Synergistic effect of K+ and PANI in vanadium oxide hydration by interlayer engineering boosts the ammonium ion storage

Aqueous ammonium‐ion (NH4+) hybrid supercapacitor (AA‐HSC), as a new type of energy storage device with great potential, is in the initial stage of rapid development. Based on its special energy storage mechanism, exploiting novel NH4+‐hosting materials is still a great challenge. Herein, vanadium oxide hydration (VOH) tuned by interlayer engineering of K+/PANI co‐intercalation, named KVO/PANI, is designed for AA‐HSC. Intercalated PANI can shield interaction between NH4+ and V–O layers to some extent and enlarge interlayer space, which improves the efficiency of reversible NH4+ (de)insertion. However, K+ enhances redox activity and electronic conductivity. The synergistic effect of co‐intercalation optimizes intercalation pseudocapacitive behavior during the (de)ammonization process, which is reported in NH4+ storage for the first time. Theoretical calculations reveal that the lowered electron transport barrier and enhanced electronic conductivity improve NH4+ kinetics and exhibit high capacitance for charge storage. The KVO/PANI can deliver the specific capacitance of 340 F g−1 at 0.5 A g−1 and retain 177 F g−1 at 10 A g−1. Pairing with activated carbon, the AA‐HSC can achieve a decent energy density of 31.8 Wh kg−1. This work gives inorganic/organic co‐intercalation that can enhance the NH4+ storage of VOH by interlayer engineering. The strategy can be used to design other materials for aqueous energy storage systems.


INTRODUCTION
Aqueous ammonium-ion (NH 4 + ) energy storage systems (ESSs) are supposed to use in grid-scale applications with one of the most promising technologies. First, the NH 4 resource and adequately unlimited in quantity. In addition, the lightweight (18 g mol −1 ) and diminutive hydrated ionic size (3.31 Å) enable quick diffusion in aqueous electrolytes. What is more, it is worth noting that NH 4 + with strong preferential orientation exhibits a tetrahedral shape. The topotactic intercalation chemistry in NH 4 + host materials may be completely different from that with spherical metal ions (Li + , Na + , etc.). [4][5][6][7][8] Overall, it is of great significance to research the feasibility of aqueous NH 4 + -storage systems and reveal its potentially remarkable binding chemistry. 9 Indeed, NH 4 + -based storage systems, including electrode materials as well as related electrolytes, have been intensively investigated recently. [10][11][12][13][14][15] However, exploring NH 4 + -storage systems is still in the early development stage. The problems, including poor stability and low capacitance, are still unresolved, which are closely related to the energy storage mechanism of hydrogen bond chemistry. Ammonium ions embedded in materials are not completely extracted, resulting in reduced reversibility and stability. 16 Besides, owing to the lack of appropriate NH 4 + host materials with capacious space structure, NH 4 + with correspondingly large ionic radius in contrast to metal ions (Na + , K + , etc.) exhibit limited specific capacity. In addition, the sluggish redox kinetics and weak electrode conductivity lead to relatively lowrate performance and barely satisfactory electrochemical performance. [17][18][19] Thus, considering the special mechanism of NH 4 + storage, it is very necessary to develop novel NH 4 + -hosting material with an appropriate structure for reversible NH 4 + (de)intercalation and excellent electrochemical performance. Up to now, different materials have been investigated for NH 4 + storage, such as vanadium-based material, 2,20 manganese-based oxides, 5 molybdenum oxide, 4,21 and Prussian blue analogs. 1,17 Among them, vanadium oxides deliver high theoretical capacity due to multi-electron redox and comparatively low molar mass. 11,22 It is widely used in ESSs with various charge carriers. [23][24][25] When applied to NH 4 + storage, vanadium oxide has abundant oxygen group and layer space, which provides the basis for NH 4 + to form hydrogen bonds and transport between layers. 