High Na‐ion conductivity and mechanical integrity of anion‐exchanged polymeric hydrogel electrolytes for flexible sodium ion hybrid energy storage

The polymeric gel electrolytes are attractive owing to their higher ionic conductivities than those of dry polymer electrolytes and lowered water activity for enlarged potential window. However, the ionic conductivity and mechanical strength of the Na‐ion conducting polymeric gel electrolytes are limited by below 20 mS cm−1 and 2.2 MPa. Herein, we demonstrate Na‐ion conducting and flexible polymeric hydrogel electrolytes of the chemically coupled poly(diallyldimethylammonium chloride)‐dextrin‐N,N′‐methylene‐bis‐acrylamide film immersed in NaClO4 solution (ex‐DDA‐Dex + NaClO4) for flexible sodium‐ion hybrid capacitors (f‐NIHC). In particular, the anion exchange reaction and synergistic interaction of ex‐DDA‐Dex with the optimum ClO4− enable to greatly improve the ionic conductivity up to 27.63 mS cm−1 at 25°C and electrochemical stability window up to 2.6 V, whereas the double networking structure leads to achieve both the mechanical strength (7.48 MPa) and softness of hydrogel electrolytes. Therefore, the f‐NIHCs with the ex‐DDA‐Dex + NaClO4 achieved high specific and high‐rate capacities of 192.04 F g−1 at 500 mA g−1 and 116.06 F g−1 at 10 000 mA g−1, respectively, delivering a large energy density of 120.03 W h kg−1 at 906 W kg−1 and long cyclability of 70% over 500 cycles as well as demonstrating functional operation under mechanical stresses.


INTRODUCTION
Electrolytes are essential components that transport ion carriers insulating electron conduction between two electrodes for energy storage applications. 13][4] However, they are limited by the narrow electrochemical window due to the water decomposition and leakage. 5,6Additionally, separators are needed to prevent short circuit and immerge electrolyte solutions, 1 but their mechanical properties are not enough to satisfy the requirement for flexible energy storage devices. 5In order to resolve these problems, the polymeric hydrogel electrolytes have been investigated owing to their higher ionic conductivities than those of dry polymer electrolytes and lowered water activity for enlarged potential window. 7,8Despite these promising merits, the polymeric hydrogel electrolytes still suffer low mechanical properties and degradation of electrochemical performances under mechanical stresses. 9Although chemical strategies, including crosslinking, 10 copolymerization, 11 and nanocomposites, 12 have been exploited to improve mechanical property, it is still difficult for the hydrogel electrolytes to overcome the tradeoff between ionic conductivity and mechanical integrity. 13erein, we demonstrate a highly ion-conducting and flexible renewable polymeric hydrogel electrolyte films prepared through the combined free radical polymerization, double networking, and anion exchange chemistries for flexible sodium ion hybrid capacitors (f-NIHC). 14,158][19] However, they are also restricted by low energy density, 20 which becomes more critical in aqueous electrolytes. 216][27] Examples of Na-ion conducting hydrogel electrolytes include polyvinyl alcohol (PVA), polyacrylic acid, polyacrylamide, agar, and chitosan-based hydrogels. 5,28,29However, their ionic conductivity and mechanical strength are limited by below 20 mS cm −1 and 2.2 MPa. 5 Furthermore, most Na-ion conducting hydrogel electrolytes are strong yet brittle, which are not suitable for f-NIHCs, and the renewable polymers are very difficult to meet the requirements for the ionic conducting and mechanically robust electrolytes.
Our hydrogel electrolytes are designed on a basis of the combined free radical polymerization, double networking, and anion exchange chemistries as illustrated in Figure 1.The double networking structure of cross-linked poly(diallyldimethylammonium chloride) (x-polyDDA) as hard network and dextrin polymers as soft network is attributed to achieving both the mechanical strength and softness of x-polyDDA/dextrin double networking (x-DDA-Dex) hydrogel electrolytes (Figure S1).More importantly, an anion exchange reaction between Cl − in x-DDA-Dex hydrogel film and polyatomic anion-herein, ClO 4 − is the optimum-allows ex-DDA-Dex to greatly improve the mechanical strength up to 7.48 MPa.The superiorities of ex-DDA-Dex hydrogel electrolytes were confirmed demonstrating a large energy density of 120.03 W h kg −1 at 906 W kg −1 , a high-rate capacity of 116.06 F g −1 at 10 000 mA g −1 , and long cyclic stability over 500 cycles as well as a functional operation of f-NIHCs under mechanical stresses.

