Cation‐Anion Redox Active Organic Complex for High Performance Aqueous Zinc Ion Battery

Organic redox compounds are attractive cathode materials in aqueous zinc‐ion batteries owing to their low cost, environmental friendliness, multiple‐electron‐transfer reactions, and resource sustainability. However, the realized energy density is constrained by the limited capacity and low voltage. Herein, copper‐tetracyanoquinodimethane (CuTCNQ), an organic charge–transfer complex is evaluated as a zinc‐ion battery cathode owing to the good electron acceptation ability in the cyano groups that improves the voltage output. Through electrochemical activation, electrolyte optimization, and adoption of graphene‐based separator, CuTCNQ‐based aqueous zinc‐ion batteries deliver much improved rate performance and cycling stability with anti‐self‐discharge properties. The structural evolution of CuTCNQ during discharge/charge are investigated by ex situ Fourier transform infra‐red (FT‐IR) spectra, ex situ X‐ray photoelectron spectroscopy (XPS), and in situ ultraviolet visible spectroscopy (UV–vis), revealing reversible redox reactions in both cuprous cations (Cu+) and organic anions (TCNQx‐1), thus delivering a high voltage output of 1.0 V and excellent discharge capacity of 158 mAh g−1. The remarkable electrochemical performance in Zn//CuTCNQ is ascribed to the strong inductive effect of cyano groups in CuTCNQ that elevated the voltage output and the graphene‐modified separator that inhibited CuTCNQ dissolution and shuttle effect in aqueous electrolytes.


Introduction
][24] For instance, 3, 4, 9, 10-perylenetetracarboxylic dianhydride (π-PMC) with an interconnected layered structure displayed remarkable capacity and superb rate capability. [25]Quinone compounds (C4Q) exhibited an impressive capacity and energy density with a fluorine-containing membrane as a separator. [26]However, remaining issues, including dissolution in electrolyte, poor electronic conductivity, and low working potential, hindered the adoption of organic compounds in AZIBs.Thus, to overcome these issues, novel organic compound-based cathode materials with high working potential and rate performance are urgently needed.
More recently, metal-organic frameworks (MOFs) started to attract researchers' interest for ZIBs because of their unique properties including tunable, ordered, and functional porosities with large surface area, which are beneficial for ions transport into the framework and facilitating the electrochemical reaction kinetics. [23,27,28]For example, Nam et al. [23] applied a 2D conductive MOF as cathode materials for AZIBs, reaching a capacity of 228 mAh g −1 at 50 mA g −1 .Pu et al. [28] demonstrated that Mn(BTC) can exhibit a Zn 2+ storage capacity of 112 mAh g −1 .These findings present that MOFs are promising candidates and may open new opportunities to assemble high-performance AZIBs.Till now, the reported MOF-based cathodes are employing hydroxy and carbonyl groups as ligands and redox centers for AZIBs.As far as we are aware, MOF cathode materials with cyano groups as Organic redox compounds are attractive cathode materials in aqueous zincion batteries owing to their low cost, environmental friendliness, multipleelectron-transfer reactions, and resource sustainability.However, the realized energy density is constrained by the limited capacity and low voltage.Herein, copper-tetracyanoquinodimethane (CuTCNQ), an organic charge-transfer complex is evaluated as a zinc-ion battery cathode owing to the good electron acceptation ability in the cyano groups that improves the voltage output.Through electrochemical activation, electrolyte optimization, and adoption of graphene-based separator, CuTCNQ-based aqueous zinc-ion batteries deliver much improved rate performance and cycling stability with anti-self-discharge properties.The structural evolution of CuTCNQ during discharge/charge are investigated by ex situ Fourier transform infra-red (FT-IR) spectra, ex situ X-ray photoelectron spectroscopy (XPS), and in situ ultraviolet visible spectroscopy (UV-vis), revealing reversible redox reactions in both cuprous cations (Cu + ) and organic anions (TCNQ x-1 ), thus delivering a high voltage output of 1.0 V and excellent discharge capacity of 158 mAh g −1 .The remarkable electrochemical performance in Zn//CuTCNQ is ascribed to the strong inductive effect of cyano groups in CuTCNQ that elevated the voltage output and the graphene-modified separator that inhibited CuTCNQ dissolution and shuttle effect in aqueous electrolytes.
ligands have not been attempted in AZIBs.][34] Profitted by the strong inductive effect from the cyano groups, CuTCNQ was deemed as the state-of-the-art organic electrode materials for monovalent ion (Li + , Na +, and K + ) batteries and exhibited excellent performance in terms of average potential and capacity. [29,30,35]Unfortunately, the electrochemical behavior of CuTCNQ for multivalent-ion batteries has not been reported, and the electrochemical behavior is still unclear.
