Sodium Nitrate/Formamide Deep Eutectic Solvent as Flame‐Retardant and Anticorrosive Electrolyte Enabling 2.6 V Safe Supercapacitors with Long Cyclic Stability

Safe operation of electrochemical capacitors (supercapacitors) is hindered by the flammability of commercial organic electrolytes. Non‐flammable Water‐in‐Salt (WIS) electrolytes are promising alternatives; however, they are plagued by the limited operation voltage window (typically ≤2.3 V) and inherent corrosion of current collectors. Herein, a novel deep eutectic solvent (DES)‐based electrolyte which uses formamide (FMD) as hydrogen‐bond donor and sodium nitrate (NaNO3) as hydrogen‐bond acceptor is demonstrated. The electrolyte exhibits the wide electrochemical stability window (3.14 V), high electrical conductivity (14.01 mS cm−1), good flame‐retardance, anticorrosive property, and ultralow cost (7% of the commercial electrolyte and 2% of WIS). Raman spectroscopy and Density Functional Theory calculations reveal that the hydrogen bonds between the FMD molecules and NO3− ions are primarily responsible for the superior stability and conductivity. The developed NaNO3/FMD‐based coin cell supercapacitor is among the best‐performing state‐of‐art DES and WIS devices, evidenced by the high voltage window (2.6 V), outstanding energy and power densities (22.77 Wh kg−1 at 630 W kg−1 and 17.37 kW kg−1 at 12.55 Wh kg−1), ultralong cyclic stability (86% after 30 000 cycles), and negligible current collector corrosion. The NaNO3/FMD industry adoption potential is demonstrated by fabricating 100 F pouch cell supercapacitors using commercial aluminum current collectors.


Introduction
[13] Although solvent-free ionic liquids (e.g., EMIM-BF 4 and EMIM-TFSI) can widen the electrochemical stability window (ESW) to 3.0-5.0V, the high cost of the complex synthesis process and the low conductivity hinder their real-world applications. [14,15]Recently, new concept of waterin-salt (WIS) electrolytes (e.g., 21 m LiTFSI) has been proposed as the promising candidates of organic solutions because of the obviously improved electrochemical stability and nonflammable nature. [16,17][20] Specifically, the ultrahigh concentration of salt inevitably leads to the high cost of electrolytes (e.g., 0.565 $ g À1 for 21 m LiTFSI electrolyte), which is much higher compared to typical commercial organic electrolytes (0.185 $ g À1 ).The aluminum current collector can be seriously corroded in aqueous solutions by a formation of aluminum oxide (Al 2 O 3 ) film as a resistive passivation layer, which reduces the conductivity of the current collector and deteriorates the supercapacitor power performance, [21] especially at high current densities.Therefore, it is still challenging to realize inexpensive and safe-to-operate electrolytes with the wide electrochemical stability window and high electrical conductivity.
[30] DES is typically a mixture of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) in specific molar ratios, where self-association occurs through hydrogen bonds (H-bonds) when the mixtures are heated. [31,32]The freezing point of DES is significantly lower than that of any individual The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/eem2.12641.DOI: 10.1002/eem2.12641Safe operation of electrochemical capacitors (supercapacitors) is hindered by the flammability of commercial organic electrolytes.Non-flammable Waterin-Salt (WIS) electrolytes are promising alternatives; however, they are plagued by the limited operation voltage window (typically ≤2.3 V) and inherent corrosion of current collectors.Herein, a novel deep eutectic solvent (DES)-based electrolyte which uses formamide (FMD) as hydrogen-bond donor and sodium nitrate (NaNO 3 ) as hydrogen-bond acceptor is demonstrated.The electrolyte exhibits the wide electrochemical stability window (3.14 V), high electrical conductivity (14.01 mS cm À1 ), good flameretardance, anticorrosive property, and ultralow cost (7% of the commercial electrolyte and 2% of WIS).Raman spectroscopy and Density Functional Theory calculations reveal that the hydrogen bonds between the FMD molecules and NO 3 À ions are primarily responsible for the superior stability and conductivity.The developed NaNO 3 /FMD-based coin cell supercapacitor is among the best-performing state-of-art DES and WIS devices, evidenced by the high voltage window (2.6 V), outstanding energy and power densities (22.77Wh kg À1 at 630 W kg À1 and 17.37 kW kg À1 at 12.55 Wh kg À1 ), ultralong cyclic stability (86% after 30 000 cycles), and negligible current collector corrosion.The NaNO 3 /FMD industry adoption potential is demonstrated by fabricating 100 F pouch cell supercapacitors using commercial aluminum current collectors.
