Significantly raising tetracyanoquinodimethane electrode performance in zinc‐ion battery at low temperatures by eliminating impurities

Applications of organic compounds‐based electrodes in aqueous zinc‐ion batteries (AZIBs) at low temperatures are severely restricted by the freezing of aqueous electrolytes and the inferior dynamic behavior of organic electrodes below zero. Herein, tetracyanoquinodimethane (TCNQ) was purified by the sublimation method and used as a cathode in AZIBs to investigate electrochemical Zn storage performance in comparison with nonpurified TCNQ at a temperature range of 25°C to −40°C. Nuclear magnetic resonance and elemental analysis prove increased purity in purified TCNQ (p‐TCNQ), whereas scanning electron microscope and Brunner−Emmet−Teller data verify reduced particle size and increased surface area of p‐TCNQ. Kinetic analysis demonstrates that p‐TCNQ is a more surface‐controlled electrode process than TCNQ and offers much higher ionic diffusivity than the latter at various temperatures. Molecular dynamics simulation validates that the existence of impurity increases the absorption energy of TCNQ in a TCNQ//Zn system that is unfavorable to Zn migration. Comprehensive analysis, including ex situ X‐ray diffraction, Fourier transform infrared, Raman, and electron spin‐resonance spectroscopy characterization confirm the high reversibility of transformation between C≡N and −C═N groups in p‐TCNQ. This work provides a simple, environmentally friendly strategy to fabricate a high‐performance AZIB at low temperatures while offering fundamental chemistry insight into organic electrode performance, thus possessing universal significance.


| INTRODUCTION
Lithium-ion batteries have accomplished great success in portable electronics, electric vehicles, and energy storage stations. However, the limited and expensive lithium metal resources bring great challenges to broad and large-scale applications. Zinc-ion batteries (ZIBs), in particular, the aqueous ZIBs (AZIBs), which are inexpensive, resource-abundant, highly safe, and ecologically friendly, [1][2][3][4][5] hold great promise as alternatives for large-scale energy storages.
Up to date, organic electrode materials as new stars have attracted great interest in AZIBs by their natural abundance, facile synthesis, diverse structure, and flexible design. Numerous organic compounds such as carboxylic hydride, polyaniline, 6 polycatechol, 7 p-Chloranil, 8 and pyrene-4,5,9,10-tetraone 9 have been reported as cathodes for AZIBs. Nonetheless, organic AZIBs at low temperatures severely suffer from easy freezing of the aqueous electrolyte below zero and pretty low electronic/ionic conductivity. To prevent freezing of the aqueous solution, breaking the hydrogen-bond network in an electrolyte is essential. Recently, Chen et al. 10 modified a 7 M ZnCl 2 electrolyte to suppress the freeze of water and depress the solid-liquid transition temperature from 0°C to -114°C. Tao et al. 11 reported 4 M Zn (BF 4 ) 2 electrolytes with a low freezing point (−122°C) and a high ion conductivity (1.47 mS cm −1 at −70°C). Specific electrolyte additives can also lower the freezing point. For instance, N,N-Dimethylformamide, ethylene glycol, and methanol were added in ZnSO 4 or ZnCl 2 electrolytes to achieve high performance in low-temperature AZIBs (LTAZIBs). [12][13][14] Apart from endeavors on electrolytes for low freezing points and high ionic conductivities, their ionic and electronic conductivity still face great challenges for further improvement to achieve highperformance organic LTAZIBs.
Tetracyanoquinodimethane (TCNQ) is an organic semiconductor and has been reported for Li/Na/K ion storage. [15][16][17][18][19] Recently, TCNQ was employed as a cathode for AZIBs at room temperature. 20 Unfortunately, it only delivered half of its theoretic capacity at a current density of 100 mA g −1 in 1 M ZnSO 4 electrolyte and the capacity dropped fast with cycles (only 50% capacity retention after 30 cycles) due to the partial dissolution of Zn-TCNQ. Later, Shen et al. 21 improved the capacity and cyclability of Zn//TCNQ battery by using 2 M Na 2 SO 4 , 1 M ZnSO 4 , and 1 M Al 2 (SO 4 ) 3 as electrolytes. Despite many reports on the energy storage performance of TCNQ cathode, the impurity effect of TCNQ on its electrochemical behavior has not been studied. We should express our deep thanks to the interesting works conducted in the last century on the purification of TCNQ, then disclosing that the purity plays a vital role in the conductivity improvement due to the low dimensional character. 22,23 In this work, highly purified TCNQ (p-TCNQ) was prepared through simple sublimation as a cathode for LTAZIBs. The batteries with p-TCNQ using 3 M ZnCl 2 + 1 vol% Polyethylene glycol (PEG) additive demonstrated a superior rate and cycling property than nonpurified TCNQ (TCI > 98%) at both room and low temperatures. The mechanism behind the improved electrochemical performance of p-TCNQ was further investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) technologies, and molecular dynamic simulation. Evolution of molecular/crystal structure and morphology of p-TCNQ during charging/discharging was examined by ex situ Fourier transform infrared (FTIR), Raman, X-ray diffraction (XRD), and scanning electron microscope (SEM) characterizations. This work provides a new opportunity for designs of high-performance AZIBs at low temperatures, while disclosing the mechanism of the improved p-TCNQ electrode performance.