2 However, the stability and reversibility in this system are still not highly desirable, due to the incomplete fracture of hydrogen bond and partial extraction of ammonium ions. At the same time, vanadium oxides deliver relatively low electroconductivity and transportation delay due to narrow interlayer spacing. It is prone to severe dissolution and irreversible phase transition, thus seriously limiting the electrochemical performance. [26][27][28][29][30] Problems mentioned above have arisen with other energy storage materials, and many endeavors have been put into solving them. [31][32][33][34][35] For example, pre-intercalation (such as metal ions and conducting polymer) is used to stabilize lay-ered structure and tune lattice spacing, which accelerates ion diffusion and charge-transfer kinetics. [36][37][38][39][40][41][42]  Inspired by above studies, the strategy of conducting polymer and metal ion co-intercalation into vanadium oxide (VOH) is applied for AA-HSCs. The interlayer engineering of K + /PANI co-intercalation for KVO/PANI optimizes NH 4 + storage. Intercalated PANI can shield the interaction between NH 4 + and V-O layers to a certain extent. It improves the efficiency of reversible insertion and extraction. The enlarged lattice spacing is in favor of enhancing ion diffusion as well as electron transfer rate. Furthermore, K + inserted into VOH enhances redox activity and electronic conductivity. Multiple characterizations and theoretical calculations have been utilized to further research corresponding mechanism. The cointercalation of K + /PANI optimizes intercalation pseudocapacitive behavior during (de)ammonization process. The KVO/PANI electrode exhibits high specific capacitance (about 340 F g −1 at 0.5 A g −1 ) and rate performance (177 F g −1 at 10 A g −1 ). In addition, the AA-HSC device exhibits excellent energy density (31.8 Wh kg −1 ) and decent capacitance retention (61%, 10 000 cycles). This strategy may carve out more ways in advanced energy storage materials.

Composition and microstructure of KVO/PANI
The detailed schematic illustration of K + and PANI intercalated into VOH to be KVO, VOH/PANI, and KVO/PANI is exhibited in Figure 1. KVO/PANI is prepared by a facile hydrothermal method; herein, V 2 O 5 with crystal water is formed and K + /PANI are pre-intercalated into the V-O F I G U R E 1 Schematic illustration of K + and PANI intercalated KVO, vanadium oxide hydration (VOH)/PANI, and KVO/PANI materials. layers to tune microstructure. At appropriate pH and temperature, AN monomer is liable to transfer π electrons and self-polymerize to form PANI, where PANI chains are in electron-deficient state. 47,48 The V-O layers with negative charge electrically attract PANI chains, which enable PANI to be intercalated into V-O layers. The intercalated PANI in VOH would serve as stabilizing "pillars" and enlarge interplanar spacing, improving the mobility of NH 4 + between layers. π-conjugated structure of PANI whittles static interaction between interlaminar ions and layers for maintaining structural stability. 49 Furthermore, pre-intercalated K + increase the proportion of V 4+ /V 5+ active sites for speeding charge transfer. The electrically charged species near the surface can aid fast ion and electron transfer processes, enhancing NH 4 + kinetics and high capacitance. 50 As control groups, KVO and VOH/PANI intercalated by K + or PANI are studied to highlight the synergistic effect of co-intercalation. The specific research studies are shown below. The crystallography, composition, and micromorphology of KVO/PANI are exhibited in Figure 2. It can be found from the X-ray diffraction (XRD) patterns ( Figure 2a  , confirming that VOH is successfully synthesized. By contrast, the (0 0 1) diffraction peak of VOH/PANI shifts toward lower diffraction angle with an enlarged interlayer spacing of 13.6 Å, manifesting the successful intercalation of PANI into the VOH lattice. The PANI serving as pillars can shield electrostatic interactions between commuting ions and V-O layers and, thus, boost kinetics. 27 However, for KVO with intercalated K + , the diffraction peak (0 0 1) shifts to larger diffraction angle. This may be because the insertion of K + increases crystal asymmetry. The structural distortion leads to an offset of diffraction peak. 43 When K + and PANI are inserted into V-O layers simultaneously, the synergistic effect of K + and PANI co-intercalation results in increased layer spacing from 11.5 Å (VOH) to 13 Å (KVO/PANI). Although KVO/PANI has a slightly smaller layer spacing compared with VOH/PANI, its electrochemical performance is significantly better. This indicates that K + intercalation can further enhance electrochemical reactivity, and specific research will be presented below. It demonstrates that the layer spacing is not a determinant of electrochemical performance. 