component. [33]DES not only shares similar physical properties with ionic liquids like a low vapor pressure and a wide operation voltage window, [34] but also has major advantages in flame retardance, low toxicity, and easy manufacturing. [35]Therefore, DESs are promising alternatives to commercial organic electrolytes.In addition, the HBD/ HBA molar ratio of DES electrolytes can be precisely designed to meet different needs, for example, lithium perchlorate (LiClO 4 ) and Nmethyl acetamide (NMA) at a molar ratio of 4.3:1 with high ESW of 3.6 V, [36] choline chloride (ChCl) and urea at a molar ratio of 2:1 featuring good flame-retardancy and anticorrosive nature. [34]he main challenge facing DES electrolytes is the low electrical conductivity, which seriously hampers its rate performance and power density in the practical energy storage applications. [37]Introducing additives is an effective approach to resolve the above issues. [37,38]Adding 5 wt% water into the ChCl/Urea DES electrolyte increased the electrical conductivity from 1.3 to 16.76 mS cm À1 . [34]However, the added water results in unstable cyclic voltage profiles and unwanted faradaic responses in device operation.Even adding considerably small amount of water (1 wt%), the capacitance retention rate after long-term cycles was obviously less than those without any water content.Chia-Wei Lien et al. [39] prepared DES electrolyte with LiClO 4 and acetamide at the molar ratio of 4.3.Although adding the acetonitrile-water mixture (molar ratio of 4.4:1) into DES could improve the electrical conductivity from 0.87 to 15.6 mS cm À1 and reduce the viscosity from 261.7 to 5.82 mPaÁs, its ESW is limited to 2.55 V and also suffers from the significantly increased risk of flammability.
Considering the "two-edged sword" effect of the above approach, it is imperative to develop new DES electrolytes, which inherently have the above desired electrochemical and physical properties.Min Zhong et al. [40] successfully prepared ChCl/ethylene glycol (EG) DES electrolyte, which shows the high conductivity up to 8 mS cm À1 .However, the operating voltage window of the ChCl/EG supercapacitor is limited to 2.0 V.The electrical conductivity of DES synthesized by tetraethylammonium chloride (TEAC) and EG reaches the highest value of 8.7 mS cm À1 among the most conductive DES electrolytes without additives, [33] while its energy density is limited by the relatively low voltage window of 1.8 V when used in supercapacitors.Besides, DES electrolytes using EG as HBD also introduce major safety issues because of the high flammability risk.Despite of the above important advances, it still remains challenging to realize DES electrolytes with the wide electrochemical stability window, high electrical conductivity, good flame retardancy, and low cost simultaneously.
In this work, a novel kind of DES electrolyte using formamide (FMD) as the HBD and sodium nitrate (NaNO 3 ) as the HBA is developed for the first time (Figure 1a).Our NaNO 3 /FMD DES electrolytes have versatile merits of the wide electrochemical stability window (3.14 V), high conductivity (14.01 mS cm À1 ), anticorrosive property and good flame-retardance.These features are superior to the current state-of-the-art nonflammable WIS and DES electrolytes.Importantly, the ultralow cost (0.0128 $ g À1 ) of the new electrolyte is only ∼ 7% of typical commercial flammable organic electrolyte (0.185 $ g À1 of 1 m TEABF 4 /ACN) and 2% of WIS electrolyte (0.565 $ g À1 of 21 m LiTFSI), making it promising for practical applications.In our NaNO 3 / FMD DES electrolytes, Raman tests show that hydrogen bonds (Hbonds) can be generated between the electropositive H atoms of FMD and highly electronegative N atoms of NO 3 À ions.This mechanism enables NO 3 À ions solvated by FMD molecules and effectively promotes the NO 3 À -Na + ion separations, thus leading to the demonstrated superior electrical conductivity.Meanwhile, such solvation structure prevents the direct contact of NO 3 À ions with the positive electrode thereby drastically reducing the ions' oxidation while widening the electrochemical stability window.These results are verified by the Density Functional Theory calculations as well as linear sweep voltammetry tests.Thanks to these merits, our NaNO 3 /FMD based AC|| AC coin cell supercapacitor using stainless steel current collector shows the high operation voltage window of 2.6 V, outstanding energy and power densities (22.77Wh kg À1 at 630 W kg À1 and 12.55 Wh kg À1 at 17.37 kW kg À1 , respectively), and ultralong cyclic stability (retention of 86% after 30 000 cycles).The achieved performance is thus among the best compared to the state-of-art DES and WIS electrolytes, and is comparable to the commercial supercapacitors based on flammable organic electrolytes.Finally, the fabricated AC||AC coin cell and ∼ 100 F pouch cell assembled using commercial aluminum current collectors demonstrate the practicality and advantages of our NaNO 3 /FMD electrolyte over the previously-reported nonflammable WIS and DES electrolytes.