Inspired by the abovementioned discussions, CuTCNQ was evaluated as the cathode for AZIBs, and the energy storage mechanisms are investigated.CuTCNQ shows multi-redox reactions around 1.2, 1.0, and 0.9 V vs. Zn/Zn 2+ with double redox active sites of Cu + cations and TCNQ x-1 anions, enabling a high voltage output of 1.0 V and excellent reversible capacity of 158 mAh g −1 at 100 mA g −1 .Importantly, with a thin graphene interlayer separator, the cell demonstrates excellent cycling stability (61 mAh g −1 retained after 500 cycles) and low self-discharge.This work not only expands the library of organic materials for AZIBs, but also supplies new insights for constructing advanced energy storage systems for reliable and large-scale electric grid applications.

Results and Discussion
The CuTCNQ was synthesized by a solution-based method and directly employed as the cathode, as illustrated in Supporting Information Scheme S1. [26] The X-ray diffraction (XRD) pattern of CuTCNQ was refined and presented in Figure 1a and Supporting Information Table S1.The calculated lattice parameters were a = 5.3317 Å, b = 5.3190 Å, c = 18.9944Å, and β = 93.0241°withRietveld refinement (Rp = 5.52%, Rwp = 8.33%).All diffraction peaks of the sample match well with CuTCNQ phase II without any impurities from phase I, which is in agreement with previous works. [29,32]According to the chemical structure of CuTCNQ phase II illustrated in Figure 1b, it shows that two kinds of adjacent TCNQ ligands coordinated with Cu + by cyano N-atoms are parallel in the same direction but in two perpendicular planes.Fourier transform infrared spectroscopy (FT-IR) was further applied to determine the chemical structure (Supporting Information Figure S1).The strong absorptions located at 2210 and 2171 cm −1 belong to the C ≡ N stretching vibrations in CuTCNQ. [36]The adsorption peaks at 1506 and 1357 cm −1 are assigned to C=C ring stretching and C-C ≡ N wing stretching, respectively. [33]The morphology and elemental distribution of as-synthesized products were then characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The typical lamella structure with side length around 3 μm can be observed from SEM (Figure 1c) and the highmagnification SEM images (inset of Figure 1c), which is the characteristic structure of phase II. [29,32]The lamella morphology of CuTCNQ was further confirmed by high-resolution TEM imaging (Figure 1d).The energy-dispersive spectroscopy (EDS) mapping of CuTCNQ is shown in Figure 1e, evidencing homogeneous distribution of Cu, C, and N elements in the lamella structure.