51 Besides, in comparison with VOH, KVO/PANI with weakened diffraction peaks reveals that the crystallinity is receded after intercalation. It can alleviate structural strain in the process of charging and discharging to some extent. Figure 2b presents Fourier transform infrared spectroscopy (FTIR) of VOH, VOH/PANI, and KVO/PANI. In the FTIR spectrum of VOH, broad peak at 1610 cm −1 verifies the presence of water molecules. The absorption peak at 1004 cm −1 is attributed to V=O stretching. The absorption peaks at 767 and 525 cm −1 emerge due to (a)symmetric vibration of F I G U R E 2 (a) X-ray diffraction (XRD) patterns of vanadium oxide hydration (VOH), KVO/PANI, KVO, and VOH/PANI; (b) Fourier transform infrared spectroscopy (FTIR) spectra of VOH, VOH/PANI, and KVO/PANI; X-ray photoelectron spectroscopy (XPS) spectra of (c) V 2p, (d) C 1s, and morphology characterization of (e) scanning electron microscope (SEM), (f and g) transmission electron microscopy (TEM) images for KVO/PANI. 36,37 The absorption peaks related to vanadium in VOH/PANI and KVO/PANI appear a slight redshift, revealing weaker V bonds due to a portion of V 5+ reduced to V 4+ . Besides, the stretching modes of quinone/benzene rings as well as C=N and C-N + bonds from PANI result in absorption peaks at FTIR spectra. Due to the interaction between PANI and V-O framework, they are slightly offset from corresponding characteristic peaks of pure PANI. 52,53 The surface chemical composition of KVO/PANI is studied by X-ray photoelectron spectroscopy (XPS). In addition to V 2p and O 1s peaks, the peaks of K 2s/2p and N 1s are shown in the survey spectra for KVO and VOH/PANI. However, KVO/PANI contains above elements without other impurities ( Figure S1a). Two absorption peaks in VOH are related to V 2p1/2 and V 2p3/2 signals ( Figure S1b). 38 The VOH contains tetravalent vanadium due to VO(O 2 ) + from reaction between V 2 O 5 and H 2 O 2 during the synthesis process. 54 Compared with VOH, V 4+ content is increased in VOH/PANI and KVO/PANI (Figure 2c). The sectional reduction from V 5+ to V 4+ manifests that a number of V 5+ receive electrons generated by AN polymerization. The electrons in N atoms from PANI can be further attracted by V cations, leading to raised protonation level of PANI and reduction of V cations. 43 Obviously, K + /PANI co-intercalation results in a larger increase in V 4+ content compared with raw material VOH and PANI-intercalated VOH/PANI. K + intercalation can further increase the active site proportion (V 4+ ) and improve redox activity, which is attributed to the conversion of vanadium from +5 to +4 to maintain the electroneutrality after K + intercalation. The reduction can insert electrons into the d-band of vanadium, which increases electron concentration and, thus, improves the conductivity of oxide material. 27 This conclusion is further confirmed by theoretical calculations below. Figure 2d and Figure S1c exhibit C 1s and N 1s spectra from PANI. The C1s spectrum includes C-O bond at 285.7 eV and C-N or C=N bond at 284.1 eV. The peak at 288.2 eV is attributed to C-C, C-H, and C=C. 39 The existence of C-O bond may be because PANI is embedded and bonded with V-O layer, which further proves the successful intercalation of PANI. In the N 1s spectrum of KVO/PANI, the energy bands at 401.3, 400.0, and 398.5 eV are corresponding to -N + , -NH-, and -N=bonds, 55 respectively. Figure S1d shows the high-resolution XPS spectrum K 2p of KVO/PANI, where the peaks at 294.9 and 292.2 eV can be ascribed to K 2p1/2 and K 2p3/2, respectively, 56 further demonstrating the successful intercalation of K + . The intercalation of PANI is further confirmed by the XPS O 1s. The O 1s spectrum of KVO/PANI is shown in Figure S1e. Crystal water and lattice oxygen make the existence of two peaks at 530.9 eV (O-H) and 529.7 eV (O-V). 57 The deconvoluted peak at 531.5 eV is attributed to C-O-V bond, 58 implying the interaction between PANI and VOH.