RESULTS AND DISCUSSION
The successful polymerization of x-DDA-Dex hydrogel was confirmed comparing the Fourier transform infrared (FT-IR) spectra of DDA monomer, polyDDA, and x-polyDDA (Figure 2a).Prior to FT-IR analysis, x-polyDDA sample was washed by deionized water (DIW) at three times to remove impurities such as unreacted monomers and initiators.The peak of all three samples at 3380 cm −1 is assigned to the stretching −OH vibration of absorbed water. 30The CH 2 bending and stretching peaks of DDA monomers at 3018 and 2950 cm −1 , respectively, were not observed for polyDDA and x-polyDDA. 31On the other hand, the C-N stretching peak of x-polyDDA at 1647 cm −1 remained intact, which suggests that nitrogen was not involved in the chemical reaction. 32These findings indicate that the chemical reaction takes place at the CH 2 as highlighted in blue color.The stretching peaks of C = C double bond at 1476 and 960 cm −1 , respectively, were weakened in a descending order from DDA monomer through polyDDA to x-polyDDA. 32The intensity decrease of these peaks signifies that the double bond of the DDA monomer is released, whereas the degree of polymerization increases.Therefore, the degree of polymerization of x-polyDDA becomes higher than that of commercial polyDDA (M w 200 000-550 000).FT-IR and Raman spectroscopies were applied to further investigate the anion exchange reaction of the x-polyDDA as shown in Figure 2B  confirms that a reaction took place between x-polyDDA and ClO 4 − , 33,34 as described in Figure 2B.This ClO 4 − peak of the x-polyDDA hydrogel films shows a marginal variation with reference to that of the ClO 4 − in the aqueous solution.
The chemical structures of x-DDA-Dex and ex-DDA-Dex hydrogels were compared using 13 C CP-MAS solidstate nuclear magnetic resonance (NMR) spectroscopy (Figure 2D).The chemical shifts of two hydrogel films are observed at 71.3, 53.1, 38.5, 30.1, and 26.7 ppm for x-DDA-Dex and at 71.3, 53.3, 38.5, 27.9, and 26.6 ppm for ex-DDA-Dex, respectively, which are assigned to the characteristic peaks of the polyDDA.These peaks can be identified at the positions of 1-5 and 6-10, respectively.The exchange reaction between Cl − and ClO  is attributed to the interaction of ClO 4 − with CH 3 at the position 7. 35 Moreover, the x-DDA-Dex and ex-DDA-Dex hydrogel films exhibit no changes in the two chemical shifts at 71.3 and 38.5 ppm.On the other hand, 30.1 and 26.7 ppm of x-DDA-Dex hydrogel at the position 4 and 5 were moved into 27.9 and 26.6 ppm of ex-DDA-Dex hydrogel at the position 9 and 10 after the anion exchange from Cl − to ClO 4 − .These findings indicate that no significant change in the structure of the polymer backbone occurs during the ion exchange reaction. 36he photographic and scanning electron microscope images of x-DDA-Dex and ex-DDA-Dex hydrogel films are shown in Figure 3.After swelling in water, the x-DDA-Dex + NaClO 4 hydrogel film exhibited a substantial weight increase from 0.09 to 1.54 g and a 10.21 times expansion in area as shown in Figure 3A.In contrast, the area of ex-DDA-Dex + NaClO 4 hydrogel films decreased by 0.96 times despite the weight increase from 0.10 to 0.  and 3.02 times of the original area, respectively.Notably, the huge expansion of the hydrogel film in DIW is attributed to its hygroscopic nature for abundant water absorption.Furthermore, the ex-DDA-Dex hydrogel films swelled in 1M NaSO 4 and NaNO 3 solutions were expanded moderately, whereas the hydrogel films were slightly contracted in area after being immersed in 1 M NaClO 4 and NaBF 4 solutions.As shown in Figure 4C, the as-swelled hydrogel films were hand-stretched using tweezers to verify their mechanical strength.The x-DDA-Dex hydrogel film swelled in water leads to a significant decrease in its mechanical properties, whereas the ex-DDA-Dex hydrogels were readily ruptured when being expanded in 1 M Na 2 SO 4 and NaNO 3 solutions.On the other hand, the as-contracted ex-DDA-Dex hydrogel film in NaClO 4 and NaBF 4 became opaque and white, demonstrating the mechanical integrity of stretching and twisting despite no significant change in the structure of the polymer backbone after anion excahnge. 36n order to further confirm the effect of anion exchange on the mechanical properties, the stress-strain curves and toughness (obtained from underlining area) of the x-DDA-Dex, ex-DDA-Dex + NaClO 4 , ex-DDA-Dex + NaBF 4 , ex-DDA-Dex + NaNO 3 , and ex-DDA-Dex + Na 2 SO 4 hydrogel films were measured at room temperature (Figure 4D and Figure S3a,b).As expected, the tensile strength (σ max ) values of the ex-DDA-Dex hydrogel films exchanged with NaNO 3 and Na 2 SO 4 were 0.32 and 0.18 MPa, respectively, higher than 0.073 MPa of x-DDA-Dex, which is consistent with stretching test in Figure 4C film demonstrates a superior capability for energy dissipation compared to other hydrogel films, reaching the maximum stress level at 103.01%strain.Additionally, the ex-DDA-Dex hydrogel film achieved the highest toughness of 5.77 MJ m −3 when exchanged with ClO 4 − ions (Figure S3a,b).