To explore the electrochemical behavior of CuTCNQ as cathode materials for ZIBs, galvanostatic discharge/charge (GDC) and cyclic voltammetry (CV) tests were performed.To optimize the electrochemical behavior of the CuTCNQ cathode, three type of electrolytes 0.5 M Zn(CF 3 SO 3 ) 2 in diglyme (DGM); 0.5 M Zn(CF 3 SO 3 ) 2 in trimethyl phosphate (TMP); 0.5 M Zn(CF 3 SO 3 ) 2 in deionized water were evaluated.The corresponding GDC profiles conducted at a low current density of 50 mA g −1 in these electrolytes are illustrated in Figure 2a-c, respectively.In the first cycle, the specific discharge capacities of Zn// CuTCNQ cells in all the electrolytes are <5 mAh g −1 , with a long flat oxidation plateau around 1.5 V (vs Zn/Zn 2+ ) during charging.The oxidation plateau in the first cycle obviously differs from the subsequent cycles, which might result from the initial electrochemical activation of CuTCNQ. [34,35]The oxidation product TCNQ 0 can be detected by Fourier-transform infrared spectroscopy (FT-IR, Supporting Information Figure S2).The adsorption peak located at 2225 cm −1 is assigned to the C ≡ N stretching vibration of TCNQ 0 . [30,38]The lamella structure of CuTCNQ is still maintained with a few cracks after initial GDC (Supporting Information Figure S3), which could be caused by the activation of CuTCNQ that results in weak coordination between metal ions and organic ligands, forming an amorphous state.This result was confirmed by the broad XRD peak of the CuTCNQ electrode after initial activation (Supporting Information Figure S4).If the working potential was narrowed to 0.1-1.4V, there are almost no capacities in the following cycles (Supporting Information Figure S5), demonstrating the necessity to electrochemically activate CuTCNQ in the AZIB system.In the second cycle, CuTCNQ displayed higher discharge capacity of 81 mAh g −1 in the aqueous electrolyte than in nonaqueous electrolytes, with three discharge plateaus located at ~1.2, ~1.0, and ~0.9 V as observed in Figure 2c.The higher capacity in the aqueous electrolyte should be related to the limited dissolution of TCNQ 0 in water than in other organic solvents, as shown in Supporting Information Figure S6.This result is consistent with the previous works using CuTCNQ as cathode materials in nonaqueous alkali-ion batteries (e.g.Li + , Na +, Energy Environ.Mater.2024, 7, e12507 2 of 8 and K + ). [29,30,35]In order to further optimize the electrochemical performance, the rate performances of CuTCNQ in the aqueous electrolyte with different salt concentrations were compared (Supporting Information Figure S7a).It shows that the CuTCNQ cathode exhibits the best rate performance when the salt concentration is 2 M, which exhibits the highest ionic conductivity (Supporting Information Figure S7b).Thus, this concentration is deemed as the optimum electrolyte formulation for further studies.The low capacity at high current density is due to the sluggish transport of Zn 2+ in micrometer scale CuTCNQ matrix and the large interfacial resistance.
The typical CV curves, rate capability, and GDC profiles of CuTCNQ in aqueous electrolyte are tested and demonstrated in Figure 2d-f.As shown in Figure 2d, the initial three CV cuves were performed at a low sweep rate of 0.2 mV s −1 .It can be observed that there is one distinctive anodic peak at 1.56 V and a small cathodic peak at 1.21 V in the first cycle.The anodic peak corresponds to the oxidation of the TCNQ ligands in CuTCNQ from TCNQ 2− to TCNQ 0 and Cu + to Cu 2+ , which agrees well with the GDC and ex situ FT-IR results in Figure 2c and Supporting Information Figure S2.After the initial activation process, cathodic peaks at 0.82 and 0.98 V can be captured; however, the cathodic peak at 1.21 V fades in the next two cycles.The damped cathodic peak could be attributed the variation of metal nodes that weakened the coordiantion in CuTCNQ, which was partially dissolved into the aqueous electrolyte.These phenomena could also be found in previously reported CuTCNQ in alkali-metal ion battery. [29,30,35]Two new anodic peaks around 1.25 and 1.45 V are present in the following two cycles, which correspond to the oxidation of TCNQ ligands and Cu + , respectively.For the second and third cycles, the cathodic and anodic peaks are almost overlapped, indicating good reversibility in activated CuTCNQ.The detailed electrochemical reaction mechanism will be discussed later.With an increased CV sweep rate, the oxidation peaks shift to higher potential, while the reduction peaks shift to lower potential due to the increased polarization (Figure 2e).The peak current raised linearly with the square root of the scan rate (Figure 2f), demonstrating that the Zn 2+ intercalaction and de-intercalation behavior are diffusion-controlled.