The elements in KVO/PANI are investigated by elemental mapping. According to elemental mapping images in Figure S2b-f, the elements are evenly distributed based on collected scanning electron microscope (SEM) image ( Figure S2a) without other impurities. At the same time, the elemental mapping of VOH has been exhibited in Figure S3. Vanadium element and oxygen element are evenly distributed, which further proves the successful synthesis of VOH. The SEM and transmission electron microscopy (TEM) are used to research morphology transition from VOH to KVO/PANI. SEM images of VOH show stratified structure ( Figure S4a,b). After the intercalation process, SEM images of KVO/PANI with different magnifications (Figure 2e and Figure S4c,d) show interconnected ribbon structures. The morphology enables electrode material to shorten diffusion channels of ions and electrons, restrain phase transitions, and add valid surface sites for NH 4 + , which is conducive to the improvement of NH 4 + storage. Compared with raw VOH, the reduction of lamellar size also promotes material to better tolerate volume expansion caused by the insertion of NH 4 + . 27 This conclusion is further confirmed by TEM results (Figure 2f,g). Ultrathin nanosheets exhibit sharp contrast between light and dark parts caused by the staggered connection of ribbon structure. This supports loose interiors, in favor of efficient insertion/extraction of ions. 48 The TEM image with high resolution (embedded in Figure 2g) shows visible lattice fringes. The interlayer spacing is 1.3 nm matching with (0 0 1) crystal face, confirming the successful expansion of V-O layer.  Figure 3b shows CV curves of KVO/PANI recorded at different potential windows. The CV curve maintains an approximately rectangular shape and has a relatively large integral area from −0.2 to 0.9 V, indicating the rationality of this working range. In this voltage window (Figure 3c), CV curves with a pair of redox peaks exhibit deflected rectangle because of extraction/insertion of NH 4 + from/into KVO/PANI electrode. 2 CV curves without obvious deformation reveal its fast kinetics and capacitive characteristic. Corresponding current increases in sequence as the addition of scan rates, implying a reversible charge storage process. 59 Notably, the response current of pure Ti electrode is almost negligible in comparison with KVO/PANI electrode ( Figure S5). Thus, the capacitance contribution from conductive substrate can be neglected. Furthermore, Figure 3d shows CV curves of VOH, KVO, VOH/PANI, and KVO/PANI at the same scan rate. Compared with VOH, PANI intercalation results in obvious redox peak on CV curve, that is, PANI intercalation enhances the extraction/insertion of NH 4 + from/into the VOH/PANI electrode. 60,61 The K +intercalated KVO presents boxlike voltammogram and more integral area, indicating unique redox pseudocapacitive characteristic. 62 However, KVO/PANI exhibits higher redox current and larger integral area. This may be because PANI intercalation enlarges VOH/PANI layer spacing, which achieves more available layer structure and benefits the transport of charge carriers. At the same time, K + intercalation further enhances the redox pseudocapacitive activity of KVO/PANI. The synergistic effect of K + /PANI co-intercalation makes KVO/PANI exhibit better electrochemical performance to store NH 4 + .

Electrochemical performance of KVO/PANI electrode
The galvanostatic charge/discharge (GCD) curves of KVO/PANI are collected at different current densities (Figure 3e). Approximately symmetric profiles indicate its reversible charge storage and capacitive feature. The calculated specific capacitance is 340 F g −1 at 0.5 A g −1 .
It outperforms many previously reported energy storage materials with NH 4 + and other typical ions, as shown in Table S1. It is worth noting that the discharge time is slightly shorter than charge time because of the partial extraction of NH 4 + after the charging process. Under the same conditions, the GCD profiles in Figure 3f reveal that the specific capacitance of KVO/PANI delivers 329 F g −1 .
