functional theory (DFT) calculations indicate that, in Gel-Tempo cathode, Tempo tends to be parallelly adsorbed on the basal plane of CNT and high Tempo content was much more conducive for enhancing the decomposition kinetics of Na 2 O 2 .In Na 2 O 2 decomposition reaction, Tempo not only acts as a mobile carrier to transfer electrons to increase the interaction areas, but also significantly reduces the Gibbs free energy.Benefiting from these merits, the fabricated quasi-solid-state SABs with high Tempo content Gel-Tempo cathode realized a state-ofthe-art cycling life of over 1746 h (72 days) in ambient, which surpass those of all the reported quasi-solid-state metal-air batteries.

Results and Discussion
Figure 1a shows the preparation procedures of Gel-Tempo cathode.Specifically, CNT and Tempo were mixed into [C 2 C 1 im]BF 4 and dispersed by a continuous ultrasound to complete a gelation.The formation of gelation was attributed to the cationic- interaction between both of Tempo and [C 2 C 1 im] cation and -electrons on CNT surface, which was verified in subsequent Raman characterization and DFT calculations. [10]As shown in Figure 1b, original CNT exhibits a highly tangled bundle morphology, [11] while CNT is dispersed into a uniform 3D network after gelation due to the cationic- interaction between [C 2 C 1 im] cations and CNT (Figure 1c). [12]Remarkably, the 3D network of CNT was still able to form in the Gel-Tempo cathode (Figure 1d), which suggests that the introduction of Tempo did not impede forming gel.As shown in Figure 1e, compared with EPR spectrum of gel, Gel-Tempo consists of three narrow superimposed peaks due to nitrogen-oxygen radical (N-O•) anisotropy of Tempo, which indicates the stable presence of Tempo in Gel-Tempo. [13]o understand the composition and formation mechanism of Gel-Tempo cathode, a series of testing techniques have been employed.As shown in Figure 2a, the crystallinity of Gel-Tempo with different Tempo addition amount was evaluated by XRD.It is seen that the characteristic peak at 22.3°(Figure S1a, Supporting Information) of CNT in Gel-Tempo-X (X represents the additive amount of Tempo in gel = 40, 80, and 100 mg) did not change significantly, while many sharp peaks were observed in XRD pattern of Gel-Tempo-120 owing to the excess of crystalline Tempo.According to the XRD pattern in Figure S1b (Supporting Information), sharp peaks at 12.2°, 16.3°, 17.1°, 21.1°,  24.7°, and 35.2°belong to Tempo phase, which is consistent with XRD pattern of Gel-Tempo-120 cathode.Therefore, the content of Tempo in Gel-Tempo cathode should be controlled within the range of 0-100 mg.XPS was further employed to understand the chemical state and surface composition of samples.XPS spectra (Figure 2b) of gel and Gel-Tempo-X show feature peaks of C 1s (284.8 eV), N 1s (398.9 eV), O 1s (532.1 eV), and F 1s (685.6 eV).The calculated data according to XPS of element atomic percentage of gel and Gel-Tempo-X was listed in Table S1 (Supporting Information).In Gel-Tempo, the content of N and F elements gradually decreased with the increase of Tempo addition amount, while the content of C and O elements increased, indicating that content of Tempo has been successfully regulated. [14]Figure 2c shows N 1s XPS spectra of gel and Gel-Tempo-X (X = 40, 80, and 100).In contrast to gel, a new characteristic peak at 401.3 eV was detected corresponding to N-O• of Tempo in Gel-Tempo-X. [15]he peak area of N-O• increased with the rise of the addition of Tempo, which may be attributed to the increased occupancy of Tempo on CNT surface.In Raman spectra, D peak (structure defect in CNT), G − peak (transverse vibrations in CNT), and G + peak (longitudinal vibrations in CNT) of gel and Gel-Tempo-X slightly shifted to higher regions compared with CNT (Figure 2df) due to a cationic- interaction during the formation process of gel. [16]It is noted that