| RESULTS AND DISCUSSION
Purification of TCNQ before its use is an important procedure in organic semiconductor fabrications 24,25 ; however, rare attention has been paid to the purity of TCNQ in battery uses up to date. The preparation of p-TCNQ illustrated in Scheme 1 shows pristine TCNQ without sublimation are brown-colored particles. After sublimation, the bright yellow powder is collected on the surface of the condenser tube as p-TCNQ.
The crystal structure and morphology of TCNQ and p-TCNQ were studied by using XRD and field emission SEM. As shown in Figure 1A, the p-TCNQ sample shows diffraction peaks at 2θ = 10.9°, 17.6°, 18.8°, 21.5°, 21.8°, S C H E M E 1 Schematic diagram for preparation of purified tetracyanoquinodimethane (p-TCNQ).
F I G U R E 1 (A) X-ray diffraction (XRD) patterns of tetracyanoquinodimethane (TCNQ) and purified TCNQ (p-TCNQ). (B) Raman spectroscopy of TCNQ and p-TCNQ. X-ray electron spectroscopy: (C) the full spectra; (D) C 1s spectra; (E) O 1s spectra; (F) N 1s spectra of TCNQ and p-TCNQ. 25.8°, 27.6°, and 30.6°, which is in good correspondence with the standard XRD pattern of TCNQ single crystal. The corresponding characteristic diffracts for p-TCNQ are assigned to (0 0 2), (1 1 1), (1 1 2), (2 0 2), (0 0 4 ), (0 2 0), (2 0 4), (0 2 3), and (3 1 0) planes of the crystal (monoclinic C 2/c space group, COD No. 2300562). However, the peak intensity of p-TCNQ on the crystal planes (0 0 2), (2 0 2), (0 0 4 ), and (3 1 0) are different from that of the nonpurified TCNQ diffraction peak, indicating that the surface preferred orientation growth of p-TCNQ may be different from that of nonpurified TCNQ. Although the XRD pattern for TCNQ shows extra sharp peaks at 22.8°and 33.1°, representing diffraction peaks of impurities in the sample. The Raman spectra of TCNQ and p-TCNQ are shown in Figure 1B. As shown, p-TCNQ shows only four peaks at 2222, 1600, 1451, and 1202 cm −1 , which are ascribed to vibrations of C≡N, C═C, C−C, and C−H bonds, respectively. TCNQ shows exceptional peaks at 1652, 1320, 1183, and 944 cm −1 , belonging to vibrations for C═O, -COO-, C−C, and pyridine skeleton bonds, respectively, which probably originated from solute or solvents (ammonium acetate, pyridine) and intermediate products (cyclohexanedione) during synthesis of TCNQ. [26][27][28] The difference between the chemical structure of TCNQ and p-TCNQ was further analyzed by X-ray photoelectron spectroscopy (XPS). According to the full spectrum ( Figure 1C), TCNQ contains extra elements such as Br, S in compassion with p-TCNQ, which is most probably due to the residual solute (NaHSO 3 and N-Bromo succinimide). [26][27][28] Highresolution C 1s XPS spectra of the TCNQ and p-TCNQ are compared in Figure 1D. As can be clearly observed, p-TCNQ exhibits fitting peaks at 284.8, 285.8, 287.1, and 289.9 eV, which are ascribed to C═C,C−C/C−H, 29 C≡N groups, and π-π* satellite peaks, respectively. In addition, the O 1s spectra in Figure 1E demonstrate that two extra peaks at 531.2 and 533.7 eV, which are attributed to -OH and C═O groups, respectively, appear for the TCNQ sample. These additional peaks should be attributed to reactants (1,4-cyclohexanediol) and intermediate residues (cyclohexanedione) during TCNQ synthesis. XPS spectra of N 1s for these two samples are shown in Figure 1F. As can be observed, the fitting peaks for N 1s at 399 eV belonged to the C≡N bond, whereas the peak for TCNQ at 399.7 eV is attributed to pyridine N, which should be attributed to the residue of pyridine during the synthesis of TCNQ. 1 H Nuclear magnetic resonance spectroscopy and elemental analysis (EA) were used to further characterize the purity of TCNQ and p-TCNQ. As can be seen from Supporting Information: Figure S1, TCNQ and p-TCNQ show 1 H peak at 7.56 p.p.m., respectively, TCNQ also displays additional peaks at 7.71 and 5.15 p.p.m., respectively, which can be attributed to its impurities. What's more, EA data in Supporting Information: Table S1 specifies that the mass ratio of N, C, and H atoms in the p-TCNQ sample (26.6%, 69.94%, 1.92%) is closer to the theoretical value (27.45%, 70.58%, 1.96%), but in contrast, the mass ratio of TCNQ is 26.42%:69.62%:1.899%, thereby clearly confirming that p-TCNQ is much purer than TCNQ. FTIR and Ultraviolet (UV) and visible spectrophotometry of TCNQ and p-TCNQ in Supporting Information: Figure S2 demonstrate that no obvious absorption difference can be observed between TCNQ and p-TCNQ samples, implying the low sensitivity of FTIR and UV technology in distinguishing impurities in organic samples.