Electrochemical Stability and its Molecular Origin
Linear sweep voltammetry (LSV) is conducted in the three-electrode system to determine the electrochemical stability window (ESW) of our NaNO 3 /FMD x electrolytes (x is the molar ratio of FMD to NaNO 3 ; x = 6, 8, and 10) at 5 mV s À1 .It should be noted that the lower NaNO 3 / FMD x ratio is not considered in this work because the stable and homogeneous liquid phase cannot be formed if x < 6.As shown in Figure 1b, the ESW values of NaNO 3 /FMD 6 , NaNO 3 /FMD 8 and NaNO 3 /FMD 10 are 3.14 V (À1.438 to 1.699 V vs Ag/AgCl), 2.92 V (À1.382 to 1.543 V vs Ag/AgCl) and 2.79 V (À1.353 to 1.433 V vs Ag/AgCl), respectively, among which NaNO 3 /FMD 6 exhibits the highest electrochemical stability.Especially, the anodic oxidation potentials of NaNO 3 /FMD x significantly decrease in the order of NaNO 3 /FMD 6 (1.699 V) > NaNO 3 /FMD 8 (1.543 V) > NaNO 3 /FMD 10 (1.433 V), compared to the slight decrease of cathodic reduction potential (from À1.438 to À1.353 V).This result defines the upper limit of the anodic potential which is achieved in the NaNO 3 /FMD 6 case, implying that the NO 3 À ions are most stable in the NaNO 3 /FMD 6 electrolyte.Density Functional Theory (DFT) calculations are performed to reveal the underlying molecular-scale mechanisms.The energy difference between the Highest Occupied Molecular Orbital (HOMO) energy and the Lowest Unoccupied Molecular Orbital (LUMO) energy of the NaNO 3 and FMD is quantified, which is directly correlated with their electrochemical stability. [41,42]Theoretical models of NaNO 3 , FMD and NaNO 3 /FMD x electrolytes are built (Figure 1c).Our DFT results show that the HOMO-LUMO energy difference of NaNO 3 (8.778eV) is much lower than that of the FMD (11.229 eV), indicating the better stability of the FMD molecule.Especially, NaNO 3 /FMD 6 exhibits the highest HOMO-LUMO energy difference of 9.506 eV compared to NaNO 3 /FMD 8 (9.355 eV) and NaNO 3 /FMD 10 (9.324 eV), which agrees with the largest ESW value of the NaNO 3 /FMD 6 electrolyte.This finding correlates with the NaNO 3 -FMD solvation structures as revealed by the following Raman spectroscopy analysis.
The Raman spectroscopy of the NaNO 3 /FMD 6 , NaNO 3 /FMD 8 , and NaNO 3 /FMD 10 electrolytes are measured to explore the microscopic solvation structure and support the DFT results, which helps explain the ESW results.As shown in Figure 1d, the addition of NO  the emergence of a weak characteristic peak at 709 cm À1 corresponding to NO 3 À ion in-plane bending and the strong characteristic peak at 1045 cm À1 corresponding to symmetric stretching vibration of NO 3 À ions.The adjacent peaks at 1092 and 1308 cm À1 are attributed to the stretching vibration of C-N bond in FMD, [43,44] and the peak at 1387 cm À1 is attributed to the deformation vibration of H-C=O in FMD, agreeing well with the previous Raman results. [43]Especially, the peak at 3195 cm À1 of FMD, representing the N-H. ..N stretching vibration, [44] shows a blue shift with the addition of NaNO 3 .Specifically, the N-H. ..N stretching vibration frequencies shift to 3320, 3318 and 3316 cm À1 for NaNO 3 /FMD 6 , NaNO 3 /FMD 8 and NaNO 3 /FMD 10 , respectively, indicating the strongest H-bond interactions in the NaNO 3 /FMD 6 electrolyte.Besides, the amplitudes of their corresponding peaks are 1540, 1431, and 1289 a.u., respectively, suggesting that NaNO 3 /FMD 6 has the highest strength and the largest number of H-bonds.
In order to further specify the relationship between the H-bonds and the stability of NaNO 3 /FMD x , Raman spectroscopy in the range from 3150 to 3450 cm À1 are selected for deconvolution (Figure 1eg).Two types of H-bonds exist in each DES system, one of which is the H-bond between FMD and NO 3 À ions (around 3320 cm À1 ) while the other is the H-bond between FMD molecules (two peaks, around 3250 and 3190 cm À1 ).It should be noted that the H-bonds between the FMD and NO 3 À ions that are located at 3320, 3316 and 3314 cm À1 in NaNO 3 /FMD 6 , NaNO 3 /FMD 8 and NaNO 3 /FMD 10 , show a significant decrease in both the amplitude and wavenumber.This result suggests that the intensive NO 3 À -FMD H-bond interactions in NaNO 3 /FMD 6 lead to the robust protective solvation structure of NO 3 À ions, which is consistent with the highest redox stability indicated by the HOMO-LUMO results.On the other hand, for H-bonds between FMD molecules, the contribution of these H-bonds area to the total H-bonds area is much less than that of the NO 3 À -FMD H-bonds area.As evidenced in Figure S1, Supporting Information, the area of Hbonds between FMD molecules only accounts for 32.7%, while the area of NO 3 À -FMD H-bonds is more than two times higher (67.3%).The latter hydrogen bonding configuration is expected to play a dominant role in the stability of NaNO 3 /FMD 6 .These Raman results are in a good accordance with the DFT results and the as-measured ESW values.
The molecular dynamics (MD) simulation is conducted to further evaluate the H-bond interaction (Figure 1h), which is characterized by the radial distribution function g(r) analysis.It is found that g(r) function shows the largest first-peak value in NaNO 3 /FMD 6 (Figure 1i), indicating the strongest interactions beteen FMD molecules and NO 3 À ions, which agrees well with the Raman and DFT results.