D, G − , and G + characteristic peaks of Gel-Tempo-X moved to a higher region than gel, implying the addition of Tempo enhanced the cationic- interaction. [17]Furthermore, with the increase of Tempo content, the characteristic peak intensity of pyrrole ring of Tempo gradually increased, which further proves the stable presence of Tempo in Gel-Tempo-X. [18]DFT calculations were performed to understand the adsorption behavior of Tempo and [C 2 C 1 im] cation on CNT surface.Figure 2g illustrates the possible adsorption types of Tempo on CNT surface, which indicates Tempo prefers lying with carbon ring paralleling to CNT due to cationic- interaction.In addition, the projected density of state (pDOS) also illustrated considerable orbital interactions between Tempo and CNT owing to the cationic- interaction (Figure S2a, Supporting information).The small unsymmetrical peak around the Fermi level from Tempo could be attributed to N-O• (Figure S2a,b, Supporting Information).The adsorption energy of Tempo was −0.52 eV, and there is no chemical bond formation in between two systems.Similarly, [C 2 C 1 im] cation is also inclined to adsorb in the manner of cationic- interaction with adsorption energy of −1.78 eV, as shown in Figure S3 (Supporting Information).The negative adsorption energies of Tempo and [C 2 C 1 im] cation implied that Tempo and [C 2 C 1 im] cation both favor physical adsorbing and were competitive on CNT surface.It was noteworthy that the higher adsorption energy of [C 2 C 1 im] cation than Tempo, which explained that [C 2 C 1 im] cations still played a major role in the gelation process of Gel-Tempo.Based on the above analyses, Tempo was physical adsorption on CNT surface rather than directly bound to CNT via a covalent bonding. [19]he physical-chemical characteristics of Gel-Tempo are shown in Figure 3.As illustrated in Figure 3a, the flame retardancy of Gel-Tempo is determined by a fire resistance test.The initial shape and size of Gel-Tempo-100 were still maintained after ignition, and thus demonstrate an excellent flame-retardant property.Moreover, TG analysis was carried out to evaluate the thermal property of Gel-Tempo-X (Figure 3b).Since Tempo crystal was decomposed at ≈62 °C and completely burnt out at 161 °C, [20] TG curves of Gel-Tempo-X show a slow mass loss at 62-332 °C owing to the loss of adsorbed Tempo.Gel-Tempo-X will decompose at higher temperature than 332 °C and is completely burnt out at around 480 °C.These above characteristics of Gel-Tempo cathode will largely eliminate the safety problem of SABs at high temperature, thus greatly widening their applications in SABs.
As shown in Figure 3c, the electronic conductivity of gel and Gel-Tempo-X was tested by the four-point probe method, and the as-obtained parameters are shown in Table S2 (Supporting Information).The conductivity of gel was much higher than that of Gel-Tempo-X, and a higher Tempo content resulted in lower electron conductivity.This suggested that the addition of Tempo could reduce the free electrons of CNT, which could be attributed to the more sufficient "cationic-" interaction in Gel-Tempo. [21]Figure 3d shows Linear Sweep Voltammetry (LSV) curves of gel and Gel-Tempo-X.Gel-Tempo-X exhibited a higher electrochemical activity (3.3 V vs Na/Na + ) compared with gel, which was favorable for decomposition of discharge products. [22]  The content of Tempo in Gel-Tempo cathode was expected to elicit evolution of their electrochemical properties.Therefore, electrochemical properties of quasi-solid-state SABs constructed with gel and Gel-Tempo-X cathodes were evaluated.Quasi-solidstate SABs with Gel-Tempo cathodes were depicted schemat-ically in Figure 4a, and electrochemical reactions occurred as follows: [23] Anode : 4Na ↔ 4Na + + 4e − (1) Scheme 1. Redox reactions at Tempo during discharge and charge in the voltage range of 1.5-4.5 V versus Na + /Na.