In order to characterize the evolution of functional groups of CuTCNQ during the second discharge/charge cycle, ex situ FT-IR of CuTCNQ at different voltage stages were collected (Figure 3a,b).As shown in Figure 3b, when discharged to 1.0 V (state a), the adsorption peak belonging to TCNQ 0 at about 2225 cm −1 is weakened. [30]wo new and strong peaks belonging to TCNQ − at 2195 and 2165 cm −1 are observed, indicating the redistribution of electrons on cyano groups in TCNQ − with intercalation of Zn ions, which is consistent with previous works. [39]When further discharged to 0.9 V (state b), the peaks at 2181 and 2131 cm −1 are ascribed to the transformation from TCNQ − to TCNQ 2− . [30,40]After fully discharged to 0.1 V (state c), the peaks at 2195 and 2165 cm −1 disappear completely, indicating that the TCNQ − is fully reduced to TCNQ 2− . [36]Besides, the peak from TCNQ 0 (2225 cm −1 ) exists in the whole discharge process, which might be due to the oxidation of TCNQ − and TCNQ 2− in the air during the ex situ FT-IR test.The ex situ FT-IR result demonstrated that the TCNQ 0 is reduced in two consecutive one-electron transfer processes with formation of radical anion and dianion.After charging to 1.5 V (state d), the peaks of TCNQ − can be observed again, demonstrating that the TCNQ 2− is oxidized to TCNQ − .After being fully charged (state e), only the peak of TCNQ 0 is remained, indicating the good reversibility of the redox behavior between TCNQ 2− /TCNQ − and TCNQ − /TCNQ 0 .In addition, the peak of C=C groups at 1542 cm −1 splits into two peaks located at 1577 cm −1 (C=C ring stretching) and 1506 cm −1 (C=C(CN) 2 stretching) during the discharge process and then disappears after charging, as shown in Supporting Information Figure S8. [40]This results from the reversible redox reaction of C ≡ N groups in ligands, demonstrating that the C ≡ N groups can work as active centers during repeated zinc ion intercalation/de-intercalation.
Furthermore, ex situ X-ray photoelectron spectroscopy (XPS) for the first two GDC curves were carried out to determine the chemical changes of N, C, Cu, and Zn in the CuTCNQ electrodes (Figure 3c-f and Supporting Information Figure S9).As shown in Figure 3c and Supporting Information Figure S9a, the N 1s peak of pristine TCNQ shows peak with binding energies at 399.3 eV along with an intramolecular "shake-up" feature at 401.8 eV.The higher binding energy feature was often excited during the photoemission process. [40,41]In the first cycle, the N 1s peak of the pristine TCNQ cathode shifts to lower binding energy (from 399.3 to 399.0 eV) during the discharge process, which is attributed to the increased electron density near the cyano groups of CuTCNQ with Zn 2+ intercalation.When the Zn 2+ is removed, the N 1s peaks shift back to 399.5 eV upon charging to 1.8 V, demonstrating the formation of TCNQ 0 from the initial electrochemical activation of CuTCNQ.In the second cycle, during the Zn 2+ intercalation (state a, b, and c) and de-intercalation (state d and e) in the CuTCNQ electrode, the N 1s peak slightly shifts to lower binding energy first and then shift back to 399.6 eV, demonstrating the restoration of TCNQ 0 .The C 1s spectra shown in Supporting Information Figure S9b and Figure 3d have the similar trend due to the change of electron density near the cyano groups of CuTCNQ during reversible intercalation and de-intercalation of Zn 2+ .These results Energy Environ.Mater.2024, 7, e12507 mentioned above are in well-accordance with FT-IR results shown in Figure 3b, indicating that the cyano groups in CuTCNQ are the redox active centers during the discharge/charge processes.To understand whether the inorganic Cu metal center of CuTCNQ participated in the redox reaction, the Cu valence change during the first two cycles was also captured by XPS and shown in Figure 3e and Supporting Information Figure S9c.For the pristine CuTCNQ, the two peaks located at 932.2 and 952.0 eV are associated to Cu2p 3/2 and Cu2p 1/2 from Cu + , respectively.In the first cycle, upon discharging to 0.1 V, no signals from Cu 0 is observed, indicating that Cu + is stable during discharge.After being charged to 1.5 V, there are no apparent Cu 2+ signals, demonstrating that the long plateau can be attributed to the redox reaction from TCNQ 2− /TCNQ − and TCNQ − /TCNQ 0 .However, after fully charged to 1.8 V, a pair of sub-peaks located at 935.4 and 955.1 eV are observed, which are attributed to the formation of Cu 2+ , manifesting a redox reaction from Cu + to Cu 2+ .The similar trend of Cu 2p peaks could also be observed in the second cycle, demonstrating that the redox reactions of Cu + and Cu 2+ in the discharge/charge proceess are highly reversible.Based on the fitting curves, the coexistence of Cu + and Cu 2+ signals suggest a partial oxidation from Cu + to Cu 2+ .The redox reaction between Cu + and Cu 2+ matches well with previous electrochemical test results, where the highest discharge plateau (Fig- ures 2c-e and 3a) results from the reduction of Cu 2+ to Cu + .These observations indicate that the four Cu-N bonds can not only stablize the structure of metal organic frameworks but also raise the output voltages.Besides, the signals of Zn 2p peaks become stronger during the discharge process and then almost disappear during charging, demonstrating the reversible intercalation/de-intercalaction of Zn 2+ into/from the CuTCNQ electrodes (Figure 3f and Supporting Information Figure S9d).According to the abovementioned analyses, both the cuprous cations (Cu + ) and organic anions (TCNQ − ) participated in the reversible storage and release of Zn 2+ when CuTCNQ was applied as the cathode in ZIBs.