Physical Propertie and Costs
In order to study the thermal property, differential scanning calorimetry (DSC) is used to cool the samples from 15 to À120 °C and then heat them to 50 °C at 5 °C min À1 .According to the data of the cooling process in Figure 2a, the peak at À14 °C of FMD is attributed to the exothermic crystallization while the peak at À58.2 °C corresponds to the crystal transformation.In contrast, the thermal performance of DES displays significant changes after the addition of NaNO 3 .Besides, NaNO 3 / FMD 8 and NaNO 3 /FMD 10 electrolytes show two exothermic crystallization peaks at different temperatures due to the existence of two types of H-bonds as measured by the Raman spectroscopy. [45,46]In the NaNO 3 /FMD 10 system, the peak of exothermic crystallization at À33.6 °C is caused by the FMD-FMD H-bond interactions, while the adjacent peak of exothermic crystallization at À40 °C corresponds to the H-bonds between the FMD and NO 3 À ions.With the increase of NaNO 3 content in NaNO 3 /FMD 8 , the temperatures of two exothermic crystallization peaks corresponding to two types of H-bonds further decrease, implying that the freezing point of NaNO 3 /FMD 8 is lowered further.Especially in the NaNO 3 /FMD 6 electrolyte, only one exothermic crystallization peak at À52.9 °C corresponding to the NO 3 À -FMD H-bonds is identified, without any evidence for the peaks attributed to FMD-FMD H-bonds.This finding suggests that most of the FMD molecules are strongly bounded to NO 3 À ions and the FMD-FMD H-bond interactions are significantly weakened, [46] which agrees with the above Raman and DFT results.Moreover, it is found that the addition of NaNO 3 significantly reduces the freezing point of DES, which decreases from À14 °C in pure FMD to NaNO 3 /FMD 6 (À52.9 °C), NaNO 3 / FMD 8 (À41.8 °C), and NaNO 3 /FMD 10 (À33.6 °C).
Electrical conductivity is a critical parameter of the ion transport property, which is crucial to determine the rate performance and power density.The room temperature conductivities of NaNO 3 / FMD 6 , NaNO 3 /FMD 8 , and NaNO 3 /FMD 10 electrolytes are 14.01, 15.32, and 15.87 mS cm À1 , respectively.As shown in Figure 2b, the logarithm of conductivity of NaNO 3 /FMD 6 exhibits a clearly linear relationship with the reciprocal of the temperature (R 2 ∼ 1), indicating the better ion transport property of the electrolyte.The relationship between the conductivity and the temperature is well fitted by the following Arrhenius equation: where σ is conductivity, σ 0 is the conductivity constant, E a is the activation energy of conductivity, R is the gas constant, and T is the absolute temperature.According to the calculation, the E a values of NaNO 3 /FMD 6 , NaNO 3 /FMD 8 , and NaNO 3 /FMD 10 electrolytes are 9.055, 7.253, and 5.470 kJ mol À1 , respectively.These values are much lower than those of typical DES electrolytes (21.17 kJ mol À1 of ChCl/Urea 2 and 13.98 kJ mol À1 of TEAC/ EG 4 ), [33,34] demonstrating that the conductivities of NaNO 3 /FMD electrolytes are less sensitive to the changes of temperature.
Viscosity is another important parameter that characterizes the ion transport property of electrolytes.The room-temperature viscosities of NaNO 3 /FMD 6 , NaNO 3 /FMD 8 , and NaNO 3 /FMD 10 electrolytes are 15.309, 7.173 and 6.313 mPa s, respectively, which are much lower than those of the previously-reported DES electrolytes (e.g., 261.7 mPa s of LiClO 4 /Acetamide 4.3 , [39] 56.99 mPa s of ChCl/Butanediol 4 , [47] 44.5 mPa s of ChCl/EG 2 , [40] 25.21 mPa s of TEAC/EG 4 and 18.13 mPa s of TEAB/EG 5 ). [33]Similarly, a linear relationship (R 2 ∼ 1) between the viscosity and temperature is presented in Figure 2c, which are well fitted by the following Arrhenius equation: where η is viscosity, η 0 is the viscosity constant, E η is the activation energy of viscosity, R is the gas constant, and T is the absolute temperature.The E η values of NaNO Walden classification diagram is a direct tool to evaluate the ionicity, which is based on the Walden relation between the conductivity and viscosity: where Λ is the molar conductivity (S cm 2 mol À1 ), η is the viscosity (Pa S À1 ), and k is the constant estimated from 1 m KCL solution. [34]As shown in Figure 2d, the solid blue line represents the ideal Walden plot of 1 m KCl solution, and the deviation from the blue line outlines the degree of ionization of the solution, which can be used to distinguish non-ionic liquids, poorly ionic liquids, good ionic liquids and superionic liquids.NaNO 3 /FMD 6 is in the category of good ionic liquids, which is much better than other typical DES electrolytes (e.g., ChCl/Urea 2 , [34] LiNO 3 /N-MAc 4 , [48] and LiTFSI/N-MAc 4 ). [49]o characterize the non-flammability of our electrolytes, the flame retardance tests are conducted and demonstrated in Figure 2e. [50]Furthermore, the flammability of our DES electrolytes is quantitatively evaluated by self-extinguishing time (SET).SET is calculated by normalizing the flame burning time to the electrolyte mass. [17,39]The SET values of NaNO 3 /FMD 6 , NaNO 3 /FMD 8 , and NaNO 3 /FMD 10 electrolytes are 0 s g À1 , indicating the good flame-retardancy nature.

Electrochemical Performance of NaNO 3 /FMD Supercapacitor
The electrochemical performance of the coin cell supercapacitor based on NaNO 3 /FMD electrolyte (noted as NaNO 3 /FMD supercapacitor) is characterized, which symmetrically uses the commercial YP-50 activated carbon (AC) as electrode material (AC||AC).The electrochemical test results of NaNO 3 /FMD 6 supercapacitor are shown in Figure 3, and the corresponding data (cyclic voltammetry plots, galvanostatic chargedischarge plots, Nyquist plots and equivalent circuit diagrams) of NaNO 3 /FMD 8 , and NaNO 3 /FMD 10 are available in Figure S4, Supporting Information.