Cathode : 4Na As shown in Figure 4b, quasi-solid-state SABs with Gel-Tempo-100 cathode exhibited an excellent open-circuit voltage of 3.04 V over 100 h without significant degradation.This indicates that Gel-Tempo cathode is extremely stable in ambient air, which means that Tempo does not react with Na 3 Zr 2 Si 2 PO 12 (NASICON) and metallic sodium (Figure S4, Supporting Information).To investigate the redox activity of Tempo in Gel-Tempo cathode, cyclic voltammetry (CV) measurements were carried out. [24]As displayed in Figure 4c, oxidation peaks were not observed in the CV curve of gel cathode, indicating the slow decomposition of Na 2 O 2 .In Gel-Tempo cathode, however, three oxidation peaks (II, III, and IV) and two reduction peaks (I and V) in CV profiles were observed and represented the redox reaction of Tempo (Scheme 1).The electron-mediating role of Tempo was confirmed by the oxidation peak III that represented the oxidation of Tempo molecules to Tempo + .Once the Tempo molecule was oxidized to generate Tempo + in the cathode, Na 2 O 2 decomposition was driven via electron mediation. [25]Tempo + oxidized Na 2 O 2 to Na + and O 2 to form Tempo − , which was subsequently oxidized in Gel-Tempo cathode (oxidation peak IV).
The Voltage curves of the batteries with gel and Gel-Tempo-X cathodes were obtained at 0.1 mA cm −2 , as depicted in Figure 4d.The ultimate discharge and charge voltages of the battery using gel cathode were 2.24 and 3.94 V, respectively, leading to a 1.70 V charge-discharge voltage gap.The polarization of the batteries with Gel-Tempo-X cathodes was improved due to the electronmediating role of Tempo.The battery with Gel-Tempo-100 cathode showed the best performance with 2.24 V discharge voltage and 3.39 V charge voltage, resulting in a charge-discharge voltage gap of 1.15 V, which demonstrated that Tempo acts as an OER catalyst in Gel-Tempo cathode.Remarkably, the dis-charge plateau of all as-assembled SABs occurred at ≈2.24 V, indicating that Tempo did not change the discharge reaction.What is more, the high Tempo content was favorable to decomposition of Na 2 O 2 caused by enhanced OER kinetics. [26]igure 4e shows the voltage-specific capacity curve during the first discharge/charge cycle under a limited discharge capacity of 3000 mAh g −1 .The charge-specific capacity was found to be 2531.6mAh g −1 for Gel-Tempo-100 cathode, 2212.5 mAh g −1 for Gel-Tempo-80 cathode, and 1812.6 mAh g −1 for Gel-Tempo-40 cathode, ≈1.66, 1.45, and 1.19 times higher than 1517.4mAh g −1 of Gel cathode, respectively.In addition, the corresponding coulombic efficiency of SABs with Gel, Gel-Tempo-40, Gel-Tempo-80, and Gel-Tempo-100 cathodes was calculated to be 50.6%,60.4%, 73.7%, and 84.5%, respectively, which indicated the higher the content of Tempo in gel cathode, the higher charge specific capacity even in the case of deeply discharged battery. [27]igure 4f shows the rate performance of quasi-solid-state SABs with different cathodes under a limited discharge capacity of 500 mAh g −1 at a current density of 0.3 mA cm −2 .At high current densities, the battery with Gel-Tempo-100 cathode exhibited a higher charging capacity of 361.5 mAh g −1 and coulomb efficiency of 72.2% than that of batteries with other three cathodes, indicating that the battery with Gel-Tempo-100 cathode had a better rate performance.This might be associated with that a high Tempo content can effectively accelerate charge transfer and enhance OER kinetics. [28]As illustrated in Figure 4g, a cycling test was conducted at a current density of 0.1 mA cm −2 for discharge-charge time of 3 h and a cut-off voltage of 4 V.The battery with gel cathode shows relatively poor cycling durability (103 cycles), while the batteries using Gel-Tempo cathode display better cycling stability (Figure S5, Supporting Information).The quasi-solid-state SAB using Gel-Tempo-100 cathode shows a remarkable cycle performance up to 1746 h (292 cycles), which is superior to previous reports (Figure 4h). [29]This excellent cycling performance can be attributed to the adequate decomposition of Na 2 O 2 during the charging process. [30]Furthermore, the catalytic role of Tempo toward decomposition of Na 2 O 2 was also studied by DFT calculations.Figure 4i shows the free energy changes of Na 2 O 2 decomposition with and without Tempo in gel cathode.Tempo played a key catalytic role in the adsorption and decomposition of Na 2 O 2 into NaO 2 and Na, and in the further decomposition into Na and O 2 .Compared with CNT catalyst, Tempo effectively reduced the overpotentials of three reaction steps by 0.53 eV, 0.71 eV, and 0.41 eV, respectively, which was conducive to the decomposition of Na 2 O 2 .Given the above results, the addition of Tempo in gel cathode was beneficial in improving the electrochemical performance of SABs.