Unfortunately, when we further examine the cycling stability of CuTCNQ at 500 mA g −1 , there was a continuous capacity fade (Supporting Information Figure S10), which is ascribed to the sparingly dissolution of Cu + and TCNQ x-1 anions in the electrolyte and shuttle effect during extended cycling.The shuttling of the intermediate products can be confirmed by the color change of the separator (covering the cathode side, Supporting Information Figure S11), the pristine colorless separator became green after a few cycles.To address this issue, separator modification strategies employed in Li-S batteries were judiciously referred.Previous works have identified that graphene-based separators can effectively mitigate the polysulfides shuttling and improve the cycling stability. [36,42,43]Given that the TCNQ x-1 anions have conjugated benzene rings, they are inclined to be absorbed on graphene through π-π stacking interaction between graphene and aromatic molecules. [44]Thus, to substantially promote the cycling stability, electrochemically exfoliated graphene was employed as an adsorption layer on separator owing to its large surface area, extended πconjugation, and abundant groups (C-OH and C(O)-O), which can deliver high sorption of metal ions and aromatic molecules. [45]The preparation of the graphene separator is simple, and the detailed process is illustrated in Supporting Information Scheme S2.The XPS results show the presence of abundant C-OH and C(O)-O groups on the surface of exfoliated graphene (Supporting Information Figure S12).The rate performance of CuTCNQ electrodes with blank and modified separator at different current densities was measured and shown in Figure 4a and Supporting Information Figure S13.Compared with the pristine separator, the cell with graphene separator exhibits better rate capability, and the mass loading of graphene interlayer is optimized to be 1 mg cm −2 .Capacities of 158, 156, 149, 116, and 93 mAh g  Energy Environ.Mater.2024, 7, e12507 frequency.In comparison to the original separator, a new semicircle at medium frequency is generated, which might be resulted from the newly formed interface between the graphene layer and glass fiber membrane. [34,35]Obviously, the cell with graphene separator shows a smaller charge-transfer resistance than the pristine one, indicating that the improved zinc ion mobility is attributed to the graphene interlayer.The typical GDC profiles of the cell with modified and pristine separator at a current density of 100 mA g −1 are compared and shown in Supporting Information Figure S15.Obviously, the polarization with the graphene interlayer is smaller than with the pristine separator, demonstrating that the graphene inlayer can promote the redox kinetics.The galvanostatic intermittent titration technique (GITT) was further carried out to explore the electrochemical kinetics of the cell with pristine and modified separator (Supporting Information Figure S16).The zinc-ion diffusion coefficient in the second cycle was calculated using the same equation as previously reported. [25]The results show that the overall zinc-ion diffusion coefficients with graphene separator are higher than those of pristine cell during the discharge and charge process, revealing enhanced zinc-ion diffusion by employing an graphene interlayer.