Firstly, the capacitive behavior and upper voltage stability limit of the coin cell are evaluated by cyclic voltammetry (CV) curves at a scan rate of 20 mV s À1 in Figure 3a.With the voltage window increasing up to 2.6 V, all CV curves are approximate rectangles in shape with the high Coulombic efficiencies ranging from 94% to 96%, indicating the good capacitive behavior of the NaNO 3 /FMD 6 supercapacitor.When the voltage window reaches 2.7 V and beyond, obvious polarization currents appear in the CV curves, leading to the rapid decrease of Coulombic efficiency (from 91% to 88%), which means the increased redox activities. [34]Therefore, the upper voltage stability limit of NaNO 3 /FMD 6 AC||AC coin cell is set to 2.6 V.The CV curves at different scan rates (20-500 mV s À1 ) within the voltage window of 2.6 V are shown in Figure S3, Supporting Information.Due to the superior electrical conductivity of NaNO 3 /FMD 6 , its CV curve at 500 mV s À1 only slightly deviates from the CV curve at 20 mV s À1 , which still approximates a rectangular shape.
In addition, galvanostatic charge-discharge (GCD) tests of the supercapacitor with NaNO 3 /FMD 6 electrolyte are conducted at different current densities (0.5-10 A g À1 ), as shown in Figure 3b.It can be seen that the GCD curve of 0.5 A g À1 remains a comparatively symmetrical triangle and its Coulombic efficiency reaches 95%.According to the GCD curves, a high specific capacitance of 90 F g À1 based on single electrode is delivered with a low voltage drop of 0.07 V.
Nyquist diagram measurements are performed to quantify the ion transport resistance during the charging/discharging process.The high frequency x-axis intercept corresponds to the electrolyte resistance (R s ), and the semicircular diameter in the high frequency range represents the charge transfer resistance (R ct ). [36,54]The Nyquist plot (Figure 3c) and equivalent circuit diagram (Figure S4g, Supporting Information) show the relatively low R s (3.7 Ω) and R ct (6.0 Ω) values of NaNO 3 / FMD 6 supercapacitor compared to the previously-reported typical DES-based supercapacitors (e.g., R s = 42 Ω and R ct = 14 Ω for ChCl/ Urea 2 , [34] R s = 3.8 Ω and R ct = 13.5 Ω for ChCl/EG 2 ). [40]These results indicate the fast ion transfer of our NaNO 3 /FMD 6 electrolyte in the nanoporous AC electrode. [55]n order to investigate the cycle life performance, Figure 3d shows the long-term cyclic stability of AC||AC coin cell using NaNO 3 /FMD 6 DES electrolyte.After 10 000 cycles of GCD tests at 5 A g À1 , the NaNO 3 /FMD 6 supercapacitor shows a high capacitance retention rate of 96%.After 20 000 cycles and 30 000 cycles, the capacitance retention rates still remain 90% and 86%, respectively, meanwhile the Coulombic efficiency is close to 100%.The Nyquist diagram in Figure 3e demonstrates that after 10 000 cycles at 5 A g À1 , the R ct of NaNO 3 / FMD 6 supercapacitor increases slightly from 6.0 to 7.8 Ω, indicating the high stability of NaNO 3 /FMD 6 electrolyte.Even after ultra-long 30 000 cycles, the NaNO 3 /FMD 6 supercapacitor can still undergo superb stable charge-discharge cycles while the R ct value only increases to 11.7 Ω.The slight impedance increase of our NaNO 3 /FMD 6 supercapacitor during the long-term cycling tests is obviously superior to previous DES and aqueous electrolyte supercapacitors (e.g., R ct increases from 14 to 29 Ω for ChCl/Urea 2 after 30 000 cycles, [34] R ct increases from 52.2 to 360.2 Ω for 1 m Na 2 SO 4 /H 2 O after 100 cycles). [21]These evidences highlight the anticorrosion property and superior stability of NaNO 3 /FMD 6 electrolyte towards the current collector, which is critically important for practical applications.
The Ragone plot in Figure 3g compares the energy and power densities of AC||AC coin cell supercapacitors using DES and WIS electrolytes.It can be clearly seen that NaNO 3 /FMD 6 supercapacitor reaches a high energy density of 22.77 Wh kg À1 at a power density of 630 W kg À1 , which is almost two times higher compared to that of the 21 m LiTFSI based supercapacitor (10.73 Wh kg À1 at 336 W kg À1 ).When the power density further increases to 17.37 kW kg À1 , the energy density reaches 12.55 Wh kg À1 for NaNO 3 /FMD 6 supercapacitor.These maximum energy and power densities are superior to the previouslyreported DES-based supercapacitors, including ChCl/Urea 2 (17.60 Wh kg À1 at 90 W kg À1 ), ChCl/Butanadiol 4 (8.93 Wh kg À1 at 998 W kg À1 ), TEAC/EG 4 (17.36Wh kg À1 at 890 W kg À1 ), and WIS-based supercapacitors like 21 m LiTFSI (11.46 Wh kg À1 at 179 W kg À1 ), 40 m HCOOK (11.14 Wh kg À1 at 1580 W kg À1 ), as well as the lowconcentration aqueous electrolyte based supercapacitor (1 m Li 2 SO 4 , [34] 7.12 Wh kg À1 at 515 W kg À1 ) and the commercial application (1 m TEABF 4 in PC, 21.87 Wh kg À1 at 1198 W kg À1 ).