To further investigate the electrochemical mechanism of quasi-solid-state SAB with Gel-Tempo-100 cathode, in situ Raman spectroscopy was used to measure discharge products (Figure 5a).The Raman patterns of the products at 12 different discharge/charge stages were detected.The intensity of peaks at 711.8 and 775.5 cm −1 corresponding to Na 2 O 2 discharge products gradually increased during the discharge, while the peak intensity of Na 2 O 2 gradually decreased during the charging process. [31]t was suggested that the generation and decomposition of Na 2 O 2 were completely reversible in quasi-solid-state SABs with Gel-Tempo-100 cathode.To better comprehend the effect of Tempo on quasi-solid-state SABs performance, the discharge product morphology of the battery using Gel-Tempo-100 cathode was investigated.As shown in Figure S6 (Supporting Information), it was obvious that the morphology of discharge products was rhombic, which was identified as Na 2 O 2 by the results of in situ Raman spectra, elemental mapping, and previous reports. [32]Furthermore, an in-depth analysis was performed based on SEM-EDS and ex situ XRD to understand the causes of quasi-solid-state SABs death.After batteries death, the discharge product consisted of an irregular spherical shell and many lozenge-shaped blocks (Figure 5b).As shown in EDS diagram, the signal intensity of Na and O elements was much higher than that of C elements.This indicated that the C element was distributed on the surface of the irregular spheres.To identify the crystalline structure and composition of discharge products, ex situ XRD was performed on the gas diffusion layer (GDL) in contact with the cathode (Figure 5c).In comparison with the pristine cathode, the diffraction curves of the discharge product after cycle belonged to Na 2 O 2 (PDF # 09-0075) and Na 2 CO 3 (PDF # 18-1205) phases. [33]he formation of Na 2 CO 3 has been attributed to the parasitic reaction of CO 2 in air (Reaction 4). [34]Therefore, the blockage of the electrode surface caused by the refractory by-product (Na 2 CO 3 ) to air was considered to be the key factor to reduce the life of quasisolid-state SABs with Gel-Tempo cathodes.
The electrochemical reaction mechanism of quasi-solid-state SABs battery using gel or Gel-Tempo cathodes is shown in Figure 6.In discharge process, as with the gel cathode, oxygen molecules get electrons on CNT of Gel-Tempo cathode, converted into superoxide radicals (O 2− ).Then O 2− further reacts with Na ions to form the discharge product Na 2 O 2 .In Gel-Tempo cathode, Tempo adsorbed on the CNT surface by cationic- interaction is also reduced to Tempo − during the discharge process.During charging, Tempo − will be oxidized to undergo Tempo − → Tempo → Tempo + conversion.Tempo + comes into contact with Na 2 O 2 deposited and oxidizes Na 2 O 2 to Na + and O 2 by electron-mediated interaction, forming Tempo − .The electronmediated action not only increases the contact area of Na 2 O 2 decomposition reaction, but also greatly reduces the Gibbs free energy of Na 2 O 2 decomposition reaction, thus completing decomposition of Na 2 O 2 .In contrast to Gel-Tempo cathode, the Na 2 O 2 is unable to completely decompose and continues to deposit in the gel cathode due to its electron-insulating properties and limited solid-solid contact with CNT.This passivates the gel cathode and impedes electron conduction of the electrode, resulting in low energy utilization rate and short cycle life of quasi-solid-state SABs.

Conclusion
In summary, Gel-tempo cathode in SABs not only exhibits a good interface compatibility, flame retardance, and high stability to air, but also has the redox activity of Tempo radical to promote decomposition of discharge product Na 2 O 2 .Experiments and DFT calculations indicate that Tempo enhances the cationic- interaction of gel and tends to parallel adsorption on basal plane of CNT.The optimum content of Tempo in Gel-Tempo cathode, detailed catalysis mechanism, and the causes of battery death were investigated.During discharging, the addition of Tempo did not change ORR reaction of the gel cathode.During charging, Tempo increased the contact area of Na 2 O 2 decomposition reaction and reduced the reaction energy barrier, thus improving the decomposition efficiency of the discharge product Na 2 O 2 .The proposed gel adsorbed redox mediators via the cationic- interaction provide a new thought and method for the design of integrated solidstate cathodes in metal-air batteries, and also provide a basis for the application of redox mediators in SABs.