In terms of cycling stability, the pristine cell delivers a low initial discharge capacity of 65 mAh g −1 and then increases to 154 mAh g −1 after few cycles at a current density of 500 mA g −1 (Supporting Information Figure S17).This is tentatively ascribed to progressive Zn 2+ ion penetration into the CuTCNQ framework under a high rate. [37]evertheless, the capacity soon decreases to 14 mAh g −1 after 100 cycles.In contrast, the Zn//CuTCNQ cell in the presence of the graphene separator displays an initial discharge capacity of 115 mAh g −1 and sustains about 94 mAh g −1 after 100 charge/discharge cycles.Besides, the SEM images of the cathode materials after 100 cycles show that CuTCNQ still partially preserved the lamellar sturcture (Supporting Information Figure S18), indicating that the presence of graphene separator does not change the morphology of CuTCNQ.At a higher current density of 2000 mA g −1 , capacity of about 61 mAh g −1 can still be maintained after 500 cycles, which is much better than that of the pristine cell (Figure 4c).The remarkable improvement demonstrates that the shuttle effect of Cu + and TCNQ x-1 anions is suppressed by the graphene separator.Figure 4d shows Raman spectra of graphene separator before and after charging to 1.8 V.The peaks observed at 1589 and 1347 cm −1 for fresh graphene separator are ascribed to the sp 2 hybridized G-band and sp 3 hybridized D-band, respectively. [45]After charging, G and D-bands shift to lower wavenumbers at 1580 and 1344 cm −1 , respectively.In addition, two new peaks present at 2225 and 1202 cm −1 are attributed to the principal vibrational modes of C ≡ N stretching and C¼CH bending in TCNQ. [35,45]Based on the abovementioned analysis, it can be concluded that TCNQ is adsorbed on graphene interlayer via π-π stacking during the discharge/charge process.Since the TCNQ chromophore is sensitive to the UV-vis radiation, in situ UV-vis absorption spectra were collected to confirm the adsorption effect of the graphene interlayer during charge/discharge.The adsorption spectra of the electrolyte were monitored by UV-vis with an in situ cuvette-cell as described previously. [29,30]The illustration of the in-situ cuvette cell is shown in Supporting Information Scheme S3.As shown in Figure 4e, it can be clearly observed that without the graphene separator, a broad peak belonging to TCNQ increases significantly after two cycles, and the color of the electrolyte solution changes to green (inset of Figure 4e), indicating that TCNQ x-1 anions are dissolved in the electrolyte.However, when the CuTCNQ electrode is isolated by a Energy Environ.Mater.2024, 7, e12507 graphene separator, the adsorption peak of TCNQ is extremely weak (Figure 4f), and the color of the electrolyte has no obvious change (inset of Figure 4f).These results prove that the graphene interlayer can effectively alleviate the shuttling effect of TCNQ species and improve the utilization of active materials.In the CV curves with graphene separator (Supporting Information Figure S19), the reduction peak at 1.21 V belonging to Cu 2+ /Cu + redox becomes more intense, indicating that the graphene separator can also suppress the shuttle of Cu ions to some extent.
In practical applications, self-discharge performance is an important parameter that is often neglected.The suppression effect of the graphene interlayer on the self-discharge of the cell was further investigated.Specifically, after being fully charged to 1.8 V, the cells were rested for 120 h to monitor the decay of open-circuit voltage (OCV).As shown in Figure 5a, the OCV declined dramatically in the early stage and then declined steadily.The final OCV of the cell with graphene interlayer is 1.25 V, surpassing that of the pristine cell (1.07 V).Correspondingly, the capacity retention after 120 h rest is 80.7% and 64.2% for the graphene and pristine separator (Figure 5b).The stable OCV demonstrated that the graphene interlayer is capable of inhibiting the self-discharge of Zn// CuTCNQ batteries.As shown in Figure 5c, the average voltage and capacity of CuTCNQ are benchmarked with literature.The average discharge voltage of CuTCNQ can reach up to ~1.0 V with a capacity of 156 mAh g −1 at 200 mA g −1 , and this result is comparable to or even better than many other organic cathode materials, including NPC (~1.18 V, 86 mAh g −1 ), [46] PQ-MCT (~0.75 V, 125 mAh g −1 ), [47] PTD-1 (~1.08 V, 123 mAh g −1 ), [48] PDBS (~0.68 V, 212 mAh g −1 ), and [49] PNI-PTFE (~0.52 V, 160 mAh g −1 ). [15,23,25,31,38,50]][54][55] Based on the loading mass of active materials, Zn//CuTCNQ device can deliver an outstanding energy density of 162 Wh kg −1 at 102 W kg −1 and a peak power density of 1600 W kg −1 at 74.4 Wh kg −1 (Figure 5d).Aqueous zinc-ion batteries (AZIBs) assembled with other cathode materials are also listed in Figure 5d.It is shown that the energy density of our Zn//CuTCNQ cell is among the best and outperforms Zn//Cu 3 (HHTP) 2 , [23] Zn//Mn (BTC), [28] Zn//PI-COF, [51] Zn//PDBS, [49] Zn//Poly(1, 5-NAPD), [18] Zn//PQ-MCT, [47] et al. [15,25] In addition, to demonstrate the practical applications of Zn//CuTCNQ AZIBs, two devices connected in series are charged to power six red light-emitting diodes (LEDs) and a humidity/ temperature sensor (Figure 5e,f) for several minutes, demonstrating the potential of CuTCNQ cathode-based AZIBs for practical applications.