In order to determine the optimal composition, σ max values of ex-DDA-Dex + NaClO 4 hydrogel films were compared varying the content of dextrin from 0 to 200 mg (Figure S3c,d).The ex-DDA-Dex + NaClO 4 Hydrogel films with the dextrin content of 0, 50, 100, 150, and 200 mg achieved 8.47, 7.82, 7.48, 5.71, and 4.44 MPa, respectively.At the higher content of dextrin, the tensile strength decreased, whereas the elongation length increased by the enhanced strain ratio.On the other hand, the toughness at each dextrin content was 0.61, 2.93, 5.77, 4.73, and 3.81 MJ m −3 , indicating the highest toughness at the dextrin content of 100 mg.bonds with the methyl group in the x-polyDDA chain. 37he ionic links pull the polymer chains closer, allowing the hydrogen bonds to form the crosslinking of x-polyDDA chains.Moreover, the hydrogen bonds linked with the backbones significantly increase the tensile strength of the ex-DDA-Dex hydrogel.Among the anion exchanged hydrogel films, the ex-DDA-Dex + NaClO hydrogel film was 27.63 mS cm −1 at room temperature, higher than 18.19 mS cm −1 of x-polyDDA + NaClO 4 hydrogel film, and twice higher than 12.65 mS cm −1 of PVA + NaClO 4 film (Figure S5).
The activation energies of the ex-DDA-Dex + NaClO 4 and PVA + NaClO 4 hydrogel films were derived from the plots of ionic conductivities (ln σ) versus temperatures (1000/T) from 30 to 80 • C following the Arrhenius equation of ln σ = −E a /RT + ln A, where σ is the ionic conductivity, T is the thermodynamic temperature, E a is the activation energy, and A is the pre-exponential factor.As shown in Figure 6B, the Arrhenius equation was well fitted (R 2 = .98),indicating the ion transport dominated by the thermally activated ion diffusion.The ex-DDA-Dex + NaClO 4 hydrogel film achieved an activation energy of 0.1460 eV, much lower than 0.3951 eV of PVA + NaClO 4 hydrogel film, indicating lower activation barrier to ion flow for faster ion transport.This observation indicates that ex-DDA-Dex + NaClO 4 hydrogel film is capable of facilitating ion diffusion through the interconnected channels.Besides, elevated temperatures promote the segmental motion of the polymer chain, which generates favorable ion conduction pathways. 14,39he LSV curve at 10 mV s −1 was measured to evaluate the electrochemical stability window of ex-DDA-Dex + NaClO 4 hydrogel film using the Ti blocking electrodes (Figure 6C).The ex-DDA-Dex + NaClO 4 hydrogel film was electrochemically stable from 0 to 2.6 V at 25 • C, which is much wider than 1.23 V of water decomposition range in aqueous electrolytes.This stability window of the ex-DDA-Dex + NaClO 4 hydrogel film is also wider than 2.2 V of PVA + NaClO 4 hydrogel film.When water is confined inside the polymeric networks of hydrogels, water decomposition reactions can be suppressed due to the reduced water activity. 40Therefore, the electrochemical stability of ex-DDA-Dex + NaClO 4 hydrogel film could be improved up to 2.6 V due to the inhibited water activity by water-in-polymer network, thereby enlarging the voltage window.