Experimental Section
Preparation of Gel-Tempo Cathode: Gel-Tempo cathode was prepared by following steps.As shown in Figure S7 (Supporting Information), first, CNT (40 mg, Superpure) and Tempo (40, 80, 100, and 120 mg, Superpure) were dispersed in 1 mL IL of [C 2 C 1 im]BF 4 with an ultrasonication for 60 min.To remove excess liquids and obtain Gel-Tempo-X (X = 40, 80, 100, and 120) cathodes, CNT/IL mixture was moved into a centrifuge tube and centrifuged at 9300 g (centrifugal force) for 20 min.The active material of CNT in as-obtained gel or Gel-Tempo-X is ≈10% of its mass (Figure S8, Supporting Information), and the mass of active material in all quasi-solid-state SABs was ≈2 mg.The CNT of gel or Gel-Tempo cathode plays a critical catalytic role in ORR processes (Figure S9, Supporting Information).
Preparation of Na 3 Zr 2 Si 2 PO 12 (NASICON): Using Na 3 PO 4 •12H 2 O (≥98.0%,Sinopharm Chemical Reagent Co., Ltd.), ZrO 2 (≥99.9%,Sinopharm Chemical Reagent Co., Ltd.), and SiO 2 (≥95.78%,Sinopharm Chemical Reagent Co., Ltd.) as raw materials, NASICON separator was prepared by high-temperature solid-phase method. [35]The stoichiometric raw materials were mixed with alcohol.Then, the mixed uniform powder was placed in a muffle furnace and sintered at 1100 °C for 12 h.The sintered product was ball milled with ethanol as the dispersant for 18 h.The powder was pressed into a disc shape by a uniaxial automatic pressurized tablet press, and then the density was enhanced by cold isostatic pressing of 200 MPa.The NASICON separator was wrapped with calcined powder and sintered at 1275 °C for 15 h.A solid-state electrolyte NASICON with an ionic conductivity of 2.7 × 10 −3 S cm −1 at 25 °C was considered as a separator to separate the anolyte from the cathode (Figure S10, Supporting Information).
The Quasi-Solid-State Na-Air Battery Assembly: An anolyte was composed of 1 m NaClO 4 in a polar aprotic solvent (1:1) of ethylene carbonate (EC), dimethyl carbonate (DMC), and 1 vol% fluoroethylene carbonate (FEC).Twenty microliters of anolyte was utilized in each quasi-solid-state SABs.The proposed quasi-solid-state SABs were constructed by gel or Gel-Tempo-X (X = 40, 80, 100, and 120) cathodes and a GDL of carbon paper.GDL facilitates the electron conduction on the gel or Gel-Tempo-X cathode surface (Figure S11, Supporting Information).The battery assembly: Na | Anolyte | NASICON | cathodes | gas diffusion layer was operated in an argon-filled high-integrity glove box.Cathodes of 0.2 g gel or Gel-Tempo-X were employed in all quasi-solid-state SABs.
Electrochemical Measurement: A battery tester (CT2001A, Wuhan LAND electronics) was used to test electrochemical performances at 30 °C in ambient air.To obtain accurately the electrochemical window, the batteries, which composed of gel or Gel-Tempo-X (X = 40, 80, and 100) cathodes, a counter electrode of sodium metal, and an enclosed working electrode of stainless steel, were used to measure by LSV under 1 to 5 V with a scan rate of 0.1 mV s −1 .The CV measurement of quasi-solid-state SABs was made with an electrochemical workstation (PARSTAT 4000 Rear Panel, AMETEK Inc.) at a scan rate of 0.1 mV s −1 .