The electrochemical performance of the NaNO 3 /FMD 6 supercapacitor using commercial aluminum current collector is tested in the Energy Environ.Mater.2024, 7, e12641 following section to demonstrate its potential suitability for the industrial manufacture.The corresponding tests of representative nonflammable WIS-based supercapacitor (e.g., 21 m LiTFSI supercapacitor) are also conducted for comparison.As shown in Figure 4a, the CV curves of NaNO 3 /FMD 6 and 21 m LiTFSI coin cell supercapacitors using aluminum current collectors exhibit typically capacitive behaviors evidenced by the rectangular shapes and high Coulombic efficiencies.The voltage window of the NaNO 3 /FMD 6 supercapacitor is 2.6 V, which is much higher than that of WIS supercapacitor (2.3 V).The Nyquist plots in Figure 4b reveal that the low R ct value of NaNO 3 /FMD 6 supercapacitor (5.7 Ω) is significantly lower than that of the WIS counterpart (8.7 Ω).
Electrolyte with good anticorrosive property towards the commercial aluminum current collector is critically important for practical applications.To demonstrate that, the long-term cycling tests of the WIS and NaNO 3 /FMD 6 supercapacitor are conducted and compared.It has been experimentally proved that the inherent water content could directly lead to the serious corrosion of metal current collectors by the formation of a resistive passivation layer (i.e., Al 2 O 3 film), which has a low electrical conductivity and deteriorate the cyclic stability. [21]As a result, the capacitance retention rate of 21 m LiTFSI supercapacitor shows a poor level of 64.9% after 10 000 cycles in Figure 4c.In comparison, due to the non-presence of water content or corrosive ingredient in NaNO 3 /FMD 6 electrolyte, a significantly improved retention rate of 82.1% is realized after 10 000 cycles, demonstrating the superior anticorrosive property towards the fragile aluminum materials.Moreover, the cyclic stability could be further improved by reducing the corrosive impurity during the manufacturing (e.g., water impurity content) and coating the aluminum surface with hydrophobic materials (e.g., graphene). [58]he Raman spectroscopy is also conducted to analyze the corrosion products of aluminum current collectors in NaNO 3 /FMD 6 and WIS electrolytes (Figure 4d). [59]Compared to the curve of original aluminum without any corrosion, there is no significant peak observed for corrosion phenomena in the curve of NaNO 3 /FMD 6 .However, the curve of WIS shows a strong peak at 744 cm À1 corresponding to the E g vibration of α-aluminum oxide (α-Al 2 O 3 ). [60]The peak at 1376 cm À1 represents the stretching vibration of Al-O in aluminum oxide, while the adjacent peaks at 1238 and 1485 cm À1 are attributed to the AlO 2 À vibration.The apparent peaks in the WIS curve indicate the much serious corrosion of aluminum current collector in 21 m LiTFSI electrolyte, which agrees with the long-term cycling results.
The low-temperature electrochemical performance of our NaNO 3 / FMD 6 electrolyte has been evaluated in Figure S5, Supporting Information.It shows good capacitance retention rate of 81% from room temperature to À20 °C.Furthermore, the electrochemical performance of our NaNO 3 /FMD 6 supercapacitor can be further improved upon adding appropriate solvents, which requires further studies.After introducing a small amount of ACN, the hybrid NaNO 3 /FMD 6 /ACN 3 DES electrolyte is synthesized with the higher electrical conductivity (19.5 mS cm À1 ) and low flammability (49 s g À1 of SET).As shown in Figure S6, Supporting Information, the hybrid DES-based supercapacitor shows the much lower electrical resistance and the outstanding rate performance with a high capacitance retention of 70% from 20 to 500 mV s À1 .63] To evaluate the practicality and commercial potential, the 100 F AC||AC pouch cell supercapacitors with the integrated aluminum current collectors are fabricated for electrochemical measurements.After cutting the aluminum plastic film open (Figure 4e), the ignition test demonstrates the excellent flame retardancy of the NaNO 3 /FMD 6 pouch cell, which ensures the safety in the future commercial applications.The Ragone plot in Figure S7, Supporting Information shows that NaNO 3 /FMD 6 pouch cell delivers the maximum energy density of 23.12 Wh kg À1 at 260 W kg À1 , and the maximum power density of 5214 W kg À1 at 11.01 Wh kg À1 , which significantly outperforms the 21 m LiTFSI pouch cell.Furthermore, two 100 F NaNO 3 /FMD 6 pouch cell supercapacitors are connected in series to power 177 blue LED bulbs in Figure 4f, which shows the practicality in real-world energy storage applications.