It is significantly higher than that of VOH (205 F g −1 ), KVO (267 F g −1 ), and VOH/PANI (255 F g −1 ). In addition, rate performance is a vital parameter to evaluate practicability. KVO/PANI delivers outstanding rate performance due to the high capacitance retention of 177 F g −1 at 10 A g −1 . Corresponding GCD curves of VOH, KVO, and VOH/PANI are shown in Figure S6. The specific capacitance comparison before and after intercalation is shown in Figure 3g. The capacitance of KVO/PANI slowly decreases compared to the other three materials, evidencing superior rate performance. It can be attributed to rapid kinetics induced by the interlayer engineering of K + /PANI co-intercalation. The expanded interlayer spacing induced by PANI intercalation enables charge storage to occur in the volume with capacious channels in crystal structure. 63 However, K + intercalation enhances redox pseudocapacitance activity. 62 The pseudocapacitive behavior can make quick charge transfer and keep capacitance even at high current densities. 64 The electrochemical impedance spectrum (EIS) is used for studying intrinsic ion diffusion and transfer behavior. Nyquist plots of materials before and after intercalation are exhibited in Figure 3h. The intercept of Nyquist plots to X axis is equal to equivalent series resistances (R s ), reflecting the internal and interfacial resistances. The diameter of semicircle reflects charge transfer resistance (R ct ). 65 Obviously, KVO/PANI has smaller Rs and Rct values than that of other materials, indicating that KVO/PANI possesses excellent electrical conductivity and charge-transfer behavior. These results further reveal that enhanced NH 4 + kinetics is closely related to the structural engineering of K + /PANI co-intercalation. The enlarged interlayer distance enables ions and electrons to commute more quickly. Most importantly, the inserted K + can connect the [VO n ] interlayers to afford more charge transfer pathways and bring about lower resistance. 66 What is more, the Warburg diffusion impedance can be used to estimate diffusion rate in electrolyte and capacitive behavior. Notably, the charge-discharge reaction rate is diffusion-controlled. 67 The ion diffusion behavior can be evaluated by the Warburg coefficient (σ w ). According to Equation (S6), ion diffusion coefficient (D EIS ) will magnify as σ w decreases. The σ w can be calculated by fitting curves of Z′ and ω −1/2 . KVO/PANI has the smallest σ w (Figure 3i) and, therefore, the largest D EIS . It further reflects that inserted K + together with PANI can boost conductivity and ionic diffusion efficiency, enabling subsequent NH 4 + to insert and extract easily. This conclusion can be further confirmed by DFT calculations below. Molecular chains in PANI have the πconjugate system, which is in favor of electron flow and conduction between layers. Therefore, KVO/PANI delivers fast reaction kinetics. Besides, the cyclic stability of (de)ammonization for KVO/PANI could support 63% initial capacitance after 10 000 cycles. Around 31% of the initial capacitance of KVO/PANI electrode is lost upon 5500 cycles, in stark comparison with that of KVO (42%) and VOH/PANI (52%) ( Figure S7). Inevitably, because of the successive charging and discharging process, the (de)ammonization results in an uninterrupted stretch of layers. Therefore, it destroys lamellar structure, and stability declines. However, KVO/PANI exhibits superior cycling performance than that of KVO and VOH/PANI. It could be attributed to the gentle transport pathway of NH 4 + in V-O layer and ultrafast kinetics of NH 4 + storage induced by K + /PANI co-intercalation. The more spacious interlayer spacing can weaken the static interaction between NH 4 + and lamella, which can mitigate structural collapse and improve material stability. To further explore capacitive and diffusion-controlled contribution for response current in NH 4 + storage, CV tests are conducted at small scan rates ( Figure S9a). 62,65 There are obvious redox peaks on CV curves, and especially, pairs of redox peaks are found at a small scan rate of 0.2 mV s −1 , corresponding to insertion or extraction of NH 4 + with lattice water co-insertion or extraction inside the KVO/PANI ( Figure S8). Specifically, the dynamic process can be analyzed according to i = a × ν b , where i and ν stand for peak current and scan rate, and a and b are coefficients. The different b value stands for diffusion-controlled and capacitive behavior. The b values for KVO/PANI range from 0.55 to 0.77 ( Figure S9b). Corresponding diffusion/non-diffusion contributions can be calculated by the equation of i = k 1 ν + k 2 ν 1/2 . Capacitance contribution accounts for about 70% of charge storage at 1.0 mV s −1 ( Figure S9c). The capacitive contribution of KVO/PANI increases significantly from 52% to 70% as the aggrandizement of scan rates ( Figure S9d). Based on above kinetics studies, non-diffusion behavior dominates the charge storage process of KVO/PANI. The NH 4 + insertion/extraction process in KVO/PANI can be considered intercalation pseudocapacitive behavior. Notably, the same tests to evaluate charge storage kinetics are conducted for VOH/PANI and KVO ( Figure S10a-c).