Characterization Techniques: X-ray diffraction (XRD; MXP3TA, Mac Science) of different cathodes was recorded by an X-ray diffractometer equipped with Cu-K radiation between 10°and 90°with a scanning rate of 0.02°s −1 .Morphologies and microstructures of samples were acquired by scanning electron microscopy (SEM; VEGA-3SBH, TESCAN) with an acceleration voltage of 5.0 kV.Transmission electron microscope (TEM; JEM-2100, JEOL) images were acquired by transmission electron microscopy with an accelerating voltage of 300 kV.X-ray photoelectron spectroscopy (XPS; PHI5000, PHI) data was collected by using an Al K line (h = 1486.6eV).Thermogravimetric (TG; MELER/1600H, Mettler Toledo) analysis measurement was conducted with a differential scanning calorimeter in ambient air from room temperature to 500 °C at a scan rate of 10 °C min −1 .In situ Raman spectra (Renishaw in Via, Renishaw) were recorded under laser excitation at 514 nm (2.41 eV) over the spectral range of 800-3000 cm −1 at room temperature (Figure S12, Supporting Information).The conductivity and resistance of gel cathode or Gel-Tempo-X (X = 40, 80, and 100) cathodes were obtained by a four-point probe method (ST-2258A, Suzhou Jingge Electronic LTD).Electron paramagnetic resonance spectroscopy (EPR; EMXnano, Bruker) was utilized to monitor the nitrogen-oxygen radicals.
DFT Calculations: Spin-polarized DFT calculations were carried out by using the Vienna ab initio simulation package (VASP).The core-electrons were described by the projector-augmented wave (PAW) method with the valence electronic states expanded in plane-wave basis sets.The cutoff energy was set to 450 eV and the force convergence criterion was set to 0.05 eV Å −1 .The surface structure optimization was conducted with a (3 × 3 × 1) k-mesh.The van-der Waals interactions were accounted by the Becke-Johnson (BJ) damping within the DFT-D3 method.Zero-point energy (ZPE) and entropy corrections were considered to calculate the free energies.The adsorption energies (Eads) were obtained as follows: E ads = E(adsorbate| substrate) − {E(adsorbate) + E(substrate)} where E(adsorbate| substrate), E(adsorbate), and E(substrate) represent the total energy of the adsorbate adsorption on the surface of substrate, the total energy of the individual adsorbate and the total energy of the substrate, respectively.

Figure 1 .
Figure 1.a) Preparation procedures of Gel-Tempo cathode.b-d) TEM images of CNT, gel, and Gel-Tempo.e) EPR spectra of gel and Gel-Tempo cathodes.

Figure 2 .
Figure 2. a) XRD patterns, b) XPS spectra, c) High-resolution N 1s XPS spectra, and d-f) Raman spectral curves of different Tempo contents.g) Adsorption energy diagram of Tempo in Gel-Tempo.

Figure 3 .
Figure 3. a) The fire resistance test of Gel-Tempo-100.b) TG curves of gel and Gel-Temp-X.c) The electronic conductivity of gel and Gel-Tempo-X.d) LSV curves of gel and Gel-Tempo-X.

Figure 4 .
Figure 4. a) Schematic illustration of quasi-solid-state SABs with Gel-Tempo cathode.b) Open-circuit potential plots.c) CV curves of batteries with gel or Gel-Tempo-100 cathodes in air atmosphere.d) Discharge-charge voltage curves at 0.1 mA cm −2 .e) Discharge-charge profiles of these batteries with cut-off voltage of 4 V and cut-off capacity of 3000 mAh g −1 .f) Discharge-charge profiles of these batteries in cut-off voltage of 4 V and cut-off capacity of 500 mAh g −1 .g) Discharge-charge cycling curves.h) Compared Histogram of cycle performance (including quasi-solid-state Li/Na/Zn-air batteries).i) Free energy diagram of the Na 2 O 2 decomposition reaction in SABs.The star "*" denotes the free site or the adsorption species.

Figure 5 .
Figure 5. Characterization of the discharge product of the quasi-solid-state SAB with Gel-Tempo-100 cathode: a) First discharge and charge curves of quasi-solid-state SAB with selective points at different stages and In situ Raman spectra.b) SEM and EDS images of SAB with Gel-Tempo-100 cathode after the battery death.c) Ex situ XRD patterns of Gel-Tempo-100 cathode before and after cycling.

Figure 6 .
Figure 6.Electrochemical reaction mechanism diagram of quasi-solid-state SABs with gel or Gel-Tempo cathodes.