Conclusion
In summary, we have reported a redox-active MOF (CuTCNQ) as a novel organic cathode for AZIBs with high performance.Interestingly, during the discharge/charge process, both Cu + cations and TCNQ x-1 anions are found to be redox-active sites, thus delivering a high capacity of 158 mAh g −1 with three obvious discharge plateaus in the voltage window of 0.1-1.8V. Benefiting from the strong inductive effect of the cyano groups in CuTCNQ, the average voltage output of Zn// CuTCNQ cell can reach up to 1.0 V. Additionally, by employing a graphene interlayer, the cycling stability and anti-self-discharge behavior were enhanced by hindering the dissolution and shuttle effect of TCNQ x-1 anions and Cu + cations through π-π interaction and coordination bonds.These findings open a new gate to design and incorporate electrode materials with double redox active sites for highperformance multivalent-ion batteries.

Experimental Section
Synthesis of CuTCNQ: CuTCNQ was synthesized using a liquid reaction as previously reported. [41]Typically, CuI (285.68 mg, 1.5 mmol) and TCNQ (306.3 mg, 1.5 mmol) with a molar ratio of 1:1 were dissolved in 100 mL and 50 mL degassed acetonitrile, respectively.Then, the CuI solution was slowly added into the TCNQ solution.The mixture solution was stirred at room temperature under N 2 atmosphere for 12 h.Then, the formed dark blue crystal powder was obtained from the solution by centrifugation and washed with ethanol several times.Preparation of graphene separator: Electrochemically exfoliated graphene was prepared as previously reported. [6]Then, the as-prepared graphene suspension was dispersed in N, N-dimethylformamide (DMF) and deposited on top of the glass fiber separator (Whatman GF/D) by the vacuum filtration process.Finally, the graphene separator was obtained after drying at 60 °C for 6 h, and the loading was adjusted from 1 to 2 mg cm −2 .
Materials characterization: X-ray diffraction (XRD) pattern of the sample was collected on a Bruker D8 Advance diffractometer (Holland) with Cu-Kα radiation.The morphologies and structures of the samples were observed using field emission scanning electron microscope (FESEM, Mira-LMS; Tescan) and transmission electron microscope (TEM, Titan G260-300).The corresponding element mappings of the sample were performed on a Titan G260-300.The Fourier-transform infrared (FT-IR) spectra were obtained on an FT-IR spectrometer (Prestige 21; Shimadzu) in a KBr pellet at room temperature, with wavenumber ranging from 750 to 4000 cm −1 .UV-vis absorbance was characterized by a UV-vis spectrophotometer (UV-2550; Shimadzu).Raman spectra were applied with a LabRam HR800 Raman spectrometer excited by a 532 nm Nd-YAG laser (Jobin-Yvon).The X-ray photoelectron spectroscopy (XPS) data were recorded on a Thermo Scientific K-Alpha XPS system (Thermo Scientific, UK) using a monochromatic Al Kɑ X-ray source (1486.6 eV).For a better comparison of the spectral shapes, the intensities of the spectra were normalized for the samples at each state.