Compared with unintercalated VOH (Figure S10d), PANI intercalation enhances the strength of redox peaks and stronger pseudocapacitance-controlled behavior. However, intercalated K + improves redox pseudocapacitance activity and leads to broad faradaic peaks and quasirectangular CV curves. 68 Thus, the synergistic effect of K + /PANI co-intercalation makes charge storage mechanism of intercalation pseudocapacitance in KVO/PANI, which is achieved by interlayer engineering to elicit an extrinsic pseudocapacitive response. Most importantly, the electrochemical performance of KVO/PANI is superior to that of VOH/PANI and KVO. Pseudocapacitive behavior is closely related to ion commute within host material. Thus, the wider layer spacing and greater redox activity in KVO/PANI are in favor of intercalation process and enabling fast kinetics. 61 Several spectroscopy tests are performed to deeply reveal the inherent electrochemical behaviors in KVO/PANI electrodes during the charging/discharging process. Figure 4a depicts the GCD curve of KVO/PANI at 1 A g −1 , and five states are selected for further studies. Intercalation chemistry generally leads to structural changes in electrode material 69 ; thus, ex situ XRD measurements have been performed, corresponding to the five states marked in Figure 4a. The (0 0 1) peak shifts slightly to a high degree in the ammonization process from 1 to 3 states (during discharge), implying a decrease in d-spacing induced by NH 4 + insertion and hydrogen bond formation. Afterward, corresponding peaks recover to a low degree in the deammonization process with NH 4 + extraction and hydrogen bond cleavage (during charge), revealing reversibly structural stability for KVO/PANI electrode (Figure 4b).
Moreover, ex situ FTIR analysis can directly validate the existence of H bond between hydrogen atoms from NH 4 + and oxygen atoms from V-O layer. Figure 4c shows the spectra of different materials at discharging to −0.2 V. The peaks located at 3035 and 3183 cm −1 are related to stretching vibrations of N-H in NH 4 + , corresponding to N-H and N-H⋅⋅⋅O. Absorption peaks at 1327 and 1400 cm −1 are corresponding bending peaks. 2 This indicates that during the discharge process, these three materials can accommodate NH 4 + embedded between layers to form hydrogen bonds. It is worth noting that strong ν(N-H) and ν(N-H⋅⋅⋅O) in KVO are found. Too strong bonding between charge carriers and hosting material is not conducive to ion transportation. Ex situ FTIR measurements of KVO/PANI are performed at different states, corresponding to five states signed in GCD curves (Figure 4d). In the ammonization process from 1 to 3 states, two peaks associated with stretching vibration of N-H and N-H⋅⋅⋅O increase gradually, implying the formation of hydrogen bonds with NH 4 + intercalation. 4 However, the relative peak intensity decreases from 3 to 5 states due to NH 4 + extraction. Besides, ex situ XPS measurement of KVO/PANI at different states is conducted to survey valence state during (de)ammonization process. The V 2p spectra show sharp V 4+ (516, 523 eV) and V 5+ (517, 524 eV) peaks at pristine state (Figure 4e). After full discharging, the content of V 4+ /V 5+ at ammoniated state exhibits an obvious increase of V 4+ due to the intercalation of NH 4 + .
However, it restores to pristine state in the charging process. The reversible charge transfer reaction in KVO/PANI reveals its superiority to host NH 4 + . The N1s spectrum can be deconvoluted into three peaks at 401.4 eV (-N+), 399.6 eV (-NH-), and 400.9 eV (-N═) (Figure 4f). The content of -NH-increases significantly after discharging, whereas these signals recover reversibly during the charging process. The spectroscopy transformation reveals that ammonium ions, as working ions, could insert and extract reversibly. 18 The C-O and O-V characteristic peaks in C 1s and O 1s XPS spectra retain in charge/discharge process, indicating that intercalated PANI is stable in V-O layers ( Figure S11). Besides, the CV curve of KVO/PANI in PVA/HCl electrolyte with the same pH as PVA/NH 4 Cl electrolyte has no obvious redox peak, and the integral area is almost negligible, indicating that the contribution from H + and H 3 O + can be neglected ( Figure S12). The above ex situ XRD, FTIR, and XPS tests imply that NH 4 + can insert and extract reversibly in KVO/PANI with the formation and fracture of hydrogen bonds during discharge-charge processes.