Conclusion
Flammability of commercial organic electrolytes causes serious safety concerns for supercapacitor applications.Although non-flammable Water-in-Salt (WIS) electrolytes are potential alternatives, they are obstructed by limited voltage window and inherent water corrosion of current collectors.In this work, a new type of deep eutectic solvent (DES)-based electrolytes is developed, which specifically uses formamide (FMD) and sodium nitrate (NaNO 3 ) as hydrogen-bond donor and acceptor (noted as NaNO 3 /FMD).This state-of-art DES electrolyte exhibits the wide electrochemical stability window (ESW, 3.14 V), high conductivity (14.01 mS cm À1 ), good flame retardancy, anticorrosive property and ultralow cost (0.0128 $ g À1 ).Raman spectroscopy and Density Functional Theory calculations suggest that the NO 3 À -FMD solvation structure constructed via the strong hydrogen bonds are primarily responsible for the superior electrical conductivity and wide ESW.Thanks to these merits, the NaNO 3 /FMD-based coin cell supercapacitor with activated carbon (AC) electrodes exhibits a wide voltage Energy Environ.Mater.2024, 7, e12641 window of 2.6 V, an outstanding cycling stability of 86% after 30 000 cycles and a high energy density of 21.09 Wh kg À1 at a power density of 291 W kg À1 .In the end, the commercial practicability of NaNO 3 / FMD electrolyte is successfully demonstrated by fabricating the coin cell and pouch cell supercapacitors using commercial aluminum current collectors.We believe that this novel type of DES electrolytes opens new opportunities for future power-demanding applications.

Experimental Section
Preparation of DES electrolyte: DES electrolytes were mixed using sodium nitrate (SCR ® , ≥99.0%) and formamide (Aladdin ® , 99%) at the molar ratio of 1:6, 1:8 and 1:10 respectively in the argon-filled glove box.In the sealed condition, the samples were stirred by PTFE A-type magnetons and heated at 80 °C for 1 h to form homogeneous and transparent solutions.Three solutions made at molar ratios of 1:6, 1:8 and 1:10 were named as NaNO 3 /FMD 6 , NaNO 3 /FMD 8 and NaNO 3 /FMD 10 respectively.
Measurement of physical properties of electrolytes: Raman spectroscopy measurements were conducted by using Germany WITec alpha 300R with a 532 nm excitation laser in the range of 100-4000 cm À1 .Differential scanning calorimetry (DSC) was measured by using NETZSCH DSC 214.The samples were sealed in aluminum crucibles and cooled from 15 to À120 °C at the rate of 5 °C min À1 then immediately heated to 50 °C at the rate of 5 °C min À1 .The electrical conductivity was measured by Leici DDSJ-308F conductivity meter and the viscosity was measured by Anton Paar Lovis 2000M micro-viscometer.For burning test, a separator (19 mm diameter of cross section) was soaked in the electrolyte and then was ignited by the lighter.
Theoretical calculation: As to the DFT calculation, the models of sodium nitrate and formamide were established by genmer program in Molclus software package.For the complexes of NaNO 3 /FMD 6 , NaNO 3 /FMD 8 and NaNO 3 /FMD 10 , 200 structures corresponding to each complex were randomly generated.Then the 200 collected structures were optimized using the GFN2-xTB method in the xTB software and the optimized structures with corresponding energy data were collected.Structures with similar energies (energy threshold = 1 kcal mol À1 ) and similar geometry (geometry threshold = 1 Angstrom) were identified as the same structure.For three complexes of NaNO 3 /FMD 6 , NaNO 3 /FMD 8 , and NaNO 3 / FMD 10 , the structure with the lowest energy value of each complex was adopted for density functional theory calculation.The lowest Unoccupied Molecular Orbitals (LUMO) energies and the Highest Occupied Molecular Orbitals (HOMO) energies were calculated using Gaussian 16 software.Specifically, the M06-2X functional and the def2-SVP basis set were adopted for all calculations.The DFT-D3 dispersion correction was applied to correct the weak interaction and improve the calculation accuracy.The IEFPCM implicit solvation model was used to account for the solvation effect of formamide.Molecular Dynamic (MD) simulations were performed with the Materials Studio ® software and COMPASS force field.Three orthorhombic cells were generated with AC program (20 NaNO 3 and 120 CH 3 NO for NaNO 3 /FMD 6 case, 20 NaNO 3 and 160 CH 3 NO for NaNO 3 / FMD 8 case, 20 NaNO 3 and 200 CH 3 NO for NaNO 3 /FMD 10 case, respectively).Periodic boundary conditions were set in all three directions and the timestep was set as 1.0 fs.All the systems were firstly equilibrated in NVE ensemble for 50 ps and then equilibrated in NPT ensemble for 50 ps at 300 K to ensure the simulation boxes reach an equilibrium state.After that, the systems were calculated in NVT at 300 K for 1 ns.The radial distribution function (RDF) and coordination structure counting were sampled and recorded by every 1000 step.