As shown in Figure 6D, the Nyquist plots were used to estimate the ionic conductivities of ex-DDA-Dex + NaClO 4 and ex-DDA-Dex + NaBF 4 hydrogel films.The ionic conductivity of ex-DDA-Dex + NaClO 4 hydrogel film was 27.63 mS cm −1 at room temperature, also order of magnitude higher than 0.46 mS cm −1 of ex-DDA-Dex + NaBF 4 hydrogel film, which implies the importance of ClO 4 − anion on ionic transport.][43] In order to demonstrate the superiority of ex-DDA-Dex + NaClO 4 hydrogel electrolyte, the f-NIHCs were fabricated configuring NVP@rGO positive electrode with AC negative electrode.The total charges of the positive and negative electrodes were balanced (q+ = q−) by adjusting the mass loading ratio of NVP@rGO to AC as 1:2.3 (Figures S6 and S7). Figure 7A displays the Nyquist plots of f-NIHCs with ex-DDA-Dex + NaClO 4 and PVA + NaClO 4 hydrogel electrolytes.Both f-NIHCs exhibited the semicircular shapes at high frequencies, which refer to the characteristic of charge transfer resistance. 44he ESR of NVP@rGO|ex-DDA-Dex + NaClO 4 hydrogel film|AC f-NHIC was measured to be 0.35 Ω, lower than NVP@rGO|PVA + NaClO 4 hydrogel film|AC f-NIHC (0.49 Ω).The charge transfer resistance of the f-NHIC with ex-DDA-Dex + NaClO 4 hydrogel film was 6.57Ω, much lower than 22.90 Ω with PVA + NaClO 4 hydrogel film, indicating the faster ion transfer of the former at the electrode/electrolyte interface.The effect of temperature on the impedance of the f-NHIC with ex-DDA-Dex + NaClO 4 hydrogel film was studied measuring the Nyquist plots at temperatures from 30 to 90 • C (Figure 7B).As the temperature of the f-NIHCs increases, the Warburg slope related to ion diffusion increases at a low frequency and the charge transfer resistance decreases, indicating the facilitated ion transport into the electrode and at the interface.Accordingly, the ESRs of the f-NHICs with ex-DDA-Dex + NaClO 4 hydrogel film were reduced from 0.32 to 0.11 Ω due to the thermally activated ion transfer when the temperatures increased from 30 to 90 • C.
In order to find a proper potential window of each electrode, the CV curves at 10 mV s −1 in a three-electrode configuration were collected using NVP@rGO or AC, Ag/AgCl, and Ti foil as working, reference, and counter electrode, respectively, in 15 M NaClO 4 WIS solution as shown in Figure 7C.The CV curves of the AC and NVP@rGO electrodes in 15 M NaClO 4 electrolytes were collected in the potential range of −0.75-0 V and 0-0.75 V, respectively, at a scan rate of 10 mV s −1 .As expected, the NVP@rGO|ex-DDA-Dex + NaClO 4 hydrogel film|AC f-NHICs in the WIS electrolyte can be stable at 1.5 V working voltage without any obvious gas evolution.The CV curve of NVP@rGO electrode exhibited the obvious redox peaks of typical faradaic reaction, whereas the CV curves of AC showed rectangular shapes, representing their behavior as electric double-layer capacitors.The CV curves of two f-NIHCs with the ex-DDA-Dex + NaClO 4 and PVA + NaClO 4 hydrogel electrolytes are measured in a two-electrode configuration as displayed in Figure 7D.Both curves do not look like a rectangular shape but show the distinctive redox peaks in the enlarged voltage window of 0-1.5 V, indicating the dominant electrochemical behavior of NVP@rGO positive electrode.The separation distance of the ex-DDA-Dex + NaClO 4 -based f-NIHC between two redox peaks of E pc and E pa is 0.12 V, which is narrower than 0.85 V with PVA + NaClO 4 , indicating more reversible and faster redox kinetics of the former rather than the latter.