The electrochemical behavior is further investigated by DFT calculations to study the synergistic effect of K + /PANI co-intercalation on the electronic structure of KVO/PANI. Electronic band structures output information about the range of energy levels that an electron can occupy, and the band gap is closely correlated with electronic conductivity. 70 Band structures of VOH, VOH/PANI, KVO, and KVO/PANI are calculated, as shown in Figure S13a-c and Figure 4g. The calculations reveal that VOH has a band gap of 2.265 eV, which is in sharp contrast to the low band gap of VOH/PANI (1.637 eV) and KVO (1.055 eV). Notably, KVO/PANI has a band gap of only 0.116 eV after K + and PANI co-intercalation, indicating that the co-intercalation process further lowers electron transport barrier and enhances electronic conductivity. It improves NH 4 + kinetics and delivers high capacitance for charge storage. Furthermore, the electronic resistance can be analyzed by density of states (DOS) plots. The DOS basically reflects the number of diverse states at a given energy level.
The calculated DOS of VOH, KVO/PANI, VOH/PANI, and KVO are shown in Figure 4h,i and Figure S13d,e. Obviously, K + and PANI co-intercalations for KVO/PANI lead to overlap near the Fermi level between V-3d and O-2p orbitals, revealing d-p hybridization and stable binding of V and O atoms. In fact, Fermi energy level is the traction for electron transport, enabling electrons to commute between anode and cathode. 26 Therefore, the DOS of KVO/PANI indicates its increased electron transfer rate, in contrast to materials without co-intercalation. In addition, charge distribution patterns can clearly exhibit the electronic structure and chemical environment of atoms. The electronic structure of KVO/PANI after NH 4 + insertion is analyzed by differential charge density ( Figure S14a), where grey, red, purple, dark grey, blue, and white balls stand for V, O, K, C, N, and H atoms, respectively. The blue and yellow regions represent loss and gain in charge. The increased charge density between the NH 4 + set and O atoms in V-O layer can be attributed to the formation of hydrogen bonds. In addition, KVO/PANI is proved to be the most stable system with the lowest energy, compared with other materials ( Figure S14b).

Electrochemical performance of KVO/PANI//AC AA-HSC device
A quasi-solid-state HSC device with PVA/NH 4 Cl gel electrolyte is fabricated using as-prepared KVO/PANI as cathode and activated carbon (AC) as anode (Figure 5a), named KVO/PANI//AC AA-HSC device, to evaluate the practi-cability of KVO/PANI. The AC is chosen as anode due to its cheap source and ideal capacitive performance. Specifically, AC exhibits triangular GCD curves and rectangular CV curves ( Figure S15a,b). It has good rate performance and electrical conductivity according to the Nyquist plot ( Figure S15c,d), revealing typically electrochemical double-layer capacitance in NH 4 + storage. In different voltage windows, no obvious deformation is found in CV curves of the HSC device until the voltage increased to 1.6 V (Figure 5b). It shows larger integral area and confirms the suitability of the voltage range. The high working voltage can be ascribed to the synergetic effect of two electric poles. The HSC device exhibits representative quasi-rectangular CV curves at 5-100 mV s −1 without distinct deformation (Figure 5c) and nearly symmetry GCD profiles at 1-10 mA cm −2 (Figure 5d), indicating its good rate capability and high Coulombic efficiency. 