The morphology of TCNQ was analyzed by SEM. As shown in Figure 2, the TCNQ are big uniform square-sized particles with a thickness of about 30 μm, whereas after sublimation, the p-TCNQ are much smaller particles with a size of around 1-10 μm; moreover, Brunner− Emmet−Teller measurement in Supporting Information: Figure S3 shows that after sublimation, the surface area of TCNQ is increased from 1.1 to 3.8 m 2 g −1 .
The electrochemical performance of p-TCNQ was tested in ZnCl 2 electrolyte, after a series of optimization (Supporting Information: Figure S4-5); 3 M ZnCl 2 (1 vol% PEG-200) was used as the electrolyte since p-TCNQ demonstrates relatively high capacity and capacity retention in this electrolyte at 0°C. PEG (200) is selected as an electrolyte additive, as it has been reported to regulate dendrite growth and improve the surface morphology of zinc electrodes by affecting the ratio of nucleation to growth, thus improving the longterm cycling stability of AZIBs. 30 Moreover, it exhibits a low freezing point of −58°C and stable cycling stability in a symmetric Zn//Zn battery for over 60 h (Supporting Information: Figure S6). Figure 3 shows the electrochemical performance of Zn//p-TCNQ battery, as can be seen from Figure 3A respectively, at the same condition. Moreover, the p-TCNQ electrode shows lower hysteresis (△V) than the TCNQ electrode at all temperatures, implying lower polarization of the p-TCNQ electrode. The rate capability of p-TCNQ and TCNQ was evaluated at 0°C and −20°C. As shown in Figure 3C,D, AT rates of 1 and 3 A g −1 under 0°C, p-TCNQ delivers capacities of 180 and 168 mAh g −1 respectively, whereas TCNQ only demonstrates capacities of 100 and 75 mAh g −1 accordingly. At a low temperature of −20°C, p-TCNQ shows a capacity of 75 mAh g −1 at 1000 mA g −1 , whereas TCNQ only delivers a capacity of 45 mAh g −1 . Besides the superior rate capability of p-TCNQ over TCNQ at low temperatures, p-TCNQ also demonstrates better cycling stability at low temperatures. As shown in Figure 3E, after 300 cycles at 1000 mA g −1 under 0°C, the capacity retentions for p-TCNQ and TCNQ are 83% and 60%, respectively. Moreover, after 300 cycles at 1000 mA g −1 under −20°C, the capacity retentions for p-TCNQ and TCNQ are 50% and 40% ( Figure 3F), respectively. Even at −40°C, p-TCNQ shows higher capacity than TCNQ (96 and 57 mAh g −1 for p-TCNQ and TCNQ, respectively, at 100 mA g −1 ), and after cycling 250 times at 100 mA g −1 , the capacity retentions for p-TCNQ and TCNQ are 94% and 93% (Supporting Information: Figure S7), respectively, demonstrating high cycling stability at low temperature.