The electrochemical performances of f-NIHC full cells were further investigated measuring the rate capability and energy and power densities as shown in Figure 8.The distinct charging and discharging peaks of CV curves at 1.16 and 0.25 V were preserved varying scan rates from 1 to 200 mV s −1 (Figure 8A), which implies the fast charge storage kinetics.Figure 8B shows the sloped profiles of charge-discharge curves, which are consistent with the redox feature of CV curves.The f-NHIC with the ex-DDA-Dex + NaClO 4 hydrogel electrolyte achieved the high specific capacitance of 192.04 F g −1 at 500 mA g −1 , preserving the high capacitance of 116.06 F g −1 even at the high current rate of 10 000 mA g −1 (Figure 8C).As shown in the Ragone plot of our f-NIHC in Figure 8D, the f-NHIC with the ex-DDA-Dex + NaClO 4 hydrogel electrolyte delivered a high power density of 12.32 kW kg −1 at 71.85 W h kg −1 and a large energy density of 120.3 W h kg −1 at 0.906 kW kg −1 .][47][48][49] The mechanical integrity and electrochemical stability of the ex-DDA-Dex + NaClO 4 hydrogel electrolyte are important for the application into flexible energy storage. 50,51As shown in Figure 9A,B, the electrochemical stability of the f-NIHC was measured under mechanical stresses varying the bend angles of the cells.As demonstrated by the bending test (Figure S8), the f-NHICs with the ex-DDA-Dex + NaClO 4 hydrogel films were bent in four steps from normal state of 0 • -90 • for a flexibility test, then returned to a normal state.All the CV curves at different bending angles were superimposed without noticeable distortion, confirming the mechanical integrity of the f-NIHC cells.The capacitance retention was estimated varying the bending angles for 40 cycles.Overall, 92% of the initial capacitance was preserved at the bending angle of 90 • , and then the capacitance reached up to 96% of the initial capacitance while returning to a normal condition.The cycle stability of the f-NHIC with the ex-DDA-Dex + NaClO 4 hydrogel film was demonstrated measuring the capacitance retention and Coulombic efficiency during 500 charging/discharging cycles at 5000 mA g −1 (Figure 9C).In aqueous electrolyte solutions, NVP@rGO electrode shows a poor cyclic stability due to the structure collapse and dissolution of vanadium-based materials as demonstrated by the capacitance retention of less than 32% after 30 cycles. 52,53Combining ex-DDA-Dex + NaClO 4 hydrogel electrolyte with NVP@rGO, the activity of the sol-vent was lowered due to the strong confinement of water inside the polymeric porous networks, which could resolve these problems of vanadium-based materials.The capacitance retention of NVP@rGO|ex-DDA-Dex + NaClO 4 hydrogel film|AC f-NIHC was 70% after 500 cycles, higher than 30% of NVP@rGO|PVA + NaClO 4 hydrogel film|AC f-NIHC, along with a high Coulombic efficiency of 98.78%.For more practical applications, the cell voltage could be enhanced by connecting several cells in a series configuration.With this tandem configuration, the f-NHIC with the ex-DDA-Dex + NaClO 4 hydrogel electrolyte succeeded in operating the 1.5 V clocks as shown in Figure S9 and supporting movie.

CONCLUSION
In this study, we have demonstrated highly Na-ion conducting and flexible ex-DDA-Dex + NaClO 4 hydrogel electrolyte films prepared through the combined free radical polymerization, double networking, and anion exchange chemistries.The synthesis procedures and chemical interactions of the double networked ex-DDA-Dex + NaClO 4 hydrogel electrolyte have been extensively investigated analyzing FT-IR, Raman, and 13 C CP MAS solid-state NMR spectra.As systematically compared with various polyatomic anions used for anion exchange reactions, the coupled ionic and hydrogen bonding interactions between ex-DDA-Dex and ClO 4 − significantly improved the electrochemical, mechanical, and thermal properties.Namely, ex-DDA-Dex + NaClO 4 achieved high tensile strength of 7.48 MPa and stretchability as well as high ionic conductivity of 27.63 mS cm −1 at 25 • C and electrochemical stability of up to 2.6 V, which were much better than conventional hydrogel electrolyte of PVA + NaClO 4 .The wide electrochemical stability window of ex-DDA-Dex + NaClO 4 allows the f-NHIC to be configured by NVP@rGO positive and AC negative electrodes at the mass loading ratio of 1-2.3.The resulting f-NHICs with the ex-DDA-Dex + NaClO 4 hydrogel electrolytes achieved the high specific capacitance of 192.04 F g −1 at 500 mA g −1 , showing the high-rate capacitance of 116.06 F g −1 even at 10 000 mA g −1 owing to the more reversible and faster charge storage kinetics rather than PVA + NaClO 4 .In particular, the f-NHICs with the ex-DDA-Dex + NaClO 4 hydrogel electrolyte delivered a large energy density of 120.3 W h kg −1 at 0.906 kW kg −1 and a high power density of 12.32 kW kg −1 at 71.85 W h kg −1 , which is superior to the previously reported Na ion energy storage devices.Furthermore, these f-NHICs with the ex-DDA-Dex + NaClO 4 hydrogel electrolyte showed a long-term cycle stability of 70% over 500 cycles at 5000 mA g −1 , as well as functional operation for practical flexible devices.Therefore, this study proposes an innovative chemistry into the design of highly Na-ion conducting and flexible hydrogel electrolytes to overcome the tradeoff of existing gel electrolytes between ionic conductivity and mechanical integrity, as well as the development of flexible Na-ion hybrid energy storage devices with the large energy and power densities.