71 As a function of GCD results, the area capacitance of HSC device is 376 mF cm −2 at 1 mA cm −2 . It maintains 209 mF cm −2 even at 10 mA cm −2 . The specific capacitance is summarized in Figure 5e, which reveals the excellent rate performance of HSC device. The Nyquist plot of HSC device consists of a quasi-semicircle followed by an almost vertical line (Figure 5f), indicating its rapid electron transfer and low resistance. It maintains 61% of specific capacitance after 10 000 repetitive cycles (Figure 5g), and corresponding Coulombic efficiency fluctuates around 100%. Figure 5h shows the Ragone plot involved energy and power densities and comparison with other NH 4 + devices. The energy density of KVO/PANI//AC AA-HSC device can reach 31.8 Wh kg −1 at the power density of 47.6 W kg −1 and output 17.7 Wh kg −1 at 476.2 W kg −1 , which is better than that of many reported devices (detailed comparison is presented in Table S2), especially NH 4 + -storage devices, as VOH/PEDOT//AC AA-HSC, 72 ACC@VPP//PTCDI AA-HSC, 20 (Na-PW)//NaTi 2 (PO 4 ) 3 (NTP) double-ion battery, 73 Zn//Na-FeHCF NH 4 + -Zn 2+ hybrid battery (ARAHB), 74 and Ni-APW//PTCDI NH 4 + battery. 1 Moreover, according to the GCD ( Figure 5i) and CV ( Figure S16) curves, devices at series as well as parallel can work well in accordance with Ohm's law. Combined with above test results, it can be concluded that assembled AA-HSC device exhibits great potential as power supply devices.

CONCLUSION
In summary, the K + /PANI co-intercalation into VOH to be KVO/PANI is achieved successfully, which optimizes the microstructure of VOH and further improves its NH 4 +storage performance as cathode material in AA-HSC. Specifically, PANI can broaden the interlayer distance of  (Table S2); (i) GCD curves of single device and that at series or parallel.
VOH and shield the electrostatic attraction of V-O layers with NH 4 + to some extent. It facilitates rapid ion transport between layers. The intercalated K + enhances redox activity and electronic conductivity, which is evidenced by theoretical calculations. Synergistic effect of K + /PANI co-intercalation optimizes intercalation pseudocapacitive response during (de)ammonization process. Capacitance, rate performance, and stability of KVO/PANI electrode are improved obviously. The KVO/PANI delivers the capacitance of 340 F g −1 at 0.5 A g −1 with a voltage potential of −0.2 to 0.9 V. Spectroscopy studies reveal that the insertion/extraction of NH 4 + into/from KVO/PANI is achieved reversibly. The assembled KVO/PANI//AC HSC device delivers excellent performance with a high capacitance of 376 mF cm −2 and a corresponding energy density of 31.8 Wh kg −1 . This work indicates that the structural optimization method of metal ion/conducting polymer (inorganic/organic) co-intercalation can effectively optimize NH 4 + -storage performance, which gives a neoteric idea for the future development of novel NH 4 + -storage systems.

Material synthesis and characterizations
The abridged general view of material synthesis is shown in Figure 1. Vanadium pentoxide (V 2 O 5 ) of 1 mmol and hydrogen peroxide (H 2 O 2 , 30%) of 17.6 mmol are put in 66 mL of deionized water (DI water). It is stirred for 30 min at room temperature. Afterward, 1 mmol potassium chloride (KCl), 60 μL aniline (AN), and moderate 3 M hydrochloric acid (HCl) are then added into above solution and adjusted pH to 3. After reacting at 120 • C for 24 h and freeze-drying, dark green KVO/PANI is obtained. At the same time, KVO and VOH/PANI are synthesized by adding KCl and AN, respectively. The details of the materials and characterizations are shown in the Supporting Information section.

Preparation of electrodes and HSC devices
The KVO/PANI electrode is prepared by blending binder, conducting material as well as active substance, and coating on Ti substrate. Notably, the slurry occupies 1 × 1 cm 2 of Ti foil. The mass of KVO/PANI is about 2-3 mg. Similarly, active carbon (AC) electrode is fabricated in the same way.
Pairing with AC electrode, the HSC devices are assembled with 1 M PVA/NH 4 Cl gel electrolyte.
The specific process and characterizations are supplemented in the Supporting Information section.

A C K N O W L E D G M E N T S
This work was supported by the Large Instrument and Equipment Open Foundation of Dalian University of Technology and Natural Science Foundation of Liaoning Province.

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.