The obviously improved electrochemical performance of p-TCNQ at low temperatures should be related to its enhanced dynamic behavior. To prove our assumption, CV curves of TCNQ and p-TCNQ were scanned at different rates under 0°C and comparatively analyzed. As shown in Figure 4A,B, both p-TCNQ and TCNQ show two reductive peaks and one oxidative peak, which are in good agreement with the discharge/charge curve in Figure 3A,B. However, the peak shape for p-TCNQ is sharp and narrow, whereas that for TCNQ is obtuse and wide. According to previous reports, 31,32 the currents (i) and scan rates (v) obey the law of Equation (1), where the value of b can be indexed to judge the energy storage types. Typically, when b is close to 1, it indicates a capacitive-controlled behavior, but when b approaches 0.5, a diffusion-controlled behavior is suggested. Figure 4C,D show the linear relationship of log(v) and log(i) of p-TCNQ ( Figure 4C) and TCNQ ( Figure 4D) at different potentials under 0°C, by which b values of Peaks 1-3 for these two samples are calculated to be 0.945/0.319/0.554 and 0.486/0.383/0.642, respectively, indicating that the kinetic behaviors for p-TCNQ and TCNQ are determined by a combination of ionic diffusion and capacitance behaviors. 33 The total capacity can be divided into the capacitive capacity (k 1 v) and diffusion-controlled capacity (k 2 v 1/2 ), as shown in Equation (2). 34,35 Figure 4E shows that the capacitance contribution ratio for the Zn//p-TCNQ battery is increased from 47.2% to 55.2% with the increase of sweep speed from 0.5 to 1.1 mV s −1 , whereas the capacitance contribution ratio of Zn//TCNQ is lower than that of Zn//p-TCNQ battery at all sweep speeds. The same performance is observed at room temperature (Supporting Information: Figure S8), demonstrating that the electrochemical reaction of p-TCNQ is a more surface-controlled electrochemical process than TCNQ. The higher proportion of surface-controlled behavior for p-TCNQ sample should be attributed to the decreased particle size and increased surface area that would facilitate ionic migration and charge transfer during a redox reaction, thus being conducive to the realization of higher capacity and better rate capability.
EIS characterization at 25°C, 0°C, −20°C, and −40°C was conducted to further explore the factor of improved electrochemical performance for p-TCNQ over TCNQ electrodes. As can be seen in Figure 5, each EIS diagram consists of a semicircle over a high-frequency region and an oblique line over a low-frequency region, which are associated with Faraday charge transfer resistance (R ct ) and Warburg impedance (Z w ), respectively. As shown, the R ct of the Zn//p-TCNQ battery (250, 505.0, 932.6, 1665.0 Ω, respectively) is much lower than that of the Zn//TCNQ battery (320, 755, 1370.0, and 2500.0 Ω, respectively) at all temperatures. To better analyze the kinetic behavior of these two samples, we calculated the diffusion coefficient of Zn 2+ by the following formula: (3) F: 96,500 C mol −1 , R: 8.314 J K −1 mol −1 , A is the electrode area which is 0.785 × 10 −4 m 2 , n is 2, C is molarity, σ is the slope of the line Z' to ω −1/2 . When taking the log value of D Zn 2+ to the plot, it can be found that p-TCNQ has a higher diffusion coefficient than TCNQ at both room and low temperatures. As presented in Figure 5C, the lower R ct and higher D Zn 2 + of p-TCNQ should be owing to the smaller particle size and high purity that enable smooth ion migration in p-TCNQ crystal, thus contributing to better electrochemical performance, especially at lower temperatures. The activation energy (E a ) is calculated according to Arrhenius equation: R: 8.314 J K −1 mol −1 , A is a constant. As shown in Figure 5D, EA values of p-TCNQ and TCNQ are 16.88 and 18.11 kJ mol −1 , respectively. The lower E a for p-TCNQ further proves better adapting of p-TCNQ to low temperatures, which is in good correspondence with the electrochemical performance shown in Figure 3.