Synthesis of x-DDA-Dex hydrogel film
For the synthesis of x-DDA-Dex hydrogel film, the x-polyDDA, which is the first network of x-DDA-Dex hydrogel, was prepared by the following method (Figure S10).The x-polyDDA hydrogel film was formed through a thermally initiated free-radical polymerization of the mixture.First, DDA, K 2 S 2 O 8 , and MBAA were used as a monomer, an initiator, and a crosslinker, respectively.First, MBAA of 30 mg (1.1 wt% of x-DDA-Dex) and K 2 S 2 O 8 of 1.5 mg (0.057 wt% of x-DDA-Dex) were added into 3.9 M of DDA aqueous solution (5 mL).This mixture was stirred and heated at 90 • C for 2 min.For the formation of second network (Figure S1), a dextrin powder (100 mg) was added into a DDA, MBAA, and KPS aqueous mixture (5.11 mL), whereas it did not react with any other reagent.The composite mixture was stirred 90 • C for 3 min until the dextrin powder is dissolved and the mixture becomes transparent.The polymerization reaction was terminated just before the viscosity of reaction mixture reached a gel level.Then, the mixture was immediately poured into a Teflon mold with a controlled thickness.The film was dried in a synthetic oven at 90

Synthesis of ex-DDA-Dex hydrogel film
For the synthesis of ion-exchanged x-DDA-Dex (ex-DDA-Dex) hydrogel films, the as-synthesized x-DDA-Dex hydrogel films were immersed in aqueous solutions including various salts and soaked for 5 min.The four types of the ex-DDA-Dex hydrogel films were synthesized performing the ion exchange reaction between Cl − of DDA and polyatomic ions of various salts (i.e., NaClO 4 , NaBF 4 , NaNO 3 , and Na 2 SO 4 ).Appending the type of salt used in the ion exchange reaction to the ex-DDA-Dex hydrogel, the ex-DDA-Dex hydrogel films were named as ex-DDA-Dex + NaClO 4 , ex-DDA-Dex + NaBF 4 , ex-DDA-Dex + NaNO 3 , ex-DDA-Dex + Na 2 SO 4 , respectively.The ex-DDA-Dex hydrogel films were peeled off from the Teflon plate and washed with DIW at three times to remove other impurities.For the measurement of solid-state 13 C CP-MAS NMR, all the samples were prepared as follows.The x-DDA-Dex and ex-DDA-Dex films were synthesized following the same synthetic procedures as mentioned above and dried the products in vacuum oven for overnight to remove water.

Preparation of electrodes
The cathode for f-NIHCs consists of 80 wt% Na 3 V 2 (PO 4 ) 3 @rGO (NVP@rGO), 54 10 wt% carbon black, and 10 wt% PVDF.The slurry of NVP@rGO was coated on Ti substrate foil (60 μm) with a mass loading of 1.2 mg cm −2 .The anode for f-NIHCs was composed of 80 wt% activated carbon (AC), 10 wt% PVDF, and 10 wt% carbone black.This slurry was coated onto Ti substrate foil (60 μm) with a mass loading of 2.8 mg cm −2 .The previous electrodes were dried at 80 • C in air for 12 h and eventually prepared into area of 10 × 20 mm 2 .

Fabrication of f-NIHCs using ex-DDA-Dex hydrogel
The f-NIHCs were composed of the ex-DDA-Dex hydrogel film (∼150 μm thickness) used as both electrolyte and separator, commercial AC (YP-50F) as anode, NVP@rGO as cathode, and Ti foil as a current collector (Figure 3A).The f-NIHCs was fabricated by sandwiching the hydrogel film between two electrodes and performing hot pressing (1 MPa) at 80 • C for 20 s.For the electrochemical measurement of neat ex-DDA-Dex hydrogel films, they were sandwiched between Ti current collectors acting as blocking electrodes.All the fabrication process was performed in air condition.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F I G U R E 1
Schematic illustration about the chemical structure of x-poly(diallyldimethylammonium chloride) (DDA), x-DDA-Dex, and the anion exchanged ex-DDA-Dex hydrogel electrolyte film.