Electrochemical tests: Linear sweep voltammetry tests were conducted at a scan rate of 5 mV s À1 using glassy carbon as working electrode (3 mm diameter of cross section), Pt plate as counter electrode and Ag/AgCl as reference electrode.The threshold of ESW was set to 0.02 mA cm À2 .NaNO 3 /FMD 6 , NaNO 3 / FMD 8 and NaNO 3 /FMD 10 electrolytes was assembled into CR2023 coin cells, which used commercial activated carbon electrodes (Kuraray ® , Japan, 2.1 mg cm À2 mass loading of active materials), commercial glass microfiber separators (Whatman ® , UK) and the stainless steel current collectors.To assess the interaction of NaNO 3 /FMD 6 and 21 m LiTFSI WIS electrolytes with commercial aluminum current collectors, the stainless steel current collectors were replaced by aluminum foil current collectors to assemble coin cells.Cyclic voltage (CV) tests, galvanostatic charge-discharge (GCD) tests, and electrochemical impedance spectroscopy (EIS) tests were conducted by Autolab electrochemical workstation (PGSTAT302N, Metrohm ® , Switzerland).The frequency range of the electrochemical impedance spectroscopy is from 10 5 to 10 À2 Hz.The long-term cycling test was conducted by Micro-current test system (CT3002A, LANHE ® , China).The 100 F pouch cells with NaNO 3 /FMD 6 and 21 m LiTFSI WIS electrolyte used 80 mm * 60 mm rectangle commercial activated carbon electrode sheets (Kuraray ® , Japan, 6.6 mg cm À2 mass loading of active materials), commercial nonwoven separators (NKK ® , Japan) and the commercial aluminum current collectors.The pouch cells adopted the aluminum electrode tabs and were sealed with aluminum plastic films.
The instantaneous specific capacitances (C CV , F g À1 ) in CV tests were calculated by the following formula: where I represents the instantaneous current (A) obtained in CV measurements, m is the mass of activated carbon (g) in electrodes and s is the scan rate (V s À1 ).
The specific capacitances (C GCD , F g À1 ) in GCD tests were calculated by the following formula: where I represents the current density (A g À1 ), t is the discharge time (s) obtained in GCD measurements, V is the voltage window (V).The energy densities (E, Wh kg À1 ) and power densities (P, W kg À1 ) of the supercapacitor were calculated by the following formulas: where C cell (F g À1 ) is the specific capacitance of the supercapacitor, V cell (V) is the voltage window, t cell (s) is the discharging time.
Corrosion tests: The NaNO 3 /FMD 6 and WIS supercapacitors with aluminum current collectors were tested by 5000 cycles of GCD at 5 A g À1 .Then the supercapacitors were disassembled to obtain the corroded current collectors.After completely washing with absolute alcohol, the aluminum current collectors were analyzed by Raman spectroscopy using HORIBA Scientific LabRAM HR with a 532 nm excitation laser.

Figure 1 .
Figure 1.Custom-designed microstructure, hydrogen bonding, and electronic configurations of the high-performance NaNO 3 /FMD x electrolytes.a) Schematic of NaNO 3 /FMD solvation microstructure at the interface and in bulk electrolyte.b) ESW values, c) the HOMO-LUMO energies and d) Raman spectroscopy of FMD and NaNO 3 /FMD electrolytes in wave number between 500 and 3750 cm À1 .e-g) Deconvolutions of H-bonds peak in the wave number range from 3150 to 3450 cm À1 .h, i) Molecular dynamics simulation.h) Snapshots of the NaNO 3 /FMD 6 , NaNO 3 /FMD 8 , and NaNO 3 /FMD 10 system.Atom Colors: C, gray; O, red; N, blue; H, white; Na, purple.i) Radial distribution functions of N FMD and N NO À 3 atoms.Abbreviations: N FMD denotes N atom from FMD molecule, and N NO À 3 denotes N atom from NO 3 À ion.

Figure 2 .
Figure 2. Physical properties and costs of NaNO 3 /FMD electrolytes.a) DSC thermogram of FMD and NaNO 3 /FMD electrolytes.b) Arrhenius plot of the electric conductivities of NaNO 3 /FMD electrolytes.c) Arrhenius plot of the viscosities of NaNO 3 /FMD electrolytes.d) Walden classification diagram at room temperature for different electrolytes.e) Ignition graph of different electrolytes.f) Comparison of ESW, electrical conductivity and cost between our NaNO 3 / FMD electrolytes and other DES and WIS electrolytes.

Figure 3 .
Figure 3. Outstanding electrochemical performance of NaNO 3 /FMD 6 AC||AC coin cell supercapacitor using stainless steel as the current collector.a) Cyclic voltammetry plot.b) Galvanostatic charge/discharge plot.c) Nyquist plot.d) Long-term cycling at 5 A g À1 and e) the corresponding Nyquist plot after 10 000 cycles and 30 000 cycles of the NaNO 3 /FMD 6 supercapacitor.f) Comparison of the voltage window and long-term cycling stability.g) Ragone plot of our NaNO 3 /FMD 6 supercapacitor and representative-electrolyte based supercapacitors (including DES, WIS and commercial organic electrolytes).

Figure 4 .
Figure 4. Potential for real-world supercapacitor applications.a) Cyclic voltammetry plot, b) Nyquist plot and c) Long-term cycling at 5 A g À1 of NaNO 3 / FMD 6 and 21 m LiTFSI AC||AC coin cell supercapacitors.d) Raman spectroscopy of aluminum current collectors (original aluminum with no corrosion, corroded aluminum in NaNO 3 /FMD 6 and WIS electrolytes).e) Flammability test of NaNO 3 /FMD 6 pouch cell supercapacitor.f) Lighting of 177 LED bulbs with two 100 F NaNO 3 /FMD 6 pouch cell supercapacitors.
3 /FMD 6 , NaNO 3 /FMD 8 and NaNO 3 /FMD 10 are 24.61,21.49, and 20.72 kJ mol À1 , respectively.The relatively low viscosities and E η values demonstrate the advantage of NaNO 3 /FMD electrolytes in enabling superior electrical conductivity and better electrochemical performance.