To deeply understand the influence of impurity on the migration of Zn 2+ in Zn//TCNQ battery, we compared the absorption (or interaction) energy in p-TCNQ/Zn and TCNQ/Zn system using molecular dynamics simulation. Figure 6 is the model that we established for these two systems; simulation details and calculation result are given in Supporting Information: Table S2. It can be calculated that the adsorption energy of Zn to p-TCNQ is −221.476 kcal/mol, which is less than −224.418 kcal/mol of the system containing impurities, turning out that the p-TCNQ is more favorable for Zn ion migration. 36 The underlying Zn-ion storage mechanism and morphological evolution of the p-TCNQ electrode during charging/ discharging were studied by ex situ ESR, FT-IR, Raman, XRD, and SEM measurements ( Figure 7B-J). The ex situ ESR technique was used to detect unpaired and delocalized electrons that reflects the redox reaction of p-TCNQ during charge/discharge process. [37][38][39][40][41] As shown in Figure 7B, the pristine p-TCNQ electrode shows weak ESR resonance due to delocalized electrons within its core. 42 As the discharging continues, electrons are constantly excited, resulting in a large number of delocalized electrons, and the content of free radicals keeps increasing, so the signal strength of ESR becomes intense. Along the charging process, the content of free radicals gradually decreases, and the signal intensity of ESR gradually weakens. At the end of charging (1.35 V vs. Zn 2+ /Zn), the ESR signal basically returns to that at the open circuit, indicating a reversible electrochemical process. Figure 7C is the ex situ FTIR spectrum for the p-TCNQ electrode. As shown, along discharge, the absorption for the N≡C-bond is shifted to a lower wavenumber; meanwhile, a characteristic peak of C═N appears near 1621 cm −1 , indicating that −C≡N is converted to C═N, which is due to the insertion of Zn 2+ . What's more, the typical absorption peak for C═C is blue-shifted along discharge, indicating that an unsaturated double bond in the benzene ring is generated. At the same time, the stretching vibration peak for C-H in the benzene ring is also generated, which further indicates the formation of the benzene ring in the discharging process. However, during the charging process, the peak for the N≡C− bond is  reversibly shifted to a higher wavenumber and the characteristic peak of the C═C bond emerged again; in addition, the vibration for alkene C-H stretching at 3046.74 cm −1 appeared, indicating that the benzene ring is converted back to the original six-membered ring structure.
The ex situ Raman spectrum in Figure 7D shows that part of the N≡C-bond is gradually changed to C═C═N and the characteristic peak of the C-C bond in the benzene ring appears near 1290 and 1320 cm −1 ; it further verifies that benzene rings are generated in the charging and discharging process. Ex situ XPS was used to further confirm the presence of the -C≡N group in the p-TCNQ electrode after full charge and discharge. Supporting Information: Figure S9 shows that at pristine state (Supporting Information: Figure S9a,d), XPS peak for -C≡N is the main functional group, while at 0.5 V (Supporting Information: Figure S9b,e), part of -C≡N is converted to C═N, symboling the reduction of TCNQ to Zn-TCNQ. At 1.35 V (Supporting Information: Figure S9c,f), C═N disappears, indicating that Zn-TCNQ is transformed into TCNQ.
Ex situ XRD characterization was conducted to study crystal change during the reaction. As shown in Figure 7E two new diffraction peaks are generated near 2θ = 9.2°and 21.8°and the peak intensity continuously enhances along discharging, indicating that a new substance is formed, which is preliminarily determined to be Zn-TCNQ. 20 When charging is completed, the diffraction peaks are recovered to that of the initial state, indicating a highly reversible process of p-TCNQ during charging/discharging. Morphology evolution during the redox reaction was illustrated by SEM images. As shown in Figure 7F-J, AT pristine state, the TCNQ particles are well embedded in the conductive carbon. When discharged to 0.77 V, rod morphology is observed on the surface of the electrode; when fully discharged to 0.5 V, more rods appeared, exhibiting the formation of Zn-TCNQ, since it was reported to be onedimensional rods. 20 When charged to 1.16 V, the TCNQ particles gradually recover. At a fully charged state (1.35 V vs. Zn 2+ /Zn), the TCNQ electrode returns to its original morphology, which also illustrates the reversibility of TCNQ along charging and discharging.

| CONCLUSIONS
In summary, we demonstrated the high performance of LTAZIBs with the use of a highly p-TCNQ as the cathode in a 3M ZnCl 2 (1 vol% PEG200) electrolyte. At 0°C, p-TCNQ delivered a capacity of 250 mAh g −1 at a current density of 100 mA g −1 and a high capacity of 160.7 mAh g −1 at a current density of 3 A g −1 . The p-TCNQ could still accomplish capacities of 172.9 and 96.6 mAh g −1 at −20°C and −40°C, respectively, at a current density of 100 mA g −1 .
The high performance of p-TCNQ at low temperatures can be attributed to the high purity, decreased particle size and increased surface area to fascinate fast ion diffusion in crystal, while enabling high discharge rates. Molecule dynamic simulation proves that the absorption energy of the p-TCNQ/Zn system is lower than that of the nonpurified one, thereby favoring Zn 2+ migration. In addition, ex situ FTIR, electron paramagnetic resonance, XRD, and Raman measurement further confirm that the reaction center is N≡C-group and the redox is mainly the transformation between p-TCNQ and Zn-TCNQ, which is highly reversible. Our work provides a new and high-performance organic electrode for LTAZIBs. More importantly, we disclose the mechanism for significantly boosting the electrode performance, thus offering a solution for the improvement of the performance of small molecule organic electrodes with universal significance.