,C.The x-polyDDA was immersed in various aqueous solutions of 1 M LiClO 4 , NaClO 4 , Al(ClO 4 ) 3 , NaBF 4 , NaNO 3 , Na 2 SO 4 , and then washed with DIW to remove any residue anion of the unreacted salt.Comparing FT-IR spectra of various salt solutions (Figure S2), the peaks associated with the salts soaked in the x-poly DDA hydrogel films are clearly identified as shown in Figure 2B.The sharp intense peak at 1086 cm −1 appeared when x-polyDDA samples were immersed in aqueous solutions of LiClO 4 , NaClO 4 , and Al(ClO 4 ) 3 containing a perchlorate anion of ClO 4 − .The x-polyDDA immersed in aqueous Na salt solutions of NaBF 4 , NaNO 3 , and Na 2 SO 4 show new peaks at 1038 cm −1 for BF 4 − and 1078 cm −1 for SO

4 −
was verified demonstrating a slight sharpening at 53.1 ppm, which
28 g.Although the as-swelled x-DDA-Dex hydrogel film is nearly transparent, the ex-DDA-Dex counterpart becomes translucent owing to the existence of exchanged ClO 4 − anions.After a freeze-drying and breaking of x-DDA-Dex hydrogel film and ex-DDA-Dex + NaClO 4 hydrogel film by liquid nitrogen, their surface and internal structures were observed in Figure 3B,C.The as-dried x-DDA-Dex hydrogel films showed an interconnected porous structure that can hold more solvents, but the low tensile strength failed in constructing self-standing electrolyte films.Energy-dispersive X-ray elemental mapping images of the x-DDA-Dex and ex-DDA-Dex + NaClO 4 hydrogel films are shown in Figure 3D,E.Indeed, Na and Cl elements of the ex-DDA-Dex + NaClO 4 hydrogel film were distributed homogeneously through the cross-sectional area, confirming the uniform distribution of the NaClO 4 salt embedded in the hydrogel film without aggregation.The effect of exchanged anions on the wettability and mechanical property of ex-DDA-Dex hydrogel film was investigated as shown in Figure 4.The Cl − ion of the x-DDA-Dex hydrogel film was exchanged in 1M aqueous solutions including NaClO 4 , NaBF 4 , Na 2 SO 4 , and NaNO 3 .Figure 4A,B shows the photographic images of x-DDA-Dex hydrogel films before and after being immersed in aqueous F I G U R E 3 (A) Photographic, (B) surface scanning electron microscope (SEM), and (C) cross-sectional SEM images of the x-diallyldimethylammonium chloride (DDA)-Dex (top) and ex-DDA-Dex + NaClO 4 (bottom) hydrogel films after swelling.(D) Cross-section energy-dispersive X-ray (EDX) mapping images of (D) the x-DDA-Dex hydrogel films and (E) ex-DDA-Dex + NaClO 4 hydrogel films observed by SEM (depicting the presence of C, N, O, Cl, and Na elements).solutions of DIW, NaClO 4 , NaBF 4 , Na 2 SO 4 , and NaNO 3 .The x-DDA-Dex hydrogel films immersed in DIW, NaClO 4 , NaBF 4 , Na 2 SO 4 , and NaNO 3 solutions underwent volume expansion or contraction by 10.21, 0.96, 0.91, 3.26, . In particular, the ex-DDA-Dex + NaClO 4 and ex-DDA-Dex + NaBF 4 exhibited the dramatic increase in σ max values up to 7.48 and 5.53 MPa, respectively, which were two orders of magnitudes greater than that of x-DDA-Dex hydrogel film.Especially, the ex-DDA-Dex + NaClO 4 hydrogel F I G U R E 4 Photographic images of the x-diallyldimethylammonium chloride (DDA)-Dex hydrogel films (A) before and (B) after immersion in deionized water (DIW), 1 M NaClO 4 , 1 M NaBF 4 , 1 M Na 2 SO 4 , and 1 M NaNO 3 aqueous solutions, (C) photographic images of the stretching test of the x-DDA-Dex hydrogel films after immersion in DIW, 1 M NaClO 4 , 1 M NaBF 4 , 1 M Na 2 SO 4 , and 1 M NaNO 3 aqueous solutions, (D) stress-strain curves of x-DDA-Dex, ex-DDA-Dex + NaClO 4 , x-DDA-Dex + NaBF 4 , x-DDA-Dex + Na 2 SO 4 , and x-DDA-Dex + NaNO 3 hydrogels, and (E) the networked structure and the interaction of the ex-DDA-Dex + NaClO 4 .