Electrochemical characterization: All the electrochemical tests were evaluated in CR2032-type coin cells at room temperature.The cathode electrodes consisted of 60 wt% CuTCNQ, 30 wt% super P conductive carbon and 10 wt% PVDF.The CuTCNQ electrodes were prepared by casting the slurry onto a carbon paper, and typical active material loading was about 0.8-1.0mg cm −2 .The zinc metal and graphene-modified Whatman glass fiber filter were used as the counter electrode and separator, respectively.In order to avoid the capacity contribution fromgraphene layer, a microporous nylon film was added between the cathode and graphene separator.A 2 M Zn(CF 3 SO 3 ) 2 aqueous solution was used as the electrolyte.The concentration of the aqueous electrolyte was also fine-tuned.0.5 M Zn(CF 3 SO 3 ) 2 in different organic solvents including trimethyl phosphate (TMP) and diglyme (DGM) were employed as non-aqueous electrolytes for comparison.The galvanostatic charge-discharge (GCD) profiles were obtained on a battery testing system (Neware CT-4008) with a voltage window from 0.1 to 1.8 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were carried out on an electrochemical workstation (Biologic SP150).EIS was recorded with an amplitude of 5 mV in the frequency range from 0.01 Hz to 100 kHz.Galvanostatic intermittent titration technique (GITT) experiments were conducted by discharging the device for 500 s at 200 mA g −1 , followed by 600 s relaxation.For the in situ UV-vis adsorption measurements, the cathode electrode was prepared by casting the slurry onto a stainless steel foil.A graphene-based separator covering the CuTCNQ electrode was encapsulated by parafilm.The parafilm near the graphene separator was drilled to allow the infiltration of the electrolyte.During ex situ FT-IR and XPS tests, the electrodes were extracted immediately after the cells reached the discharge/charge state, then washed with distilled water, and dried in vacuum oven.The energy density and power denstiy are calculated by the following equations [56] : where the E s , P s , C s , V a , M p , and t respresent the specific energy density, average specific power density, specific capacity of the cathode material, average voltage, mass loading of the CuTCNQ active material in the cathode, and discharge time, respectively.

Figure 1 .
Figure 1.a) X-ray diffraction pattern and Rietveld refinement of the obtained sample.b) Chemical structures of CuTCNQ (phase II).c) Scanning electron microscopy images of CuTCNQ, inset is the high magnification.d) Transmission electron microscopy (TEM) image and e) TEM-energy-dispersive spectroscopy mapping of Cu, C, and N elements of CuTCNQ.
−1 can be obtained at respective current densities of 100, 200, 500, 1000, and 2000 mA g −1 .The representative GDC profiles are displayed in Figure 4b.All the curves exhibit obvious plateaus, demonstrating fast Zn 2+ diffusion kinetics during the discharge/charge process.The polarization increases along with current density, which should be caused by the poor conductivity of charging products.To understand the charge transfer kinetics, electrochemical impedance spectra (EIS) were collected and illustrated in Supporting Information Figure S14.The EIS profiles for the cell with a modified separator show two obvious semicircles in high and medium frequency and a straight line in low

Figure 4 .
Figure 4. a) Rate performance of CuTCNQ electrodes with pristine and graphene separator.b) GCD curves of CuTCNQ with graphene separator.c) Long cycling life of CuTCNQ with pristine and graphene separator at 2000 mA g −1 .d) Raman spectra of the graphene separator before and after charging to 1.8 V.In situ UV-vis spectra of CuTCNQ for e) pristine and f) graphene separator.The insets in Figure 4e,f are the images of the cuvette cell with pristine and graphene separator after two cycles.

Figure 5 .
Figure 5. a) Self discharge test and b) galvanostatic discharge/charge curves of CuTCNQ with pristine and graphene separator.c) Comparison of CuTCNQ with representative reported organic cathodes.d) Ragone plot of CuTCNQ in this work compared with the representative organic cathode materials reported in literature.Demostration of two charged Zn-CuTCNQ batteries connected in series to power e) six red LEDs and f) a humidity/temperature sensor.