Figure
Figure 4E illustrates two modes of the interaction between the exchanged ClO 4 − anions and the x-polyDDA of the ex-DDA-Dex + NaClO 4 hydrogel film.First, the single bonded oxygen atom in ClO 4 − ionically links with the quaternary ammonium in the x-polyDDA ring.Second, the double bonded oxygen atoms in ClO 4 − forms hydrogen

4 F
4 hydrogel film achieved the similar physical properties to those of ex-DDA-Dex + NaBF 4 in terms of stretchability, color, and mechanical strength.Accordingly, the x-DDA-Dex hydrogel film actively interact with NaClO 4 and NaBF 4 in two modes as discussed above, whereas NaNO 3 and Na 2 SO I G U R E 5 TGA curves of polyvinyl alcohol (PVA) + NaClO 4 , x-diallyldimethylammonium chloride (DDA)-Dex, and ex-DDA-Dex + NaClO 4 hydrogels.salts displayed no strong interaction resulting in the moderate swelling and weak mechanical property.In order to examine thermal stabilities, TGA curves of the x-DDA-Dex, ex-DDA-Dex + NaClO 4 , and PVA + NaClO 4 hydrogels were collected at a heating rate of 10 • C min −1 in air condition as shown in Figure 5.The PVA + NaClO 4 hydrogel experienced thermal decomposition starting from 251.23 to 330 • C. The weight loss of the x-DDA-Dex and ex-DDA-Dex from 25 to 100 • C is attributed to the evaporation of moisture from the film.The x-DDA-Dex hydrogel underwent thermal decomposition at stage I over a wide temperature range from 202.78 to 413.38 • C, whereas the ex-DDA-Dex + NaClO 4 hydrogel began to be decomposed at 359.39 • C and endured thermal decomposition up to 410.2 • C. The thermal decomposition of x-DDA-Dex hydrogel at stage I is attributed to the chemical reaction, where the methyl groups of x-polyDDA are decomposed and combined with Cl − ions (Figure S4). 38On the other hand, the thermal decomposition of ex-DDA-Dex + NaClO 4 hydrogel at stage I is due to the degradation reaction, where the methyl groups of x-polyDDA are decomposed and combined with ClO 4 − ions.In the stage II, both x-DDA-Dex and ex-DDA-Dex + NaClO 4 hydrogels underwent decomposition in a range from 410 to 460.0 • C. The thermal decomposition for both samples at stage II is attributed to a weight loss of the remaining material in the hydrogel.These findings indicate that the ex-DDA-Dex + NaClO 4 hydrogel showed a significant improvement in the thermal stability after the anion exchange of x-DDA-Dex hydrogel.Furthermore, the ex-DDA-Dex + NaClO 4 hydrogel is more thermally stable than PVA + NaClO 4 hydrogel.As shown in Figure 6, the electrochemical properties of the ex-DDA-Dex + NaClO 4 hydrogel films were analyzed measuring ionic conductivity, activation energy, and electrochemical potential window.The Nyquist plots of ex-DDA-Dex + NaClO 4 hydrogel films were collected at the frequencies ranging from 0.1 Hz to 1 MHz in the temperatures range of 30-90 • C after conditioning them in an aqueous 15 M NaClO 4 water in salt (WIS) solution.The Nyquist plots of ex-DDA-Dex + NaClO 4 hydrogel films were also measured to compare with those of PVA + NaClO 4 hydrogel films against Ti blocking electrodes in a flexible cell configuration.As the temperatures increased, the equivalent series resistances (ESRs) decreased from 0.27 Ω at 30 • C to 0.115 Ω at 90 • C. In particular, the ionic conductivity of ex-DDA-Dex + NaClO 4
• C for 8 h to remove moisture and to participate both monomers and crosslinking agents in the reaction.During the process, the thickness of x-DDA-Dex hydrogel film was controlled in a range from 100 to 150 μm.The thickness of x-DDA-Dex hydrogel film was varied according to the types of exchanged anions.When the larger size and mass of ClO 4 − anion than those of Cl − anion were exchanged, the thickness of the x-DDA-Dex hydrogel film increased as further supported by the weights of x-DDA-Dex and ex-DDA-Dex + NaClO 4 hydrogel films (Figure S11).The weight of x-DDA-Dex hydrogel film increased by 21.33% after the anion exchange reaction into ex-DDA-Dex + NaClO 4 .
This research is supported by both the financial support from the National Research Foundation (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2020R1A3B2079803) and "Rediscovery of the Past R&D Result" through the Ministry of Trade, Industry and Energy (MOTIE) and the Korea Institute for Advancement of Technology (KIAT) (Grant No.: P0026069), Republic of Korea.